Rosser, Zoe Catherine (1998) The role of disturbance in community dynamics and structuring of tropical rainforest stream communities. PhD thesis, James Cook University.

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Abstract

This study examined the role of disturbance in the community dynamics of benthic macroinvertebrates in tropical rainforest streams, incorporating sites in the Australian wet tropics and Papua New Guinea. The focus was on physical disturbance through the movement of the substratum, as this is likely to be the most ecologically relevant measure of disturbance to benthic stream fauna. The approach combined long-term sampling following severe, natural floods, with smaller-scale experimental manipulations, permitting faunal response to disturbance to be examined at three spatial scales: stream-wide, stream section and individual stone.

Streams within the Australian Wet Tropics are characterised by strongly seasonal patterns of stream flow, with high and variable flows during the summer wet season and low and relatively stable flows during the dry season. Continuous, long-term sampling of macroinvertebrate stone fauna within the major habitats, riffles and pools, revealed that the influence of disturbance on the temporal variation in benthic community structure is dependent on the magnitude of the flood and differs between habitats. When floods are severe, resulting in substantial alteration to the habitat, seasonal cycles in community structure are most distinct, with a relatively distinct, highly resilient fauna (i.e. able to recover rapidly following disturbance) present during the wet season shifting to less resilient assemblages during the dry season. During years where the magnitude of disturbance by floods is reduced, the temporal shift in assemblage dominance is less marked, and highly fluctuating communities (e.g. species turnover — indicated by multivariate ordinations) suggest increased prominence of biotic interactions as habitats contract during the dry season. The fauna on stones was remarkably resistant, particularly in pool habitats, and recovered rapidly (within 21 days in the pool and 45 days in the riffle) to pre-flood densities and richness following substratum-moving floods. The relatively high resistance and resilience in pool habitats compared to riffles may not only reflect intrinsic characteristics of the assemblages (which were relatively distinct), but also the reduced impact of floods in pool habitats, which tended to accumulate fauna. This suggests an important refugial role, of pools, and the subsequent redistribution from these areas is very likely an essential part of the recovery process following flood disturbance in these streams.

As the spatial scale of disturbance is reduced to the stream section and individual stone, faunal recovery becomes more rapid (2 days — 24 hrs), reflecting the proximity of colonists to the disturbed patch, and small-scale rather than large-scale recolonisation processes are likely to increase in importance (e.g. benthic movements rather than drift). The composition and recolonisation characteristics of the fauna are influenced by the position of the disturbed patch within the stream. Along a longitudinal gradient, incorporating headwater, mid-reach and lower-reach sites, faunal resistance and resilience was highest at the lower-reach site, corresponding to longitudinal differences in the natural flood regime and stability characteristics of the surrounding substratum.

The temporal frequency of disturbance influenced faunal responses as indicated by experiments which directly manipulated the frequency of substratum disturbance, and by comparisons between streams with different historical patterns of natural disturbance. Increasing disturbance frequency per se — the number of times individual stones are tumbled — increased the susceptibility of macroinvertebrates to disturbance. The influence of different historical frequencies of disturbance was examined by comparing faunal response to experimental disturbance in aseasonal streams in Papua New Guinea, characterised by a high annual frequency of natural flood disturbance, with a seasonal stream in the Wet Tropics which had a low annual frequency of disturbance. Sites in PNG, with a short inter-flood period (i.e. frequently disturbed), supported a fauna that was much more resistant and resilient to disturbance, compared with the streams in the Wet Tropics. The historical pattern of disturbance is likely to provide a strong selective force for the fauna inhabiting the frequently disturbed PNG streams, promoting characteristics such as high mobility and species turnover, which are likely to enhance persistence in these environments.

Disturbance is likely to play an important role in the maintenance of the very high benthic diversities observed in streams of the Australian Wet Tropics. The results of this study show that at all spatial scales, disturbance tends to increase the spatial variability of individuals on stones, which may create opportunities for the coexistence of rarer species. The combination of abiotic disturbance during the wet season and increasing biotic interactions during the dry season may provide the basis for the particularly high diversity in these streams.

In summary, disturbance involving the physical movement of the substratum is clearly a major factor influencing the dynamics of benthic communities in tropical streams. However, the results of this study indicate that macroinvertebrate response to disturbance is strongly influenced by the characteristics of the disturbance, such as magnitude and frequency and also by the intrinsic nature of the habitat.

Item ID:	27177
Item Type:	Thesis (PhD)
Keywords:	community dynamics; seasonal cycles; assemblage dominance; benthic macroinvertebrates; tropical rainforest streams; wet tropics; Queensland; Papua New Guinea; physical disturbance; benthic stream fauna; floods; pool habitats
Date Deposited:	03 Jun 2013 02:24
FoR Codes:	05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 33% 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 34% 05 ENVIRONMENTAL SCIENCES > 0599 Other Environmental Sciences > 059999 Environmental Sciences not elsewhere classified @ 33%
SEO Codes:	96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 51% 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 49%
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