Download lecture 25 ch 22 community development

LECTURE 20 CH 22 COMMUNITY DEVELOPMENT
Long-term changes due to climate change
During glacial advances and retreats
Changes in location of communities (or biomes) and species composition
Biomes did not move intact; individualistic view
Differences in migration rates and location among species
Expect similar changes due to future climate change
Succession: Changes on scale of 1-500 yrs due to disturbances and changes in land forms
Directional replacement of species
Theoretically ending in climax community that is self-replacing
Sere = all stages of successional change
Disturbance initiates succession; occur in all communities and vary in:
Examples of disturbance; they differ in
Size (scale)
Frequency/predictability
Intensity
Level of environmental heterogeneity
Remnant biological neighborhood and seed sources
Initial stages depend on intensity and extent of disturbance
Types of succession
Primary
On new bare substrate; no soil
No seed bank; seeds dispersed from outside
Extreme abiotic conditions
Very slow
Secondary
After disturbance to existing vegetation
Soil remains; seed bank may remain
. ‘old field’ succession
Micro-succession and ‘gap dynamics’ in forests
Forest as shifting mosaic of different successional ages
How long does succession last? (When) is climax community reached?
Disturbance at any stage can set back to earlier seral stage
Periodic disturbance can keep at subclimax stage
Predictable patterns in succession
Communities are most diverse, complex, and productive at intermediate stages.
Ratio of biomass to productivity increases.
Net accumulation of biomass slows and stops.
Greater proportion of nutrients resides in organic matter than in soil.
Animal communities follow stages of plant succession
How study succession?
Direct – can do for early stages
Indirect for longer periods
Chronosequence – infer change through time by comparing plots of different age
Tree cores
Life history differences influence species place in sere
r-selected pioneer species in early succession
many, small seeds with high dispersal ability
long seed dormancy
rapid growth and small mature size
low shade tolerance
K-selected species in late succession: opposite to above
Tradeoff between colonizing vs. competitive ability
Mechanims governing succession
Facilitation – (drives ‘relay floristics’); common in primary succession
Inhibition – can lead to ‘arrested succession’
‘Jump-starting’ succession in restoration ecology
Tolerance – (explains ‘initial floristics’); passive
Changing views of succession
Clements: Succession as part of superorganism concept
Succession as growth, development, maturation, and reproduction
Directional/ predictable
Monolithic theory of climax: all seres lead to same climax
Climatic climax driven by common climate and topography
Acknowledged may be some edaphic climaxes
Rejection of above
Variation in end point depending on variation in area of disturbance and in
conditions in early succession
Alternative pathways and alternative stable states
Open continuum of endpoint within region
Revision of older classical views and current questions
Acknowledge incredible complexity and system-specificity
Are there variable or predictable end points?
How frequent are communities disturbed?
How often do stable climax communities develop?
What is role of chance and non-equilibrium forces in controlling direction
and makeup of community?
A call for pluralism: which model best fits observations and at which time?