How Does Habitat Patch Size Affect Animal Movement? An Experiment With Darkling Beetles

Ecology, Oct, 1999 by Nancy E. McIntyre, John A. Wiens

DISCUSSION

Both the existence of heterogeneity and the scale (grain) of heterogeneity had important effects on animal movement patterns. There were significant differences between the homogeneous sand control and the four heterogeneous grass-sand treatments, in terms of the number of beetles that traversed the extent of the EMS. There were also significant differences between the control and treatments and among the four treatments for various movement characteristics [ILLUSTRATION FOR FIGURE 2 OMITTED], demonstrating that the presence of spatial heterogeneity affects animal behaviors. Furthermore, the significant differences among the treatments in Fig. 2 (manifested as peaked distributions in [ILLUSTRATION FOR FIGURES 2D, E, and F OMITTED]) demonstrate that animals respond differently to different grains of heterogeneity. The similarity of responses to 0.125-m and 1-m treatments illustrates that landscapes may be structurally dissimilar and yet functionally equivalent, at least for movement at these scales. Beetles concentrated their time in movement on sand, taking fewer time steps on grass, which resulted in more linear pathways in landscapes with no grass patches or a few large patches than when there were numerous small patches.

These peaked distributions may be interpreted relative to how beetles may perceive patches of different sizes as having different ecological functions. Small grass patches may have been perceived simply as obstacles because they contain insufficient resources, whereas larger patches may have been viewed as colonizable habitat. This would explain the increase in response to patch size observed for most variables between the 0.125-m and 0.25-m treatments. When a habitat patch's area is quite large, however, a beetle may use habitat in a lesser proportion than predicted by the habitat's size. This would explain why landscapes with large and small patches have pathway fractal dimensions statistically indistinguishable from one another [ILLUSTRATION FOR FIGURE 3 OMITTED], and yet these landscapes with large patches evoke different responses than do landscapes with patches of intermediate size [ILLUSTRATION FOR FIGURE 2 OMITTED]. Thus, different mechanisms or constraints may be acting at different spatial scales to create such peaked responses.

Both the landscapes with the smallest grass patches and the landscapes with the largest patches evoked the most convoluted movement paths [ILLUSTRATION FOR FIGURE 3 OMITTED], once again suggesting that patches of different sizes may have assumed different roles to beetles. However, the nonlinear response that we observed in movement pathways was more complex than a simple reaction to patch aggregation. Three patch sizes were statistically indistinguishable (0.125 x 0.125, 0.50 x 0.50, and 1 x 1 m), but were separated by an intermediate size (0.25 x 0.25 m), suggesting that there may have been a tradeoff between attraction to grass as habitat and avoidance of grass because it is more difficult to move through than is sand. The greater viscosity of grass compared to sand (Crist et al. 1992, Wiens et al. 1997) probably accounted for the more convoluted and localized beetle movements in the more finely patterned landscapes, where the numerous grass patches interrupted the flow of movement. This response to grass may also explain why fewer beetles reached the boundary of the EMS in the treatments as compared to the control. Because patch boundary presence and type are thought to have important effects upon animal dispersal and patch colonization (Gardner et al. 1992, Wiens 1992), this aspect of landscape patchiness merits further research.