Cost analysis of remotely sensed foraging paths in patchy landscapes with plant anti-herbivore defenses (Patagonia, Argentina)

Abstract

We developed metrics at a landscape scale to evaluate the costs and rewards experienced by large herbivores while foraging in natural vegetation with patchy anti-herbivore plant structures. We show an application of these metrics to the analysis of 16,000 records of positions at successive 1 min intervals of free-ranging ewes (Ovis aries) harnessed with Global-Positioning-System (GPS) loggers, in a large paddock of the Patagonian Monte shrublands (Argentina). Dominant shrubs in the area display numerous anti-herbivore defenses (spiny-resinous leaves, thorny stems, etc.) protecting them from grazing and herbivore trampling. Preferred grasses and forbs constitute a minor part of aboveground plant biomass and grow in relatively open areas among or around shrub patches. We mapped the movement speed of ewes onto high-resolution aerial photographs of the grazed paddocks and estimated costs and rewards along their paths based on algorithms of surface cost theory. Ewes explored areas of sparse vegetation at low speeds compatible with predominant grazing, and increased their speed when crossing denser shrubby patches. The cost algorithm was applied to evaluate daily searching costs as well as grazing rewards in relation to the length of daily searching paths. The observed path lengths and search speeds were consistent with those that compensate costs and rewards of the grazing activities as estimated by the surface cost analysis. We conclude that the technique presented here constitutes a valuable tool to quantify the effect of landscape characteristics on behavioral traits of grazing animals in similar environments.