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Rob JM Franken
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Thesis

Proefschrift.


Franken, R.J.M. 2008. Habitat variation and life history strategies of benthic invertebrates. Thesis, Wageningen University.
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Abstract:

The adaptive capability of organisms to environmental conditions is essential to their distribution, growth and productivity, and reproductive potential. This thesis deals with differences in the life history strategies (suites of species traits) of three benthic invertebrate shredders - the freshwater shrimp Gammarus pulex, the freshwater louse Asellus aquaticus and the stonefly Nemoura cinerea - and their response to variation in the environment, described by the major habitat factors in small temperate streams: organic matter dynamics, current velocity and substrate type. Current theory predicts that allochthonous inputs (leaf litter) are more important than autochthonous sources (primary production by algae) to communities of small deciduous streams, based on terrestrial-aquatic linkages. In this thesis, effects of canopy opening on stream functioning were studied in the field and laboratory. Moderate differences in riparian canopy cover did affect shredder-detritivore population dynamics and suggests that even detrital-based and shredder-dominated food webs can be influenced by the indirect effects of light. Light intensity affected both leaf-biofilm quality and consumer behaviour and affected several aspects of the decomposition-consumer interaction. The results suggest that algae can be an important component of the nutritional value of the leaf-biofilm for benthic invertebrates, directly as an additional food source and indirectly through a link with bacteria and/or fungi. The results further suggest that light, by its effect on the biofilms on leaf surfaces, might be a more important factor in small streams than is usually assumed. Hydraulic and mineral substrate conditions interact to affect interstitial refugium availability because each depends partially on the other. To understand the mechanisms involved in refugium use, hydraulic (near-bed current velocity) and mineral substrate conditions were experimentally manipulated in indoor, artificial channels. Coarse gravel interstices positively affected shrimp and stonefly performance (growth/development, feeding, and behaviour), and there was a possible confounding effect of direct exposure on sand substrates. The underlying mechanism of interstitial refugium use might be associated with the adaptation of organisms to seek refuge from predators. The hydraulic and substrate conditions further resulted in adaptive phenotypic plasticity among stoneflies: variation in allometry and accelerated development. Different mechanisms were involved in the response to current velocity; the increase in velocity probably facilitated oxygen uptake, but also increased the risk of dislodgement, in which the energetic costs associated with resisting current flow were dependent on morphological traits (species-specific threshold). The same applies for the effect of substrate, which was attributed to either the presence of interstitial refugia or to substrate particle size itself, and might be associated with a species’ ability to move and orientate. In conclusion, this thesis supports the view that linking functional species traits to the environment will aid in identifying underlying mechanisms of species response, and, consequently, will advance the scientific basis of applied ecology.
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