Cropland restricts occurrence and alters spatial ecology near the mule deer geographical range limit

Abstract

Background: Habitat fragmentation can influence the spatial ecology of wildlife populations, with downstream effects on population dynamics and sustainability. Row-crop farming is a common anthropogenic landscape alteration, yet the effects on animal movement and space use is understudied in some species. Cropland can benefit wildlife nutritionally but may result in habitat loss because of changes in landscape composition and human disturbance.

Methods: We quantified the influence of cropland presence and coverage on mule deer spatial ecology in the southern Great Plains. We GPS-collared 146 adult mule deer in four regions of the Texas Panhandle and monitored movement relative to spatio-temporal fluctuations in cropland and particular crop species availability for 2 years. We modeled the effects of cropland on space use and resource selection at multiple spatio-temporal scales to characterize population and individual habitat components of mule deer.

Results: We observed a functional response in cropland use, where at low coverage, use was proportional to availability but decreased with > 20% cropland coverage at the home range and within-home range scales. Few mule deer exhibited long-distance movements towards cropland. Individuals within 1.6 km of cropland exhibited greater cropland use, whereas deer > 4.2 km from cropland rarely used these areas. At the population level, mule deer selected cropland during the winter and late summer, probably for nutritional benefit when rangeland nutrients are low. At a finer scale, step-selection functions identified individual heterogeneity in crop species selection. Winter wheat, alfalfa, and fallow fields had greater use relative to other crop types. Generally, 15-60% of mule deer with access to cropland selected alfalfa year-round, and up to 63% of deer selected winter wheat post-reproduction.

Conclusions: Our results suggest that at a low spatial coverage, cropland alters the spatial ecology of mule deer at several spatio-temporal scales and may provide nutritional benefits, but at a cost of lost habitat when cropland exceeds 20% of the landscape. Declining groundwater resources and an exponentially growing human population will alter future farming practices. Understanding how species occupying these environments, such as mule deer, are influenced by human-induced landscape changes can enhance mitigation of human-wildlife interactions and aid conservation actions as policy and social pressures shape future agricultural practices.

Keywords: Odocoileus hemionus; Cropland; Functional response; Great plains; Ogallala aquifer; Resource selection; Row-crop farming; Step selection.

Fig. 1

The Texas Panhandle with four study sites (purple) used to assess the influence of cropland on mule deer spatial ecology using 146 mule deer from 2015–2019. Much of the Panhandle is on a caprock escarpment with very little variation in topography (grayscale) and is dominated by cropland (green) which sits atop the rapidly depleting Ogallala Aquifer (blue)

Fig. 2

Average home range size of 146 mule deer in the southern Great Plains from 2015–2019 (A) and the relationship between cropland use and average distance of the home range from cropland (B). Mule deer in this system display a functional response of use based on cropland availability at both the home range scale (C & D) and within home range scale (E & F; Available steps generated for step-selection functions). B indicates a piecewise regression to identify breaks in cropland use based on distance classes from cropland. C and E were fitted with a second-order polynomial regression and D and F were fitted with a piecewise regression to identify the point of deviation from a proportional linear relationship (dashed black line) between use and availability

Fig. 3

Inversed selection coefficients from distance variables and 95% confidence intervals per month and sex from population level resource selection modeling at landscape scale (A) and step-selection selection modeling at the within home range scale (B). The horizontal black line indicates no effect whereas above the line indicates selection and below the line avoidance of the resource relative to availability. Conservation Reserve Program (CRP) or Fallow indicates fields enrolled in CRP or were dormant with native vegetation regrowth

Fig. 4

(A) Proportion of individuals that select (green), avoid (red), or show no effect (gray) of specific cropland types based on individual based step selection modeling. (B) Proportional use of major crop types observed in our study by mule deer per month. Black points represent population averages. Proportional use is roughly equivalent to the proportion of time spent in each cover type. Both selection and use results represent only individuals that occupied areas < 4.23 km from cropland. Females are the left column and males are the right column, respectively. Winter wheat was only available October through April, corn and cotton were available May through September, and alfalfa was available year-round. Conservation Reserve Program (CRP) or Fallow indicates fields enrolled in CRP or were dormant with native vegetation regrowth