Distribution of North American seed dispersers: A mismatch of richness between vertebrates and the plants they disperse
Plants and their mutualist seed dispersers are products of diffuse coevolutionary processes. Under some circumstances, like a changing climate during glacial cycles, the current geographic distributions of plants and their dispersers are not only a result of coevolution, but also determined by species-specific responses to climate. In this study we sought to determine the relationship between diversity of vertebrate seed dispersers and the plants they disperse across temperate North America. With strong mutualisms between plant and disperser, a relationship between their diversities would be expected. We predicted there would be strong positive correlations with animal-dispersed plant richness and richness of dispersers.
We limited our study to vertebrate seed dispersers and the vascular plants they disperse through frugivory and scatter-hoarding. We compiled distribution data from BirdLife international and NatureServe for avian species, the IUCN Red List for mammals and plant data from the North American Seed Dispersal project. Individual distributions were rasterized, overlaid, and added to enumerate the richness across North America. Generalized linear models were then used to determine the correlation between frugivorous vertebrate and scatter-hoarding animal richness to vertebrate dispersed plants. Climate and geographic variables were also used to look for any correlations with disperser richness.
To our surprise, vertebrate disperser richness is not correlated with vertebrate dispersed plant richness. However, there are obvious patterns of dispersal richness across North America. Total frugivore and frugivorous bird richness has a strong negative correlation with latitude and is concentrated in the southern United States. Particularly, the highest richness is found in the American southwest. Frugivorous mammal richness is patchier, with hot spots again in the American southwest, but additional richness hotspots in the Pacific Northwest, northern forests in Alberta, and eastern Lake Superior. Scatter-hoarding animal richness is also negatively correlated with latitude, but more concentrated than frugivore richness, with the majority of the richness being found in the American southwest into the Colorado Plateau and southern Rockies. Both scatter-hoarding bird and mammal richness follow a similar distribution as described above, however scatter-hoarding birds are largely absent from the North American deserts. Despite there being no correlation between disperser and plant richness, other variables such as temperature, precipitation, and elevation are strongly correlated with disperser richness. This suggest coevolution from the formed mutualism between plants and vertebrates in North America may be weak compared to environmental factors which better describe the diversity of each group.