Biogeography

Example of biogeographic map: distribution of the main terrestrial zoogeographic regions.

The biogeography is a scientific discipline that studies the distribution of living beings on Earth, as well as the processes that have originated it, that modify it and that can contribute to its development. It is also an interdisciplinary science, which is both a branch of geography and biology, receiving its foundations from specialties such as botany, zoology, ecology, evolutionary biology and other sciences such as geology.

General considerations

The distribution of living beings is the result of biological evolution and the dispersion of lineages, of global and regional climatic evolution, and of the evolution of the distribution of land and seas, mainly due to avatars of orogeny and plate tectonics. Biogeography is a historical science, that is, it deals with the study of systems whose evolution has followed a unique trajectory, which must be studied specifically, and its knowledge cannot be obtained deductively from general principles. The living beings present in a specific region cannot be deduced from geographical factors, but must be empirically examined.

The surface of the Earth is not uniform, the same conditions do not exist in different places. The first and fundamental distinction is between the aquatic environment and the aerial or terrestrial environment. In both cases, a first fundamental factor is the availability of primary energy, which enters the ecosystem through the primary producers, which is generally sunlight. The distribution of this factor follows a latitudinal gradient, in which energy and temperature are maximum in the equatorial regions and decrease in the polar direction. The seasonality varies at the same time, which becomes more marked the further we move away from the equator. In terrestrial environments, the second major factor is the distribution of rainfall, or rather the balance between rainfall and evapotranspiration, with an intertropical strip and two temperate ones characterized by maximum humidity. In the oceans, the second major factor is the very uneven distribution of nutrients, with more productive and diverse ecosystems in relatively cold waters, but fertilized by nutrient upwelling from the bottom.

Biogeography does not only study the distribution of species and taxa of a higher category, their areas, which is dealt with by the specialty called chorology, but also the distribution of ecosystems and biomes. Although reality is always complex, science must perform simplifying operations to make it accessible for study and, above all, to achieve useful descriptions. For biogeography the task is to define areas that are relatively homogeneous and distinct from the surrounding ones, which are characterized by more or less uniform values of the factors, and by equally homogeneous biota and ecosystems. These areas, more or less idealized, are likely to be presented cartographically. On the other hand, the geographical study of environmental and ecological diversity must consider the differences of scale; since the area that appears homogeneous on a continental map, for example as a Mediterranean forest, is actually a mosaic of situations on a lower scale, with special environments such as gallery forests, on the banks of rivers, or salt marshes in basins salinized endorheic; or differences due to a marked relief, such as the one between sunny (on the slopes that face the equator) and shady (on the opposite).

Biogeography has to take into account, for the interpretation of its object of study, the human factor. Humanity has significantly altered terrestrial environments, and now also oceanic ones, since the Upper Paleolithic or since the end of the last glacial period. Even before the current demographic and industrial explosion, it was impossible to find a single corner of the continents that did not keep the memory of human alteration, although awareness of this fact is recent. Currently, the proportion of areas that deserve to be called natural is already very small, and what we find instead are anthropized environments to varying degrees.

Ecological and historical biogeography

Biogeography has been divided into two branches, known as historical biogeography and ecological biogeography. Ecological biogeography studies the distribution of living beings in time and space, and each of these branches relies more on one of these elements, historical biogeography focuses more on time, looking for how the distributions of species to its current state. Ecological biogeography using techniques, such as the theory of ecological tolerance, is based more on the spatial distribution of living things at the present time. Some consider these two branches irreconcilable, however each is the complement of the other.

History of biogeography

The naturalist Alfred Russel Wallace.

The first question that the history of this discipline asks us is to what extent religion influenced or continues to influence the ideas that have been raised in it. From one point of view, the idea of a center for the creation of species and from there their dispersion to the rest of the planet was the axis of the first ideas about the distribution of living beings, but even though these ideas apparently were left behind with the appearance of some naturalists such as Buffon, there was a notion that the main axis of the distribution was the dispersion of the species, this idea was indirectly influenced by religious and philosophical ideas.

It was not until the introduction of the vicariancist ideas of Alfred Russel Wallace in the 19th century that the approach began to truly change. It is at this point where a new stage in the history of biogeography is marked, accompanied by the new paradigm of biology, the theory of evolution, although some authors had already raised evolutionary ideas before Darwin, but without having specified them or only as isolated examples. And surely evolution changed biogeography as it changed every other branch of biology. «The biogeography of Charles Darwin and Wallace would predominate for almost a century, annihilating the idea of dispersion in this science and basically limiting it to ecological aspects» The end of the so-called Darwinian biogeography ends in the stage of contemporary biogeography, where the Factors that previously were left as chance products, also as in all sciences, are being changed by technological development and thought. In this case, the plate tectonic theory is taken into account, the technology for phylogenetic analysis is available, and some theories that are considered obsolete are rejected. It is for biogeography a scientific revolution, which leads to a paradigm shift. The results are numerous different approaches, based on different search and analysis criteria, among which panbiogeography and cladist biogeography stand out. The latter bases its method on three pillars: the cladist method, plate tectonics, and the criticism of the dispersionist model made by León Croizat, and it is considered one of the main current schools of historical biogeography, partly due to the impact it has had on Cladism had in systematics, which is closely related to biogeography, since they are even areas of the same authors.