Part 4: In which we discuss Species and Areas of Endemism.
If biogeography is to address how and why species are distributed across the globe, then the definition of species deserves some attention. A philosophical approach would describe a species as a single lineage of ancestor-descendant populations which maintains its identity from other such lineages, and which also has its own unique evolutionary tendencies and historical fate.
This idea is known as the Evolutionary Species Concept and is often used in conjunction with the Phylogenetic Species Concept This notion defines a lineage as a species if it has a unique apomorphy, a character-state exclusive to a single, terminal taxon, such as the loss of limbs in the evolution of snakes. An apomorphy is also called a derived trait, whereas an ancestral or primitive trait is called a plesiomorphy. A plesiomorphy of a smaller group is synapomorphy of a larger group, and cannot tell us anything about relationships. If a character-state originated very early in a group’s evolutionary history, such as the presence of limbs in an ancestral reptile that gave rise to dinosaurs, crocodiles, lizards, and snakes, then it provides no indication that the group formed of these reptiles is a clade to the exclusion of other groups.
This definition of a species was not the first to be fostered by biologists. The earliest proposition is termed the Biological Species Concept. It defines a species as members of a population that actually or potentially interbreed in nature. While this concept has been very influential in the growth of evolutionary theory, it has some series limitations. For instance, the ability to interbreed is a primitive trait (a plesiomorphy). It cannot be applied to extinct groups since we cannot test the reproductive potential of fossils. It also neglects classification of unisexual organisms.
In order to address some of the limitations of the Biological Species Concept, other “species concepts” have been proposed. The Morphological Species Concept is a more subjective classification based on anatomical (morphological) criteria. Organisms are considered to be the same species if they appear identical and look different from other sets of organisms. According to this concept, separate species are recognized based solely on phenotypic similarly: if two animals look the same but are prevented from mating with each other by some barrier, they would still be considered the same species. While appearance is certainly helpful in identification of species, it should not be used a solitary measure. Organisms like the Western and Eastern meadowlarks may look almost identical, but they do no interbreed because their songs are unique to each species. Organisms may also look very different, such as two individual ants, but both belong to the same species serving different functions in their population.
Another approach, called the Ecological Species Concept, defines a species as a set of organisms adapted to a particular niche in the environment. Ecological and evolutionary processes control how resources are divided up and tend to produce discrete phenetic clusters, which we recognize as species. The problem with this concept is that if two organisms such as a lion and a tiger that occupy the same niche, use the same resources, play similar roles, and can even interbreed, then they would be considered the same species. Of course we know that this is not true, but the idea is important to the overall description of a species.
Species distribution may cover very large or very small areas across the globe. A species exhibits cosmopolitan distribution when its range extends to all or most of the appropriate habitats on earth: humans, for instance, are a cosmopolitan species. When a species is confined to a particular area, however, it is said to be endemic to that area: all species of lemur are endemic to the island of Madagascar. In fact, islands often show high levels of endemism compared to less isolated geographical regions. High degrees of isolation result in more endemic species because each population is given the opportunity to follow its unique evolutionary trajectory. Isolated “islands” can also include mountain peaks, desert springs, and expanses of land that are surrounded by man-made barriers. Endemic species often have small ranges in terms of homeland size and species distribution; in other words, endemic species are a type of steno-taxa with a narrow range.
Levels of endemism depend on variability of isolation and area. Older spans of isolation will result in more endemic species: the longer a population has been isolated, the more unique it will become. Larger areas also tend to show more endemism than small areas. While not always true, this characterization is based on the notion that larger areas would have more resources available to its biota, and so would have a lot of species. Older areas will have more endemism than younger areas because populations will have had more time to speciate. A high rate of speciation in an area will produce more species endemic to that area. On the other hand, a low rate of extinction will also yield more endemism. The net value of an area’s endemism relies on more species being formed than are disappearing.
Levels of endemism are dependent not only on isolation variability. A stable environmental climate will allow a region’s biota to become particularly adapted to that region. More endemism is reported in areas where seasonality is less pronounced and where the environment and topography are more variable. Higher niche availability broadens the span of species types suited to the particular climate of the isolated region.
Endemism thrives in areas with slow immigration rates. An immigrant species will remain indigenous to its homeland, but will no longer be endemic to that region if it begins to inhabit other regions. Because endemism depends on this “loyalty” of a species to its origin, the expression of low agility in a species greatly increases its endemic probability. However, endemism is not unique to species that originate in a particular area. When a species has originated in an area of endemism, it is said to be autochthonous to that area. When a species has dispersed into an area of endemism, it is then allochthonous to that area.
The relationship between species and the concept of endemism exists because species are the basic units of classification. Endemism tends to describe taxa at the level of species; there are many species of bat endemic to Madagascar, but the order Chiroptera is widespread. In isolated areas of endemism it is likely to find very similar species as populations react and adapt to the same environment. With a species defined as a particular descendant taxon with a unique evolutionary heritage, and possessing an apomorphy with which it is distinguished from other such groups, it is now a more manageable task to discuss its distribution.