Linking movement ecology and biogeography: The fascinating case of afrotropical ducks

Background/Question/Methods

Biogeography has long acknowledged movement as an important influence on patterns of species occurrences. Incorporation of movement into biogeographic studies has often, however, been undertaken in a relatively simple way. We argue that unpacking the concept of dispersal into the fundamental elements of movement (internal drivers, external drivers, navigation capacity, and motion capacity) provides a better basis for contrasting competing hypotheses in biogeographic studies. We illustrate the value of taking a deeper perspective on movement with a case study that uses extensive satellite telemetry data from 26 Afrotropical ducks of two species, Red-billed Teal Anas erythrorhyncha and Egyptian Goose Alopochen aegyptiaca, at three different southern African study sites spread across a 17-degree latitudinal gradient. Movement paths from individual birds were analysed using Matlab to obtain estimates of a range of relevant parameters for each bird (e.g., mean and total distances moved, speed of movement, degree of resighting point aggregation, and turning angles). These data were then compared by species and site.

Results/Conclusions

Unexpectedly, Red-billed Teal, a ‘highly nomadic’ species, moved with a relatively high degree of consistency at each of our three different study latitudes, while Egyptian Geese, a ‘resident’ species, showed latitude-dependent variation in their movements. However, much of the latitude-related variation for Egyptian Geese was driven by their annual moult migrations, rather than directly by fluctuations in resource availability. Internal factors appear to dominate movements and probably determine the species ranges of both of our study species. Biogeographic patterns in this taxon may thus be best understood (and modelled) as a long-term response to environmental stochasticity, rather than as a deliberate selection of optimal habitat. More generally, our analysis suggests several ways in which movement ecology and biogeography could be more tightly linked, to the benefit of both areas of research.