In my last post, I introduced biogeography as a field which studies how populations and species are distributed. Distribution is influenced by many factors including biotic factors. It is a catch-all term which refers to factors relating or due to the biology of the the species, and may include dispersal ability, interspecies interactions, and breeding systems.
Dispersal ability is a measure of how well a species can spread from its original home and includes aspects such as fecundity, migratory ability, adaptability and gamete dispersal. Fecundity describes the reproductive ability of a species. Migratory ability describes how mobile a species is, and how able it is to move from one habitat to another. Adaptability measures how well species can adapt to new habitats, such as after a dispersal event. Gamete dispersal looks at how far a species can disperse its gametes and how well those gametes do after dispersal.
Elephants are not very fecund, for example. They take many years to mature and when they do mature, they do not produce many young. Low fecundity combined with slow maturity limits their potential dispersal ability and is a reason why populations are so vulnerable to hunting.
On the other hand, many wind-dispersed plants have a strong dispersal ability – their gametes can go wherever the wind takes them. Dandelions are an example of a plant which is widely dispersed and distributed. Thus we can find dandelions in lawns across North America. But plants do not generally migrate, unless they are able to reproduce from shoots or branches which fall off and are carried by other vectors.
How does the species interact with other species? Predator-prey relationships can limit the spread of populations, either through the limitation of food sources for the predator or through prevention of unchecked growth by the prey. A good example would be the sea otter-urchin-kelp relationship.
Sea otters in the North-East Pacific have historically kept sea urchin populations low through predation. Urchins feed on kelp as a primary food source. When sea urchins are few, herbivory levels on kelp are low, allowing kelp populations to expand and diversify over time. But in the last century or so, local populations of sea otters have become extinct due to hunting pressures. As a result, the sea urchin populations have expanded and the kelp populations are diminished and diversity is reduced to a few hardy species.
For plants, interspecies interaction can also include the interaction with animal pollinators. Many plants, especially those endemic to islands such as Hawaii, have very specific pollinators. Orchids are especially notorious for specifically catering to certain species of bees or moths for pollination. Sometimes the shape of the flower may only allow a certain insect inside, or perhaps the scent produced only attracts certain moths. Dispersal is then limited to areas where these pollinators are present.
Seed-bearing plants are possessed with a variety of different breeding systems. The breeding system of a species will often determine how successfully a species will distribute.
Some species are unisexual, that is, male and female plants are separate. Holly is an example of a unisexual plant. For unisexual plants, colonizing a new area requires (at least) two “dispersal events” to deliver a male and female plant in order for a sexually reproducing population to establish.
Other plants are bisexual – male and female are present on the same individual. Rosaceae, the family containing apples and roses, are an example of a bisexual plant. Within these plants, some are able to fertilize themselves (“self-compatible”) while some are not (“self-incompatible”). Self-compatible, bisexual plants are the best dispersers. Only one seed is needed to successfully start a new sexually reproducing population
All these biotic factors, and more, can change the distribution of populations over time. But it’s not enough to be biologically able or unable to spread to new environments – sometimes it is the environment itself that limits or encourages population growth. My next post will look at some abiotic factors that affect the distribution of species.