Science 7: Unit A – Interactions and Ecosystems Topic 6: Succession and Change in Ecosystems

Succession Succession occurs when changes in an ecosystem lead to some species replace other species in the ecosystem. Eg. A fire burns through a forest, destroying trees and grass. A few years later, scrubs and bushes are growing in the same location.

Primary Succession Primary Succession is the gradual growth of organisms in an area where no life previously existed. Lichens, moss, and ferns are usually the first organisms to grow on bare rock. As these organisms do establish themselves, the break down rock and make the ecosystem more suitable for other, more complex species of life.

Ferns Growing in bare rock

Secondary Succession Secondary succession is the gradual growth of organisms in an area that already hosted life. The example of the forest fire is an example of secondary succession. Some species of bushes grow best in soil where there is large amounts of ash.

Grasses and Bushes Growing in an area previously burned.

Human Effects on Succession Because humans are having a wide effect on many ecosystems, we are influencing the process of succession in these ecosystems. Certain species of animals succeed in an ecosystem because they can adapt to the changes we cause in an ecosystem. These species are called generalists because they are a jack of all trades that scavenge for scraps of food we leave behind.

Generalists tend to be Scavengers Generalists are species that can adapt to wide range of conditions. They include: rats, raccoons, cockroaches, seagulls, pigeons. Many generalists are scavengers. These animals are succeeding against native animals such as owls, wolves, foxes, etc. Many of the animals suffering are carnivores and herbivores. These animals cannot adapt as easily to living with humans.

Pest Control Using chemicals such as DDT to kill pests leads to quite a few problems. Pollution, resistance, bioaccumulation, and damage to non-pest organisms. When you use a pesticide it can affect organisms that you don’t want to harm, such as your crops. Also, using pesticides can lead to the pest species developing resistance to the chemical.

Biological Control Another way to eliminate pests is to use their predators as a natural pesticide. Beetles and lady-bugs are very effective at controlling pest plants and animals. There is still a risk with biological control. Introducing species into a new area could have unpredicted effects.

Introduced Species An introduced species is one that is brought to a new ecosystem with humans. Often introduced species do damage to the ecosystem by harming the organisms there. Eg. Settlers brought rabbits with them to Australia. The rabbits escaped into the wild, and without predators in Australia, began to grow quickly in numbers, feasting on the grasses. This lead to a drop in the herbivore population in that part of Australia.

Species in Danger Threatened Species – a species whose numbers are dropping. Endangered Species – a species whose numbers are so low that they are in danger of becoming extinct or extirpated in an area. Extinct - a species which has been completely wiped out. Extirpated – a species which has been wiped out in an area or ecosystem.

Causes of Extinction Humans have had a large impact on recent extinctions. This happens in a numbers of ways: 1. Loss of habitat and food sources. If the organism loses its home, everything else will suffer. 2. Over-hunting by humans. 3. Competition by introduced species. An introduced species can eat all of the species’ food, or the species itself.

How Can We Help? By designating certain areas as wildlife preserves. Wildlife preserves are areas that we leave completely alone. They are not parks because people are not even supposed to go camping there. Habitats are preserved and introduced species are kept from spreading into the preserve.