To what extent could edge effects and habitat fragmentation diminish the potential benefits of land sparing?

Abstract

Land sharing and land sparing are contrasting proposals for minimising the impacts of agriculture on wild species. Edge effects (biophysical gradients near habitat boundaries) might reduce population sizes on spared land, particularly in highly-fragmented landscapes, so might change conclusions about whether land sparing or land sharing is better for species' persistence. We assessed this possibility by modelling the population sizes of 120 Ghanaian bird species in the presence of a range of hypothetical edge effects under land-sparing and land-sharing strategies, and at different levels of habitat fragmentation and agricultural production. We found that edge effects can reduce population densities on spared land, and in highly-fragmented landscapes can-at modest levels of agricultural production combined with high edge penetration distances-cause the optimal strategy to switch from land sparing to land sharing. Nevertheless, land sparing maximised population sizes for more species in most cases tested. This conclusion was best supported for sensitive species with small global geographical ranges, which are likely to include those of greatest future conservation concern. The size of patches of spared land affected conservation outcomes: population sizes were maximised under a land-sparing strategy that spared large blocks of natural habitat of ~1,000 or, better, ~10,000 ha. To effect land sparing in practice would require policies that promoted both increases in agricultural yield and the establishment or protection of natural habitats on spared land. Because the optimum scale of patches of spared land for edgesensitive species is generally larger than the size of individual farms, policies that facilitate coordinated action by farmers or other land managers might be required.

Land‐sparing agriculture sustains higher levels of avian functional diversity than land sharing

“…Furthermore, trait abundances conserved under land sparing declined markedly under high production scenarios, where spared lands represent only a small proportion of the landscape. This result supports findings of other studies, emphasizing the importance of sparing large blocks of natural habitat for effective biodiversity conservation via land sparing (Edwards et al, ; Gilroy, Woodcock et al, ; Lamb, Balmford, Green, & Phalan, ).…”

“…Lamb et al () showed that for Ghanaian birds the relative benefits of sparing versus sharing are partially influenced by habitat quality, with land sharing marginally outperforming land sparing when spared lands constitute severely fragmented landscapes and suffer acute edge effects. Because rare and disturbance‐sensitive species are most likely to be affected by fragmentation (Banks‐Leite, Ewers, & Metzger, ) and contribute disproportionately to FD by possessing unique traits necessary for ecosystem function (Leitão et al, ), fragmentation effects could be especially important in degrading the value of sparing for FD.…”

The role of livestock intensification and landscape structure in maintaining tropical biodiversity

“…Alternatively, if species are strongly impacted by forest loss, then land sparing will be less harmful as it minimises natural habitat loss while intensifying production elsewhere (Edwards et al., ; Paul & Knoke, ; Phalan et al., ). The relative benefits of sparing and sharing will also vary with the effects of landscape structure: land sparing is likely to be less harmful if forest fragmentation has large negative impacts on biodiversity (Gilroy, Edwards, Medina, Haugaasen, & Edwards, ; Green et al., ; Lamb, Balmford, Green, & Phalan, ). However, the exact impacts of fragmentation are unclear (Fahrig, ), and it is possible that a land sharing approach that conserves highly fragmented habitats could be less harmful.…”

“…The relative benefits of sparing and sharing will also vary with the effects of landscape structure: land sparing is likely to be less harmful if forest fragmentation has large negative impacts on biodiversity (Gilroy, Edwards, Medina, Haugaasen, & Edwards, 2014;Green et al, 2005;Lamb, Balmford, Green, & Phalan, 2016). However, the exact impacts of fragmentation are unclear (Fahrig, 2017), and it is possible that a land sharing approach that conserves highly fragmented habitats could be less harmful.…”

What Have We Learned from the Land Sparing-sharing Model?

“…The original formulation of the land sparing-sharing framework omits some complexities, but it can be and already has been modified to incorporate many of these, including spatial configuration and the influence of edge effects [60,61]; the influence of changing diets and reducing food waste [50]; inclusion of some ecosystem services [4,7,50,62]; prediction of the effects of specific public policies [63]; and application to forestry [64,65], urban planning [66][67][68][69] and marine conservation [70]. It is a model, and so all assumptions can be varied and tested.…”