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Arthropods as ecological indicators of disturbance in forest ecosystems

Extracted from the paper “Arthropods as ecological indicators of sustainability in Canadian forests”, by David W. Langor (Natural Resources Canada, Canadian Forest Service, Edmonton, Alberta, Canada) and John R. Spence (Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada), presented at the XII World Forestry Congress.

Since knowledge of forest ecology and the capacity to describe and measure functions that maintain desired forest features are limited, changes in distribution and abundance of organisms can be examined as indicators of change in ecological functioning. Such indicators make it possible to detect and thus mitigate anthropogenic perturbations that threaten “natural” systems, and also to confirm ecological recovery after ecosystem perturbation. Early efforts to use forest fauna as ecological indicators of disturbance in Canadian forest ecosystems have focused primarily on vertebrates. Yet arthropods, mainly insects, have features that make them interesting as potential indicators:

• around two-thirds of faunal species in Canadian forests are arthropods, and they have a diverse range of functions;
• it is inexpensive to gather samples reflective of populations;
• few species undertake large-scale migrations, ensuring that population changes are reliably attributed to local changes;
• their sensitivity to environment makes it feasible to identify which aspects of environmental change are responsible for faunal change.

Development of ecological indicators is a systematic process involving several essential steps, each of which offers significant challenges.

Indicator selection. Effective indicators must be economically and logistically viable and biologically efficacious. In Canada, the greatest challenge in the choice of taxa or assemblages for study is the difficulty of arthropod identification. Consequently, most work has focused on a few relatively well-known groups: litter-dwelling beetles (Carabidae, Staphylinidae) and spiders; saproxylic beetles associated with dead wood; moths and butterflies; and soil-dwelling mites and springtails. Other groups may offer value as ecological indicators, but investigation is often hampered by taxonomic impediments.

Data collection and interpretation. Data are required to understand the range of natural variation (RNV) in species abundance and to elucidate relationships between the indicators and selected abiotic or biotic variables. The RNV serves as a baseline against which to compare responses to anthropogenic disturbances. Recent studies have provided baseline data about RNV in relation to natural disturbances and quantified arthropod responses to human disturbances.

This step presents significant challenges. First, trapping methods and sampling protocols have inherent biases that must be recognized for correct interpretation of data. Second, seasonal variation in arthropod populations is high, so sampling over a short part of the activity season will not allow accurate assessment of the presence/absence or relative abundance of species, nor allow for meaningful comparisons over space and time as required by monitoring goals. Third, the cost of processing samples is high and may limit the scope of projects and slow the accumulation of data. These challenges usually limit arthropod studies to relatively small spatial scales.

Assessment of robustness and repre sentativeness. The usefulness of an indicator depends on its robustness and representativeness. Robustness is a measure of how well the results of small-scale work may be scaled up additively to represent situations at larger scales. Meta-analyses of multiple data sets over increasing scales might be used to test the spatial robustness of indicators. The degree to which an indicator represents responses of other groups may now be examined, as several studies in Canada have measured the responses of multiple taxonomic groups to disturbances. Thus, individual groups can be assessed for their suitability as indicators of broader ecosystem responses. For carabid beetles in particular, sufficient data exist to permit meta-analysis of their robustness and representativeness.

Biomonitoring. Biomonitoring is the systematic assessment of a suite of proven ecological indicators over multiple spatial and temporal scales to detect incipient change in ecosystem structure, function and composition in response to natural and anthropogenic influences. To date, few terrestrial arthropod ecological indicators have been used in monitoring, and the potential for their future application rests on the outcome of the ecological indicator selection process. The indicator value of groups currently under study should be demonstrated before investment is made in the study of new groups, or in the use of arthropods in monitoring programmes.

Use in management. The cost of using arthropod indicators to guide management would be substantial. It may be more econo-mical to focus on habitats (or other ecological surrogates) as indicators. Improving ecological knowledge of arthropod distribution and habitats could lead to much improved ecological classification systems and identification of habitats threatened by forestry activity. However, the congruence between habitat state and species state must be continually verified to ensure that the surrogacy system remains ecologically relevant.

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