J OURNAL OF C RUSTACEAN B IOLOGY, 33(2), 286-292, 2013
EFFECTS OF RECREATIONAL USE ON BRANCHIOPOD EGG AND EPHIPPIA
DENSITY, BLACK ROCK DESERT-HIGH ROCK CANYON EMIGRANT TRAILS
NATIONAL CONSERVATION AREA, NEVADA, USA
Donald W. Sada 1,∗ , Christopher Rosamond 1 , and Kenneth D. Adams 2
1 Division
2 Division
of Hydrologic Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, USA
of Earth and Ecosystem Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, USA
ABSTRACT
Numerous plays playas occur on valley floors of endorehic basins in arid regions of the western USA. Their openness makes them attractive
for hiking, vehicle travel, military, and other uses when dry. Branchiopod crustacean dormant egg banks survive in these systems and are
a rich food resource for migrating birds. Brachniecta gigas Lynch, 1937, B. mackini Dexter, 1956, and Lepiduras lemmoni Holmes, 1895,
and Moina cf. macrocopa (Straus, 1820) occur in the Black Rock Desert playa, Nevada, USA. We collected playa egg bank samples to
determine effects of human use in three studies. We compared intact egg and ephippia density in virgin playa areas with: 1) a heavily used
vehicle track, and 2) recreational camping and vehicle activity mitigated by dust abatement in Black Rock City (site of the Burning Man
Festival). We also attempted to quantify changes in intact egg and ephippia density through repeated vehicle travel over a track on virgin
playa. We found no observed decrease in intact egg or ephippia density attributed to incrementally increased vehicle travel over virgin
playa, which may be attributed to strength of the playa substrate matrix. Differences in intact egg and ephippia density were substantially
lower in heavily used vehicle tracks than in adjacent playa, but differences were not statistically significant. Density of intact eggs and
ephippia in Black Rock City were lower following the festival, but differences were statistically significant only for eggs in camping areas
and not roads. Weak effects observed on these roads may be attributed to dust abatement that maintained substrate density.
K EY W ORDS: playa branchiopods, playa ecology
DOI: 10.1163/1937240X-00002130
I NTRODUCTION
Desert playas of the western U.S. arid lands are dynamic,
harsh ecological systems that are exposed to lengthy aridity, aeolian forces, and periodic inundation (flooding) by
salty, turbid, alkaline water during periods of high precipitation. A number of studies have examined their environmental characteristics such as hydrology, geologic processes,
substrates, and dust emission (Neal and Motts, 1967; Neal
et al., 1968; Motts, 1970; Reynolds et al., 2007). When
dry, their openness makes them attractive for hiking, vehicle travel, military activity, festivals, and other uses that
disturb substrates. Branchiopoda include diverse orders: tadpole shrimp (Notostraca), fairy shrimp (Anostraca), smooth
clam shrimp (Laevicaudata), and the spiny clam shrimp and
water fleas (Diplostraca, suborders Spinicaudata, Cyclestherida, and Caldocera) occupy playas and other intermittent desert aquatic systems (Eng et al., 1990; Samraoui et
al., 2006; Brendonck et al., 2008). These crustaceans have a
common life history where adults grow and reproduce during inundations, and lay eggs that survive through drought
and incubate quickly when again inundated. The ecology of
rare and endangered branchiopods in the western U.S. has
been actively studied (Eriksen and Belk, 1999), and physiological requirements of many playa species is well known
(Bowen et al., 1988; Broch, 1989; Hathaway and Simovich,
∗ Corresponding
1996). Playas support an ecosystem of producers (phytoplankton, bacteria, other microbes) and consumers (branchiopods) during inundation periods that are a rich food resource for migrating birds (Williams et al., 1998; Plissner et
al., 2000; Boros et al., 2006; Samraoui et al., 2006), hence
they provide a link between these ephemeral wetlands and
the surrounding terrestrial ecosystem.
Few studies have examined branchiopod egg fragility and
their susceptibility to damage. Eirksen et al. (1986) found
anostracan eggs were susceptible to damage by off-highway
vehicles and Hathaway et al. (1996) quantified the amount
of force required to damage branchiopod eggs. No studies
have considered the influence of vehicle use on cladocera
ephippia, or influences of other playa anthropogenic uses on
branchiopod eggs. The Black Rock Desert playa (BRP) is in
the Black Rock Desert-High Rock Canyon National Conservation Area (Pershing and Washoe Counties, Nevada), and
it is one of the largest, nearly flat surfaces in interior North
America. It is a remnant of Pleistocene Lake Lahontan (Reheis, 1999) and attracts a number of human activities including off-highway vehicle travel, the Burning Man Festival,
world land speed record trials, and other miscellaneous activities (see Fox, 2002). The duration and frequency of BRP
use has increased over the past 25 years, but little is known
about the influence of these activities on playa life, and no
previous studies have examined BRP ecology. The poten-
author; e-mail: [email protected]
© The Crustacean Society, 2013. Published by Brill NV, Leiden
DOI:10.1163/1937240X-00002130
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SADA ET AL.: PROBLEM EFFECTS ON BRANCHIOPODS
Fig. 1. Black Rock Playa location. The pentagram encompasses Black
Rock City.
tial impact of human activity on the playa is of concern because studies show that branchiopod eggs are crushed by vehicle use, and increasing use of BRP for a number of activities may affect branchiopod abundance. We conducted studies on BRP during 2006 and 2007 on branchiopod egg and
ephippia abundance and distribution, and to examine the effect of anthropogenic activities camping and vehicle use during three studies by: 1) comparing densities in a heavily used
vehicle track and adjacent virgin playa, i.e., playa with no
visible disturbance by any human activity since the most recent inundation, 2) comparing densities in Black Rock City
roads and camping areas before and following the Burning
Man Festival, and 3) quantifying changes in density through
repeated travel over a track in virgin playa.
E NVIRONMENTAL S ETTING
The Black Rock Desert is in northwestern Nevada, USA, and
includes a playa covering approximately 2600 km2 (Fig. 1).
It lies at approximately 1190 m elevation and annual precipitation is less than 25 cm. Maximum summer temperature
rarely exceeds 35°C and minimum winter temperatures are
often below −20°C. The playa is dry during summer and
inundated only when winter and spring precipitation from
surrounding drainages supply sufficient water. Extensive inundation is not caused by single precipitation events. The
playa substrate is dominated by clay (Table 1).
The BRP is wettest during winter and early spring; deep
mud always makes vehicle travel impossible. Human use
and its intensity also vary spatially on the playa. It is
typically greatest during the summer and autumn along
tracks to scenic destinations, and in Black Rock City, where
foot traffic, camping, and vehicle traffic by more than 40 000
people occurs annually for a minimum of 1 week during
Table 1.
strates.
Particle size distribution (±1 standard deviation) of BRP sub-
Particle size
Sand (>62.5 μm)
Silt (>15 μm and >3 μm)
Fines (3 μm)
Clay (<3 μm)
Composition (%)
3.1 (1.2)
10.3 (4.1)
21.2 (4.8)
65.3 (7.5)
the Burning Man Festival in late summer. There is little
annual change in the location of Black Rock City, which
consists of surveyed, concentric roads that are separated
by recreational vehicle and tent camping sites. Movement
during the Festival is by bicycle or foot, and motorized
vehicle use is limited to camp site access and egress before
and following the Festival, respectively. City roads are
wetted several times each day to minimize dust, but camp
areas are not wetted (U.S. Bureau of Land Management,
2001). Use of BRP is lowest in remote areas that are distant
from Black Rock City and paved or graded roads that border
west and east sides of the playa.
Playa substrate durability and hardness change between
periods of inundation. The playa substrate is hard in summers that follow flooding and friability increases annually
without inundation due to erosion from wind, freezing and
thawing, and the natural contraction and expansion of soil.
Evidence of vehicle traffic is minimal in summers after
flooding, suggesting that the disturbance patterns are eroded
by water. Durability and hardness decrease annually between inundations, which is evident by increasing occurrence of porous substrate and transitory dunes. Visual evidence of vehicle traffic also increases over these periods
(Sada field notes, 2007, Adams, 2009).
M ATERIALS AND M ETHODS
Playa Inundation
The frequency and spatial extent of playa inundation was mapped for the
years 1973-2008 from LANDSAT imagery. Over this period approximately
195 images were examined with a photo frequency of approximately every
16 days, but the number of scenes processed for each year ranged from zero
(1991) to 15 (1995), depending on the presence and duration of inundation
and image quality (usually cloudiness). Scenes were stretched to accentuate
water, clipped, then imported into ArcGIS® for mapping.
Playa topography was determined from approximately 26 000 points
collected using a roving, survey-grade global positioning system (GPS;
Thales Navigation, with a stated accuracy and precision of 1 cm) mounted
to a vehicle roof rack, and a base station set at benchmarks. Most points
were collected over four days during the summer of 2006, producing data
that were merged with high precision GPS data collected in a similar
manner during a 2001. Spatial and temporal variability in water depth on
the playa were quantified by overlaying contours derived from these surveys
with digitized water surface areas compiled from LANDSAT. Precipitation
during 2006 was relatively great, maximum water depth over BRP was
approximately 1 m, and approximately 310 km2 of the playa was inundated.
Playa inundation images showed this level of inundation had not occurred
since 2001. Precipitation was much less in 2007 when less than 20 km2 of
playa was inundated.
Branchiopod Studies
Adult branchiopods were collected when the BRP was flooded in early May
2006 from a kayak using a 120 cm2 × 200 μm mesh net. Collections were
placed in 90 percent ethanol and returned to Desert Research Institute (DRI)
where they were identified and archived.
Branchiopod eggs and ephippia were sampled during 2006 and 2007
and analyzed in the laboratory. Eggs and ephippia were separated from the
substrate by soaking for one hour in de-ionzed water, following guidelines
of Eirksen et al. (1988). After saturation, each sample was gently rinsed
through a series of three sieves (0.7 mm, 150 μm, and 106 μm, respectively)
using a low volume pump. Material retained on each sieve was carefully
rinsed into a sorting dish and placed under a compound microscope
where broken eggs and ephippia were tallied and removed using a plastic
transfer pipette. It was not possible to determine the cause of breakage
and fragments were often small portions of eggs, which prevented using
broken egg and broken ephippia tallies to assess the effects of human use
on BRP branchiopods. In their study examining effects of vehicles on playa
branchiopod eggs, Eirksen et al. (1988) also discounted broken material. We
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Table 2. The number of substrate samples collected from study sites to
compare differences in the number of intact branchiopod eggs and epphipia
in virgin playa and areas used by vehicles and recreationists. Each sample
is a composite of five 3.8 cm wide by 2.5 cm deep substrate samples.
Study site
Sample size
SM/DHS Road
Black Rock City
Experimental Track
11
40
36
employed a quality assurance protocol where 10 percent of samples were
additionally processed to determine whether the original worker missed any
eggs or ephippia. All sorted eggs and ephippia were archived in 80 percent
ethanol. Eggs and ephippia from other substrate samples were incubated in
the laboratory to identify species in the playa substrate.
We compared the number of intact eggs and ephippia in unimpacted
playa substrates with a heavily used vehicle track (Trego Road); examined
Black Rock City roads and camping areas before and following the Festival,
and the change in egg and ephippia density during repeated vehicle travel.
The number of samples collected for each study is shown in Table 2. Each
sample consisted of six composited, 3.8 cm wide × 2.5 cm deep substrate
samples spaced every 50 cm along a transect. Transects for Trego Road and
Experimental Track studies were placed in the track and along transects
50 m away in virgin playa. Transects in Black Rock City were oriented
perpendicular to mapped roads and extended an equal distance into adjacent
camping areas.
The Trego Road study was conducted during July 2006. The Black
Rock City study was conducted in 2006 during July (pre-Festival) and late
September (post-Festival). The Experimental Track study was conducted in
July 2007.
The Experimental Track study examined changes in intact egg and
ephippia density in virgin playa and following 6, 12, 25, 50, and 100 passes
over a straight track by a 2007 Chevy Colorado Crew Cab truck (weighing
2000 kg and exerting a downward force of approximately 2 kg/cm2 )
traveling 24 km/h. The track was marked with flags placed at 3 m intervals
and sample location for each treatment was randomly selected along the
track. Collections for each treatment consisted of six samples, three of
which were taken from left and right tracks, respectively. Control samples
were taken on adjacent virgin playa.
All data were log(x + 1) transformed before statistical analysis. Data
normality was tested using the Klomogrov-Smirnoff test. No distributions
were normal. The Kruskal-Wallis one-way analysis of variance (ANOVA)
non-parametric test was used to determine the statistical significance.
R ESULTS
Playa Inundation
Playa inundation occurred frequently, and in differing
amounts, from 1972-2008 (Fig. 2). Maximum water coverage was approximately 300 km2 over this period, with a volume of 15 000 000 m3 , which occurred during three years;
smaller amounts of the playa were covered 27 years, and the
playa was dry for five years. The relationship between surface area and maximum water depth over this period was
determined by regression of playa surface elevation measured during GPS surveys and surface area that was digitized from LANSAT images, which is described by y =
79.6x2 − 189188.3x + 112440237.2 (r2 = 0.997). Maximum estimated water depth between late 1972 and 2008 was
1 m, and depths of 0.5 m occurred 15 times from the period from December 1972 through December 2008. Depths
approximating 1 meter occurred only three times over this
period.
Fig. 2. Surface water coverage of the Black Rock Playa from 1972-2007
as determined by digitizing the extent of water shown by satellite images.
Branchiopod Studies
Spatial Variability in Abundance.—Adult B. mackini, B.
gigas, and L. lemmoni were collected during kayak surveys.
Many B. mackini and Moina sp., and a single L. lemmoni
were reared in laboratory aquaria. Rearing studies did not
produce any B. gigas, and a single B. gigas egg was found
in playa substrate studies. The paucity of B. gigas eggs was
not unexpected due to the low ratio of B. gigas to B. mackini
(∼1:40 000) reported from studies in other playas (Broch,
1989).
A total of 966 eggs of B. mackini and 401 ephippia of
Moina sp. were collected from 87 playa samples. There was
spatial variability among virgin playa samples for eggs and
ephippia (Table 3). Eggs were most abundant in Black Rock
City, followed by the site near Trego Road, then the Experimental Track, while ephippia density was greatest at the
Experimental Track. Differences between egg and ephippia
density were statistically significant adjacent to the Experimental Track (p < 0.02, df = 1, 12, Kruskal-Wallis oneway analysis of variance [ANOVA]) and highly significant
at Black Rock City (p < 0.001, df = 1, 39, Kruskal-Wallis
one-way ANOVA). Differences between eggs and ephippia
near Trego Road were not statistically significant (p > 0.12,
df = 1, 12, Kruskal-Wallis one-way ANOVA). Differences
in egg density among these sites were highly significant (p <
0.001, df = 2, 31, Kruskal-Wallis one-way ANOVA), but
differences were not statistically significant (p > 0.24, df =
2, 32, Kruskal-Wallis one-way ANOVA) for only ephippia.
It is difficult to discern reasons for this spatial variability,
but they may include a number of factors such as patchy egg
and ephippia distribution, differences in the frequency and
Table 3. Sample size, mean ± 1 standard error, and range (in parentheses)
in density of B. mackini eggs and Moina sp. ephippia in virgin’ Black Rock
Playa substrates sampled during 2006.
Sample area
N
B. mackini eggs
Black Rock City
Trego Road
Experimental Track
20 26.7 ± 4.9 (0-53)
6 7.10 ± 3.1 (3-21)
6 3.3 ± 1.0 (1-7)
Moina sp. ephippia
2.0 ± 0.4 (0-7)
2.3 ± 1.0 (0-60)
7.5 ± 1.1 (5-12)
SADA ET AL.: PROBLEM EFFECTS ON BRANCHIOPODS
289
Fig. 3. Mean (+1 standard error [SE]) number of intact B. mackini eggs
and Moina sp. ephippia in substrate samples collected in the Trego Road
track (Road) and adjacent virgin playa (Playa) during the autumn of 2007.
Fig. 5. Mean density (±1 SE) of B. mackini eggs in playa samples
collected during the Experimental Track study on the BRP during the
summer of 2007. Regression equation: y = −0.0035x + 3.7528, r2 = 0.075.
duration of site inundation, substrate chemistry, concentration in areas that pond the longest where adults have greater
egg production time, eggs concentrating in wind rows, etc.
Additional studies are needed to fully understand how these,
and potentially other, factors affect spatial variability of eggs
and ephippia.
from statistical significance for Moina sp. (p > 0.4, df =
1, 19, Kruskal-Wallis one-way ANOVA). Approximately
50 percent fewer eggs of B. mackini occurred in camp
areas following the Burning Man Festival, while the number
occurring after the event was approximately 30 percent
fewer in the roads (Fig. 4). Although statistical differences
were not documented, the impact the Festival has on intact
egg abundance may be biologically important and influence
the abundance of adult B. mackini in subsequent years.
Statistical differences between B. mackini egg density before
and after the Festival were highly significant in camp areas
(p = 0.003, df = 1, 19, Kruskal-Wallis one-way ANOVA)
and significant (p 0. 05, df = 1, 18, Kruskal-Wallis oneway ANOVA) in roads. The number of ephippia was lower
in camp areas following the Festival and slightly greater in
roads but differences were not significant for either camp
areas or roads (p > 0.17, df = 1, 15, Kruskal-Wallis oneway ANOVA).
Trego Road.—The number of eggs and ephippia in the Trego
Road track was lower than in adjacent virgin playa (Fig. 3).
These differences were nearly statistically significant for
eggs of B. mackini (p = 0.052, df = 1, 11, KruskalWallis one-way ANOVA) and not statistically significant
for ephippia of Moina sp. (p > 0.08, df = 1, 11, KruskalWallis one-way ANOVA) (Fig. 4). Lack of significance for
these comparisons may be attributed to high variance among
samples of eggs and ephippia.
Black Rock City.—Samples from Black Rock City were
taken at 1190.7 m elevation, which is a height flooded 16
times between late 1972 and late 2008. Differences between
egg and ephippia density in road and camp areas before
the Burning Man Festival were not statistically significant
(p > 0.43, df = 1, 19, Kruskal-Wallis one-way ANOVA)
(Fig. 4). Differences between the two areas were almost
significant following the event for B. mackini (p > 0.07,
df = 1, 19, Kruskal-Wallis one-way ANOVA) and distant
Fig. 4. Mean (+1 SE) number of intact B. mackini eggs and Moina sp.
ephippia in substrate samples collected in Black Rock City camp areas and
roads before and after the Burning Man Festival.
Experimental Track.—The Experimental Track test was
conducted in a portion of virgin playa approximately 8.5 km
northeast of Black Rock City. Following treatment, the playa
surface had been visually altered by repeated truck passage,
but repeated travel caused no discernable change in either
egg or epphipia density (p > 0.58, df = 5, 36, Kruskal-Wallis
one-way ANOVA) (Figs. 5 and 6).
Fig. 6. Mean density (±1 SE) of Moina sp. ephippia in playa samples
collected during the Experimental Track study on BRP during the summer
of 2007. Regression equation: y = 0.0014x + 3.0037, r2 = 0.006.
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D ISCUSSION
Great Basin playas were first introduced in the scientific literature by G. K. Gilbert in his description of Lake Bonneville in northern Utah (Gilbert, 1890). He was intrigued by
the geological formation, aridity of the region, and evidence
of past, more mesic climates as shown by ancient shoreline terraces carved on mountains surrounding most playas.
Since this description, playas have intrigued most visitors
and many are now used by a variety of activities (Nea, 1998;
Fox, 2002). Although playas are ancient, dry lake beds, they
vary widely in substrates, inundation periodicity, and hydrology (Güven and Kerr, 1966; Neal et al., 1968; Motts,
1970; Scuderi et al., 2010), which are influenced by surrounding geology, hydrologic and geologic processes, substrate composition and density, water chemistry, and climate
(Motts, 1970; Reynolds et al., 2007). All of these factors affect playa ecology by creating distinct physicochemical environments for each playa. Since all stages of branchiopod
life history are affected by these factors, they influence structure of branchiopod assemblages (Bowen et al., 1988; Eng
et al., 1990; Gonzalez et al., 1996). Biological surveys in
the Great Basin in surrounding region have not examined
all playas, and a complete list of playa branchiopod species
cannot be compiled for the region. Recent descriptions of
B. raptor Rogers, Quinney, Weaver, and Olesen, 2006, as
well as B. hiberna Rogers and Fugate, 2001 suggest that additional taxa may occur in the region, but collections from
playas in northern Nevada document the presence of B. gigas, B. mackini, and L. lemmoni (A. Shawl, personal communication, Nevada Department of Wildlife, 2005), which
suggests that species collected during our studies on the BRP
are relatively widespread in the region.
A number of studies have examined the effects of mechanical human use on desert upland systems, and documented changes in vegetation and substrate density that alter functional characteristics of vegetation communities and
substrate properties (Wilshire and Nakata, 1976; Webb and
Wilshire, 1983; Caldwell et al., 2006). Few studies have examined the effects of human disturbance on playa substrates
and their biota. Eirksen et al. (1988) found that approximately 30 percent of anostracan eggs were damaged or destroyed on Bicycle Dry Lake playa (San Bernardino County,
California) with 20 passes by a 1974 Toyota Corolla Sedan
(weighing 972 kg and exerting a downward force of approximately 3 kg/cm2 ). In a laboratory study using eggs from
eight species, Hathaway et al. (1996) found that the force
crushing individual eggs differed among species and that
forces less than one newton and 0.1 newton damaged dry
and wet eggs, respectively.
Some of our studies on the Black Rock Desert generally
confirmed observations by Eirksen et al. (1986), but vehicle
effects in Black Rock City appeared to be mitigated by dust
abatement. Additionally, eggs in some portions of the playa
seemed to be less susceptible to vehicle use than others,
which may be attributed to spatial differences in substrate
matrix over the playa. We also observed that ephippia
were more resilient to disturbance than eggs, but both egg
and ephippia density were lower in playa affected by use,
indicating that both are affected by vehicles but that eggs are
more vulnerable to disturbance.
The Burning Man Festival decreased egg density in
Black Rock City roads and camping areas, but had little
effect on ephippia. Effects on eggs were surprisingly greater
in camping areas than roads. This may be attributed to
continuous disturbance of the substrate matrix in camping
areas, as compared to roads where water was added to
control dust, which appeared to strengthen playa substrates.
This strengthening is also suggested by and the higher
number of eggs in roads than camp areas, which suggests
that wetting for dust control did not moisten substrates
sufficiently to soften or fracture eggs. Observations also
suggested that polygons characterizing undisturbed playa
were present on wetted portions of the playa were hard
before, during, and after the Festival. This contrasted with
observations in camp areas where polygons were absent and
substrates were sandy and loose after the Festival (Sada field
notes, 2006). These findings suggest that periodic wetting
for dust control may armor playa substrate and protect
eggs. The increased susceptibility of wet eggs to fracture
(Hathaway et al., 1996) suggests that egg fracture may
increase if too much water is added during dust control or
any other activity associated with recreational use of the
playa.
Results from the Experimental Track study indicated that
vehicle travel had no influence on BRP egg or ephippia density, which contrasted with conclusions of some components
of our other studies and with Eirksen et al. (1986). Reasons
for these differences are unknown but may be attributed to
several factors. The BRP trials used a vehicle that applied approximately 2/3 of the downward force of the vehicle used
by Eirksen et al. (1986), which may have been less force than
needed to damage BRP eggs. There may also be differences
between BRP and Bicycle Dry Lake substrates such that the
BRP substrate matrix is more resilient to disturbance. Also,
the substrate matrix at the track site may be more resilient
to disturbance than other BRP study areas, e.g., Black Rock
City and Trego Road. None of these complicating factors
could have been anticipated before the study, and they were
realized only after laboratory assessment.
A number of poorly understood factors may have affected
our observations. In our Trego Road study, it was difficult to
discern if eggs and ephippia were present in tracks because
they had survived vehicle use or if they were recent arrivals
that were captured in track depressions during wind events.
Eirksen et al. (1986) also speculated that eggs may be dense
in ruts following inundation, but subsequently destroyed
when vehicle use begins as the playa dry. Insight into
these questions could be provided by comparing egg density
in ruts following inundation and before vehicle use with
densities following seasonal use. Quantifying wind dispersal
dynamics of eggs would provide insight into their deposition
in depressions. Populations may also be influenced by
dispersal to and from nearby playas. Vanschoenwinkel et
al. (2008) found propagule dispersal was common between
nearby (2 m to 16 m distant) pools, but Brendonk and
Riddoch (1999) concluded passive dispersal of propagules
was rare when compared to active dispersal through aquatic
connectivity.
Use of BRP has increased over the past 25 years and it
is now annually visited by tens of thousands of people and
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thousands of vehicles. Most use is focused at Burning Man
City, which covers approximately three percent of the playa.
Other uses are comparatively short-term and involve fewer
people, but their impact on branchiopods may be equivalent
because these activities encompass a much greater area and
may involve release of pollutants from vehicles and other
pollutants associated with camping. Information from our
studies indicates that the location of disturbance may have
an important influence on branchiopod abundance. Activities where eggs and ephippia are scarce are likely to exhibit less of an influence on abundance than activities affecting areas with higher branchiopod density. Egg density was
higher at Black Rock City than at other sites, but there may
be other areas where the playa supports even higher branchiopod density. While our 2006 and 2007 studies found a
negative impact of human use on BRP branchiopods, this
information weakly addresses the effect of use on branchiopod abundance on BRP. These studies were limited spatially
and temporally, focused on conditions following winter and
spring inundation and a period when the playa was relatively hard, and samples were limited to small portions of
the playa. Better insight into the influence of playa use on
long-term variability in branchiopod abundance can be provided by sampling a greater proportion of the playa to quantify spatial variability in egg density and to identify areas
with highest and lowest density. It would also be beneficial
to sample for additional years following inundation in virgin
and disturbed playa to quantitatively examine relationships
between natural changes in branchiopod and playa substrate
density between inundations and track the effect of cumulative years of activity on branchiopods.
There is little doubt that human uses affect playa branchiopods. Additional work is needed to more thoroughly examine the potential effect of uses other than vehicles, and
if the effects of use may be mitigated by treating substrate
surfaces with water and/or limiting use to areas where reproductive activity is lowest or the substrate matrix is strongest.
The long-term consequences of disturbance on population
size are also relatively unstudied. Hairston and De Stasio
(1988) suggested that disturbance may be buffered by a large
bank of eggs deposited during previous years and Eriksen
and Belk (1999) noted that all eggs may not hatch during
each inundation. Both of these strategies may increase the
likelihood of populations persisting through lengthy, nonreproductive periods. The strong reproductive capacity and
potentially large size of playa populations also suggests that
egg loss from disturbance of small portions of a playa may
moderately affect adult abundance.
As with most species and communities, long-term biological effects of disturbance are a function of frequency,
duration, and magnitude (Pickett and White, 1985; Pimm,
1991). Populations are resilient when factors are insufficient
to greatly degrade habitat quality or reduce genetic viability
such that growth and reproduction can no longer occur. In
playa systems, effects of human use on branchiopod populations will be greatest when: 1) areas of the playa with highest egg density are affected, 2) large portions of playa are
affected, 3) frequency of reproduction is reduced by lengthy
periods between inundation, and 4) egg abundance declines
due to years of cumulative activity between inundations. The
effects of use on BRP may also be compounded by annual,
incremental transition from a hard, consolidated playa immediately following inundation to an increasingly soft and
friable playa. This is because eggs in friable substrates s are
more vulnerable to disturbance than eggs protected in hard
substrate. Additional studies are needed to address these issues in context of how branchiopod population size responds
to human disturbance.
ACKNOWLEDGEMENTS
Funding for the work was provided by the U.S. Bureau of Land Management, Winnemucca District, Winnemucca, Nevada through Great
Basin Cooperative Ecosystem Study Unit, Cooperative Agreement No.
LA08AC14343. Field and laboratory studies were assisted by D. Henneberry, C. Jacobs, and L. Newton. Comments by C. Rogers and an anonymous reviewer greatly improved the quality of this manuscript.
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R ECEIVED: 1 February 2012.
ACCEPTED: 6 November 2012.
AVAILABLE ONLINE: 19 December 2012.