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

Alice Helliwell

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.

As much as ecosystems are varied, ecologists have recognised common and essential features which characterise all ecosystems and all allow one to make the following statements. All ecosystems require an influx of energy in the form of light and heat radiation from the sun. Solar energy is the ultimate and only significant source of energy in any ecosystem. This solar energy is henceforth converted, by producers, into organic compounds such as C6H12O6, which store chemical potential energy within their molecular bonds. Producers are autotrophs- organisms that are independent of outside sources for organic food, manufacturing their own organic material from inorganic sources. Photoautotrophs such as plants utilise this solar energy to combine carbon dioxide and water (in the presence of chlorophyll) into sugars. Chemoautotrophs such as chemosynthetic bacteria and caroteniod-bearing purple bacteria utilise other sources of energy to synthesise organic matter although, in comparison to the energy entering the ecosystem via solar radiation, making a relatively insignificant contribution to the total of an ecosystem’s energy relations. Heterotrophs (organisms with a requirement for energy-rich organic molecules) then consume the producers, meeting their energy needs indirectly. Such organisms are termed primary consumers e.g. the herbivorous Lapland reindeer, which consume photosynthetic Reindeer Moss (which is, paradoxically, a lichen,) are in turn eaten by omnivorous secondary consumers; humans. In this manner, food chains are inherent to ecosystems, describing the energy flows between organisms:

Autotroph → Heterotroph Producer → Consumer Producer → Herbivore → Carnivore

Energy losses within food chains, increasing with subsequent trophic levels, are also fundamental to the

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Autotroph → Heterotroph Producer → Consumer Producer → Herbivore → Carnivore

Energy losses within food chains, increasing with subsequent trophic levels, are also fundamental to the

energy-based interactions of all ecosystems. Decomposers also feature in ecosystems, breaking down biomass into its basic elements, thus restoring nutrients to primary producers. Ecosystems also feature, in comparison to the non-cyclic and unidirectional flow of energy through food chains, the cyclic flow of minerals through the system. In addition one may conclude that in all ecosystems, no organism is sufficient unto itself, similarly no ecosystems are entirely self-sufficient. Instead, ecosystems form a contiguity or continuousness with each other, as biotic and abiotic factors move across supposed ecosystem boundaries. I will now proceed to describe the term community, an essential element of all ecosystems.

A community consists of all the plants and animals living in a habitat, or more simply all the species that occur together in space and time. Ecologists, when studying communities tend to focus on how the groupings of species (assemblages) are distributed in nature and how such groupings are influenced by interactions between species and by the physical forces of their environment. For example, Gibson and Phillipson set about constructing a vegetation map of the Aldabra atolls of the Indian Ocean. The atolls, ring-like coral islands and reefs which enclose lagoons, contains extremely unique and rare communities in which herbivore-plant interactions are dominated by the population of 150, 000 giant tortoises (Geochelone gigantae) which reside on the islands. Gibson and Phillipson found that the determinants of spatial vegetation included both topological changes and exposure to wind (abiotic factors) as well as biotic factors such as the numbers of tortoises evident in a particular area. Topological changes varied from the seacoasts to the lagoon shores, whereby the pattern of a gradual replacement of mixed scrub species for P.acidula was observable. 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) is the maximum possible value for D, occurring when all individuals in the sample are completely evenly distributed amongst categories/species. E is a value that resides between 0 and 1.

Nonetheless, it is important to note that communities are more than the sum of their constituent species. Communities consist of the sum of various species and their subsequent interactions (e.g. parasitism, predation and competition.) Such interactions reveal the emergent properties of the community. By studying a variety of communities, the ecologist endeavours to discover repeating patterns of emergent and collective properties that may be predicted in a ‘community X’, regardless of the species assemblage.

Communities may be proverbially thought to be separated by clear, sharp boundaries where groups of species lie adjacent but do not intergrade into one another. This is in fact very rare and exceptional, for example, the boundary between aquatic and terrestrial communities appearing clear yet being frequently crossed by frogs and otters, or insects who sped their larval lives in water only to later spend their winged stages on land and in the air. Thus, at a given location, by virtue of mainly its physical characteristics, a community will possess a reasonably predictable association of species, although a given species that occurs within this association and in this area is likely to similarly occur with another group of species under different environmental conditions. In this manner, community boundaries probably do not exist, although a continuum of how sharply each community is defined is evident. As a result, ecologists focus their investigations into how these communities merge or grade into one another.

Under the definition of what constitutes a community, groups of species where said to occur in the same space and time. The space where an organism lives is termed its habitat, scrubs, mangrove swamps and coastal dunes comprising the habitats of Aldabra giant tortoises. An organism’s niche moreover, is the adaptive behaviour and interacting roles it plays with other individuals within this habitat, normally referring to the organism’s feeding habits. For example, the habitat of the straw-coloured fruit bat includes forest and savannah areas of the southwestern Arabian Peninsula as well as south of the African Sahara and well as Madagascar. Straw-coloured fruit bats’ diverse roosting habitats include caves, trees and lofts. The niche of the straw-coloured fruit bat involves extracting juices from fruits such as apples, bananas and grapes (as opposed to the long-tongued fruit bat of Malaysia which extorts nectar from the beremban flower) in addition to consuming the blossoms and young shoots of the silk cotton tree. A guild is a term used to describe a group of organisms with similar niches i.e. a group of species that exploit the same class of environment resources in a similar manner. The species that make up a guild need not be taxonomically related, for example, the fig-eating guild of Borneo being comprised of the area’s fruit bats, arboreal mammals and birds. The term guild also implies a certain level of competition between the species that exploit the same resource. Competition refers to the negative interaction between individuals whereby organisms inhibit each other due to the use of a shared resource. Intraspecific competition occurs between members of the same species whereas interspecific competition occurs between individuals of divergent species. Competition takes the form of interference in which there is direct interaction between individuals (e.g. fighting) compared to indirect, exploitative competition that causes the depletion of one group’s food/ nutrient/ light resources by another group. 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.