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Fattorini et al.
are more sensitive to the number of land cover categories
existing on an island than to their proportional extent. Ten-
ebrionids are usually detritivorous insects, and most spe-
cies can exploit a number of different biotopes (e.g. under
bark or stones, on foliage, into ant, mammal and bird nests,
into the sand of river banks and coastal dunes, etc.) in sev-
eral kinds of land cover categories. Even if land cover cat-
egories used in this study are coarse in comparison to the
insect scale , each category have different keystone struc-
tures (e.g., microclimate conditions, food resources, and
soil characteristics). Thus, the number of biotopes that can
be used by tenebrionids increases with the number of land
cover units, but this effect is only slightly affected by the
extent of each land cover unit. This happens because, even
when a certain land cover category covers a small surface
at the scale of island area, this would still be sufficient to
include a good number of biotopes at the scale of the in-
sect. Looking at the percentage of variance in species rich-
ness explained by area and number of land cover catego-
ries, and the correlation between area and land cover cat-
egories, it is clear that the number of land cover categories
is highly related to area, but also explains something more
for Tenebrionidae. In this respect, the number of land cov-
er categories encompasses all the variance explained by
the area as a correlate.
When using indices of proportional richness, diversi-
ty, evenness and dominance to express environmental het-
erogeneity/homogeneity, we found that tenebrionid rich-
ness correlated positively with environmental heterogene-
ity (richness, diversity and evenness), and negatively with
environmental homogeneity (dominance). This means that
environmental heterogeneity promotes species richness,
whereas homogenization has a negative impact. The posi-
tive relationship between environmental heterogeneity and
tenebrionid richness is also in accordance with the fact that
landscape diversity typically leads to an increment in the
proportion of generalist insect species (Jonsen & Fahrig
1997, Krauss et al. 2003).
Kadmon and Allouche (2007) suggested that species
richness should follow a hump-shaped distribution in rela-
tion to increasing biotope numbers (‘habitat diversity’ ac-
cording to authors’ use). According to this model, species
diversity initially increases with number of biotopes, from
a very simple island into a biotope-wise more complex is-
land, until a maximum species richness is reached, and
then declines because more biotope types reduce the total
area of any single biotope type, in turn reducing the suit-
able biotope for any given species. This model has been
however criticized by Hortal et al. (2009, 2013), who, in
contrast, found that species richness on islands usually in-
creases with number of “habitats” (=biotopes) and that it
never decreases. Results obtained from the tenebrionids of
the circumsicilian islands support the findings of Hortal et
al. (2009, 2013), not only as regarding for the number of
landscape units, but also for the various measures of envi-
ronmental diversity. However, relationships between spe-
8
cies richness and environmental heterogeneity or homo-
geneity were not linear, as in the models used by Hortal et
al. (2009, 2013), but follow a power function model. This
indicates that, at least in our study, environmental homog-
enization may determine a rapid, non linear decline in spe-
cies richness. These findings may have important implica-
tions in conservation biology, which could be better clari-
fied by future research applying our analytical framework
to other archipelagos and biota.
Aknowledgments – We are grateful to P. Leo and P. Lo Cascio
for discussion about the taxonomic status of certain populations
of tenebrionid beetles on the circumsicilian islands. We would
like to thank two anonymous referees for providing us with con-
structive comments and suggestions on a previous version of the
paper.
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