Boundary effects between aquatic and terrestrial ecosystems in a Mediterranean landscape

Background/Question/Methods We studied seasonal patterns of the boundary effect between aquatic (river, wetlands, and wet meadows) and forest habitats under a Mediterranean climate in the upper basin of the South Fork Eel River of in Northern California. We quantified insect activity-abundance and fluxes between three small (<100 m²) wetlands, three meadows and six river reaches and adjacent forest. Insect horizontal abundance and fluxes were sampled using 2-sided sticky traps set 0.5 m above the ground at 6 intervals along 150 m transects from wetland or river shorelines to adjacent meadow or forest habitat upslope. We sampled twice a month from April to July and once a month from August to March for three consecutive years. Ca. 3,000 traps and 35,000 insects were identified (to infraorder, superfamily or family levels according to their abundance).

Results/Conclusions Boundaries had major effects on insect dispersal in river and in wet meadow locations, and almost no effect on wetland locations. We did not find net fluxes of insects either from the river to the forest or vice-versa. However, the boundary between river and forest showed a permeability gradient related to the body-size of the insects. Small (<1mg) insects (gnats, midges, and hump-backed flies) showed lower ability to cross the aquatic terrestrial boundary than larger ones (wasps, beetles, and muscoid flies). But this effect switched over the seasons, concentrating small insects close the river across the boundary. In addition to the effects related to the body size, we found that in wet meadows  most of terrestrial insects have extended residence times in or around the boundary, and did not reach far (<30 m) into the meadow. In general our results show that river has few interactions with surrounding forest, and this is mediated by a body-size limiting permeability. The results also show the important fluxes between wet meadows and forest may be neutralized by strong boundary effects regardless the body-size.