Despland, E., Collett, M., and Simpson, S.J. 2000. Small-scale processes
in desert locust swarm formation: how vegetation patterns influence
gregarization. Oikos 88:652-662.
Desert locusts ( Scistocerca gregaria ) change phase in response
to population density: 'solitarious' insects avoid one another, but when
crowded they shift to the gregarious phase and aggregate. This
individual-level process is the basis for population-level responses that
may ultimately include swarm formation. We have recently developed an
individual-based model of locust behavior in which contagious resource
distribution leads to phase change. This model shows how population
gregarization can result from simple processes operating at the individual
level. In the present study, we performed a series of laboratory
experiments in which vegeation pattern and locust phase state were
assigned quantitative, measurable indices. The pattern of distribution of
the resource was represented via fractal dimension: the phase state was
evaluated using a behavioral assay based on logistic regression analysis.
Locusts were exposed to different patterns of food resource in an
artificial arena, after which their behavioral phase state was assayed.
These experiments showed that when the distribution of the vegetation was
patchy, locusts were more active, experienced higher levels of crowding,
and became more gregarious. These results are consistent with simulation
predictions and field observations, and demonstrate that small-scale
vegetation distribution influences individual behavior and phase state and
plays a role in population-level responses.