Communities

1. Communities Definition: Community- a collection of species living in the same area that may interact.

2. 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.

3. 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.

4. 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?

5. 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?

6. Community Interactions • Competition- negative interactions between species (interspecific competition) or within species (intraspecific competition). Example: competition for food and living space. • Predation- organisms that feed on other organisms. Herbivores feed on plants, carnivores feed on animals, and detritivores feed on the remains of organisms. • Symbiosis- interactions between organisms that are beneficial. Example: mutualism occurs when both interacting organisms benefit. • Allelopathy- the occurrence of one species can “poison” a habitat for other species.

7. 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:

8. 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.

9. Definitions Seral 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).

10. 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.

11. 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.

12. 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).

13. 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.