The effects of climatic fluctuations and extreme events on running water ecosystems

Abstract

Most research on the effects of environmental change in freshwaters has focused on incremental changes in average conditions, rather than fluctuations or extreme events such as heatwaves, cold snaps, droughts, floods or wildfires, which may have even more profound consequences. Such events are commonly predicted to increase in frequency, intensity and duration with global climate change, with many systems being exposed to conditions with no recent historical precedent. We propose a mechanistic framework for predicting potential impacts of environmental fluctuations on running-water ecosystems by scaling up effects of fluctuations from individuals to entire ecosystems. This framework requires integration of four key components: effects of the environment on individual metabolism, metabolic and biomechanical constraints on fluctuating species interactions, assembly dynamics of local food webs, and mapping the dynamics of the meta-community onto ecosystem function. We illustrate the framework by developing a mathematical model of environmental fluctuations on dynamically assembling food webs. We highlight (currently limited) empirical evidence for emerging insights and theoretical predictions. For example, widely supported predictions about the effects of environmental fluctuations are: high vulnerability of species with high per capita metabolic demands such as large-bodied ones at the top of food webs; simplification of food web network structure and impaired energetic transfer efficiency; and reduced resilience and top-down relative to bottom-up regulation of food web and ecosystem processes. We conclude by identifying key questions and challenges that need to be addressed to develop more accurate and predictive bio-assessments of the effects of fluctuations, and implications of fluctuations for management practices in an increasingly uncertain world.

Keywords: biodiversity; community assembly; ecosystem functioning; food webs; metabolism; resilience.

Figure 1.

A conceptual framework for studying the effects of environmental fluctuations on running water ecosystems. ‘2D’ and ‘3D’ refer to two and three spatial dimensions, respectively. Both, temperature and hydrological fluctuations affect individuals, which are then propagated through species interactions to higher levels of organization. Species interactions are likely to be disrupted also due to inter-specific mismatches arising from the differing tolerances, physiologies or biomechanics of predator and prey. Note also that ectotherm thermal performance curves are typically asymmetric (as shown)—i.e. heat waves are likely to have far stronger impacts than cold spells on species and interactions.

Figure 2.

The effect of environmental fluctuations on dynamically assembled model ecosystems. Changes in key food web features in response to increasing intensity of environmental fluctuations (environmental variance, ; equation S3) are shown at different carrying capacities (K). Size-based properties—mean size and consumer–resource size ration—are on log₁₀ scale. The bars represent 95% confidence intervals around the mean of 200 community assembly simulations. Each model community at the end of a simulation is at immigration–extinction equilibrium. Model structure and parametrizations are detailed in the electronic supplementary material, appendix S1.

Figure 3.

Conceptual diagram of shifts in body mass and abundance under two scenarios of combined extreme events (cf. figure 1). The white squares represent taxa associated with more lentic conditions, the coloured circles represent taxa associated with more lotic conditions. Green nodes represent producers, while red nodes represent consumers. The red boxes represent endotherms; the blue boxes ectotherms. Note, responses can be multifaceted and include species loss (especially among the higher trophic levels and endotherms), population declines (or occasionally increases, if predator release occurs), as well as changes in links and higher-level properties related to system complexity and energy flux.

Figure 4.

Fluctuations in abundance of an apex predator in a riverine ecosystem in response to environmental fluctuations and extreme events. The black line shows counts of grey heron breeding pairs in the UK 1928–2012. Red line is a LOWESS smoother. Note the particularly sharp drop following the exceptionally severe winter in 1963, which led to extensive and protracted freezing of inland waters (source: BTO).

Figure 5.

The correlated abundance of two trophic levels over 13 years following an extreme event in the Glenfinish River in Ireland. Abundance (number m⁻²) is in log₁₀ scale. Significant break points in time series trend determined through circular binary segmentation analysis are shown by the red horizontal lines; the mean of all time series is shown by the black horizontal line. The grey line corresponds to a LOWESS smoothing. The vertical blue line indicates the catastrophic flood event in 1986 (adapted from [29]).

Figure 6.

Hypothetical effects of extreme events on food web structure in a riverine landscape. The global web (top left) is altered by the hydrology of the system such that different regional (e.g. lower reaches and floodplain habitats) or local (e.g. individual headwater streams) communities are found under drought (top right), baseflow (middle right) or flood (bottom right) conditions. Note that increased habitat volume and beta diversity may increase regional web size under flood conditions, even if local webs may lose species.