RESPONSE OF A TERRESTRIAL MOLLUSC COMMUNITY TO AN
AUTUMN PRESCRIBED BURN IN A RARE WETLAND PRAIRIE
OF WESTERN OREGON, USA
PAUL M. SEVERNS
US Army Corps of Engineers, Lookout Point Reservoir, Willamette Valley Projects,Lowell, OR 97452, USA
Current address: Department of Botany and Plant Pathology, Oregon State University, 2082 Cordley Hall, Corvallis, OR 97331, USA
(Received 26 August 2004; accepted 25 November 2004)
ABSTRACT
Conservation and management of grasslands may involve the use of prescribed fire to reinstate a
historical disturbance regime recently suppressed by humans. I used traps to describe the terrestrial
mollusc community in a rare wetland prairie ecosystem of western Oregon, USA over a 3-year period in
an adjacent burned and unburned wetland prairie parcel beginning 1 year following an autumn
prescribed fire. Species richness was lower throughout the burned area for the duration of the study
period and mollusc abundance was lower in the first postburn year, but steadily increased over time,
surpassing the adjacent burned area by the third postburn year. According to Multi-response
Permutation Procedure, the mollusc community in the adjacent burned area differed significantly from
the unburned prairie each year since the burn, suggesting that fire history may structure the wetland
prairie mollusc community. Indicator species analysis identified that Deroceras reticulatum and Monadenia
fidelis were indicator species for unburned wetland prairie, while Catinella rhederi and Vertigo modesta were
indicator species for burned habitat at the study site. Since fire appears to decrease wetland prairie
mollusc diversity and abundance, prescribed burns should be conducted in accordance with refuges, to
provide a source population for colonizing molluscs and for other animals with unknown responses to fire.
INTRODUCTION
With an increasing rate of global grassland loss through
conversion to agricultural lands, urbanization, invasion of exotic
species, and suppression of historical disturbance regimes
(D’Antonio & Vitousek, 1992; Bakker & Berendse, 1999), the
preservation and maintenance of native grasslands may involve
the use of prescribed burning. Grasslands of western North
America are thought to have evolved under a frequent fire-return
interval, initiated by native Americans who used fire as an aid in
hunting and farming (Barrett & Arno, 1982; Agee, 1993; Keeley,
2002). Generally, prescribed fires are believed to benefit grassland flora by reinstating a natural and historic disturbance
regime that maintained ecosystem diversity and structure (Howe,
1994; Brockway et al., 2002). Often, the use of prescribed fire is
targeted towards the benefit of rare, threatened and endangered
plant species that appear to respond favourably to the
disturbance (Olff & Ritchie, 1998; Lesica, 1999; Pendergrass
et al., 1999; Kettle et al., 2000; Gray et al., 2003). However,
comparably little effort is directed at assessing the effects of
prescribed fire on taxonomic groups that are sensitive to
desiccation, like terrestrial molluscs.
Willamette Valley wetland prairie of western Oregon, USA, is
an expansive ecosystem occupying a part of the nearly 200-km
long Willamette Valley flood plain. Historically maintained by
annual and semi-annual anthropogenic fires that consumed both
wetland and upland prairies (Boyd, 1986), the recent practice of
fire suppression, urbanization, agricultural development and
succession of wetland prairies has resulted in a reduction of size to
less than 1% of its former expanse (Christy & Alverson, 1994).
This substantial loss of habitat has probably resulted in the
extirpation of some wetland prairie species, and contributed to
Correspondence: e-mail: [email protected]
the listing of at least 10 plants as endangered, threatened or
species of concern (Oregon Natural Heritage Program, 2001).
Willamette Valley grassland species may have evolved firecoping mechanisms or have behaviours that allow them to exploit
vacant niches, due to the history of frequent anthropogenic
burns. The native wetland prairie plants Lomatium bradshawii
(Rose) Math. and Const. (Pendergrass et al., 1999), Camassia
quamash (Pursh) Greene (Taylor, 2000), and Vaccinium caespitosum
Michx. (Pendergrass, 1996) respond positively to fire by
increasing reproduction and clonal spread. Moreover, a local
race of day-flying moth (Severns, 2003) and an endangered,
endemic butterfly (in Schultz & Crone, 1998) prefer to lay eggs
on host plants that were burned the previous year. The
aforementioned species’ responses to fire suggest that prescribed
burning may be beneficial in Willamette Valley grasslands.
However, prescribed fires in other temperate North American
grasslands are also known to negatively impact insects (Panzer &
Schwartz, 2000; Panzer, 2002) and there is little known about
mollusc response to fires (Nekola, 2002; Kiss & Magnin, 2003).
Remnant Willamette Valley wetland prairies are often
managed through autumn prescribed fires to reduce the cover
of invading woody plants and are typically low intensity burns,
ranging from 61 8C to 154 8C near the soil surface (Pendergrass,
1996). As a management practice proposed to maintain and
encourage the native wetland prairie plant community, considerable effort has been invested in determining the effects of
prescribed fire on the plant community (Pendergrass, 1996;
Taylor, 1999) and some focal plant species (Pendergrass et al.,
1999; Maret & Wilson, 2000; Clark & Wilson, 2001). Only one
published study addressed the effects of wetland prairie fire on a
non-plant organism (Severns, 2003), demonstrating that the
faunal community response to prescribed burning is largely
unknown. A wetland prairie prescribed burn in the autumn of
1998 created an opportunity to assess the potential positive
Journal of Molluscan Studies (2005) 71: 181–187
q The Author 2005. Published by Oxford University Studies on behalf of The Malacological Society of London, all rights reserved.
doi: 10.1093/mollus/eyi021
P.M. SEVERNS
seasons increases, graminoids become increasingly dominant
while the cover of forbs increases in the few years following a
burn, but then decreases over time. Within the burn treatment
area at the mollusc study site, forb cover was approximately 20%
after 6 years following the burn in 1998 and graminoid cover was
nearly 40%. However, the unburned portion of the study site
(. 50 years since the last burn) had nearly equal graminoid cover
to the burned area (ca 40%), but forbs covered less than half of
the area (ca 10%) than they did in the burned habitat (Holt &
Severns, in press).
and negative effects of fire on a local terrestrial mollusc
community in the years following a burn.
Cameron (1986) and Ruesink (1995) mention the potential
effect of fires on terrestrial molluscs in passing, implying that fire is
mortal to terrestrial gastropods, but give no direct data to
support this claim. Nekola (2002) and Kiss & Magnin (2003)
directly assessed the impact of fire on molluscs and found that
species richness and abundance can be reduced in response to
wildfires and prescribed burning, over time scales ranging from a
few to 10 years depending on the habitat and mollusc assemblage.
Since autumn prescribed burns are used to maintain and manage
the Willamette Valley wetland prairie ecosystem for native
species diversity, I wanted to describe how the management
practice could structure the terrestrial mollusc community and if
prescribed burns could be detrimental to the conservation of
wetland prairie molluscs. Terrestrial molluscs are sensitive to
water loss and heat (Cameron, 1970; Barnhart, 1989) and species
dependant upon thatch for resting sites may be excluded
following fires that consume the detritus layer (Nekola 2002).
Therefore, I hypothesized that prescribed burning would result
in an overall decrease of mollusc abundance and lower species
richness, which should result in a temporary restructuring of the
gastropod community.
Sampling the mollusc community
Strict collection rules in the Fern Ridge Research Natural Area
limit the applicable techniques for terrestrial mollusc sampling to
non-destructive methods, therefore collection of soils samples and
litter for smaller molluscs could not be undertaken. Sampling of
wetland prairie terrestrial molluscs was limited to in situ hand
searches, which can be inconsistent between multiple observers
and may lead to the bias of conspicuous taxa depending on past
experience with mollusc searching (Ward-Booth & Dussart,
2001). Instead of performing a timed one-time or periodic search
on different patches of ground within a defined habitat common
to mollusc diversity studies (Ports, 1996; Barker & Mayhill, 1999;
Nekola, 1999; Schilthuizen & Rutjes, 2001, Cameron et al., 2003;
Kiss & Magnin, 2003), I used traps to repeatedly and frequently
sample the same patch of ground within the study area over
multiple years. Coverboards, 0.6 £ 0.6 m pieces of 1-cm thick
plywood, were placed on the ground in the study site as traps to
sample different regions of the prairie. Coverboards (sensu Gleich
& Gilbert, 1976; Boag & Wishart, 1982) help to reduce multiple
observer search bias and provide a consistent, repeatable
sampling effort (Boag, 1982). Although coverboards may be
biased towards sampling slugs and away from capturing
pupillids, the technique is repeatable, standardized and provides
a consistent manner of sampling between habitats. There were 21
coverboards in the unburned part of the prairie that predated the
prescribed burn, and a group of eight coverboards in the burned
area that postdated the fire. Trap size was unequal due to the
presence of multiple rare protected plant species growing in the
burned area. The distance between coverboards ranged from
3.5 m to 22 m. The unburned and burned trap transects started
approximately 30 m from the burn edge and extended up to
100 m into the centre of each area. I placed coverboards in the
burned area in July 1999, which corresponded with the inactive
season for wetland prairie terrestrial molluscs. Hand removal of
all vegetation beneath the coverboards created similar habitat
under the traps and aided in mollusc detection.
I sampled coverboards weekly through three mollusc active
seasons (October – June) beginning 1 year following the prescribed burn. In the first post-burn year (1999/2000), I sampled
the traps on 42 occasions, while in the second post-burn year
(2000/2001) the site was visited 41 times, and 29 times in the third
post-burn year (2001/2002). I hereafter refer to the first, second,
and third post-burn years as yr 1, yr 2 and yr 3, respectively.
Traps were checked before 13.00 h on days following nights that
were suitable for mollusc activity. The number and species of
terrestrial molluscs encountered on the top and underside of the
coverboard, as well as the ground beneath the coverboard, was
recorded. I left live molluscs in situ so that the removal of
individuals from the population would not affect species
distributions, relative abundance and expose any rare species to
local extirpation. Dead snails were counted and then discarded
from beneath the coverboard so they would not be recounted.
I identified molluscs using Pilsbry (1939, 1940, 1946, 1948) and
Burch & Pearce (1990).
MATERIAL AND METHODS
Habitat and study site description
Wetland prairie is a unique ecosystem that has standing water
from late autumn (November) to late spring (May) and dries to
hard, occasionally cracked soil conditions in the summer.
Standing water accumulates due to a semi-impermeable clay
layer 1 – 3 m below the surface, creating a perched water table
(Finley, 1995). As precipitation increases, the water table level
rises to expose and surround tussocks of the dominant native
grass, Deschampsia cespitosa L. (Beauv.), creating small islands of
terrestrial habitat. Perennially dry mounds (2 –15 m in diameter)
are scattered throughout the study site and support a different
flora that has characteristics of Willamette Valley upland
prairies. Throughout the majority of the rainy season, soils are
completely saturated at the study site. Even soils on the
perennially dry mounds can be saturated, forcing molluscs that
would otherwise be fossorial to the surface, leaving little latitude
for mollusc burrowing.
The 10-ha study site is located approximately 10 km west of
Eugene, Oregon, USA, in the southern Willamette Valley on a
portion of the Fern Ridge Research Natural Area. Half of the site
was burned on 30 October 1998, and the other half of the study
site was left unburned. The burned part of the study site was also
burned in 1988, 1990 and 1992, but the unburned portion of the
site has not been burned in the last 50 years. Both burned and
unburned areas are dominated by the native plants Deschampsia
cespitosa, Camassia quamash, Rosa nutkana Presl., Grindelia integrifolia
DC., Juncus nevadensis Wats. and the non-native Rubus armeniacus
L., Rosa eglanteria L. and Anthoxanthum odoratum L. The rare
endemic plants Lomatium bradshawii, Aster curtus Cronq. and
Horkelia congesta Hook., an endangered and two species of
concern, respectively, grow in the burned portion of the study
site.
Secondary succession occurs following prescribed burns in the
wet prairie, resulting in different stages of dominance by
graminoids and forbs over time following a fire. Recently burned
wet prairie parcels usually lack a thatch layer and most of the
aboveground plant biomass is absent. The resultant habitat is
comprised of approximately 30% bare ground, 20% standing
water, nearly equal cover of moss and graminoids (ca 20%) and
about 10% is covered by forbs in the 6– 9 months following a
burn (Holt & Severns, in press). As the number of growing
182
PRESCRIBED BURNING AND WETLAND PRAIRIE TERRESTRIAL MOLLUSCS
Table 1. List of species encountered in the burned (n ¼ 8) and unburned
(n ¼ 21) traps coverboard arrays and percentage of exotic species
captured by year.
Analysis
Multi-response Permutation Procedure (MRPP) (Mielke, 1984;
Mielke & Berry, 2001) was used to test for significant differences
in the mollusc community between the adjacent burned and
unburned areas. MRPP is a non-parametric multivariate test
for differences between groups that yields a P value and a
chance-corrected within-group agreement statistic (A), which
describes the within-group homogeneity compared with what is
expected from randomized data. The A statistic explains how
groups differ from each other; A ¼ 0 for equal homogeneity
within groups, A , 0 for differences within groups, or A ¼ 1 for
groups that are identical. I used Sørensen distances in the
MRPP test and species count data for all three years for each
trap were log transformed (log x þ 1) to conserve absolute
mollusc abundance in the main matrix. The main matrix
consisted of 29 rows (traps) and 10 columns (species) and a
second matrix was coded to correspond to treatment, burned or
unburned. I ran multiple MRPP analyses to test for differences
within each year between treatments and for an overall
difference between habitats by combining all species count
data for the three sample years.
I followed the MRPP analysis with indicator species analysis
(Dufrêne & Legendre, 1997) to identify different species between
the burned and unburned area that were responsible for
differences between groups in the MRPP analysis. Indicator
species analysis (ISA) ranks species based on their fidelity to a
priori groups and their proportional abundance of the species to
the a priori groups. To have a high indicator value (IVmax), species
must have high fidelity to an a priori group and also have a high
proportional abundance within the group. Indicator values
range from 100 for a perfect indicator species to zero for a species
with no indicator value. IVmax observed values are then
evaluated for statistical significance against IVmax estimated
values from Monte Carlo randomizations. I used the same main
and secondary matrices for ISA as the MRPP test and assessed
observed IVmax values for significance using 10,000 Monte Carlo
randomizations. PC-ORD (McCune & Mefford, 1999) was used
for all multivariate analyses.
I used trap efficiency, the mean number of molluscs captured
per trap in the burned and unburned areas as an indicator of
abundance. Monthly mean trap efficiency for the burned and
unburned areas (^ SE) were plotted over time for the 3-year
sample period to show any patterns associated with recolonization of the burned area.
Burned
Unburned
Species
Yr 1
Yr 2
Yr 3
Yr 1
Yr 2
Yr 3
Vespericola cf depressa
75
146
168
512
279
305
(Pilsbry & Henderson)
Deroceras laeve (Müller)
31
21
66
81
54
39
Catinella rehderi (Pilsbry)
42
55
51
12
9
14
Vertigo modesta (Say)
0
19
29
8
9
3
Monadenia fidelis (Gray)
0
0
0
24
4
3
Prophysaon andersoni (Cooper)
0
0
1
2
13
3
Cochlicopa lubrica (Müller)
0
1
7
0
3
6
Deroceras reticulatum (Müller)*
Arion hortensis Férussac*
Arion ater L.*
3
1
4
112
86
41
0
0
0
29
18
0
1
5
0
3
5
6
Exotic species (%)
2.6
2.4
1.2
18.4
22.7
11.2
*Non-native mollusc species.
burned habitat in all 3 years (Fig. 1), suggesting that the two slug
species responded differently to the prescribed burn.
MRPP revealed that the mollusc community in the adjacent
burned and unburned areas were significantly different from each
other when all three years of data was combined as well as when
years 1, 2 and 3 were considered separately (Table 2). Indicator
species analysis detected four species that may be indicators of
burned and unburned wetland prairies. The exotic slug species
Deroceras reticulatum and the native snail Monadenia fidelis were
significant indicators of unburned wetland prairie habitat
(Table 3). Two native snails, Catinella rehderi and Vertigo modesta
were significant indicator species for the burned habitat
(Table 3). Although the exotic slug Arion hortensis and the native
slug Prophysaon andersoni were encountered primarily in the
unburned habitat (Table 1), their relative abundance within
the unburned prairie was not great enough for these species to be
significant indicator species.
Mollusc abundance
The mean monthly trap efficiency changed in the burned habitat
as time following the prescribed burn increased, suggesting a
burn effect. In yr 1 the unburned area had a greater overall trap
efficiency than the burned area, but in yr 2 the burned and
unburned habitats had approximately the same overall trap
RESULTS
Species richness was lower in the burned area compared with the
unburned area in all 3 years (Table 1). One exotic species, Arion
hortensis, and one native species, Monadenia fidelis, were not
captured in the burned area throughout the 3 years of monitoring
despite having intermediate abundance in the adjacent
unburned habitat (Table 1). The percentage of exotic species
that comprised the mollusc community in the unburned area was
approximately 10 times greater than the percentage of exotic
molluscs captured in the burned area (Table 1), suggesting that
the native and exotic species responded differently to the
prescribed burn. When the percentage of the two dominant
slug species, Deroceras reticulatum and Deroceras laeve (exotic and
native species, respectively), were calculated as the proportion of
the total yearly captures within each habitat, differences in
community dominance appeared to be related to the burn
treatment. Generally, Deroceras laeve was proportionally represented with twice the abundance of captures in the burned area
than it was in the unburned habitat (Fig. 1). Conversely,
Deroceras reticulatum was at least eight times more abundant within
the mollusc community in the unburned area than it was in the
Figure 1. The percentage of Deroceras laeve (DELA) and Deroceras
reticulatum (DERE) from the total number of mollusc captures in the
burned and unburned habitat for years 1– 3.
183
P.M. SEVERNS
this study, nor could the effect of fire history be addressed. It does,
however, appear that the effects of wetland prairie prescribed
fires may last for at least 4 years and potentially longer, assuming
the response at the study site is typical for wetland prairies.
The primary difference in the wetland prairie mollusc
community between the burned and unburned habitats
appeared to be the lack of exotic species in the burned area
and the abundance of two smaller snail species that were not
common in the unburned area. The exotic slugs, Deroceras
reticulatum and Arion hortensis, were commonly found in the
unburned area but not in the burned habitat. Furthermore,
D. reticulatum was the second most abundant species at the site
(Table 1) and was an indicator species for unburned habitat, so
its absence in the burned habitat is noteworthy. In the burned
area, however, there was a pronounced elevation in abundance of
two smaller native snails, Catinella rehderi and Vertigo modesta
(Table 1). Both of these native species are small, less than 5 mm
spire height as adults, and were indicator species for burned
habitat. Elevated abundance of C. rehderi and V. modesta in the
burned area may be due to the lack of dominant competitors and
vacant niches left open following the burn. Such instances of
ecological release following disturbance are known to occur in
many communities (Davies, Gale & Lees, 1996; DeVries, Walla
& Greeney, 1999; Kruess & Tscharntke, 2002), especially in
secondary succession following fires (Menges & Kohfeldt, 1995;
McCullough, Werner & Neumann, 1998; Fredericksen &
Fredericksen, 2002).
Burned area trap efficiency increased over time, demonstrating
that molluscs were actively colonizing the burned area in yr 1 and
by yr 3 the burned area abundance surpassed that of the
unburned habitat in yr 3 (Fig. 2). In light of a severe drought that
occurred during the mollusc-active season in yr 2 and that
affected the wetland prairie mollusc community (Severns,
manuscript in review), it is curious that mollusc abundance in
the burned area continued to increase while abundance in the
unburned area remained similar or slightly decreased during and
following the drought (Fig. 2). Moreover, the species that
appeared to increase in the burned area during the drought were
generally the native species (Table 1). The increase in mollusc
abundance may be the result of greater rates of population
growth for species inhabiting the burned area. With the
exception of Vertigo modesta and Deroceras laeve, all species from
the wetland prairie readily consumed Romaine lettuce in
captivity (unpublished data), suggesting that most of the species
in the study are herbivores to some degree. Following prescribed
fires wetland prairies there is generally a large amount of bare
ground available for seedlings and in the burned study area there
appeared to be more forbs present than in the unburned habitat
Table 2. Results from MRPP analysis of burned versus unburned habitat.
A statistic
P value
Year 1
0.0581
0.003
Year 2
0.0432
0.011
Year 3
0.0825
0.001
Overall
0.0409
0.021
efficiency (Fig. 2). However in yr 3, 4 years following the fire, the
unburned area had a substantially lower trap efficiency than the
adjacent burned area (Fig. 2). The unburned habitat appeared
to have a decreasing trend in trap efficiency over time while in the
burned habitat the trap efficiency steadily grew (Fig. 2),
suggesting a continued fire effect on mollusc abundance 4 years
following the prescribed burn.
DISCUSSION
The wetland prairie mollusc community in this study did not
appear to be catastrophically perturbed in the few years following
the wet prairie prescribed fire, but the two adjacent habitats had
different assemblages. Although species richness in the burned
portion of the prairie never reached that of the unburned habitat,
two species (Arion hortensis and Monadenia fidelis) were not
captured in the burned area throughout the 3 years of visiting
the site. Furthermore, the consistently low trap efficiency of exotic
species in the burned area and the low relative abundance of
Monadenia fidelis (Table 1) suggests that the effect of prescribed
burning extends beyond a few years for some of the wetland
prairie species. Nekola (2002) reported an approximate 30%
decrease in terrestrial mollusc richness from previously burned
grasslands compared with similar unburned habitats. Moreover,
there was evidence for a resilient negative effect of fires on mollusc
richness and abundance up to 15 years following the prescribed
burn (Nekola, 2002). Kiss & Magnin (2003) also found that
species richness was lower in burned open forests and garrigue
than in unburned habitat, and that species richness increased
with the time since burn over a number of sites that varied from 1
to 10 years since the most recent fire. The loss of terrestrial
mollusc species richness following prescribed burning in the
wetland prairie is consistent with trends described in drier
grasslands (Nekola, 2002) and forests (Kiss & Magnin, 2003).
The permanence of a burn effect on the wetland prairie mollusc
community cannot be determined in the time period spanned in
Table 3. Results from indicator species analysis with 10,000 Monte Carlo
randomizations for IVmax expected.
Indicator
IVmax
IVmax
Standard
group
observed
expected
deviation
VESP
Unburned
52.3
57.6
5.7
DERE
Unburned
87.9
63.8
9.7
0.017
DELA
Burn
61.5
65.4
8.9
0.651
MOFI
Unburned
70.4
57.8
5.4
0.027
VEMO
Burn
80.7
67.3
8.4
0.032
CARH
Burn
88.4
65.4
9.9
0.004
ARAT
Burn
50.5
56.9
4.7
1.000
ARHO
Unburned
77.9
78.9
5.3
0.640
PRAN
Unburned
63.4
57.7
4.8
0.188
COLU
Unburned
58.3
55.9
3.8
0.240
Species
P value
0.815
*VESP, Vespericola cf depressa; DERE, Deroceras reticulatum; DELA,
Deroceras laeve; MOFI, Monadenia fidelis; VEMO, Vertigo modesta; CARH,
Catinella rehderi; ARAT, Arion ater; ARHO, Arion hortensis; PRAN,
Prophysaon andersoni; COLU, Cochlicopa lubrica.
Figure 2. Mean monthly trap efficiency (^SE) through three active
seasons beginning one year following the prescribed burn in the autumn
of 1998. The dark, solid line represents the unburned habitat and the
dotted line shows the adjacent burned habitat.
184
PRESCRIBED BURNING AND WETLAND PRAIRIE TERRESTRIAL MOLLUSCS
captures of these species 200 km to the north of the study site in
the late 1920s for D. reticulatum and A. hortensis, and 1946 for Arion
ater. These capture dates are well after the beginning of fire
suppression in the Willamette Valley, which began in the early
1850s, so exotic molluscs in wetland prairies are unlikely to have
experienced the same historical burning regime as the native
wetland mollusc species. Due to a lack of shared fire history in the
Willamette Valley, exotic molluscs may be prone to disproportionate mortality in the presence of prescribed burning when
compared with native species, and native species may be able to
utilize resources in recently burned areas more efficiently than
exotic species. More directed research into the behaviour,
survival and population growth of exotic and native wetland
prairie molluscs is needed to verify any trends in disproportionate
mortality and the slow colonization of exotic species into burned
habitat.
Unfortunately, I was unable to obtain preburn and burn year
data for the study site. Many of the conclusions in this study are
based on the assumption that the underlying distribution of
mollusc abundance and species richness in the preburn mollusc
community would be similar to the unburned area in the absence
of the fire. With these assumptions in mind, however, there
are management recommendations that can be made. The
Willamette Valley wetland prairie is a highly fragmented and
rare ecosystem containing a number of rare and endemic species.
Prescribed burns that consume entire wetland prairie parcels
may unnecessarily increase the possibility of mollusc species loss.
This wetland prairie study and two other experiments that
directly investigated the effect of fire on mollusc communities all
resulted in lower species richness and mollusc abundance over the
first few years following a fire (Nekola, 2002; Kiss & Magnin,
2003). In a highly fragmented ecosystem, like Willamette Valley
wetland prairie, prescribed fires that consume entire parcels that
do not have adjacent unburned habitat for colonizing molluscs
may lead to population bottlenecks and loss of mollusc
abundance. Repeated and frequent burning of parcels without
suitable fire refuges may result in local species loss and extinction
if unique species are locally distributed. Historically, the
anthropogenic fires consumed only a portion of the grasslands
in the Willamette Valley (Boyd, 1986), leaving a mosaic of
burned and unburned fragments where molluscs could find
refuge. To mimic this historical burn-mosaic of wetland prairie
habitat and conserve molluscs, future prescribed fires should be
conducted in accordance with artificial refuges, to maintain
potential source populations for molluscs and other fauna whose
response to fire is unknown.
throughout the study period (Holt & Severns, in press). When
given a choice, herbivorous terrestrial molluscs often prefer
seedlings or younger, tender vegetation to older plant material
(Westerbergh & Nyberg, 1995; Clear-Hill & Silvertown, 1997;
Scheidel & Bruelheide, 1999). Wetland prairie fires consume
aboveground biomass resulting in a flush of germinants and new
plant growth (Pendergrass, 1996; Maret & Wilson, 2000; Clark
& Wilson, 2001), so mollusc preference for recently burned
habitat may occur if food palatability is important. Some
molluscs also prefer plants that are higher in nitrogen content
(Iglesias & Castillejo, 1999; Cook et al., 2000), which may occur
in plants growing in recently burned areas. Fires create an
ammonium pool in the soil, leading to a pulse of nitrates that
plants incorporate into their foliar tissues (Ojima et al., 1994;
Grogan, Burns et al., 2000; Radho-Toly, Majer & Yates, 2001)
resulting in potentially more nutritious food for terrestrial
molluscs in recently burned habitats.
Colonization of the burned area and population growth
appear to be slower for the exotic species, suggesting that
burning may have a suppressive effect on the distribution of
exotic molluscs. South (1965) demonstrated that Deroceras
reticulatum had a low dispersal capacity when compared with
another slug of similar size, perhaps explaining the slow
colonization time of this dominant species at the study site.
However, differences in shelter preferences between exotic and
native species may also explain the lower abundance of exotic
molluscs in the burned habitat. Many of the native species were
encountered at the openings of small-mammal burrows and
tunnels that were scattered throughout the burned and unburned
habitats. Presumably, if native molluscs sheltered in smallmammal tunnels they should be able to survive the prescribed
fires in the dormant season. However, exotic species were
observed either beneath thatch or at the base of bunch-grass
tussocks (Severns, in preparation). In the unburned area there
was abundant thatch cover and the tussocks were comprised of
dead as well as live grass blades, providing ample cover and
shelter sites for the exotic species. The prescribed fire consumed
the thatch that was probably the primary shelter for the exotic
species, thereby limiting the distribution and colonization of
thatch-dependant slugs. Shelter is often an important determinant in the distribution of terrestrial molluscs (South, 1965;
Ledergerber et al., 1997; Frank, 1998; Hommay, Lorvelec &
Jacky, 1998) and Nekola (2002) demonstrated that the thatch
and duff layer was critically important to the abundance of
terrestrial molluscs in burned grasslands. The potential separation of resting habitats for different mollusc species and low
dispersal capacity may explain the observed differences in relative
trap efficiency between the burned and unburned habitat in this
study.
Prescribed burning is sometimes rationalized among land
managers as a practice that discourages exotic species and
encourages native ones. The relative abundance of exotic
molluscs in the burned area never approached that of the
unburned habitat in the time following the burn, despite the
apparent recolonization of the burned area by native molluscs.
Moreover, when the two dominant slugs (one exotic and the
other a native species) were compared between the burned and
unburned areas, the two species appeared to hold a different
position within the community depending on habitat type
(Fig. 1). Although a clear mechanism for explaining the
dominance of the native slug Deroceras laeve in the burned habitat
and the dominance of the exotic slug Deroceras reticulatum in the
unburned habitat could not be ascertained, behaviours of native
molluscs may have been influenced by strong selection pressures
accompanying the frequent historical anthropogenic burns in the
Willamette Valley. Deroceras reticulatum, Arion ater, and Arion
hortensis are relatively recent arrivals to the Pacific northwest
region of the United States. Pilsbry (1948) lists the first known
ACKNOWLEDGEMENTS
I thank Kat and James Beal for access to the study site and
support of this project. J. V. Syring, H. Sweet, C. D. Benfield,
A. Swengel, J. C. Nekola and one anonymous reviewer provided
comments that helped to improve this manuscript. I greatly
appreciate the help of J. S. Applegarth, T. J. Frest and B. Roth
who kindly identified many of the terrestrial molluscs in
this study.
REFERENCES
AGEE, J.K. 1993. Fire ecology of Pacific Northwest forests. Island Press,
Washington D.C.
BAKKER, J.P. & BERENDSE, F. 1999. Constraints in the restoration of
ecological diversity in grassland and heathland communities. Trends in
Ecology and Evolution, 14: 63 –68.
BARKER, G.M. & MAYHILL, P.C. 1999. Patterns of diversity and
habitat relationships in terrestrial mollusc communities of the
Pukeamaru Ecological District, northeastern New Zealand. Journal
of Biogeography, 26: 215 –238.
185
P.M. SEVERNS
pine- wiregrass ecosystem: responses to fire frequency and population
size. Natural Areas Journal, 23: 210 –219.
BARNHART, M.C. 1989. Evaporative water loss from minute terrestrial
snails (Pulmonata: Pupillidae). Veliger, 32: 16–20.
BARRETT, S. & ARNO, S. 1982. Indian fires as an ecological influence
in the northern Rockies. Journal of Forestry, 80: 647–651.
BOAG, D.A. 1982. Overcoming sampling bias in studies of terrestrial
gastropods. Canadian Journal of Zoology, 60: 1289–1292.
BOAG, D.A. & WISHART, W.D. 1982. Distribution and
abundance of terrestrial gastropods on a winter range of bighorn
sheep in southwestern Alberta. Canadian Journal of Zoology, 60:
2633–2640.
BOYD, R. 1986. Strategies of Indian burning in the Willamette Valley.
Canadian Journal of Anthropology, 5: 65–86.
BROCKWAY, D.G., GATEWOOD, R.G. & PARIS, R.B. 2002.
Restoring fire as an ecological process in shortgrass prairie ecosystems:
initial effects of prescribed burning during dormant and growing
seasons. Journal of Environmental Management, 65: 135–152.
BURCH, J.B. & PEARCE, T.A. 1990. Terrestrial Gastropoda. In: Soil
biology guide (D.L. Dindal, ed.), 201–309. John Wiley, New York.
CAMERON, R.A.D. 1970. The survival, weight loss and behaviour of
three species of land snail in conditions of low humidity. Journal of
Zoology, 160: 143–157.
CAMERON, R.A.D. 1986. Environment and diversities of
forest snail faunas from coastal British Columbia. Malacologia, 27:
341–355.
CAMERON, R.A.D., MYLONAS, M., TRIANTIS, K.,
PARMAKELIS, A. & VARDINOYANNIS, K. 2003. Land-snail
diversity in a square kilometere of Cretan Maquis: modest species
richness, high density and local homogeneity. Journal of Molluscan
Studies, 69: 93 –99.
CHRISTY, J.A. & ALVERSON, E.R. 1994. Saving the Valley’s wet prairie.
The Nature Conservancy, Oregon Chapter Newsletter, Portland
Oregon.
CLARK, D.L. & WILSON, M.V. 2001. Fire, mowing, and handremoval of woody species in restoring a native wetland prairie in the
Willamette Valley of Oregon. Wetlands, 21: 135– 144.
CLEAR-HILL, B.H. & SILVERTOWN, J. 1997. Higher order
interaction between molluscs and sheep affecting seedling numbers
in grasslands. Acta Oecologica, 18: 587–596.
COOK, R.T., BAILEY, S.E.R., MCCROHAN, C.R., NASH, B. &
WOODHOUSE, R.M. 2000. The influence of nutritional status on
the feeding behaviour of the field slug, Deroceras reticulatum (Müller).
Animal Behaviour, 59: 167 –176.
D’ANTONIO, C.M. & VITOUSEK, P.M. 1992. Biological invasions by
exotic grasses, the grass/fire cycle, and global change. Annual Review of
Ecology and Systematics, 23: 63–87.
DAVIES, P., GALE, C.H. & LESS, M. 1996. Quantitative studies of
modern wet-ground molluscan faunas from Bossington Hampshire.
Journal of Biogeography, 23: 371–377.
DE VRIES, P.J., WALLA, T.R. & GREENEY, H.F. 1999. Species
diversity in spatial and temporal dimensions of fruit-feeding butterflies
from two Ecuadorian rainforests. Biological Journal of the Linnean Society,
68: 333– 353.
DUFRÊNE, M. & LEGENDRE, P. 1997. Species assemblages and
indicator species: the need for a flexible asymmetric approach.
Ecological Monographs, 67: 345 –366.
FINLEY, K.F. 1995. Hydrology and related soil features of three
Willamette Valley wetland prairies. MSc thesis, Oregon State University,
Corvallis.
FRANK, T. 1998. Slug damage and number of slugs (Gastropoda:
Pulmonata) in winter wheat in fields with sown wildflower strips.
Journal of Molluscan Studies, 64: 319 –328.
FREDERICKSEN, N.J. & FREDERICKSEN, T.S. 2002. Terrestrial
wildlife response to logging and fire in a Bolivian tropical humid
forest. Biodiversity and Conservation, 11: 27–38.
GLEICH, J.G. & GILBERT, F.F. 1976. A survey of terrestrial
gastropods from central Maine. Canadian Journal of Zoology, 54:
620–627.
GRAY, J.B., WENTWORTH, T.R. & BROWNIE, C. 2003. Extinction,
colonization, and persistence of rare vascular flora in the longleaf
GROGAN, P., BURNS, T.D. & CHAPIN, F.S. 2000. Fire effects on
ecosystem nitrogen cycling in a Californian bishop pine forest.
Oecologia, 122: 537–549.
HOLT, E.A., SEVERNS, P.M. The effects of prescribed burning on wet
prairie lichen communities. Natural Areas Journal, in press.
HOMMAY, G., LORVELEC, O. & JACKY, F. 1998. Daily activity
rhythm and the use of shelter in the slugs Deroceras reticulatum and Arion
distinctus under laboratory conditions. Annals of Applied Biology, 132:
167 –185.
HOWE, H.F. 1994. Managing species diversity in tallgrass
prairie: assumptions and implications. Conservation Biology, 8:
691 –704.
IGLESIAS, J. & CASTILLEJO, J. 1999. Field observations on feeding of
the land snail Helix aspera Müller. Journal of Molluscan Studies, 65:
411 –423.
KEELEY, J.E. 2002. Native American impacts on fire regimes of the
California coastal ranges. Journal of Biogeography, 29: 303–320.
KETTLE, W.D., ALEXANDER, H.M. & PITTMAN, G.L. 2000. An
11-year ecological study of a rare prairie perennial (Asclepias meadii):
implications for monitoring and management. American Midland
Naturalist, 144: 66 –77.
KISS, L. & MAGNIN, F. 2003. The Impact of Fire on some
Mediterranean land snail communities and patterns of post-fire
recolonization. Journal of Molluscan Studies, 69: 43–53.
KRUESS, A. & TSCHARNTKE, T. 2002. Grazing intensity and the
diversity of grasshoppers, butterflies, and trap-nesting bees and wasps.
Conservation Biology, 16: 1570–1580.
LEDERGERBER, S., BAMINGER, H., BISENBERGER, A.,
KLEEWEIN, D., SATTMAN, H. & BAUR, B. 1997. Differences
in resting-site preference in two coexisting land snails, Arianta
Arbustorum and Arianta chamaeleon (Helicidae), on alpine slopes. Journal
of Molluscan Studies, 63: 1–8.
LESICA, P. 1999. Effects of fire on the demography of the endangered,
geophytic herb Silene spaldingii (Caryophyllaceae). American Journal of
Botany, 86: 996–1002.
MARET, M.P. & WILSON, M.V. 2000. Fire and seedling population
dynamics in western Oregon prairies. Journal of Vegetation Science, 11:
307 –311.
MC CULLOUGH, D.G., WERNER, R.A. & NEUMANN, D. 1998. Fire
and insects in northern and boreal forest ecosystems of North
America. Annual Review of Entomology, 43: 107–127.
MC CUNE, B. & MEFFORD, M.J. 1999. PC-ORD. Multivariate analysis of
ecological data. Mjm Software Design, Gleneden Beach, Oregon, USA,
Version 4.25.
MENGES, E.S. & KOHFELDT, N. 1995. Life history strategies of
Florida scrub plants in relation to fire. Bulletin of the Torrey Botanical
Club, 122: 282 –297.
MIELKE, P.W. 1984. Meteorological applications of permutation
techniques based on distance functions. In: Handbook of statistics
(P.R. Krishnaiah & P.K. Sen, eds), 4: 813–830. Elsevier Science.
MIELKE, P.W. & BERRY, K.J. 2001. Permutation methods: a distance
function approach. Springer Series in Statistics.
NEKOLA, J.C. 1999. Terrestrial gastropod richness of carbonate cliff and
associated habitats in the Great Lakes region of North America.
Malacologia, 41: 231–252.
NEKOLA, J.C. 2002. Effects of fire management on the richness and
abundance of central North American grassland snail faunas. Animal
Biodiversity and Conservation, 25: 53 –66.
OJIMA, D.S., SCHIMEL, D.S., PARTON, W.J. & OWENSBY, C.E.
1994. Long-and short-term effects of fire on nitrogen cycling in
tallgrass prairie. Biogeochemistry, 24: 67–84.
OLFF, H. & RITCHIE, M.E. 1998. Effects of herbivores
on grassland plant diversity. Trends in Ecology and Evolution, 13:
261 –265.
OREGON NATURAL HERITAGE PROGRAM. 2001. Rare, threatened,
and endangered plants and animals of Oregon. Oregon Natural Heritage
Program, Portland.
186
PRESCRIBED BURNING AND WETLAND PRAIRIE TERRESTRIAL MOLLUSCS
PANZER, R. & SCHWARTZ, M. 2000. Effects of management
burning on prairie insect species richness within a system of
small, highly fragmented reserves. Biological Conservation, 96:
363 –369.
PANZER, R. 2002. Compatibility of prescribed burning with the
conservation of insects in small, isolated prairie reserves. Conservation
Biology, 16: 1296–1307.
PENDERGRASS, K.L. 1996. Vegetation composition and response of fire of
native Willamette Valley wetland prairies. MSc thesis, Oregon State
University, Corvallis.
PENDERGRASS, K.L., MILLER, P.M., KAUFFMAN, J.B. & KAYE,
T.N. 1999. The role of prescribed burning in maintenance of an
endangered plant species Lomatium bradshawii. Ecological Applications, 9:
1420–1429.
PILSBRY, H.A. 1939. Land mollusca of North America (north of
Mexico). Academy of Natural Sciences of Philadelphia Monograph, Series 3,
1: 1–573.
PILSBRY, H.A. 1940. Land Mollusca of North America (north of
Mexico). Academy of Natural Sciences of Philadelphia Monograph, Series 3,
1: 575–994.
PILSBRY, H.A. 1946. Land Mollusca of North America (north of
Mexico). Academy of Natural Sciences of Philadelphia Monograph, Series 3,
2: 1–520.
PILSBRY, H.A. 1948. Land Mollusca of North America (north of
Mexico). Academy of Natural Sciences of Philadelphia Monograph, Series 3,
2: 521–1113.
PORTS, M.A. 1996. Habitat affinities and distributions of land
gastropods from the Ruby Mountains and east Humboldt Range of
northeastern Nevada. Veliger, 39: 335–341.
RADHO-TOLY, S., MAJER, J.D. & YATES, C. 2001. Impact of fire on
leaf nutrients, arthropod fauna and herbivory of native and exotic
eucalypts in Kings Park, Perth, Western Australia. Austral Ecology, 26:
500–506.
RUESINK, J.L. 1995. Snail faunas on chalk grassland: site comparisons
and implications for management. Journal of Molluscan Studies, 61:
9–20.
SCHEIDEL, U. & BRUELHEIDE, H. 1999. Selective slug grazing on
montane meadow plants. Journal of Ecology, 87: 828– 838.
SCHILTHUIZEN, M. & RUTJES, H.A. 2001. Land snail diversity in a
square kilometere of tropical rainforest in Sabah, Malaysian Borneo.
Journal of Molluscan Studies, 67: 417 –423.
SCHULTZ, C.B. & CRONE, E.E. 1998. Burning prairie to restore
butterfly habitat: a modeling approach to management tradeoffs for
the Fender’s blue. Restoration Ecology, 6: 244 –252.
SEVERNS, P.M. 2003. The effects of a fall prescribed burn on Hemileuca
eglanterina Boisduval (Saturniidae). Journal of the Lepidopterists’ Society,
57: 137 –143.
SOUTH, A. 1965. Biology and ecology of Agriolimax reticulatus (Müll.)
and other slugs: spatial distribution. Journal of Animal Ecology, 34:
403–417.
TAYLOR, T.H. 2000. Long-term vegetation response to fire of Willamette Valley
wet prairie species. MSc thesis, University of Oregon, Eugene.
WESTERBERGH, A. & NYBERG, A.B. 1995. Selective grazing of
hairless Silene dioica plants by land gastropods. Oikos, 73:
289 – 298.
WARD-BOOTH, J.F. & DUSSART, G.B.J. 2001. Consistency in
hand-searching for snails. Journal of Molluscan Studies, 67: 502– 506.
187