Environmental Engineering Reference
In-Depth Information
Evenness
indices accomplish this by scaling the heterogeneity indices to a
theoretical maximal value of diversity when all species are equally repre-
sented in the sample. In this case, large index values imply that the species
are equally represented or equally abundant in a sample; small index values
imply that there are a few species that are highly abundant and many that
are rare. Ultimately, the choice of index to describe diversity depends largely
on whether one is interested in emphasizing common species or rare species
in an area.
The Ecology of Rarity
Each of these indices share one important feature:They are devoid of any
biology other than measures and weightings of simple relative abundance.
This can be problematic, especially for those indices that attempt to give
greater weight to rare species.The problem arises because species rarity, in
conservation, is often equated with being particularly fragile or threatened
by humans and thus deserving of conservation concern.Yet there are many
ecological reasons why particular species might be rare. So, viewing all rare
species as a target for conservation concern can mislead conservation efforts
(Rabinowitz et al. 1986).
Rabinowitz et al. identify seven different causes for species rarity based
on contingency among three features of species populations: (1) their geo-
graphic range, (2) their habitat specificity, and (3) their local population size.
Within this contingency, one might find that, at one extreme, a species has
a broad geographic range but is rare within that range because it has very
specific habitat requirements. A case example is the woodland caribou
(
Rangifer tarandus caribou
), which occurs throughout the Canadian boreal
forest but in extremely low population densities.This species is vulnerable
to predation, competition with other ungulate species, and disturbances
(Courtois et al. 2004).Woodland caribou especially prefer mature and over-
mature lichen-covered conifer stands with irregular structure.These habi-
tats are less suitable for other ungulates (Courtois et al. 2004). Moreover,
such habitats tend to be widely dispersed on the landscape into isolated,
small pockets that can only support low local population densities. Caribou
also roam widely across the landscape as they move from pocket to pocket
of habitat.This limitation on population size, together with the roaming be-
havior also reduces encounter frequency with natural predators (Courtois
et al. 2004). Rarity due to a preference for a widely dispersed habitat type
and migration among those habitat pockets may in this case simply be a
