Part 2: We can digest the science of biogeography using five key observations.
The non-randomness of distribution patterns.
Random distribution is rare in nature because while it occurs where resources and conditions are unchanging. it neglects strong social interactions of species.
Uniform distribution is more common and occurs where the distance between individuals in a population is maximized. Territoriality and competition will tend to drive individuals further from one another.
The most common form of distribution, however, is clumped distribution, where the distance between individuals is minimized. Animals will “clump” around essential resources when they are less plentiful. Individuals will also aggregate if juveniles are not able to leave the population, which is beneficial to the group. Species distribution patterns allow for the spatial arrangement of taxa.
Different assemblages occur in different regions, but they often serve different functions.
Grazers, for example, regulate overgrowth of grasses, shrubs, and algae depending on where they live. Grazers include a wide array of organisms, as different as elephants in Africa and wallabies in Australia, but they play similar roles in their environments. For instance, aquatic grazers help to regulate the effects of unwanted algal blooms in marine systems. Removing organisms such as crustaceans and larval fishes from their habitats will allow algae to rapidly propagate and potentially cause an algal bloom. A bloom can produce toxins or cause mechanical damage to other organisms.
Grazing is necessary to control plant populations in an ecosystem, but it can also lead to problems such as soil erosion and the loss of biodiversity. The number of grazers in the food chain is regulated by the number of predators. Predatory animals are also widespread and diverse. Wherever they are found, all predators help to prevent grazers from depleting available resources. If a keystone predator is removed from its habitat, lower trophic levels will experience a loss of species diversity. For example, by removing sea otters from a coastal ecosystem, sea urchin will thrive an populate out of control, freely grazing on benthic algae and depleting it from their environment.
The previous discussion cannot be explained by distance alone.
Both time and the environment must also be considered when discussing roles and assemblages. Convergent evolution is one possible explanation for the presence of similar functions in unrelated organisms in different environments. The development of the “camera” eye of cephalopods, cnidarians, and vertebrates is an example of convergent evolution, having arisen through different processes in each group as a common reaction to their environments. These and all other processes occur during the passage of time. The distribution of life changes with time because as the environment changes, so too does its interaction with its biota.
Biotas are clearly different when comparing the past to the present.
The occurrences of speciation, the production of species, and extinction, the removal of an entire lineage, over time depicts a biological distribution specific to its place in time. 230 million to 65 million years ago, during the time we refer to as the Mesozoic Era, the biota included dinosaurs and mostly very small mammals, lizards, and snakes. The earlier part of this period was dominated by gymnosperms. However, by the end of this era the dinosaurs had gone entirely extinct, save for their surviving avian descendants. Mammals had the opportunity to grow larger and plants experienced a tremendous change in biodiversity. By then, angiosperms were the more plentiful vegetation, as is true today. More organisms had the opportunity to diversify and speciate, creating an entirely new distribution of biota.
The current distribution of biota is different from the same biota in the past.
Dispersal events occur when individuals relocated to a different habitat, often in search of resources that have been depleted in their own. As individuals disperse the distribution of the species is altered over time. Also, as the surface of the earth changes due to such phenomena as continental drift and erosion, geographical regions that were once connected may become separated; conversely, separated regions may eventually make contact with one another.