Functional redundancy and how river ecosystems respond to stress
A well preserved section of the Segura River in Murcia, Spain, showing a riparian area dominated by black poplar, ashes and willows.
This week we have a guest post written by Daniel Bruno and Cayetano Gutiérrez-Cánovas on their new Journal of Applied Ecology paper which examines the potential of using the functional redundancy concept to assess how river ecosystems respond to stress.
Daniel is a researcher at the University of Murcia in Spain. His work explores river conservation and restoration, riparian ecology and the use of indicators to assess ecological status. Tano is a research associate at the University of Cardiff in Wales, whose work in the MARS project analyses how aquatic ecosystems respond to multiple stressor interactions at different spatial and temporal scales.
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The world’s ecosystems are experiencing an unprecedented increase in the number and variety of impacts that alter their ecological functioning. Such ecosystem alterations include changes to primary production, pollination, nutrient cycling and organic matter decomposition, among others.
Traditionally, ecologists have used species composition and other taxonomical approaches as indicators for ecosystem health. For example, some species are more likely to be abundant in disturbed sites, while others are very sensitive to human-caused stress. However, these techniques are not able to tell us how ecosystem functions might change after the disturbance, which would be far more useful for environmental managers in predicting and reacting to such impacts.
In a new study recently published in the Journal of Applied Ecology, we tested the potential of new functional indicators for assessing how river ecosystems respond to stress. We found that some of these indicators were able to detect single and combined effect of stressors, which may allow a better understanding of how freshwater ecosystem functioning responds to human pressures. Among them, functional redundancy is the most promising indicator as it relates positively to stability, resistance and resilience in ecosystems with a high sensitivity to stress.
The functional indicators used here account for the variability of the biological attributes of riparian vegetation (plants growing on land along river banks) that relate with ecosystem functioning, such as size, growth rate or leaf surface.
These measures have several advantages when compared to species-based tools. They have broader spatial utility (species composition varies more spatially than their biological attributes); better comparison among taxonomic groups (biological attributes as size are shared among all kind of organisms) and can be linked directly with ecosystem processes. There are many such links: leaf nutrient content affects in-stream processes like decomposition; larger trees cause more shadow which can alter river temperature and production and contribute to significant habitat modifications like natural, woody dams.
Riparian vegetation beside an intermittent river (Corneros River in Murcia, Spain), affected by drought in the Segura Basin study area.
Our study was conducted in a Mediterranean climate river basin (the Segura River) located in the southeast of Spain, where agricultural intensification, dams and natural droughts are the main causes of ecosystem stress. We produced different metrics to account for the multiple functional traits and aspects of the riparian plants. Then, we compared how those metrics responded to the single and combined stressors impacting on the ecosystem.
There were two main groups of functional indicators. First, measures of functional diversity which described the variability of biological attributes driving ecosystem functions. To measure functional diversity we calculated functional richness (community functional variability), functional evenness (how individuals or species are distributed among functional types) and functional dispersion (mean functional similarity among species);
Second, we used measures of functional redundancy, which can be defined as the number of species performing similar roles in an ecosystem, for example nutrient cycling, sediment fixation or climate regulation. Higher values of redundancy can mean increased long-term stability of related ecosystem functioning. This means that in functionally redundant ecosystems it may be possible for populations of some species to highly stressed or even made locally extinct, with little or no impact on ecosystem functioning.
One of the ways to estimate functional redundancy classifies species into functional groups which make similar contributions to ecosystem functioning. In our study, we classified riparian plants in five major functional groups: large highly water demanding trees; water demanding shrubs; evergreen shrubs; climbing plants; and drought-adapted vegetation.
Reach of the Mundo river in Albacete, Spain, which is disturbed by agricultural intensification. Note the scarcity of woody riparian vegetation in the left margin being dominated by evergreen shrubs and invasive species (e.g. Arundo donax).
Our results showed that stressors, when considered individually, caused general marked declines in functional indicators, with a variety in the size of decline. Generally, agricultural intensification was the most influential stressor for riparian functionality, followed by natural droughts. Hydrological regulation weakly affected functional indicators.
Functional redundancy was the most sensitive indicator in response to single and combined environmental filters. Combined effects on functional redundancy resulted from the interaction between agricultural impacts and droughts; and agricultural impacts and flow regulation. This suggests that such stressors should be considered together for the most accurate understanding of their effects on ecosystem health.
The most significant implication of these results is that functional redundancy can be used to identify which ecosystem functions are at the highest risk when an ecosystem is stressed. When we lose species from the same functional group (species playing a similar ecosystem function), this causes a decrease in functional redundancy. As such, using this functional index provides a valuable early warning system before ecosystem function begin to decline. Therefore, incorporating functional redundancy into river evaluation and management planning may help us to anticipate the effects caused by the ongoing global change.
Predicted functional redundancy for riparian communities in the Segura Basin study area.
An important advantage of the functional redundancy approach is its accurate response to combined stressors (best model explained near to 60% of its variability). We predicted the functional redundancy values for the entire river network (see figure above). For this forecast, we used a large dataset of sites from which agricultural intensification, flow regulation and natural drought were estimated, constituting a potential basis for biomonitoring and environmental management at the basin scale. This map is useful to detect the most impacted river reaches, to plan restoration measures, as well as to conserve the reaches with the best ecological functioning.
The predictors used here are low-cost, coarse-grain variables that are easy to obtain from digital maps and environmental databases. We also provide an open statistical method (R-scripts) to estimate the functional features and run the models showed in the study, allowing administrations and ecologists to extend this method to their study areas. Therefore, although the sensitiveness of functional redundancy to human impact must be specifically compared with other traditional biomonitoring tools and river types, it can be considered as an ecologically-sound measure able to detect ecological responses to single and multiple stressors.
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