Communities in Ecology

Lesson # 28

There is a tremendous variability in the mix of species found in different communities.

However, a common feature of many type of communities is that they are dominated by only a few species

Ex: One biologist, examined a single tree and found an estimated of 1 700 species of beetle living in it. These beetles were only one part of the larger group of insects living in the tree.

Ecological Dominants:

They are the small number of species that are abundant in a given community.

These generally are plants, but they can be other life forms, as corals that are the ecological dominants in coral reefs.

It is all the populations of all the species living in one area.Community:

There are species who may not be numerous in a given area, but who play a role in a community that cannot be assumed by any other community member. Each of these is a keystone species. Keystone species: It is a species whose absence from a community would bring about significant change in that community.

When the predatory see start Pisaster ochraceus was removed from a small area of rocky shore along the Pacific coast of the United States, the species composition of the community changed drastically, more so than if one of the other species had been removed.

BiodiversityThere are three principal types of biodiversity

a) Species diversity: The diversity of species in a given area

b) Geographic diversity: The distribution of species across a given area

c) Genetic diversity: The diversity of alleles or variants of genes in a population

High species diversity Low species diversity

High geographic diversity Low geographic diversity

High genetic diversity Low genetic diversity

Importance of the Biodiversity1- Diverse communities tend to be more productive communities.Productivity in Ecology: It is a measure of how much of the sun’s energy plants are able to capture and then turn into living material or biomass.2- Species diversity leads to greater stability and greater capacity to recovery from, say, a drought

Tropical rain forests are the most productive biome type in the world, meaning they produce more biomass per square meter then any other. No other ecosystem can touch them in terms of species diversity. Ex: In the tropical forest of Amazon, more than 300 species of trees have been identify in a single hectare of land.

Two Important Community ConceptsHabitat

Niche

It is the physical surroundings in which a species can normally be found (it is the species’ preferred type of neighborhood)

It includes where the organism makes a living, what it does to obtain resources, and how it deals with competition for these resources (it is an organism’s occupation)

Ex: The horseshoe crab has found a niche walking on the bottom of shallow costal waters, feeding on items that range from algae to small invertebrates

Ex: Horseshoe crabs inhabit coastal regions of eastern North America and the Indo-Pacific

Types of Interactions among Community Members

1- Competition2- Predation and Parasitism

3- Mutualism4- Commensalism

1- CompetitionIt is the battle of two or more species for vital resources when their niches overlap to some degree in a community Competition does not last long in nature because in the competition for vital resources the result is that the losing species is driven to local extinction.Competitive Exclusion Principle:When two species compete for the same limited, vital resource, one will always outcompete the other and thus bring about the latter’s local extinction. Nevertheless, competitive exclusion has been observed in nature, often when humans have a hand in things.

The kudzu plant was introduced in the American South in the 1930s and has since spread at a rapid rate, locally eliminating many plants in its path.

There are numerous instances in nature in which two related species use the same kinds of resources from the same habitat over an extended period of time but will divide the resources up such that neither of the species undergoes local extinction. This phenomenon is called coexistence through resource partitioning.Coexistence: Sharing of habitat.Resource Partitioning: It is the dividing up of scarce resources among species that have similar requirements.

Cape Maywarbler

Bay-breastedwarbler

Myrtlewarbler

Feeding patterns of different species of warbles. All of them ate caterpillars, but from substantially different parts of the trees

When two species compete for the same limited, vital resource, one will always drive the other to localextinction—as the paramecium P. aurelia did to the parameciumP. caudatum. This is the competitive exclusion principle at work.

Competitive Exclusion

Resource PartitioningConversely, when Gause put P. aurelia together with anotherparamecium, P. bursaria, thetwo species divided up the habitat, and both survived. This is a demonstration of resource partitioning.

2- Predation and ParasitismIt is when one organism feeds on parts or all of a second organism (predator eats the pray)

The pray can be other animals, plants, protists; and the definition includes animals consuming whole plants or their seeds

Parasitism:It is a variety of predation in which the predator feeds on pray but does not kill it immediately, and may not ever kill it.The prey of a parasite is known as the host.

A parasite can use a host not only as a food source but as a vehicle to facilitate its reproduction.

3- MutualismIt is an interaction between individuals of two species that is beneficial for both individuals

In this relationship, plants supply fungi with food produced in photosynthesis, while the fungi supply plants with minerals and water.

Up to 90 percent of seed plants live in a cooperative association with fungi.

4- CommensalismIt is an interaction between individuals of two species in which one benefits while the other is neither harmed nor helped.

The cattle egret, Bubulcus ibis, is a classic example of commensalism. It forages in fields among cattle and horses, feeding on insects stirred up by the grazing animals. The egret benefits from this relationship, while the livestock are typically unaffected by it.