The Effects of Habitat Geometry on Territorial Defense Costs: Intruder Pressure in Bounded Habitats

Computer simulation models were used to explore the effect of habitat geometry on intruder pressure for territories in different locations within a patch of uniformly high quality habitat, for territories in patches of different shapes and sizes, and for patches surrounded by different types of suboptimal habitats. In models in which the edges of the habitat patch are impermeable (hard-edged), intruders do not leave the central territorial habitat, H, and intruder pressure is lower for territories on the edge of H than for more centrally located territories. Average intruder pressure for any given loop of territories ($IP¯i$, for loop = i) is positively related to both the proportion of territories on the edge of the patch (ESR) and the average distance moved by intruders. In models in which the edges of H are permeable (soft-edged), intruders are able to move between H and the surrounding habitats, which were of two types: sinks (no intruders generated there) and reserves (a source of intruders). The presence of sinks dramatically reduces both $IP¯i$ and average intruder pressure over H ($IPH¯$) as compared to hard-edged habitats, and both $IP¯i$ and $IPH¯$ are negatively related to the proportion of territories on the edge of the patch (ESR). Conversely, $IP¯i$ and $IPH¯$ are positively related to ESR when the surrounding habitat acts as an intruder-reserve. Data from empirical studies of territorial species agree with many of the direct and indirect qualitative predictions of these models. The effects of habitat geometry on defense costs may be important in many territorial species, and should be taken into account in future studies.