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Have you ever gone in your backyard and discovered a wild raspberry bush that has never been there before? How did it get there? Why is there only one bush year after year? In this lesson, you’ll study how plant and animal species colonize in new areas through dispersal and immigration and how some species thrive in a new area while others fail.

Dispersal Mechanisms

When organisms reproduce, there is a very important issue that must be addressed: where are the offspring going to live? Related to that issue is the question of how the offspring are going to get there. Highly mobile organisms like most vertebrates have a lot of flexibility because they are mobile for most of their lives and can, to varying degrees, pick up and move whenever it is advantageous. However, for organisms whose adult form is immobile, like barnacles, corals and terrestrial plants, dispersal, or the spread of organisms to new areas, is key to their reproductive success. Be careful not to confuse dispersal with dispersion, which, you may remember, is the pattern of spacing of individuals within a population.

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I don’t really have a trick to remember the difference; you’ll just have to remember that dispersal refers to the actual process by which organisms spread and dispersion describes the pattern in which organisms are already spread within a population.Many immobile aquatic species, like barnacles and corals, solve the dispersal problem by releasing sperm and eggs into the water in a coordinated fashion with all of the other members of their population. Such events result in millions of zygotes, or fertilized eggs, which then quickly develop into mobile or free-floating larval forms. In this way, the offspring are carried far and wide through the ocean, and those lucky enough to land on a habitable spot attach themselves and grow into their immobile adult form.If the offspring end up in a spot that doesn’t already have organisms of their species and they survive to adulthood and start a new population there, they have colonized a new habitat, or started a population in a habitat where the species was not already present. Some terrestrial plants use a similar strategy by releasing seeds or spores that are designed to be carried long distances by the wind and air currents. Coconut palms use water to disperse their seeds, the coconuts themselves, which float in the ocean water and can be carried to beaches far away from their parent plant.

Still other plants rely on animals to spread their seeds. Many types of grasses and other small plants have seeds that stick to animal fur, while many types of trees and bushes produce fruit that contains seeds. When animals eat the fruit, the seeds are usually also consumed. Such seeds are resistant to digestive enzymes, so they survive the journey through an animal’s digestive system and are able to germinate from the animal feces in a new location surrounded by an abundance of natural fertilizer.

Habitat Fragmentation

So what’s to say that the seed ends up in a location that is suitable for a plant to survive? Not much.

Other than plants that simply drop their seeds on the ground right underneath the parent, plants and immobile animals, like corals and barnacles, have very little control over where their progeny end up. They may end up in a favorable habitat or they may not. Not all habitats are the same. In addition, habitats are fragmented, and people are fragmenting them even more by several means, including building communities, logging and converting natural habitats to farmland.

This means that, at least for the terrestrial habitats that are becoming more fragmented, dispersal is perhaps now a bigger factor than ever when it comes to determining an organism’s fitness. But how do we go about studying dispersal and the effect it has on individual populations and communities?

Island Biogeography

Example of a biogeography graph
Biogeography Graph

In the 1960s a theory emerged that attempted to explain the effect of dispersal on biodiversity within biogeographical islands: the theory of island biogeography, which is an ecological theory that estimates the number of species that can live on an island based on its size and proximity to a mainland source of species. This theory distilled biodiversity on an island, or, for that matter, any isolated habitat, down to just two factors: the size of the habitat and how far away a source of species capable of living in that habitat is. It assumed that evolution, niche differentiation and annual migrations were not occurring and that all types of organisms have the same rate of immigration, or a one-time migration of an organism or group of organisms into an area.Now, this is an awful lot of assumptions to make, and there are even more that I didn’t mention. Even the ecologists that came up with the theory of island biogeography knew that these assumptions weren’t true and that all of these other factors were in fact playing a role in species diversity on islands.

However, none of these other factors were easily incorporated into the theory, or were very predictable for that matter, and the size of the island and the proximity to the mainland seemed to be the two largest factors affecting species diversity.So, the island biogeography theory hypothesized that these two factors alone could be used to create a graph that could roughly predict the number of species that could be found on any given island by finding the balance between colonization and extinction. On this graph, the rates of extinction and colonization are represented on the Y-axis and the number of species are represented on the X-axis. Two lines are drawn on the graph, one line that represents the rate of colonization from the mainland and one line that represents the rate of extinction of species on the island.The rate of colonization is highest when the number of species on the island is lowest because, although immigration is assumed to be constant, the likelihood that a species is new is mostly dependent on the number of species that are already there. In addition, the rate of colonization is also assumed to be directly related to how close the island is to the mainland. So, the colonization line can be adjusted up for islands that are close to the mainland or adjusted down for islands that are far from the mainland.

Likewise, the extinction line can be adjusted up for small islands and down for large islands because the larger the island, the more habitat, niches, resources and space there will be to support larger populations and communities. The larger the populations and communities, the less likely it is that extinctions will occur. The point at which the colonization and extinction lines meet is the point of equilibrium between colonization and extinction, and it predicts the number of species that can be found on the island.

Biogeography graph showing points of equilibrium between colonization and extinction
Biogeography Graph Showing Points of Equilibrium

So, if we take a look at our island biogeography chart here, we can see that small islands far from the mainland should have the fewest number of species, as predicted by this point of intersection here, while large islands close to the mainland should have the largest number of species, as predicted by this point of intersection all the way over here. Even though the island biogeography theory makes a lot of assumptions that don’t hold true in nature, actual field experiments showed that it did do a fair job of predicting the relative number of species that would populate an island, at least in the short term over time periods when evolution and niche differentiation wouldn’t be expected to have an effect. In addition, the island biogeography theory has also served as an important starting point from which other ecologists have constructed their own, often more complex, ecological theories that are related to species diversity, colonization and population survival.

Lesson Summary

In summary, dispersal, or the spread of organisms to new areas, is an important factor in the reproductive success of an organism. Be careful not to confuse dispersal with dispersion, which you may remember is the pattern of spacing of individuals within a population. Highly mobile organisms, like most vertebrates, have a lot of flexibility because they are mobile for most of their lives and can, to varying degrees, pick up and move whenever it is advantageous. However, for organisms whose adult form is immobile, like barnacles, corals and terrestrial plants, dispersal must occur soon after fertilization when their offspring are just seeds, spores or larvae that have not yet established themselves.Immigration is a one-time migration of an organism or group of organisms into an area. If immigration occurs in an area that isn’t already occupied by that species, it can result in colonization, which is when an organism or group of organisms starts a population in a habitat where the species was not already present.

An important factor that affects dispersal, immigration, colonization and populations in general is habitat fragmentation. In addition, human activities, such as building communities, logging and converting natural habitats to farmland, has resulted in further habitat fragmentation in many areas around the globe. This means that, at least for the terrestrial habitats that are becoming more fragmented, dispersal is perhaps now a bigger factor than ever when it comes to determining an organism’s fitness.

Island biogeography is an ecological theory that estimates the number of species that can live on an island based on its size and proximity to a mainland source of species. This theory distilled biodiversity on an island, or, for that matter, any isolated habitat, down to just two factors: the size of the habitat and how far away a source of species capable of living in that habitat is. Island biogeography predicts that larger islands and islands that are closer to the mainland will have greater species diversity than islands that are smaller or further away from the mainland. Even though island biogeography takes a rather simplified approach to species diversity and does not take into account many relevant biological processes, it does have some predictive value, especially over a short timeframe when evolution and niche differentiation do not have enough time to factor into the equation. In addition, the theory has provided a starting point for other ecologists to use as a basis to develop their own theories and experiments.

Lesson Objectives

After watching this lesson, you should be able to:

  • Describe dispersal and how it might occur for immobile species
  • Define colonization and immigration
  • Explain the effects of habitat fragmentation
  • Summarize island biogeography and its significance

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