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Ecological Successionsuccession Ecologicalecological Successionsuccession AgriculturalAgricultural EcologyEcology ByBy Dr.Dr. RensenRensen ZengZeng DepartmentDepartment ofof EcologyEcology CollegeCollege ofof AgricultureAgriculture South China Agricultural University Email: [email protected] ChapterChapter 1616 EcologicalEcological successionsuccession EcologicalEcological successionsuccession • Ecological succession is defined as a continuous, unidirectional, sequential change in the species composition of a natural community • This sequence of community is termed as a sere （演替系列）, and culminates in the climax community （顶极群落） LevelsLevels ofof bioticbiotic organizationorganization – Individual – Population – Ecosystem • Concept of Community • Ecosystem concept AutogenicAutogenic ((自生）自生） successionsuccession • Is self-driven, resulting from the interaction between organisms and their environment. • Primary succession occurs on a newly formed substrate such as glacial till (冰川 沉积物）. • • Secondary succession follows disturbance, for example by flooding, fire or human activity VegetationVegetation DynamicsDynamics && PlantPlant SuccessionSuccession • No vegetation is static with time. There is always changes happening in vegetation and the subject that studies changes in vegetation is called vegetation dynamics. • Plant succession: Directional cumulative changes in species composition through time. • Time: 5 – 500 years • Seasonal changes in species composition a year is not considered as succession. 原生演替原生演替 Hydrarc succession: Beginning in wet (hydric) site. eg. pond, lake, bog. Xerarc succession: Beginning in dry (Xeric) site eg. rock surface（旱生演替） TwoTwo aspectsaspects ofof successionsuccession i) Sequence of plant, animal and microbial communities that occupy an area over time ii) The processes by which this biotic communities replace each other and by which the physical environment becomes altered over time. In the later sense the product of succession is called sere, the characteristic sequence of biotic communities that successively occupy and replace each other in an environment over time. The various communities that together make up the sere is called seral stages BasisBasis ofof classificationclassification ofof successionsuccession Initial condition (living propagules): Primary succession（原生演替）: when it begins at a place devoid of plant propagules and microbs. eg. succession after volcanic eruption, landslide, glacier retreats. Secondary succession: （次生演替）begins at a place that already has plant propagules and microbs. eg. succession after fire, logging, insect epidemic. InitialInitial conditioncondition (habitat(habitat moisture)moisture) Xerarc succession: Beginning in dry (Xeric) site eg. rock surface（旱生演替） Hydrarc succession: Beginning in wet (hydric) site. eg. pond, lake, bog. Mesarc succession: Beginning in moist (mesic) site. eg. forest, old fields. The resulting seres called Xerosere, hydrosere and mesosere. Ecological Convergence ConceptConcept ofof ClimaxClimax A self replacing seral stage that is relatively stable to other seral stage. eg if a dominant sp shows an inverted J curve the it is in a climax stage as opposed to a bell curve where the species is not self replacing and may be at a seral stage. ComplexityComplexity ofof ClimaxClimax CommunityCommunity Progressive: Results into climax community with high structural and functional diversity. eg. Succession after a natural fire in black spruce forest near Thunder Bay. Retrogressive: Results into climax community with lower and structural and functional diversity. eg. Succession after clearcutting in black spruce forest with dense Kalmia understory in Newfoundland. DrivingDriving forceforce behindbehind successionsuccession Autogenic: Habitat biotic factor eg. replacement of existing community by the next results from the afterlife effects of the former that makes the habitat unsuitable to continue. Allogenic: Habitat abiotic factors such as soil parent material, pH, nutrients drives the direction of succession. eg. filling of a lake with sediment and the further modified by the colonizing plants. Biogenic: When a sudden interference with an autogenic and allogenic succession by a living organism that becomes a major agent of succession eg. herbivore, insect infestation etc. ProcessesProcesses inin SuccessionSuccession • Nudation • Colonization • Establishment • Reaction • Displacement • Facilitation • Recolonization MechanismMechanism ofof SuccessionSuccession • Colonization (vegetative, seeds) • Alteration of habitat (physical and chemical characteristics) • Displacement of species (competition, allelopathy, insects, animals) Mechanisms,Mechanisms, PathwaysPathways && ModelsModels • Mechanisms: Process or interaction that contributes to successional change eg. competition, predation, organic accumulation. • Pathways: Temporal pattern of ecosystem change ie. the sequence of plant community and habitat changes over time. • Models: Abstract conceptual representation to describe successional pathways & mechanisms eg. Word, pictorial, diagrammatic and computer based models ClassicalClassical andand modernmodern conceptsconcepts andand modelsmodels ofof successionsuccession Early references to succession: Theoprestus (300 BC): Pattern of development of bog vegetation. Thorea (1863): succession of hardwood stands replaced after logging pine in NE USA. Coined the term forest succession. Cowles and Clements: developed the formal concept of succession after studying sand dunes of lake Michigan. Clements (1916) Monoclimax theory of succession MonoclimaxMonoclimax TheoryTheory • Species composition and structure of the terminal community of all seres is determined by the regional macroclimate. • Plants of climax community can reproduce beneath their own shade and maintain the composition indefinitely. • The linear development towards climax can be arrested by SERECLIMAX: lichen a steep exposed rock, springfed bog SUBCLIMAX: regularly burned Calluna heathland DISCLIMAX: communities are held in a stable early stage eg. farmlands, subalpine meadows by burrowing animals. TwoTwo TenetsTenets ofof ClementsianClementsian ofof SuccessionSuccession (Clements 1916) 1) The community as superorganism 2) The climatic climax is a predictable endpoint He emphasized the biotic processes and de-emphasized the importance of abiotic factors that drive succession ReasonsReasons forfor PolyclimaxPolyclimax TheoryTheory (Tansley 1920, 1935 1939,1941) Many factors can intervene to prevent an area from reaching the climatic climax eg. 1. frequent natural fires maintain grassland or forest of mid-seral stage, 2. outcrops of serpentine soils rich in Mg or limestone soils rich in Ca 3. Areas of then soil and steep topography support stable communities that are different from those that are on flat valley-bottom. Climax vegetation of these sites are all floristically different from the regional climax TheThe PolyclimaxPolyclimax TheoryTheory Succession in a region does not lead towards a single climax but tward a mosaic of different climax communities determined by the mosaics of habitats Therefore in addition to climatic climax there may be: Pyral climax (Fire) Edaphic climax (Soil) Biotic climax (Animal influence) TheThe ClimaxClimax PatternPattern HypothesisHypothesis (Whittaker(Whittaker 1953)1953) Vegetation is a complex pattern of integrating communities rather than a mosaic of distinct communities. This hypothesis has not proven as popular and persistent as either the monoclimax or the polyclimax theory. ClimaxClimax RecentRecent ModelsModels ofof SuccessionSuccession • Recognition of the importance of individual species (Gleason 1926, 1927, 1929) • Importance of life history characteristics (Drury and Nesbitt 1973) • Plant strategies (CRS of Grime 1974, 1979) • Plant strategies and changing resource level (Resource ratio hypothesis of Tilman 1982, 1985, 1988,190a,b) • Three pathway model: Facilitation, Tolerance and Inhibition pathway (Connell and Slatyer 1977) VitalVital AttributesAttributes ModelModel ofof SuccessionSuccession (Noble and Slatyer 1977, 1979) 1. Mode of persistence (four classes) D–Species (dispersed by seeds) S–Species (stored seeds , seed bank) C–Species (canopy seeds) V–Species (vegetatively regenerating) 2. Conditions for establishment (three classes) T–Species (able to tolerate competition, establish any time) I–Species (intolerant of competition, establish at initial stage) R–species (Establish under other species after habitat change) 3. Timing of critical life history events (five classes) o- time when disturbance occurs p- time taken for propagules to arrive m- time taken for individuals to rich reproductive maturity l- life span, after which individuals are lost from the community e- time taken for species to be locally extinct. RatesRates ofof SuccessionalSuccessional ChangeChange • Climate and soil condition • Degree of environmental change during succession • Rate at which biota change the environment • Longevity of the dominant species • Degree to which the communities dominate the site resist invasion • Frequency and severity of disturbance. ChangesChanges inin EcosystemEcosystem FunctionFunction DuringDuring SuccessionSuccession • Ecological energetics • Biogeochemistry i) Geochemical cycle ii) Biogeochemical cycle iii) Biochemical or internal cycle ForestForest SeralSeral StagesStages 1. Stand initiation 2. Understory suppression 3. Stand differentiation 4. Understory reinitiation These correspond to: 1. Regeneration stage 2. Canopy closure stage 3. Self-thinning stage 4. Reestablishment of understory in gaps
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