دانلود رایگان مقاله تاثیرات آبشاری عنکبوت در شبکه غذایی کف جنگل

Abstract

Spiders have cascading effects on forest-floor food webs and ecosystem processes by depressing soil fauna densities and altering species composition. However, it remains unclear how spiders with different foraging strategies influence cascading effects on decomposition. In addition, prolonged droughts would likely have important consequences for trophic interactions in detritus-based food webs. In the present study, we evaluated how interactions between spider predation and drought affect litter decomposition in a tropical forest floor. We manipulated densities of dominant spiders with actively hunting or sit-and-wait foraging strategies in microcosms which mimicked the tropical-forest floor. We found a positive trophic cascade on litter decomposition rates triggered by actively hunting spiders under ambient and reduced moisture. However, sit-and-wait spiders showed no cascading effects on litter decomposition under ambient and drought conditions. Our findings suggest that trophic interactions in detritus-based food webs should be considered to better understand litter decomposition in the face of environmental change.

Discussion

In this study, we found a positive trophic cascade on litter decomposition rates triggered by actively hunting spiders under ambient and drought moisture, which could be explained by theirindirect beneficial effects onOribatid abundance.However,sit-and-waitspiders had no cascading effects on litter decomposition under ambient or drought conditions. One possible explanation for indirect beneficial effects of spiders on oribatids is that mobility of Acari is very low, between 1 and 5 cm/day (Berthet 1964), thus they are unlikely to encountersit-and-waitspiders.Chemical defenses of Acari may also reduce attacks by actively hunting spiders (Nentwig 1987). This is consistent with previous findings that the presence of salamanders (Ensatina eschscholtzii) decreased densities of Entomobryidae springtails and adult Fig. 3. Spider-induced trophic cascade effects on leaf litter decomposition rate under ambient and drought conditions expressed as trophic cascade index. A trophic cascade index significantly >0 indicates positive effects of treatments on litter decomposition, an index value significantly <0 indicates negative effects of treatments on litter decomposition. Asterisks (*) indicate significance differences between index and dotted line (zero) (P < 0.05). Data are expressed as mean ± s.e.m. (n = 10). The P-values indicate significance of differences in trophic cascade index between ambient and drought conditions. beetles, while increasing the density of oribatid mites (Best & Welsh 2014). The removal of these two soil invertebrates by salamanders probably opened up resourcesfor the smaller and more numerous mites, allowing their increase (competitive release)(Best&Welsh 2014). Oribatidmites can increase litter mass loss by stimulating microbial respiration rates, altering microbial extracellular enzymes, and other transformations of litter chemistry (Wickings & Grandy 2011). In this study, the AH spider treatment had higher Oribatid density compared with the control treatment under ambient and drought conditions. Therefore, actively hunting spiders had a positive trophic cascade on litter decomposition rates. Lawrence and Wise (2004) also found that wandering spiders enhanced decomposition.