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Fragmenta entomologica, 47 (1): 1-14 (2015) eISSN: 2284-4880 (online version)
pISSN: 0429-288X (print version)
Research article

Submitted: March 28th, 2015 - Accepted: May 25th, 2015 - Published: June 30th, 2015

Calling for a new strategy to measure environmental (habitat) diversity in
Island Biogeography: a case study of Mediterranean tenebrionids
(Coleoptera: Tenebrionidae)

Simone FATTORINI 1,*, Leonardo DAPPORTO 2, Giovanni STRONA 3, Paulo A. V. BORGES 1

1 CE3C, Centre for Ecology, Evolution and Environmental Changes/Azorean Biodiversity Group and Universidade dos Açores -
Departamento de Ciências Agrárias - 9700-042 Angra do Heroísmo, Açores, Portugal - simone.fattorini@gmail.com; pborges@uac.pt
2 Department of Biological and Medical Sciences, Oxford Brookes University - Headington, Oxford OX30BP, UK - leondap@gmail.com
3 European Commission, Joint Research Centre, Institute for Environment and Sustainability - Via E. Fermi 2749, 21027 Ispra (VA),

Italy - giovanni.strona@jrc.ec.europa.eu
* Corresponding author

Abstract
Many recent researches in island biogeography attempted to disentangle the effects of area per se and “habitat diversity” on species rich-
ness. However, the expression “habitat diversity” in this context should be avoided, because habitats can be only recognized by referring
to the resources needed by a particular species. What is really measured in such researches is some form of “environmental heterogene-
ity”. Although habitat heterogeneity can be measured in various ways, most researches in island biogeography simply used the number of
biotopes (typically classified as land cover categories). However, not all biotopes have the same surface.On the basis of the area occupied
by each land cover category, it is possible to calculate indices of environmental diversity, evenness and dominance, as commonly done
in community ecology research. These indices can be used to investigate the role of environmental diversity in determining species rich-
ness. We used the tenebrionid beetles inhabiting twenty-five small islands around Sicily (Central Mediterranean) to illustrate these con-
cepts. We found that both area per se and environmental heterogeneity contributed to determine species richness. Moreover, we found
that the relationship between species richness and environmental homogeneity followed a power function model. This indicates that en-
vironmental homogenization may determine a rapid, non linear decline in species richness.

Key words: diversity, environmental heterogeneity, evenness, habitat hypothesis, structural equations, Sicily.

Introduction versity” hypotheses and the best analytical procedures to
test their (relative) magnitude.
The species-area relationship, i.e. the law modelling how In this paper, we describe a comprehensive framework
the number of species in a community increases with area, to deal with these issues, focusing on a group of island in-
is one of the best documented patterns in island ecology sects as a case study.
(Whittaker et al. 2008, Whittaker & Triantis 2012, Trian-
tis et al. 2012). Larger islands may host more species be- What is a habitat?
cause they provide larger targets for dispersing individuals The expression “habitat diversity” refers to the idea that
and, since they can support larger populations, the extinc- a region can be partitioned into a number of habitats, and
tion rates for individual species become lower (MacArthur that each of them can be occupied by a subsample of the
& Wilson 1967, Ricklefs & Lovette 1999; Whittaker & total number of species living in the region as a whole.
Fernández-Palacios 2007). However, since also the extent Larger areas are more likely to host more habitats, and
and variety of resources a species can use increase with hence more species.
area, one could not exclude that resource availability, and Actually, many authors describe habitats in terms of
not area, is the key to species coexistence (Rosenzweig environmental features occurring in specific areas (e.g.,
1995, Ricklefs & Lovette 1999). This additional explana- number of plant associations, number of soil types, veg-
tion is known as the “habitat diversity” hypothesis, and etation structure, land-uses, climate, etc.). The concept of
has received great attention in several recent studies (e.g., habitat has been defined in various and contrasting ways.
Triantis et al. 2005, 2006, Hortal et al. 2009, 2013 for re- For example, Dennis (2010) lists nine different definitions
views). However, most of them have highlighted various given between the 1960s and the 2000s. According to the
problems in unraveling the “area per se” and “habitat di- Dictionary of Ecology (Allaby 2010), the habitat is “The

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