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Community (ecology)

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"Ecological community" redirects here. For human community organized around economic and ecological sustainability, see ecovillage.

A bear with a salmon in their mouth, having caught the fish from a river. Interspecific interactions such as predation are a key aspect of community ecology.

In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".

Community ecology or synecology is the study of the interactions between species in communities on many spatial and temporal scales, including the distribution, structure, abundance, demography, and interactions between coexisting populations.[1] The primary focus of community ecology is on the interactions between populations as determined by specific genotypic and phenotypic characteristics.

Community ecology also takes into account abiotic factors e.g. annual temperature or soil pH. Abiotic factors filter the species that are present in the community. For example the difference in plants present in the desert compared to the tropical rainforest is dictated by the annual precipitation. These non-living factors also influence the way species interact with each other [2].

Community ecology has its origin in European plant sociology. Modern community ecology examines patterns such as variation in species richness, equitability, productivity and food web structure (see community structure); it also examines processes such as predator–prey population dynamics or succession.

On a deeper level the meaning and value of the community concept in ecology is up for debate. Communities have traditionally been understood on a fine scale in terms of local processes constructing (or destructing) an assemblage of species, such as the way climate change is likely to affect the make-up of grass communities.[3] Recently this local community focus has been criticised. Robert Ricklefs has argued that it is more useful to think of communities on a regional scale, drawing on evolutionary taxonomy and biogeography,[1] where some species or clades evolve and others go extinct.[4]

Niche

Within the community, each species occupies a niche. A species' niche determines how it interacts with the environment around it and its role within the community. By having different niches species are able to coexist.[5] This is known as niche partitioning. For example, the time of day a species hunts or the prey it hunts.

Niche partitioning the reduces competition between species.[6] Such that species are able to coexist as they suppress their own growth more than they limit the growth of other species. The competition within a species is greater than the competition between species. Infraspecific competition is greater than interspecific.

The number of niches present in a community determines the number of species present. If two species have the exact same niche (e.g. the same food demands) then one species will outcompete the other. The more niches filled, the higher the biodiversity of the community.

Theories

Holistic theory

Holistic theory refers to the idea that a community is defined by the interactions between the organisms in it. All species are interdependent, each playing a vital role in the working of the community. Due to this communities are repeatable and easy to identify, with similar abiotic factors controlling throughout.

Clements developed the holistic (or organismic) concept of community, as if it was a superorganism or discrete unit, with sharp boundaries. [7] Clements proposed this theory after noticing that certain plant species were regularly found together in habitats, he concluded that the species were dependent on each other. Formation of communities is non-random and involves coevolution. [8]

The Holistic theory stems from the greater thinking of Holism; which refers to a system's with many parts all of which are required for the functioning of the system.

Individualistic theory

Gleason developed the individualistic (also known as open or continuum) concept of community, with the abundance of a population of a species changing gradually along complex environmental gradients. [9] Each species changes independently in relation to other species present along the gradient. [10] Association of species is random and due to coincidence. Varying environmental conditions and each species' probability of arriving and becoming established along the gradient influence the community composition. [11]

Individualistic theory proposes that communities can exist as continuous entities, in addition to the discrete groups referred to in the holistic theory.

Neutral theory

Hubbell introduced the neutral theory of ecology. Within the community (or metacommunity), species are functionally equivalent, and the abundance of a population of a species changes by stochastic demographic processes (i.e., random births and deaths).[12] Equivalence of the species in the community leads to ecological drift. Ecological drift leads to species' populations randomly fluctuating, whilst the overall number of individuals in the community remains constant. When an individual dies, there is an equal chance of each species colonising that plot. Stochastic changes can cause species within the community to go extinct, however this can take a long time if there are many individuals of that species.

Species can coexist because they are similar, resources and conditions apply a filter to the type of species that are present in the community. Each population has the same adaptive value (competitive and dispersal abilities) and resources demand. Local and regional composition represent a balance between speciation or dispersal (which increase diversity), and random extinctions (which decrease diversity).[13]


Interspecific interactions

Species interact in various ways: competition, predation, parasitism, mutualism, commensalism, etc. The organization of a biological community with respect to ecological interactions is referred to as community structure.

Interactions Species 1
Negative Neutral Positive
Species 2 Negative Competition Amensalism Predation/Parasitism
Neutral Amensalism Neutralism Commensalism
Positive Predation/Parasitism Commensalism Mutualism

Competition

Species can compete with each other for finite resources. It is considered to be an important limiting factor of population size, biomass and species richness. Many types of competition have been described, but proving the existence of these interactions is a matter of debate. Direct competition has been observed between individuals, populations and species, but there is little evidence that competition has been the driving force in the evolution of large groups.[14]

Lion eating their prey
  1. Interference competition: occurs when an individual of one species directly interferes with an individual of another species. This can be for food or for territory. Examples include a lion chasing a hyena from a kill, or a plant releasing allelopathic chemicals to impede the growth of a competing species.
  2. Apparent competition: occurs when two species share a predator. The populations of both species can be depressed by predation without direct exploitative competition.[15]
  3. Exploitative competition: This occurs via the consumption of resources. When an individual of one species consumes a resource (e.g., food, shelter, sunlight, etc.), that resource is no longer available to be consumed by a member of a second species. Exploitative competition is thought to be more common in nature, but care must be taken to distinguish it from apparent competition. Exploitative competition varies:
  • complete symmetric - all individuals receive the same amount of resources, irrespective of their size
  • perfect size symmetric - all individuals exploit the same amount of resource per unit biomass
  • absolute size-asymmetric - the largest individuals exploit all the available resource. [16]
The degree of size asymmetry has major effects on the structure and diversity of ecological communities

Predation

Predation is hunting another species for food. This is a positive–negative interaction, the predator species benefits while the prey species is harmed. Some predators kill their prey before eating them, also known as kill and consume. For example, a hawk catching and killing a mouse.

Other predators are parasites that feed on prey while alive, for example a vampire bat feeding on a cow. Parasitism can however lead to death of the host organism over time.

Another example is the feeding on plants of herbivores, for example a cow grazing Predation may affect the population size of predators and prey and the number of species coexisting in a community.

Mutualism

Mutualism is an interaction between species in which both benefit. Examples include Rhizobium bacteria growing in nodules on the roots of legumes and insects pollinating the flowers of angiosperms.

Commensalism

Commensalism is a type of relationship among organisms in which one organism benefits while the other organism is neither benefited nor harmed. The organism that benefited is called the commensal while the other organism that is neither benefited nor harmed is called the host. For example, an epiphytic orchid attached to the tree for support benefits the orchid but neither harms nor benefits the tree. The opposite of commensalism is amensalism, an interspecific relationship in which a product of one organism has a negative effect on another organism but the original organism is unaffected.[17]

Parasitism

Parasitism is an interaction in which one organism, the host, is harmed while the other, the parasite, benefits.


Community structure

A major research theme among community ecology has been whether ecological communities have a (nonrandom) structure and, if so how to characterise this structure. Forms of community structure include aggregation[18] and nestedness.

See also

References

  1. ^ a b Sahney, S.; Benton, M. J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society B: Biological Sciences. 275 (1636): 759–65. doi:10.1098/rspb.2007.1370. PMC 2596898. PMID 18198148.
  2. ^ Dunson, William A.; Travis, Joseph (November 1991). "The Role of Abiotic Factors in Community Organization". The American Naturalist. 138 (5). doi:10.1086/285270.
  3. ^ Grime J. P.; et al. (2008). "Long-term resistance to simulated climate change in an infertile grassland". PNAS. 105 (29): 10028–10032. doi:10.1073/pnas.0711567105. PMC 2481365. PMID 18606995.
  4. ^ Ricklefs R.E. (2008). "Disintegration of the Ecological Community". American Naturalist. 172 (6): 741–750. doi:10.1086/593002. PMID 18954264.
  5. ^ Albrecht, M.; Gotelli, N.J. (2001). "Spatial and temporal niche partitioning in grassland ants". Oecologia: 134–141. doi:10.1007/s004420000494.
  6. ^ Cloyed, Carl S.; Eason, Perri K. (2017). "Niche partitioning and the role of intraspecific niche variation in structuring a guild of generalist anurans". Royal Society Open Science: 170060. doi:10.1098/rsos.170060.
  7. ^ Hanspach, Jan; Hartel, Tibor; et al. (2014). "A holistic approach to studying social-ecological systems and its application to southern Transylvania". Ecology and Society. 19 (4). doi:10.5751/ES-06915-190432.
  8. ^ Shipley, Bill; Keddy, Paul A. (April 1987). "The individualistic and community-unit concepts as falsifiable hypotheses". Vegetatio. 69: 47–55. doi:10.1007/BF00038686.
  9. ^ Verhoef, Herman A. "Community Ecology". Retrieved 8 March 2015.
  10. ^ "What is vegetation classification?". International Association for Vegetation Science (IAVS). Retrieved 8 March 2015.
  11. ^ McINTOSH, ROBERT P. (1995). "H. A. GLEASON'S 'INDIVIDUALISTIC CONCEPT' AND THEORY OF ANIMAL COMMUNITIES: A CONTINUING CONTROVERSY". Biological Reviews: 317–357. doi:10.1111/j.1469-185X.1995.tb01069.x.
  12. ^ Hubbell, Stephen P. (2001). The unified neutral theory of biodiversity and biogeography (Print on Demand. ed.). Princeton [u.a.]: Princeton Univ. Press. ISBN 978-0691021287.
  13. ^ Vellend, Mark (June 2010). "Conceptual synthesis in community ecology". The Quarterly Review of Biology. 85 (2): 183–206. doi:10.1086/652373. PMID 20565040.
  14. ^ Sahney, S., Benton, M.J. and Ferry, P.A. (2010). "Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land". Biology Letters. 6 (4): 544–547. doi:10.1098/rsbl.2009.1024. PMC 2936204. PMID 20106856.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ^ Holt R.D. (1977). "Predation, apparent competition, and the structure of prey communities". Theoretical Population Biology. 12 (2): 197–229. doi:10.1016/0040-5809(77)90042-9. PMID 929457.
  16. ^ del Río, Miren; Condés, Sonia; Pretzsch, Hans (2014). "Analyzing size-symmetric vs. size-asymmetric and intra- vs. inter-specific competition in beech (Fagus sylvatica L.) mixed stands". Forest Ecology and Management. doi:10.1016/j.foreco.2014.03.047.
  17. ^ Willey, Joanne M.; Sherwood, Linda M.; Woolverton Cristopher J. (2011). Microbiology. Prescott's. pp. 713–738.
  18. ^ Poulin, R. (2006) Evolutionary Ecology of Parasites Princeton University Press

Further reading

  • Akin, Wallace E. (1991). Global Patterns: Climate, Vegetation, and Soils. University of Oklahoma Press. ISBN 0-8061-2309-5.
  • Barbour, Burke, and Pitts, 1987. Terrestrial Plant Ecology, 2nd ed. Cummings, Menlo Park, CA.
  • Morin, Peter J. (1999). Community Ecology. Wiley-Blackwell Press. ISBN 978-0-86542-350-3.
  • Odum, E. P. (1959) Fundamentals of ecology. W. B. Saunders Co., Philadelphia and London.
  • Ricklefs, R.E. (2005) The Economy of Nature, 6th ed. WH Freeman, USA.
  • Ricketts, Taylor H., Eric Dinerstein, David M. Olson, Colby J. Loucks et al. (WWF) (1999). Terrestrial Ecoregions of North America: a conservation assessment. Island Press. ISBN 1-55963-722-6.
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