Do forest patches influence leaf breakdown and macroinvertebrate communities in agricultural landscapes?

Background/Question/Methods

It is widely recognized that stream ecosystems are intimately associated with riparian vegetation.  However, the removal of forests in agricultural landscapes has decoupled riparian forests and streams which has influenced various aspects of stream ecosystems including nutrient concentrations, substrate composition, animal communities (e.g., fish and macroinvertebrates), and the breakdown of organic matter (e.g., leaves).  Although these effects are pronounced in agricultural landscapes, forest patches are often scattered throughout these landscapes and may maintain linkages between streams and forests that are otherwise absent.  However, little is known about how forest patch size influences these linkages or how quickly these linkages recover in agricultural landscapes.  To investigate the functional linkages between forest patches and streams in agricultural landscapes, we studied leaf litter breakdown of tuliptree leaves (Liriodendron tulipifera L.) and macroinvertebrate communities colonizing these leaves in streams transitioning from agricultural to forested riparian areas.  Our primary hypothesis in this study was that there are systematic changes in leaf breakdown and macroinvertebrate community composition with increasing distance into forest patches.  

Results/Conclusions

Contrary to our hypothesis, we found no systematic trends in leaf breakdown in streams transitioning from agriculture to forest patches.  Further, we found no overall difference in breakdown rates along this transition.  In contrast to patterns in leaf breakdown, macroinvertebrate communities exhibited strong trends with increasing distance into forest patches.  Linear regressions revealed that taxa that are tolerant of human disturbance such as Chironomids decreased with distance into a forest patch, while more sensitive taxa such as Plecopterans increased with distance into patches.  There were also clear shifts in macroinvertebrate diversity, richness, and community composition with increasing distance into forest patches.  We found evidence for increasing diversity (Simpson and Shannon-Wiener), but decreasing taxa richness with increasing distance into a forest patch.  At both sites, NMDS ordination plots indicate that there are systematic shifts in species composition with increasing distance into forest patches.  Macroinvertebrate results were complemented by shifts in physicochemical parameters (e.g., nutrients and substrate) that may act as drivers of the observed trends in macroinvertebrate communities. Overall, our results indicate that while these forest patches may not appreciably influence ecosystem processes such as leaf breakdown, they can strongly influence macroinvertebrate communities and potentially provide refugia for sensitive species.