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The role of habitat structure in a freshwater food web.

Warfe, DM 2002 , 'The role of habitat structure in a freshwater food web.', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Warfe, DM
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