Foraging Ecology of the Feral Horses of Sable Island

Abstract

A foundational theory in the field of density-dependent foraging ecology is that of the Ideal Free Distribution, or IFD. The problem of pattern and scale in ecology has also received much attention. However, less is known of how IFD might scale in space and time. My thesis primarily focuses on expanding on previous research using IFD as a null model of intraspecific competition to describe the distribution and abundance of individuals within a landscape by testing for potential drivers of IFD-departure at the small scale. As the only terrestrial mammal of Sable Island, Nova Scotia, the island’s feral horses be studied without confounding effects of predation risk or inter-specific competition. I used the fitness proxy of hourly net energetic gain to build on previous research at the sub-population level and coarse-scaled habitat occupancy (Objective 1). Additionally, I explored how gastro-intestinal parasites might influence outward energetics of horses (Objective 2) and examined short-term (summer) patterns of resource selection in relation to energy content specific to vegetation communities (Objective 3). In 2018, I carried out a study on a focal group of 67 females, quantifying their instantaneous and daily energy intake rates, energy expenditures, parasite loads from strongyle fecal egg counts (FECs), and observed use of vegetation communities. When considering the extent of the whole island, horses did not select for communities based on energetic value but those potentially linked to water availability. However, western horses (high water availability, conspecific density, and FEC) had a higher rate of net energy intake (kJ/hr) than did those in the east there being a negative relationship in energy intake and FEC. The gradient I observed in net energy intake therefore indicates departure from IFD for short-term foraging behaviour using this currency. While IFD-departure at the large scale is known from heterogeneity in annual population growth rates, my research sheds light on fine-scale decisions and their constraints. I conclude that such constraints on behaviour and fitness at the level of the individual resulting in heterogeneity of net energetic gain across the population are likely foundational to the processes from which patterns at larger ecological scales emerge.