What comes to mind when you hear the word "cascade', a waterfall maybe? Then you are on the right track. In ecology the term "cascade effect" refers to a series of secondary extinctions(disappearance) that are triggered by the extinction of a primary or key species in the ecosystem.

In this article I will look at some of the causes of the cascade effect.

There are about four main causes for the ecological cascade effect. The first cause of the cascade effect is a dependency on a few specific food items. Of course animals must have alternative food sources, especially those that rely on fruit that are seasonal, so when they are out of season these animals would starve. Alternative food sources can be destroyed in a variety of ways, such as, by bushfires either naturally or maliciously set. So that if a large enough area is destroyed the cascade effect can locally occur.

The cascading effect also occurs when a mutualistic relationship ends. Mutualism is a type of relationship where two or more organisms or species benefit from each others presence or the services they provide. An example of a mutualistic relationship would be between water buffalo and cattle egrets. These large animals depend on the birds to free them of parasites such as ticks, while the birds depend on the ticks from these animals as a food source. Therefore if the birds were to be wiped out by a disease, then these large animals could become ill or die because of the parasite overload.

The last cause of the cascade effect which I will look at is the introduction of alien or invasive species to an area. These species are not native to the area and may have entered the country as someone's pet and because they became too burdensome to care for, they were abandoned. Another scenario would be that they were brought in unknowingly, which is the case with many marine organisms that hitch a ride on the undersides of boats or ships or in ballast water. These invasive species are quite aggressive and soon out-compete the native species for their resources, so that the other species that depend on the native species either as a source of food or for some service will go locally extinct.

However, the main cause of the cascade effect is the loss of top predators which function as key species in the ecosystems. This has the effect of causing a population explosion of prey organisms lower down the food web.

These animals end up exploiting their own food sources so that when the food source is gone they go extinct because of starvation. These top predators might be removed by human activities like hunting. This also can happen in the marine environment where shark fishing becomes extreme, sharks being the top predator in the marine food web.

Plants also undergo the cascade effect as well, for instance, some plant species in the understory layer of forests depend on trees in the canopy to provide shade. These understory species are dependent on shade and will not survive in full sunlight, and the animals that depend on these plants in the understory will also go extinct.

So removal of the canopy layer can be regarded as the end of a mutualistic relationship. It is mutualistic in nature in that when these understory plants die or shed their leaves they supply the canopy trees with nutrients.

Plants also experience this effect when their mutualistic relationship with animals is disrupted.

An example of this is that many plant species depend on animals for pollination, so when the pollinators stop visiting the plants they cannot reproduce and will become extinct in that area as will the animals that depend on them. A problem is that some plants have evolved to attract a certain type of pollinator or have become so specialised as to attract a certain species.

But the good news is that the extinctions that comprise the cascade effect are local, so that there are still populations of these species around to continue its presence on the planet.

Unfortunately some of the causes of this effect are easily replicated, for instance, bush fires or the clearing of land (habitat for organisms).

Also certain activities like hunting where key predators are targeted and thus the cause and the effect will be greater, probably resulting in this species being totally wiped out from the planet.

So we can see from the range of examples that the relationships between organisms are complex and connected and one of the main results of the cascade effect is the loss of biodiversity.