Biocomplexity in dryland river systems : the influence of flow regime on ecosystem character and foodweb structure

Abstract

Riverine landscapes are amongst the most complex and diverse ecosystems on earth. They are characterised by dynamic patch mosaics that interact at multiple scales in space and time to produce patterns and process that govern the structure and function of the ecosystems they support. The flow regime, and in particular hydrological connectivity, maintains multiple, interactive pathways across the riverine landscape. The movement of water acts in concert with other biophysical components resulting in feedbacks between connectivity and spatiotemporal heterogeneity, leading to landscape scale patterns and processes responsible for creating biocomplexity. Understanding, describing, quantifying and monitoring the biocomplexity of riverine landscapes is necessary for the management of these systems, with the ultimate goal of reaching sustainability in this increasingly resource dependant world. Analysis of foodweb dynamics provides insight into the complexities of the structure and function of riverine landscapes, since they form the foundations of ecosystems and are intrinsically linked with their physio-chemical environment. It is widely accepted that flow regime influences aquatic foodwebs; however, the roles of specific features of the flow regime at varying temporal scales in influencing facets of the foodweb hierarchy are less well understood. The hierarchical nature of the flow regime and ecosystems necessitates a hierarchical approach for investigating the effect the flow regime has on ecosystem structure. This thesis uses a hierarchical framework to explore relationships between the flow regime and ecosystem character in an attempt to explain patterns of community structure and function in order to elucidate the underlying mechanisms responsible for the biocomplexity that typifies riverine landscapes. Relationships are explored by investigating the influence of hydrological connectivity across a range of temporal scales on community structure, trophic interactions and foodweb structure in the Lower Balonne, a dryland river system in south eastern Australia. This is achieved by describing the character of the flow regime at different temporal scales; investigating relationships between various hydrological connectivity parameters, physical habitat character and community structure of in-channel waterholes acting as refugia; and exploring the role of hydrological connectivity in shaping foodweb structure at multiple temporal scales. The hierarchical approach applied in this study revealed that several features of the flow regime, operating at two spatial scales, are influencing the character of foodwebs in 16 waterholes. Several features of the long-term flow regime including flow variability, low-flow variability and duration of connections influence what species exist in the waterholes, shape fish assemblages with regard to species richness, presence and abundance, and control food chain length, mean trophic position and the level of omnivory. In contrast, the character of the flow-pulse exerts little influence on species assemblage or foodweb structure, but variables such as days since last pulse and frequency of connections, influence the size structure of fish communities and generate responses in trophic interactions among taxa (trophic assemblage) within the constraints of foodweb structure determined by long-term flow regime. It can be concluded that the character of the long-term flow regime sets the boundaries which determine overall structure of riverine foodwebs and within these bounds, flow-pulse scale characteristics create variation that is reflected in responses of taxa within the community to short-term connectivity conditions. The complex hierarchical interactions between the flow regime and ecosystem character is presented in a simplified model. The model summarises how various aspects of the flow regime exert top-down constraints and bottom-up influences on different features of ecosystem and foodweb structure at multiple temporal scales. It also provides an example of how a hierarchical study design can be used to explore complex interactions that influence the structure and function of riverine ecosystems. The examination of foodwebs has proven to be a useful way to assess ecosystem character since their overall structure is not influenced by current hydrological conditions, yet dynamics within foodweb structure, such as the trophic assemblage, do respond to short-term hydrological changes and thus give insight into how shorter-term connectivity influences trophic interactions among taxa within a community. This study has highlighted how an understanding of foodwebs can aid in our quest to quantify biocomplexity and elucidate the processes responsible for creating and maintaining this elusive yet essential feature of riverine landscapes.

Date of Award	2009
Original language	English
Supervisor	Martin THOMS (Supervisor), Michael A. Reid (Supervisor) & James Thorp (Supervisor)