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Transcript
Chapter 17 Ecosystem Change • LongLong-term changes in the physical environment • Changes in the genetic constitution of organisms Billings, W. D. 1938. The structure and development of old-field shortleaf pine stands and certain associated physical h i l properties ti off the th soil. il Ecological Monographs 8:437-499. • Changes in the types, numbers and groupings of organisms 1 Nothing in biology makes sense except in the light of evolution. Ecological Succession Theodosius Dobzhansky (1900-1975) Pioneer of the use of fruit flies in genetics experiments. i t In 1937 he published one of the major works of the modern evolutionary synthesis, the synthesis of evolutionary biology with genetics, entitled Genetics and the Origin of Species, which amongst other things defined evolution as "a change in the frequency of an allele within a gene pool". Succession Succession • Sequence of plant, animal, and microbial communities that successively occupy an area over a period of time. • Process of change by which biotic communities replace each other and by which the physical environment becomes altered over a period of time. 2 Sere Seral Stages • Product of succession. • Product of succession. • The characteristics sequence of biotic communities that successively y occupy py and replace each other in a particular environment over time after either disturbance of the original community or the formation of a new, previously uncolonized environment. • Various communities that together make up a sere. Primary Succession Example: plant seral stages Seral Stage Soil Modification Bare rock or soil Algal, lichen, moss community Grass, forb, fern community Small shrub community Large shrub community Small tree species community Large tree species community Final tree species community little or no soil create simple soil accumulate and deepen soil developing soil profile accumulating organic material deeper, more complex soils fully developed soil profile deep, stabilized soil profile Secondary Succession • Begins in environments that lack organic matter • Not yet altered by living organisms Dry (xeric) environments Moist (mesic) environments Very wet (hydric) environments • Begins in environment that has already been modified by living organisms Abandoned Fields Clearcuts Succession: xerarch mesarch hydrarch Seres: mesoseres hydroseres xeroseres 3 Why Change Occurs??? Why Change Occurs??? • Change in environment (physical, biotic) caused by physical processes external to the biotic community y • Change in environment (physical, biotic) caused by resident organisms => Allogenic Succession SEDIMENT => Autogenic Succession Changes in biota Lake Changes in biota Bog Bog Climax Communities Relatively stable communities, representing either the final or an indefinitely prolonged stage of a sere. Vegetation Dynamics Forest Successional Retrogression • So far, we have discussed forward development of the ecosystem toward a climax condition. • Retrogression: effects of disturbance changing the seral condition back to an earlier stage (fire destroying a climax community => pioneer successional community). Classical concepts & theories • Some researchers prefer to use term “Vegetation Dynamics” rather than succession,, (see ( pp. pp 466 Kimmins). ) 4 Classical concepts & theories 1863 - American philosopher and naturalist Thoreau Monoclimax theory • Composition and structure of species of the climax is determined by regional macroclimate. • Plants in climax reproduce successfully under their own shade. • coined term “forest succession” (even-aged hardwood stands replacing pine stands after logging in N.E. US) • Community is maintained indefinitely under prevailing climatic conditions. Ecological convergence principle • If succession is regulated mainly by climate, all biotic community (in all site types) within a climate region will converge in structure and species composition to one eventual climax community - the climatic climax Kimmins, Fig. 17.2 Polyclimax theory Many factors can intervene to prevent reaching the climatic climax condition. Pyral climax - frequent natural fires Biotic climax - grazing animals Edaphic climax - Serpentine soils (mg) and limestone soils (calcium) in areas of acid igneous rocks result in different local climaxes. What is a climax community? • Self-replacing, stable seral stage. • Insufficient allogenic and autogenic agents to promote change. • Persists over several generations. • If community is disturbed, replaced by individuals of the same species. 5 Approaching the Climax • Succession continues until the addition of new species and exclusion of established species no longer changes the environment: – at this point succession reaches climax: • the community growth form has come into equilibrium with its physical environment Continuum concept The Climax Pattern Hypothesis • Monoclimax and polyclimax theories assume discrete mutually exclusive plant communities. • But vegetation forms a continuum wherein species are distributed and replace each other independently along environmental gradients. The Climax Pattern Hypothesis • Vegetation is seen as a complex pattern of integrating communities and not a mosaic of distinct communities communities. • Conclusion: Highly unlikely that one theory of succession will apply equally well everywhere. 6