Download BIO 212 Environmental Biology Fall LECTURE 5 ECOLOGICAL

BIO 212
Environmental Biology
LECTURE 5
ECOLOGICAL SUCCESSION
Fall
I. GENERAL CONCEPTS ABOUT SUCCESSION
A. Definition-Succession is the non-seasonal, directional and continuous pattern of colonization and
extinction on a site by species populations.
B. Primary vs. Secondary succession
1. Primary succession: the establishment of vegetation on land not previously occupied by vegetation
and where no previous soil development has occurred.
2. Secondary succession: the establishment of vegetation on a site which has soil, but the previous
vegetation has been destroyed by some disturbance (fire, hurricane, agriculture).
C. Disturbance changes ecosystem process.
1. Impact of disturbance on hydrology and nutrients. Lesson is that the plants hold nutrients at a site;
a. Either as part of the standing biomass.
b. Or by preventing erosion or leaching. (Lessons of Hubbard Brook Experimental Forest)
c. Leaching is the downward movement of nutrients through the soil. Also includes sideways
movement as well.
2. Removal of vegetation at a site increases the temperature.
a. This can make it difficult for new species to colonize the site (seed germ. and seedlings survival).
b. Too hot and dry for the organic matter to decompose, thus slowing down the return of nutrients to the
soil.
D. Typical trends in succession.
1. Soils- Increase in soil depth, increase in development of soil horizons, increase in organic matter.
a. Nutrients in soil decrease. Pulse of nutrients at first (quickly from fire, slowly from decomposition),
followed by absorption by perennial plants over time.
2. Inorganic nutrients- the pool of inorganic nutrients in soil and within plants increases, with a greater
fraction of this pool held in the tissues of the plants. (Compare field to forest; young forest to old forest)
3. Vegetation sequence from annuals to biennials to perennial herbs, to woody plants.
a. The woody plants may proceed from shrubs to softwoods (e.g. pines) to hardwoods (e.g. oaks).
4. Species richness of plant community increases, then decreases.
a. Number of ‘old field species’ (annuals, biennials, grasses) decrease.
b. Total number of all species (plants, animals, fungi, insects…) increases to mid-successional
community, then declines.
c. This is the intermediate disturbance hypothesis.
d. The greatest number of species will be found in communities that are intermediate between
frequently disturbed and not disturbed at all.
5. Productivity of the vegetation increases- expressed as the rate of formation of organic matter per unit
area.
a. Productivity does slow down for very old communities.
6. Greater productivity, and greater size of plants leads to greater biomass present in the community.
Bigger trees.
II. EXAMPLE OF SUCCESSION
1. Seral stage- The sequence of communities, e.g., from grassland to shrubland to forest, is known as a
sere (from the Latin word series), and each of the changes (i.e. each different community) is called a
seral stage.
A. Old field succession. The classic example of secondary succession. Based on an old field in North
Carolina.
Year 1- after disturbance of the site, the area is invaded by annuals. Also present are the biennials.
Year 2- After the 2nd year, the annuals decline in size and numbers. Biennials are blooming and producing
seed.
Year 3 and beyond- Slowly, the annuals and biennials give way to the perennials.
a. Plant species richness is high during the first 2 years, then declines during the grass and perennial
stage.
b. After succession of herbs and grasses, abandoned farmland is invaded by shortleaf pine.
c. Pine seedlings are effective competitors for soil water and eventually shade herbs and grasses
2
d. Dominance of pines declines as hardwoods enter forest. After 50 years, oaks are important
understory species.
B. Succession does not necessarily have a definite starting point.
1. Large or small cleared patches may be quickly colonized by the sprouting of seeds that have been
dormant in the leaf litter for several years. This is known as a seed bank.
C. Concept of Climax- when does it end?
When succession comes to an end, it is called Climax: the cessation of directional change in presence and
abundance of species over time; a stable equilibrium will occur if individuals which die are replaced
on a one-to-one basis by young of the same species.
1. Replaceability, not persistence, is the key.
2. You’ll see that this model does not always exist in reality. A mature forest may have patches of
different aged trees.
3. It is more appropriate to think of a plant community that exhibits relatively little change as a mature
community.
III. PLANT STRATEGIES AND SUCCESSION
A. Early successional species lead a fugitive lifestyle. Their continued survival depends on dispersal to other
disturbed sites. They cannot persist in competition with late successional species.
1. Seeds of shade intolerant species are dispersed by wind, they have a greater chance of landing on a
recently disturbed site with plenty of light.
2. In contrast, seeds that are shade tolerant have larger seeds. They do not disperse as far, from the parent
plant. They can germinate and grow beneath the parent plant.
B. In general, animals follow the succession of plants at a site. However, animals can impact the composition of
future forests.
LIFE HISTORY TRAITS OF EARLY AND LATE SUCCESSIONAL PLANTS
Attribute
Seeds
Dispersal in time
Distance
Dispersal mechanism
Viability
Seed Germination enhanced by
Light
Seed number
Seed size
Early Successional
Late Successional
Well dispersed
Large
wind, birds, bats
Long
Poorly dispersed
Small
gravity, mammals
Short
Yes
Many
Small
No
Few
Large
Structure & Strategy
Shade tolerance
Mature size
Structural strength
Growth rate
Potential life span
Net Primary Production
Resource acquisition rate
Recovery from nutrient stress
Low
Small
low (herbaceous)
Rapid
Short
High
High
Fast
High
Large
High (woody)
Slow
Long
Low
Low
Slow
Ecosystem
Major site of nutrient storage
Role of detritus
Mineral cycles
Plant species diversity
Soil
Minor
Rapid, leaky
High
Plant Biomass
Important
Slow, tight
Low
IV. ANIMALS FOLLOW PLANTS
A. As the vegetation species change, so do the animal species.
Rev. 10/09