Seasonal dynamics in the assembly of arthropod community structure and trophic structure

Background/Question/Methods

Temporal dynamics over a growing season are an integral part of arthropod community assembly patterns. We examined the hypothesis that arthropod trophic constraints follow predictable patterns over a growing season in an extremely hot, managed riparian stand of a foundation tree species, Populus fremontii. We believe that these trophic constraints will often help predict community assembly across seasons. While these patterns have been established in more moderate environments with shorter growing seasons, it is unknown how arthropods respond to a long growing season with extreme temperatures (>49 C) occurring in summer months. We found an ideal sampling system in the Fremont cottonwood (P. fremontii) trees planted for riparian habitat restoration on the Cibola National Wildlife Refuge, AZ. Our study measured the assembly of canopy arthropod communities across a growing season by monthly sampling of a consistent set of cotton wood trees with nondestructive visual techniques for 12 months. We used structural equation modeling to compare the effects of several factors thought to be important to arthropod community assembly and trophic levels including tree genetics, temperature, canopy cover, and wind speed.

Results/Conclusions

We found two main patterns. 1. Arthropod trophic structure followed similar patterns as those found in milder environments with shorter growing seasons, and these patterns extended proportionately over the 10 month growing season. Herbivores were most abundant in the early months of the year, predators became more numerous in the hot summer months and then a new group of herbivores dominated the community in the final few months. 2. Our structural equation models indicate that the importance of factors affecting community assembly vary among trophic levels. We conclude that many arthropod communities may follow similar seasonal assembly rules, regardless of environmental or seasonal variation. These results argue that, while we need more information to predict arthropod community assemblage and trophic structure, sampling time may be one of the most important factors driving the patterns we find. Therefore, initial surveys to understand these dynamics are important before making broad statements about community patterns.