30 Sep 2003 15:53 AR AR200-ES34-18.tex AR200-ES34-18.sgm LaTeX2e(2002/01/18) P1: GCE
10.1146/annurev.ecolsys.34.011802.132419
Annu. Rev. Ecol. Evol. Syst. 2003. 34:487—515
doi: 10.1146/annurev.ecolsys.34.011802.132419
Copyright c© 2003 by Annual Reviews. All rights reserved
First published online as a Review in Advance on August 14, 2003
EFFECTS OF HABITAT FRAGMENTATION ON
BIODIVERSITY
Lenore Fahrig
Ottawa-Carleton Institute of Biology, Carleton University, Ottawa, Ontario,
Canada K1S 5B6; email: Lenore Fahrig@carleton.ca
Key Words habitat loss, landscape scale, habitat con guration, patch size, patch
isolation, extinction threshold, landscape complementation
n Abstract The literature on effects of habitat fragmentation on biodiversity is
huge. It is also very diverse, with different authors measuring fragmentation in dif-
ferent ways and, as a consequence, drawing different conclusions regarding both the
magnitude and direction of its effects. Habitat fragmentation is usually de ned as a
landscape-scale process involving both habitat loss and the breaking apart of habi-
tat. Results of empirical studies of habitat fragmentation are often dif cult to inter-
pret because (a) many researchers measure fragmentation at the patch scale, not the
landscape scale and (b) most researchers measure fragmentation in ways that do not
distinguish between habitat loss and habitat fragmentation per se, i.e., the breaking
apart of habitat after controlling for habitat loss. Empirical studies to date suggest
that habitat loss has large, consistently negative effects on biodiversity. Habitat frag-
mentation per se has much weaker effects on biodiversity that are at least as likely
to be positive as negative. Therefore, to correctly interpret the in uence of habitat
fragmentation on biodiversity, the effects of these two components of fragmentation
must be measured independently. More studies of the independent effects of habitat
loss and fragmentation per se are needed to determine the factors that lead to positive
versus negative effects of fragmentation per se. I suggest that the term fragmen-
tation should be reserved for the breaking apart of habitat, independent of habitat
loss.
INTRODUCTION
A recent search of the Cambridge Scienti c Abstracts database revealed over 1600
articles containing the phrase habitat fragmentation. The task of reviewing this
literature is daunting not only because of its size but also because different authors
use different de nitions of habitat fragmentation, and they measure fragmentation
in different ways and at different spatial scales.
This diversity of de nitions of habitat fragmentation can be readily seen in
the titles of some articles. For example, Impacts of habitat fragmentation and
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488 FAHRIG
patch size... (Collingham & Huntly 2000) suggests that habitat fragmenta-
tion and patch size are two different things. However, other authors actually use
patch size to measure habitat fragmentation (e.g., Golden & Crist 2000, Hovel &
Lipicus 2001). The effects of forest fragmentation and isolation ... (Good-
man & Rakotodravony 2000) suggests that forest fragmentation and isolation are
different, in contrast to authors who use forest isolation as a measure of for-
est fragmentation (e.g., Mossman & Waser 2001, Rukke 2000). Effect of land
cover, habitat fragmentation, and... (Laakkonen et al. 2001) contrasts with many
authors who equate landscape fragmentation with land cover (e.g., Carlson &
Hartman 2001; Fuller 2001; Gibbs 1998, 2001; Golden & Crist 2000; Hargis
et al. 1999; Robinson et al. 1995; Summerville & Crist 2001; Virg«os 2001). The
in uence of forest fragmentation and landscape pattern ... (Hargis et al. 1999)
contrasts with researchers who de ne fragmentation as an aspect of landscape
pattern (e.g., Wolff et al. 1997, Trzcinski et al. 1999). As a nal example, Effects
of experimental habitat fragmentation and connectivity... (Ims & Andreassen
1999) suggests that habitat fragmentation and connectivity can be examined in-
dependently, whereas some researchers actually de ne fragmentation as a dis-
ruption in landscape connectivity (With et al. 1997; see also Young & Jarvis
2001).
My goal in this review is to discuss the information available on the effects
of habitat fragmentation on biodiversity. To meet this objective I rst need to ex-
amine the different ways in which habitat fragmentation is conceptualized and
measured. Of course, the concept of biodiversity is probably at least as wide-
ranging as the concept of habitat fragmentation. However, I do not deal with
the issues surrounding the concept of biodiversity. Instead, I include any eco-
logical response variable that is or can be related to biological diversity (see
Table 1).
To determine current usage of the term habitat fragmentation, I conducted
a search of the Cambridge Scienti c Abstracts (Biological Sciences) database
on 11 April 2002 for papers containing either habitat fragmentation, forest
fragmentation, or landscape fragmentation in the title of the paper. I reviewed
in detail the most recent 100 resulting papers, irrespective of the journal in which
they appeared. I limited this search to papers containing fragmentation in the
title to ensure that my sample included only papers that are directly on the subject
of habitat fragmentation. The results are summarized in Table 1.
I then surveyed the broader ecological literature to ask the following: How
strong are the effects of habitat fragmentation on biodiversity, and are the effects
negative or positive? Habitat fragmentation is generally thought to have a large,
negative effect on biodiversity and is therefore widely viewed as an aspect of habitat
degradation (Haila 2002). However, as I show, this conclusion is generally valid
only for conceptualizations of fragmentation that are inseparable from habitat loss.
Other ways of conceptualizing habitat fragmentation lead to other conclusions. I
end the paper with recommendations.