PARENT SESSION
Contributed Oral Session 10: Aquatic Trophic Systems I
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 519 B, Level 5, Palais des congrès de Montréal

Living on ecosystem boundaries: The impacts of floods on populations of a New Zealand fishing spider, Dolomedes aquaticus.

Greenwood, Michelle ^*,1, McIntosh, Angus¹, ¹ University of Canterbury, Christchurch, New Zealand

ABSTRACT- The cursorial fishing spider Dolomedes aquaticus lives and hunts among riparian gravel banks alongside New Zealand rivers, which are often physically disturbed by unpredictable floods, especially in alpine areas. Flood disturbances are likely to influence populations of these spiders both directly through mortality caused by moving substrata and indirectly through alterations to habitat and food availability. Populations of D. aquaticus were surveyed at 13 streams forming a disturbance gradient in a subalpine region of the South Island. Biomass of Dolomedes in a 100m reach peaked in rivers with intermediate bed stability and declined in the more disturbed and stable streams surveyed. The biomass of potential prey (aquatic and terrestrial invertebrates) was higher in stable rivers than the more disturbed ones. Therefore, the decline in spider biomass in more stable rivers was hypothesised to be caused by habitat limitation. This hypothesis was supported by measurements of usable habitat (i.e. unembedded, large rocks), which declined as bed stability increased. A field manipulation of habitat availability performed in four stable streams also provided evidence of habitat limitation in these rivers. Species living on ecosystem boundaries can gain advantage from spatial subsidies of resources. However, disturbance in an adjacent ecosystem can limit populations through mortality and by altering habitat conditions at the boundary.

Key words: disturbance, Dolomedes aquaticus, ecosystem boundaries