The influence of allochthonous input from streams on terrestrial guilds of arthropods

Background/Question/Methods

Arthropods with complex life-cycles can migrate to terrestrial environments in some instar, and may be important allochthonous resources to generalist predators that forage in water-land interface. The resource input increases the availability of prey on terrestrial ecosystems. Consequently, predators may respond to resource input through changes in their performance and density. In these regions, predators may decrease populations of insect herbivores on vegetation, with potential cascading effects on herbivory rates. We investigated with a large-scale experimental approach (ie, isolating stretches of streams) if the allochthonous resources from streams change the abundance of terrestrial arthropod predators, phytophagous and detritivorous that live on plants. We expect higher abundance of predators in areas with allochthonous input which, by its turn, decrease the number of herbivorous and detritivorous arthropods. We covered overall 180 m from a river in a Tropical forest in Southeast Brazil in four blocks. Each block consisted of 45 m covered and 45 m opened areas. In the terrestrial area adjacent to each block we established four plots with six experimental plants. We sampled arthropods associated with these plants every other month, during a total of seven samplings. The arthropods were identifying until family taxonomic group and split into guilds – predators, phytophagous and detritivorous. We performed the analyses among and within guild and plant species for each different sampling period. We considered as co-variable the number of leaves of each plant.

Results/Conclusions

We did not find significant differences in the abundance of guilds between experimental and control treatments in our preliminary results. It is probably the consequence of a low input from streams to terrestrial ecosystems during this period. However we just analyzed three sample datas, that were in the months of june, august and october of 2010, of a total of seven sample datas. The months analyzed are not in the main reproductive period of most of the species in the area, and the allochtonous input from streams can be very seasonal. We expect the effects of cross-systems subsidies to be strong at the wet and hot season in Tropical forests. Previous studies made on Temperate forests have shown that the hot season is when the flux of cross-system subsidies from the rivers is higher. The knowledge generated by this study will be important for understanding ecological processes at interfaces between rivers and terrestrial environments.