The role of habitat structure in a freshwater food web.

Habitat structure refers to the nature ofthe physical structure that provides an environment for biotic communities. Much of the research in marine and freshwater systems notes the importance of habitat in community organisation (for example, fish predators are commonly less effective as habitat structure increases), but few studies have specifically described the mechanisms by which it influences trophic interactions and thereby community structure. My research investigated the role of macrophyte structure in trophic interactions and community structure in the macrophyte beds of a lowland river. One of the problems in assessing the role ofhabitat structure is the confusion over the definition, and therefore the measurement, of habitat structure, particularly in a way that allows comparison between different habitats and systems. I defined habitat structure as a combination of the qualitative and quantitative components of structure, so where macrophytes provide the habitat, this refers to their shape and density. While macrophyte density is relatively straightforward to quantify, macrophyte shape is more problematic which has lead to a variety of system-specific measures. I tested nine different indices of habitat complexity to determine which would best describe plant shape and best relate to the macroinvertebrate distribution on different macrophytes. I found a high degree of intercorrelation and redundancy between the structural indices such that they could be organised into two suites: one describing the interstitial space and the surface rugosity at coarse scales, the other describing the "whole plant" attributes of surface area and plant volume and the surface rugosity at fine scales. In particular, there were two indices which fell into both suites, an index of refuge space from predation, and the surface rugosity at 5 x magnification. Both these indices were also the most highly related to macroinvertebrate abundance and taxon richness, so I suggest they should be incorporated in the development of a broadly applicable index of macrophyte shape. As macroinvertebrates responded to the refuge role of macrophytes, I tested if differences in both macrophyte density and macrophyte shape had any effect on the prey-capture success of two predators, the southern pygmy perch and a predatory damselfly. I used two predators to address the impacts of multiple predators; ifhabitat structure can mediate the outcomes of predator-prey interactions, then it may also affect the outcomes of predator-predator interactions. I tested predator success in three macrophyte shapes at each of five macrophyte densities in a tank experiment. Surprisingly, there was no effect of plant density, but plant shape was important as fewer prey were captured, by each predator in isolation and by both predators combined, in the most structurally complex plant. This indicated that a more structurally complex plant can negatively affect the prey-capture success of predators, and also that macrophyte shape can mediate the outcomes of predator interactions. The implications of this laboratory experiment prompted a field experiment to determine ifthe influence ofmacrophyte shape on fish predator success translated to field conditions and affected the macroinvertebrate and periphyton communities in macrophyte beds. I conducted a two-factorial, repeated measures, randomised complete block experiment using floating cages in existing macrophyte beds. I tested the factors ofmacrophyte shape (three types) and the presence or absence of fish predators using the native southern pygmy perch. I ran the experiment for eight months, sampling the macroinvertebrate and periphyton communities at 2, 6, 1 0, 26 and 30 weeks. Macrophyte shape had strong, consistent effects on both the macroinvertebrate and periphyton communities; both were most abundant on the most structurally complex plant. In contrast, pygmy perch affected only a subset of the macroinvertebrate community and had minor indirect effects on the periphyton composition. Contrary to expectations though, pygmy perch had their strongest effects on vulnerable invertebrate herbivores in the most structurally complex plant. I concluded that in this system, macrophyte shape has a stronger influence than macrophyte density on trophic interactions, and constitutes a clear regulating influence on the macroinvertebrate and periphyton communities such that it precludes the conditions most likely to reveal strong effects of fish predation