7.4.4 Results
When settlement was random, average downstream drift distances decreased by roughly 50% when the proportion of DWZ was increased from 5% to 15%, but spatial patchiness had little effect. This picture was complicated by restricting settlement to DWZ. Increasing the proportion of DWZ again halved drift distances, but at each proportion, the average drift distance was roughly doubled when there were fewer, but larger obstacles (Figure 7.5). The spread of drift distances also increased greatly with fewer (larger) obstacles, particularly at lower (5%) obstacle densities, and again when settlement occurred only in DWZ (Figure 7.6). Thus, even with these simple movement rules, spatial patterning of settlement sites appears to have a strong bearing on the potential drift distances of our model organisms. According to the model, the effects of spatial patchiness are contingent on the realities of where organisms actually settle in streams, and whether this is restricted to particular areas such as DWZ. Testing this assumption is a critical challenge. Nevertheless, the results of this modelling provide a simple explanation for discrepancies in previous empirical studies (e.g. Lancaster et al., 1996).