Alejandro Baladrón Julian, M.S.
Did you know that small amounts of pollution can dramatically change the species composition of a natural habitat? Biodiversity indicators are excellent tools for assessing which way landscape units are heading in terms of habitat quality. Biodiversity can be easily calculated by using rapid assessment indicators such as species richness and species diversity indices. By comparing results across landscape units, we can predict biodiversity patterns, detect management gaps in natural areas, or implement appropriate conservation planning guidelines. However, results obtained through the use of rapid assessment indicators require cautious interpretation. Sometimes it is assumed that habitats exhibiting very high values of species diversity are of excellent quality. However, that assumption is not always correct.
Understanding Disturbance & Species Diversity
All species have tolerance limits for environmental factors beyond which individuals cannot survive, grow, or reproduce. Disturbance sources, such as pollution and land use changes, can disrupt normal ranges of environmental factors affecting biological communities. Biomonitoring protocols help measure the quality of habitats by studying the attributes of biological communities, including their diversity in terms of species taxa or functional groups. However, the relationship between biodiversity and intensity or frequency of disturbance is not always linear, as is depicted in the parabolic graph (Fig.1) of the Intermediate Disturbance Hypothesis.
Fig. 1. Graph showing variation of diversity with disturbance according to the Intermediate Disturbance Hypothesis: a) low levels of disturbance allow competition reducing diversity; b) greatest biological diversity occur at intermediate levels of disturbance as a result of loss of the most sensitive species and invasion of opportunistic species; c) high levels of disturbance reduce diversity.[/caption]
Low Intensity Disturbance
Physical disturbance prevents a competitively dominant species from excluding other species from the community, and there is a trade-off between a species’ ability to compete and their ability to tolerate disturbance. For example when disturbance conditions in a river are of low-intensity or infrequent, species diversity may also be low because only the best competitors persist. Headwater streams are usually good places to find highly specialized pollution-intolerant taxa including giant stoneflies and flathead mayflies, as long as there are no cattle or other sources of disturbance.
High Intensity Disturbance
When disturbances have a high intensity, diversity is reduced through the loss of species that are particularly sensitive to disturbance, which tend to be native species well-adapted to pristine conditions. For instance, streams draining heavily urbanized catchments are almost always exposed to high disturbance regimes due to frequent pollution loads entering the streams through sewer systems. As a result, macroinvertebrate communities in urban streams are usually dominated by large numbers of pollution tolerant taxa including tubificid worms, leeches, moth flies, and rattailed maggots.
Moderate Intensity Disturbance
Disturbance scenarios of moderate intensity have been widely discussed in scientific literature because of their implications for species diversity. At intermediate levels of disturbance, there is a balance between competitive exclusion and loss of competitive dominant species due to disturbance. In this scenario, conditions favor the coexistence of competitive species and disturbance-tolerant species. As a result, a peak in diversity may occur at intermediate intensities of disturbance.
Streams affected by moderate intensity disturbances can show an array of native aquatic invertebrates. The majority are usually competitively dominant, with the addition of a few opportunistic species, which are usually benefited by increased disturbance. Ecologists have observed certain streams located on agricultural lands with the ability to support a high diversity of aquatic invertebrates, as long as they are not affected by aggressive practices like the excessive use of pesticides in row crops.
When we evaluate water quality conditions of streams by analyzing macroinvertebrate-based bioindicators, higher diversity may be found in slightly polluted sections of the stream rather than in pristine sections, where we may find only native species. Therefore we may incorrectly conclude that overall habitat quality is higher in polluted sections like those affected by stormwater outfalls. An incorrect interpretation of biodiversity results would prevent us from providing an accurate stream assessment of water quality status.
Analyzing and Interpreting Biodiversity Data
Rapid assessment biodiversity indicators, such as species richness and evenness, can significantly reduce costs of environmental studies and are strongly recommended for pilot or reconnaissance studies. However, their capacity to detect differences in habitat quality is sometimes limited and may not fulfill the most demanding project goals. Rapid assessment procedures can be substituted with studies based on hypothesis testing, replicate samples, and multivariate analysis approaches which increase the accuracy of results but also raise project costs.
It is also possible to increase the reliability of results and reduce projects costs by using multiple diversity and community composition indices to assess habitat quality. For example, Great Ecology uses a comprehensive set of benthic metrics to periodically assess the quality of restored open water habitats and tidal areas at the Woodbridge Waterfront Park projectsite in New Jersey. Benthic metrics are measurements of the structure, function, or other characteristics of the macroinvertebrate community that change in some predictable way to increased disturbance. Our set of benthic metrics includes measures of species richness, diversity, dominance, presence/absence of tolerant or intolerant species, functional feeding groups, and macroinvertebrate-based indicators of water quality. A broad spectrum of metrics provides more accurate information to address our clients’ restoration goals as compared to using just species richness and diversity indices. Understanding what makes a quality habitat requires an awareness of local disturbances and a comprehensive knowledge of the best practices to approach each particular case.
About the Author
Alejandro Baladrón Julian is an environmental scientist specializing in hydrology. He has extensive experience performing Environmental Impact Assessments and hydrology modeling, including the design of drainage networks for highway and railway lines, the analysis of evacuation capacity in wastewater drainage systems, and flood risk studies.