Ch19 Ecological Succession and Community Development1.pdf

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University of Toronto St. George
University College Courses
Spencer Barrett

BIO120H © Lis| Page 111 chapter 19: ECOLOGICAL SUCCESSION AND COMMUNITY DEVELOPMENT  Krakatau island (Indonesia) exploded on Aug. 27, 1883  all life was obliterated  it was a great chance for scientists to study the development of biological communities on a newly formed, raw terrain of volcanic ash  nearest sources of colonists were from Sumatra and Java (40 km away)  sea-dispersed plants were first to show up (made up 10 out of 24 species by 1886)  wind-dispersed grasses and ferns – seeds and spores could be blown across ocean  wind-dispersed tree species arrived; closed forest developed across the island by 1920s  pioneering species were pushed to marginal habitats or disappeared  birds and bats were attracted to the trees (fruit-eating species)  brought seeds of animal-dispersed trees and shrubs  animal-dispersed outnumber sea- and wind-dispersed plants now  island fragments are constantly changing due to continuing eruptions  island will continue to change  communities exist in a state of continuous flux, but indiv. are replaced by their own kind  disturbance: an event that causes rapid or marked change in a population or community, often thought of as displacing an ecological system from its equilibrium  after a habitat is disturbed, pioneering species adapted to disturbed habitats are successively replaced by other species as the community attains its former structure and composition  species successively replace one another in the process of succession  succession: a sequence of changes in the species composition of a community initiated by a disturbance, progresses to a stable state  climax community: the ultimate association of species achieved after succession  the end point of a successional sequence, or sere; a community that has reached a steady state under a particular set of environmental conditions  Frederic E. Clements THE CONCEPT OF THE SERE INCLUDES ALL THE STAGES OF SUCCESSIONAL CHANGE  any new habitat attracts colonizers (adapted to be good pioneers)  rates of succession slow as slower-growing plants appear  transition from abandoned field to mature forest takes only one of several successional sequences that may lead to similar climax communities w/in a given biome  sere: a series of stages of community change in a successional sequence leading toward a stable state  although initial stages of the sere depend on the beginning habitat, with time, seres converge on similar climax communities  seres have been classified into 2 types according to origin Primary Succession  primary succession: succession in a newly formed or exposed habitat devoid of life  ex. natural conversion of ponds in north-temperate and boreal climates to dry lands  deep ponds (kettle holes) left by glaciers and those formed by beaver dams undergo bog succession  rooted aquatic plants establish at the edge of the pond  some species of sedges form mats on the water surface and eventually cover pond BIO120H © Li| Page 2011  produces a firm layer of vege.  “quaking bog”  detritus produced by sedge mat accumulates at pond bottom (anaerobic, thus no microbial decomposition occurs down there)  as a bog accumulates sediments and detritus, sphagnum moss and shrubs establish along edges  add to development of a soil  black spruce and larch  birch, maple, fir  over thousands of years, aquatic habitat becomes terrestrial as organic detritus accumulates and soil rises above water table Secondary Succession  secondary succession: succession in a habitat that has been disturbed, but in which some aspects of the community remain  even when reseeding initiates a secondary sere, the type of disturbance and size of gap it creates influences the first species to establish  disturbance and recovery occur quickly in marine habitats  marine invertebrates have inversely related colonizing and competitive abilities  created bare patches  on rock occupied by sessile invertebrates and;  filled in quickly by tunicates and sponges (highly successful competitors)  gap size didn’t influence succession  bryozoans and polychaete larvae attempted to colonize but were overgrown  on artificial hard surfaces, ex. ceramic tiles, placed in sand away from colonists  gap size had huge effect on pattern of colonization  tunicates and sponges don’t disperse well, thus, didn’t Michael Keough colonize them (influence of gap  bryozoans and polychaetes had chance size/isolation on  larger gaps = larger targets – colonized by T & S succession in  smaller gaps – colonized by B & P (once established, can marine habitats) prevent the colonization of T & S)  B & P are disturbance-adapted species ecologists in the field  weed a.k.a. ruderals: an organism, generally having strong powers of dispersal, that is capable of living in highly disturbed habitats  colonize open patches quickly  mature and produce offspring at an earlier age  eliminated by slowly colonizing superior competitors  require frequent disturbances to stay in sys.  size of gap also influences whether predators & herbivores will be active there  some feed in large gaps – easy to find  others are vulnerable to predation and need the cover of intact habitat, from whose edges they venture to feed – graze small gaps and gap edges than the centre of large gaps BIO120H © Li| Page 3011 The Climax Community  succession leads to the climax community (ultimate expression of community dvlpmt)  each sere develop under a particular set of envmtal conditions, but all progress toward similar climax states  Clements believed that climate alone determined the nature of the local climax and that diff. climax states were discrete, recognizable, and separate from one another (closed community)  communities are more commonly viewed as open sys. today  alternative “climax” communities are possible, resulting from factors ex. intensity of a disturbance, size of the gap it produces, physical conditions during early succession  continuum index: the scale of an environmental gradient based on changes in physical characteristics or community composition along that gradient SUCCESSION ENSUES AS COLONISTS ALTER ENVIRONMENTAL CONDITIONS  a species’ presence in a sere is determined by: 1. how it readily invades a newly formed/ disturbed habitat 2. how it responds to changes that occur in the envmt over the course of succession  those that disperse and grow rapidly dominate the early stages of a sere  they sometimes modify envmts that allow later-stage species to become established o ex. herbs shad soil to help it retain moisture  others inhibit others into a sere by competing more effectively or by direct interfrnc.  Joseph Connell and R.R. O. Slatyerlassified the processes governing the course of succession into 3 categories: facilitation, inhibition, tolerance  describes the effect of one species on the prob. of a second becoming established and whether that effect is +ve, -ve, or neutral  facilitation: a process by which one species increases the probability of another species becoming established, particularly during early succession  ex. alder trees harbour N-fixing bacteria in roots which provide soil w/ N, facilitates N-limited plants i.e. spruce, which eventually replace them  T. A. Dean and LL. E. Hurd(fac
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