Patchy Landscapes and Animal Movements: Do Beetles Percolate?

Abstract

Simple neutral percolation models of ecological landscapes predict a critical threshold of landscape connectivity at ca 60% coverage of cells that are permeable to movement. Using such models as an inspiration, we conducted a field experiment in which we employed an experimental model system of tenebrionid beetles (Eleodes obsoleta) moving through mosaics of grass and bare ground in which grass coverage was systematically varied from 0% to 80% in a random pattern. Three parameters of beetle movement pathways (mean step length, mean vector length, and net displacement rate) all showed a sharp reduction between 0% and 20% grass cover and no differences over further 20% coverage increments; the mean fractal dimension of pathways (which could not be derived for the 0% coverage treatment) also did not differ with increasing grass coverage from 20% to 80%. The proportion of time spent by beetles in grass patches did not increase with increasing grass coverage, although individuals spent significantly more time steps in grass in the 20% grass-cover treatment than would be expected by chance. The distance moved per time step was greater when beetles moved over bare ground than in grass, but the mean step length while on grass was significantly lower when grass constituted only 20% of the experimental landscape than when more grass was present, perhaps because beetles stopped more frequently on grass when little of it was present. The threshold in beetle movement through the experimental landscapes differed markedly from that predicted by simple neutral percolation models, probably because, in contrast to the models, both cover types were permeable to beetle movements and individuals moved nonrandomly. The results of this experiment indicate that definitions of landscape connectivity depend on both the spatial pattern of the landscape and how individuals move within and among patches. Moreover, it appears that movement patterns within a particular patch may be contingent on the characteristics of the surrounding landscape. As a consequence, a land-cover map alone may not predict whether, to a particular kind of organism, a landscape is fragmented or connected, and the spatial distribution of individuals in a population may not map closely onto the distribution of suitable habitat patches. This finding suggests that assessments of metapopulation structure must consider both overall landscape patterns and the nonlinear responses of organisms to such patterns.