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Measuring influence of environmental heterogeneity on island beetles
when direct and indirect effects were summed (a1b1+b2 = potential array of features that can be used to express en-
0.889), was greater (1.57 times) than the effect of environ- vironmental heterogeneity is virtually infinite. Thus, the
mental heterogeneity (b1), although less markedly than for characteristics to be measured must be reduced to a subset
the aforementioned indices of diversity or evenness. chosen by researchers according to their subjective per-
In summary, the use of various measures of environ- ception of the environment. Although an enormous vari-
mental heterogeneity led to different results as regards the ety of environmental characteristics can be easily meas-
importance of direct and indirect effects of area. Howev- ured using standardized techniques, this does not imply
er, although the total effect of area was invariably greater that every approach to measure environmental heteroge-
than the effect of environmental heterogeneity, area exerts neity is appropriate for every group of organisms. On the
from 24.7 to 58.1% of its overall effect on species number other hand, since environmental heterogeneity is measured
through a powerful effect on environmental heterogeneity/ independently from species requirements, comparing the
homogeneity, which indicates that environmental hetero- effects of different measures of environmental heterogene-
geneity is actually important in determining tenebrionid ity on species richness (or other measures of alpha diversi-
diversity. Yet, the direct influence of area on species rich- ty) of a given animal group may help identifying which as-
ness was higher than the direct effect of environment when pects of the environment are more important for the target
maximum elevation and dominance indices were used, it species assemblages (e.g. guilds or communities). In the
was similar when Margalef richness index was used, and it study case presented here, we considered various meas-
was lower when diversity or evenness indices were used, ures of environmental heterogeneity: maximum island ele-
and it was much lower when number of land cover was used. vation, number of land cover categories, and a series of in-
dices that use the proportional surface extent of land cover
Discussion categories to express their richness, proportional diversity,
evenness and dominance. It is well known that larger is-
Many studies attempted to assess the importance of envi- lands show higher levels of environmental heterogeneity
ronmental heterogeneity in explaining variations of spe- (Harner & Harper 1976, Rafe et al. 1985, Gibson 1986,
cies richness in different areas and animal groups (e.g., Rosenzweig 1995). We confirmed this trend here since all
Buckley 1982, Rafe et al. 1985, Deshaye & Morisset the measures of environmental heterogeneity/homogene-
1988, Kohn & Walsh 1994, Tjørve 2002). However, most ity we used were correlated with island area. A strong link
of these researches are hampered by two main problems: between environmental heterogeneity and area can sim-
first, habitat requirements may differ enormously among ply result as a probabilistic phenomenon leading to a more
species; second, there is no consensus among ecologists likely occurrence of rare biotopes or landscapes in larger
about the definition and recognition of habitats (Rosenz- areas (Whitehead & Jones 1969, Kohn & Walsh 1994).
weig 1995, Hall et al. 1997, Dennis et al. 2003). The latter The same may hold for other environmental categoriza-
problem is largely a reflection of the first one, because hab- tions: for example, elevational range, which tends to in-
itats can be defined, and hence recognized, only with refer- crease with island area (Fattorini 2002a,b, Steinbauer et al.
ence to species, with each species having its own habitat. 2013). Altitude, in turn, determines different climatic lev-
Moreover, a given guild may have very specific environ- els and high variability in sun exposure, slope, and geolog-
mental requirements but select slightly differentiated mi- ical structure. The complex intercorrelation between area
cro-biotopes, so that habitat categorization may have to be and environmental heterogeneity makes it difficult to dis-
adapted accordingly (such as in the case of peculiar cases, entangle the relative importance of these factors in regulat-
as those of canopy and cave animals). ing species richness. Typically, structural equation models
We agree in the use of the word “habitat” to indi- are suggested as a good statistical way to distinguish the
cate the resources used by a species as recommended by role of multiple collinear variables (such as area and en-
Dennis and coworkers (Dennis 2010; Dennis et al. 2003, vironmental heterogeneity) in respect to various response
2014), and propose to use “environmental heterogeneity” variables (Sokal & Rohlf 1995, Grace & Pugesek 1997,
instead of “habitat diversity” to express the environmen- 1998, Legendre & Legendre 1998, Triantis et al. 2005).
tal complexity of an area and to any kind of variability in The results of structural equation models for the tene-
environmental characteristics that can affect species pres- brionid richness on the circumsicilian islands indicate that
ence. This includes the range of environmental conditions the variance in species richness explained by the intercor-
(which can include number of biotopes, landscape units, relation of area and environmental heterogeneity, and en-
plant associations, soil types, etc. and their proportional vironmental heterogeneity and area separately, vary sub-
contribution), as well as their spatial configuration, and stantially according to the measure used. When expressed
their variation over time. as number of land cover categories, habitat heterogeneity
While the habitat is a characteristic of the species, overwhelmed the influence of area, whose influence be-
these measures of environmental heterogeneity depend came statistically not significant, while in all other cases
on the characteristics of the environment. However, the both area and environmental heterogeneity exerted a sig-
nificant effect. This may suggest that tenebrionid beetles
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