| dc.description.abstract | The spatial distribution of tree cutting activity by
beaver (Castor canadensis Kuhl) and tree availability were
measured at two beaver colonies near Thunder Bay, Ontario.
Beaver maximize the net rate of energy acquisition
(e/t) when they minimize the distance travelled from the
lodge, their central place, while obtaining a balanced
diet. However, beaver must travel through two mediums to
obtain terrestrial forage so minimum distance is a function
of the relative costs of swimming and walking. A continuum
of relative costs was used to generate two contrasting
foraging models: 1) swimming equalled the cost of walking
and the foraging path was a straight line from the lodge to
a tree; 2) swimming was costless and the foraging path was
the shortest distance from water to a tree.
The models were compared for relative goodness of fit
with the observed foraging pattern using chi-square and
linear regression goodness of fit tests. The water costless
model was the best fit model and empirically supported the
implicit assumption of most workers that the pond is the
effective central place. In practical terms 'water costless*
was judged to be indistinguishable from a 5-10 times
advantage for swimming over walking. Although experimental
confirmation is required, this advantage was considered too
large to be explained by energy or time savings and was
therefore not an optimization of e/t. It was tentatively concluded that the pond should be viewed as a refuge from
predators which probably constrain the relative availability
of terrestrial forage to beaver.
A test for optimal foraging, given the predation constraint,
showed that beaver were maximizing e/t at Northbranch
Pond. At Pinetop Pond, a relatively old site, beaver
expanded their refuge to obtain patches of relatively high
food quality. They could have realized higher e/t with
perfect knowledge of their site, but apparently they maximized
e/t with a patch use strategy that minimized search
time.
A review of beaver literature discussed optimization
processes at four levels: evolution of body form and
function, the individual lifespan, yearly, and seasonally.
Arguments of particular interest were developed for dispersal
(population regulation), central place, and the size-distance
relationship. | |
