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Fattorini et al.
living place of an organism or community, characterized
by its physical or biotic properties”. Thus, a habitat can be
only recognized by referring to the resources needed by a
particular species or to a group of species requiring similar
sets of resources (Dennis 2010, Dennis et al. 2003, 2014).
Thus, “habitat diversity” is an intrinsically ill-formulated
concept, at least as used in current ecological literature,
and should not be used to qualify general features of ar-
eas. A more appropriate expression to be used in lieu of
“habitat diversity” would be “environmental heterogene-
ity” (see Looijen 1998, Hortal et al. 2013, Stein & Kreft
2014), because it refers to the variety of environmental as-
pects that may characterize an area.
What is categorized?
We said that larger areas can host more species because
they tend to provide organisms with more places where to
find necessary resources in quality, quantity and stability.
One of the most commonly measures of “habitat diversi-
ty” is the number of biotopes (e.g., Kohn & Walsh, 1994,
Ricklefs & Lovette 1999), which can be considered a very
basic measure of environmental heterogeneity. While hab-
itats exist only in relation to a species, recognition of bio-
topes is rooted to a particular place, and is therefore ap-
propriate to study if species richness increases with extent
and variety of resources. Biotope, as in the case of a habi-
tat, has also been described in different ways. Some defi-
nitions of biotope relate it to habitat and the two words are
frequently treated as being synonymous (see Dennis 2010;
Dennis et al. 2014). In fact, the two variable terms habitat
and biotope describe very different phenomena. Habitat,
defined as that collection of resources and conditions that
ensures the persistence of a population at a site (resource-
based definition), is a species-specific term and can be a
very complex physical construct. By contrast, biotope, de-
fined as a region (area, space) that is distinguished by par-
ticular environmental conditions, is a community-specific
term and is a relatively simpler concept to apply. For this
reason, biotopes are frequently used as surrogates for hab-
itats, although this can be dangerous, because dimensions
of the biotope may exceed that of the habitat space or it
may fail to embrace all of resources and conditions needed
by a species (see Dennis et al. 2014).
Although a big effort has been spent to reach some con-
sensus for the recognition of broad biotope types (e.g., the
European Corine Land Cover classification), definitions
and identification criteria of biotopes still vary consider-
ably among different studies. For examples, biotopes have
been identified in terms of land cover categories, land-
scape units, vegetation types, soil types, etc. depending on
the spatial scale and on the target taxon. Biotopes are not
the only aspect of the environment that can be used to de-
fine environmental heterogeneity. Variability of edaphic
and climatic factors, elevational ranges within geographi-
cal units (e.g. cells or islands), precipitation and tempera-
ture ranges, depth of litter, size of rocks, type of substrates,
2
etc. can all determine environmental heterogeneity (Solem
et al. 1981, Ricklefs & Lovette 1999, Schoener et al. 2001,
Kocher & Williams 2000, Stein & Kreft 2014). The choice
is not trivial, and will necessarily vary according to data
availability, and to the hypotheses being tested in a partic-
ular study.
What is measured?
Most studies dealing with “habitat diversity” tend to indi-
cate the number of biotopes as the “number of habitats”.
When using biotopes to measure environmental heteroge-
neity, usually ecologists focus on their number. Howev-
er, this approach may not be the best choice. For exam-
ple, it does not consider the relative extent of different bio-
topes, an aspect that can be of fundamental importance in
sustaining viable populations. In addition, two areas may
host the same number of biotopes, but they might differ
in the relative area and spatial arrangement of these bio-
topes, thus determining different levels of environmental
heterogeneity. Thus, some authors have used a “habitat di-
versity index” (HD) which takes into account the relative
area covered by each “habitat” (= biotope) thus weight-
ing the relative contribution of each biotope (e.g. Rick-
lefs & Lovette 1999, Fox & Fox 2000). Advances made
in the field of community ecology could be helpful to this
purpose. It has been long recognized that species number
cannot fully describe the diversity of a community. For
this, ecologists use a set of indices to quantify diversity,
not only in terms of richness but also in terms of species
dominance and evenness (equitability). Most of these in-
dices are not specific for biological communities, but can
be also applied to quantify environmental diversity, dom-
inance and evenness. Yet, up to date, this idea has been
little pursued in biogeographical research and the number
of biotopes still remains the most widely used measure of
environmental diversity (see Fattorini 2006a; Tognelli &
Kelt 2004, Triantis et al. 2005, 2006, Hortal et al. 2009,
and references therein). Shannon index (or its modifica-
tions) is commonly used in landscape ecology (see Farina
1998, Luoto et al. 2001, Burel & Baudry 2003, Dušek &
Popelková 2012 for a discussion) and, less frequently, in
biogeography (Lobo & Martín-Piera 2002, Nogués-Bravo
& Martínez-Rica 2004, Stefanescu et al. 2004, Maes et al.
2005). However, there are several other ‘neglected’ indi-
ces developed in community ecology which could provide
important information to quantify environmental diversity
in biogeographical analyses.
The expression “environmental diversity” has been
sometimes used as synonym of “habitat diversity” (e.g.
Triantis et al. 2005, 2006), and the use of “habitat diver-
sity” has been defended on the basis of its relatively wide
use (Panitsa et al. 2006). Like for species richness, also
“environmental diversity” can be better expressed in terms
of richness, evenness and dominance. Thus, a more gen-
eral expression, such as “environmental heterogeneity”
would be preferable (considering that dominance indi-