Evidence of niche partitioning among small mammals in the Eastern Mojave Desert from scat distribution - UC ...
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Evidence of niche partitioning among small mammals in the Eastern
Mojave Desert from scat distribution
Jacob Ferrall1, Lucy Malamud-Roam2, Roxana Ramirez3, Zishi Wu4
1
University of California, San Diego; 2University of California, Santa Cruz;
3
University of California, Riverside; 4University of California, Davis
Species that occupy similar ecological niches rely on niche partitioning to avoid direct
competition. Our study aimed to detect niche partitioning between four abundant small
mammal species residing in the Eastern Mojave Desert. By varying resource usage over
space and time, the desert cottontail (Sylvilagus audubonii), the blacktailed jackrabbit
(Lepus californicus), the desert woodrat (Neotoma lepida), and the white-tailed antelope
squirrel (Ammospermophilus leucurus) are able to coexist along the mountains’ ridge. We
investigated their niche partitioning through a scat distribution survey. We found
evidence for spatial niche partitioning between jackrabbits and squirrels, and temporal
niche partitioning between woodrats and cottontails. Jackrabbits and squirrels never
defecated near each other, indicating a spatial difference. Cottontails scat piles increased
in abundance around larger middens and were most often in closest proximity to that of
woodrats, sometimes in latrines, revealing that they share the same area but avoid direct
competition through temporal niche partitioning. Our study supplements understanding
of how small desert mammals subsist with each other given limited resources.
Keywords: niche partitioning, small mammals, latrines, middens, scat distribution
INTRODUCTION organisms occupy different spaces in the
same habitat, and temporal partitioning, in
An ecological niche refers to a species’ which multiple species rely on the same
position in the ecosystem (Schoenherr resources but their niches are separated by
1992). The term encompasses all the the time that they are active (Albrecht 2001).
interactions between a species and the Spatial niche partitioning is exemplified
biotic and abiotic factors that affect it. when closely related bird species occupy
Ecological and evolutionary theory dictates different heights on the same tree (Lara et
that only one species can occupy an al. 2015). Temporal niche partitioning is
ecological niche at a time. The coexistence of exemplified by owls and hawks, both of
two species in the same habitat is obtained which rely on similar resources, but hunt at
through niche partitioning, the different times of day (Richard and Meslow,
differentiating of niches and resource use so 1984). Temporal niche partitioning is
that one species does not out-compete the common in habitats occupied by both
other. Two of the main types of niche diurnal and nocturnal species.
partitioning are spatial partitioning, in which
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CEC Research https://doi.org/10.21973/N3G660 Winter 2021 Vol. 5 Issue 2 1/9
Niche partitioning is particularly important since they are easily alerted to human
when the environment is harsh and presence and are active at different times of
resources are scarce, such as in desert the day, so we used scat and nests as
ecosystems (Schoenherr 1992). A diverse indicators of animal activity. Scat sampling
array of flora and fauna compete to survive has been widely used for diet analysis or
in the resource-scarce uplands of the population estimation in carnivores and
Eastern Mojave, including many species of rodents, and the place of defecation
small mammal, who forage amongst its effectively reflects the active area of the
shrubs and cacti. The desert woodrat animals (Garrote 2014). Therefore, we used
(Neotoma lepida) (henceforth “woodrat”) the proximity of scat piles between species
shares a habitat with the desert cottontail to show how associated the species are. For
(Sylvilagus audubonii) (henceforth nesting evidence, the burrows of cottontails,
“cottontail”), the black-tailed jackrabbit (Lepus jackrabbits, and squirrels were hard to
californicus) (henceforth “jackrabbit”), and distinguish, but woodrats create
the white-tailed antelope squirrel conspicuous nests or middens out of various
(Ammospermophilus leucurus) (henceforth plant debris between rocks and under
“squirrel”) (Burt 1980). These four species plants. In the American Southwest region,
occupy a similar ecological niche with woodrats construct their middens beneath
overlapping diets. The woodrat’s diet yuccas (Yucca spp.) and other shrubs,
consists of cholla cacti (Cylindropuntia spp.), covering them in the spines of cholla
Mojave yucca (Yucca schidigera), seeds, (Cylindropuntia spp.) for protection
nuts, and any available green vegetation (Schoenherr 1992). In addition, woodrats are
(Burt 1980). Cottontails mostly feed on territorial with a small range, and they
grasses but also feed on the leaves and peas mostly forage within 10 meters from their
of mesquite (Prosopis spp.), barks, fallen middens (Jameson 2004), making middens a
fruit, and twigs of shrubs (Turkowski 1975). good marker of woodrat activity. Few
Jackrabbits are known to forage on studies have been conducted on how
herbaceous vegetation such as grasses and woodrat middens impact the activities of
forbs during the spring and summer, but other small mammals in their proximity.
switch to the buds, bark, and leaves of woody We were also interested in woodrat
plants in the fall and winter (Foresman middens because woodrats deposit scent
2012). White-tailed antelope squirrels eat marks around their middens to indicate
mostly fruit, seeds, plants, and insects territory ownership. The places where scat
(Bradley 1968). Some squirrels plant accumulated from repeated defecation and
foraging preferences include Mojave yucca urination are called latrines (Kays 2002). The
(Yucca schidigera), blackbrush (Coleogyne strong odor of latinres can mark territory,
ramosissima) and catclaw acacia (Acacia communicate sex differences to potential
greggii). mates of the same species, or communicate
In this study, we assessed if woodrats, with other species in the same environment
cottontails, jackrabbits, and squirrels exhibit (Hirsch et al. 2014). Many animals behave
niche partitioning to avoid direct correspondingly to scent marks to either
competition for space and resources. It was track prey or avoid predators and
difficult to directly observe animal behavior competitors (Arakawa 2008). Research has
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CEC Research https://doi.org/10.21973/N3G660 Winter 2021 Vol. 5 Issue 2 2/9
shown that subordinate mice species avoid METHODS
the scentmark of dominant mice species to
lessen intraspecific competition. Therefore, 2.1 Study System.
understanding how different small
mammals adjust their behavior near a The study was conducted in the Eastern
midden or latrine could further our Mojave Desert at the Sweeney Granite
knowledge of spatial niche partitioning and Mountains Desert Research Center (34°48'N
how coexistence in a desert ecosystem is 115°37'W) (henceforth “the Granite
balanced over time. We examined the scat Mountains”) in February 2021. A few plant
density of small mammals around middens communities are characteristic of the range;
to evaluate if the presence of middens has an single-leaf pinyon pines (Pinus monophylla)
effect on the activities of non-woodrat species. (henceforth “pinyons”) and California
We predicted that larger woodrat middens junipers (Juniperus californica) (henceforth
would have more fresh woodrat scat under “junipers”) speckle the mountains’ ridge,
them, signifying more woodrat activity, and and the descending wash hosts an array of
therefore the surrounding niche would be desert scrub species. The flatlands
dominated by woodrats. We speculated that surrounding the wash host an abundance of
larger middens would have fewer scat piles Mojave yucca (Yucca schidigera), creosote
of non-woodrat species around them bush (Larrea tridentata), buckhorn cholla
because middens may act as a territorial (Cylindropuntia acanthocarpa), and honey
marker for the woodrats (Arakawa 2008). mesquite (Prosopis glandulosa). Desert
We predicted that woodrat and cottontail woodrats (Neotoma lepida) are a common
scat piles would be nearer to each other than inhabitant of the pinyon-juniper-covered
to jackrabbit or squirrel scat piles. A previous hillsides of the Eastern Mojave. They obtain
study in Idaho found only Nuttall cottontail necessary food and water from surrounding
(Sylvilagus nuttallii) activities around bushy- vegetation, as well as materials for building
tailed woodrat (Neotoma cinerea) middens, their middens.
despite there being three rabbit species,
2.2 Sampling Design
including jackrabbits, living in the area
(Johnson and Hansen 1979). Squirrels’ ability Plant community composition and
to climb trees could mean their diet often available midden-building material, changes
extends to the seeds of pinyon pines, which as one moves further from the ridge. We
grow on elevated terrain (Bradley 1968). The were curious to see if this change would
difference in feeding preference may impact woodrat abundance and/or
separate the location of squirrels from the behavior, so we decided to survey plots of
rest of the species. We also hypothesized varying distances from the ridge (Fig. 1). We
that the scat of woodrats and cottontails surveyed eleven 30 m x 30 m plots, which we
would most frequently be found beneath marked with transect tape. The closest
plant cover, so they can hide from predators as distance from each plot to the ridge was
they defecate (Foresman 2012). Meanwhile, recorded. The nearest plot was 10 meters
we expected to see the majority of from the ridge and the furthest was 250
jackrabbit scat in the open, as jackrabbits are meters. In each plot, every fresh woodrat,
too large to fit beneath most desert shrubs.
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CEC Research https://doi.org/10.21973/N3G660 Winter 2021 Vol. 5 Issue 2 3/9cottontail, jackrabbit, and squirrel scat pile estimated the amount of scat we could see
that contained more than 10 pellets was surrounding or within the midden. We then
identified by sight from a comprehensive recorded the average of our two estimates.
walking survey and marked with a flag. Every In addition, we counted the number of
active woodrat midden was flagged with flagged scat piles within two meters of the
another color. Fresh woodrat scat, as well as midden’s edge to calculate the density of
fresh bite marks in surrounding vegetation scat piles around middens, noting the
indicated active middens. Scat color, species that had produced each one.
composition/solidity, and moisture level are
all indicators of scat age (Elbroch 2012). For 2.4 Scat-Centered Survey
our study, we wanted to focus only on fresh
scat, indicative of recent animal activity. We We identified the species of each scat pile.
classified only dark brown except for the scat The round scat of cottontails and jackrabbits
of jackrabbits, which is lighter-brown when could be distinguished by size difference,
fresh, soft, moist scat as “fresh”. A midden- and the scat of woodrats and cottontails
centered and scat-centered survey were could be easily identified based on size and
each conducted subsequently. shape (Fig. 2). After selecting a focal scat
pile, we then identified the species of the
nearest scat pile. We also recorded if the pile
was in an open area or an area covered by
vegetation.
Figure 2. Fecal pellet identification. (A) Desert
Figure 1. A satellite view of Granite Mountains taken woodrat: oval shape, ~1 cm in length, brown. (B)
with Google Earth with the eleven 30m x 30m plots Desert cottontail: round shape, ~0.6 cm in diameter,
marked by boxes. Granite Mountains are located in brown (C) blacktailed jackrabbit: round shape, ~1cm
the Eastern Mojave Desert in California. Plots were in in diameter, light brown (D) white-tailed antelope
flatlands with varying distance to the east of a squirrel:oval shape, ~1.5 cm in length, black.
mountain ridge.
2.5 Statistical Tests
2.3 Midden-Centered Survey
We used JMP Pro 15 (SAS Institute inc.) for
The diameter of each midden at its widest all of our analysis. Linear regressions were
point was measured with transact tape, and used to test for a relationship between
an estimate of the amount of woodrat scat midden size and the density of scat piles of
in the midden area was recorded. In teams different species around the midden, as well
of two, we circled each midden and roughly
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CEC Research https://doi.org/10.21973/N3G660 Winter 2021 Vol. 5 Issue 2 4/9as midden size and the number of woodrat
scat pellets under the midden. An ANOVA
was used to analyze if distribution of scat
piles among the four species differed
depending on the distance to the ridge. Chi-
squared tests were used to assess if each
species defecated closer to a particular other
species by comparing the percentage of the
nearest scat pile from a specific species to
the expected value if the scats were
distributed randomly. Another chi-squared
test was used to examine whether the scat
Figure 3A. (Top) Midden diameter vs. the number of
piles of each species had differences in being woodrat scat pellets around the midden. This linear
located in the open or under plant cover. regression shows that the size of the middens had a
positive relationship with the amount of woodrat scat
RESULTS under and around them (N=76, R²=0.17, pCottontail scat was more likely to be in closest proximity to that of woodrats (N=387, χ =420, p
shrubs such as yucca, mesquite, and cacti, more forbs (Flinders and Crawford 1977).
which are densely populated in the flat area The research was conducted in a high plain
at the bottom of the ridge (Foresman 2012). in Texas, a very different ecosystem than the
Therefore, the spatial niche partitioning Mojave Desert with vegetation mainly
between them could be driven by composed of various grass species.
preferences in feeding between the two However, it still indicates that feeding
species. With the ability to climb trees, differentiation exists between cottontails
squirrels can access more pine seeds than and jackrabbits. For our study system, future
rabbits and rats. They may prefer to forage research could compare the proportion of
more along the ridge where pinyon pines are cacti, mesquite, and yucca materials in fecal
more common, competitors are fewer, and pellets of the two species to further our
shrubs are also accessible. understanding on feeding differentiation
Despite the foraging area for both squirrels between them in a desert ecosystem.
and jackrabbits overlapping with that of For woodrats and cottontails, their scat
cottontails and woodrats, there was also piles were more evenly distributed across
evidence for spatial niche partitioning based the landscape, and no differences were
on the type of area they defecated in. found between the two species in terms of
Cottontails and woodrats defecated mostly distance from the ridge. This suggested that
under cover and often together in latrines, woodrats and cottontails share the same
while jackrabbits and squirrels defecated foraging area, and this idea was further
mostly in open areas. Such a behavioral supported by the closest scat pile species.
difference between jackrabbits and We were more likely to find woodrats being
cottontails is particularly interesting since the nearest scat pile species to a cottontail
both species belong to the family Leporidae. scat pile than if all four species were
We speculated that partitioning was due to defecating randomly on the landscape and
the differences in foraging strategies. vice versa. This is consistent with the study
Cottontails are known to frequently utilize done on bushytailed woodrats (Neotoma
protected cover, and jackrabbits are larger in cinerea) and Nuttall cottontails (Sylvilagus
size than cottontails (Vaughan 1972). This nuttallii) in Idaho, where among blacktailed
might mean the tunnels created by small jackrabbits, pigmy rabbits, and cottontails,
rodents under shrubs are often too small for only cottontails were observed near the
jackrabbits to enter, forcing them to feed in woodrat middens (Johnson and Hansen
more open areas where plants are more 1979). The sharing of active areas could be
easily accessible to them. Additionally, food an indicator of direct competition, however,
preferences could be another factor in there is temporal niche partitioning between
determining where each species defecates. cottontails and woodrats that weakens such
The types of plants that cottontails and competition. Cottontails and Woodrats have
jackrabbits consume are highly overlapped, a similar diet, but woodrats are active at
but a previous study showed that the night while cottontails are active during the
proportion of plant species in their fecal day. Furthermore, the presence of mixed
pellets differed significantly: jackrabbits scats from the two species in the latrines
ingested greater proportions of grasses and even suggested their tendency to defecate in
woody plants while cottontails ingested the exact same area repeatedly, potentially
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CEC Research https://doi.org/10.21973/N3G660 Winter 2021 Vol. 5 Issue 2 7/9indicating interspecific communication. The feeding preferences are significantly
separation between diurnal and nocturnal different.
activity amongst small mammals has Larger middens had more woodrat scat
previously been observed to occur among under them, so it is possible that cottontails
different species of spiny mice (Acomys spp.) might be tracking the scent mark of
through odor cues traced in their scat (Haim woodrats to find middens in which they can
and Rozenfeld 1993). The shared latrines of feed upon the host plant. If there is
woodrats and cottontails may signify that interspecies communication to cottontails
they too partition their niches temporally, as occurring from the scent of woodrat scat,
opposed to the spatial partitioning observed then it may be revealed by examining their
in jackrabbits and squirrels. latrines (Schoener 1973). If interspecies
The interaction between woodrats and communication is found, it implies that the
cottontails can also be reflected by latrines are used to facilitate temporal niche
cottontails increasing activity around larger partitioning. Evidence of different species
middens. Contrary to our predictions that sharing latrines to convey information of
other species would be deterred by woodrat their presence was found in some solitary
middens, we found larger middens carnivores such as pampas cats (Leopardus
associated with a greater density of pajeros) and culpeos (Lycalopex culpaeus),
cottontail scat piles within two meters from as well as honey badgers (Mellivora
their edge. This is probably because larger capensis) and meerkats (Suricata suricatta)
middens are generally found under larger (Berg 2003; Jordan 2005). Observing how
host plants, providing more sources for many scat pellets from each species are in a
herbivory. Woodrats have small home latrine across several seasons can reveal if
ranges and they mostly forage within 10 the species uses the latrine to communicate
meters from their middens, so the host plant to the opposite sex of their own species,
of the midden acts as a food source for them with potentially more pellets during mating
(Jameson 2004). Middens are mostly found season. If the woodrats and cottontails
residing under yucca and cholla cacti, and consistently use the same latrine with
both are important food resources for desert equivalent frequency year-round, then there
cottontails (Turkowski 1975). Additionally, may be interspecies communication
the enriched soil under woodrat middens between them rather than only intraspecies
(Whitford and Steinberger 2010) could be communication for mating.
cultivating better conditions for vegetation
around the midden, thus attracting more ACKNOWLEDGMENTS
cottontails that feed on the vegetation
around the midden. A possible future course This work was performed at the University
of study could focus on this phenomenon of California’s Sweeney Granite Mountains
and investigate the effects of woodrat Desert Research Center, doi:
middens on their surrounding plant 10.21973/N3S942. We would like to extend
community. Furthermore, comparing the our gratitude to Sarah Kingston and Tim
fecal pellet composition between woodrats Miller for the opportunity to conduct our
and cottontails in a desert environment own research and enhance our knowledge of
could be a good indicator of whether their the natural world.
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