The design of protected areas is an important field of research in conservation biology. According to the International Union for Conservation of Nature (IUCN), a protected area can be defined as;
“…a clearly defined geographical space, recognised, dedicated and managed, through legal or other effective means, to achieve the long-term conservation of nature with associated ecosystem services and cultural values.”
Before the 20th century, there were only a handful of protected areas around the world, although the concept of maintaining areas for the purpose of preserving natural values is not necessarily a recent phenomenon. Among the many historical examples of such areas include the sacred groves of Asia and Africa and the establishment of royal hunting forests introduced in the Frankish kingdoms of continental Europe in the seventh century. However, the first protected areas that were introduced for the purpose of mitigating the effects of habitat loss and the preservation of their dramatic landscapes and associated biodiversity were Yosemite and Yellowstone National Parks, both of which were designated in 1864 and 1872 in North America respectively. They were then followed by the establishment of Royal National Park near Sydney, Australia, in 1879, and later on by Kruger National Park in South Africa in 1892 (Hugh P Possingham et. al, 2006). Today, there are approximately 200,000 protected areas in the world, which cover around 14.6% of the world’s total land mass and around 2.8% of the world’s oceans. Their presence does not only provide a means for conserving nature but also provide a wide range social, environmental and economic benefits to people and communities worldwide that serve as a means to deal with more pressing challenges such as food and water security, human health and well-being, disaster risk reduction and climate change (IUCN, 2014). The key issues that are concerned with reserve design involve size, shape, and positioning of such protected areas so as to optimize their ability to protect biodiversity. The four R’s of designing network reserve is a good way of representing these issues in an orderly manner and they include the following;
– Representation, a reserve should contain as many aspects of biodiversity (species, populations, habitat, etc.) as possible.
– Resiliency, a reserve must be sufficiently large and well managed to maintain all aspects of biodiversity in adequate condition for the benefit of present and future generations.
– Redundancy, a network of protected areas must represent each aspect of biodiversity so as to ensure the long-term existence of the unit in the face of future uncertainties.
– Reality, there must be sufficient funds and political will to acquire and subsequently manage the protected area.
The SLOSS debate;
It is a conceived notion that larger is better. Most conservation biologists also recommend that protected areas be as large and numerous as possible. However, due to economic and political constraints other reserve designs have been contemplated which are more controversial. Controversy over the design of protected areas involves the following key aspects;
– What is more suitable, one large reserve, or a number of smaller ones of the same total area? This issue is commonly denoted by the acronym SLOSS, which refers to single large, or several small. According to ecological theory, populations in larger protected areas should have a smaller risk of extinction, compared to those in smaller reserves. However, if there are populations in several different reserves, their redundancy might actually prevent extinction in the event of a loss in one reserve;
Jared Diamond, in a paper entitled “The island dilemma: Lessons of modern biogeographic studies for the design of natural reserves” (1975). Diamond proposed that a single large land reserve would be more beneficial in terms of species richness and diversity than several smaller reserves. This claim was based on the theory of island biogeography, which was proposed by Robert MacArthur and E. O. Wilson (1967) in the self-entitled book “The Theory of Island Biogeography”. MacArthur and Wilson’s theory suggested that species richness is in dynamic equilibrium between immigration and extinction and is as such affected island size and isolation. Therefore the larger the island and the closer it is to other islands, the more species that it can support. Diamond envisioned that a system of natural reserves, each surrounded by altered habitat, resembles a system of islands from the point of view of species restricted to natural habitats. Diamond’s findings basically corroborated with MacArthur and Wilson’s theory that the number of species that a reserve can hold at equilibrium is a function of its area and its isolation. Hence, larger reserves and reserves located close to other reserves can hold more species (Diamond, 1975).
Wilson and Daniel Simberloff, a student of his at the time, had already experimentally tested the theory of island biogeography, in a paper entitled “Experimental zoogeography of islands – A two?year record of colonization” (1970). However, Simberloff and Lawrence Abele in an article entitled “Island biogeography theory and conservation practice” (1976) suggested that the idea behind the theory of island biogeography was merely based on the assumption that larger reserves contain more species than smaller ones. They pointed out that it could be the case that, for smaller reserves having unshared species, two or more smaller reserves might actually possess more species than a single large reserve (D. Simberloff & L. Abele, 1976). Simberloff had thus challenged his previously held notion that a single larger reserve is inherently better than several smaller reserves and thus laying the groundwork for what was to be one of the most heated controversies in conservation history, known as the SLOSS Debate.
– Reserves can also be designed to have less edge habitats, transition habitats between ecosystems known as ecotones. An edge habitats often act as conduits for the introduction of invasive species and predators, which could cause some significant problems in protected areas;
In 1985, Bruce A. Wilcox and Dennis D. Murphy responded to Simberloff and Abele’s claims with a paper entitled “Conservation strategy – effects of fragmentation on extinction”. In it Wilcox and Murphy did not assess the SLOSS issue per se, but rather they pointed out the assertion made by Simberloff and Abele that habitat fragmentation has no detrimental effect on most species, as smaller fragmented reserves that have unshared species might harbour more species when compared to a single large reserve, and therefore should not be considered when designing natural reserves. Wilcox and Murphy argued that this assertion was in contradiction to the prevailing view, at the time, that habitat fragmentation negatively affects population survival and is t probably a major threat to the loss of global biological diversity. They go as far as to say that the SLOSS issue is not equivalent to, or at best a special case of, the problem of habitat fragmentation (Bruce A. Wilcox & Dennis D. Murphy, 1985).
Wilcox and Murphy’s assertions paved the way for the establishment of the Biological Dynamics of Forest Fragments Project (BDFFP) in 1979 by Thomas Lovejoy and Richard Bierregaard. The project was set at an area covering roughly 1,000 km2 in the north of Manaus in the central Amazonian rainforest, Brazil. Ranchers in the area chosen for study were in the process of clearing the land for development but Lovejoy managed to strike a deal with them to leave a number of small areas untouched. Lovejoy and Bierregaard, along with a team of researchers segregated the plots by dripping burning rubber onto forest debris around their perimeter. The experiment was set up to test fundamental theories about the viability of small, disconnected ecosystems caused by the effects of habitat fragmentation. By 1983, results showed that there was a substantial loss of key species within the edges of these plots. These early results suggested that scientists at the time were underestimating the broader impacts of fragmentation and by 2003 Lovejoy had recorded a 50% decline in the number of bird species living beneath the canopy in the plots over the first 15 years of isolation. The cause for these substantial declines in species richness was attributed to “edge effects”. Such effects included sunlight and air circulation such that as the pastures and forest edges heated up each day, the air over those regions rises, drawing cool moist air out of the forest. This sudden introduction of hot dry air has a significant impact on large hardwood trees such as mahogany and ebony. Moreover, the open fields also expose the forest to wind forces, which blow down trees and further opens up the canopy, which would have a significant impact on any arboreal species that might reside there (J. Tollefson, 2013).
The metapopulation perspective;
During the mid-1980s there was a shift from the theory of island biogeography which places an emphasis on the equilibrium of species richness, to that of populations, which focuses on individual species (T.Good & J. P. Rodríguez , 2012). This approach to the SLOSS debate is particularly concerned with the patchiness of populations in space, and the role of these patches in population dynamics, stability, coexistence of species, and the maintenance of biodiversity. A patch, in this manner, is essentially an area of suitable habitat for a particular species or particular collection of species i.e. local population, ideally bounded by unsuitable habitat, or habitat with different physical properties. The collection of all local populations, known as the metapopulation, can only persists if local extinction is balanced by re-colonization from surrounding patches. A “classical metapopulation” satisfies the following conditions;
(i) Local populations are partially isolated from one another and are frequently capable of sustaining themselves for several to many generations in the absence of immigration from other local populations
(ii) Local population extinction occurs on a time scale of several to many generations.
(iii) Migration between local populations leads to re-establishment of local populations following local extinction.
How the balance between these conditions is achieved is the principle behind metapopulation theory (Chesson, 2013).
Most human activities such as urban development tend to fragment natural habitats in such a way as to artificially create metapopulations or indeed decrease the density of local populations within an existing metapopulations thus endangering the continued survival of natural populations. However, it is widely accepted that essentially all natural populations are patchily distributed in space in some way or another, and that patchiness in space and time has functional roles in population dynamics over time (Chesson, 2013). The importance of patchiness can be observed from the point of view of breeding populations such as the Western Snowy Plover, Charadrius alexandrinus nivosus. This avian species is one of many species to be included on the Endangered Species List in the USA and is currently listed as vulnerable. Its inclusion on the Endangered Species List was partly due to the analysis of their metapopulation dynamics which highlighted the need increased management of the species and its respective habitats. Habitat degradation caused by human activity, together with expanding predator populations, resulted in a decline in nesting areas and ultimately the size of the breeding population (H. Resit Akc¸akaya et al., 2006).
An already sparse population that was evenly distributed over an area may have a low reproductive rate because males and females do not encounter each other as much for many eggs to be fertilized. The importance of patchiness comes into play such that high local concentrations or aggregations of the species in smaller areas may decrease the problem of low encounter rates between males and females (Chesson, 2013).
Based on the observation, that an increasing number of patches will result in a higher chance of finding a mate when compared to one large area in which the population is evenly distributed, metapopulation models projected that recovery of the species was possible if each breeding pair had produced at least one or more viable offspring into the population. However, this was only possible if intensive short-term management of such areas was enforced such as the setting of area enclosures and predator control methods (H. Resit Akc¸akaya et al., 2006). Habitat fragmentation is generally believed to be detrimental to the persistence of populations and indeed there are some instances in which species may suffer from the effects of habitat fragmentation. These include the risk of increased local extinction due to the impact of demographic stochasticity, which is the variability in population growth, on smaller habitat patches. However, as seen in the example of the Western Snowy Plover, the re-colonization of empty habitat patches by dispersers from extant patches may cause the population to persist at the metapopulation level. This brings into question as to what degree does habitat fragmentation become harmful (S. R. Zhou & G. Wang, 2006).
In a paper written by Shu-Rong Zhou and Gang Wang (2006), they assess the issue of reserve design in the context of metapopulation dynamics with particular concern for target species subjected to what is known as the Allee effect, which refers to the correlation between population size and the mean individual fitness. Zhou and Wang suggested that the survival of a metapopulation increases as the number of designated reserve areas increase however, this then decreases as the size of each reserve decreases. Moreover they argue that the metapopulation cannot persist for long irrespective of how many reserves there are, if the Allee effect is too severe. The significance of such an effect when approaching the issue of reserve design from a metapopulation perspective is such that for optimal persistence of a metapopulation there has to be an intermediate number of reserves in which the Allee effect is expressed in a moderate manner as it allows for an equilibrium to be achieved between extinction and re-colonization (S. R. Zhou & G. Wang, 2006).
The single species focus of many metapopulation studies is a significant limitation when considering reserve design. In fact, much of conservation management is concerned with communities rather than an individual population. Even where a single species is targeted for conservation, its survival and fecundity will often depend on interspecific competition within the trophic level or predation from higher trophic levels. For example, in the case of wild dogs, an important objective is to restore their ecological role as predators. This requires research into the viability of wild dog to prey interactions in confined areas. It is often conceived that the reason as to why metapopulation models tend to focus on single-species dynamics is because these are better understood than complex food-web and ecosystem processes (H. Resit Akc¸akaya et al., 2006).
By adding more species into the system more dimensions need to be factored into the analysis to account for both exclusive and shared occupancy of suitable habitat patches. This therefore, increases the number of parameters needed for estimation and thus may introduce errors in the projected models. The general lack of understanding and data on multispecies interactions means that few empirical metapopulation studies have sufficient parameter estimates to model community dynamics (H. Resit Akc¸akaya et al., 2006).
The metapopulation concept is important in conservation biology, especially when considering the design of natural reserves as habitat fragmentation creates distinct local populations even if they could not be defined before human interference (Chesson, 2013). However, the limitations imposed by the focus on single species in many metapopulation studies do not take into account community dynamics, which are important when considering the design of natural reserves.