2. 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
8. 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…
24. 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…
25. 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
27. 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
28. 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)
30. 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
31. 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
32. 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
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).