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Define and describe the following: ecosystem, community, assemblage, guild, niche and habitat.

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Alice Helliwell Define and describe the following: ecosystem, community, assemblage, guild, niche and habitat. The term ecosystem is a holistic concept concerning plants, the animals habitually associated with them and all the chemical and physical components of the immediate environment, which together form a recognisable, fairly self-contained entity (Tansley 1935). The relationships between living entities in the ecosystem are manifested, not in a vacuum, but in physio-chemical settings i.e. a stage consisting of non-living (abiotic) environmental substances and gradients. This refers to basic inorganic elements and compounds such as water, carbon dioxide, calcium, carbonates and phosphates, or organic compounds, prevalent as by-products of organism activity or decay. Ecological relationships are also shaped by physical factors, such as edaphic conditions, gradients of current, wind speed and direction, moisture and solar radiation with its concomitants of light and heat. Within this arena act the biotic or living components i.e. plants, animals and microorganisms, all interrelating via energy-dependent relationships. These intra-biotic relationships as well as the relationships of living organisms with abiotic factors together comprise the ecosystem. Although ecosystems are often regarded as more or less independent parts of the biosphere (e.g. a forest, ocean, grassland, etc) and are conventionally divided into two parts: that of the biological community (a term I shall shortly define) and the physical environment, this creates a problem. Ecological communities and their environments should not, I believe, be studied as entirely separate entities as no individual, population or community exists in isolation from its environment. ...read more.


This occurred with increasing concentrations of salt in ground water and decreasing height of land above sea level. The deep lens of the Aldabra atolls accounted for the immense diversity of flora. The extent of shelter from dry, salt-laden southeast trade winds also affected the spatial distribution of the island's vegetation, most species being restricted to sheltered areas. Vegetation was also affected by interactions with tortoises, notably through the reptiles' grazing and trampling habits. Species such as Melanthera biflora where restricted to areas of little tortoise grazing as opposed to other species which grew only in areas of intense tortoise activity. The plant species Lagrezia oligomeroides appeared in divergent growth forms when ungrazed and grazed areas were compared. Communities are composed of individuals and populations, populations being comprised of an interbreeding group of individuals of the same species, usually found in an area that is describable. Ecologists studying communities may therefore measure its collective properties, such as species diversity and community biomass. One of the simplest measures of community diversity is Simpson's Index (D), a non-parametric approach to determining sample heterogeneity, which gives the probability that, of two individuals in a random sample, both are of the same category or species. S = total number of species in the community P = proportion of individuals in each species that contribute to the total sample Equitability (E) ...read more.


It follows that if two competing species coexist in a stable environment, this coexistence will have occurred through niche differentiation i.e. the use of slightly different resources or conditions e.g. utilising different food sources, habitats or times for activity. Without such differentiation, one species will eliminate the other- the crux of the competitive exclusion theory. So, according to competition theory, many of the niches that species currently occupy are the result of competition between species that occurred in the past or at present. An example of niche differentiation is evident between the beetles Oryzaephilus sp. and Rhizopertha sp. who both coexist in stored wheat grain. Although both species of adult beetles feed off the outside of wheat grain, the larvae of Rhizopertha sp. feed off the inside of the grain, allowing the Oryzaephilus sp. larvae to consume the outer coating of wheat grain too. In conclusion, it is clear that the term ecosystem (all the interacting and interdependent biotic and abiotic factors in a particular area that make up a self-contained system, in terms of energy flow) encompasses the concepts of community, assemblage, guild, habitat and niche within its definition. As we have seen, ecosystems may be fundamentally split into two portions: its communities as well as the habitats or environments these biotic factors are found to live in. Within communities, assemblages of species are present, each occupying a specific niche. Guilds thus arise between assemblage groups when resources are exploited in similar ways. ...read more.

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