Introduction to Biogeography and Conservation Biology
The first requirement in any subject is to define the limits and the connections in it. The textbook standard definition of biogeography is that it is the study of the distributions of species over space and time and the causes of those distributions. It has two basic components: descriptive biogeography and ecological biogeography. The first part studies the geography of species and the second part deals in the causes of those distributions. Clearly the first part is a necessary component, but it is the second part that is more exciting. The subject as a whole is dynamic because its subjects - biological populations and species- respond dynamically to causative factors over time and space, and because it is a relatively young science.
The time scales which you will see are important in biogeography range from days, weeks, or a few months, i.e. periods we can describe as ecological time, to millions of years, which constitute evolutionary or geological time scales. Physical events or changes driving these dynamics range from rapid effects of nonindigenous species (also called alien species) introductions, which may occur over short periods, to continental drift and plate tectonics, in which the most recent continental movements have occurred over the last 200-230 million years.
To indicate the dynamic character of biogeography, we need only consider the recent history of the subject, and the impact of the theory of continental drift on biogeography. The concept of a plastic mantle for the earth, plate tectonics, and continents moving, carried on the plates was only developed during the 20th century. The theory was initially proposed by Wegener in 1912. As recently as 1957 there is clear evidence that theory had not been widely assimilated and accepted. One of the leading biogeographers of the 20th century, Philip Darlington rejected the concept of continental drift in the first version (1957) of his classic book. Other biogeographers, as well as Darlington in a revised edition (1965), rewrote ideas about explanations to include the impact of continental drift. Today, no biogeographer questions the impact of drift.
Similarly, we can look back to Darwin to appreciate his recognition of islands as important natural experiments in ecology and evolution. However, island biogeography was initially viewed as an interesting sidelight to larger problems of continental distribution. Now island processes are seen as important in understanding the population dynamics and genetics of species having patchy distributions, which, at one scale or another, characterizes the distributions of almost all species. Islands are now seen to include situations as diverse as oceanic or fresh water islands or lakes in a terrestrial matrix, habitat islands, peninsulas, woodlots, clumps of zebra mussels on lakebeds of the Great Lakes, and even patches in the abundance distribution of species scattered widely over an area. Lake Huron is an aquatic island on the North American continent, Manitoulin Island is an island in Lake Huron, and lakes are biological islands on Manitoulin Island. Island concepts are also important in considering how best to preserve species and communities as humans impose themselves on natural environments. We can trace the recognition of the broad importance of islands to important publications by Robert MacArthur (1963) and Edward Wilson (in 1967).
The theory of island biogeography is a critical component in the design of natural preserves and in assessing the likelihood (or time) of persistence for endangered populations. However, even as it is indicated as important, it should also be recognized that aspects of the theory are still very contentious. There was an extensive literature, mostly from the early 1980's, arguing whether the statistical tests of observations based on the model, particularly occupancy and coexistence on islands, were correct. Dan Simberloff argued that appropriate null hypotheses, against which to test observations and model predictions, had not been formulated. Related arguments about whether communities are random assemblages from pools of potential colonists, or whether there are discernable assembly rules that can predict which species (or at least species from particular ecological groups) will be represented together in established communities. Some aspects of biogeography remain highly relevant to conservation programs today, although the same issues can be addressed through studies of sub-divided populations, called metapopulations.
The science of biogeography can only realistically have begun when its students got past simply describing and naming new species, and into describing habitat characteristics and relationships among species (e.g. competition, predation, mutualism), dispersal capabilities in relation to geological events, climate and climate change, and continental drift. That is not an exhaustive list, but fairly inclusive. The first attempts at this sort of analysis are traceable to phytogeography in the first two decades of the 19th century. In 1805 Alexander von Humboldt developed quantitative indicators of relationships between plant species and climate, indicating an initial subdivision of climate types. Humboldt made extensive collections of plants and associated environmental variables throughout Latin America during a 5 year research trip, from tropical forests to Peruvian alpine communities. That first classification was rapidly developed further, particularly by DeCandolle in 1813, but summarized more thoroughly in Candolle (1855).
Not long afterward, first Lyell, then Darwin and Wallace, turned the world of geology, biology and biogeography on its collective ear. Lyell proposed gradual change in the geographical features of the earth. In the process, he developed the concept of uniformatarianism, the idea that processes today are identical to processes operating in the past. It is that concept that allows us to infer history from observations made in contemporary time. If the forces were different, then science, and certainly biogeography, would make little progress. Uniformatarianism does not state that the way things are happening now is identical to the way they happened in the past, but rather that the forces or processes that determine pattern today are identical to the processes which drove changes in the past. Darwin and Wallace, in their separate collecting trips in the new world and southeast Asia respectively, observed species distributions which were critical to the development of the theory of evolution. Darwin noted the presence of shells of marine gastropods high in the Andes in southern Argentina and Chile, and recognized that these areas must once have been marine for shells to be found there. That is, a significant proportion of his information was biogeographical. One of the results of the revision in biological thought caused by the theory of evolution was a parallel revision in biogeography, which could be argued to begin with the works of Ernst Haeckel. He named a special discipline called chorology, which was the study of the spatial distributions of organisms and their causes. One of the major components explaining the change in species' distributions was evolution, and we now call the subject biogeography, which he called chorology.
We all know that one of the driving forces which caused Darwin to publish his theory was the parallel development of a nearly identical theory by Alfred Russell Wallace. Darwin is regarded as the father of the theory of evolution, but both men made very substantial contributions. Wallace and Darwin differed in their beliefs concerning the forces which drove evolutionary change. We know Darwin's belief about biological interactions driving evolution. Wallace felt that abiotic forces were of great importance. One probable reason is differences in the organisms whose evolution was studied. Wallace made his observations mostly on insects, which are more likely to be affected by climatic change or difference. Both Darwin and Wallace made numerous further contributions, but Wallace is regarded as the father of zoogeography. Because animals are mobile, and because so long was spent simply categorizing insects (because there are so many), 'animal biogeography' took longer to get off the ground. The earliest, still valuable animal biogeography is Wallace's master work, The Geographical Distribution of Animals. In it Wallace described the transition in terrestrial fauna between Australia, with its associated islands, and the islands extending out from southeast Asia towards Australia. The exchange of fauna (excepting animals capable of flight or transferred by man) is extremely limited. Two lines have since been drawn by later biogeographers to demark this transition. Weber's line encloses the region in which the mammalian fauna is exclusively Australasian, and west of the Celebes, between Bali and Lompok, is Wallace's line, which marks the outer limit of the Asian (or Oriental) mammalian fauna. In the narrow zone between these boundaries there is limited mixing; the area is called Wallacea.
Figure 1 - A map of Austalasia, with the positions of Weber's and Wallace's lines marked.
It could be argued that the 60-70 years following Wallace's zoogeographic work represented a period of consolidation, of data gathering, which was the basis for the radical developments of the last 20-30 years. That, however, would miss various important contributions which occurred in the late 19th and earlier 20th centuries. Among these are included:
1. Bergmann's rule which states that warm blooded animals from cooler climates have larger body sizes and lower surface to volume ratios. That change in body plan is clearly adaptive to restrict heat loss in cooler climes, and to maximize heat dissipation in warmer areas. A good comparison to indicate the difference is to compare the body form of arctic hares (larger bodies, generally 'more spherical') with those of temperate cottontails or jackrabbits (smaller bodies, on average 'longer and leaner').
2. Allen's rule which says that warm blooded animals will have more compact extremities in cold climates than warm ones. The underlying reasoning is the same, i.e. optimization of design for heat dissipation in relation to climate. Following the same comparisons, limbs and ears are shorter in the arctic hare, longer and thinner in the cottontail, and notably long in the jackrabbit.
3. Merriam's classification of altitudinal and latitudinal vegetation types and zones, termed life zones, and their relationship to temperature and rainfall. Most modern diagrams of Merriam's zonation present it as a three axis system, in which the climatic axis is potential evapotranspiration. This represented an advance on the earlier plant biogeography. Merriam attempted to generalize his classification scheme to animals, and failed.
Figure 2 - Merriam's life zones.
Finally, we come to the quantitative theory of biogeography, traceable to the monograph by MacArthur and Wilson. While controversies have since developed about the breadth with which the basic theory can be applied, much of its basic structure deserves the same comment that Huxley made upon reading Darwin's The Origin of Species: "stupid of me not to think of that myself".
The history of conservation biology parallels that of biogeography to at least the degree that they overlap. There are unique aspects, however. In this case we are interested in the history of extinctions and the recognition of human impact. Humans (native Americans and Inuit) are widely regarded as responsible for the extinction of a variety of large mammals in North America including mastodons, tapirs, glyptodonts, and giant ground sloths. Humans have long cut forests. In Greek times, the forests of the Baltic area and those in southern Asia were cut for ship building. Tropical forest has been cut for centuries in the course of slash and burn agriculture. When one patch used for subsistence agriculture gave out, a farmer moved to a nearby patch, cut the trees down, then burned the logs, releasing the nutrients tied up in the biomass. Until the soil hardened (laterization) and nutrients percolated down or washed away, the farmer used the patch. When yield dropped, he moved on again. The contemporary problem is the effect of increased population size, meaning larger areas cut and more frequent exhaustion of areas, combined with other impacts mentioned earlier.
There are some common themes. One of the most important is the 'tragedy of the commons'. If everyone takes advantage of common, public areas, each thinking his or her small impact is not important, the summed result is the destruction of the commons. Garrett Hardin, in one of the seminal papers of conservation ethics, wrote about the occurrence of exactly this phenomenon in the commons of New England towns, where grazing one extra cow was thought to be insignificant. The same thing happened in Europe, where royal preserves and the manor lands of the wealthy were unavailable, and the public lands (the commons) were deforested to provide charcoal for heating and industry. There the industrial revolution was the last straw, and Great Britain was largely deforested by the end of the 18th century. It is not that the conservation ethic has not been recognized for far longer than the discipline has existed; it is that scientific efforts to develop a framework for broad principles of conservation have only developed recently.
Conservation biology is a much newer discipline than biogeography. It is always difficult to set a time of origin for something as abstract as a discipline, but the best guess would be to say that conservation biology came into existence as a distinct discipline with the recognition in the 1970's that the rate of extinction of species globally, partially to largely due to the influence of human activities and population growth, is now high enough to parallel rates which were previously only 'seen' during some of the megafaunal extinctions of the past. Therefore, conservation biologists describe the current scenario as a "biodiversity crisis". The rates they quote are almost certainly conservative and lower than actual rates of species loss. The reason is that the areas of greatest diversity, e.g. tropical rain forest and coral reefs, are also areas with the greatest numbers of as yet undescribed species and simultaneously areas under severe pressure from human activities. Tropical rain forest is being cut for fuel wood, lumber, and conversion to pasture land at a rate which will lead to the elimination of all but small protected reserves within about the next 50 years. Coral reefs are affected by physical and chemical degradation due to human activities on nearby terrestrial areas.
Conservation biology is a synthetic science, built from ecology, population biology, population genetics, biogeography, economics, anthropology, philosophy, and probably other disciplines with the intention of developing principles and strategies to preserve diversity. Different approaches may attempt to maintain the diversity of species directly, or through maintenance of a diversity of habitats. It must maintain a balance between the potential desire to preserve everything in a pristine natural state, and the political desire to permit intensive development. The buzzwords among those who work hardest at rational balance are "sustainable development", though there is not yet an established theoretical basis for it.
The main objectives of Conservation Biology are to:
· Human population growth has accelerated during the past 400 years, and 100 years in particular; and resource demands have risen accordingly;
· Present threats to biodiversity are unprecedented; never in human history have so many species and habitats been threatened;
· Many of the threats to biodiversity are synergistic (additively or multiplicatively)
o e.g. eutrophication (i.e. nutrient enrichment) and overfishing;
o e.g. species invasion and climate change (e.g. malaria in North America)
o species invasion and overharvesting: e.g. extinction on Feb. 1, 1996 of Polynesian tree snail (Partula partula). This species succumbed to combined pressures of human exploitation of its pretty shells, and a nonindigenous mollusc predator introduced for biocontrol of a different species;.
o Environmental deterioration may signal pending human misery
1) e.g. history of humans and environmental disaster that occurred on Easter Island (south Pacific)
2) recent history of the Aral Sea. Salinity of this inland 'sea' has increased tremendously as humans appropriated (diverted) water inflows for agriculture irrigation purposes. As the basin's surface area and volume have declined, it has become increasingly inhospitable to human usage (e.g. as a source of fish).
Conservation Biology is a very new discipline; its most prominent journal Conservation Biology was created in only 1987 and the Society for Conservation Biology (created by Michael Soulé, Paul Ehrlich and Jared Diamond) was founded in 1985. The society has grown explosively since then.
Basic Principles of Conservation Biology (Meffe and Carroll 1997)
These principles are largely an offshoot of the statement by the famous ecologist G.E. Hutchinson in which he stressed the 'ecological theater and the evolutionary play'. Ecological events help shape evolutionary patterns of species and communities; in order that evolution be permitted to occur more or less unencumbered, we must not destroy the habitats and species that facilitate evolution.
Richard Primack (1994) has also established basic principles:
Candolle, A. de. 1855. Géographie botanique raisonée. 2 vol. Paris, Masson Editeur.
Clausen, Keck, and Heisey. 1948. Experimental studies on the nature of species. III. Environmental responses of climatic races of Achillea. Carnagie Institute, Pub. no.581.
Darlington, P. 1957. Biogeography of the Southern End of the World. Harvard Univ. Press, Cambridge, MA.
Hardin, G. 1968. The tragedy of the commons. Science 162:1243-1248.
MacArthur, R.H. and E.O. Wilson. 1963. An equilibrium theory of insular zoogeography. Evolution 17:373-387.
MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton Univ. Press. 203p., Princeton, NJ.
Meffe, G. and C.R. Carroll. 1997. Principles of Conservation Biology. Sinauer, Sunderland, MA.
Primack, R.B. 1994. Essentials of Conservation Biology. Sinauer, Sunderland, MA.