Time and space are two extremely important concepts that are central to formulating theories and models in biotic conservation. Following along the themes expounded by Schumm and Lichty (1963?) in “Time, Space and Causality”: “It is the purpose of this discussion to demonstrate the importance of both time and space (area) to the study of geomorphic systems. We believe that distinctions between cause and effect in the molding of landforms depend on the span of time involved and on the size of the geomorphic systems under consideration” (Pp. 110). Although the paper quoted above is only concerned with geomorphology, the arguments being made are logical with the substitution of biotic conservation in place of geomorphic systems.
Knowledge and understanding about a given ecosystem or species is crucial to its conservation Short term studies do not allow for a reasonable gain in knowledge to allow intelligent decisions regarding preservation. Paul Ehrlich (1994) states, it is ‘difficult to draw conclusions on ecosystem impacts without detailed knowledge of the system. Observations taken over a short interval (a “snapshot”) will often miss essential elements, as will a lack of understanding of keystone roles”. Time is important to gather the data and information necessary to understand nature.
Another major practical problem in conservation biology is to be able to predict the survival times-“lifetimes”-for small populations under alternative proposed management regimes. This terminology is referred to as “minimum viable population” or “MVP”. It is commonly defined as the minimum number of individuals of a certain species required to ensure that the population will survive over the next 100 years. In constructing such a concept, the underlying assumption is that there is some threshold for the number individuals that will insure that the population will endure in a viable state for a given period of time.
The processes of determining MVP, called “population vulnerability analysis” is not an easy task. Earlier works focused on different aspects to determine MVP. MacArthur and Wilson (1967) in their “Theory of Island Biogeography” focused on demography as the determining factor. The central point of their analysis was based on birth and death processes.
Other researchers such as Schoenwald-Cox et al (1983) accentuated a genetic approach. They asserted that if a population dropped below a certain size then what is know as genetic “bottlenecking” would occur. The analogy is one which all the genetic resources of a species are condensed into a few individuals, hence the “bottlenecking” effect. The limiting of genetic variation to a few species can have deleterious effects such as inbreeding and loss of adaptability to stochastic events.
Mark Schaffer (1981) was the first researcher to advocate an overall systems approach. He concluded there were four forces that independently contributed to the extinction of a population that needed to be taken into consideration when trying to determine MVP. Along with demographic and genetic stochasticity, he also included environmental stochasticity (those environmental shocks received by all members of the population) and catastrophes.
Determining the minimum viable population is a complex issue. The four forces mentioned above are influenced greatly by the characteristics of space in which they are considered.
As the research of MacArthur and Wilson (1967) on islands has demonstrated, the larger a given island, the more number of species it can support. Unfortunately, this increase has been shown to be linear. A ten fold increase in space will only support a two fold increase in species diversity (Caughley and Sinclair 1994).
The conceptual framework of the Theory of Island Biogeography has been the model for the analysis of the effects of fragmentation in mainland sites. Increasing anthropogenic influences have resulted in a process termed “insularization” – the fragmentation of natural habitat into isolated pieces. Like the water surrounding a natural island, the human altered environments serve as barriers to many of the natural processes.
Based on the many models and theories generated from the Theory of Island Biogeography, there are conceptual questions that have arisen when attempting to create nature reserves. Out of these are three main queries:
- How much of the available habitat must be set aside as reserves and in what distribution of sizes?
- Should reserves be clustered together in close proximity to each other, or spread out over a broad area?
- What is the optimum shape for reserves
The second part of the first question is frequently referred to as the “SLOSS” question, an acronym for Single Large or Several Small reserves. While it has been established that larger areas of land can support a greater species diversity than can smaller areas, the establishment of one large reserve is not always the best option.
The main reason why establishing one large reserve is not always the optimum solution is simply a lack of adequate natural space. The ability to create a reserve large enough to allow for the survival of all species is simple not a reality for most areas. In his analysis of wildlife parks in the western portion of North America, W.D. Newmark (1987: pp. 432) concluded: “The natural post-establishment loss of mammalian species in western North American national parks indicates that virtually all western North American national parks were too small to maintain the mammalian faunal assemblage found at the time of park establishment”.
The establishment of large reserves is especially unfruitful when the conservation of large mammals is the main concern. Referring back to the concept of MVP, many large mammals require enormous tracts of land to exist. For example, one pair of mountain lions alone requires 30 square kilometers to survive (Caughley and Sinclair 1994). Multiply that by the minimum number of lions needed in a given area to meet MVP criteria and it results in a huge requirement of land to be committed into a reserve. A requirement that is not realistic.
What can then be established as a compromise is a set of smaller reserves that are interconnected via a network of undisturbed natural areas called wildlife corridors. The corridors (as the name implies) serve as passageways between the reserves and have the effect of allowing the two reserves to become one large “pseudoreserve”.
Behind this concept is the assumption that species do not uniformly inhabit space. Even within a suitable habitat not all areas are use. Competition, environmentally variability inhibit occupation of certain areas. Building upon this, reserves can then be established in the only most suitable areas with links to connect them.
However, this solution is not a panacea. As many benefits corridors may have, there is an equal number of detriments. These included exposure of diseased individuals, pests, exotics; promotion of fire and other abiotic disturbances, and an increase of a phenomenon known as the “edge effect”. The edge effect are effects that result from the proximity of natural areas to altered areas such as feral predation and anthropogenic disturbance.
Thus, each conservation situation calls for a different approach to the SLOSS question and the other two main questions as outlined. In the end there will always be a trade-off between increased heterogeneity of habitat by creating smaller, but more, reserves, versus sustaining low-density and therefore area-sensitive species, by maintaining fewer, larger reserves.
Biotic conservation is a complex process. It involves the consideration of the long-term needs of both natural and anthropogenic uses on a spatial level. The Theory of Island Biogeography has been heavily leaned upon in the formulation of models and concepts for the conservation of fragmented natural landscapes which mimic the processes of islands. In order to create a natural reserve which will incur the least amount of species loss, consideration must be given to the minimum viable population of each species. This is greatly influenced by the size, shape and proximity of reserves. A successful reserve will be able to support a species diversity comparable to the undisturbed landscaped over the long-term.
Caughley, G, and A.R. Sinclair. 1994. Wildlife ecology and management. Oxford: Blackwell Scientific Publications.
Ehrlich, P. 1994. Foreword. in E.D. Schulze and H.A. Mooney (eds.) Biodiversity and Ecosystem Function. Berlin: Springer-Verlag.
MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton: Princeton UP.
Newmark, W.D. 1987. A land-bridge island perspective on mammalian extinctions in western North American parks. Nature. 325(29): 430-432.
Schoenwald-Cox, C.M., Chambers, S.M., MacBryde, F., and L. Thomas (eds.). 1983. Genetics and conservation: A reference for managing wild animal and plant populations. California: Benjamin/Cummings.
Schumm, S.A., and R.W. Lichty. 1963(?). Time, Space and Causality in Geomorphology.
Shaffer, M.L. 1981. Minimum viable population sizes for species conservation. Bioscience 31(2): 131-134.
Soule, M.E. (ed.). 1986. Conservation biology: the science of scarcity and diversity. Michigan: University of Michigan.