Je. Gross et al., MOVEMENT RULES FOR HERBIVORES IN SPATIALLY HETEROGENEOUS ENVIRONMENTS- RESPONSES TO SMALL-SCALE PATTERN, Landscape ecology, 10(4), 1995, pp. 209-217
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.