Ecological responses to habitat fragmentation per se

Professor Lenore Fahrig, of the Department of Biology at Carleton University and co-director of the Geomatics and Landscape Ecology Research Laboratory there, wrote a paper challenging the conventional belief that fragmentation of natural areas is bad for their preservation.  On the contrary, for the same amount of habitat, she finds that positive effects of fragmentation far outweigh the negative ones.

Her conclusion is based on a rigorous analysis of 118 studies that report a total of 381 effects. Her criteria include a definition of negative/positive in terms of “abundance, species richness, movement success, etc.” Only statistically significant effects were counted.

Ecological Responses to Habitat Fragmentation per se, Annual Reviews of Ecology, Evolution and Systematics, 48, 2017 (1.2 MB; 23 pp. + Figures, to p45)

The Abstract follows.

Prof. Fahrig was interviewed on CBC Radio’s Ottawa Morning on February 6, 2017 (7m45s).

Comment: These findings could easily be misunderstood as saying fragmentation is good for the environment (so no harm in interrupting the landscape with country lot estates, for example?) On the contrary, as the Abstract repeats, the conclusion is rather that small patches should be valued as important.   Also, if the choice is between keeping two relatively large patches (“land sparing”) or the same amount of acreage but fragmented in 18 pieces (“land sharing”), then the latter will likely have more positive effect than the former. (Ref. Figure 2 in the paper.)

Erwin

5 March 2017

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Abstract
I reviewed empirical studies finding significant ecological responses to habitat fragmentation per se, i.e. significant responses to fragmentation independent of the effects of habitat amount (hereafter, ‘habitat fragmentation’). I asked, (1) are most significant responses to habitat fragmentation negative or positive?, and (2) are there particular attributes of species or landscapes that lead to a predominance of negative or positive significant responses? I found 118 studies reporting 381 significant responses to habitat fragmentation independent of habitat amount. Seventy-six percent of these responses were positive. Most significant fragmentation effects were positive, irrespective of how the authors controlled for habitat amount, the measure of fragmentation, the taxonomic group, the type of response variable, or the degree of specialization or conservation status of the species or species group. There was also no support for predictions that most significant responses to fragmentation should be negative in the tropics, for species with larger movement ranges, or when habitat amount is low; most significant fragmentation effects were positive in all these cases. Thus, while 24% of significant responses to habitat fragmentation were negative, I found no conditions in which most responses were negative. Authors suggest a wide range of possible explanations for significant positive responses to habitat fragmentation: increased functional connectivity, habitat diversity, positive edge effects, stability of predator-prey/host-parasitoid systems, reduced competition, spreading of risk, and landscape complementation. A consistent preponderance of positive significant responses to fragmentation implies there is no justification for assigning lower conservation value to a small patch than to an equivalent area within a large patch – just the opposite. It also suggests land-sharing will usually provide higher ecological value than land-sparing.

Key Words: landscape pattern, landscape structure, landscape configuration, landscape complementation, landscape connectivity, landscape heterogeneity, patch area, patch isolation, edge effect, SLOSS

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