Putting disagreement into context
Highlighting these three principles (Fig. 1) is important because
ecologists and conservation biologists have long discussed how best to
manage native habitat to sustain biodiversity. Earlier discussions
revolved around SLOSS (Diamond 1975; Simberloff & Abele 1976) – should
conservation prioritize ‘a Single Large o r Several Small’ habitat
patches? Through time, SLOSS matured into a debate around the effects of
habitat fragmentation relative to effects of habitat amount. And more
recently, the debate has been on whether habitat fragmentation has
positive or negative effects on biodiversity (Fletcher et al.2018; Fahrig et al. 2019). The problem is, while disagreement is
healthy in an academic setting, it fails to provide pragmatic solutions
for management and policy making, when those solutions exist.
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 (Shafer 1995;
Fahrig et al. 2019), whereas others have suggested that reduced
patch sizes inevitably depauperate biodiversity even if the total amount
of habitat remains unchanged (Fletcher et al. 2018; Bateman &
Balmford 2023). 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 (Betts et al.2019; Banks-Leite et al. 2022) or intraspecific variation
(Bellotto-Trigo et al. 2023), and some theoretical, including
issues of spatial scaling (Fahrig 2023; Riva & Fahrig 2023a). Authors
even differ in what they consider relevant habitat, from “at
least 100-1000 ha ” (Balmford 2021) to “smaller than 1 ha ”
(Riva & Fahrig 2023b).
The existence of different schools of thought might cast doubt on the
generality of the principles we propose, yet this is a misconception.
Embracing the principles we outline (Fig. 1) instead helps to put
disagreements into perspective. For instance, there is no debate about
the need to conserve habitat: the effects of increasing native habitat
on biodiversity are overwhelmingly positive. It is true that large areas
of nature are important and must be protected (Haddad et al.2015; Bateman & Balmford 2023), as much as it is true that ensuring the
conservation or restoration of multiple small habitat patches is
fundamental for global conservation, particularly in extensively
modified regions (Arroyo-Rodríguez et al. 2020; Riva & Fahrig
2022). These are neither incompatible nor competing strategies; they are
complementary approaches to protect biodiversity across all regions.
Disagreement can be 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.
The risks of ignoring these principles are clear. Habitat existing as
small patches is often deemed less valuable than large swaths of habitat
in less modified regions (Bateman & Balmford 2023), which is
inadvertently leading to widespread cumulative loss of habitat from
millions of small patches across the globe. For instance, smaller
(< 1000 ha) forest patches are more likely to suffer a given
amount of habitat loss than larger (> 10,000 ha) patches
(Riva et al. 2022). While the recent agreement of the parties
involved in COP 15 is agnostic on patch area, policies that protect only
patches larger than a minimum size are widespread [see (Riva & Fahrig
2023b) for examples in Mexico, US, Canada, Australia, and Europe].
Such policies hinder biodiversity conservation because they fail to
protect biodiversity in highly-modified regions where protection is
clearly needed. Similarly, suggesting that habitat protection should
occur primarily in biodiversity-rich regions and/or large habitat
patches (Bateman & Balmford 2023) risks neglecting extensive areas of
the planet with unique flora and fauna but also large anthropogenic
footprints (Haddad et al. 2015). Finally, failing to maintain
small habitat patches reduces landscape connectivity among larger
patches due to the loss of “stepping stones” (Terborgh 1974), 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 few regions (Haddad et al. 2015), and their conversion to human
land uses is placing many species – most of which have not yet been
identified to science (Hortal et al. 2015) – at risk. For
instance, continued deforestation in the Amazon has been predicted to
trigger an ecosystem state-shift. This biome persists thanks to feedback
between vegetation and climate (Albert et al. 2023). Losing 20%
of the Amazonian forest could trigger a shift from forest into savanna,
a death-knell for the forest-dependent species of the Amazon (Albertet al. 2023). 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 (Scholtz &
Twidwell 2022). Beyond biodiversity, loss of these extensive
natural habitats would bring significant losses to the economic,
cultural, and ecological identity of large regions (Scholtz & Twidwell
2022; Albert et al. 2023).
Protecting
biodiversity with people and for people
While the principles we propose are essential to sustain biodiversity,
conservation is destined to fail unless the rights and needs of people
also enter the equation. This implies that the three principles, even if
best for biodiversity, cannot be always applied. Tradeoffs with other
priorities in landscape management must also be considered. For
instance, the provision of food, water, shelter, and energy to humans
often implies the sacrifice of large areas of native habitat. How can we
sustain biodiversity, while at the same time supporting the needs of an
increasing global human population?
Careful planning that does not affect the total area reserved to nature
can optimize conservation investments. For example, natural habitats can
be maintained within agricultural landscapes to sustain several crucial
services (e.g., pollination, pest control, and nutrient retention). In
the Midwestern US, removing from crop production sub-field areas that
are consistently under-yielding makes conservation possible across
millions of hectares (Basso & Antle 2020). Avoiding growing food
in such locations can reduce the total surface of land needed to feed
humanity. As a further example, restoration of small (≤ 0.16 ha) forests
in oil palm plantations can enhance biodiversity and multiple ecosystem
services without compromising yield (Zemp et al. 2023).Thus, it is possible to reduce the area allocated to land used by people
and increase land for nature, while also guaranteeing the services that
people rely on.
Because area-based conservation actions are intertwined with
socio-political dynamics and ethics (Richardson et al.2023), they require integrating biodiversity policy with other
human goals, e.g., the United Nations sustainable development goals such
as “Zero hunger” and “Clean water and sanitation”. In some regions
this can result in situations where actions to sustain wilderness are
not always desirable for people. For instance, human-wildlife conflicts
are more likely in human-occupied regions containing significant
wilderness areas. This complicates global conservation of large
carnivores, especially in the global South where regulations on land use
have large impacts on the ability of many people to gain a living.
Conservation action must therefore be implemented equitably, not only
for ethical reasons, but also because a loss of social legitimacy often
causes nature reserves to be disregarded both legally and practically.
Consideration of aspects beyond – but dependent on – biodiversity must
therefore be central in the dialogue around how to implement area-based
conservation efforts. This dialogue requires weighing different
conservation, ethical, social, and economic priorities, but we stress
that the principles we champion here must be central to the process of
weighing these different priorities. This is because failing to halt
biodiversity loss entails a risk of societal collapse as most ecosystem
services supporting human societies would disappear (Tilman et
al. 2014).