PerspectiveIs habitat fragmentation good for biodiversity?
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.
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