Mechanisms of niche partitioning between in the East Mojave

Ciera Castillo1, Katherine Hernandez2, Isabelle Panza1, Alexa Rowland1

1University of California, Santa Cruz, 2University of California, San Diego,

ABSTRACT

Understanding how species can coexistence in light of competition has long been a focus of ecological study. One of the many mechanisms that have been proposed to explain how species with overlapping niches can coexist is that they differ in their behavioral patterns. In our study we investigated how rodents in the genera Dipodomys and Peromyscus partition their habitat spatially and how their behaviors differ. To test for behavioral differences and spatial habitat partitioning in these species we monitored using traps and wildlife cameras and mapped their habitat use in space. We found that Peromyscus displayed risk averse strategies in their foraging and habitat use whereas Dipodomys displayed more variability in their movements and were more likely to take risks. These interspecific patterns help us to better understand community ecology as a whole, and can have broad implications for the structure of communities in sensitive desert habitats.

INTRODUCTION found that spatial partitioning can Co-occurring species can avoid costly enable coexistence on a single resource. competitive interactions via spatial, Schroder and Rosenzweig (1974) temporal, or resource-associated niche trapping records suggest that one partitioning (Schoener 1974; Wiens species of Desert prefers a 1977), and which can further lead to grassier habitat, and D. merriami a stable species coexistence (Albrecht and habitat dominated by creosote bush. Gotelli 2001).The mechanisms allowing Their findings suggest that these species for coexistence of species, and avoid competition through habitat particularly the number of species selection which is one mechanism of coexisting in a given habitat, have been species coexistence. explored from many viewpoints (Hafner Desert rodents have been 1977). Horn and MacArther (1972) fundamental in exploring the underlying mechanics of community ecology given

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the coexistence of a diverse assemblage behavior. Understanding how these of granivorous species within a mechanisms contribute to the relatively simple desert ecosystem coexistence of these two competing (Kotler and Brown 1988). The Mojave rodent genera can help further Desert houses diverse assemblages of elucidate the outcomes of shared granivorous rodents, including several niches, and furthermore adds to our species of Dipodomys, Peromyscus, etc., understanding of community-level and whose communities vary with micro patterns of resource use. and macro-habitat heterogeneity (Stevens and Tello 2009). Coexistence METHODS among these granivorous rodents occur Study Area via a combination of mechanisms, including spatial and temporal niche We conducted our study at the partitioning (Kotler and Brown 1988). Sweeney Granite Mountains Desert Dipodomys and Peromyscus are two Research Center in the East Mojave major genera of nocturnal rodents in Desert, California, U.S.A. (34° 48’ 20” N, the Mojave whose diets rely on heavily 115° 39’ 50” W). We conducted our on seeds and whose ranges often study during the fall (November 3–8, overlap (Kotler and Brown 1988; 2017), with average nightly Stevens and Tello 2009). The use of temperatures ranging from 13 degrees shared habitat, resources, and similar C. Our study area was located in mixed foraging behavior between Dipodomys desert scrub, with dominant plant and Peromyscus presents a unique species consisting of creosote bush system in which to explore the different (), Mojave yucca (Yucca mechanisms leading to species schidigera), and buckhorn cholla coexistence. Despite an abundance of (Cylindropuntia acanthocarpa). studies examining niche partitioning in Experimental Design desert rodents, there remains a lack of In order to investigate mechanisms of clarification on the mechanisms spatial partitioning among Peromyscus underlying the coexistence of and Dipodomys rodents, we conducted Dipodomys and Peromyscus in the mark-recapture sampling and tracked . individual movements over the course In this study, we investigated spatial of 5 nights. We conducted mark- and temporal mechanisms of niche recapture sampling within a 48 m x 48 partitioning in Peromyscus and m grid containing 25 plot points Dipodomys. Specifically we ask if regularly spaced 12 m apart. At each Peromyscus and Dipodomys exhibit plot point, we placed a Sherman live spatial partitioning in home range and trap filled with cotton balls to provide habitat use. Secondly, we ask if warmth and baited with oats and Peromyscus and Dipodomys exhibit peanut butter to attract granivorous temporal partitioning in foraging rodents. We set traps at sunset (~5

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p.m.) and returned at approximately However, recapped individuals were 8:30 p.m. each night to check traps. again dipped in UV dust and associated Captured rodents were identified to information regarding location species. Rodents not identified as and distance traveled were recorded as Dipodomys or Peromyscus were additional information on the home released without further processing as range of that individual. they were not focal in our study. In order to investigate temporal niche Rodents identified as Dipodomys or partitioning between Dipodomys or Peromyscus, were processed according Peromyscus and to monitor rodent to the following protocol. First, we foraging behavior, we placed four recorded life history characteristics to camera traps in an area away from our assist with species identification, grid, each with a quarter cup of bait including the weight of the rodent, the (oats and peanut butter). We set length of its tail (and length of the apical camera traps to record 30 s videos with tuft for Dipodomys), the length of the 5 seconds gaps in-between triggered hind foot, the length of the ear, and recordings. For three consecutive count of toes for Dipodomys. Secondly, nights, we turned on the cameras we individually marked rodents at their around 4:45 p.m., baited camera traps, first capture with a unique pattern and returned the following morning consisting of a series of dots and lines around 9 a.m. to retrieve the camera marked using black and blue sharpie on memory cards. From the camera trap the ventral side of the tail. Lastly, we data, we recorded the time of a visit dipped the rodent into a bag of UV dust from Peromyscus and Dipodomys, the (pink for Peromyscus and yellow for frequency of visits, and duration of a Dipodomys), and released them at the visit measured as the number of times a spot where they were captured. We camera was consecutively triggered by followed their path with a UV flashlight an individual before it left the site. (51 LED, 395 nM) for as far as possible, Data from camera traps and Sherman often ending at the entrance of a live traps were analyzed in JMP using a burrow. We recorded these on t-test comparing number of recordings a gridded map, giving each burrow a of an individual per visit, and by chi- unique identifier and spatial coordinates squared test comparing the average in reference to our trapping grid. We proportion of visits to the baited camera also recorded the estimated distance (as trap sites between genus. Recapture the crow flies) of the UV trail, which we rates between genus was analyzed using used as a proxy for foraging distance or a chi-squared test. home range. On subsequent trapping We used R Statistical Software days, recaptured individuals were programming (v. 3.4.2) to conduct all identified by their unique symbol on the spatial visualization and analyses. In ventral side of the tail but no other life order to visualize individual home range history characteristics were recorded. we generated Minimum Convex

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Polygons (MCP; package ggConvexHull) 1). Home range estimates ranged around locations where individuals were between 5 m2 and 400 m2 in Peromyscus trapped by Sherman live trapping or and 22 m2 for a single Dipodomys followed by UV dust trails to burrows. individual. We found significantly more We calculated home range using MCP visits to baited camera traps by methodology (package “adehabitatHR”, Dipodomys than by Peromyscus (N = 52, function mcp, percent = 100). Home P = 0.04; Fig 2). We also found that range estimates required a minimum of there was a marginally significant five tracking points, and thus, we were difference in duration time of visits only able to estimate home range for between Dipodomys and Peromyscus; four Peromyscus and a single specifically, Dipodomys were observed Dipodomys individual. spending more time foraging at camera trap sites than the Peromyscus (P = .08, RESULTS t = -1.8; Fig 3). Finally, Dipodomys had a Estimated spatial territories of significantly lower rate of recaptures Peromyscus and Dipodomys indicated than Peromyscus (N = 48, P = 0.03; Fig little overlap between home ranges and 4). habitat use between the two genera (Fig

Figure 1. Habitat partitioning between Dipodomys and Peromyscus. Grid represents our study plot in the field with dimensions extended from 48 m x 48 m to 60 m x 60 m in order to incorporate burrows found outside of our study plot. Each unit represents a meter. Minimal convex polygons were made following individuals from traps at

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which they were captured to the burrows they retreated to. Shaded regions are the calculated home ranges of an individual.

Figure 4. Average duration of Dipodomys and Figure 2. Average proportion of visits to camera Peromyscus visits to baited camera traps. trap sites between Dipodomys and Shaded bars are representations of mean from Peromyscus. Shaded bars are representations of individuals with binomial error bars included. mean from individuals with standard error bars included.

Figure 3. Average duration of Dipodomys and Peromyscus visits to baited camera traps. Shaded bars are representations of mean from individuals with standard error bars included.

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DISCUSSION not directly within their territory, while Our results suggest that coexistence Dipodomys may not. Thus, Dipodomys between Dipodomys and Peromyscus likely are able to exploit high-risk may be mediated by spatial niche foraging opportunities while partitioning of habitat. Dipodomys are Peromyscus may be restricted to low- considerably larger than Peromyscus, risk foraging opportunities. These and consequently have much larger observations concur with a study by home ranges (what are their estimated Price and Brown (1983), which found home ranges?) that exceed the area of that the interspecific differences in our trapping grid (Scheibe 1984). Larger microhabitat affinity in desert rodents granivores, such as Dipodomys, tend to contributes to their coexistence and create “patches” in which they forage, genus characterization. relative to their size and other Overall, our study reveals complex neighboring rodents (Price and Brown niche partitioning in Dipodomys and 1983). Our results concur with these Peromyscus. The two genera have very studies, and suggests that spatial similar diets and life histories, but by partitioning does occur between partitioning their habitat through spatial Dipodomys and Peromyscus, and that and behavioral means they may reduce their foraging territories are likely costly energy expenditures associated relative to their respective sizes. with competition. Because of the harsh In addition, we did not find evidence conditions and limited resources in the for temporal niche partitioning between Mojave desert, it is unsurprising to find Dipodomys and Peromyscus. However, such dynamic interactions arise. we did find that Dipodomys were However, it is worth researching how approaching and spending a longer similar multi-level partitioning occurs in amount of time feeding at camera traps other desert habitats, where other than Peromyscus. This suggests a rodent genera take precedence. difference in foraging behavior that ACKNOWLEDGEMENTS extends beyond temporal or spatial partitioning. Peromyscus, by having a This work was performed at the specific territory and visible hesitancy University of California’s Sweeney while feeding, is more risk averse in its Granite Mountains Desert Research foraging behavior than Dipodomys. Center, doi:10.21973/N3S942. However, Peromyscus were recaptured far more than Dipodomys, and were REFERENCES consistently recaptured in the same Albrecht, M. & Gotelli, N. J. 2001. Spatial and area of the grid. Thus, the differences in Temporal Niche Partitioning in Grassland foraging behavior between these two Ants. Oecologia 126, 134–141. genera also likely contribute to further Brown, J. H., Reichman, O. J. & Davidson, D. W. niche partitioning. Peromyscus may be 1979. Granivory in desert ecosystems. more likely to give up a food resource

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