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).