Protected-Area Design: The Ecology of Drawing Lines on a Map

Topic: Conservation Planning Last updated: 2026-06-15

A line on a map looks like an administrative act, but where it falls is an ecological one. A reserve that is too small to hold a viable population, too elongated to shelter an interior, too isolated to receive immigrants, or too poorly placed to capture the habitats a species actually uses may carry the legal status of protection while failing at the biological task it was created for. The recognition that the geometry and placement of reserves — not merely their existence — determines whether they conserve what they were meant to conserve is one of the foundational insights of conservation biology, articulated in the discipline's earliest synthesis [Soulé & Wilcox 1980]. Protected-area design asks a set of questions that are quantitative and ecological rather than bureaucratic: how large must a reserve be, what shape should it take, where should it sit relative to others, and how should a network of sites together represent the full diversity of a region. Over half a century these questions have generated a body of theory, a famous and instructive debate, a suite of planning algorithms, and a set of global targets — and they have forced conservationists to confront the people who live within and around the lines they draw.

Reserves as Islands: The Biogeographic Foundation

The intellectual scaffolding of reserve design was borrowed from the study of real islands. MacArthur and Wilson's equilibrium theory of island biogeography proposed that the number of species on an island reflects a dynamic balance between the rate at which new species colonize and the rate at which resident species go extinct, with immigration falling as an island sits farther from a source of colonists and extinction rising as an island shrinks [MacArthur & Wilson 1967]. The theory rested on the long-observed species–area relationship, the empirical regularity that the number of species S increases with area A according to a power function of the form S = cA^z, where c and z are fitted constants — meaning that larger areas hold more species, but with diminishing returns as area grows [MacArthur & Wilson 1967]. The conceptual leap that made this body of theory central to conservation was the realization that a protected area embedded in a landscape of farmland, pasture, and development is, functionally, an island of habitat in a sea of inhospitable terrain [Soulé & Wilcox 1980]. If that analogy holds, equilibrium theory carries a direct prediction: a reserve that is larger and less isolated should, over time, retain more species, while a small, isolated reserve should bleed species through local extinction until it settles at an impoverished equilibrium [MacArthur & Wilson 1967]. The species–area relationship also implies that habitat loss carries a quantifiable cost in species, since reducing A reduces the equilibrium S the remaining patch can sustain [Soulé & Wilcox 1980].

Diamond's Rules and the SLOSS Debate

From this theory came prescription. Diamond translated equilibrium biogeography into a set of geometric design "rules," arguing that, all else equal, a single large reserve is preferable to several small ones of the same total area, that a compact shape is preferable to an elongated one, that reserves placed closer together or linked by corridors are preferable to scattered ones, and that round reserves minimize the harmful effects of edges [Diamond 1975]. The single-large argument followed from the species–area relationship and from the expectation that larger areas support larger, more extinction-resistant populations of wide-ranging species [Diamond 1975]. These rules were intuitive, influential, and almost immediately contested. Critics pointed out that the species–area relationship cuts both ways: because several small reserves can sample a greater variety of habitats and capture species that do not co-occur, a set of small sites may, through complementarity, hold more total species than a single large reserve of equal area. This tension crystallized into the SLOSS debate — Single Large Or Several Small — which became one of the most durable arguments in the field [Margules & Pressey 2000].

The debate did not resolve in favor of one side so much as it dissolved into a recognition that the answer is context-dependent. Whether a single large or several small reserves conserve more depends on what is being conserved: wide-ranging species with large area requirements — large carnivores above all — need the contiguous space that only a large reserve provides, while diverse assemblages of habitat-specialist plants and invertebrates may be better captured by a scattered set of sites spanning environmental variation [Diamond 1975; Margules & Pressey 2000]. It also depends on the total area available, on the spatial pattern of the species in question, and on whether the small sites can be connected so that they function as a network rather than as isolated fragments vulnerable to the very extinction dynamics equilibrium theory predicts [MacArthur & Wilson 1967]. The lasting contribution of SLOSS was not a universal rule but the insight that reserve configuration must be evaluated against explicit conservation targets rather than asserted from geometry alone [Margules & Pressey 2000].

Shape, Edges, and the Problem of the Interior

If size answers part of the design question, shape answers another. The boundary of a reserve is an ecological zone in its own right, where the conditions of the protected interior give way to the altered microclimate, light, wind, predators, and human disturbance of the surrounding matrix — the so-called edge effect. Because a compact, round reserve minimizes the length of perimeter relative to the area enclosed, it shelters a larger interior core than an elongated reserve of identical area, which may be all edge and no interior [Diamond 1975]. This geometry matters acutely for interior-dependent species. Forest birds and other organisms that require deep-interior conditions can be effectively excluded from a reserve that, despite covering substantial area, is too narrow or too convoluted to contain habitat far enough from the boundary to escape edge influence [Diamond 1975]. The same logic establishes the concept of a minimum patch size below which interior conditions disappear altogether and the reserve supports only edge-tolerant generalists. Buffer zones address the problem from the outside: by surrounding a strictly protected core with a ring of compatible, lower-intensity land use, a buffer absorbs disturbance and pushes the hostile matrix outward, enlarging the effective interior without requiring that the entire area be placed under the strictest protection [Soulé & Wilcox 1980]. Shape, in short, is not an aesthetic property of a reserve but a determinant of how much of its mapped area is biologically functional.

Systematic Conservation Planning

For much of the twentieth century, protected areas accumulated opportunistically — established on scenic land, on terrain too steep or remote to farm, or on parcels that happened to be available, with little regard for whether the resulting estate actually represented a region's biodiversity. Margules and Pressey named and codified the alternative: systematic conservation planning, a structured, target-driven, and transparent process for selecting reserve networks [Margules & Pressey 2000]. At its core lies the principle of complementarity — the idea that each new site added to a network should be chosen for the contribution it makes to the features not yet represented, rather than for its richness in isolation. A site dense with species already captured elsewhere adds little; a species-poor site holding the one feature absent from the existing network may be irreplaceable [Margules & Pressey 2000]. Built on complementarity, optimization algorithms can assemble a set of sites that represents every targeted biodiversity feature — habitats, species, communities — at the minimum total area or cost, a combinatorial problem for which mathematical methods and dedicated software were developed [Possingham, Ball & Andelman 2000]. The contrast with ad hoc protection is sharp: where opportunistic reservation protects whatever land is left over after other uses have claimed the productive ground, systematic planning begins from explicit representation targets and asks which combination of sites achieves them most efficiently, making the trade-offs between conservation gain and cost visible rather than implicit [Margules & Pressey 2000].

Representation, Replication, and Gap Analysis

The goal that systematic planning operationalizes is representation: the principle that a protected-area network should contain a sample of every habitat type, ecosystem, and species in a region, rather than over-protecting a few photogenic landscapes while leaving entire biomes unguarded [Margules & Pressey 2000]. Representation is paired with replication — the recognition that a single protected example of a habitat is vulnerable to a single catastrophe such as fire, disease, or extreme weather, so that multiple, spatially separated samples of each feature spread risk and improve the odds of long-term persistence [Possingham, Ball & Andelman 2000]. The diagnostic tool for measuring how well a network achieves representation is gap analysis, which overlays the existing protected estate onto maps of species ranges and habitat distributions to identify the "gaps" — the features that fall outside any protected area and are therefore unrepresented [Rodrigues et al. 2004]. Gap analysis turns the abstract goal of representation into a concrete, mappable accounting exercise: it asks not how much land is protected but whether the protected land actually covers the biodiversity that protection is meant to secure, and it reliably reveals that the two are far from the same thing [Rodrigues et al. 2004].

Connectivity and Landscape Context

No reserve is self-contained. Equilibrium theory's prediction that isolation drives small populations toward extinction implies that a reserve network functions only if the sites within it remain demographically and genetically connected, so that immigration can rescue declining populations and recolonize patches where local extinction has occurred [MacArthur & Wilson 1967]. Connectivity is therefore not an optional enhancement to a network but a condition of its viability, and it is achieved through corridors of habitat linking core areas and through a matrix — the human-modified land between reserves — that is permeable enough for organisms to cross [Diamond 1975]. The need for connectivity is most acute for the species whose area requirements exceed anything a single reserve can offer. Large carnivores routinely range across territories larger than the parks meant to hold them, so that their persistence depends on networks of linked reserves embedded in a landscape they can move through, not on any one protected area however large [Soulé & Wilcox 1980]. This places the surrounding landscape at the center of design. The viability of a reserve is shaped as much by the land-use intensity, fragmentation, and hostility of its matrix as by anything inside its boundaries, which means that protected-area design cannot stop at the fence line but must consider the configuration of the whole landscape in which the reserve is set [Margules & Pressey 2000].

The 30×30 Target and Other Effective Measures

The cumulative science of representation, connectivity, and effectiveness now sits behind a global policy target. In 2022 the parties to the Convention on Biological Diversity adopted the Kunming-Montreal Global Biodiversity Framework, whose Target 3 commits signatories to ensuring that at least 30 percent of terrestrial, inland water, coastal, and marine areas are effectively conserved and managed by 2030 — the headline figure widely known as 30×30 [Convention on Biological Diversity 2022]. The framework is explicit that the target is not satisfied by area alone: the protected and conserved areas must be ecologically representative, well connected, and equitably governed, importing directly into international commitment the design principles of representation and connectivity developed over the preceding decades [Convention on Biological Diversity 2022]. The framework also formally recognizes that conservation need not occur only through legally designated protected areas. Other Effective Area-based Conservation Measures (OECMs) — sites that deliver sustained biodiversity outcomes through governance and management even though conservation is not their primary stated purpose — count toward the 30 percent, broadening the toolkit beyond the conventional park and acknowledging the contribution of lands held and managed by communities, the military, private owners, and others [Convention on Biological Diversity 2022]. The emphasis on connectivity and representation within the target reflects the long-standing finding that the configuration and ecological quality of protected areas, not their summed extent, determine whether they conserve biodiversity [Margules & Pressey 2000].

Paper Parks and the Gaps in the Existing Network

A target expressed in percentages invites a hard question: protected on paper, or protected in fact? Designation confers legal status, but a reserve that lacks enforcement, funding, and effective management may continue to lose the species it nominally protects — the phenomenon of the "paper park." Even setting management aside, the global network as designed leaves systematic gaps in what it covers. In the most comprehensive assessment of its kind, Rodrigues and colleagues compared the global protected-area network against the ranges of terrestrial vertebrates and found that the network failed to represent a substantial fraction of species: at least 12 percent of threatened birds and roughly a quarter of threatened mammals had no part of their range inside any protected area — they were "gap species," entirely unrepresented despite a global estate of tens of thousands of reserves [Rodrigues et al. 2004]. The finding demonstrated that expanding protected-area coverage without systematic, complementarity-based targeting tends to add more of what is already represented while leaving the most vulnerable and narrowly distributed species outside the system altogether [Rodrigues et al. 2004]. It is a direct empirical vindication of systematic conservation planning over opportunistic protection: a network can grow in area while stagnating in representation, and only deliberate gap-driven selection closes the holes that ad hoc protection leaves [Margules & Pressey 2000; Rodrigues et al. 2004].

The Social Dimension: Fortress Conservation and Community Stewardship

The lines that designers draw enclose not only habitats but people, and the history of protected areas carries a contested social legacy. The "fortress conservation" model — the establishment of strictly protected areas from which human residence and use are excluded, sometimes through the displacement of the communities who had long inhabited the land — treated people as a threat to be kept outside the fence. The discipline that grew up around reserve design recognized from early on that conservation operating at landscape scale is inseparable from the human communities who share those landscapes, and that durable protection depends on their participation rather than their exclusion [Soulé & Wilcox 1980]. The contrasting tradition of community-based conservation, including Indigenous and Community Conserved Areas (ICCAs) — territories and areas governed and managed by Indigenous peoples and local communities — has accumulated evidence that locally stewarded lands can sustain biodiversity at levels comparable to formally designated state protected areas, and that Indigenous-governed territories frequently retain ecological integrity equal to or exceeding that of conventional parks [Convention on Biological Diversity 2022]. The Kunming-Montreal framework reflects this shift in its insistence that area-based conservation be equitably governed and respect the rights of Indigenous peoples and local communities, and in its formal recognition of OECMs as a vehicle through which community-stewarded lands count toward global targets [Convention on Biological Diversity 2022]. The honest reading of this history is that protected-area design is not a purely technical optimization. A network that is ecologically sound on paper but socially unjust in practice tends to fail, because the people excluded from it bear costs they have little reason to accept, and the evidence increasingly indicates that recognizing and supporting community and Indigenous stewardship is not a concession to be traded against conservation goals but, frequently, a route to achieving them [Convention on Biological Diversity 2022].

References

Convention on Biological Diversity (2022) Kunming-Montreal Global Biodiversity Framework. CBD/COP/DEC/15/4. Montreal.

Diamond, J.M. (1975) "The island dilemma: Lessons of modern biogeographic studies for the design of natural reserves." Biological Conservation 7:129–146. DOI: 10.1016/0006-3207(75)90052-X

MacArthur, R.H. & Wilson, E.O. (1967) The Theory of Island Biogeography. Princeton University Press, Princeton.

Margules, C.R. & Pressey, R.L. (2000) "Systematic conservation planning." Nature 405:243–253. DOI: 10.1038/35012251

Possingham, H.P., Ball, I.R. & Andelman, S. (2000) "Mathematical methods for identifying representative reserve networks." In S. Ferson & M. Burgman (eds.), Quantitative Methods for Conservation Biology. Springer, New York. pp. 291–306.

Rodrigues, A.S.L., Akcakaya, H.R., Andelman, S.J., Bakarr, M.I., Boitani, L., Brooks, T.M., Chanson, J.S., Fishpool, L.D.C., Da Fonseca, G.A.B., Gaston, K.J., Hoffmann, M., Marini, M.A., Mehlman, D.W., Pearson, D.L., Pilgrim, J.D., Pressey, R.L., Schipper, J. & Vié, J.-C. (2004) "Effectiveness of the global protected area network in representing species diversity." Nature 428:640–643. DOI: 10.1038/nature02422

Soulé, M.E. & Wilcox, B.A. (eds.) (1980) Conservation Biology: An Evolutionary-Ecological Perspective. Sinauer Associates, Sunderland, MA.