Dry washes direct movement of pollinators of the desert shrub, Acacia greggii A Gray

Background/Question/Methods

In the face of changing landscape and climate, determining the degree to which environment shapes the movement patterns of species is essential. Desert ecosystems are increasingly affected by human-driven environmental pressures. In the American southwest, there has been rising aridity and development over the last 20 years. Plant species face a greater risk to changing environments due to their limited ability to move and frequent dependence on pollinators to maintain genetic connectivity. Here we study pollen flow patterns in a representative Mojave shrub species, Acacia greggii A Gray. We have shown in previous work that genetic variability in A. greggii adults is best explained by the dry wash within which a tree occurs and the elevation at which it is found. We attributed this pattern to a combination of rainfall events moving seeds downstream and pollen flow restricted by elevation due to phenological separation.  We test this hypothesis of separate pollen and seed movement patterns by genotyping 240 seeds from 20 maternal trees in parallel dry washes to determine if the current pattern of pollen flow shows a significant directional influence on movement instead of having a uniform (isotropic) pattern.

Results/Conclusions

Our results show non-uniform (anisotropic) movement, however, the dominant direction is not what was expected. We see that pollen is moving preferentially along the path of the dry washes, and not across an elevation gradient as expected, given strong phenological separation. Average dispersal distance along washes is 100m, compared to 10m across washes. These results demonstrate that resource corridors may promote pollen-flow by directing pollinator movement. This highly anisotropic pollen movement would be under-estimated in traditional isotropic models.  Our results demonstrate the impact of desert washes as gene flow corridors, which may mitigate the otherwise depauperate desert ecosystems. Thus, even during climate warming, desert washes may promote genetic connectivity as long as sufficient vegetation remains within them to support pollinator communities.