The Character and Resilience of an Anthropocene River

Abstract

<p>Rivers are complex adaptive systems that display process-response properties. Disturbance is a key component of rivers as process-response systems; as a result, rivers are disturbance-driven ecosystems. A disturbance is any relatively discrete event that disrupts ecosystem, community, or population structure through changes in the physical environment and/or governing resources. Increasingly, the role of humans as drivers of change is important when considering rivers as disturbancedriven systems. Large-scale anthropogenic stressors have the potential to flip river ecosystems into another regime, which has a different set of structures, functions, and interactions, from which recovery cannot occur – they are “Anthropocene” rivers. The increasing scope and intensity of human activities underscores the need to examine process-response relationships in highly modified – Anthropocene – systems. This thesis proposes that Anthropocene Rivers are in a new state, or regime, and as a result, their character is fundamentally different from less-altered rivers, at the rivernetwork scale. Using resilience thinking as a framework, this thesis demonstrates the move to a new Anthropocene regime, and then describes the character of this new regime.</p> <p>To date, the study of disturbance in river ecosystems has been limited. The concept of equilibrium and rivers as gradient- or continuum-based systems that return to ‘normal’ conditions underpinned the study of disturbance in river systems. Moreover, research of disturbance in river systems tended to focus on single disturbances and events at relatively small spatial and temporal scales, in pristine systems. Increasingly, river ecosystems are subject to a host of multiple anthropogenic disturbances including climate change and land-use modification, which changes their disturbance regimes. This means we need an alternative approach for studying disturbance in Anthropocene Rivers. This thesis advocates for a new paradigm for studying disturbance in Anthropocene rivers by blending key tenets from several disparate disciplines and concepts – including systems theory, resilience thinking, landscape ecology, and interdisciplinary river science – to examine and understand responses to disturbance in an Anthropocene River. This thesis examines the structure, function, and interactions of an Anthropocene river ecosystem from an interdisciplinary river-science perspective. </p> <p>A multiple-lines-of-evidence approach has been taken in this thesis. First, changes in fish assemblages in response to multiple anthropogenic stressors were explored from a 60-year data set for the Illinois River (Illinois, USA). Despite a legacy of stressors, the highly modified Illinois River responded to multiple additional stressors, including two policies to improve water quality and bigheaded carp invasion. The response in fish diversity (Shannon H′) showed multiple thresholds, which suggests a complex response over time. There were also differences in the rate and direction of change between thresholds, and the variance of response at two spatial scales and two ecological levels of organization. Overall, nine response trajectories were observed. The observed changes in fish diversity in the Illinois River since 1959 do not conform to current ecosystem-response models. These differences in ecosystem structure along the Illinois River suggest a shift to a new regime.</p> <p>Second, food webs were developed for different stressor phases to investigate changes in ecosystem function in response to anthropogenic stressors over a 150-year period in the Illinois River. Stable isotope ratios, community niche space, basal resource contribution to higher levels in the food web, mean trophic position of fish functional feeding guilds, and food chain length were all used to show pronounced and persistent differences in ecosystem function in the Illinois River. Moreover, the river did not return to pre-stressor conditions following major systematic attempts at restoration. Differences in trophic status and food web character also varied between the upper- and lower-river zones of the Illinois River. The spatial divergence in response to multiple stressors highlights a complex response to anthropogenic stressors in the Illinois River. These differences in ecosystem function along the Illinois River provide another line of evidence suggesting a shift to a new regime – an Anthropocene regime – and one that has a reduced capacity to absorb additional disturbances.</p> <p>Third, physical heterogeneity is a strong driver of ecosystem function in rivers, but it is not clear whether this relationship persists in Anthropocene Rivers. This thesis examines spatial heterogeneity in the physical template and ecosystem function (i.e., tropic status and food web character) of the Illinois River. A systemic reduction in physical heterogeneity resulted in simplification of the physical environment of the Illinois River. This has implications for ecosystem function because of the strong positive relationship between physical heterogeneity and food chain length. The reduced spatial heterogeneity of ecosystem function in the Illinois River is reflective of ecosystem homogenization.</p> <p>Fourth, this thesis also examines the character of fish growth along the entire main channel of the Illinois River. Four species of fish from different functional feeding guilds were collected from each of six functional process zones (FPZs) – unique largescale hydrogeomorphic patches – along the entire length of the Illinois River, and three growth metrics were calculated: growth rate (<i>k</i>), maximum size (<i>L∞</i> ), and a relative growth index. The majority (7 of 12) of species-growth metric combinations did not differ among FPZs. Of the five species-growth metric combinations that were different, none exhibited more than three distinct groups of values. The limited difference in growth along the main channel of the Illinois River reflects a homogenization of ecosystem function, and is associated with the systemic simplification of physical heterogeneity of the river channel. The fishes studied from the Illinois River also tended to have faster growth rates (<i>k</i>) and smaller maximum sizes (<i>L∞</i> ) relative to other North American freshwater ecosystems. These results reveal spatial constraints to life-history traits and changes to ecosystem interactions, which are further evidence of being in a new regime. </p> <p>The multiple lines of evidence presented in this thesis strongly indicate a regime shift has occurred in the Illinois River; one that is driven by a legacy of anthropogenic stressors. This new Anthropocene regime of the Illinois River is different than the previous regime, as demonstrated via changes in fish biodiversity (i.e., ecosystem structure), and trophic status and food web character (i.e., ecosystem function). Moreover, despite system-wide improvements in water quality, ecosystem structure and function did not return to the previous regime. Further, this new regime has a reduced ability to adapt to additional anthropogenic disturbances – i.e., there has been a loss of resilience. Invasive species that entered the Illinois River in the early 1990s flipped this river ecosystem into another regime. This thesis also demonstrates that the physical template of the entire Illinois River was simplified through a system-wide decrease in physical heterogeneity, which was associated with homogenization of trophic status and food web character and fish growth along the entire main river channel of the Illinois River. </p> <p>Overall, this thesis contributes to our philosophical understanding of river science, by demonstrating a new interdisciplinary framework used to define Anthropocene Rivers. From a resilience-thinking perspective, this thesis demonstrates the Illinois River is an Anthropocene River, fundamentally different in complexity, adaptive capacity, and system-level relationships than more ‘natural’ or ‘pristine’ rivers, and is not just a river of the Anthropocene epoch. This thesis also challenges traditional views of equilibrium in ecosystems, especially as they pertain to Anthropocene Rivers. The philosophical contributions of this thesis have important management implications in a rehabilitation context, given the irreversibility of uses and changes that prevent the restoration of river systems to ‘natural’ states. Efforts to manage and rehabilitate the Illinois River and other Anthropocene River systems must find ways to increase heterogeneity at multiple scales by reconnecting or improving habitats lost or degraded as a result of anthropogenic activities. Management actions focused on enhancing heterogeneity will provide more niches, and consequently greater species diversity (i.e., ecosystem structure), and a greater diversity of ecosystem processes (i.e., ecosystem function), which not only increases redundancy, but also increases resilience to future disturbances like climate change. Ultimately, this thesis demonstrates a new paradigm that improves our knowledge of Anthropocene Rivers, and how disturbance is studied and understood in Anthropocene River systems. This shift to a new paradigm is needed to improve the understanding, management, and protection of river systems, which are an integral and critical part of our global environment.</p>