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Effect of drying flow regimes on macroinvertebrates in headwater streams

Carey, Nicole (2021) Effect of drying flow regimes on macroinvertebrates in headwater streams. PhD thesis, Murdoch University.

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In many regions, global warming is causing drying in perennial streams and prolonged drying in intermittent streams. Flow regime strongly influences stream ecosystems, so changing regimes are predicted to alter stream biodiversity and function. This thesis investigated the effect of flow regime change (perennial to intermittent) on macroinvertebrate communities in headwater streams in south western Australia (SWA), a region that has already experienced substantial climatic drying. I used a multiple before-after, control-impact design where controls comprised stream reaches that remained perennial and impact reaches were perennial until 2000s and then became intermittent. Before-drying information came from Bunn’s 1985 PhD thesis and I collected after-drying data from the same reaches in 2016-2017. Many species (including SWA endemics) were lost from streams that became intermittent, which also supported drying-adapted species, including new arrivals. Regardless of flow regime, high species turnover characterised both spatial and temporal beta-diversity in assemblages. Species traits did not adequately predict persistence under intermittency. Of the 13 formerly most-common species, seven were restricted to perennially flowing streams, and only four (two mayflies, one stonefly, one amphipod) adapted their life histories to intermittency. Bunn (1985) showed that leaf-litter breakdown was moderated by macroinvertebrates in these streams. By repeating his experiment, I showed that leaf-litter breakdown processes had remained similar in intermittent and perennial streams, despite the loss of most large macroinvertebrate shredders. To survive drying, invertebrates used a similar range of drought refuges identified in other studies. Altogether, these results demonstrate profound alterations to macroinvertebrate communities in streams forced into intermittency, particularly the loss of relictual and functionally important species. Given that current understanding of traits did not predict species persistence, life history research should focus on understanding trait flexibility. Identification and protection of evolutionary refugia is urgent in regions undergoing climatic drying because substantial biodiversity loss is imminent there.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Environmental and Conservation Sciences
United Nations SDGs: Goal 14: Life Below Water
Supervisor(s): Robson, Belinda, Chester, Edwin and Chambers, Jane
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