EFFECTS OF CUBAN TREEFROG (OSTEOPILUS SEPTENTRIONALIS) REMOVAL
ON NATIVE FLORIDA HYLA POPULATIONS
by
Miranda Cunningham
A Thesis Submitted to the Faculty of
The Charles E. Schmidt College of Science
In Partial Fulfillment of the Requirements for the Degree of
Master of Science
Florida Atlantic University
Boca Raton, FL
May 2015
Copyright 2015 by Miranda Cunningham
ii
ACKNOWLEDGEMENTS
The author wishes to express sincere gratitude to her committee members for all
of their guidance and support. The author is grateful to the Florida Park Service and
Jonathan Dickinson State Park for allowing research to be conducted on state land. Last
but not least, the author wishes to thank all of the park staff and volunteers that helped
collect the field information, especially Ernie Cowan and Jeff Bach.
iv
ABSTRACT
Author:
Miranda Cunningham
Title:
Effects of Cuban Treefrog (Osteopilus Septentrionalis) Removal
on Native Florida Hyla Populations
Institution:
Florida Atlantic University
Thesis Advisor:
Dr. Jon Moore
Degree:
Master of Science
Year:
2015
Invasive species are one of the major threats to biodiversity and understanding the
effects any one invasive species has on members of its new ecosystem can help land
managers decide how to best use their limited resources. This study attempted to show
the effect Cuban Treefrogs (Osteopilus Septentrionalis) were having on native Florida
hylids. For a year, Cuban Treefrogs were removed from three cypress domes and
monitored in three other cypress domes, a change in the native population in the
experimental domes was the eventual desired effect. Due to weather issues and low
native hylid numbers no effect was shown, however due to environmental constraints an
effect could not be ruled out either.
v
DEDICATION
This manuscript is dedicated to my family, especially my husband, Rob, who has
encouraged and helped beyond what one could ever hope for, and to my daughter,
Sophie, whose life has brought meaning to mine. I also dedicate this work to the late
Hank Smith, whose belief in me both professionally and academically made much of this
work possible.
EFFECTS OF CUBAN TREEFROG (OSTEOPILUS SEPTENTRIONALIS) REMOVAL
ON NATIVE FLORIDA HYLA POPULATIONS
List of Tables ..................................................................................................................... ix
List of Figures ..................................................................................................................... x
Introduction ......................................................................................................................... 1
The Impacts of Invasive Species ..................................................................................... 2
Control of Invasive Species ............................................................................................. 3
Cuban Treefrog Life History ........................................................................................... 3
Florida Native Treefrogs Life Histories .......................................................................... 5
Background and Rationale .................................................................................................. 7
Study Site ............................................................................................................................ 8
Study Objectives ............................................................................................................... 10
Hypothesis ..................................................................................................................... 11
Methods............................................................................................................................. 12
The Cypress Domes ....................................................................................................... 12
PVC Pipe Refugia .......................................................................................................... 13
Statistical Analyses ........................................................................................................... 16
Results ............................................................................................................................... 17
Discussion ......................................................................................................................... 37
Appendix ........................................................................................................................... 46
vii
References ......................................................................................................................... 51
viii
LIST OF TABLES
Table 1. Date of Surveys .................................................................................................. 15
Table 2. Average Number of Cuban Treefrogs and Native Hylids Caught by Dome. .... 19
Table 3. Average Number of Native Species Caught by Dome ...................................... 21
Table 4. Study P-values and F ratios. .............................................................................. 23
ix
LIST OF FIGURES
Figure 1. Locations in Jonathan Dickinson State Park of Six Cypress Domes. ................. 9
Figure 2. Initial Numbers of Native Hylids in All Six Domes ......................................... 18
Figure 3. Initial Numbers of Cuban Treefrogs (Os) in All Six Domes............................. 18
Figure 4. Average Number of Native Hylids and O. septentrionalis Caught Over All
17 Surveys ................................................................................................................... 19
Figure 5. Total Number of Hylids Caught Across All Domes and Surveys. .................... 20
Figure 6. Total Number of O. septentrionalis Versus Native Hylids Average by
Dome.. ......................................................................................................................... 20
Figure 7. Average Number of Native Hylids Caught in Surveys by Dome. .................... 21
Figure 8. Total number of frogs by species in each dome for all surveys. ....................... 22
Figure 9. Osteopilus septentrionalis in Control Versus Removal Domes. ...................... 24
Figure 10. Number of Cuban Treefrogs (O. septentrionalis) Control Versus
Removal Domes Prior to Removal. ............................................................................ 25
Figure 11. Total Number of O. septentrionalis Captured in Control and Removal
Domes by Survey ........................................................................................................ 25
Figure 12. Combined Numbers of All Four Native Hylid Species in Control Versus
Removal Domes .......................................................................................................... 26
Figure 13. Treatment by Survey for All Four Native Hylid Species Combined. ............ 26
Figure 14. Hyla femoralis in Control Versus Removal Domes ....................................... 27
x
Figure 15. Treatment by Survey for H. femoralis ............................................................ 27
Figure 16. Hyla gratiosa in Control Versus Removal Domes ......................................... 28
Figure 17. Treatment by Survey for H. gratiosa. ............................................................ 28
Figure 18. Hyla squirella in Control Versus Removal Domes ........................................ 29
Figure 19. Treatment by Survey for H. squirella............................................................. 29
Figure 20. Hyla cinerea in Control Versus Removal Domes. ......................................... 30
Figure 21. Treatment by Survey for H. cinerea.. ............................................................. 30
Figure 22. Average SVL of O. septentrionalis Difference Between Control and
Experimental Treatment.............................................................................................. 31
Figure 23. Average SVL of O. septentrionalis Treatment by Survey ............................. 31
Figure 24. Total number of O. septentrionalis in Control Versus Experimental
Treatment .................................................................................................................... 32
Figure 25. Osteopilus septentrionalis Treatment by Survey Excluding Pre-and PostRemoval Data.............................................................................................................. 32
Figure 26. Ratio of O. septentrionalis in Control Domes Versus Removal Domes in
Pre- and Post-Treatment Surveys................................................................................ 33
Figure 27. Number of O. septentrionalis for the 17 Surveys........................................... 34
Figure 28. Average Number of Treefrogs Per Survey for Control Domes, Grouped
by Months Since a Freeze Event ................................................................................. 35
Figure 29. Only Surveys Were Domes Differed in Moisture Levels, in All Other
Surveys Domes Were Either All Wet or All Dry ....................................................... 36
Figure 30. Number of O. septentrionalis Caught in Control Domes Only, Wet
Versus When They Were Dry ..................................................................................... 36
xi
Figure 31. Average Annual Rain Fall by Month in West Palm Beach, Florida, From
1981-2010. .................................................................................................................. 39
xii
INTRODUCTION
Invasive plants and animals pose several of threats to indigenous species,
ecosystems, human activities, and are hard to control (Simberloff et al., 1997). The
economic impacts invasive species have in control costs, in native species losses, and
agricultural damages are over 126 billion dollars a year in the United States alone
(Pimentel et al., 2005). In Florida, plants like Lygodium microphyllum (Cavanilles,
1810) cover cypress strands causing headaches for prescribed burners. Feral hogs (Sus
scrofa; Reichenbach, 1846) cause major ecological ground disturbances (Engeman et al.,
2004a). Cuban Treefrogs (Osteopilus septentrionalis; Duméril and Bibron, 1841), the
focus of this study, not only compete with, but also prey on, native hylids (Meshaka,
2001 and 2011). Florida is one of the leading states for invasive species and South
Florida is the top region in the country for exotic animals (Ferriter et al., 2005). Many
consider invasive species to be one of the top threats to biodiversity, second only to
habitat destruction and fragmentation (Vitousek et al., 1996). Not all non-indigenous
species become established, nor do all that do survive become invasive causing problems
ecologically (Simberloff et al., 1997). Many are innocuous or their threat is at least to
this point unknown. However, for those species that do become invasive and cause
problems within an ecosystem, management action is needed. Removal is often a just a
stop gap of maintaining a population at an acceptable low level, though eradication is
usually not an option for a well-established invasive species (Simberloff et al., 2005).
1
The impacts an invasive species has on native species can be divided into two
basic categories, direct and indirect effects (Simberloff et al., 1997). A direct effect is
when one species impacts another through predation, parasitism, or competition for the
same food source or reproductive habitat (Groom et al., 2006; Moon et al., 2012). In
contrast an indirect effect is when one species impacts another through the use of a third
party (Moon et al., 2012). Observing the direct effects of an invasive species is often a
much easier task than that of indirect effects. One example of a direct effect is that of the
invasive Cane Toads (Rhinella marina; Linnaeus, 1758) in tropical Australia, they have
caused the major decline of several predatory monitors who eat the toads and then die of
the toxic poisons released by the toads (Doody et al., 2012). In many places the toads
have caused a direct decrease in the predators by as much as 97% (Doody et al., 2012).
Some indirect effects include apparent competition and trophic cascades (Groom et al.,
2006, Moon et al., 2012) that are much more difficult to show because they require a
greater understanding of community and ecosystem (Doody et al., 2012) rather than the
simpler two species interaction. An indirect effect of Cane Toad invasion occurred with
the Common Tree Snake (Dendrelaphis punctulatus; Gray, 1826), which is a prey species
for monitors. As the monitor population was significantly decreased by the arrival of the
Cane Toad, the Common Tree Snake saw an increase in its population, most likely from
the release of predation by the monitors (Doody et al., 2012). Many invasive species
cause both direct and indirect effects though understanding the complete picture is often
beyond the scope of one study. This project is an investigation to better understand the
relationships, direct or indirect, between the Cuban Treefrog (O. septentrionalis)
population and the native hylid treefrog populations.
2
Control for invasive species has three main parts; prevention, eradication, and
maintenance at a low level (Simberloff et al., 2005). Common problems to all three
include: 1) multiple agencies and stakeholders, which can produce jurisdictional
conflicts, 2) funding and adequate resources, and 3) disagreement of the threat
assessment of any one invasive species (Simberloff et al., 2005). The first part of control
is preventing an invasive species gaining entry into a foreign land and has little to do with
property managers. Prevention has more to do with state and federal policies and
regulations along with the private industry self regulating (Simberloff et al., 2005).
When these policies and regulations fail, it leaves property managers to pick up the
pieces in the next two phases. Eradication is most likely to be successful when
infestations are caught early, but efforts are often stymied by lack of sufficient resources
(Simberloff et al., 2005). Finally, maintenance at a low level is where many invasive
species programs find themselves. Common maintenance control methods include
mechanical, chemical and biological (The National Invasive Species Council, 2005).
Both mechanical and chemical control can be labor intensive and expensive, but are more
traditional methods with less controversy than biological controls. Biocontrols can be
effective if they work (Simberloff et al., 2005). The problem with biocontrols is that,
more often than not, they do not work, are expensive to develop, and in some cases the
biocontrols can do more harm than good and can eliminate native species instead of
invasive (Cowie, 2001). For this study, the control method used to remove Cuban
Treefrogs is mechanical.
The Cuban Treefrog, the focus of this study, was first discovered in Key West in
1931 (Barbour, 1931) and was found in mainland South Florida in the 1940’s in Dade
3
County (Meshaka, 2001). In 1994 the Cuban Treefrog was captured at Jonathan
Dickinson State Park (JDSP) in Martin County; however, it was thought to only exist in
ruderal areas particularly around buildings and not much of a threat to invade the natural
habitats (Timmerman et al., 1994). However, a recent study showed that Cuban
Treefrogs were found in cypress domes, far from any developed areas (Rossmanith and
Cunningham, unpublished data).
Originally a native of Cuba, the Cayman Islands, the Bahamas and the Isle of
Pines, the Cuban Treefrog is a large hylid of the West Indian genus Osteopilus (Meshaka,
2001). There are seven other species in this genus and all eight share the distinct
characteristic of having the skin fused to the skull, making them easily distinguishable
from other hylids (Faivovich et al., 2005). The Cuban Treefrog also has large toe pads in
comparison to other Florida hylids and in size dwarfs the native Florida tree frog species,
with an average size range of 2.5 to 10 cm and some females can reach 15 cm (Johnson,
2010). The largest native, Barking Treefrog (Hyla gratiosa; LeConte, 1857), is at most
6.9 cm (Wright and Wright, 1942). Female Cuban Treefrogs are a third to twice as large
as males (McGarrity and Johnson, 2009). They are an aggressive predators, shown to be
a frog eater and cannibal (Meshaka, 2001; Wyatt and Forys, 2004). Their diet also
includes many different invertebrates, small snakes and lizards (Johnson, 2010). Studies
imply that when Cuban Treefrogs establish in an ecosystem, the native tree frog
population goes down (Meshaka, 2001; Rice et al., 2011). Homeowners in Florida have
claimed for years, that Cuban Treefrogs have replaced the native frogs they used to see
around their homes (Johnson, 2010). The goal of this study is to experimentally test the
effect of Cuban Treefrogs on native tree frog species.
4
The four native tree frog species of Jonathan Dickinson State Park, Green
Treefrog (H. cinerea; Schneider, 1799), Pine Woods Treefrog (H. femoralis; Bosc, 1800),
Barking Treefrog (H. gratiosa) and Squirrel Treefrog (H. squirella; Bosc, 1800), are
similar to each other. All have a diet which consists of insects. All breed in the spring
and summer, ranging from March to September (Wright and Wright, 1942). Tadpoles
generally metamorphose around the same length of 50-75 days (Wright and Wright,
1942). The Barking Treefrog has the shortest time to metamorphosis with the low end of
its range at 40 days and Pine Woods Treefrog the longest having a range up to 75 days
(Wright and Wright, 1942). Frog size and egg production are the biggest differences
among the native hylids. Pine Woods Treefrog and Squirrel Treefrog are the smallest in
size with a range of 2.5 – 3.8 cm and 2.2 – 4.1 cm, respectively. Green Treefrog is a
medium to large frog at 3.2 – 6.4 cm and Barking Treefrog is the largest at 5.1 – 6.9 cm
(Wright and Wright, 1942). Egg production can be singular (Barking Treefrog and
Squirrel Treefrog) or in small films (Green Treefrog and Pine Woods Treefrog) (Wright
and Wright, 1942). Native hylids can all be found in moist areas in trees, swamps or
wetlands, with Barking Treefrog and Pine Woods Treefrog also being in trees in drier
systems like pine flatwoods.
The Cuban Treefrog has characteristics predisposing it to invading novel
ecosystems. Cuban Treefrog has a high fecundity laying up to 16,000 eggs for a large
female in a year (Meshaka, 2001), short generation times (Meshaka, 2001) a broad diet
(Meshaka, 2001), and is very adaptable to many habitats including urban areas and
natural ecosystems (Meshaka, 2001). Their ability to coexist with humans and human
activity has helped to expand their range to peninsular Florida, much of the Caribbean,
5
and Hawaii (Salinas, 2006). Cuban Treefrogs are stowaways in cars, on shipments of
nursery plants, and even on boat trailers (Johnson, 2010). Cuban Treefrog are also a part
of the pet trade, which is believed to be the route of introduction into Hawaii (Meshaka,
2001). The Cuban Treefrog invasion north of peninsular Florida is limited by climate,
but breeding populations have been established from a line south of Jacksonville to
Gainesville to Cedar Key (Johnson, 2010) and though successive freezes in 2010 and
2011 may have had an impact on northern populations, rebound is almost certain
(Johnson, 2010). One fear with Cuban Treefrogs and other tropical invaders is that with
climate change their range, which is currently limited by temperature, will begin to
increase northward (Rödder and Weinsheimer, 2009; Johnson, 2010).
Aspects of this invasive species’ behavior can be exploited in studies of its
population biology. The Cuban Treefrog is active mostly at night and seeks refuge
during the day in tight spaces (Meshaka, 2001). They can breed year-round with an
emphasis on the late spring and summer months, coinciding with the Florida wet season
(Meshaka, 2001). Cuban Treefrogs can live in a wide variety of habitats from suburbia
and agricultural sites, to natural environments like cypress domes, pinelands and
hammocks (McGarrity and Johnson, 2009). Although study of Cuban Treefrog behavior
requires nighttime observations, measurement of abundance and presence of Cuban
Treefrogs only needs daytime checks of refugia or artificial refugia established for this
purpose. Boughton et al. (2000) developed the polyvinyl chloride (PVC) pipe refugia for
tree frog species surveys done during the daytime
6
BACKGROUND AND RATIONALE
Simberloff et al. (2005) suggests that there are three stages to preventing
invasion: 1. Prevention 2. Eradicate shortly after arrival or before establishment and 3.
Manage at low levels. The Cuban Treefrog invasion in peninsular Florida is in stage 3
but is not to this point really being managed at any level in most places. Much of this is
due to a lack of concrete evidence on the impact that Cuban Treefrogs are having on the
native tree frog populations or on ecosystems as a whole. They are not much more than a
nuisance to the public and to this point have no known agricultural impact, so
economically there are no major costs associated with Cuban Treefrogs (Johnson, 2010).
Land management agencies have a limited number of resources to spend on exotic
species. Typically those resources are spent in areas where known positive gains will
come or known negative impacts will occur if nothing is done. For Cuban Treefrogs
neither of these scenarios is known with any great confidence so agencies largely ignore
Cuban Treefrogs and spend resources on more high profile and understood species like
feral hogs or Burmese Pythons (Python molurus bivittatus; Kuhl, 1820). It has been
shown that removal of Cuban Treefrogs does have a positive impact on some native tree
frog abundance (Rice et al., 2011) but not necessarily survival. More studies are needed
to determine the effects of Cuban Treefrog invasion, so the priority for control can be
properly determined.
7
STUDY SITE
The study area was contained entirely within Jonathan Dickinson State Park
(JDSP), which is managed by the Florida Park Service (FPS) under the Florida
Department of Environmental Protection (FDEP). JDSP is a 4,036 ha park in southern
Martin County in southeast Florida (Florida Park Service, 2012). JDSP was originally
used as a top-secret military base during World War II. After the base was closed, it was
turned over to the state in 1947 and opened as a state park in 1950. JDSP is known for its
large intact sand pine scrub on the eastern edge of the park and the National Wild and
Scenic Loxahatchee River in the western portion of the park. Beside these two rare
ecosystems, one can find 11 other natural communities including pine flatwoods, cypress
domes and strands, wet prairies and depression marshes (Florida Park Service, 2012).
The park is home to over 40 designated plants and over 35 designated animals but, at the
same time, at least 150 non-native plants and 18 non-native animals including Cuban
8
Treefrog are found in the park (Florida Park Service, 2012).
Figure 1. Locations in Jonathan Dickinson State Park of six cypress domes used in this study. Odd
numbers represent control domes, even numbers represent removal domes.
9
STUDY OBJECTIVES
A recent
study in the park tested a more efficient and safer technique for capturing
Cuban Treefrogs, since it was to be done during the day versus at night, and laid some
ground-work for this study. For Cuban Treefrogs, the park has no real removal method
employed and these frogs are now believed to be found throughout the park. Recent
studies have estimated population densities for specific locations (Campbell et al, 2010;
Rice et al., 2011); however no such study has been done at JDSP. A preliminary study
conducted at JDSP to test removal techniques and to find natural areas colonized by the
Cuban Treefrog (Rossmanith and Cunningham, unpublished data) formed the foundation
for this thesis project. The two capture techniques used were the traditional visual
encounter survey done at night and PVC pipes checked during the day (Moulton et al.
1996; Boughton et al., 2000; Campbell et al., 2010). It was found that the PVC pipes
were overwhelmingly the most effective technique for catching Cuban Treefrogs
(Rossmanith and Cunningham, in review). Not only were more frogs captured, but it was
safer and more efficient (Rossmanith and Cunningham, unpublished data), therefore this
present study used only the PVC pipes as the trapping technique. The natural areas tested
were cypress domes since an entire dome could be surveyed relatively easily. Cypress
domes will also be the natural area of choice for this project. The preliminary study
answered the questions; where are Cuban Treefrogs and how do we catch them? In this
work, I will address the following hypotheses:
10
Ho: Removal of Cuban Treefrogs has no effect on population size of native hylid
treefrogs.
Ha: Removal of Cuban Treefrogs is followed by an increase in population sizes of native
hylid treefrogs.
11
METHODS
The project included six cypress domes located in the park. The cypress domes
were chosen as the test community because it is known habitat for both native treefrogs
and Cuban Treefrogs (Rossmanith and Cunningham, unpublished data) and each dome
could be sampled completely. Six cypress domes were used, three control and three
removal sites (Fig. 1). The domes had three different pairs of sizes; small (1 and 2),
medium (3 and 4), and large (5 and 6) with one of each pair being either a control or a
removal dome. The control domes were, one (0.09 ha), three (0.14 ha), five (0.15 ha) and
the removal domes were two (0.08 ha), four (0.13 ha), and six (0.18 ha). JDSP is over
11,000 acres and the domes were chosen based on access, size, and distance apart from
each other so that there would be no movement of Cuban Treefrogs between domes.
Vegetation in the domes is similar with a canopy of Cypress Trees (Taxodium ascendens;
Brongniart, 1833) and an under story dominated by Sawgrass (Cladium jamaicense;
Crantz, 1766), Pond Apple (Annona glabra; Linnaeus, 1753), Swamp Rosemallow
(Hibiscus grandiflorus; Michaux, 1803) and greenbrier (Smilax sp.). There was a
vegetative density difference, with two domes having a much less dense under story but
both had a highly dense island in the middle of the dome. Water level was recorded for
each survey as being either wet or dry. Wet surveys had standing water across the
majority of the dome. Densities of the Cuban Treefrog varied based on the size of the
domes with larger domes having the largest population of Cuban Treefrogs. The domes
were then blindly assigned to control or to removal based on size with one of each size
12
being in either control or removal. In the control sites Cuban Treefrogs were marked
with a unique toe clip (Donnelly et al., 1994) and released and native hylids were
weighed and measured and released. In the removal domes Cuban Treefrogs were
euthanized and native hylids were again weighed and measured and released. Native
hylids were not marked because of a previous study (Rossmanith and Cunningham,
unpublished data) finding marks not effective for the smaller native hylids, based on the
size of toe pads and frequency of capture. Weight using a ± 0.1 g Pesola scale and the
snout-vent length (SVL) using ± 0.1 mm calipers were also taken for all frogs. The frogs
were shaken from the pipes and into plastic bags where the measurements took place.
Released frogs were let go onto the nearest natural feature, not released back into the
pipe. The pipe was rinsed if water was available, filled if water was available, and hung
back into place. If no water was available, all pipes were emptied and hung back dry.
Frogs were captured in PVC pipe refugia that were placed vertically in the domes.
All pipes were all 0.61 m in length. Three diameters were equally represented: 2.54 cm,
3.81 cm and 5.08 cm. The different pipe diameters were a holdover from a previous pilot
study conducted by the Florida Park Service and was meant to account for the possible
different body sizes of frogs (Zacharow et al, 2003; Rossmanith and Cunningham,
unpublished data). The pipes were capped at the bottom and two holes were drilled 10.2
cm up from the bottom and hung using inverted peg board hooks. The caps provide a
moist environment and the holes prevent the entire pipe from filling up (Boughton et al.,
2000). The pipes were set out at a density of 1096 total pipes per ha (Rossmanith and
Cunningham, unpublished data) and the typical dome was around 0.15 ha. The pipes
were open for a total of two years and 11 months (5/2008 – 4/2011). Table 1 lists the
13
dates of survey and whether or not the domes were wet at the time. Any one survey
typically ranged over a few days, with the exception of survey 3 which was done in the
aftermath of Tropical Storm Fay.
After the initial set up of the pipes, they sat open for an entire month before being
checked to allow the frogs to find and begin using them. After the first month they were
checked once a month for four months to establish a baseline estimated population. After
the initial four months, the removal study began for the next year, with each dome being
checked once a month. Then the pipes remained open and unchecked for another year to
see what the recovery of the Cuban Treefrogs would be in the removal sites. This
allowed for a real life management scenario of removal and then a break in management.
After one year of recovery, the pipes were checked for another four months for the final
population estimation.
14
15
Table 1. Date of Surveys
Survey #
Year
Month / Date
1
2008
5/7 to 5/8
2
2008
6/3 to 6/6
3
2008
8/21 to 9/4
4
2008
10/7 to 10/15
5
2009
3/3 to 3/5
6
2009
4/6 to 4/8
7
2009
5/5 to 5/7
8
2009
6/18 to 6/22
9
2009
7/22 to 7/23
10
2009
8/31 to 9/2
11
2009
10/17 to 10/24
12
2009
12/7 to12/9
13
2010
1/24 to 1/27
14
2011
1/24 to 1/25
15
2011
2/23 to 2/25
16
2011
3/22 to 3/23
17
2011
4/20 to 4/21
Dome 1
Dry
Wet
Wet
Wet
Dry
Dry
Dry
Dry
Wet
Wet
Dry
Wet
Wet
Dry
Dry
Dry
Dry
Dome 2
Dry
Wet
Wet
Wet
Dry
Dry
Dry
Wet
Wet
Wet
Wet
Wet
Wet
Dry
Dry
Dry
Dry
Dome 3
NS
Wet
Wet
Wet
Dry
Dry
Dry
Wet
Wet
Wet
Wet
Dry
Wet
Dry
Dry
Dry
Dry
Dome 4
Dry
Wet
Wet
Wet
Dry
Dry
Dry
Wet
Wet
Wet
Wet
Wet
Wet
Dry
Dry
Dry
Dry
List if dates for surveys and whether the domes were wet or dry at time of survey. NS – No survey
Dome 5
NS
Wet
Wet
Wet
Dry
Dry
Dry
Wet
Wet
Wet
Dry
Wet
Wet
Dry
Dry
Dry
Dry
Dome 6
Dry
Dry
Wet
Wet
Dry
Dry
Dry
Wet
Wet
Wet
Dry
Dry
Dry
Dry
Dry
Dry
Dry
STATISTICAL ANALYSES
Analysis of variance (ANOVA), as implemented by JMP software (student
Edition 8), was used to analyze the experimental results for this project. Repeated
measures two-way ANOVA was used to analyze treatment by survey, dome by survey
and treatment by dome. Due to low degrees of freedom no three-way combination of
treatment by survey by dome was done. The two-way combination of treatment by
survey was the best way to see the effect of the treatment. Other ANOVAs included the
ratios between control and removal of Cuban Treefrogs and Pine Woods Treefrog in the
pre-surveys and post-surveys. Due to lack of frogs this could not be done with the other
native hylids. In addition to ANOVA analyses, least squares regression was used to
examine the relationship between frog numbers and months since a freeze event,
independent of treatment; for this analysis, the raw count data for each dome was
adjusted to a unit dome size, and all variables were normalized to a standard score.
16
RESULTS
Basic informational results which did not have any analysis done include pre
removal numbers for native hylids (Figure 2) and for Cuban Treefrogs (Figure 3). The
numbers exclude the first survey since it was not done at all six domes. The average
number of Cuban Treefrogs and native hylids caught in all six domes are shown in Figure
4 and Table 2; this includes all surveys done in each dome. Since it was an average the
first survey was not excluded for domes one, two, four and six; those domes were divided
by 17 and domes three and five were divided by 16. The total number of Cuban
Treefrogs caught across all domes and the overall number of native hylids caught across
all domes are shown in Figure 5, and in Figure 6 the total number of Cuban Treefrogs and
native hylids caught in each dome are shown. The average number of native hylids by
species per dome over the entire study is shown in Table 3 and Figure 7. Finally, in
Figure 8 shows the breakdown of frogs by species caught in each dome for each survey.
17
Figure 2. Initial numbers of native hylids in all six domes. Number of captures from surveys 2-4.
Control domes - 1, 3, and 5. Removal domes - 2, 4, and 6.
Figure 3. Initial numbers of Cuban Treefrogs (Os) in all six domes. Number of captures from
surveys 2-4. Control domes – 1, 3, and 5. Removal domes – 2, 4, and 6.
18
Figure 4. Average number of native hylids and O. septentrionalis caught over all 17 surveys except in
domes three and five was over 16 surveys. Control domes – 1, 3, and 5. Removal domes – 2, 4, and 6.
Table 2. Average number of captures of O. septentrionalis by dome and average
number of combined totals for native hylids (±SE).
Avg. #
Avg. # Os
Domes
Natives
Captures
Captures
1
8.00 ± 1.80
3.41 ± 0.62
2
2.70 ± 0.65
2.76 ± 0.57
3
19.87 ± 4.06
3.50 ± 1.14
4
5.35 ± 1.55
4.64 ± 1.30
5
18.75 ± 2.77
9.68 ± 2.94
6
20.88 ± 4.28
2.41 ± 0.92
Os – Osteopilus septentrionalis, Control domes – 1, 3, and 5. Removal domes – 2, 4, and 6.
19
Figure 5. Total number of hylids caught across all domes and surveys. P value – 0.002, F ratio –
4.317.
Figure 6. Total number of O. septentrionalis versus native hylids average by dome. Control domes –
1, 3, and 5. Removal domes – 2, 4, and 6. Os – Osteopilus septentrionalis, Hf – Hyla femoralis, Hg –
Hyla gratiosa, Hs – Hyla squirella, Hc – Hyla cinerea.
20
Table 3. By dome the average number of native species (±SE)
Avg. # Hf
Avg. # Hg Avg. # Hs
Avg. # Hc
Domes
captured
captured
captured
captured
1
3.35 ± 0.64 0.05 ± 0.06
0
0
2
2.58 ± 0.53 0.18 ± 0.09
0
0
3
1.58 ± 0.56 0.29 ± 0.22 0.18 ± 0.10 1.24 ± 0.58
4
4.05 ± 1.34 0.47 ± 0.15 0.12 ± 0.08
0
5
1.41 ± 0.56 0.12 ± 0.09 5.53 ± 2.24 2.06 ± 0.67
6
2.12 ± 0.94
0
0.18 ± 0.18 0.12 ± 0.12
Hf – Hyla femoralis, Hg – Hyla gratiosa, Hs – Hyla squirella, Hc – Hyla cinerea. Control domes – 1, 3, and 5.
Removal domes – 2, 4, and 6.
Figure 7. Average number of native hylids caught over all 17 surveys by dome. Control domes – 1, 3,
and 5. Removal domes – 2, 4, and 6. Hf – Hyla femoralis, Hg – Hyla gratiosa, Hs – Hyla squirella, Hc
– Hyla cinerea.
21
Figure 8. Total number of frogs by species in each dome for all surveys. Red lines indicate freeze
events. Surveys 5-13 represent the removal phase. There is a one year break between surveys 13 and
14. Os – Osteopilus septentrionalis, Hf – Hyla femoralis, Hg – Hyla gratiosa, Hs – Hyla squirella, Hc –
Hyla cinerea
Analyses were done on a set of data that included 17 surveys in six different
cypress domes over almost three years. The data set includes three phases; phase 1 were
22
surveys 1-4 and represent the initial four months meant to establish a population estimate
for each dome, phase two was the removal phase as surveys 5-13 and phase three was the
second census study meant to establish the post-recovery population estimates. Table 4
provides a list of significant p values for these data
Table 4. Study P-values and F ratios.
Variables
Avg. Os
SVL TxS
Avg. Os
SVL T
Os TxS
Os T
Natives TxS
Natives T
Hs TxS
Hs T
Hc TxS
Hc T
Hf TxS
Hf T
Hg TxS
Hg T
Natural Set of Data
Treatment and
Recovery
Treatment
P value
F ratio
P value
F ratio
P value
F ratio
0.1137
1.5329
0.3281
4.2833
0.025*
2.5697
0.3899
0.8125
0.0232*
0.4856
0.0542
0.6452
0.0305*
0.0095*
0.0001*
0.9905
0.1821
0.9940
0.5246
0.7489
0.6703
5.3970
0.9822
3.8377
0.8327
4.8840
2.2914
17.2544
0.3401
1.1817
0.3401
0.4091
0.0478*
0.6214
0.0020*
0.5679
0.0284*
0.6861
0.0331*
0.0371*
0.0002*
0.9730
0.2897
0.9878
0.6643
4.1085
0.8281
10.6311
0.8841
5.0812
0.7611
4.7922
2.0565
16.7202
0.3556
1.1489
0.2937
0.1905
0.0072*
0.5962
0.0087*
0.9507
0.3854
0.3209
0.1006
0.4117
0.0139*
0.9251
0.7011
1
1
8.1262
0.8126
7.7112
0.3257
0.7721
1.2101
2.8406
1.0602
6.6852
0.3785
0.1497
0
0
SVL – Snout Vent Length, Os – Osteopilus septentrionalis, Hs – Hyla squirella, Hc – Hyla cinerea, Hf –
Hyla femoralis, Hg – Hyla gratiosa. TxS – treatment by survey, T – treatment. * - significant P values.
Natural Set of Data includes all surveys, Treatment and recovery includes surveys 5-17, and treatment
includes surveys 5-13.
Three major findings were evident. First, there were fewer Cuban Treefrogs in the
treatment domes versus the control dome during the removal and recovery period (Figure
9). In other words, Cuban Treefrogs were found less commonly in areas where they were
removed. Prior to removal there was no statistical difference between the numbers of
Cuban Treefrogs found in the control domes versus the removal domes (205 and 206
frogs respectively) (Figure 10). However, two-way ANOVA with a cross of treatment
and survey showed no statistically significant difference (Figure 11). So over time,
23
which is represented by the individual surveys, there was no significant difference
between the number of Cuban Treefrogs found in the control domes versus the removal
domes.
Figure 9. Osteopilus septentrionalis in control versus removal domes. Number includes totals for
three control domes and three removal domes during removal and recovery phase. P value – 0.002, F
ratio – 10.631.
24
Figure 10. Number of Cuban Treefrogs (O. septentrionalis) control versus removal domes prior to
removal.
Figure 11. Total Number of O. septentrionalis captured in control and removal domes by survey, not
the statistical model. Shaded box indicates the removal phase of the study (P value – 0.812, F ratio –
0.670, these values are from the ANOVA model run). Red lines represent freeze events.
Second, there were more native frogs in the control domes versus the removal
domes during the removal and recovery phase (Figure 12 and Figure 13). This is the
opposite of what was expected; over time the treatment of removal of Cuban Treefrogs
had no effect on the total native population (Table 4 and Figure 13). There was actually a
25
significant in increase in Green Treefrogs over time in the control domes, but not in the
treatment domes (Table 4 and Figure 21)
Figure 12. Combined numbers of all four native hylid species in control versus removal domes.
Number includes totals for three control domes and three removal domes during removal and
recovery phase. P value – 0.028, F ratio – 5.081.
Figure 13. Treatment by survey for all four native hylid species combined. P value – 0.486, F ratio –
0.982.
Third, when looking at individual species, Pine Woods Treefrog and Barking
Treefrog were more commonly encountered in treatment domes than in control domes,
but not significantly (Figures 14 and 16 respectively), therefore removal of Cuban
26
Treefrog in those domes did not greatly affect their numbers (Figures 15 and 17).
Squirrel Treefrog and Green Treefrog were significantly more prevalent in control domes
versus treatment domes (Figures 18 and 20 respectively). Green Treefrog also had
significant difference between the two-way analysis of treatment and survey (Table 4 and
Figure 21) but not the Squirrel Treefrog (Table 4 and Figure 19).
Figure 14. Hyla femoralis in control versus removal domes. Number includes totals for three control
domes and three removal domes. P value – 0.289, F ratio – 1.149.
Figure 15. Treatment by survey for H. femoralis. P value – 0.991, F ratio – 0.340.
27
Figure 16. Hyla gratiosa in control versus removal domes. Number includes totals for three control
domes and three removal domes during removal and recovery phase. P value – 0.664, F ratio – 0.191
Figure 17. Treatment by survey for H. gratiosa. P value – 0.994, F ratio – 0.313.
28
Figure 18. Hyla squirella in control versus removal domes. Number includes totals for three control
domes and three removal domes during removal and recovery phase. P value – 0.033, F ratio –
4.792.
Figure 19. Treatment by survey for H. squirella. P value – 0.645, F ratio – 0.833.
29
Figure 20. Hyla cinerea in control versus removal domes. Number includes totals for three control
domes and three removal domes during removal and recovery phase. P value – 0.0002, F ratio
16.720.
Figure 21. Treatment by survey for H. cinerea. P value – 0.010, F ratio – 2.291.
Other analyses were run on data that focused just on the experimental surveys.
The recovery data was the data collected one year after removal of frogs ended and was
intended to see the population recovery of Cuban Treefrog. By excluding the recovery
data and the pre-removal date, it was found that there were smaller, on average, Cuban
Treefrogs in the treatment domes (Figures 22 and 23), but not fewer Cuban Treefrogs
30
when looked at through the two way repeated measures ANOVA of treatment by survey
(Figures 24 and 25).
Figure 22. Average SVL of O. septentrionalis difference between control and experimental
treatment, excluding the pre-removal and recovery data. P value – 0.007, F ratio – 8.126.
Figure 23. Average SVL of O. septentrionalis treatment by survey, excluding the pre-removal and
recovery data. P value – 0.025, F ratio – 2.569.
31
Figure 24. Total number of O. septentrionalis in control versus experimental treatment, excluding
pre-removal and post-removal data. P value – 0.009, F ratio – 7.711.
Figure 25. Osteopilus septentrionalis treatment by survey excluding pre-removal and post-removal
data. P value – 0.596, F ratio –0.813.
Finally the Jolly-Seber Stochastic Method for predicting populations was not
possible due to the inconsistent number of recaptures. Instead, to see a change in the
beginning populations versus recovery populations of the control and removal domes the
ratio of Cuban Treefrogs between control and removal domes was calculated for pre32
treatment and post-treatment surveys (Figure 26) and an ANOVA was used to compare
the ratios in paired pre- and post-treatment surveys. The first pair of surveys in each
phase were excluded, since data was not collected for domes in the first pre-treatment
survey. By a factor of almost seven to one, more Cuban Treefrogs were found in the
control domes versus the removal domes during the post-treatment phase (P value –
0.001, F ratio – 72.009).
Figure 26. Ratio of O. septentrionalis in control domes versus removal domes in pre- and posttreatment surveys.
During the three years in which this study took place, several natural events
occurred which directly impacted the hylids in this study. First, there were three major
freeze events in the middle of this study, which caused a significant die off of Cuban
Treefrogs (Figure 27). Between surveys 4 and 5, 12 and 13 and before 14 were when
these freezes occurred and they were important because of when in the study it took
place. Survey 5 marks the beginning of Cuban Treefrogs removal. Survey 13 was the
last survey where Cuban Treefrogs were removed. Survey 14 was the start of the
33
recovery data (after a 1-year interval). The impact of the freezes on the experimental
results could not be controlled for statistically, but will be examined further in the
discussion.
To determine the relationship between freeze events and frog numbers
independent of any removal effect, survey data from control domes was examined
relative to the number of months since a freeze event (Figure 28). Least squares
regression analysis of control dome surveys grouped by months since a freeze showed a
highly significant, but opposite, relationship between frog numbers and freeze events for
Cuban Treefrogs and Pine Woods Treefrogs; standardized regression coefficients were
0.642 (P value - <0.0001) and -0.426 (P value -0.0023), respectively. No statistically
significant relationship between months since freeze and frog numbers was found for any
native frogs other than Pine Woods Treefrog.
Figure 27. Number of O. septentrionalis for the 17 surveys. Each survey includes the total number of
Os for all six domes combined. Red lines indicate freeze events.
34
Figure 28. Average number of treefrogs per survey for control domes, grouped by months since a
freeze event. Os – O. septentrionalis, Hf – H. femoralis, Hs – H. squirella, Hc – H. cinerea. H. gratiosa
not shown due to multiple zero values.
Water readings were also recorded during each survey and analyzed for each
dome as either wet or dry. The domes were either all wet or all dry, except in surveys
two, eight, eleven, twelve and thirteen (Figure 29). Only 4 surveys occurred when all six
domes were wet, and 7 surveys occurred when all six domes were dry. Dome 6 was the
driest dome, having 11 surveys that were dry and domes 2 and 4 were the wettest with 9
surveys that were wet. There were overall more Cuban Treefrogs captured across all
surveys and control domes when it was wet versus when it was dry but it was not
significant (P value – 0.077, F ratio – 3.229) (Figure 30). Water level data was not used
for statistical analyses because the experimental design for this study used control domes
to account for factors other than removal, including water levels that were expected to
affect frog populations.
35
Figure 29. Only surveys were domes differed in moisture levels, in all other surveys domes were
either all wet or all dry. Domes that are wet or dry by survey, dry =1 and wet = 2. Control domes –
1, 3, and 5. Removal domes – 2, 4, and 6.
Figure 30. Number of O. septentrionalis caught in control domes only, wet versus when they were
dry. (P value – 0.077, F ratio – 3.229)
36
DISCUSSION
This study did not find that Cuban Treefrogs were having an impact on native tree
frogs. However, as is often the case in experimental ecology it was not possible to
control for environmental factors that could confound treatment effects; although this
study failed to confirm an impact it does not rule out possible impacts.
This study attempted to experimentally demonstrate the impact of the Cuban
Treefrog on native hylid species. The null hypothesis was that the Cuban Treefrogs have
no significant effect on native hylids. The results in this study agree with that hypothesis;
more native species were found in the control domes than in the removal domes (Figure
12). Though a previous study found evidence that the invasive Cuban Treefrog had a
negative effect on native hylids (Rice et. al. 2011), the present study did not reveal any
significant effect. However, the experimental results may have been confounded by
environmental disturbances, or conditions that vary annually and seasonally, but could
not be controlled for statistically given the relatively short time-frame of the study.
Another consideration is the timing of the removals; they were done about once a month
over a year, whereas another study (Rice et al., 2011) removed twice a month for year
and did show a positive response by natives. Removing twice a month so that frogs are
removed quicker, could mimic a more natural event like a freeze, where a massive die off
of Cuban Treefrogs occurs in a very short period of time versus a slow steady removal,
which allows for some recovery.
37
The three freeze events during the course of this study appeared to have had a
major impact on data collection. The timing of the freeze events occurred at points in the
study which did not allow for an accurate account of the dead frogs, many of which were
so decomposed that the species of frog could not be determined. In addition, the freeze
events derailed the mark and recapture portion of the study, which could have provided
another statistic for comparison between the control and treatment domes.
The freeze events could help shape management efforts in controlling Cuban
Treefrogs. If mimicking a freeze event allows managers to do multiple removals in a
month and have a better result than one removal a month multiple times over a year, it
could make scheduling removal times easier for management. This could mean land
managers can devote time and man hours at a key point or two in the year rather than
dedicating time throughout the entire year. Based on this study, that time might be in
fall, both the 4th and the 11th survey (Figure 26) took place in October and they yielded
the two highest numbers of Cuban Treefrogs. October marks the end of breeding season
for Cuban Treefrogs (Meshaka, 2001) even though it is capable of breeding year round.
October is also when wet season rainfall totals begin to drop heading into the dry season
(Figure 31). Future studies of mass removals during different seasons, may yield the
best use of time for land managers in removal efforts for Cuban Treefrogs.
38
Figure 31. Average annual rain fall in West Palm Beach, Florida, from 1981-2010. Provided by the
Florida Climate Center.
Removals may only be part of the solution and are not the best long term solution.
Focusing on a more overall ecosystem management may help to control or contain Cuban
Treefrogs. An improved and sustained fire management program could help to reduce
immigration into domes from the surrounding pine flatwoods. A sustained fire program
could help to reduce tree cover and snags which would reduce day time refugia for Cuban
Treefrogs. With less day time refugia, there could be less Cuban Treefrogs in habitats
surrounding cypress domes and thus reducing immigration. With less immigration
removal efforts could be more successful. The most important management could be
hydrological restoration, which could mean that some of the domes are wetter longer
allowing for more predatory fish. Predatory fish and their presence is one of the control
theories for Cuban Treefrogs (Meshaka, 2001). The nice thing about other management
efforts, besides specific removal, is that many will be done or planned to be done
regardless of Cuban Treefrog presence or not since they are an overall ecosystem benefit.
39
Another issue for the study could also be pipe bias. Rossmanith and Cunningham
(unpublished data) show that Cuban Treefrogs were more commonly encountered in
pipes than in the visual encounter survey, but the native hylids were not. Therefore, a
change in the population of the native hylids may not have been accurately captured by
the pipes. Similar to the pipe bias is the overall starting population of native hylids
within each dome (Figure 2). Some domes had small native populations to begin with
(domes 3, 5 and 6) or at least few found by the pipes suggesting that there was just not
much of a native population left to rebound. The use of frog loggers, which can
automatically record frog calls, in future studies could overcome some pipe bias of native
hylids and help establish a population in a dome or help in confirming its absence. The
length of time for the study may have also been an issue, in that it just did allow enough
time for an impacted and barely hanging on native population to recover. Increasing the
timeline of removal may have a positive impact on the native populations.
One major finding in this study was not the impact that the removal of Cuban
Treefrogs was or was not having on native hyla, but what impact it was having on the
Cuban Treefrog population in those removal domes. The average SVL in the removal
domes during the time of the experiment was smaller than that of the control dome. This
suggests that slowly the Cuban Treefrog population may have been on the decline and
had the removal process continued then that may have been seen in the two-way ANOVA
in the number of Cuban Treefrogs in the treatment domes. Immigration is going to play a
factor in the decline, the domes were nowhere near a closed system and overcoming the
immigration could take longer than the length of this study. This could explain why a
year was not long enough to make an impact on the population as a whole.
40
The biggest question as to whether Cuban Treefrogs have a negative impact on
native hylids is raised by the control dome on the edge of Eaglesview recreation area
(Dome 5). This dome has the closest proximity to structures and to people and yet had the
highest number of native hylids on average over the entire course of the study, by more
than two-fold (Figure 4). It also had the third highest average number of Cuban
Treefrogs (Table 2). This dome also had high diversity, with all four native hylids being
found in the dome at some point in the study. This dome does raise a lot of questions on
Cuban Treefrogs’ possible effects on natives. One question that should be asked is what
the carrying capacity of a particular dome is? If that dome has enough resources to
handle both Cuban Treefrogs and a healthy population of native hylids then maybe the
direct effect of Cuban Treefrogs on native hylids is not that great. Perhaps, the effect of
Cuban Treefrogs on native hylids are more pronounced in degraded systems or systems
that have a lower carrying capacity where predation and competition can have a larger
impact. A study of plant diversity and density and an invertebrate study could determine
a carrying capacity for a particular dome or system. This could lead land managers to
focus on areas where natives might have a harder time surviving.
One basic fact cannot be ignored and that is Cuban Treefrogs were by far the most
dominant tree frog in the domes (Figure 5) by almost 3 to 1. With a ratio of Cuban
Treefrogs to native hylids that was highly significant (P value – 0.002, F ratio – 4.317).
Cuban Treefrogs were, on average, the most common frog in the control domes and in
one removal dome (Figure 4). The ratio of Cuban Treefrogs in the control domes versus
the removal domes in the pre versus post treatment surveys was also significant (P value
– 0.001, F ratio – 72.009). Caution needs to be taken though when looking at the
41
numbers here, since the surveys were done at different times of the year, 2-4 were in the
summer and fall and surveys 15-17 were in the winter and spring. A freeze also occurred
just two months prior to survey 15. It is interesting to note that, although a lot of
environmental factors could be contributing to this significant ratio, almost seven to one
Cuban Treefrogs occurred in the control versus removal domes. The removal domes by
far did not recover as quickly as the control domes when it came to major freeze events
leaving, perhaps, a legacy of removal. In the pretreatment surveys the ratio was nearly
one to one (Figure 26) and in post treatment it is between five and seven to one, and it
would be tempting to conclude that it was due to removal but that just is not the case.
Domes 2 and 4 had few frogs overall, and the Cuban Treefrogs were believed to have
been decimated by the first freeze event of this study. The population of Cuban
Treefrogs in those two domes did not recover, but it cannot be determined if that was due
solely to the freeze or a combination of freeze and removal efforts.
The Cuban Treefrogs that were removed from the domes during this study were
donated to a biology lab at Florida Atlantic University where they had their stomach
contents analyzed. Of the 135 frogs examined only 94 (70%) were listed as having
stomach contents and of those only 58 (62%) had identifiable parts. Of those 58 frogs
only one had a known frog part in its stomach a Pine Woods Treefrog. Invertebrates
made up most of the stomach contents. This could be due to low native hylid populations
in the domes so there just were not many natives left to be eaten. It could also suggest
that Cuban Treefrog impact on native hylids has to do more with direct competition and
only occasionally predation. If competition is a major effect on native hylids, Cuban
Treefrog may have a greater impact on Green Treefrogs and Barking Treefrogs which are
42
closer in size to Cuban Treefrogs. This could be why Rice et al. (2011) showed that
Green Treefrogs went up in abundance once Cuban Treefrog removals began. If direct
competition were the major impact of Cuban Treefrog on native hylids it could explain
why on average there were more Pine Woods Treefrog in all the domes than any other
native tree frog species (except for dome 5 where Squirrel Treefrog was the highest, a
frog comparable in size to Pine Woods Treefrog) (Table 3 and Figure 7). Rice et al.
(2011) also showed that Squirrel Treefrog went up in abundance as well once Cuban
Treefrog was removed, but for both frogs this occurred only in one site. This suggests
that the relationship between native hylids and Cuban Treefrog is a complex one that
might be more site specific. Competition, predation, breeding and sometimes a
combination of all of the above have been anecdotal reasons as to why Cuban Treefrog is
having an impact on native hylids (Babbitt and Meshaka 2000; Salinas, 2006).
It is often difficult to quantify the impacts an invasive, especially an amphibian,
can have on an ecosystem or even on an individual species. A deeper understanding of
an invasive species’ impact may take a better understanding of the life histories of an
invasive but perhaps more importantly its native competitors. The American Bullfrog
(Rana catesbeiana; Shaw, 1802) is one of the most harmful invaders to freshwater
systems worldwide (Abbey-Lambertz et al., 2014). In Oregon it was found to affect one
native species, the Oregon Spotted Frog (R. pretiosa; Baird and Girard 1853), extirpating
it from 70% of its native range, but had smaller effect on another, The Red-legged Frog
(R. aurora aurora; Baird and Girard, 1852; Pearl et al., 2004). The difference in effects
was based on a greater common use of similar habitats by the Oregon Spotted Frog with
the American Bullfrog.
43
Overlapping with juvenile habitats may explain the decline of some native
conspecific. In the case of the Green Anole (Anolis carolinensis; Voigt, 1832) and the
Brown Anole (A. sagrei; Duméril and Bibron, 1837) it is suggested that in disturbed areas
the juvenile Green Anole is preyed upon by the adult invasive Brown Anole, as they
share the same habitat (Echternacht, 1999). Once an adult, Green Anole occupies a
different though still overlapping niche of Brown Anole, recruitment of Green Anole is
significantly reduced and it eventually leads to the collapse of the population in disturbed
sites (Echternacht, 1999).
Predation has been invoked as a major way invasive species lead to extirpation
and extinction. While competition has not been directly linked to the cause of extinction
of a native species, it has caused population declines (Davis, 2003). One reason as to
why competition does not get the lions’ share of the blame is that it perhaps takes longer
to cause an extinction than the quicker direct cause by predation or habitat loss (Davis,
2003). If this is the case, and it is the competition between Cuban Treefrogs and native
hylids that is having a greater impact than it would seem, long term studies would be
needed in order to show that interaction. The design of this study was more likely to
detect a predation interaction than competition.
Billions of dollars are spent each year on managing invasive species and being
able to show a negative impact on native species or systems helps to focus some of that
money. Removal of Feral Hogs (Sus scrofa) was in part justified by placing a monetary
value on the wetlands that the hogs damaged (Engeman et al. 2004b). It is also easy to
justify removal of high profile invasive species like the Burmese Python (Python molurus
bivittatus) not because of their ecological impacts but as a public safety issue to motor
44
vehicles and human safety (Harvey et al. 2013). Old World Climbing Fern is a plant that
can cover the ground and reaches to the top of the canopy in cypress domes and sloughs,
tree islands in the Everglades, and open wetlands (Langland and Hutchinson, 2013). The
impact can be devastating for the system by killing trees and creating thick mats that
cover and shade out plants including rare and endemic species (Langland and
Hutchinson, 2013). Old World Climbing Fern also creates an issue for fire managers by
carrying fire into systems that might not naturally burn and by compromising fire control
by carrying fire across fire lines (Langland and Hutchinson, 2013). Based on these
multilayered negative effects is an easy choice for land managers to try to control Old
World Climbing Fern and its spread. With Feral Hogs, Burmese Pythons and Old World
Climbing Fern the damage is often easy to see, making the choice obvious to spend
money on their management. For other species the impacts are not so clear cut. Cuban
Treefrogs are one of those species, which is why continued studies are needed to better
establish their effects on native species and support the argument for a larger slice of the
management pie.
45
APPENDIX
46
Raw data, number of frogs caught per survey by dome.
47
Treatment
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
Dome
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
Year
2008
2008
2008
2008
2009
2009
2009
2009
2009
2009
2009
2009
2010
2011
2011
2011
2011
2008
2008
2008
2008
2009
2009
2009
2009
2009
2009
2009
2009
Month
5
6
8
10
3
4
5
6
7
9
10
12
1
1
2
3
4
5
6
8
10
3
4
5
6
7
9
10
12
Survey #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
3
4
5
6
7
8
9
10
11
12
OS
4
17
11
15
7
7
4
8
7
14
28
13
1
0
0
0
0
2
5
10
5
2
1
0
1
1
3
5
7
HF
6
0
0
2
3
4
2
4
1
3
3
1
4
11
6
4
3
2
1
0
1
5
5
0
2
0
1
3
3
HG
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
HS
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
HC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
48
Treatment
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
Dome
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
Year
2010
2011
2011
2011
2011
2008
2008
2008
2008
2009
2009
2009
2009
2009
2009
2009
2009
2010
2011
2011
2011
2011
2008
2008
2008
2008
2009
2009
2009
2009
2009
Month
1
1
2
3
4
5
6
9
10
3
4
5
6
7
8
10
12
1
1
2
3
4
5
6
8
10
3
4
5
6
7
Survey #
13
14
15
16
17
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
3
4
5
6
7
8
9
OS
0
1
1
1
1
0
23
43
35
28
25
16
20
6
5
48
48
7
3
3
4
4
3
22
10
20
6
4
2
8
3
HF
2
8
3
5
3
0
0
0
0
2
2
0
1
0
7
0
0
5
5
3
2
0
13
1
2
0
5
4
2
0
1
HG
0
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
2
0
0
0
0
0
1
1
0
0
0
0
HS
0
0
0
0
0
0
0
0
0
1
0
0
0
0
1
0
0
0
1
0
0
0
0
0
0
1
0
1
0
0
0
HC
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
3
0
1
9
3
2
2
0
0
0
0
0
0
0
0
0
49
Treatment
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
Dome
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
Year
2009
2009
2009
2010
2011
2011
2011
2011
2008
2008
2008
2008
2009
2009
2009
2009
2009
2009
2009
2009
2010
2011
2011
2011
2011
2008
2008
2008
2008
2009
2009
Month
9
10
12
1
1
2
3
4
5
6
9
10
3
4
5
6
7
8
10
12
1
1
2
3
4
5
6
9
10
3
4
Survey #
10
11
12
13
14
15
16
17
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
3
4
5
6
OS
3
7
2
0
0
0
0
1
0
17
20
24
28
23
21
13
11
15
38
44
5
7
11
11
12
15
30
44
60
28
21
HF
1
8
0
21
6
2
1
2
0
0
0
0
3
3
0
0
0
0
3
0
8
4
1
1
1
0
0
0
0
0
0
HG
0
1
0
0
2
1
1
1
0
0
0
0
0
0
0
0
0
0
1
0
0
1
0
0
0
0
0
0
0
0
0
HS
0
0
0
0
0
0
0
0
0
2
0
0
6
0
0
2
0
0
1
4
3
32
16
11
17
0
0
0
0
0
3
HC
0
0
0
0
0
0
0
0
0
0
0
1
3
0
0
0
0
1
4
4
4
10
4
3
1
0
0
0
0
0
0
Treatment
2
2
2
2
2
2
2
2
2
2
2
Dome
6
6
6
6
6
6
6
6
6
6
6
Year
2009
2009
2009
2009
2009
2009
2010
2011
2011
2011
2011
Month
5
6
7
9
10
12
1
1
2
3
4
Survey #
7
8
9
10
11
12
13
14
15
16
17
OS
9
19
24
30
43
27
2
0
1
1
1
HF
0
0
2
0
0
0
3
13
10
3
5
HG
0
0
0
0
0
0
0
0
0
0
0
HS
0
0
0
0
0
0
0
0
0
0
0
HC
0
0
0
0
0
0
2
0
0
0
0
50
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60