Controls of food chain length in an urban desert environment

Author's: John L. Sabo, Kevin E. McCluney
Summary: Understanding the determinants of the length of food chains is of fundamental importance to ecologists. Food chain length influences the potential complexity of the community, patterns of biomass in each trophic level, and possible biomagnification of harmful substances (e.g. mercury or DDT, Post 2002). There are many factors that may affect food chain length and debate over the importance of each has recently intensified (Post 2002). Additionally, very little is known about the functioning of urban ecological communities. Here we examine the influence of resource availability (water), disturbance (wind), and ecosystem size (number of plants) on the food chain length of arthropod communities inhabiting brittlebush (Encelia farinosa) at the Desert Botanical Gardens, a remnant desert site located in the urban Phoenix, AZ area. To do this we set up a field experiment consisting of 108 sets of replicate brittlebush locations, subject to three levels of watering (resource availability; one, three, or seven times per week), leaf blower wind (disturbance; none, once, or twice per month), and number of plants (ecosystem size; 1, 2, or 3 plants in sets). Our preliminary results suggest that water increases and wind decreases food chain length in brittlebush arthropod communities, and that there is a potential interaction between these two variables. Our results suggest no support for an effect of ecosystem size (number of plants in replicate sets) on FCL, though total plant volume may exert a heretofore unmeasured effect. Overall, our results suggest strong climatic control on the length of food chains on this naturally occurring plant in desert cities.