Here, Peter G. Kevan, School of Environmental Sciences, University of Guelph, with Charlotte Coates, explores the issue of measuring ecosystem health (no longer a metaphor) and functionality against biodiversity and how this could be used in environmental policy
The idea of ecosystem health has a had a chequered history with sometimes vociferous debate as to its meaning: it has had the hallmarks of being a debacle. We explain that the idea has merit in biology, biodiversity, ecology, evolution, environmentalism and policy and can be used objectively for temporal and spatial comparisons. Although ‘debacle’ is often used to describe some chaotic failure, it has an ecological (and rather Canadian-relevant) meaning of breaching an ice dam on a waterway and the ensuing spate.
Biodiversity and the functionality of ecosystems seem to go hand-in-hand as both are diminishing as human pressures increasingly afflict the world’s biota. Methods to reverse the alarming trends remain problematic. The close relationship between biodiversity and ecosystem function is not questioned by ecologists. However, measuring that relationship as a diagnostic tool to assess both together is not well considered.
Several indices can measure the diversity of the biota in any ecosystem. Alpha-diversity is usually expressed as the number of species in a particular area, i.e., species richness. Beta-diversity is defined by the difference in species richness between areas at a given time or within areas across different times. Gamma-diversity simply increases the geographic scale of species richness. None of which address the complex problems of ecosystem functionality. Numerous ecological indices include the numbers of individuals of each species present in a particular place or time.
Those indicators’ calculation, assessment, and comparisons have become part of the established ecological methodology. Specifically, ecologist Evelyn C. Pielou researched the relationship between biodiversity and ecosystem function, i.e., diversity, abundance and activity of interacting organisms. Ecology has been defined in various ways; but let us consider Pielou’s definition: “Ecology is the study of the diversity, abundance, and activities of life.” Ecosystem functionality must encompass all three aspects.
How might we measure or compare ecosystem functionality in a way that provides a diagnostic tool for ecologists, conservationists and policy? The concept of ‘ecosystem health’ excited debate among ecologists in the 20th Century even though the idea, as a metaphor, existed since Aldo Leopold a century before. At times, the debate became quite acrimonious with views ranging from the extreme that ‘health’ and ‘ecosystem’ should not occur in the same sentence, let alone in juxtaposition to interest in how one could diagnose the ecosystemic condition.
Some of the attempts to assess ecosystemic conditions are circular. For example, eutrophication indicates poor ecosystem health and vice versa: i.e., the diagnostic criterion is symptomatic. Resilience, whereby ecosystems can self-restore after the stress has ended, is not in itself a measure of ecosystem health, just of recovery.
Ecosystem health
The term ‘ecosystem health’ is still regarded as a ‘metaphor’ by many, despite the erroneous contentions of others. Attempts to assess ecosystem health with objectively independent tools and provide comparisons between ecosystems that are hypothesized to be healthy vs. unhealthy removes the concept from the realm of metaphor. There are diagnostic tools to measure the health of individuals and populations, notably by considering the rates of affliction, as in epidemiology. It can be postulated that departures from a uniform, standard characteristic of all ecosystems can be measured and compared as a diagnostic tool for assessing ecosystem health.
The classic log-normal relationship describes how diversity and abundance interact and provides a model for deeper consideration than it has usually received in ecology. The log-normal relationship describes the biodiversity of a given ecosystem as having a few abundant species, increasingly more species with lessening abundance and a diminishing number of rarer species.
If ecologists sample ecosystems with the log-normal relationship with diversity and abundance in mind, it is axiomatic that the collection of the rarest species becomes unlikely. This effect is well known to ecologists sampling environments and finding a species saturation curve: further sampling yields more species. However, the curve approaches an asymptote as the sampling effort increases.
The log-normal relationship also describes ecosystem activity, represented by the relative abundances of the various species, i.e., the outcome of their interactions through cooperation for resources. That is, the assemblage comprises species that have varying degrees of niche overlap. The log-normal curve is far from static as far as biodiversity and abundance are concerned, but the form of the curve (abundance) would be similar regardless of the species present. Some have casually dismissed the log-normal relationship as a simple outcome of the Central Limit Theorem (CLT) despite being beneficial in describing ecological relationships. Conversely, the log-normal distribution of diversity and abundance has been presented as having fundamental meaning in evolutionary ecology. Perhaps the outcome of evolutionary and ecological processes can be considered as an outcome of the CLT?
When the log-normal relationship is regarded as a natural outcome of ecological and evolutionary processes, it must be assumed that the species present are interacting. The species must share resources and activities to influence each other. Ecologists use various terms (e.g., taxocene (members are taxonomically related), guild (members share roles) to describe such assemblages, but in log-normal relations, there is an assumed hierarchy of niche overlap regardless of taxonomy or a particular role, but no two species occupy the same unique niche.
In co-evolving ecosystems, it is logical to suppose that species assemblages would follow the log-normal distribution of diversity and abundance by a hierarchy of niche overlap regardless of the seral stage (except species-poor and simple stages) and species present. When ecosystems become perturbed, naturally, or unnaturally, the abundances of species change. During perturbation, the log-normal relationship of species abundance and diversity may change, thus substantiating symptoms of ill-health. That does not require that ill-health results in a change in the log-normal relationship.
A salient example of the use of the log-normal relationship in assessing ecosystem health is that of the assemblage of pollinating bees on lowbush blueberry heaths in the Maritimes under the stress of insecticide use to protect nearby forests from spruce budworm. Kevan et al. (1997) found that the bee assemblages on unaffected heaths were log-normal for diversity and abundance, but those affected by insecticide were not. After insecticide use finished in a particular area, the assemblage of bees became log-normal after a year or two.
Ecosystem stress caused by insecticide use resulted in downstream effects in diminished blueberry production to the extent that province-wide (New Brunswick vs. Nova Scotia and Maine) economic losses were invoked (Kevan 1977; Kevan and Oppermann 1980). Based on those findings, and with the Ecosystem Health program involvement at the University of Guelph, methods of measuring ecosystem health through a diagnostic tool were discussed. An emergent tool was discovered by re-evaluating the log-normal relationship of diversity and abundance in the context of ecology and evolution.
Ensuring effective environmental policy
It is a great hindrance to effective environmental policy that there remains a bias based on a misunderstanding about ecosystem health and how it can be measured. The log-normal relationship of species diversity and abundance is probably one of many ways. Pollination can illustrate many problems in ecosystem sustainability in natural and managed environments (IPBES 2017). The term ‘ecosystem health’ has been applied, notably regarding policy issues (IPBES 2016). Even so, the terminology used is lax.
Those attempted definitions often suffer from vague terms, such as ‘balanced’ or ‘natural’. The result is that legal criteria are open to interpretation by regulators and the regulated, meaning the status quo is preserved regardless of its ongoing ecological effects. Inevitably, such loose definitions have led to mixed and inconsistent results, making it difficult for biologists and policy-makers, to compare issues in meaningful commensurate ways reliably. Even though pollination has provided a rigorous model for defining and addressing ecosystem health utilising objective and established ecological principles, the methods have been largely overlooked.
References
- Belaoussoff S and Kevan PG. 1998. Toward an ecological approach for the assessment of ecosystem health. Ecosystem Health, 4: 4–8.
- IPBES. 2016. The assessment report of the intergovernmental science-policy platform on biodiversity and ecosystem services on pollinators, pollination, and food production. 40 pp. Summary for Policy Makers. Bonn: Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. https://www.researchgate.net/publication/310132044_IPBES_2016_Summary_for_policymakers_of_the_assessment_report_of_the_Intergovernmental_Science-Policy_Platform_on_Biodiversity_and_Ecosystem_Services_on_pollinators_pollination_and_food_production_2016#fullTextFileContent
- IPBES. 2017. The assessment report of the intergovernmental science-policy platform on biodiversity and ecosystem services on pollinators, pollination and food production. Bonn: Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, 556pp. https://doi.org/10.5281/zenodo.3402856 or https://www.ipbes.net/sites/default/files/downloads/pdf/individual_chapters_pollination_20170305.pdf.
- Kevan PG, Greco CF and Belaoussoff S. 1997. Log-Normality of biodiversity and abundance in diagnosis and measuring of ecosystemic health: Pesticide stress on pollinators on blueberry heaths. Journal of Applied Ecology, 34: 1122–1136.
- Kevan PG. 1977. Blueberry crops in Nova Scotia and New Brunswick – pesticide and crops reduction. Canadian Journal of Agricultural Economics, 25: 61–64.
- Kevan PG and LaBerge WE. 1979. Demise and recovery of native pollinator populations through pesticide use and some economic implications. Proceedings of the 4th International Symposium on Pollination. Maryland Agriculture Experimental Station Special Miscellaneous Publication, 1: 489–508.
- Kevan PG and Oppermann EB. 1980. Blueberry production in New Brunswick, Nova Scotia and Maine: a reply to Wood et al. Canadian Journal of Agricultural Economics, 28: 81–83.
- Pielou EC. 1975. Ecological Diversity. John Wiley & Sons, New York.
- Pielou EC. 1977. Mathematical Ecology. Wiley-Interscience Publications, John Wiley & Sons, New York.
Acknowledgements
This is a modified version of an article that originally appeared in the Newsletter of the Biological Survey of Canada 28 Volume 40 (1) Summer 2021 pp. 28-30. The authors retain the copyright of the material published here. Permission has been granted to publish this version of the article in Open Access Government.
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