Insect fluxes from forest toward open habitats in a Mediterranean landscape

Background/Question/Methods We studied seasonal patterns in the insect fluxes from aquatic (river and wetland), meadow, and forest habitats under a Mediterranean climate in the upper basin of the South Fork Eel River of in Northern California. We quantified insect fluxes between six small wetlands, three meadows and six river reaches and adjacent forest. Insect horizontal 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. Ca. 3,000 traps and 60,000 insects were identified (to suborder or lower taxonomic levels for the most abundant groups).

Results/Conclusions Comparisons of insect densities on incoming vs outgoing sides of sticky traps revealed no preferential movement of terrestrial or aquatic insects towards or away from of aquatic ecosystems (river and wetlands). We did find a consistent net flux of insects moving from forest to meadows, 102±12 (mean±SE) mg of insect biomass m^-2d^-1 during the wet season (March to June), and 73±13 mg insect m^-2d^-1 during the dry season (July to October). Insect accumulated in the vicinity of forest edges, ~300 mg insect m^-2d^-1 versus ~200 mg insect m^-2d^-1 within the forest or in the meadow. Insect activity-abundance was highest in the dry season (77±10 mg insect m^-2d^-1 in forest > 150 m from the river and 118±9 mg insect m^-2d^-1 in forest adjacent to the river). Light gradients or local wind turbulence might induce behavioral or forced movement of insects from more vegetated to more open parts of the landscape.  Insect fluxes in turn could affect habitat use and foraging patterns of insectivores, and might increase nutrient loading in the open habitats soils.