Topic 5 Community Ecology

Interdependent communities

Communities in which species depend on each other to exist

Independent communities

Communities in which species do not depend on each other to exist

zonation

• zones in which each species flourishes

• reflects different tolerance ranges for temperature and moisture availability, as well as different abilities to compete with other species for resources

Ecotone

A boundary created by sharp changes in environmental conditions over a relatively short distance, accompanied by a major change in the composition of species

Keystone Species

A species that substantially affects the structure of communities despite the fact that individuals of the species might not be particularly numerous

Direct Effect

An interaction between two species that does not involve other species

Indirect Effect

An interaction between two species that involves one or more intermediate species

Trophic Cascade

Indirect effects in a community that are initiated by a predator

Density-Mediated Indirect Effect

An indirect effect caused by changes in the density of an intermediate species

Trait-Mediated Indirect Effect

An indirect effect caused by changes in the traits of an intermediate species

Bottom-Up Control

When the abundances of trophic groups in nature are determined by the amount of energy available from the producers in a community

Top-Down Control

When the abundance of trophic groups is determined by the existence of predators at the top of the food web

Succession

The process by which the species composition of a community changes over time

Climax Community

The final seral stage in the process of succession

Seral Stage

Each stage of community change during the process of succession

Pioneer Species

The earliest species to arrive at a site

Primary Succession

The development of communities in habitats that are initially devoid of plants and organic soil, such as sand dunes, lava flows, and bare rock

Secondary Succession

The development of communities in habitats that have been disturbed and include no plants but still contain an organic soil

Facilitation

A mechanism of succession in which the presence of one species increases the probability that a second species can become established

Inhibition

A mechanism of succession in which one species decreases the probability that a second species will become established

Priority Effect

When the arrival of one species at a site affects the subsequent colonization of other species

Tolerance

A mechanism of succession in which the probability that a species can become established depends on its dispersal ability and its ability to persist under the physical conditions of the environment

Community Stability

The ability of a community to maintain a particular species composition

Community Resistance

The amount that a community changes when acted upon by some disturbance, such as the addition or removal of a species

Community Resilience

The ability of a community to return to its original state after being disturbed

Alternative Stable State

When a community is disturbed so much that the species composition and relative abundance of populations in the community change, and the new community structure is resistant to further change

Landscape Ecology

The field of study that is focused on the spatial arrangement of habitats at different scales and how this influences individuals, populations, communities, and ecosystems

Legacy Effect

A long-lasting influence of historical processes on the current ecology of an area

Habitat Heterogeneity

the variability in physical and biological features within an area, including different vegetation types, soil types, moisture levels, and food sources

Alpha/Local Diversity

The number of species in a relatively small area of homogeneous habitat, such as a stream

Gamma/Regional Diversity

The number of species in all the habitats that comprise a large geographic area

Beta Diversity

The number of species that differ in occurrence between two habitats

Equilibrium Theory of Island Biogeography

A theory stating that the number of species on an island reflects a balance between the colonization of new species and the extinction of existing species

Fragmentation Effects

• The total amount of habitat decreases

• The number of habitat patches

  increases

• The average patch size decreases

• The amount of edge

  habitat increases

• Patch isolation increases

achieving an equilibrium number of species on an island

• As the number of species living on an island increases, the rate of colonization by new species from the regional pool declines

• At the same time, the rate of extinctions of species living on the island increases

• The equilibrium number of species, occurs where the two curves cross and the opposing processes are balanced

effect of island size on the equilibrium number of species

• smaller islands support smaller populations that are more prone to extinction

• smaller islands have steeper extinction curves

• smaller islands contain fewer species at equilibrium than larger islands

effects of island isolation on the equilibrium number of species

• Islands that are far from a source of new colonizing species experience lower rates of colonization than islands that are close

• far islands contain fewer species at equilibrium than near islands

the combined effects of island size and isolation on species richness

• small islands that are far from a continent should have the lowest number of species at equilibrium

• large islands that are close to a continent should have the highest number of species

experimental test of island biogeography theory

• Researchers in the Florida Keys constructed a scaffold frame around islands and covered the scaffold with tarps to act as a tent when fumigating the islands

• Fumigating the islands removed most of the arthropods

• Over the course of a year, researchers returned to determine how many

  species of arthropods had recolonized

• By the end of the experiment, the islands had nearly recovered their original number of species, as indicated by the dashed lines, with the near islands containing a higher number of species than the far islands

latitudinal trends in biodiversity

• The number of different species in a given area increases as you move closer to the equator

• The number of mammals and birds also increases as we move from east to west in North America

• trees and amphibians are more diverse in the moister eastern half of North America than in the drier, more mountainous western regions

Glacier Bay Succession

• Sites that had been exposed for 35 to 45 years had tall species of willow and alder shrubs and black cottonwood trees

• Sites older than 100 years contained Sitka spruce trees

• sites older than 160 years had hemlock in the understory

• By examining changes in the plant community at sites that had been exposed for different lengths of time, he was able to hypothesize how the forests of Alaska responded to the massive disturbance of an advancing and retreating glacier

Terrestrial Succession

• two types

  • primary & secondary

Lake Michigan Succession

• The oldest dunes were far away from the current shoreline, while the youngest dunes were close to the shoreline

• Younger dunes had scattered plants such as beachgrass and bluestem grasses that represented the earliest stages of succession

• Further away from the water, older dunes contained larger and more abundant plants that included herbs and several species of shrubs

• Beyond these plants, still older dunes had pine trees, while the oldest dunes had beech, oak, maple, and hemlock trees

Intertidal Succession

• succession can occur much more quickly, in part because the generation time of the dominant species is much shorter

• powerful waves that occur during storms commonly remove organisms that are attached to boulders

• the first species to arrive was a green alga known as sea lettuce

• Over the next year, sea lettuce came to dominate the rocky habitat and largely prevented a competing species of red alga from colonizing

• as the sea lettuce became more dominant, it attracted crabs that eat it, which cleared areas on the boulders for the less edible red algae to colonize

Lake Succession

• The classical explanation for succession in these habitats describes a gradual and steady accumulation of organic matter that eventually fills in the basin and converts it to a terrestrial habitat

• More recent studies have demonstrated that the process can occur in occasional large bursts when multiyear droughts allow vegetation to extend along the dry part of the basin

  • When water becomes abundant again, the extended vegetation floats on the water’s surface and grows in thickness

  • Multiple drought events allow the vegetation to expand; eventually it covers the water’s surface and fills in the basin