Predicting how species ranges will be affected by global change requires understanding the drivers of species’ spatial dynamics (i.e., abundances across space and time). Recent theoretical and empirical studies of spatial dynamics have focused on the role of abiotic factors (e.g., temperature, resource availability) in the expansion of individual populations into unoccupied habitats. However, virtually all species grow in, and disperse to, habitats occupied by other species. Since species interactions such as competition are known to affect population growth and dispersal, biotic factors may play a significant role in spatial dynamics.
We examined experimentally how competition between two species affected spatial dynamics, including the formation of range boundaries in expanding populations. We established replicate populations of a flour beetle species (Tribolium castaneum) and, over 8 generations, observed their spread across 16-patch, linear landscapes with a closely-related competitor (Tribolium confusum) either present or absent in part of the range. In addition to comparing rates of expansion between treatments, we fully described the spatial dynamics in our system by fitting a set of spatially explicit, stochastic community models to the experimental data.
Results/Conclusions
We found that competition significantly limited expansion across the experimental landscapes. While initial expansion rates were similar between treatments, once T. castaneum populations mixed with T. confusum, its expansion was largely halted compared to single-species controls. Thus, competition acted as a strong barrier to range expansion.
As in previous work, the growth component in the best-fit community model included demographic stochasticity, stochastic sex determination, demographic heterogeneity, and environmental stochasticity. In addition, the best-fit model included non-uniform dispersal, potentially because patch conditions, including presence of the competing species, affected spread rates. Simulating the best-fit stochastic model, we found that competition in our system can lead to quasi-stable range boundaries between the two species. Our results highlight the importance of considering biotic factors such as competition when describing and predicting the spatial dynamics and range expansion of species.