Evaluating barriers to dispersal: weirs and tributaries in the montane rivers of the Australian Alps
AuthorBrooks, Andrew John
AffiliationResource Management and Geography
Document TypePhD thesis
Access StatusOpen Access
© 2020 Andrew John Brooks
Dispersal involves the movement of individuals between established populations or colonisation of uninhabited areas and is a key organising process underpinning patterns in populations and communities. Dispersal is a fundamental component of metapopulation and metacommunity theory, central to explaining the underlying the patterns of abundance and distribution of species, both spatially and temporally. Furthermore, dispersal is a main determinant of community assembly, governing how communities are constructed and maintained through immigration. Barriers that limit or prevent species from dispersing to a location can have a major influence on population dynamics and how communities assemble. In a restoration context, this can result in delayed or poor restoration outcomes due to a lack of dispersing colonists preventing colonisation. In freshwater environments, one key hypothesis describing how dams and weirs affect rivers is that they disrupt longitudinal connectivity, fragmenting river ecosystems, potentially limiting dispersal of biota along rivers. In this thesis, I asked three main questions: 1) Are there natural barriers to dispersal in advective systems? 2) Do artificial barriers hinder dispersal more than natural barriers? 3) Do artificial barriers constrain dispersal in ways that affect population and communities, especially community assembly? The study focused on rivers located mostly within Kosciuszko National Park, in the Snowy Mountains region of south-east Australia. Many rivers in this area are affected by the Snowy Mountains Hydro-electric Scheme and associated culverts, weirs and dams, which capture and divert numerous alpine and montane streams, severing flow connections between much of the Snowy River catchment and its headwaters. First, I tested how species dispersal between suitable habitats was influenced by the characteristics of the intervening matrix of unsuitable habitat. Specifically, I examined whether stream insect drift was constrained by natural river features, potentially limiting connectivity within rivers. I found that natural, slow moving pools may limit the connectivity of benthic invertebrate populations in rivers by reducing drift rates between riffle habitats. Furthermore, I determined that the hydraulic conditions within a pool limit drift dispersal and total distance between riffle habitat patches was not an important factor in limiting dispersal via drift. Secondly, I tested whether species dispersal was constrained by a human-made barrier, potentially limiting connectivity between populations more than natural landscape structures. In a natural river system, I studied whether a weir and associated pool reduced the drift rates of insects to a greater degree than natural pools. This component addressed a major knowledge gap about the effect of weir structures on downstream dispersal of stream biota. The weir consistently reduced numbers of drifting insects for 3 of 4 study taxa, exceeding the reduction of drifters in natural pools. The morphology of the weir pool was substantially deeper and wider and slower than the majority of studied natural pools. The combined effects of much lower average water velocity, multiple large low velocity areas within the weir pool and the weir wall were likely to be central causes of the reduction stream insect drift through the weir. Lastly, in a multi-year study, I tested whether and how the removal of dispersal constraints affected community assembly in new habitats and whether changed dispersal can alter existing communities. In this study, I investigated the patterns and mechanisms of freshwater invertebrate community assembly after the reintroduction of water to 2 streams downstream of weirs that were previously dry for over 50 years. Colonisation of the newly formed habitat in the tributaries downstream of the weirs was rapid and strongly influenced by dispersal via drift from upstream. Even with some dispersal constraint via reduced drift rates, the new communities rapidly resembled unimpacted communities that were the source of colonists. In the regulated rivers, a reduction in environmental constraints had a much greater influence on the trophic structure of established communities than increased dispersal from the newly formed community in the tributaries. Collectively, this research has overturned important, pre-existing assumptions about dispersal in advective systems by identifying and quantifying the influence of different types of barriers on movement and the effect of altered dispersal rates on community assembly. This information can inform the development of river restoration strategies in rivers affected by weirs and dams and elucidate how and why restoration measures may have been unsuccessful in the past.
Keywordscommunity assembly; colonisation; dispersal; freshwater invertebrates; barriers
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