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Minimum area requirements

The smaller an area, the more likely it becomes that populations of species will go extinct. Many conservationists are concerned about the viability of an area. MacArther, R.H and Wilson, E.O. (1967) in “The Theory of Island Biogeography” have triggered a worldwide discussion on minimum sizes of protected areas. Having a broad representation of species in a protected areas system by itself is not enough if many of those species will not be able to survive. While having the right ecosystems are the first condition required for the survival of the species of a country, it is also necessary to asses how much of each ecosystem is needed to warrant the conservation of species over a prolonged period of time. It is therefore necessary to assess how much is needed of every ecosystem for different species to survive.

Figure 6: Number of species on an island after MacArthur and Wilson (1967).

We depart from the assumption that all ecosystems – even the very small ones - are viable, but not each size is suitable for maintaining all the species that are associated with it. When an ecosystem decreases in size or undergoes ecological change, some species may go extinct, but a reduced or altered ecosystem continues to be viable for the remaining and probably also for some new species. Therefore, “viability” rather relates to the individual species belonging to an ecosystem and it varies widely per species. When this document relates to “viability” it refers to the durable condition of an area to facilitate the survival of the majority of the species or an assemblage belonging to an ecosystem class. The question is how large must an ecosystem be, for a species to survive.

These examples show that every organism has different requirements regarding its population size and its distribution across one specific or more ecosystems. The challenge is to find ways that lead to the conservation of the largest variety of species possible, of both species that live in high and in low densities, while making the smallest possible territorial demands.

Minimum area requirements (MARs) of the species are needed to calculate the minimum sizes of protected areas. The MAR can be calculated by multiplying the MVP with the reversed fraction of the density of the effective population. It usually requires some analysis to determine whether density data found in literature concerns the total population or the effective population – e.g. the density of breeding pairs of a bird species relates to the effective population, while a population density derived from data from a "camera trap" relates to the total population. Ranges or home-ranges of animals, often mentioned in literature, should not be used as indicators of population density. Many animals have partially overlapping home ranges, while the ranges of males and females often show major differences in size and the sexes overlap.

Many of the remaining ecosystems with their corresponding species assemblages are too small to durably accommodate top predators. Writing off smaller ecosystems on the assumption that most of them would collapse (see chapter #) in absence of large predators would a priori write off millions of species from the American Hemisphere.

A fundamental question is, how much area is needed of each ecosystem to allow the required life-supporting ecological processes to perpetuate spontaneously or independently. To that end, some concept of minimum area requirements had to be established for ecosystems as well. To that end, the current theories of population dynamics alone don’t provide workable criteria, reason to develop some new concepts.

When species are eliminated from an ecosystem, certain shifts of population densities will always take place; some more species may disappear in their wake while new ones may move in, often not desirable ones from a human point of view. The degree of change will be different in each case, depending on a large variety of factors. One of those factors is that more often than not, small protected areas are usually islands in a "sea" of production land, and according to the island theory, they are likely to lose species. It is safe to assume that almost all small protected areas end up in a state of "extinction dept" (Dobson, 1996). However, without complementary small protected areas to protect the ecosystems absent in the large protected areas, all species depending on those absent ecological conditions would be lost. Therefore it is necessary to protect those ecosystems that are absent or underrepresented in large protected areas elsewhere in small protected areas, even though a part of their species will not be able to survive. Additionally, some extinctions can be prevented through management measures, but this is often costly and in many developing countries, the required finances for management are usually absent and know how is inadequate everywhere.

Terrestrial ecosystems

Some ecosystems are usually found in rather small spaces, such as montane ecosystems on some mountain tops or some isolated small waterbodies, while other ones – such as non-inundated prairies, savannahs and lowland tropical rain forests - occupy enormous continuous spaces. If one were to define a minimum size to fit all ecosystems, it would have to be rather large; much larger than the sizes in which those smaller ecosystems are often found. An attempt to deal with ecosystem requirements of different levels of classification has been made by TNC (Secaira et al. 2001), when observing that for the higher (coarser) levels of the hierarchy, ecosystems require different "typical sizes". They argued that species adapted to living in typically small ecosystems need less space to "be representative and viable" than species in large composed ecosystems.

In the elaboration of the concept, Secaira et al. (2001) are not very systematic. They argue that if the typical area of an ecosystem is between 10 and 2,000 ha, the minimum size should still be 2,000 ha for areas from 2,000 ha to 200,000 ha they suggests that the minimum size should be 5,000 and a next level up (they speak about "ecoregions") one should take 10,000 ha. These sizes are not corroborated with any criteria, and in this document we make an attempt to further the concept.

The minimum size of an ecosystem should be large enough for the survival of the majority of the species that belong to that ecosystem without human intervention (management). That means that those species must have viable populations, which requires that all natural ecological processes must proceed spontaneously. In naturally small ecosystems, the species that depend on such ecosystems must have low MARs or otherwise they could not persist under those space-restricted conditions, whereas a number of species primarily depending on large ecosystems may have much greater MARs.

Such criteria are likely to vary regionally as may appear from the following example. Paramos in Central America are typically small ecosystems, but – depending on the level of classification detail – they are probably typically medium size ecosystems in the vastness of the Andes region. The categorization of the typical sizes will need to be done on the bases of expert assessment, considering currently existing sizes, probable historical occurrence and ecological characteristics of each ecosystem.

As there is a principle difference in connectivity between terrestrial ecosystems and aquatic systems, the typical minimum sizes are dealt with separately. Most aquatic systems are so well connected, that even if the different recognised ecosystems are separated over significant distances, the populations are still connected by water.

Typically large terrestrial ecosystems

This document uses the case of Central America (Vreugdenhil 2002) as a very suitable example as it spans about 1,500 km of length, has a high level of detail for an ecosystem map based on satellite imagery and is of a very recent production. There are 11 ecosystems of more than 200,000 ha left, of which the "Tropical evergreen broad-leaved lowland forest, well-drained" is the largest. The tropical semi-deciduous broad-leaved well-drained lowland forest is slightly under that size, but this ecosystem must have been well spread along the Pacific lowlands. All these ecosystems are considered typically large ecosystems and are shown in Table 6. When analysing these very large ecosystems in Central America, all of them also occur in smaller natural patches within much larger clusters of natural ecosystems of different composition. A few of them – the ones marked TLE in Table 1, occur or probably have occurred historically as very large ecosystems almost anywhere before conversion of land for production purposes. The other ones exist in smaller sizes anywhere between what have been considered typically large or typically small ecosystems. Those are considered typically medium size ecosystems that shall be dealt with in later paragraphs.

What would be a responsible minimum area requirement of those ecosystems? The largest ecosystem, IA1a(1)(a), still occurs without major interruptions – except for the Panama Canal - from Colombia to the border of Costa Rica, and in the past it may have extended into Honduras and maybe, even Southern Guatemala. A case exists of well-documented isolation of this ecosystem for a period of about eighty years: Barro Colorado in Panamá, a 1,564 ha patch of forest, isolated from its surroundings by the rising water of the Panamá Canal since 1914. The area has been intensively studied since it was declared a biological reserve in 1923 ( 2002). Scientists have documented that 18 species of birds out of a total of 318 (, or 5%, have been lost since its detailed observation started in the early nineteen twenties. This suggests a considerable resilience of many species to size reduction through physical isolation.

An area of 10,000 ha – 6 times the size of Barro Colorado – of a stand-alone typically large ecosystem would provide viability to the majority of its animal populations with densities of 1 individual per 5 ha or denser, which is enough for most herbivores except the very large ones, as well as the medium-size predatory mammals.


        Table 8: Very large ecosystems in Central America with suggested typical dimension categories





        Tropical evergreen broad-leaved lowland forest, well-drained


        Tropical evergreen broad-leaved lowland forest, moderately drained


        Tropical evergreen broad-leaved submontane forest


        Tropical evergreen seasonal broad-leaved lowland forest, well-drained, Mosquitia variant


        Tropical evergreen seasonal broad-leaved lowland forest, well-drained, on rolling karstic hills

        IA2a(1)(a)K -r

        Tropical evergreen seasonal broad-leaved lowland forest, moderately drained


        Tropical evergreen seasonal broad-leaved lowland forest on calcareous soils


        Tropical evergreen seasonal broad-leaved lowland forest on calcareous soils


        Tropical evergreen seasonal needle-leaved submontane forest


        Tropical semi-deciduous broad-leaved well-drained lowland forest


        Short-grass waterlogged savannah with needle-leaved trees, Mosquitia variant


        Tectonic lake



This should be enough for the survival of most species. In the American Hemisphere, this does not provide MARs for the following species with large area requirements: Puma, Felis concolor, Jaguar, Pantera onca, Wolf, Canis lupus, Brown Bear, Ursus americanus, Grizzly Bear, Ursus arctos, Spectacled Bear, Tremarctos ornatus, Tapir, Tapirus ssp., Bison, Bison bison, (Nowak 1999), Harpey Eagle, Harpia harpya , (Grzimek et al. 1973) and King Vulture, Sarcoramphus papa, both condors, and probably some other species of mega-birds of prey. It would, however, under very favorable conditions provide MARs for many larger mammals, including all primates, all Deer species, both Peccaries, Tayassu ssp., and the Giant Ant Eater, Myrmecophaga tridactyla (See also Box 11). Under less favorable conditions, fewer species may maintain their population at a viable level. It should be emphasised, however, that if this targeted ecosystem size were designed to provide shelter for the last remaining populations of the larger mammals, it would be grossly undersized and inadequate. That, however, is not the purpose. It is meant as a building block for assessing ecosystem viability in protected area systems, which only in their combined composition maximise both species diversity and survival durability of all species, including the ones that require large territories.

Embedded ecosystemsSpecies are usually associated with one specific ecosystem, while in reality, many species live in habitats consisting of mosaics of one or more small-sized ecosystems embedded in one or more different ecosystems. Most mapped ecosystems are artificially cut up, while in reality, many species are distributed along gliding scales of density along gradual changes, which in turn, lead to development of meta-populations. As a result, individual species distributions unavoidably deviate from the mapped ecosystems and even many ecologically selective species belonging to small ecosystems, also occur in parts of neighboring ecosystems, albeit in different densities. It is very common that mapped small ecosystems embedded in larger ecosystems are complemented by finer-grained mosaics of similar conditions that can't be mapped, and which allow species to live in mosaics of much larger territories than an ecosystems map seems to suggest. Such small patches of ecosystems embedded in intact large ecosystems provide viable conditions for populations of organisms that have developed under those circumstances.

Typically small ecosystems (TSE) In principle, these ecosystems represent relatively rare conditions, such as high elevation levels, permanently isolated fresh water systems as well as ecological transitions or ecotones. These ecosystems often occur in areas of 5000 ha or less. With less common ecological conditions, one may expect relatively low species diversity but high endemism and occurrence of relative rare species. It would make sense that the species dependant on those conditions would occur in relatively high densities, that would allow for their continuation required for their populations to be viable, probably showing densities much higher than one adult per ha. Obviously, species requiring large territories cannot depend on typically small ecosystems. Some mobile species that live in low densities may require very small ecosystems for a specific function in their lifecycle, like reproduction or for bridging an unfavorable season in their habitat– while foraging in a much larger range. For TSE (not belonging to small islands and not embedded in larger ecosystems), it would be wise to strive for a minimum area of 1,000 ha if such ecosystems are isolated in a small protected area or located along the edges of a larger protected area. This would allow for MVPs of species requiring 1 individual per 0.5 ha. Embedded or on small islands, these ecosystems would not require a minimum size, as they occur in the size of their original development, which per definition should be considered viable. Examples are high elevation tropical peat bogs and dwarf shrub communities. When occurring at the outer limit of a natural habitat, an extra zone of a minimum of 200 m (see edge effects) should be allowed for an ecosystem to meet the 1,000 ha criterion.

Populations of species on small islands Much has been written on populations on islands since Diamond (1975). Given the special ecological position of islands, it would make sense to consider their ecosystems to be different from mainland ecosystems, even if their physiognomic and ecological conditions are very similar to those on the mainland. This means that frequently, ecosystems of small islands will fall in the category of typically small ecosystems. Particular attention needs to be paid to individual populations of endemic species or subspecies and it must be made sure that protected habitats exist where a specific MVP can survive. Also, be aware that usually, altitudinal zonation on islands is mostly compressed as compared to high continental mountains (van Steenis 1961, Grubb 1971), so that elevation zones cannot be compared directly to those at the same elevation on the mainland.

Typically medium size terrestrial ecosystems (TME) Between typically large and small terrestrial ecosystems are the typically medium size ecosystems, ranging from 5,000 to 200,000 ha. The characterization of this ecosystem size category is obviously between the other two. Given the clearly more restricted size of this category, migration is likely to be more restrictive and some endemism is more likely to have developed. By taking the lower level of the category as the MAR, this should allow for a reasonable safety level for MVPs of any organism living at densities of one reproducing pair per 2.5 ha. Under stand-alone conditions, this would probably still allow for prolonged survival of most medium sized ungulates (except tapirs) of the American Hemisphere south of the United States. Possibly, under very favorable conditions, one of the heaviest of the American monkeys, the Howler monkey (Alouatta palliata), may occasionally have MVPs under those conditions, assuming an average total population density (N) of about 1-3 individuals per ha (e.g. Higgins et al. 2000), as well as all other – smaller – New World monkeys; only the still larger Spider monkeys (Ateles spp.) may not. Giant anteaters, Myrmecophaga tridactyla, sometimes believed to live in large territories, have densities recorded of 1.3 per ha, which would allow for MVPs, which would also be the case for sloths, Megalonychidae, with densities recorded at 6-7 per ha, (Nowak 1995). Aquatic ecosystems Some of the most threatened ecosystems on earth include freshwater ecosystems, coastal ecosystems, wetlands and coral reefs (Glowka et al. 1994), and special attention must be given to the conservation of such ecosystems. This is not easy, as it is rarely possible to consider complete water systems. One must consider both the quantitative and qualitative aspects of the water system (ranging from watersheds, estuaries, coastal waters to minuscule isolated pools) as a whole, in which many recognised ecosystems are ecologically-connected inter-dependent subsystems.

Limnic ecosystems In limnic watersystems, ecosystems may be very small and specific species may be associated with them for at least part of their life-cycle. These ecosystems are often linear in shape and too small to be recognised and delineated from satellite images. Even though species may have specific ecological preferences, many populations of aquatic species cover much larger areas than the ecosystems where the majority of them are found during a specific time of their lifecycle. In other words, most small aquatic ecosystems will be embedded in larger aquatic ecosystems and their organisms, usually will have viable populations, if they live in a healthy watersystem. Protected areas rarely encompass complete watersystems, and their viability must be assessed on a case-by-case basis, taking into consideration the human activities that take place in a watersystem. Furthermore, dimensions of water bodies may vary greatly over time, depending on seasonal water tables and the meandering of rivers. Usually integral water management of an entire watershed is required to warrant the integrity of flora and fauna of gazetted wetland protected areas.

Transitional water-land ecotones are essential, but not all shore territory needs to be included in a protected areas system and special connectivity between such zones in one protected area usually is not essential, as non-territorially connected "stepping stones" are usually sufficient to connect populations over large distances. Furthermore, unlike terrestrial ecosystems, open water ecosystems are heavily dominated by fauna. With most limnic fauna being poikilothermic, species apparently can live at much larger densities then endoterm fauna species (Dobson 1996) in terrestrial ecosystems. No data have been found on fish species that could not survive in an isolated water of less than 1,000 ha with an MVP of a few thousand, but this requires further review. Thirdly, most terrestrial ecosystems are traversed by rivers, and they practically always include aquatic ecosystems. These aquatic elements surrounded by predominantly terrestrial ecosystems are usually part of watersystems that reach far beyond the protected area, and consequently, the viability of the aquatic species in such areas are subject to the integrity of those entire watersystems, or at least their upstream part. Given these considerations, no minimum area is considered necessary for aquatic ecosystems. Special consideration should be made for the few species that do require larger ranges, like the fresh water dolphins and manatees. Those species fall in the same concern as migratory fish (e.g. catfishes in the tropics and salmonoids in temperate climates): species that require species oriented attention, wielding protection or use-regulation and measures, such as fish-ladders, to warrant connectivity.

Marine and estuarine systems Marine ecosystems are part of enormous connected spaces with most species either being extraordinarily mobile, spanning very large areas or even entire oceans or enjoying very effective dispersal resulting from oceanic currents. When one speaks about marine protected areas, in practical terms, one must speak about protected areas in the exclusive economic zones of nations (UN, no date) of 200 nautical miles, as beyond that zone, individual nations lack sufficient jurisdiction to regulate ecosystem protection. Usually it is not (politically) possible to apply the traditional protected area concept to marine areas. The IUCN Centre for Mediterranean Cooperation (2003) convened a workshop in Malaga, Spain to address this issue and concluded that a worldwide network of Marine Protected Areas is required in the "High Seas", as they include rare and unique ecosystems and species found nowhere else on the planet. Those areas need to be created outside the territorial sea and Exclusive Economic Zones and above the "outer continental shelf" of coastal nations.

Within the coastal seas, restrictions to access are often far less accepted then on land. Protected areas usually involve a set of negotiated specific restrictions, to regulate resource use, rather than the access, that is commonly known for parts of protected areas. In a sense, providing shelter to organisms in marine protected areas is similar to providing shelter to migratory terrestrial fauna (e.g. migrating ungulates, shore birds, passerines). One provides oases of protection to populations that are exploited elsewhere as opposed to resident populations in self-sustained protected areas.

Estuarine areas vary greatly in size, but most would be in the range of typically medium size ecosystems. Intertidal zones and brackish inland lakes often are medium size (Including mangroves and extended mud-flat areas "Wadden"). Usually, brackish inland lakes are at least periodically connected and aquatic organisms move back and forth between brackish and freshwater conditions, brackish conditions and the sea, or all three. Dimensions of the water bodies may vary greatly in time, either through tidal movements, seasonal influx of water and morphological variation of the area.

Large protected areas The size of a country permitting, one should strive for at least one area or complex of contiguous areas of a minimum of 1 percent of the national territory, or a minimum of 1,000,000 ha, preferably larger, in which large birds of prey and mammalian predators may keep up a healthy population and where large herbivores may roam.

In general, animals with large territories are more ecologically tolerant and may span plural habitats and be resilient to human presence if not hunted. Many may leave natural habitats and roam through rural areas. If left alone, individuals may connect with populations of their kind in other protected areas, thus breaking their genetic isolation. In most rural societies, farmers are inclined to hunt down every predator that roams the region, and most farmers don’t enjoy the visit of a herd of elephants either. Their need for hunting might be diverted if farmers are compensated for the occasional kill of a domestic animal or crop loss (Vreugdenhil 1992), e.g. in the Netherlands farmers are compensated for crop damaged caused by wintering geese (van Dijk, pers. com.). This will be dealt with in Chapter #.

This page  is part of our web-book on Biodiversity Conservation. For organized reading go to our on-line Table of Content, or download our book in pdf format.


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