Environmental Engineering Reference
In-Depth Information
habitat types will cause the area affect on diversity to be confounded or
washed-out by the effect of changing habitat and the different kinds of
species associated with those habitats.This is not to downplay the impor-
tance of diversity patterns caused by
the mosaic of habitat types on a land-
scape, called beta (
One ecological pattern that per-
haps comes closest to being law-
like is the relationship between the
size of an area and the diversity of
species within that area, called the
species-area relationship.
Predictive reliability—in our partic-
ular case that increasing size of an
area supports an increasing diver-
sity of species, up to an upper
limit—has profound implications
for conservation and management.
) diversity. But,
beta diversity requires scaling from
local areas or habitats to the land-
scape, a subject that will be treated in
detail in chapter 8.
β
Diversity Indices
There are various indices or measures
of alpha diversity that are used by
ecologists, each of which have differ-
ent kinds of information content.The
most commonly employed diversity
index is called Species Richness. It is a simple count of the number of
species in an area.This index gives equal weighting to all species, whether
they occur frequently and thereby dominate an area or they are rare. Be-
cause it does not account for commonness or rarity, Species Richness can
be conflated by the contribution of rare species to the measure of species
diversity. In many cases, we want to understand the richness of species rel-
ative to their relative abundance.
Ecologists have proposed other diversity indices that combine Species
Richness with various weightings for relative abundance.The first kind of
indices, called
heterogeneity
or
diversity
indices (Krebs 1989), quantify either
the likelihood that two individuals sampled randomly from an area are not
the same species (Simpson's index), or the likelihood that one cannot pre-
dict to which species the next individual collected in an area belongs (Shan-
non-Weiner index). In both cases, larger values of the indices imply more
heterogeneity, and hence diversity, than do smaller values of the indices.
These two indices differ in their sensitivity to the weighting given to rare
species.The Shannon-Weiner index is most sensitive to changes in the num-
ber of rare species sampled in an area whereas Simpson's index is most sen-
sitive to changes in abundant species (Krebs 1989). Finally, ecologists have
long known that natural communities have a few dominant species and
many rare species and so wished to quantify such unequal representation.
