Population Dynamics of Browsing and Grazing Ungulates in the Anthropocene

Abstract

In this Chapter, we describe patterns of ungulate population dynamics, intrinsic and extrinsic causal factors underlying population growth, and consequences of variation in these causal factors in the face of anthropogenic change. We group ungulates as grazers and browsers, and review how each main functional feeding group copes with spatial and temporal variability of forage availability. Densities of browsers and grazers are highly variable in space and time, with the highest densities (top 10{\%}) realized within specific body mass ranges. Among browsers, highest densities are usually found in smaller-bodied species (range: 20--233 kg, median: 45 kg), whereas highest densities for grazers are realized in larger species and within a wider body mass range (17--325 kg, median: 137.5 kg). A literature review of demographic processes (births, deaths, and movements) governing population dynamics suggests that direct effects of environmental variation on demographic rates, cohort effects, and indirect effects of perturbations on the age structure, all influence population growth rates. Additionally, the role of direct versus indirect effects can depend on life history strategies. Which specific demographic processes are most important to population growth rate are largely context dependent. Population growth rates of browsing and grazing ungulates are strongly influenced by environmental variation, with primary productivity---which varies strongly in space and time---the fundamental factor influencing the carrying capacity of a given area. Competition, direct and indirect effects of predation, and diseases can lower population densities below their resource-determined potential. Resource availability, predation, diseases, and perturbations of the environment (e.g. drought, fire, and land use change) interact synergistically in their regulation of herbivore populations to create indirect-, additive-, reciprocal-, and interaction-modifying relationships. In particular, human-caused perturbations (land use and climate change, introduction of livestock, and direct exploitation) may directly or indirectly affect both ``bottom up'' and ``top down'' regulation. A qualitative threat review indicates that obligate grazers in sub-tropical regions may be particularly threatened given the scale and diversity of anthropogenic perturbations projected to be influential.