The influence of landscape heterogeneity and local habitat effects on the response to competitive pressures in metapopulations

Ecological modellers move along the continuum between mean-field models and spatially explicit models in order to formally express and test hypotheses regarding the effect of landscape heterogeneity on ecosystem patterns and behaviour, but care must be taken when doing so. With careful and explicit cross-checking between models as their structure evolves and the spatial complexity increases, our understanding of the model assumptions can similarly evolve and reveal otherwise unnoticed dynamics in the modelled system. The patch-occupancy metapopulation model was extended to include a spatially explicit heterogeneous resource substrate, following an incremental series of increasingly complex models using cellular automaton and coupled map lattice techniques. The extension process was monitored to track the meaning of model parameters and the resultant interpretation of relevant ecological processes.When colonization was modelled using local population frequencies rather than global averages, the aggregate results were similar to mean-field approximations, though local variability occurred around the equilibrium frequency. Relative differences in dispersal range, as expressed by local neighbourhood size, became more important in reducing competitive pressure on the fugitive species. Decoupling the system-wide disturbance parameter into species-specific parameters created a dramatic shift in population dynamics and revealed an important model assumption. Although the parameter is apparently specified at the landscape scale, investigation of the decoupled dynamics revealed that it is in fact a patch scale parameter, integrated over space and time in the coupled form. In reconciling results across the model series, it appeared that under competitive pressure a fugitive species’ resilience to disturbance or resource limitations may be more important than its ease of dispersal. The model was further refined to include an explicit, heterogeneous resource substrate, a resource-limiting effect on dispersal propensity and resilience, and a temporally variable climate signal. The fugitive species experienced reduced competitive pressure in regions with reduced resources (i.e. marginal habitat), especially when its dispersal range is long relative to the superior competitor. From a modelling perspective, this suggests that with sufficient spatial detail and model complexity, it may be possible to harmonize the extinction–colonization and source–sink perspectives on coexistence and competition into a unified model.