Community Ecology

Unit : Ecology

Chapter: Community ecology

Reference: Community ecology, Structure of a Community

Species Richness, Species Diversity, Foundation Species

Keystone Species, Community interactions, Climax community

 Learning objectives

  • To understand the structure of community
  • To learn about community interactions

Community ecology

Community ecology is the science and theory that examines how populations of organisms interact and respond to their non-living surroundings.

Structure of a Community

  • A community is made up of all the populations of various species that dwell in the same area and interact whereas the population is a collection of creatures from the same species living in the same region and interacting with one another
  • A community is made up of all an area's biotic components.
  • Organisms in a stand of pine trees, on a coral reef, and in a cave, a valley, a lake, or a stream are examples of communities.
  • The structure of a community can be defined using Species richness, or the number of species present, and species diversity, which is a measure of both species’ richness and species evenness
  • Many variables influence community structure, including abiotic influences, and species interactions, disturbance levels, and random events.
  • Foundation species and keystone species, for example, play critical roles in shaping the structure of their ecosystems.

Species Richness

  • The number of distinct species in a community is referred to as species richness.
  • If we identified 202020 species in one community and 200200200 species in another, the second community would have significantly more species diversity than the first.
  • The most diverse communities are found around the equator, where there is abundant solar energy (supporting high primary productivity), mild temperatures, abundant rainfall, and little seasonal variation.
  • The communities with the least diversity of species are found in the poles, which receive less solar energy and are therefore colder, drier, and less conducive to life.

Species Diversity

  • The number of species in a community is a measure of its complexity.
  • It is determined by the number of distinct species in the community (species richness) as well as the relative abundances of those species (species evenness).
  • Species diversity is increased when there are more species and their abundances are more evenly distributed.
  • Ecologists believe that ecological groups with greater diversity are more stable (i.e., better able to rebound after a disruption) than those with less diversity

Foundation Species

  • A foundation species plays a crucial role in the formation and definition of a community.
  • Foundation species frequently operate by altering the environment in order to assist the other organisms that make up the community.
  • Kelp (brown algae) is a foundation species that supports the kelp forests off the California coast.
  • Kelps provide habitats that allow other creatures in the kelp forest ecosystem to survive.

Another foundation species is the corals of a coral reef.

  • Living and dead coral exoskeletons make up most of the reef structure, which protects other species from waves and ocean currents.

Keystone Species

  • A keystone species is one that, in relation to its biomass or abundance, has a disproportionately large impact on community structure.
  • Keystone species differ from foundation species in two respects.
  • They are more likely to belong to higher trophic levels (to be top predators).
  • The most well-known example of a keystone species is the intertidal sea star Pisaster ochraceus, which may be found in the northwest United States.
  • The sea stars were taken from the intertidal zone where they dwelt in a typical community ecology experiment.
  • As a result, the populations of their prey (mussels) rose, changing the community's species makeup and drastically lowering species diversity.
  • After sea stars were present, around 252525 species of barnacles and algae were found in the lower intertidal zone, but when they disappeared, the mussel population surged downward and nearly completely supplanted these other species.
  • When a keystone species is lost, a significant fall in diversity or collapse of community structure is frequent.
  • The loss of diversity occurred in this situation because the mussels pushed out other species that could typically coexist because the sea stars kept the mussels in control.

Community interactions

  • Parasitism- one species (Parasite) is benefitted and the other (Host) is harmed (+, –). Ex. Cuscuta, Petromyzon, hagfishes, koel or cuckoo is a brood parasite, liver fluke. The parasites have following types of special adaptations for their survival
  • Commensalism-one species is benefitted and other is neutral i.e., neither benefitted nor harmed (+ ,0). Ex. egret and the cattle, orchid growing as an epiphyte on mango tree, clown fish hiding in the colony of sea anemone, barnacle growing on the back of whale, orchid growing as an epiphyte on mango tree.
  • Mutualism- interaction both the species are benefitted (+, +). Ex. Lichens, Mycorrhizae, female wasp pollinates the fig inflorescence while searching for suitable site for egg laying. The Mediterranean orchid Ophrys employs ‘sexual deceit’ to get pollination done by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour, and markings. The male bee is attracted to what it perceives as a female, ‘pseudo copulates’ with the flower, and during that process is dusted with pollen from the flower. When this same bee ‘pseudo copulates’ with another flower, it transfers pollen to it and thus, pollinates the flower
  • Predation- interspecific interaction also one species is benefitted and the other is harmed (+, –). Ex. tiger and deer, cat and mouse, and lion and zebra, introduction of prickly pear (cactus) into Australia; Nile perch into Lake Victoria (East Africa) and Eichhornia into water bodies in India. The other examples of exotic species that spread rapidly due to absence of predators are Parthenium, Lantana and Clarias etc.
  • Competition- interspecific process in which the fitness of one species (measured in terms of ‘r’) is significantly lower in the presence of another species. It is (–, –) relationship. Ex. the Flamingos (birds), visiting S. American lakes, compete with the resident fishes for the common food, zooplanktons.
  • Ammensalism- association one species is harmed but the other remains neutral (–, 0). Ex. Penicillium notatum growing close to bacteria.

Ecological succession

  • Series of biotic communities at same site, one after the other till a climax community develops which does not change further because it is perfect harmony with environment of area.
  • Pioneer community- first biotic community that develop in bare area.

Climax community- a stable, self-perpetuating, and final biotic community that develop at the end of biotic succession and is in perfect harmony with physical environment.

  • Seral or transitional communities- various biotic communities that develop during biotic succession
  • Sere- entire sequence of development stages of biotic succession from pioneer to climax community.

Succession is of two types-

  • Primary succession- It is the ecological succession occurring in an area where no organisms are found, like bare rocks.
  • Secondary succession- This type of succession takes place in those areas where all the previous biotic communities have been destroyed. Ex. burned forests, flooded fields.

Solved examples

Example 1. Rabbits and deer eat lettuce leaves from a garden.

What ecological interaction describes the relationship between rabbits and deer?

  1. Mutualism b) Competition c) Commensalism d)Predation

Solution 1: b. There is a competition between rabbits and deer for eating lettuce.

Example 2. A keystone species is…

a) the top carnivore. 

b) has the largest population.

c) has the most specialized niche

d)determines numbers and types of other species.

Solution 2: d. A keystone species determines numbers and types of other species.

Summary

  • Community ecology is the science and theory that examines how populations of organisms interact and respond to their non-living surroundings
  • The structure of a community can be defined using species richness, species diversity, foundation, and keystone species.
  • The number of distinct species in a community is referred to as species richness.
  • Foundation species frequently operate by altering the environment in order to assist the other organisms that make up the community.

 

 

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