Site factors affecting reproduction among subpopulations of Acacia greggii A Gray in a desert ecosystem

Background/Question/Methods

Connectivity among subpopulations of plants is a key determinant to local viability and the maintenance of genetic variation within a region. Desert ecosystems are comprised of patchy landscapes where the attraction of pollinators to a given site can be critical to plant species. For these insect-pollinated species connectivity and ultimately reproductive success are shaped by the landscape matrix promoting movement among sites and site-specific characteristics affecting both attraction to pollinators and the conditions for reproduction. Key factors include pollinator availability, flower abundance per site, tree density, and elevation (a surrogate for temperature and radiation). This study examines the effect of these factors on the reproduction of Acacia greggii A Gray, a patchy gregarious insect-pollinated species. Our experimental design included 21 sites in The Mojave National Preserve monitored in the summer of 2010. At five adults per site we measured the number of seeds per inflorescence on branches that allowed pollinators (three per plant) and those that excluded pollinators (three per plant), excluding herbivores in both cases.

Results/Conclusions

The ANOVA demonstrated a significant effect of site and treatment on the number of seeds per inflorescence indicating a pollinator and possible outcrossing dependence for reproduction. A regression analysis indicated that, in branches allowing pollinator access, cumulative floral abundance per site was the most important factor influencing reproductive success of individuals, even when controlling for individual floral display. Further genetic studies will test the extent to which outcrossing and effective number of pollen sources account for this result. This study illustrates the importance of examining site-specific factors when considering connectivity within patchy ecosystems. Understanding reproductive limitations in this representative species of the American Southwest is essential as large scale development on previously preserved public land and increasing aridity will likely lead to greater isolation of this and other species.