The notion and utility of indicator species in insects

I have just returned from the International Congress of Entomology in Foz do Iguasu, Brazil; the theme of this quadrennial congress of world leaders in entomology was “Biodiversity: challenge for the third millennium”. As such, there was a great deal of discussion (formal and informal) about the issues of inventorying arthropod biodiversity and the proper implementation of the concept of indicator taxa.

The discussion of inventory revolved around the usual issue of the limiting problem of available taxonomists to do the appropriate identifications, which requires in most instances the descriptions of heretofore unnamed taxa. All of this discussion was focused on “third world” tropical biodiversity issues. To be quite frank, the bandwagon mentality here was simply to generate lengthy species lists – a highly questionable political strategy at the very least, one that might work in the tropics but would not be politically acceptable in North America. Scientists at the British Museum of Natural History (London) seem to be the most organized driving force behind this type of approach; they have published (or are publishing) several books to try to establish programmatic approaches to systematized inventory methodology in order to compare disparate localities. Especially (and exceptionally) the work out of the British Museum utilizes the additional aspect of a functional ecological approach and categorization of species.

Indicator taxa have been utilized in a number of different ways throughout the world. As such, definition of precise aspects is critical to useful implementation. There are two basic types of “indicator” applications:

A)            using the presence of particular rare species to indicate the supposed possible co-occurrence of additional unrelated rare species that are not inventoried directly (used for delineation of potential nature reserves);

B)            using the local species richness of one taxon to represent the local species richness of total taxa (used to understand the pattern of biodiversity across the landscape, and usually with respect to quantifying the biotic response to a management protocol).

The first type of “indication” is currently particularly argumentative. In the British Isles (the best known biological region of the world) analysis has shown that rare species do NOT have overlapping distributions. The facts are not in dispute, but discussion centers around how these data are to be applied in other parts of the world. No one would argue that a number of rare species would co-occur on the top of Mt. Kilimanjaro considering the whole of East Africa; this is an instance of the uniqueness of a climatological type. However, within a widespread environmental type (or closely similar types) is there co-occurrence of rare species? THIS is the question that the John Lawton (keynote plenary speaker) analysis addresses. Is his analysis applicable to the Pacific Northwest, in particular to the Klamath/Siskiyou? Probably – BOTH, yes and no. One has to examine the causal effects of what makes species rare to understand this issue. The British Isles (100% of their surface area) have been heavily impacted by Homo sapiens for a VERY long time; as such, the occurrence of rare species is a product of both the “original” stochastic distribution of antediluvian rare species and the effect of the pattern of landscape fragmentation over the last 1000 years in particular. Island biogeographic theory predicts that each species will become locally extinct or will colonize a habitat fragment independently of every other species. In the case of rare species in the British Isles the theory and the data agree nicely. However, in the Pacific Northwest, heavy impact by Man has been confined to the lower altitudes and has not affected most localities for more than two rotations. Clearly at lower altitudes, there should be co-occurrence of rare species in the remnant islands of oak forest – simply because the climato-edaphic region has been nearly entirely converted to tramp species; in these regions the oak-forest-adapted species richness should correlate with increasing size of the habitat island and decreasing inter-island distance. In this instance type-A indicator species usage would be valid. In contrast, it could be argued that in the majority of the mixed conifer belt of the Klamath/Siskiyou forest we still have the basically stochastic distribution of pre-colonial rare species and that an indicator species approach would not be valid since the distribution of rare species would be independent (with the exception of climato-edaphically unique regions – i.e., summit of Mt Ashland). It is clear that 99% of land set-asides will continue to be based on the distribution and abundance of charismatic vertebrate species, but it is also clear that vertebrate diversity patterns are not always coupled with invertebrate diversity. Proper ecosystem management calls for indicator (Type-A) analysis to determine if there are regions of invertebrate concern NOT included in previously set-asides for vertebrate diversity (i.e., owls and salmon).