Aquatic-terrestrial linkages

The population dynamics of a riparian fishing spider: interactive effects of flow-related disturbance on cross-ecosystem subsidies and spider habitat

The transfer of prey between ecosystems can have dramatic consequences for both systems. Landscape-scale factors influence the importance, direction and magnitude of energy flows, but may also alter the ability of consumer organisms to respond. As part of her PhD research, Michelle Greenwood (see Greenwood 2007, pdf, 7.0mb) used flood- and drying- disturbance gradients to investigate interactions between these two processes on populations of a riparian fishing spider, Dolomedes aquaticus (left).

The abundance of winged aquatic insects (i.e., Dolomedes aquaticus prey) was markedly higher at less flood-prone rivers and declined with increasing flood disturbance. Larger spider populations were expected at stable rivers due to high aquatic prey abundance. However, an experimental manipulation of Dolomedes habitat revealed that the lack of floods at these sites led to habitat-limited populations. A peak shaped relationship of spider biomass and abundance was found, with the largest spider populations at intermediately disturbed rivers (Greenwood & McIntosh 2008, pdf, 300kb). In addition, patchy habitat most likely caused the small-scale (4 m2) spider aggregation at the most stable and disturbed rivers. These patterns were also associated with strong interactions between the spiders. Stable isotope analysis of field collected spiders and an experimental manipulation of spider densities and food availability indicated that cannibalism rates were likely to be significantly higher at stable and disturbed rivers than those intermediate on the disturbance gradient.

Cannibalism mesocosm experiment, set up at the University of Canterbury field station at Cass.

To separate the effects of habitat availability and aquatic prey abundance Michelle used drying rivers, as the amount of aquatic insect prey alters as the water recedes. Desiccation mortality and low aquatic prey biomass most likely caused the spiders’ spatial distribution and size class structure to alter in drying river reaches.
Cross-ecosystem transfers of prey had large impacts on the distribution, cannibalism rates and life history traits of D. aquaticus but their effects were modified by the nature of the ecosystem boundary. Thus, river the flow regime controlled the magnitude of the subsidy and its use by a consumer.

Searching for spiders at Bruce Stream, upper Waimakariri River catchment

People involved in this project

University of Canterbury
Michelle Greenwood (Completed PhD student)
Angus McIntosh
Jon Harding
Simon Pollard

Publications

Greenwood 2007. The population dynamics of a riparian spider: interactive effects of flow-related disturbance on cross-ecosystem subsidies and spider habitat. PhD thesis, University of Canterbury (pdf, 7.0mb).
Greenwood & McIntosh 2008. Flooding impacts on responses of a riparian consumer to cross-ecosystem subsidies. Ecology 89: 1489-1496 (pdf, 300kb).