Species Diversity Within and Among Ecosystems - Ecological Systems - page 255

Definition of Species Diversity
Species diversity is a function of species richness, the number of species in a given
locality and species evenness, the degree to which the relative abundances of
species are similar [ 1 , 2 ]. While this notion may be easy to conceptualize, it has
proven difficult, and at time contentious, to quantify [ 1 , 2 ]. Commonly used
methods include constructing mathematical indices known as diversity indexes
(the Shannon, Simpson, and Margalef indexes being the most widely used) or
comparing observed patterns of species abundance to theoretical models [ 3 ].
There is no single best metric and often commonly used ones are chosen because
they are familiar and not necessarily because they are the most appropriate [ 1 ]. The
scale of assessment may range from within a single site or habitat (known as a
diversity) to the difference between two or more sites ( b diversity) [ 4 ], which can
then be combined to give g diversity - the diversity of the landscape. Early research
tended to be concentrated on largely a diversity, but there has been a marked
increase in studies investigating b and g diversity during the last decade [ 1 ].
The species richness aspect of diversity is often considered the iconic measure of
biodiversity since it is well defined and aligns with our intuitive sense of the concept
[ 1 , 5 ]. As the species level is the level at which living organisms are most widely
known, species have become the major taxonomic rank for describing biodiversity.
Along with populations, it is also the level that most scientists, managers, and
policy makers use when referring to biological diversity, and it provides a useful
means for both monitoring and priority setting [ 6 ]. As the species level is one of
three fundamental levels of biodiversity (the others are the genetic level and the
ecosystem/community level), species diversity is thus an important component of
conservation. This entry will refer to species diversity in the less restrictive sense, that
is, the variety of species that inhabit our planet.
Introduction
Ever since naturalists began classifying living organisms, there has been no con-
sensus on how best to define a species [ 7 ]; Mayden [ 8 ] listed 24 different named
species concepts and in a partial listing, Mallet [ 9 ] indentified 15. This longstanding
failure of scientists to agree on how to identify species is commonly referred to as
the species problem, with taxonomists being described as either “lumpers” or
“splitters” with the former approach resulting in lower numbers of species and
the latter higher. However, despite this proliferation of definitions, most
taxonomists agree that species represent a distinct genetic lineage, interact with
the environment in similar ways, and are reproductively compatible [ 1 , 5 , 7 ]. More
importantly, whether based on gene flow, ecological separation, or morphological
distinctiveness, most concepts tend to give similar results due to the independent
evolutionary history that has resulted in unique morphological, ecological, and