Brief notes 3

Island Biogeography.

Continental islands usually suffer some extinctions as they are supersaturated at separation. Once equilibrium is reached extinctions usually balance immigration. Extinctions are few at first because there are few species and plenty of resources, increasing later as more species go extinct and resources and niches are occupied.

Water balance in insects

Insects do not regulate their water balance by sweating or panting, fluid loss is regulated mainly by the rate of secretion of the Malpighian tubules and absorption in the rectum. When they have excess water a diuretic hormone is released which increases the volume of urine entering the gut. When water is in short supply due to environmental conditions, or to the type of food intake, insects are able to survive by absorbing water from the rectal lumen, even against a concentration gradient. Some insects, e.g. locusts can decrease their respiratory loss six-fold to minimize water loss.

Salvinia molesta and Cyrtobagous salviniae, an example of successful biocontrol

Salvinia molesta is a floating aquatic fern native to South America, but spread accidentally since 1939 into many tropical rivers, lakes and canals. Its growth is favoured by warm, nitrogen rich water, and it can spread vegetatively. It has no natural enemies outside South America, so rapidly became a serious pest completely blocking waterways and disrupting the livelihood of people who depended on the water for transport, irrigation and food, especially fishing, rice and sago palm. The problem was especially bad in the Sepik River in Papua New Guinea where some villages had to be abandoned as the locals could no longer make a living. By 1980s 2000 km2 of water surface was covered. This was too expensive for manual, mechanical or herbicide removal. Biocontrol was the answer.

In Brazil the fern is controlled by a weevil, but it appeared that certain Salvinias have their own species specific weevils. So the Salvinia had to be correctly identified, then its specific weevil identified. This took until 1978. Previously the Salvinia had been mis-identified. Once the correct weevil had been found and released success was rapid. In Papua New Guinea the villagers were able to re-occupy their villages and live their lives as they had done before. In Sri Lanka the economics of bio control were studied in detail and a cost/benefit analysis was done. Returns for financial investment were 53:1, and labour investment was 1678:1. the team responsible for the ecological research leading to the bio control were awarded the UNESCO Silence prize in 1985. It was recognised that taxonomists had made essential contributions by establishing the true identity of the salvinia and the weevil. The weevil in question was new to science.

Garadoume in Niger

Garadoume is located in a river valley, the river is dry for part of the year. The problems faced are lack of water and soil erosion. Bankettes, low stone dykes, were built on the ground raised by a plough along the contours of a very gentle slope. The bankettes were made every 40 metres, and they help to catch silt etc. Crops can be planted within 15 metres of the bankette catchments area. Across the valley floor windbreaks were planted every 100 m to stop topsoil erosion and provide fuel wood. Crops can be grown between the windbreaks. The bankette terraces were made by women's groups. Along the contours of the very gentle slope crescent-shaped mounds of stones help to increase infiltration of water and the trees planted in the depressions help to stabilise the soil. The whole programme was very labour intensive and the locals were involved at every step. Most of the work and organisation was carried out by women. The latest danger is that gravel and sand from the dry river bed is being taken by a big construction company. The women say this makes the river flow faster and therefore there is less water infiltration. They have the support of the local chief, and have dug deep pits in the riverbed to make it impassible to trucks. All of this work has cemented a good community spirit and given the women a feeling of power and achievement.

The causes of microevolution

Genetic drift - Random changes in a small gene pool due to sampling errors in the propagation of alleles.
Gene flow - Change in gene pools due to immigration or emigration of individuals between populations.
Mutation pressure - Change in allelic frequencies due to net mutation.
Nonrandon mating - Inbreeding or selection of mates for specific phenotypes (assortative mating) reduces the frequency of heterozygous individuals.
Natural selection - Differential reproductive success increases frequencies of some alleles and diminishes others.

Autogenic, allogenic and secondary succession

Allogenic succession is driven by external influences which alter conditions e.g. salt marsh to woodland. 

Autogenic succession of sp. is driven by processes within the community itself e.g. shallow pond to bog to scrub.

Secondary succession occurs on land that has been cleared e.g. by fire of flora and fauna, but which still has viable seeds and spores in the soil.