Effects of pH and resources on a processing chain interaction in simulated treeholes

1.  Both resources and abiotic factors may affect biotic interactions. One interaction that occurs in treehole habitats involves leaf shredders that facilitate growth of detritivores, and it may be affected by both leaf litter quantity and changes in water quality.
2.  Water chemistry in central Pennsylvania treeholes has been impacted by acid deposition, and the most common insects therein have differential survival under low pH conditions. Experimental microcosms that mimic treehole habitats were used to test the hypothesis that this abiotic factor, pH, also affects facilitative interactions. Leaf litter resources and pH were varied independently of presence of leaf-shredding scirtid beetles ( Helodes pulchella and Prionocyphon discoideus ), and the mosquito Aedes triseriatus , to examine interactions among pH, resources and insects.
3.  pH affected the interaction between the insects, such that effects of scirtids were more evident at pH 4·5 than at 6·5. Female mosquitoes were larger in the presence of scirtids, low resource and low pH conditions than in absence of scirtids, low resource and low pH conditions.
4.  There were also effects of A. triseriatus on scirtids. The size of individual scirtids was smaller in the presence of A. triseriatus , but total scirtid biomass was unaffected as survival was also higher in the presence of A. triseriatus .
5.  The effects observed on a resource-mediated biotic interaction led to the conclusion that this interaction is pH dependent, and gives support to the concept that abiotic factors play a role in determining the outcome of biotic interactions, and that acidification can have complex effects on communities.