Population density and vegetation resources influence demography in a hibernating herbivorous mammal

TL;DR

This 32-year study investigates how vegetation phenology and population density influence demography and functional traits in Columbian ground squirrels, revealing complex ecological interactions affecting survival and reproduction.

Contribution

It uniquely integrates multiple environmental variables over a long period to analyze their combined effects on small mammal demography and traits.

Findings

Higher population density reduces survival and breeding success.

Earlier vegetation start improves adult female and juvenile survival.

Later vegetation timing benefits all squirrels' survival.

Abstract

Demography of herbivorous mammal populations may be affected by changes in predation, population density, harvesting, and climate. Whereas numerous studies have focused on the effect of single environmental variables on individual demographic processes, attempts to integrate the consequences of several environmental variables on numerous functional traits and demographic rates are rare. Over a 32-year period, we examined how forage availability (vegetation assessed through NDVI) and population density affected the functional traits and demographic rates of a population of Columbian ground squirrels (Urocitellus columbianus), an herbivorous hibernating rodent. We focused on mean population phenology, body mass, breeding success and survival. We found a negative effect of population density on demographic rates, including on breeding success and pup and adult survival to the next year. We found diverging effects of vegetation phenology on demographic rates: positive effects of earlier start to growing season on adult female and juvenile survival, but no clear effect on male survival. Interestingly, neither population density nor vegetation affected population phenology or body condition in the following year. Vegetative growth rate had a positive influence on female mass gain (somatic investment) over a season, but vegetative growth rate and biomass, surprisingly, had negative effects on the survival of young through their first hibernation. Later vegetative timing during the year had a positive influence on survival for all ground squirrels. Thus, ground squirrels appeared to benefit more from later timing of vegetation than increases in vegetative biomass per se. Our study provides evidence for complex ecological effects of vegetation and population density on functional traits and demographic rates of small mammal populations.