The role of microhabitat and food availability in determining riparian invertebrate distributions on gravel bars: a habitat manipulation experiment

Exposed riverine sediments (ERS) are an important ecotone, where aquatic and terrestrial habitats and species interact. Within an area of ERS, invertebrate species richness tends to be highest along the water's edge ecotone. In this article, we discuss a habitat manipulation experiment which was conducted to test the hypotheses that ERS invertebrate distribution is linked to: (1) the availability of food items (emerging and stranded aquatic invertebrates) and (2) favourable physical microhabitat and microclimate (temperature and moisture). Four experimental plots were created (wet, wet-fed, dry-fed and dry control plot), replicated three times. The plots were wetted using a capillary pump system, and fed with dried blood worms (Chironomids) for 28 days. Sediment temperature was measured at 15-min intervals. Hand searches were undertaken on 25% of each plot after 7, 14, 21 and 30 days. Significant temperature differences were observed between the wet and dry sediment and the air temperature. Wet plots were 1.9 degrees C cooler on average than the dry plots and had a smaller temperature range; all plots remained significantly warmer than the air temperature. The wet and wet-fed plots yielded significantly greater numbers of beetles than the dry and dry-fed plots; however, no significant difference was found between the wet and wet-fed plots. Spiders were significantly associated with dry plots and showed no response to food variability. These results suggest that micro-environment and not food availability is the main driving factor underlying beetle distribution and movement, although it is possible that they are using lower temperatures and increased moisture as a cue for aquatic food availability. Copyright (C) 2011 John Wiley & Sons, Ltd.