Elsevier

Biological Conservation

Volume 226, October 2018, Pages 9-15
Biological Conservation

Perspective
Is habitat fragmentation good for biodiversity?

https://doi.org/10.1016/j.biocon.2018.07.022Get rights and content

Highlights

  • Habitat loss and fragmentation have long been considered to have negative effects on biodiversity.

  • Yet recent review by Fahrig (2017) argues that in fact habitat fragmentation has largely positive effects on biodiversity.

  • We highlight empirical and theoretical counter evidence that illustrate negative effects of fragmentation can be common.

  • We argue that positive effects can often be misleading or not of conservation importance.

  • We provide six key reasons why the conclusions in Fahrig (2017) should not be used in conservation decision-making.

Abstract

Habitat loss is a primary threat to biodiversity across the planet, yet contentious debate has ensued on the importance of habitat fragmentation ‘per se’ (i.e., altered spatial configuration of habitat for a given amount of habitat loss). Based on a review of landscape-scale investigations, Fahrig (2017; Ecological responses to habitat fragmentation per se. Annual Review of Ecology, Evolution, and Systematics 48:1-23) reports that biodiversity responses to habitat fragmentation ‘per se’ are more often positive rather than negative and concludes that the widespread belief in negative fragmentation effects is a ‘zombie idea’. We show that Fahrig's conclusions are drawn from a narrow and potentially biased subset of available evidence, which ignore much of the observational, experimental and theoretical evidence for negative effects of altered habitat configuration. We therefore argue that Fahrig's conclusions should be interpreted cautiously as they could be misconstrued by policy makers and managers, and we provide six arguments why they should not be applied in conservation decision-making. Reconciling the scientific disagreement, and informing conservation more effectively, will require research that goes beyond statistical and correlative approaches. This includes a more prudent use of data and conceptual models that appropriately partition direct vs indirect influences of habitat loss and altered spatial configuration, and more clearly discriminate the mechanisms underpinning any changes. Incorporating these issues will deliver greater mechanistic understanding and more predictive power to address the conservation issues arising from habitat loss and fragmentation.

Introduction

Land-use change is impacting biodiversity across the planet (Newbold et al., 2015). There is no question that the extent and condition of native vegetation has declined precipitously in recent decades, such that most species now live in fragmented patches of degraded habitat, subject to rising threats from the surrounding anthropogenic matrix (Haddad et al., 2015; Pfeifer et al., 2017). Conservation threat assessments in fragmented landscapes repeatedly emphasize that there are multiple causal agents of biodiversity decline that operate in complex and often synergistic ways (e.g., Cote et al., 2016; Laurance and Useche, 2009).

It is surprising, then, that claims have been made that habitat loss, and not the configuration of remaining habitat, is sufficient to explain effects of land clearing on biodiversity loss, whereas the effects of habitat fragmentation (i.e., altered spatial configuration of habitat for a given amount of habitat loss) are often ‘weak’ or ‘absent’ (Fahrig, 2003, p. 508). The argument is that the effects of habitat loss are overwhelming and that the complexity of effects due to habitat fragmentation, such as declining patch areas, reductions in connectivity, or increasing edge effects, are not needed to explain patterns of biodiversity change in most landscapes. These claims have had a major impact in focusing efforts on understanding the effects of habitat loss relative to habitat fragmentation (see summary in Hadley and Betts, 2016), and it is clear that habitat loss has severe effects on biodiversity (e.g., Brooks et al., 2002; Schipper et al., 2008), as emphasized in Fahrig (2003). However, a large body of evidence runs counter to claims that habitat fragmentation effects are weak or absent. Not only have the pattern and process of habitat fragmentation been shown to have substantial and lasting effects on biodiversity (e.g., Haddad et al., 2015), but also the spatial configuration of habitat loss has been shown to influence how habitat loss effects extend into remaining habitat (Barlow et al., 2016; Pfeifer et al., 2017).

The viewpoint that fragmentation is not important has arisen primarily because statistical models that attempt to partition ‘independent’ effects of habitat loss from habitat fragmentation tend to show greater effects of habitat loss (Fahrig, 2003). These models would be valid if the processes of habitat loss and fragmentation were conceptually and empirically independent, and the resulting spatial patterns of habitat amount and configuration could be treated as statistically independent (Koper et al., 2007; Smith et al., 2009). However, others have argued that habitat loss and fragmentation are frequently linked, such that statistical independence of the resulting patterns must be explicitly tested rather than assumed (Didham et al., 2012). In fact, landscapes across most regions of the world exhibit very high collinearity between habitat amount and configuration (e.g., Cushman et al., 2008; Liu et al., 2016). Because of these real-world patterns, Ruffell et al. (2016) argue that the causal basis of this collinearity should be incorporated explicitly into statistical models, most logically by partitioning the direct vs indirect mechanisms by which habitat loss influences ecological responses via the mediating effects of altered habitat configuration.

Even though there is apparent disparity in philosophical and analytical perspectives, it is important to point out that both perspectives share a fundamental motivation for discriminating the effects of habitat amount and configuration: to allow more targeted and cost-effective use of scarce conservation resources on the factor(s) of greatest importance for biodiversity loss (Fahrig, 2003; Ruffell et al., 2016). After all, conservation strategies may well differ in their effectiveness when focusing on mitigating habitat loss versus changes in habitat configuration (Villard and Metzger, 2014). The ‘loss versus fragmentation’ question has consequently become a major focus of research within landscape ecology and conservation (Hadley and Betts, 2016).

Now, however, Fahrig (2017) has made a new claim in a review of studies that attempt to separate the effects of habitat fragmentation ‘per se’ from habitat loss. Fahrig concludes that the weight of evidence supports largely positive effects of habitat fragmentation ‘per se’ on biodiversity, and that the negative effect of habitat fragmentation on biodiversity is a “zombie idea” – a concept that is repeatedly refuted but yet somehow survives (Quiggen, 2010). Fahrig then casts a wide net for other so-called ‘zombie’ ideas: large patches contain more species than several small patches of similar combined area, edge effects are typically negative, habitat fragmentation reduces connectivity, habitat specialists have stronger negative responses to habitat fragmentation relative to generalists, and negative effects of habitat fragmentation are stronger in the tropics and at low levels of habitat amount (Table 1).

These assertions, if supported, would be remarkable for two reasons. First, they run counter to mainstream empirical and theoretical research on diverse components of habitat configuration effects (e.g., Haddad et al., 2015; Tilman and Lehman, 1997), suggesting the ecological research community has been mired in consensus and blind to the positive effects of habitat fragmentation. Second, they have major implications for the management of the world's fragmented ecosystems.

Given the importance of these issues, we re-evaluate Fahrig's assessment. First, we discuss why the review process utilized by Fahrig likely biased the findings and led to unwarranted conclusions. Second, we address the origins of the conflicting viewpoints, illustrating that there is ample empirical evidence and theory that laid the foundation for the idea of negative effects of habitat fragmentation that were not acknowledged in Fahrig (2017) (see Table 1 for a non-exhaustive list of summaries). Third, we discuss why these conclusions should not be applied to conservation in fragmented landscapes. We conclude by highlighting areas of consensus to help advance the conceptual understanding and applied relevance of habitat fragmentation effects.

Section snippets

The review and conclusions on fragmentation effects

Over the past two decades, several reviews and meta-analyses have suggested that the effects of different spatial components of habitat fragmentation, such as habitat edge or isolation, have undesirable or variable effects on ecological responses (Debinski and Holt, 2000; Ewers and Didham, 2006; Fletcher Jr. et al., 2016; Fletcher Jr. et al., 2007; Gilbert-Norton et al., 2010; Haddad et al., 2015; Pfeifer et al., 2017; Ries et al., 2004; Ries et al., 2017). Yet in some of these reviews there

Are these conclusions warranted?

The results in Fahrig (2017) were surprising, yet the review's main conclusions come from a narrow subset of literature and do not provide reliable evidence or sufficient context to dismiss the negative effects of fragmentation as a ‘zombie’ idea. We focus on three key reasons why this is the case: 1) the search terms and review criteria led to the omission of key literature; 2) the use of a vote-counting approach likely biased the relative weighting of findings; and 3) there has been no

Origins of conflicting viewpoints

Fahrig (2017) argues several reasons why most other researchers erroneously believe that habitat fragmentation has negative effects. One point Fahrig raises (p.2, 18) is that early conceptual work relevant to habitat fragmentation confounded habitat patchiness with habitat amount (e.g., den Boer, 1968; MacArthur and Wilson, 1967). Decades of advances in metapopulation and metacommunity theory show clearly that effects of habitat fragmentation can increase extinction rates and decrease

Implications for management and conservation in the real world

We believe that the overall goal for most science on habitat fragmentation is to gain a deeper mechanistic understanding of why habitat configuration effects occur, how they might mediate the relationship between habitat loss and biodiversity decline, and ways to mitigate the impacts of habitat loss and related land-use change (e.g., via conservation corridors). Ignoring or diminishing the importance of spatial configuration effects as a core part of that mechanistic understanding comes with

Conclusions and moving forward

We agree with Fahrig that habitat loss is well known to have large negative effects on biodiversity, and that small fragments can have conservation value for biodiversity and ecosystem services (e.g., Mitchell et al., 2014). We also agree that the term ‘habitat fragmentation’ is often used interchangeably as both a loose catch-phrase to refer to the overall process of changing amount and configuration of habitat through time, and as a more refined characterization of altered spatial

Acknowledgements

This research was supported by the National Science Foundation (DEB-1655555 to RF and RH). AG is supported by the Liber Ero chair in Biodiversity Conservation. RP is supported by a research fellowship from CNPQ (Conselho Nacional de Desenvolvimento Científico e Tecnológico)308205/2014-6.

Competing interests

The authors declare no competing interests.

References (74)

  • C. Banks-Leite et al.

    Unraveling the drivers of community dissimilarity and species extinction in fragmented landscapes

    Ecology

    (2012)
  • J. Barlow et al.

    Anthropogenic disturbance in tropical forests can double biodiversity loss from deforestation

    Nature

    (2016)
  • M.G. Betts et al.

    A species-centered approach for uncovering generalities in organism responses to habitat loss and fragmentation

    Ecography

    (2014)
  • T.M. Brooks et al.

    Habitat loss and extinction in the hotspots of biodiversity

    Conserv. Biol.

    (2002)
  • R.A. Chisholm et al.

    Species-area relationships and biodiversity loss in fragmented landscapes

    Ecol. Lett.

    (2018)
  • I.M. Cote et al.

    Interactions among ecosystem stressors and their importance in conservation

    Proc. R. Soc. B Biol. Sci.

    (2016)
  • S.A. Cushman et al.

    Separating the effects of habitat area, fragmentation and matrix resistance on genetic differentiation in complex landscapes

    Landsc. Ecol.

    (2012)
  • D.M. Debinski et al.

    A survey and overview of habitat fragmentation experiments

    Conserv. Biol.

    (2000)
  • P.J. den Boer

    Spreading of risk and stabilization of animal numbers

    Acta Biotheor.

    (1968)
  • R.K. Didham et al.

    Rethinking the conceptual foundations of habitat fragmentation research

    Oikos

    (2012)
  • C. Echeverria et al.

    Impacts of forest fragmentation on species composition and forest structure in the temperate landscape of southern Chile

    Glob. Ecol. Biogeogr.

    (2007)
  • R.M. Ewers et al.

    Confounding factors in the detection of species responses to habitat fragmentation

    Biol. Rev.

    (2006)
  • R.M. Ewers et al.

    Synergistic interactions between edge and area effects in a heavily fragmented landscape

    Ecology

    (2007)
  • L. Fahrig

    Effects of habitat fragmentation on biodiversity

    Annu. Rev. Ecol. Evol. Syst.

    (2003)
  • L. Fahrig

    Rethinking patch size and isolation effects: the habitat amount hypothesis

    J. Biogeogr.

    (2013)
  • L. Fahrig

    Ecological responses to habitat fragmentation per se

    Annu. Rev. Ecol. Evol. Syst.

    (2017)
  • J. Fischer et al.

    Land sparing versus land sharing: moving forward

    Conserv. Lett.

    (2014)
  • R.J. Fletcher

    Multiple edge effects and their implications in fragmented landscapes

    J. Anim. Ecol.

    (2005)
  • R.J. Fletcher

    Emergent properties of conspecific attraction in fragmented landscapes

    Am. Nat.

    (2006)
  • R.J. Fletcher et al.

    The role of habitat area and edge in fragmented landscapes: definitively distinct or inevitably intertwined?

    Can. J. Zool.

    (2007)
  • R.J. Fletcher et al.

    The matrix alters the role of path redundancy on patch colonization rates

    Ecology

    (2014)
  • R.J. Fletcher et al.

    Divergent perspectives on landscape connectivity reveal consistent effects from genes to communities

    Curr. Landscape Ecol. Rep.

    (2016)
  • L. Gilbert-Norton et al.

    A meta-analytic review of corridor effectiveness

    Conserv. Biol.

    (2010)
  • A. Gonzalez et al.

    Metapopulation dynamics, abundance, and distribution in a microecosystem

    Science

    (1998)
  • J. Gurevitch et al.

    Meta-analysis and the science of research synthesis

    Nature

    (2018)
  • N.M. Haddad et al.

    Habitat fragmentation and its lasting impact on Earth

    Sci. Adv.

    (2015)
  • N.M. Haddad et al.

    Experimental evidence does not support the habitat amount hypothesis

    Ecography

    (2017)
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