The habitat heterogeneity hypothesis, which states that more structurally complex habitats contain more diverse niche spaces and so support greater biodiversity, is one of the fundamental concepts of ecological theory. With genes controlling some physical and structural aspects of arthropod habitat, increased genetic diversity should lead to increased habitat complexity and thus to increased biodiversity. This study tests the further hypothesis that the genetic diversity of natural stands of Fremont cottonwood trees (Populus fremontii) will explain more of the variance in arthropod diversity than structural factors. This is likely due to varying amounts of genetic diversity supporting diverse niche spaces and thus affecting arthropod diversity differently in different stands. We nondestructively sampled, over a large geographic area, the canopy arthropod communities of trees in wild stands of Fremont cottonwoods with naturally varying levels of genetic diversity. We used amplified fragment length polymorphisms (AFLPs) as a measure of genetic diversity in each stand. We also collected data on arthropod habitat and food resources. We included all data in a structural equation model to assess the importance of each factor on stand-level arthropod diversity.
Results/Conclusions

We found that stand-level genetic diversity explained the most variation (35%) in arthropod biodiversity. Geographic location, a composite variable including latitude, longitude, elevation, and Palmer drought index information, explained 10%. Habitat resources, which include tree height, DBH, canopy cover, and temperature, explained 6%. Food resources, which include carbon to nitrogen levels, photosynthesis rate, and phosphorus levels, explained 2% of the arthropod biodiversity. Overall, our model explained 53% of arthropod diversity in wild stands of Fremont cottonwood trees across much of their range in the West. This research is important to addressing what factors affect how communities are formed, a key frontier in ecology. In addition, it expands the habitat heterogeneity hypothesis to the genetic level and addresses the effects of genetic diversity on arthropod communities through non-structural means. As we explained 53% of arthropod diversity in our system, there are clearly more factors affecting arthropods to be addressed in future work. However, the knowledge gained from this study that can be incorporated into restoring habitats with varying levels of genetic diversity may be critical to managers who strive to support biodiversity by conserving and replanting native habitat, especially in threatened ecosystems, like those of the riparian Southwest.