Research Accomplishment Reports 2007

Impacts of Interactions among Generalist Arthropod Predators in Two Complex Food Webs: Vegetable-Crop Gardens and Forest-Floor Leaf Litter

D.H. Wise
Department of Entomology

 

Project Description

Ants and spiders are abundant terrestrial intraguild predators, yet the strength and consequences of interactions between them are largely unknown. In the leaf-litter food web of a deciduous forest in Kentucky, we tested direct and indirect effects of ants on spiders and shared prey (Collembola) by experimentally subsidizing ants in open plots in two field experiments. Ant activity was increased, and the density of ants in the litter was doubled, by placing carbohydrate and protein baits in the center of each plot. Gnaphosa spiders were almost twice as abundant and Schizocosa spiders were half as abundant in baited plots relative to controls. There were more tomocerid Collembola in baited plots, suggesting possible indirect effects on Collembola caused by ant-spider interactions.

The second experiment showed large ants, primarily Camponotus, could alone induce similar effects on spiders. Gnaphosa biomass density was almost twice as high in the plots where large ants were more active, whereas Schizocosa biomass density was reduced by half in these plots. Although tomocerid densities did not differ between treatments, tomocerid numbers were negatively correlated with the activity of Formica, another large ant species. Path analysis failed to support the hypothesis that the ant Camponotus indirectly affected tomocerid Collembola through effects on densities of spiders. These results illustrate the complexity of interactions between and within two major IG predator groups with disparate predatory behaviors, that will have consequences for the forest-floor food web.

To examine impacts of predicted changes in precipitation on the rate of decay of canopy leaves, we placed litterbags in replicated, fenced low-rainfall and high-rainfall plots. Unfenced plots served as an ambient treatment. Litter in high-rainfall and ambient plots decayed 50% and 78% faster, respectively, than litter in low-rainfall plots. Litter in ambient plots disappeared 20% faster than in high-rainfall treatment, perhaps via greater leaching during heavy rainfall. Ambient rainfall during the experiment was similar in total amount, but more variable in intensity and timing, to the high-rainfall treatment. Changes in rainfall predicted with global climate change will likely strongly alter litter decay rates in deciduous forests.

We studied the small-scale spatial patterns of spider webs in alfalfa and the influence of small-scale variation in prey availability by comparing web distributions to the pattern of sticky-trap captures of Aphididae and Diptera. Cutting of alfalfa reduced overall density of web-building spiders but had no immediate impact on web spatial distribution. Aphid availability was highest before the alfalfa was cut and was clumped at a scale of 66 cm. Spider webs were not clumped at any scale or date. Webs were regularly distributed at smaller distances (<20 cm) immediately before and after cutting. Because cursorial and web-building spiders were most active during this period, we hypothesize the development of small-scale regularity in web locations was driven by intraguild interactions.

Impact

Linyphiid spiders are potentially important in the control of alfalfa pests in Kentucky. Basic knowledge of the densities and dispersion patterns of these spiders, and insights into how rapidly pre-harvest conditions are re-established after cutting, are critical to the effective incorporation of linyphiid spiders into the biological control component of alfalfa IPM. Because much of Kentucky is forested, the health of Kentucky forests is important to the economy and quality of life in Kentucky. An important process in forests, one that affects forest productivity, is the release to the soil of nutrients from the decomposition of leaf litter and other detritus. Arthropods in the litter community play an important role in this process.

Understanding how altered rainfall predicted by models of global climate change may affect interactions among arthropods inhabiting forest floor leaf litter, with ultimate effects on litter decomposition, is critical to predicting how human-caused environmental change may impact the health and productivity of a major Kentucky resource: its deciduous forests.

Publications

Moya-Larano J. and Wise D. H. 2007. Two simple strategies of analysis to increase the power of experiments with multiple response variables. Basic and Applied Ecology 8: 398-410.

Birkhofer K, Scheu S, Wise D.H. 2007. Small-scale spatial pattern of web-building spiders (Araneae) in Alfalfa: Relationship to disturbance from cutting, prey availability, and intraguild interactions. Environmental Entomology 36:801-810

Moya-Larano J. and Wise D.H. 2007. Direct and indirect effects of ants on a forest-floor food web. Ecology 88: 1454-1465.

Lensing J.R. and Wise D.H. 2007. Impact of changes in rainfall amounts predicted by climate-change models on decomposition in a deciduous forest. Applied Soil Ecology 35: 523-534.