Section 20-2 + 3 Patterns & Succession

1. Section 20-2 + 3 Patterns & Succession

2. Species Richness Species richness refers to the number of different species found within a community. Species richness varies with latitude being the greatest near the equator and decreases towards the poles. This is based on the fact that there is a more stable climate for food production and fewer instances of disturbances over time in the tropics.

3. Species Evenness Species Evenness is the relative number of members within each population for a species. It provides information on the abundance and stability of the species within a particular ecosystem. Generally species evenness is inversely related to richness; it is greater at the poles and decreases towards the equator.

4. Habitat Size vs. Richness Another pattern that relates to species richness is available space. The species-area effect shows that richness decreases when habitats are restricted in size like on islands or from human encroachment. Many tertiary consumers require larger habitats that when fragmented reduce species diversity.

5. Species Interactions vs. Richness Predator-prey relationships influence both populations. In a healthy food web there are multiple prey species for each predator which allows more predators to co-exist within a community. Predators improve the overall health of the prey species by preventing overpopulation that could result in the exclusion of other prey species from the area.

6. Disturbances & Richness Periodic disturbances occur in every community and test the resilience of species. Maladapted species will disappear as a result leading to a decrease in richness. Some species require disturbances to thrive and reproduce and can move into new areas thus increasing richness.

7. Stability & Richness Stability is like homeostasis at the community level. The maintenance of constant conditions favors both an increase in richness and evenness. The longer conditions stay the same, the more adapted populations become and the more population growth can occur. Recovery from disturbance also increases with higher richness.

8. Ecological Succession Ecological succession is the process of a change in species and conditions that occurs over time. It occurs more rapidly after a disturbance when it is necessary to rebuild ecosystems. The process takes centuries to occur to create or restore an ecosystem.

9. Primary Succession When succession occurs in an area that was not previously inhabited then it undergoes primary succession. Examples include on bare rock, sand dunes, or a volcanic island. The initial stages take a long time because there is no soil present and must be created by the erosion of larger rocks.

10. Primary Succession The first organisms that move into the area are called pioneer species and serve the purpose of creating soil. Examples include autotrophic bacteria, lichens (fungi + algae), and/or moss plants. Pioneer species are r-selected. They are small, grow quickly and reproduce quickly to begin the succession process.

11. Secondary Succession Secondary succession occurs when a previously colonized area undergoes a disturbance and starts to rebuild. Examples include abandoned fields, unused dirt roads and a forest after a fire where soil is still present. Pioneer species in this case tend to be annual plants that then get out-competed and replaced by other plants eventually.

12. Secondary Succession The pioneer species are replaced by transitional species such as perennial grasses and flowers. After several decades a climax community develops that includes shrubs, conifer treesand deciduous trees. Other organisms such as invertebrate and vertebrate animals move in as food chains develop during the succession process.