Assessing ecological integrity

June 3, 2018

Lake Trout by Tim Bartley

The integrity of ecosystems has been a focal point for environmental scientists and managers for the past 50 years.  'Ecosystem integrity’ can be thought of as the ability of an ecosystem to support and maintain ecological processes (such as the water cycle) as well as a diversity of living organisms.  When an ecosystem’s integrity is diminished, the capacity of that ecosystem to support life is also diminished (Karr, 1993).

Ecosystems are extremely complex and therefore environmental professionals interested in assessing an ecosystem's integrity often rely on indices or indicators to provide an estimate.  One of the earliest indices, the Index of Biotic Integrity (IBI), has been used extensively since the 1980s and was developed by Karr (1981) to evaluate aquatic ecosystems.  The index uses a number of biotic variables, in this case, variables related to fish species, to calculate an estimate of integrity.  Variables include abundance of fish, number of fish species, and proportion of species tolerant to human disturbance.  The IBI has been criticized over the years for issues such as a lack of objectivity and statistical rigor.  Due to the importance of having an accurate and reliable estimate of ecosystem integrity, several variations on the IBI have been developed (Capmourteres et al., 2018).

One such alternative approach was elaborated in a recent paper by Capmourteres et al. (2018).  In order to compare against the results of the IBI, the authors used most of the data Karr (1981) employed with some additional measures, all of which were based on waterbodies in Northeastern Illinois.  Data included biotic variables, for instance, number of fish species, abundance of fish, proportion of hybrid species, trophic levels, proportion of intolerant species, and proportion of native and exotic species.  Environmental variables were also used:  water quality (e.g., fluoride, total phosphorus, ammonia, pH, water temperature, dissolved oxygen) and the proportion of area covered by forest and wetlands versus urban areas within the vicinity of locations where fish were captured.  The authors encountered some issues when calculating the IBI including obtaining the same IBI value for ecosystems that are, in reality, at differing levels of ecological integrity, index values that mask the potential influence of exotic species, and conflicting IBI scores (e.g., presence of a species that could indicate both high and low integrity).

With the help of a statistical technique commonly used in community ecology called 'canonical correspondence analysis' (CCA), Capmourteres et al. (2018) were able to identify patterns in the biotic data and how these related to patterns in the environmental data. The results from CCA served as a guide for 'structural equation modelling' (SEM) - a relatively new approach for addressing complex ecological questions where multiple variables are at play. Overall, the analyses showed that water quality influenced variables associated with the diversity of fish species and also the abundance of fish. These effects translated to changes in trophic interactions between fish.  

The authors state that their approach can complement existing indices of ecological integrity, such as the IBI, by addressing some of the issues outlined above, elucidating broad patterns that may exist with multiple ecological variables, and by providing insights into cause and effect relationships. 


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Capmourteres, V., Rooney, N., and Anand, M. (2018) Assessing the causal relationships of ecological integrity: a re-evaluation of Karr's iconic Index of Biotic Integrity. Ecosphere. 9(3):1-19.

Karr, J.R. (1981) Assessment of biotic integrity using fish communities. Fisheries. 6:21-27.

Karr, J.R. (1993) Defining and assessing ecological integrity: beyond water quality. Environmental Toxicology and Chemistry. 12:1521-1531.