What is a community?


A community is an assemblage of species living close enough together for interaction. There are different interactions that occur between species of a community; some key relationships in the life of an organism are its interactions with organisms in other species. There are interspecific interactions which include competition (a -/- interaction),
predation ( a +/- interaction), mutualism (a +/+ interaction), and commensalism (a +/0 interaction). Interspecific interaction can affect the survival and reproduction of each species and the effects can be summarized as positive (+), negative(-), or no effect(0).

Interspecific Interactions


Interspecific competition is an interaction that occurs when resources are limited. Organims from different compete for a resource that limits their growth and survival. For example, oxygen is very abundant so species would have no need to compete for it, but there are other resources that are can be short in supply and competed for. Grass is a food that grasshoppers and Bison both eat in the Great Plains, so naturally there will be competition between the two species for the grass.
Picture3.jpgBut two species cannot compete for the same resource in the same location forever, can they? Ecologist G.F. Gause studied this question using related species of protists. He put the two species together and added the same amount of food everyday. What eventually happened was the protists P. caudatum suddenly started dieing down and soon became extinct. Gause concluded that protitst P. aurelia were more efficient in using and obtaining the food, so they rapidly outgrew the competing species. This outcome is called competitive exclusion. The main idea is that two species competing for the same limiting resources cannot coexist in the same place, there is always a local elimination of a competing species.

The total of a species' use of biotic and abiotic resources is called the species' ecological niche. An ecological niche can also be thought of as an organism's ecological role, or how the individual firs into an ecosystem. The niche concept can be brought into the competitive exclusion principle, which states that two species cannot coexist permanently in a community if their niches are identical. Ecologically similar species and coexist in a community if there are one more significant differences in their niches. Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community.

A species' fundamental niche is the niche potentially occupied by that species. A realized niche is the niche actually occupied by that species. As a result of competition, a species' fundamental niche may differ from its realized niche, like for example the presence of one barnacle species limits the realized niche of another species.

Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric population of the same two species. The variation in beak size between populations of two species of Galapagos finches is an example of character displacement.


Predation refers to interaction where one species, the predator, kills and eat the other species, the prey. Predators develop feeding adaptions like claws, teeth, fangs, poison, and stingers. Rattlesnakes have toxins that they inject into their prey that find by heat-sensing organs. Prey also get adaptions to help avoid being eaten. They develop behavioral defenses including hiding, fleeing, forming groups, self-defense mechanisms, and alarm calls. Animals also have morphological and physiological defense adaptions.

Prey can develop a camouflage colored body, or cryptic coloration, which makes them difficult to spot. Animals with chemical defense often exhibit bright warning coloration called aposematic coloration. Predators are usually cautious in dealing with prey that display that coloration.In some cases, a prey species may gains significant protection by mimicking the appearance of another species.

Batesian mimicry is when one species, usually harmless, develops features that make it look like members of another known dangerous species. A hawkmoth larva can imitate a poisonous snake puffing its head and thorax when it is in the presence of a predator so larva can avoid being eaten.

Mullerian mimicry is a phenomenon where two harmful species, which may or may not be closely relates, mimic each other in their external appearance to scare away predators. There are two major types of mullerian mimicry that organisms can express: aposematism and crypsis mimicry. Aposematism mimicry is a strategy where dangerous organisms, like wasps, signal their defenses to predators through bright colors such as yellow, orange, purple, or red. The opposite of this is called crypsis, where an organism attempts to survive by attracting as little attention as possible. So they have attributes like camoflage which helps them hide. There are cases where organisms do both of aposematism and crypsis mimicry by hiding and looking inconspicous until the they are found, at which point they flash warning colors. This dual strategy is found in many amphibians and snakes.


An interaction where a herbivore eats parts of a plant or alga is called Herbivory. The plants (prey) being eaten cannot obviously run away from predators so they may have thorns or poison in them. Predators develop features to help them find and eat plants. Many herbivourous insects have chemical sensors to help them differentiate between toxic and non-toxic plants.


Symbiosis is a relationship where two or more species live in direct and intimate contact with one another.

Parasitism is a type of symbiotic relationship between organisms of different species in which one, the parasite, benefits from a prolonged, close association with the other, the host, which is harmed. In general, parasites are much smaller than their hosts, show a high degree of specialization for their mode of life and reproduce more quickly and in greater numbers than their hosts. Classic examples of parasitism include the interactions between vertebrate hosts and such diverse animals as the tapeworms, flukes, Plasmodium species and fleas.

Endoparasites are parasites that live within the body of their host (tapeworms or other parasitic worms) and ectoparasites are parasite that live on the external surface of a host (ticks, lice, or fleas). Some parasites change the behavior of the host to increase their own fitness. Tapeworm, for example, live inside the intestines of humans and farm animals. They obtain digested food and shelter from their hosts so the hosts are likely to suffer from malnutrition. Ticks that live on moose take blood from the moose which makes them weaks, causing hair loss. Now the moose can die from cold weather or predation by wolves.

Mutualism is any relationship between two species of organisms that benefits both species. Mutualism can be obligate, where one species cannot survive without the other, or faculative, where both species can survive alone. When nitrogen fixing bacteria live on plant roots, the bacteria get a place to live and the plants get nitrogen made by the bacteria; this is an example of mutualism.

In commensalism, one species benefits and the other is neither harmed nor helped. Epiphytic plants (such as many orchids) which grow on trees or birds that live in holes in trees are common examples of commensalism.
Orchids often grow on trees, using them to anchor themselves to get closer to the sun which is helping the orchid, but not affecting the host tree.

Facilitation describes an interaction where one species can have positive effects on another species without direct and intimate contact. For example, the black rush makes the soil more hospitable for other plant species.

Species Diversity and trophic structure

Two fundamental features of community structure are species diversity and feeding relationships. Species diversity is the variety of life forms that make up the community. It is dependent on both the species richness and the

relative abundance of different species. Species richness is defined as the total number of different species in the community. Relative abundance is the proportion each species represent of the total individuals in the community. Diversity can be compared using a diversity index, which is based on species richness and relative abundance. One commonely used index is the Shannon diversity index (H) which states:

H = –(pA ln pA + pB ln pB + pC ln pC + …)

where A,B,C are the species, p is the relative abundance of each species, and ln is the natural logarithm.The higher H is, the more diverse the community is.

Determining the number and abundance of species in a community is difficult, especially for small organisms. Molecular tools can be used to help determine microbial diversity.

Diversity and Community Stability

Ecologists change diversity in experimental communities to study the potential benefits of diversity. For example, plant diversity has been manipulated at cedar Creek Natural History Area in Minnesota for two decades!
Communities with higher diversity are:
--More productive and more stable in their productivity.
--Better able to withstand and recover from environemntal stresses
--More resistant to invasive species, organisms that become established outside their native range.

Trophic structure

Trophic structure is the feeding relationships between organisms in a community. what defines different trophic structures (or systems) is the amount of energy available to the organisms in the different trophic levels. The available energy of all/any trophic structures is almost always represented by a pyramid. Food energy is transferred from primary producers to primary consumers. then from primary consumers the secondary, tertiary, and quaternary consumers. Food chains link trophic levels from producers to the top carnivores. A food web is a series of food chains. Specifically, A food web is a model that shows all the possible feeding relationships between organisms living in an ecosystem.
Food webs can be simplified by:
–-Grouping species with similar trophic relationships into broad functional groups
-–Isolating a portion of a community that interacts very little with the rest of the community

Food chain length

Each food chain in a food web is usually only a few links long. Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis. The energetic hypothesis states that the length is limited by ineficient energy transfer. The dynamic stability hypothesis proposes that long food chains are less stable than short ones. Out of both the hypothesizes, most research and data support the energetic hypothesis. Certain species have a very large impact on community structure because they are highly abundant.

Species with a large impact
Dominant species are those that are most abundant or have the highest biomass. Because of this, dominant species exert powerful control over the occurrence and distribution of other species. For example, sugar maples have a major impact on shading and soil nutrient availability eastern North America affecting the distribution of other plant species. There is no definite explanation for why species become in control in a community, but there are hypotheses. One hypothesis suggests that dominant species are most competitive in exploiting resources. Another hypothesis is that they are most successful at avoiding predators.

Keystone species display strong control on a community by their ecological roles or niches. In comparison to dominant species, they are not necessarily abundant in a community.

Ecosystem engineers (or also called foundation species) cause physical changes in the environment that affect community structure. The effects of ecosystem engineers can benefit a community or give it drawbacks. As an example, beavers build damns and create ponds, which transform areas of a forest into wetlands.

Bottum-Up and Top-down models
The bottom-up model proposes V --> H linkages, in which the presence or absence of mineral nutrients (N) controls plant (V) numbers, which control herbivore (H) numbers, which control predator (P) numbers. Basically:
N --> V --> H --> P. In comparison, the top-down model states that it is mainly predation that controls community organization. Predators limit herbivores, which limit plants, which limit nutrient levels through the uptake of
nutrients during growth and reproduction. Simplified, the model is N <-- V <-- H <-- P.

Biomanipulation can help restore polluted communities by using the top-down model.
In a Finnish lake, blooms of cyanobacteria (primary producers) occurred when zooplankton (primary consumers) were eaten by large populations of roach fish (secondary consumers).
The addition of pike perch (tertiary consumers) controlled roach populations, allowed zooplankton to increase and ended cyanobacterial blooms.

Disturbance influences species diversity and composition

A long time ago, ecologists agreed with the fact that communities are in a state of equilibrium, but the nonequilbrium model states otherwise. the nonequilibrium modeldescribes communities as contantly changing after being buffeted by disturbances.
A disturbance is an event that changes a community, removes organisms from it, and alters resource availability. Fire is a significant disturbance in most terrestrial ecosystems. The intermediate disturbance hypothesis suggests that moderate levels of disturbance can raise greater diversity than either high or low levels of disturbance.
•High levels of disturbance exclude many slow-growing species
•Low levels of disturbance allow dominant species to exclude less competitive species
Ecological succession is the sequence of community and ecosystem changes after a disturbance.
Primary succession occurs where no soil exists when succession begins.
Secondary succession begins in an area where soil remains after a disturbance.

Human Disturbance

Humans have the greatest impact on biological communities all over the world. Humam disturbance to communities reduce species diversity. Humans turn tropical forests ito vast barren areas for lumber, cattle grazing, and farmland. The boats that drag nets on the seafloor to scrape up life forms in the ocean is a major disturbance in marine ecosystems.

Biogeographic factors affect community diversity

  • Latitude and area two key factors that effect a community's species diversity. Capture4.JPG
  • Species richness is especially great in the tropics and generally declines along an equatorial-polar gradient.
  • Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate.
  • Temperate and polar communities have started over repeatedly following glaciations
  • The greater age of tropical environments may account for the greater species richness
  • In the tropics, the growing season is longer such that biological time is faster

  • Climate is likely the primary cause of the latitudinal gradient in biodiversity.
  • Two main climatic factors correlated with biodiversity are solar energy and water availability.
  • They can be considered together by measuring a community’s rate of evapotranspiration.
  • Evapotranspiration is evaporation of water from soil plus transpiration of water from plants.
  • The species-area curve tells the idea that, all other factors being equal, a larger geographic area has more species.
A species-area curve of North American breeding birds supports this idea.

Island Equilibrium Model

  • Species richness on islands depends on island size, distance from the mainland, immigration, and extinction.
  • The equilibrium model of island biogeography that species richness on an island levels off at a dynamic equilibrium point.



Pathogens alter and drastically change communities. For an example. coral reef communities are being hit by the white-band disease, which is decimating them. Zoonotic pathogens have been transferred from other animals to humans. The transfer of pathogenscan be direct or through an intermediate species called a vector. Many of today’s emerging human diseases are zoonotic. Avian flu is a highly contagious virus of birds. Ecologists are studying the potential spread of the virus from Asia to North America through migrating birds.