PS 8-98 - Food webs in the litter: Effects of food addition on decomposition and ant communities in coffee agroecosystems

Monday, August 8, 2011
Exhibit Hall 3, Austin Convention Center
Stacy M. Philpott, Environmental Studies, University of California, Santa Cruz, Santa Cruz, CA, Cody J. Murnen, University of Toledo, David J. Gonthier, School of Natural Resources and Environment, University of Michigan, Ann Arbor, MI and Gabriel H. Domínguez, Finca Irlanda Research Station

Community assembly of organisms is mediated by several factors including resource availability. For leaf litter ants, abundance and diversity of food and nest resources can affect richness, abundance, composition, and colony growth. Yet, the relative importance of different resources may change with habitat. Further, high abundance of food resources can accelerate decomposition of leaf-litter nesting resources. Thus resource availability affects brown food webs in complex ways. We examined impacts of adding food resources on leaf litter ant communities in forests and coffee agroecosystems differing in shade management. Specifically, we examined whether food addition affected colony abundance, richness, species composition, or colony growth and how responses to food addition differed by habitat. We also examined whether food addition affected litter decomposition or occupation of artificial nests by ants. We established 40 1x1 m plots in six sites in three habitats (high- and low-shade coffee and forest) and added 5g of dead mashed insects to half of plots (food+) every four days for two months. We harvested all ant colonies by visual inspection of leaf litter and nest cavities and counted all ant individuals per colony. We also estimated decomposition rates and occupation of artificial nests in food+ and control plots.


Food addition had limited effects on ant communities. There were more colonies in food+ (0.86±0.11) than control plots (0.67±0.10) but the difference was not significant. There were more colonies in forests (1.03±0.15) and in high-shade coffee plots (0.78 ± 0.12) than in low-shade coffee (0.49±0.10) but no difference in colonies between forests and high-shade coffee.  Species richness did not differ in food+ (29 species) or control (27) plots, or between habitats (forest: 28, high-shade coffee: 21, low-shade coffee: 19), however composition differed between forests and the two coffee habitats. Worker:brood ratios did not differ in food+ and control plots in any habitat indicating that colony growth is not strongly affected by food addition. However, there were significantly more brood per worker in high-shade coffee compared with low-shade coffee and forest. Ants only occupied 5 (out of 398) artificial nests; 4 in control and 1 in a food+ plot. Low occupation of artificial nests may be due to high natural nests availability. Decomposition was faster in high-shade coffee, but food addition did not affect decomposition in any habitat. Thus, food addition did not strongly affect leaf litter ants in this study, but habitat type impacted some characteristics of ant communities.

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