Topic 5.2 / Option G.3 Human Impact on Ecosystems 2 Invasive species, Biomagnification and the Ozone layer Assessment Statements: G.3.4 – G.3.11
Threats to biodiversity: introduced species When species are intentionally or unintentionally moved from their native habitat, their populations are often not kept in check by the predators, parasites and pathogens from their native habitats. Often such species can spread and take over ecosystems, outcompeting natives for resources, increasing predation and reducing biodiversity by causing extinction of species that are not adapted to the non-native. Researchers are developing ways to reduce invasive non- native species through biological control. This means using one organism to control another. This can help reduce numbers of the pest species, but introducing another species can create more problems if the control species itself becomes a pest.
Examples of predation and competition by introduced species The brown tree snake accidentally traveled to Guam and has since caused 12 bird and 6 lizard species to go extinct as a result of predation. Reed canary grass outcompetes many native wetland plants and creates a vegetative monoculture. European starlings compete with and kill many native songbirds. Domesticated cats kill many native (and also non-native) songbird species. Zebra mussels outcompete many local species of benthic invertebrates, destroying the base of river, stream and wetland food webs, threatening native fish.
Examples of biological control Gone right (so far) Alligator weed clogs up many wetlands in florida. After the alligator weed flea beetle was introduced, the weed was successfully controlled. The alfalfa weevil has been controlled with the introduction of some natural predators The European corn borer has been controlled through the introduction of a chinese wasp. Gone wrong The cane toad was introduced in australia to control the cane beetle. The toad also eats native insects, and competes and spreads disease to native reptiles and amphibians. Mongoose was introduced in Hawaii to control rats, but it has also destroyed native bird populations.
Biomagnification Biomagnification is the buildup of toxic chemicals in higher trophic levels. The chemical is present in very low levels in producers, but does not break down. The chemical is passed up the trophic levels and accumulates in the tissues of organisms. When top level consumers eat the biomass from lower trophic levels, they accumulate the chemical in higher concentrations, therefore they are the most affected.
Examples of biomagnification: DDT DDT (a pesticide) was used to kill mosquitos and other crop pests, and was magnified in tertiary consumers such as birds of prey and seabirds. DDT interferes with the deposit of calcium in eggshells. Weakened eggshells break when the birds attempt to sit on them. DDT was banned in the US in 1971 due mostly to a book called “Silent Spring” by Rachel Carson. It is still produced and shipped to other countries.
Examples of Biomagnification: Mercury Mercury is a biproduct of many industries and is released into the atmosphere with air pollution. When it settles into water, it interacts with bacteria to become methyl mercury, which is toxic. Methyl mercury accumulates in all fish from contaminated waters, but is especially concentrated in fish and marine mammals at high trophic levels. Methyl mercury causes birth defects and damages the immune and nervous systems, especially of embryos and young children.
The cause of ozone depletion Chlorofluorocarbons (CFCs) from refrigerants and aerosol cans has build up in the atmosphere since around 1975. The chlorine atoms react with ozone (O 3 ) in the stratosphere and bind to one of the O, leaving ClO and O 2. Two ClO bind together to form Cl2O 2, and in the presence of sunlight, dissociate to 2Cl and O 2. Thus ozone is broken down, but the chlorine is still free to attack more ozone.
The effects of ozone depletion Ozone exists in the stratosphere, and absorbs and helps reflect UV radiation from the sun. With a thinner ozone layer, more UV radiation will reach the earth and the living things there. UV radiation is a mutagen. It penetrates cells and causes mutations in the DNA. In humans, this usually results in skin cancer. Animals and plants will also suffer the effects of excessive UV radiation in the form of more cancers and disrupted DNA, reducing productivity. Perhaps the most disturbing effect will be on phytoplankton, which form the basis for all oceanic food webs. The exact effects are not known, but it is likely they will be damaged.
How to help stop ozone depletion CFCs were banned in many countries and chemical companies are using alternatives to refrigerants and aerosols. Refrigerants may be recycled rather than dumped or released into the atmosphere. Non-aerosol propellants may be used. Cardboard may be used instead of styrofoam, which is manufactured using CFCs. CFCs take up to 7 years to actually reach the upper atmosphere, but there is evidence that the hole is growing at a slower rate and may even be healing itself. Don’t vote for politicians who want to lower environmental standards