Seasonal variation of reciprocal subsidies between coupled ecosystems in a Mediterranean climate

Background/Question/Methods   Nakano and Murakami (2001) showed reciprocal subsidies between terrestrial and river ecosystems to be strong and seasonally complementary under a northern continental climate in the Horonai Stream (42°43'N, 141°36'E) in Hokkaido, Japan. We studied seasonal patterns in the insect fluxes from river, wetland, and forest habitats under a Mediterranean climate in and near the South Fork Eel River of in Northern California (39°44'N, 123°37'W). We quantified insect emergence and fluxes between six wetlands and six river reaches and adjacent, forest. Insect horizontal fluxes were sampled using sticky traps along 150 m transects from the moister to the dryer habitats. We sampled twice a month from April to July and once a month from August to March. Ca. 1600 traps and 30,000 insects were identified (to suborder or lower taxonomic levels for the most abundant groups).

Results/Conclusions   Production peaks in forest, wetland, and river ecosystems were partially offset seasonally. Wetland insect fluxes were highest in mid-July, with secondary peaks in mid-April and September, when river fluxes were lowest. Forest production peaked in early-June. The maximum river production occurred in early July as observed during previous years. In the wet season, insects were uniformly distributed among forest, river, and wetlands. Insect fluxes from forest to river and forest to river (averaged over 7 months) were similar (11 insects m-2d-1), but insect fluxes from wetland to forest were on average lower (8.5 insects m-2d-1) than from forest to wetland (13 insects m-2d-1). From late April through May, when the whole landscape was moist, there was no spatial variation in insect abundance-activity along forest, wetland, or river transects, and abundances averaged 315 insects m-2d-1. As the uplands dried out, from June to September, insect abundance remained high in wetlands and near the river, but dropped in the forest to average 32 insects m-2d-1. In contrast to the continental Hokkaido ecosystem, the peaks of river versus terrestrial insect fluxes were less offset in our Mediterranean system, and the seasonality of terrestrial versus river peaks was reversed. The wetlands, with three abundance peaks distributed through spring, summer, and fall, maintained insect fluxes when river and forest fluxes were low. Wetlands represent a miniscule proportion of this steep, seasonally dry landscape, but may supply critical food resources to insectivores during seasonal bottlenecks. We are investigating how insectivorous birds and bats track and respond to these seasonal landscape shifts in food supply sources.