Ecological Succession

Plant succession

Succession• Disturbance of a community is usually followed

by recovery, called ecological succession.• The sequence of succession is driven by the

interactions among dispersal, ecological tolerances, and competitive ability.

– Primary succession-the sequence of species on newly exposed landforms that have not previously been influenced by a community, e.g., areas exposed by glacial retreat.

– Secondary succession occurs in cases which vegetation of an area has been partially or completely removed, but where soil, seeds, and spores remain.

• Early in succession, species are generally excellent dispersers and good at tolerating harsh environments, but not the best interspecific competitors.

• As ecological succession progresses, they are replaced with species which are superior competitors, (but not as good at dispersing and more specialized to deal with the microenvironments created by other species likely to be present with them).

• Early species modify their environment in such a way as to make it possible for the next round of species. These, in turn, make their own replacement by superior competitors possible.

A climax community is a more or less permanent and final stage of a particular succession, often characteristic of a restricted area.

Climax communities are characterized by slow rates of change, compared with more dynamic, earlier stages.

They are dominated by species tolerant of competition for resources.

An Influential ecologist named F.E. Clements argued that communities work like an integrated machine. These “closed” communities had a predictable composition.

According to Clements, there was only one true climax in any given climatic region, which was the endpoint of all successions.

Other influential ecologists, including Gleason, hypothesized that random events determined the composition of communities.

He recognized that a single climatic area could contain a variety of specific climax types.

• Evidence suggests that for many habitats, Gleason was right, many habitats never return to their original state after being disturbed beyond a certain point.– For example; very severe forest fires have

reduced spruce woodlands to a terrain of rocks, shrubs and forbs.

• An incredibly rapid glacial retreat is occurring in Glacier Bay, Alaska. In just 200 years, a glacier that once filled the entire bay has retreated over 100km, exposing new landforms to primary succession.– Clements would have predicted that succession today would follow

the sequence of ecological succession that has occurred in the past for other parts of Alaska.

– In fact, three different successional patterns seem to be occurring at once, depending upon local conditions. Thus, Clements’ view of succession is somewhat of an oversimplification.

• Trophic structure is the hierarchy of feeding. It describes who eats whom

• (a trophic interaction is a transfer of energy: i.e., eating, decomposing, obtaining energy via photosynthesis).

• For every community, a diagram of trophic interactions called a food web.

• Energy flows from the bottom to the top.

A Simple Food Web Killer Whales SharksHarbor Seals Yellowfin Tuna

Mackerel Cod Halibut

Zooplankton

Unicellular Algae and Diatoms

One path through a food web is a food chain.

Killer Whales

Harbor Seals

Mackerel

Zooplankton

Phytoplankton

• The niche concept is very important in community ecology.

• A niche is an organism’s habitat and its way of making a living.

• An organism’s niche is reflected by its place in a food web: i.e, what it eats, what it competes with, what eats it.

• Each organism has the potential to create niches for others.

• Keystone species are disproportionately important in communities.

• Generally, keystone species act to maintain species diversity.

• The extinction of a keystone species eliminates the niches of many other species.

• Frequently, a keystone species modifies the environment in such a way that other organisms are able to live, in other cases, the keystone species is a predator that maintains diversity at a certain trophic level.

Examples of Keystone Species

• California Sea Otters: This species preys upon sea urchins, allowing kelp forests to become established.

• Pisaster Starfish: Grazing by Pisaster prevents the establishment of dense mussel beds, allowing other species to colonize rocks on the pacific coast

• “Mangrove” trees: Actually, many species of trees are called mangrove trees. Their seeds disperse in salt water. They take root and form a dense forest in saltwater shallows, allowing other species to thrive