MOVEMENT RULES FOR HERBIVORES IN SPATIALLY HETEROGENEOUS ENVIRONMENTS- RESPONSES TO SMALL-SCALE PATTERN

Foraging herbivores respond to the spatial pattern of resources at a v ariety of scales. At small scales of space and time, existing models c apture the essence of the feeding process and successfully predict int ake rates. Models that operate over larger scales have not exhibited a similar success, in part because we have a limited understanding of t he rules used by animals to make decisions in spatially complex enviro nments, or of the consequences of departing from these rules. To evalu ate the rules that large herbivores use when navigating between forage s, we examined movements of bighorn sheep foraging on apparent prey (a lfalfa plants) in hand-constructed patches of plants. Observations of movements and path lengths were compared to simulations that used a va riety of different rules-of-thumb to determine a search path. Rules us ed in simulations ranged from a random walk with various detection dis tances, to more complicated rules that solved a variant of the travell ing salesman problem. Simulations of a random walk yielded movement le ngths that exceeded observations by a factor of 3 for long detection d istances, and by 30-fold for short detection distances. Observed move distances were most closely approximated by simulations based on a nea rest-neighbor rule - over 75% of all moves by bighorn sheep were to th e closest available plant. Movement rules based on random walks are cl early inappropriate for many herbivores that typically consume visuall y apparent plants, and we suggest the use of a nearest-neighbor rule f or modelling foraging by large herbivores.