Does a species' dispersal ability affect how much it fills its potential range?
The degree to which a species’ dispersal ability influences its geographic range is not fully understood. Some large fruited trees (e.g. Osage Orange, Honey Locust) are rarely dispersed today, and are proposed to be relics once dispersed by Pleistocene megafauna. Given the paucity of dispersers available to these trees since deglaciation, it has proposed that their modern ranges are reduced relative to their potential ranges with available dispersers. Using bioclimatic envelope models, we calculated potential ranges for 180 Eastern North American trees based on their natural distributions mapped by E.L. Little. Following convention that temperate trees are limited by cold stress, drought tolerance, and growing season competitiveness, three mechanistic climate factors were used (winter minimum temp, water balance, growing degree days) to build potential ranges. A species’ realized range area was then divided by the area of their potential range. We hypothesized that species adapted to megafauna dispersal would realized a lower proportion of their potential range relative to trees with extant dispersal mechanisms.
The proportion a species fills of its potential range is more strongly correlated with the size of its realized geographic range than with dispersal mechanism. Species with small ranges fill little of their potential range (e.g. Osage Orange, at 27% filled), while widely distributed species realize a larger percentage (e.g. Red maple, 84% filled). Surprisingly, dispersal mechanism appears to have little influence on range filling, as species with megafauna dispersal adaptations fill their ranges similarly to other species; including those with light, wind dispersed seeds. Most trees do not fill the majority of their potential range (average 49% filled), which suggests tree species in North America are not in equilibrium with climate. This may be due to postglacial dispersal lags, as has been argued for European taxa, however, given that range filling is highly correlated with range size, species’ natural history traits may be an important factor in determining range filling. Understanding what factors determine restricted species’ ranges has important implications for conservation of biodiversity and rare endemics.