Displacement of Native Riparian Shrubs by Woody Exotics: Effects on Arthropod and Pollinator Community Composition

摘要

Throughout the southwestern U.S., riparian gallery forests of cottonwood and willow are being invaded by woody exotics, primarily Russian olive and salt cedar. We wondered what effect this might have on native pollinator populations. Pollinators are indispensable contributors to biodiversity, ecosystem health, and human food production. Recent declines in pollinator abundance and health, such as catastrophic declines in honey bee populations due to Colony Collapse Disorder, has renewed interest in native pollinators and the ecosystem services they provide. Insects were collected from willow, Russian olive and salt cedar throughout April and May of 1997 and 1998 using sweep nets. For each collection day, nets were swept over the target shrubs for a specified number for passes to ensure equal collection effort. Insects were counted and identified to family. Total numbers were adjusted by number of sweep-days. Total insect abundance was greatest for willows (33.5 insects per sweep-day), followed by Russian olive (18.0) and salt cedar (6.8). Willows also had the greatest number of insect orders and families represented. Of the four primary insect pollinator orders, willow had the greatest numbers of dipterans, hymenopterans, and lepidopterans collected per sweep-day. Russian olive had the greatest number of coleopterans. When ants and chalcids were excluded from the hymenopterans, willows still had the greatest numbers and proportions of hymenopterans caught. It appears that the willow habitat is important to pollinating insects, especially bees. In contrast, saltcedar consistently had the lowest numbers and proportions of all four of the pollinator orders.