Biogeography

Have you ever wondered how and why species are distributed in certain ways?

For example, why a species may be present in one river basin but not another river basin that is in close proximity? Perhaps you have thought about species endemic to only specific habitats or islands.

In the following article, we will discuss the study of biogeography, including its definitions, variations, and relation to ecology and evolution. In addition, we will discuss some examples related to biogeography.

Definition of biogeography

Biogeography is a field of evolutionary biology and geography that looks at the geographic distribution of species over time.

Biogeography looks at how the physical environment has and continues to shape the distribution of animal and plant species. Evolution, extinction, speciation, and dispersal all play significant roles in the study of biogeography.

Types of biogeography

The study is typically split into three different types: conservation biogeography, ecological biogeography, and historical biogeography.

Conservation biogeography

Conservation biogeography is the newest form of biogeography, melding the fundamentals of biogeography with conservation concerns. This type of biogeography looks at how the changes in species distribution over time may have affected or continue to affect their conservation status.

Ecological biogeography

Ecological biogeography is concerned with the geographic distribution of species at the present time and takes into account the habitat and physiological requirements of the species being studied.

Historical biogeography

Historical biogeography also referred to as paleobiogeography, is concerned with the historical distribution of species. It looks at how and why the historical distribution of species may have changed over time.

While historical biogeography can deal with extinct species, it also concerns the historical distributions of extant species.

Examples of the use of biogeography in studying species distribution

Examples of biogeography can be seen in the following regarding saltwater crocodiles and the Wallace line in studying species distribution.

Saltwater crocodile

The saltwater crocodile (Crocodylus porosus) has the widest distribution of any living crocodilian species, with a range that extends from eastern India in the west to Vanuatu in the east, including much of the tropical world in between.

In the case of the saltwater crocodile, the primary causes of the reduction in the range are anthropogenic and include hide-hunting, habitat destruction, and deliberate eradication efforts, among others. Adult saltwater crocodiles can and do prey upon humans, thus the species was quickly eradicated from most major population centers (e.g., Bangkok, Ho Chi Minh City, Jakarta, etc.).

Unfortunately, as the human population continued to expand, the destruction of the species of mangrove and freshwater swamp habitat lead to further declines in distribution. In the case of China, the massive migration of populations southward during the Song Dynasty may have contributed to the decline.

While the species is currently listed as Least Concern by the IUCN (International Union for Conservation of Nature), due to a pre-exploitation-sized population in northern Australia and rebounding populations in other areas nearby, it is considered rare in many other regions (e.g., Bangladesh and Myanmar) and extinct in the countries above.

As you can see, in this case, all three types of biogeography are involved, particularly historical biogeography (historical range), but also conservation biogeography (reintroduction efforts, current conservation status), and ecological biogeography (current range).

Figure 1: The current (green), possible (yellow), and historic (orange) geographic distribution of the saltwater crocodile.

The Wallace Line

The Wallace Line, named after famous naturalist Alfred Russel Wallace, refers to the biogeographical boundary that separates the Asian and Australasian biological regions. The Wallace Line passes through the Makassar Strait, between Borneo and Sulawesi, south through the Java Sea and between the islands of Bali and Lombok.

Thousands of years ago, when sea levels were lower, Borneo, Java, and Sumatra islands were all part of a greater landmass connected to the mainland, called Sundaland.

Though the islands became isolated, they still contained wildlife from the Asian mainland.

In contrast, islands to the east of the Wallace Line were not connected to Sundaland and thus contained different wildlife. These eastern islands, including New Guinea and Sulawesi, contain wildlife not found on the Asian mainland, such as kangaroos (on New Guinea), cuscus, and more. New Guinea and some of the nearby islands were once connected to Australia, and are even today only separated by around 150km of the ocean in the Torres Strait north of Queensland's Cape York Peninsula.

Animals that are capable of wider dispersal and swimming (e.g., saltwater crocodiles and some snake and lizard species), on the other hand, are found throughout both regions. As you can see, studying the wildlife to the east and west of the Wallace Line encompasses all three types of biogeography to give us a better understanding of how and why species distributions have changed over time and how this knowledge can help inform future conservation measures.

Figure 2: Map illustrating the Wallace Line (red line) and approximate locations of former exposed land (black lines) prior to sea level rise.

Biogeography helps us understand evolution.

Studying biogeography allows us to better understand how and why species evolved into what they are today and how they may continue to evolve in response to the current environment and projected future selective pressures. Let's look at an example of how a species' geographic distribution may affect its evolution.

How does biogeography factor into this? It has been found that a jaguar's habitat dramatically influences whether or not the melanistic characteristic is selected for or against it.

In the Pantanal wetlands of Brazil and Bolivia, melanistic jaguars are either very rare or entirely absent, likely because the habitat is much more open and sparsely forested. In such a habitat, melanistic jaguars would be easily spotted by potential prey species, reducing the fitness of such individuals. In addition, melanistic jaguars would be far easier for human poachers to spot, further reducing fitness.

However, in densely forested areas, such as the Amazon, the melanistic trait is beneficial and selected for. Having a melanistic coat renders the jaguar more challenging to spot in dense forests, increasing its chances of successful predation and reducing the chances of being hunted by humans.

Global Ecology and Biogeography

Global Ecology and Biogeography is a monthly scientific publication that started in 1991 to focus on macroecology, a type of ecology.

Macroecology is concerned with how organisms interact with their environment and how these interactions shape various aspects of their natural history, including geographic distribution. Macroecology contains essential elements of ecological biogeography.