Influence of flood disturbances and biotic interactions on the microdistribution of stream invertebrates

Most living communities form a temporally shifting patchwork of
irregularly distributed organisms. Besides many habitat-specific
biotic and abiotic environmental conditions, two key drivers are
known to shape community structure: abiotic disturbance and biotic
interactions (most notably competition and predation). Few other
ecosystems possess either the frequency or intensity of
disturbances observed in running waters. Therefore, disturbance
(mainly in the form of floods) is discussed to be the dominant
organizing factor in streams and rivers. The aim of my thesis was
to investigate the interplay between flood disturbances and biotic
interactions in determining the small-scale distribution of benthic
invertebrate communities in streams. Especially during small and
mid-sized floods, the high shear forces that move and rearrange
parts of the stream bed result in a complex mosaic of small (≤ 1
m2) bed patches that experience scour, sediment deposition or
remain undisturbed (“local disturbance history”). In my thesis, I
found that local disturbance history patterns caused by natural
floods (Chapter 1) or created experimentally (Chapters 2, 3 and 5)
played an important role for the distribution of mobile
invertebrates. Further, stable bed patches seemed to act as
invertebrate refugia during and shortly after floods and, in the
longer term, several common invertebrate taxa preferably colonized
depositional or scour patches. Various habitat parameters such as
current velocity, substratum size or food resources were also
partly responsible for the heterogeneous distribution of stream
invertebrates (Chapters 1, 2 and 5). The combined findings of my
manipulative experiments described in Chapters 2 and 5 suggest that
immediate, 'direct' effects of local disturbance on the
invertebrates (mostly negative, i.e. density reductions in
disturbed bed patches) are often in the longer term (several weeks
after a flood) replaced by 'indirect' effects mediated via
disturbance-induced changes in habitat parameters such as current
velocity, substratum size and resource availability. Previous
studies indicate that biotic interactions such as competition,
grazing and predation can also be important determinants of the
distribution of stream biota. However, although most streams are
subject to considerable discharge variations, almost all of these
earlier studies were performed in streams or artificial channels
with permanently stable flow, or during long periods of stable flow
in periodically disturbed streams. To date it is still unclear if
biotic interactions are also important in frequently disturbed
streams. To begin closing this knowledge gap, I conducted three
experiments that examined the interactive effects of physical
disturbance and interspecific competition on benthic stream
invertebrates and algae. Singular (Chapter 3) and repeated (Chapter
4) local disturbances were combined with frequent manual removals
of the most common invertebrate taxa. Disturbance played an
important role for the microdistribution of invertebrates in all
experiments. By contrast, competition was only found to be an
important driver in shaping community composition in a stable
stream (Chapter 4). In both experiments conducted in frequently
disturbed streams, I found no evidence that competition influenced
the invertebrate community (Chapters 3 and 4). Moreover, there were
hardly any interactions between disturbance and competition
treatments. Collectively, the results from previous research
conducted in stable streams and my own experiments support the
hypothesis that the importance of competition in shaping aquatic
communities should decrease with increasing frequency or intensity
of disturbance. In my last experiment (Chapter 5), I examined the
separate and interactive effects of patchy bed disturbance and fish
predation on benthic invertebrates and algae. While experimental
disturbance had strong and lasting effects on the benthic
community, effects of local fish exclusion were weaker. Moreover,
effects of fish predation on invertebrate and algal densities were
generally present or absent regardless of the disturbance history
of the studied patches of stream bed. These results emphasize the
pervasive importance of patchy bed disturbances for the
microdistribution of stream organisms and also indicate a notable,
but less prevalent, influence of fish exclusion at the patch scale
on this microdistribution. Collectively, my findings on the
interplay between disturbance and competition or predation confirm
the key role of local disturbance history for the small-scale
distribution of stream invertebrates both in stable and in
frequently disturbed streams (Chapters 3, 4 and 5). Furthermore,
local habitat parameters such as current velocity or food resources
may define suitable bed patches for stream invertebrates, but
several of these parameters themselves seem to be influenced by
local disturbance history, as well. Finally, the frequency and/or
intensity of such disturbances may determine whether populations
become so dense that competition or predation can strongly
influence the structure of the benthic stream community.