Communities

Definition:Definition:

Community- a Community- a collection of collection of species living species living in the same in the same area that may area that may interact.interact.

The Ecological Niche• Niche- the range of conditions (ecological amplitude) tolerated (or preferred)

by species within a community; a hypervolume (like a graph with more than three dimensions).

• By graphing the range of conditions over which a species occurs, we gain an understanding of the species’ niche.

• The graph below shows the Blue-gray Gnatcatcher with respect to two dimensions: populations and forest type; populations are greatest in deciduous forests and are absent from conifer forests.

Blue-gray Gnatcatcher

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1 1.5 2 2.5

Forest Type

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sity

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1 = deciduous 2 = mixed 3 = conifer

Communities and Niches

• Within communities, species have different niches (they use habitats differently).

• Example: the Red-eyed Vireo (above) lives in interior forests, and the Warbling Vireo (below) uses forest edges.

• By having different niches, species subdivide the habitat among them.

• Subdivision can occur over living space or food.

Community Relationships-

Considering Two Dimensions

• Species and communities are influenced by a variety of habitat factors.

• In the adjacent graphs, species richness in a bird community is related to a measure of local temperature.

• What is the relationship of temperature (larger values are associated with lower local temperatures) to species richness in summer vs. winter?

Community Relationships- Considering Three or More Dimensions

• Species within communities are influenced by more than one factor simultaneously, and three-dimensional graphs illustrate this.

• In the adjacent graph, bird species richness is influenced not only to season but also geographic region (northeastern vs. southeastern Connecticut).

• How does the relationship of species richness to season change between regions?

Community InteractionsCommunity Interactions

• CompetitionCompetition- negative interactions between species - negative interactions between species (interspecific competition) or within species (intraspecific (interspecific competition) or within species (intraspecific competition). Example: competition for food and living competition). Example: competition for food and living space.space.

• PredationPredation- organisms that feed on other organisms. - organisms that feed on other organisms. HerbivoresHerbivores feed on plants, feed on plants, carnivorescarnivores feed on animals, and feed on animals, and detritivoresdetritivores feed on the remains of organisms. feed on the remains of organisms.

• SymbiosisSymbiosis- interactions between organisms that are - interactions between organisms that are beneficial. Example: beneficial. Example: mutualismmutualism occurs when both occurs when both interacting organisms benefit.interacting organisms benefit.

• AllelopathyAllelopathy- the occurrence of one species can “poison” a - the occurrence of one species can “poison” a habitat for other species.habitat for other species.

Environmental Gradients• As habitat conditions change, the species present change (species with

different ecological niches appear) , leading to the development of different communities.

• Example: a forested slope in southern New England:

Community Development

• The process of community development over time is called succession. There are two types.

• Primary succession- the developmental sequence that follows initial colonization of mineral substrates by living organisms.

• Secondary succession- the sequence that occurs following disturbance of a pre-existing community.

DefinitionsSeral stage- a recognizable stage in a successional sequence.Climax community- that stage at which a community reaches

a steady state (above left).Types of primary succession:

1. Xerarch- succession in uplands (above right- the dark gray soil was deposited during an ancient glaciation and the lighter soil above it was deposited after the most recent glaciation, about 17,000 years ago).2. Hydrarch- succession in wetlands (below right- in bog).

Primary Succession• After glacial times, the landscape of

southern New England was colonized first by, small, primitive plant-like forms.

• These forms changed their environment, and were then succeeded by herbaceous plants and trees over a period of roughly 10,000 years (below).

• Some peaks in the Berkshire Mts. (right) have been in early to mid-succession for thousands of years.

Secondary Succession• After the early 1800s, much farmland in

southern New England was abandoned.• Bare mineral soil was first colonized by

annual weeds, which were then replaced by perennial herbs within 1-2 years.

• After 5-10 years, woody plants invade the meadow, thereby becoming an old field.

• After about 25 years, the canopy of the old field closes into a young forest.

• After 50 years, the forest matures, and by 150 years an old growth forest develops.

Factors Influencing Succession

• Disturbance, such as through animal activity, fire, agriculture, logging.

• Previous land use (e.g. land once used as a pasture will develop differently than land used as a cornfield). Note the wide growth form of the tree at left that began growing when the area was a pasture.

• The nature of surrounding habitats (e.g. an adjacent forest provides seeds that can germinate in an old field).

Setting Back Succession

• Before (left)- the volcano of Anatahan Island as it was in 1990. Note that vegetation is present to the bottom of the crater.

• After (above)- a massive eruption in 2004 eliminated all vegetation, bringing the island back to a bare mineral state in which primary succession was again initiated.