Putting disagreement into context
Highlighting these three, unequivocal principles is important because
ecologists and conservation biologists have long discussed how best to
manage native habitat to sustain biodiversity (5, 6) . Earlier
discussions revolved around the SLOSS question (should conservation
prioritize ‘a single large or several small’ habitat patches?). Through
time, SLOSS matured into a conversation around the effects of habitat
fragmentation relative to effects of habitat amount. And more recently,
the discussion among scientists has focused on whether habitat
fragmentation may even have a positive effect on species. The problem
is, while disagreement is healthy in an academic setting, it confuses
management and policy making.
Still, the extensive body of literature addressing these topics has not
been sufficient to reach consensus on them. Some scientists have
concluded that landscapes containing many small patches of native
habitat can sustain rare and/or habitat specialist species (e.g., due to
beta diversity patterns or spatio-temporal dynamics across patches)(5, 9) , whereas others have suggested that the detrimental
effects of reduced patch size inevitably depauperate biodiversity even
at high cumulative habitat areas (e.g., due to extinction debt that
might be paid in landscapes containing many small patches) (6,
8) . Underlying different perspectives are several factors determining
our understanding of patterns in species occurrence and biodiversity.
Some of these factors are contextual to different ecosystems, including
biogeographical differences or intraspecific variation, and some
theoretical, including issues of spatial scaling or the balance between
deterministic and stochastic forces across landscapes.
Despite the different schools of thought in this field, there is no
debate about the need to conserve habitat: the effects for biodiversity
of increasing native habitat area are overwhelmingly positive.
Furthermore, embracing the three principles we outline (Fig. 1) helps to
put disagreements in the conservation literature into perspective. For
instance, it is true that large areas of nature are important and must
be protected (2) , as much as it is true that ensuring the
conservation or restoration of multiple small habitat patches is
fundamental for global biodiversity conservation, particularly in
regions with high land use (9) . These are neither incompatible
nor competing strategies; instead, they are complementary approaches to
protect biodiversity across all regions. Healthy disagreement can be
wrongly translated into a false dichotomy between the protection of
large or small patches, a mistake that must be avoided at all costs for
the sake of biodiversity conservation because both are important.
Habitat existing as small patches is often deemed less valuable than
large swaths of habitat in less modified regions (8) , which is
inadvertently leading to widespread cumulative losses of habitat from
millions of small patches across the globe. For instance, a recent
global analysis found that smaller (< 1000 ha) forest patches
are more likely to suffer a given amount of habitat loss than larger
(> 10,000 ha) patches (4) . Policies that protect
only patches larger than a minimum size hinder biodiversity conservation
as they fail to protect biodiversity in highly-modified regions that
clearly need protection. In Southern Ontario, Canada, where only about
15% of native habitat remains, about 95% of recent wetland habitat
loss has occurred below the 2-ha minimum area criterion used to identify
“Provincially Significant Wetlands” (10) . Failing to maintain
small habitat patches also reduces landscape connectivity among larger
patches, due to the loss of “stepping stones” (Fig. 1.3), where small
patches distributed through a landscape can facilitate movement between
larger patches.
At the same time, very large tracts of native habitat are now limited to
a small number of regions (2) , and their continued conversion to
human land uses is placing biodiversity at risk. For instance, continued
deforestation in the Amazon could trigger an ecosystem state-shift,
because the persistence of this biome depends on feedbacks between
vegetation and climate (11) . Losing even a fifth of the Amazonian
forest could shift the remaining area of forest into savanna, a
death-knell for the forest-dependent species of the Amazon (11).Similarly, while the few remaining extensive grasslands worldwide
sequester large amounts of carbon and host unique species, they remain
poorly protected and continue to shrink (12). Beyond biodiversity
per se, loss of these extensive, natural habitats would bring
significant losses to the economic, cultural, and ecological identity of
large regions (11, 12).
Ultimately, initiatives that set targets for habitat protection in the
coming decades require protection of both small and large areas of
native habitat. In ecoregions where vast areas of native habitat still
exist, ongoing habitat transformation needs to stop to both protect
biodiversity and reduce the pace of climate change. In ecoregions with
high land use, urgent protection and restoration of large numbers of
small patches is needed to sustain not only biodiversity but also
crucial ecosystem services including soil retention, water security,
pest control, pollination, and human well-being and health.