Over half a century ago, Biogeography appeared in the scientific landscape as a new modern science, as a response to the urge to understand and manage the impact of human activity on the planet, which was causing the alteration of the habitats of species. But what exactly is Biogeography? It’s a science that deals with the analysis and explanation of the distribution patterns of the different species that inhabit the Earth, and interprets the changes that have taken place in the past and the ones that are happening nowadays. In other words, it’s the study of the geographical distributions of living beings or ‘the geography of life’.
Throughout the years, Biogeography has tried to answer many questions about the distribution of species: why are they confined to their current distribution? Why can a species live in a specific location and what stops it from occupying other areas? What impact have historical events, such as continental drift, glaciations or the more recent climate change had on the distribution of species? In order to answer these and other questions, it’s important to know about the movements that allowed for species diversification – a series of biogeographic processes and ecological mechanisms that we’ll try to explain in this article.
Long-distance dispersals in short periods of time
The mechanism of species dispersal consists of a series of important movements, long journeys that can lead to the colonization of new territories and habitats by a new species. Long-distance dispersals have great geographical reach (sometimes, the species travel thousands of kilometres or miles) and take place in a relatively short period of time. In this sense, it’s common for colonisations to be completed in just a few months – even days! In any case, and in order to ensure that the dispersal process is effective, the species must successfully establish itself in the newly conquered distribution areas.
The dispersal phenomenon (or immigration phenomenon, if we look at it from the point of view of the area reached) shows that most living beings have a natural tendency to extend their distribution area, to broaden their territories, usually through their descendants. All organisms, both mobile and sessile, have a certain dispersal capacity, a key skill when there are alterations and transformations in their biological niches – something that’s likely to happen, eventually.
The conquest of the oceanic islands that have never been part of a continent is a clear example of colonisation by dispersal.
In any case, these dispersal processes only involve a few small groups of individuals, rather than entire populations. The process can take place in two different ways: actively or passively. Active dispersal is carried out by the large flying species of the planet, such as birds, bats or large insects, as well as by those species that swim every now and then. On the other hand, passive dispersal takes place, for example, when organisms are transported by another organism, when they’re spread by the wind or water currents, or when they travel attached to feathers or fur.
The dissemination process or the propagation of a species
After dispersal, the dissemination process takes place; this is when the species propagates throughout the colonised territory. This mechanism is considerably slow, but also progressive, and the process may require several generations (and centuries) to be completed. Unlike the dispersal processes, in which only small groups of individuals are involved, dissemination can require entire populations.
This ecological process was very common during the biological recolonisations of the postglacial era, when a lot of species that had been forced to migrate due to the climatic conditions moved back to their territories. On the other hand, introduced species usually develop dissemination mechanisms in their new distribution areas too, provided the introduction has been successful. This is the case, for example, of the common starling (Sturnus vulgaris), a species that has been introduced and has successfully established in many different regions, such as North America or Australia.
A slow and progressive movement: secular migration
Secular migration is the slow and progressive movement of populations and species over long distances. In terms of geological time, secular migration is a considerably slow process, which can take from thousands to millions of years, and will require hundreds of generations. In fact, this phenomenon can lead to evolutionary changes in species as they colonise new territories; that is, species can evolve ‘en route’, and can even go extinct in their original distributions.
This is exactly what happened to camelids millions of years ago. Camelids are a family of species – that originally inhabited the Nearctic realm (the region of North America) – that began a secular migration process, for several reasons, moving gradually and colonising new territories and habitats. After thousands of years, they managed to reach extremely distant regions, such as the Palearctic or South America, regions that are still part of their distribution area. During their migration, they evolved and diversified, and went extinct in their distribution of origin.
Periodic or cyclical movements: zoological migrations
Zoological migrations are the cyclical or periodic movements of species, related to their life cycle or reproductive function. As these migrations are genetically determined, they’re fairly predictable. Birds, insects, fish, mammals… Many animal species migrate every year, being one of the most important and studied phenomena in the animal kingdom.
Translated by Carlos Heras