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Ecology, derived from the Greek word "oikos" (meaning home) is the scientific study of the distribution and abundance of organisms. Where organisms occur and how many there are is determined by the interactions between them and their environment. An organism's environment is made up of other organisms (biotic factors) on the one hand and physical and chemical phenomena (abiotic factors) on the other. The combined biological and physical constituents of the environment are referred to as an ecosystem. Within an ecosystem, ecology can deal with individual organisms, populations (individuals of the same species occurring together in space and time) or communities (assemblages of species populations). The actual place where an organism lives is its habitat, whereas the term ecological niche describes 'how' an organism lives (its tolerances and requirements defining the resources and conditions it needs to survive). Ecology is furthermore concerned with the pathways of energy and matter flowing through an ecosystem, for example through looking at food webs or nutrient cycles. The applications of ecology are manifold and include biodiversity conservation, management of habitats and protected areas, pest control and harvest management to name but a few.
In terms of evolutionary convergence, the repeated conquest of new habitats can provide fascinating insights. For example, terrestrialisation or taking to the sky (in the form of gliding or powered flight) has occurred multiple times independently in different groups of animals. It is also particularly instructive to explore parallels between communities of organisms inhabiting similar environments.
Mediterranean-type ecosystems, which are characterised by long hot, dry summers and short winters with variable rainfall, occur in different regions of the world, and the plant communities there show a number of striking similarities, notably drought-evading annuals and evergreen shrubs.
In ancient times, Madagascar and New Zealand were both home to giant herbivorous birds, against which the resident plants (that are only very distantly related to each other) evolved defences, such as tough wiry branches with widely spaced leaves. With these giant birds now extinct, the wire anatomy still remains as a ghost of past adaptation.
Bacteria show an extraordinarily wide distribution and have colonised even the most extreme of environments. For example, thermophilic bacteria flourish in high-temperature environments, while halotolerant bacteria can cope with extreme salinities. These extremophiles represent a key example of convergence as they have evolved independently in each of the two major subgroups of bacteria, the Archaea and Eubacteria.
Further interesting examples of ecological convergence can be found among the birds (e.g. between the nectar-feeding hummingbirds and sunbirds) and snakes (e.g. between African and European whip snakes).