Comparing the responses of functional redundancy and functional diversity indices to stress

In this post Daniel Bruno discusses his paper ‘Impacts of environmental filters on functional redundancy in riparian vegetation

The world’s ecosystems are experiencing an unprecedented increase in the amount and variety of impacts (global change) which is leading to an unprecedented biodiversity loss and modification of ecosystem functioning (e.g. changes in primary production, pollination, nutrient cycling and organic matter decomposition). Accordingly, there is a long-standing interest among environmental scientists in understanding and predicting ecosystem responses to environmental changes. Traditionally, we have used species composition and richness as indicators of ecosystem health ignoring other ecosystem properties. However, these techniques are not able to inform about how disturbances alter ecosystem functions which would have a high interest for environmental managers. Thus, it is necessary to move forward in the detection and prediction of ecosystem responses to multiple environmental stressors in order to guide conservation efforts and the management of natural resources.

During the last decade, there has been a growing development of trait-based approaches and whole-community functional indices (e.g. functional richness) to explore the environmental effects of human activity. However, this new knowledge has not been translated into biomonitoring. The incorporation of functional measures to conventional tools provides a great opportunity of improving ecosystems management in a cost-effective way since they can add some extra complementary information. 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.

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Well-preserved river reach (Segura river, SE Spain) showing a riparian area dominated by black poplar, ashes and willows (large, water-demanding trees and shrubs).

In our new study recently published in Journal of Applied Ecology, we tested the potential of functional indicators for assessing how river ecosystems respond to stress. We performed our study using riparian vegetation because it is a key component in the functioning of rivers, which are one of the most diverse but threatened habitats worldwide. However, in comparison with terrestrial ecosystems, aquatic conservation science is still lagging in quality and quantity of empirical studies. 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 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. Then, we compared how those metrics responded to the single and combined stressors impacting on the ecosystem.

Reach of the Mundo river (Albacete, Spain) disturbed by agricultural land use. Note the scarcity of woody riparian vegetation in the left margin being dominated by evergreen shrubs and invasive species.

There were two main groups of metrics. First, measures of functional diversity which described the variability of biological attributes driving ecosystem functions i.e. 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 (e.g. on nutrient cycling, sediment fixation or climate regulation). Higher values of redundancy can mean increased long-term stability of related ecosystem functioning.

Riparian vegetation affected by flow regulation and dominated by the invasive species Arundo donax.

We found that some of these indicators were able to detect single and combined effects of stressors, which may allow a better understanding of how freshwater ecosystem functioning responds to human pressures. Our results showed that stressors, when considered individually, caused general marked declines in functional indicators. As the main outcome, functional redundancy was the most sensitive indicator in response to single and combined environmental filters. In general, agriculture was the most influential stressor for riparian functionality, followed by natural droughts. Thus, temporary streams flowing through an agricultural, regulated basin had reduced values of functional redundancy. On the other hand, free-flowing medium-sized, perennial water courses flowing through unaltered sub-basins displayed higher values of functional redundancy and potentially greater stability against human impacts.

Thus, functional redundancy is a promising indicator as it relates positively to stability, resistance and resilience in ecosystems showing also a high sensitivity to stress. The loss of functionally equivalent species causes a decrease in functional redundancy greater than that observed in functional diversity. The most significant implication of these results is that functional redundancy can be used as a valuable early warning before losing ecosystem functions as a consequence of increased stress. Therefore, incorporating functional redundancy into river evaluation and management planning may help us to anticipate the effects caused by the ongoing global change.


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Predicted functional redundancy for riparian communities in the study area (Segura basin).

In addition, an important advantage of the functional redundancy approach is its accurate response to combined stressors. The response of functional redundancy can be predicted for entire river networks, such as the study area (in the image above), constituting a potential useful tool for biomonitoring and environmental management at the basin scale. The predictors used here are low-cost, coarse-grain variables that are easy to obtain from digital maps and environmental databases. The resulting map can be used to detect more impacted river reaches and improve them through restoration measures, as well as to conserve the reaches with better functional conditions. Therefore, although the sensitiveness of functional redundancy to human impact must be specifically compared with other traditional biomonitoring tools, it can be considered as an ecologically-sound measure able to detect ecological responses to single and multiple stressors.

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