Ecological Connectivity in Braided River-Scapes
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Ecological connectivity in braided riverscapes A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology by Duncan Gray University of Canterbury 2010 Contents General acknowledgment 3 Abstract 5 General introduction 7 Chapter 1 14 Braided river ecology: a literature review of physical habitats and aquatic invertebrate communities Chapter 2 90 Braided river benthic diversity at multiple spatial scales: a hierarchical analysis of ß diversity in complex floodplain systems Chapter 3 129 Multi-scaled environmental determinism of benthic invertebrate communities in braided rivers: evidence for complex hierarchical controls on local communities Prologue to chapters 4 & 5 165 Chapter 4 171 Carbon cycling in braided river floodplain ecosystems: rapid out-gassing of carbon dioxide transmits δ13C gradient through the stream food-web Chapter 5 207 Riparian vegetation mediates stream community use of carbon: invertebrate response to successional change in a braided river floodplain spring Synthesis and synopsis 239 Appendix 1 251 A new species of Zelandobius (Plecoptera: Gripopterygidae: Antarctoperlinae) from the upper Rangitata River, Canterbury, New Zealand Appendix 2 264 Namanereis tiriteae, New Zealand’s freshwater polychaete: new distribution records and review of biology Appendix 3 281 Presence/absence of taxa across the eleven rivers included in my survey 2 General acknowledgement It has been 4 years since I wrote one of these. A period of time that stretches back into dim memory, yet seems inadequate to contain events that have occurred. Theses and life have progressed, along with the passage of time. Whilst the latter is constant and inevitable, the former were more erratic and certainly not passive processes. Accordingly, I want to thank the people and organisations who have smoothed the passage of my scientific endeavors, and those other people to whom I rush home to forget all about empiricisim. I have been very fortunate in my choice of supervisory panel. Jon Harding, Mike Winterbourn (UC both), Mike Scarsbrook (ex-NIWA) and Graham Fenwick (NIWA) have all, in their own way, been quite pivotal to this thesis. Special mention must go to Jon and Mike W whom not only bear the brunt of editing and administration, but are cool heads in a crisis (usually mine) and good company throughout. Constant and inevitable. The freshwater ecology research group (FERG) has been my home and extended family for 7 years now. This is a wonderful group of people, constantly in flux, constantly challenging, yet as supportive a base as any family. Certain people are highlighted in the individual chapter acknowledgments, others are not. I have been privileged to spend time with them all. Particular mention must go to Jo O’Cock, Tanya Blakely, Amy Whitehead, Peter McHugh, Michelle Greenwood, Justin Kitto, Rebecca Campbell, Linda Morris, Hamish Greig and Troy Watson. This thesis would not have been possible without the generous support of the Miss E.L. Hellaby Indigenous Grassland Research Trust, the Department of Conservation, the Brian Mason Scientific and Technical Trust and the FORST funded Groundwater Ecosystems Progamme at NIWA. Beyond the double doors of level five, Old Zoology, is the world at large. The infrastructure of the School of Biological Sciences, the University and the wider scientific community. Although accusations of insularity are sometimes levelled at the 3 latter I have always found people interested and enthusiastic, a bit busy, but in no way elitist or aloof. From 9 to 5 ish I feel very much part of a community. Travis Horton and Dave Arscott deserve particular mention for generosity with their time, insight and expertise. Twenty-four hours a day I have a family outside of science. They have enjoyed with me the highs and lows of research, to them I owe the greatest thanks, for their tolerance, patience, support and love. Bridget, Lucia, Lauren and Honor. Also, the Allen family and the Gray’s, for all the socks, baby sitting and inclusion, from near and far. The last seven years for me have been inspired by a landscape. This land is not pristine, but neither is it an example of a place long associated with people. Whilst, few populated regions of the world have reached a balance between nature and man, New Zealand still has the capacity for rapid and dramatic change. We have the opportunity to influence this change. We are going to need all the goodwill, enthusiasm and empathy available. Thanks all What next? 4 Abstract Rivers are hierarchical networks that integrate both large and small scale processes within catchments. They are highly influenced by variation in flow and are characterised by strong longitudinal movement of materials. I conducted an extensive literature review that indicated braided rivers lie at the upper end of the river complexity gradient due to the addition of strong lateral and vertical connectivity with their floodplains. The management of these rivers requires an understanding of the connective linkages that drive complexity, however in developed regions few braided river systems remain intact. The large number of relatively pristine braided rivers in New Zealand provided a unique opportunity to study physical and biotic patterns in these large dynamic systems. Initially I examined horizontal connectivity through patterns in regional and local diversity in eleven braided rivers in the North and South islands of New Zealand. Subsequently, the next component of my thesis focused on vertical connectivity through intensive investigations of energy pathways and the recipient spring stream food-webs. The eleven river survey included sampling of multiple reaches and habitats (main channels, side braids, spring sources, spring streams and ponds) and confirmed the importance of lateral habitats to invertebrate diversity. However, I found that large spatial scales made a greater contribution to diversity than small scales, such that major differences occurred between rivers rather than habitats. This result suggested either a role for catchment-scale factors, such as flow, or biogeographic patterning. Subsequent analysis of the relationships between invertebrate diversity and the physical environment indicated strong regulation by flow variability, but also biogeographic community patterns. Braided rivers are clearly disturbance dominated ecosystems, however the effects of disturbance are manifest in different ways across the riverscape. The role of vertical hydrological connectivity in linking the different components of the floodplain was investigated by tracing carbon pathways from the terrestrial floodplain to spring-fed streams and their communities. Using δ13C isotope signatures it was possible to show that inorganic carbon in groundwater was derived from terrestrial vegetation and subsequently incorporated into spring stream food-webs. However, the degree to which a 5 stream community uses groundwater as opposed to allochthonous carbon is affected by the successional stage of riparian vegetation, a function of the shifting habitat mosaic that is regulated primarily by flow variation and sediment dynamics. In summary, the structure of braided river ecosystems is regulated primarily at the catchment scale, but connectivity at smaller scales plays an important role in determining ecological structure and function. 6 General Introduction Braided rivers were once a dramatic component of landscapes around the globe (Tockner et al. 2003). However, in many areas of the world anthropogenic modification of catchments and rivers has drastically altered them (Ward & Stanford 1995; Tockner & Stanford 2002; Nilsson et al. 2005). The modification of these complex natural systems has markedly reduced the physical heterogeneity and biotic diversity of modern landscapes. Rivers are important not only for their intrinsic values, but because they serve to integrate phenomena which occur across entire catchments and beyond. This concept was originally axiomated for modern stream ecologists by Hynes (1975) who wrote that “in every respect the valley rules the stream”. In braided rivers this is particularly important because of the high connectivity between habitats found both across the floodplain and longitudinally along the riverine system. The conceptualisation of connectivity by stream ecologists has borrowed from a subset of general systems theory that deals with hierarchies (Allen & Starr 1982; Stanford & Ward 1993). Organisms, habitats, or stream reaches are nested within physical and organisational elements, which exist at a higher level within the hierarchy. In ecological systems generally, it has become apparent that linkage and regulation feedbacks propagate patterns and process up as well as down the scales of this hierarchy (Harrison & Cornell 2008). Thus, hierarchically scaled approaches are highly appropriate for rivers, which form dendritic networks and are open, “leaky” systems within an often highly heterogeneous matrix (Pringle et al. 1988; Power & Dietrich 2002; Lowe et al. 2006; Grant et al. 2007). The role of disturbance has also emerged as a major paradigm in ecology (Connell 1978), and particularly stream ecology (Resh et al. 1988; Death & Winterbourn 1995; Townsend et al. 1997; Lake 2000). Disturbance can dictate the structure of biotic communities at both ecological and evolutionary scales (Lancaster & Belyea 1997; Diaz et al. 2008), and braided rivers probably represent the quintessential disturbance-dominated freshwater ecosystem (Scrimgeour & Winterbourn