19: Disturbance and Succession

-Succession in a Community where the previous community or climax community has been removed. This can include human disturbance, or disturbance from fires, floods, severe storms, etc...
-Regeneration following a disturbance: remove vegetation, not the soil.

-Succession based on biotic modification of the environment.

-Changes in species composition associated with abiotic changes in the environment, such as global drying or warming trends that change an area's vegetation.

Nudation: exposure of the soil surface

Migration: of seeds, propagules, seed bank...

Germination

Competition

Reaction: autogenic effects of plants on the environment

Stabilization: results in a climax community

-In desert scrub communities, for example, creosote bush (Larrea tridentata) is a dominant species. Birds and rodents shelter in Larrea, leaving droppings and seeds of other species. When Opuntia cactus seeds germinate they grow well under Larrea. Larrea eventually seneces, exposing Opuntia. Opuntia falls over and eventually dies, leaving open space where Larrea colonizes best.

Toposequence: study communities of different ages that have had similar disturbances. Main problem is that humans can't control conditions at the establishment of the sequence. This may partially be solved by replicated observations within community age groups.

Chronosequence: follow a community from the initial disturbance through time. Here the main problem is that this takes a long time, longer than the lifetime of most ecologists!

-Facilitation: early species facilitate colonization of later species. Later species - stronger competitors for resources, exclude early species.
-Tolerance: first-come, first-served. All species can colonize regardless of other species present. Competition, longevity determine climax community.
-Inhibition: early species inhibit/ block colonization by later species. Random colonization order is important. Longevity drives succession.

-By removing early species and monitoring the rate of colonization of later species.

-They found a combination of factors affecting successional sequence. Alder (N-fixing plant) reduces the nutrient limitations on spruce and aiding its colonization. However, net effect of alder was inhibitory.  Alder has dense canopy, blocking out sunlight for spruce. Alder leaf litter inhibits spruce germination. Herbivores preferred alders over spruce.

-It showed primary succession.  Plant species rapidly increased early, then decreased over time.
-Soil depth, organic content, moisture, and nitrogen increased.

1. Communities are most diverse, complex, and productive at intermediate stages.
2. Ratio biomass / productivity increases. Biomass increases, productivity increases.
3. Net accumulation biomass slows, stops.
4. Greater proportion nutrients in organic matter than soil.

1.Initial exposed soil
2.Colonization: arrival of seeds, organisms
3.Competition
4.Reaction: effects of organisms on the environment
5.Stabilization: results in a climax community

-It showed a facilitation model: colonists modify habitat, help aid in the settlement of next species until climax community is achieved.

-It proposed that facilitated interactions between species are not that important.  Instead, he argued that stochastic events, who got there first, is more important in succession.

-Egler added that the longevity of species is determined by competition.

(1) Founder Controlled: would rely more on stochastic factors..which species colonized a place first.. and this would correlate with vagility
(2) Dominance Controlled: control would relate more to competitive ability

-Life histories

-Good colonizers, grow rapidly.

-Tolerate poor resources, dominate competition.

-They have larger seeds and seedlings.
-Seed mortality rate is inversely proportional to seed weight.  Lighter seeds have higher mortality than heavier seeds.

-Secondary succession.

-Viewed succession as uni-directional and predictable.
-Many seres in different local environments converge into the same climax community.
-End point (climax) is determined by topography and climate.

-Variation in size of disturbance and conditions in early succession can lead to alternative stable states.

-It was replaced by open continuum within a region, where climax communities represent a continuum of vegetation types.

-Accelerate arrival of seeds.