Direct Measurement Versus Surrogate Indicator Species for Evaluating Environmental Change and Biodiversity Loss

The enormity and complexity of problems like environmental degradation and biodiversity loss have led to the development of indicator species and other surrogate approaches to track changes in environments and/or in biodiversity. Under these approaches particular species or groups of species are used as proxies for other biota, particular environmental conditions, or for environmental change. The indicator species approach contrasts with a direct measurement approach in which the focus is on a single entity or a highly targeted subset of entities in a given ecosystem but no surrogacy relationships with unmeasured entities are assumed. Here, we present a broad philosophical discussion of the indicator species and direct measurement approaches because their relative advantages and disadvantages are not well understood by many researchers, resource managers and policy makers. A goal of the direct measurement approach is to demonstrate a causal relationship between key attributes of the target ecosystem system (for example, particular environmental conditions) and the entities selected for measurement. The key steps in the approach are based on the fundamental scientific principles of hypothesis testing and associated direct measurement that drive research activities, management activities and monitoring programs. The direct measurement approach is based on four critical assumptions:(1) the 'right' entities to measure have been selected, (2) these entities are well known, (3) there is sufficient understanding about key ecological processes and (4) the entities selected can be accurately measured. The direct measurement approach is reductionist and many elements of the biota, many biotic processes and environmental factors must be ignored because of practical considerations. The steps in applying the indicator species approach are broadly similar to the direct measurement approach, except surrogacy relationships also must be quantified between a supposed indicator species or indicator group and the factors for which it is purported to be a proxy. Such quantification needs to occur via: (1) determining the taxonomic, spatial and temporal bounds for which a surrogacy relationship does and does not hold. That is, the extent of transferability of a given surrogate such as an indicator species to other biotic groups, to landscapes, ecosystems, environmental circumstances or over time in the same location can be determined; and (2) determining the ecological mechanisms underpinning a surrogacy relationship (for example, through fundamental studies of community structure). Very few studies have rigorously addressed these two tasks, despite the extremely widespread use of the indicator species approach and similar kinds of surrogate schemes in virtually all fields of environmental, resource and conservation management. We argue that this has the potential to create significant problems; thus, the use of an indicator species approach needs to be better justified. Attempts to quantify surrogacy relationships may reveal that, in some circumstances, the alternative of direct measurement of particular entities of environmental or conservation interest will be the best option.