Fig 10.8 Graphs showing changes in biomass and diversity with succession.
Fig 10.9 (a) Hypothesized changes in soil nitrogen during the course of soil development. (b) Change in total soil phosphorus over time with soil development. (Source : P. M. Vitousek and P. S. White, 1981, “Process Studies in Forest Succession,” in D. C. West, H. H. Shugart, and D. B. Botkin, eds. Forest Succession: Concepts and Applications [New York: Springer-Verlag, 1981], Figure 17.1, p. 269.)
Fig 10.7 Diagram of bog succession. Open water (a) is transformed through formation of a floating mat of sedge and deposition of sediments (b) into wetland forest (c).
Succession Slides 2012
Population Changes in Context - in Communities!
Succession Defined:• The gradual, sequential change in the relative abundances of the dominant species in a biological community following a disturbance…• Primary succession: beginning from an abiotic environment following a cataclysmic disturbance• Secondary succession: beginning from a major disturbance, but all forms of life are not destroyed
Primary Succession (forest)• Colonization: of bare rock, tiny seedless plants like mosses, and lichens, “pioneer species”• Early: plants typically small with short lifecycles (annuals), rapid seed dispersal, “environmental stabilizers”• Middle: plants typically longer lived, slower seed dispersal (herbs, shurbs, perennials)• Late: plant species are those associated with older, more mature ecosystem-largest vegetation (trees)• “Climax Community” mature forest in this case (but varies by biome) *Note: Consumers and decomposer populations will also vary as producer populations change…as well as nutrient cycling…
Is a Climax Community Always Inevitable?• New research suggests that we cannot always project the course of a given succession or view it as preordained• Communities are always subject to disturbances and we cannot always know the outcome• Disturbances can be beneficial for communities…
The Intermediate Disturbance Hypothesis• Hypothesis: Communities that experience fairly frequent but moderate disturbances have the greatest species diversity• Reasoning: Moderate disturbances are large enough to create openings for colonizing species in disturbed areas, but mild and infrequent enough to allow the survival of some mature species in undisturbed areas
Ecotones• Disturbances often create ecotones, but they also exist as natural transitions between biomes or ecosystems• An ecotone is a transition area between two adjacent ecological communities – a sharp boundary or a gradual blending effect – particularly significant for mobile animals, as they can exploit more than one set of habitats within a short distance – this can produce an edge effect along the boundary line, with the area possibly displaying a greater than usual diversity of species
Edge Effects• Disturbances can fragment ecosystems and create edge effects• An edge effect describes the differing abiotic and biotic conditions that exist at a border between contrasting environments in an ecosystem – the increased light, greater wind and temperature extremes and lower humidity at the boundaries of fragments favor some plant species over others (native colonizing species or invasive species) – this can make the combination of species present near the boundary different from that inside the fragment (more diverse or less depending on the factors)
The Island Theory of Biogeography• Small islands have fewer habitat types and greater risk of extinction-smaller islands have less genetic diversity too• The more distant the island, the less chance of reaching it• Over time, an island tends to maintain a constant number of species, because the rate at which species are added is about the same as the rate at which other species become extinct
The Island Theory of Biogeography• Species often evolve to a smaller size on islands because islands often have a limited supply of food, fewer predators, and fewer species competing for the same resources• Examples include island foxes, pygmy mammoths, and a dwarf human species Homo floresiensis
The Island Theory of Biogeography• Island concepts can also be applied to “ecological islands”• Ecological islands are comparatively small habitats separated from a major habitat of the same kind – A small stand of trees within a prairie is a forest island