Biological Processes in Biogeography Introduction

--PART III-- Biological processes in biogeography Introduction The uneveness of species distributions over the globe forms a basic characteristic of living organisms. Biogeographers have frequently tried to explain these distributions by reference to preconceived biogeo graphic processes. This has often resulted in authoritarian narratives that really neither test the process invoked, nor propose possible alterna tives. A more rigorous explanatory app.roach is to search for consistent patterns amongst the various species distributions and then use these patterns to discover the processes which underlie them. Indeed, the discovery of such processes is a prime function of biogeographic analysis. Rosen (Chapter 2) conjectures as to whether there are any uniquely biogeographical processes, but whether there are or not, geographical (tectonic, eustatic, climatic, oceanographic), evolutionary (adaptation, speciation, extinction) and ecological (dispersal, biotic interactions) processes are all implicated in pattern forming. Intuitively, the assump tion is that large scale abiotic processes exert their effect in conjunction with evolutionary processes, through vicariance or through merging of biotas, whilst ecological processes cause changes in pattern through biotic interactions and colonizing jumps and are in tum affected by small scale abiotic factors. ADAPTATION AND ECOLOGY At a very basic level, to understand why a particular organism or group of organisms is found in a specific place, we must know something about their general biology i.e. the nature of the adaptations which allow the species to live where it does. Different genetic strains or phenotypes of a species, often called ecotypes, can themselves be differentially adapted to their local environmental conditions and hence be restricted to certain habitats. These intraspecific habitat and regional differences tend to broaden the ecological tolerance ranges of many species. This intraspecific level of adaptation is explored in Chapter 6. In broad biogeographic terms, adaptation is important on two dif ferent scales; a spatial one, influencing distribution patterns at a given time; and a temporal one, involving the influence of a change in conditions on adaptation and the indirect effect on distribution patterns 150 Biological processes in biogeography over time. But how can we define the organism's world in order to understand the biogeographic role of the process of adaptation? This is the problem of defining the ecological niche. We can conceptualize two states of the niche following Hutchinson (1957). The potential, or fundamental niche represents the sum of a species' physiological, morphological and developmental adaptations (or tolerances), and hence the range of environments within which the species can potential ly survive and replace itself indefinitely. This potential range is usually reduced to an actual or realized niche by the activity of sympatric species (including competitors, predators and parasites) or by some other factors. In the biogeographic context, we are mainly concerned with the spatial aspect of the niche, the habitat and range dimensions. The fundamental niche can describe the maximum range, encompassing all suitable habitats, over the globe. The realized niche describes the actual range the species occupies, controlled partly by activities of other species at a local level and at a global level by geographical barriers, dispersal ability and chance historical events, such as vicariance. All of these determine the presence or absence of a particular species in otherwise suitable habitats. Because most species occupy only a limited part of their potential global range and because adaptation leads towards accommodation of organisms to their environment, we often see that distantly related organisms have independently evolved similar form and function to fill similar ecological niches in similar habitats around the world, i.e. ecological convergence. The important feature of such convergence in the present context is in the power of natural selection to shape a species population to fit into the prevailing environment (within the constraints imposed by the very nature of the organism). These ideas can be extended to convergence at the community level, although the evidence is equivocal (Chapter 9). What happens when environments change either in abiotic para meters (e.g. climate) or biotic parameters (e.g. intensification of interac tions or changing resource levels)? This question was considered in the short term by Fisher (1930) using a simple theoretical model of adapta tion (Chapter 6). It is based on the premise that no organisms will be perfectly adapted to the present conditions, since adaptations (due to a time lag in response) reflect conformity to past environmental pressures. Over longer time scales, Van Valen (1973) views the environment as constantly decaying with respect to adaptations of existing species, so natural selection operates only to enable the species to maintain its current state of adaptation rather than improve it (Red Queen hypoth esis; see also Chapter 8). Sufficient plasticity in behaviour or physiology can allow short-term tracking of, or acclimation to, small scale changes Introduction 151 in environments over ecological time (a slight, short-term or temporary shift in the realized niche). Niche shifts, leading to a reduction in habitat ranges in the face of competition, provide a pertinent example and are considered in Chapter 9. Longer term changes in the species, in response to larger or longer term environmental change, require permanent (i.e. genetic) alterations to their biology and result in a permanent shift in the fundamental niche. What effect can long-term changes in the environment outlined in many of the chapters have on the biogeography of species? If species are able to re-adapt, or track the environment, as outlined above, the species population can maintain its spatial occupation in the face of changing conditions, by moving into a new fundamental niche. But what if the species population cannot re-adapt sufficiently in the face of environmental change? Changing conditions can be countered, to some extent, if the species is able to shift its spatial occupation in relation to a shift in its required conditions. The species may thus retain its niche, but alter its geographical distribution. This kind of forced range shift has been documented as an effect of glaciations, forcing temperate organ isms to shift latitudinally as the temperate zones moved (e.g. trees (Davis, 1986), insects (Coope, 1987)). Continual changes in conditions might also act to reduce the size of the realized niche of a species population, causing gradual restriction of its range to those places where conditions have not changed, and eventually create refugia for species or even whole biota (Chapter 10). EXTINCTION Adaptation may be viewed as a very slow environmentally driven rearrangement of niches accompanied by gradually changing species, which are always incompletely adapted and change their spatial occupa tions in time with shifting adaptive zones. Ultimately, if conditions continue to change and a species has nowhere left to go (i.e. its niche disappears), then extinction will ensue. As pointed out in Chapter 8, the speed of extinction will depend on the intensity and rate of environ mental change, and different species will respond to this differently, so extinction episodes will not affect all species equally. Extinction, as a process, can thus be seen to act as a filter, affecting different species populations differentially. One obvious effect therefore is to change the species pool of an area, which will undoubtedly have a concomitant effect on distributions of other species (Fig. 1.1). Three levels of extinction can be determined, largely based on differences in taxonomic, spatial and temporal scales. Local extinction of one species may broaden the ecological opportunities open to similar 152 Biological processes in biogeography species, and may also enable a previously excluded species to establish itself (expansion of a realized niche towards a fundamental niche - known as competitive release). This process is discussed in Chapter 9. On a larger scale, local and sometimes global extinctions of species may be viewed as resulting from competition with ecologically similar species. Knoll (1984), for example, has suggested that vascular plant radiations actually caused extinctions of previously dominant groups through competition over time. Extinctions at this level would again lead to ecological readjustments amongst the remaining species, which in turn would influence distribution patterns. Ultimately, extinctions of lineages would occur, e.g. dinosaurs and ammonites. The explanations of these so-called mass extinctions are varied and subject to considerable debate (Chapter 8). Such large scale extinctions might then allow great adaptive radiation amongst the surviving species and groups. SPECIATION As demonstrated in the fossil record, extinction is the ultimate fate of all lineages and species. However, whilst the process of extinction has acted to reduce the available species on the earth, and greatly influenced their distribution, its action has been countered by enrichment of biotas through the process of speciation. This process is important in biogeography in the provision of raw material for distribution and in the alteration of the biotic environment and hence distribution patterns of species

Load more