Bio Chapter 19.doc

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22 Apr 2012
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Chapter 19 – Ecological Succession and Community Development
(392-399, A; 400-402, C; 403-408, A/B)
While communities exist in a continuous state of flux – species die and are replaced by new ones –
communities seem not to change appreciably over time. It is only when a community is disturbed (e.g. by volcano or fire) that
considerable changes can be observed. To begin, pioneering species adapted to disturbed habitats will move in, they are then replaced
by other species and eventually the community regains its former structure. Succession: the sequence of changes in community
composition initiated by disturbance. Climax community: the “ultimate” or “final” association of species or community composition
attained after disturbance.
Pioneering species –which are attracted to a disturbed habitat because of their ability to withstand such conditions – will modify a
habitat. Plants may affect shade level, contribute to detritus in soil, alter moisture content, etc. These modifications may make the
habitat unsuitable for the pioneering species, but make it more suitable for species that follow. Sere: a successive sequence which
occurs after disturbance. Sequences of seres and the course of community development depend on how a community starts off before
succession.
Primary succession: the establishment and development of communities in newly formed or recently disturbed habitats.
Secondary succession: follows rapidly if a disturbance leaves some organisms in place or proceeds after primary succession in very
severely disturbed habitats. It is the process of reestablishing a community’s structure by reseeding, etc. There are various stages
involved in secondary succession as different plant species have an easier time establishing themselves under different conditions
(some require specific moisture content, shade, etc.).
Climax community: the ultimate, final or resultant community structure or species association after a disturbance; communities are
essentially mosaics of various successive stages all progressing towards the climax.
Facilitation: process by which one species increases the probability that another species will become established. Inhibition: can
prevent progression towards a climax community; species inhibit the growth or establishment of other species. Priority effect:
observed from inhibition – the outcome of an interaction between two species will depend on which becomes established first, i.e. if
you “get there first”, you win. Tolerance: the ability of a plant to disperse and tolerate physical conditions as it established itself.
Succession follows regular stages of progression in terrestrial systems. Generally, pioneering species are well-adapted to colonize
uninhabited habitats; they have impressive dispersal ability (many small seeds which can actually remain dormant in forests for years
in seeds banks until some disturbance creates bare-soil conditions), they grow quickly.
Conversely, climax species grow and disperse more slowly, they have a high shade tolerance early in life (they have larger seeds
which allow them withstand low light conditions) and larger size than pioneering species when they reach maturity (they produce
elaborate root and stem structures for support). They are suited to prosper in conditions created by pioneering species.
As a community gets closer and closer to its climax, the rate of succession slows down and new species have a less dramatic effect on
things like soil moisture and light intensity. The time required to reach the climax depends on the initial quality of the habitat. While it
seems a well-defined target, climax is an elusive concept, particularly elusive due to human activities which keep communities in a
constant state of flux; while communities tends towards equilibrium, they are generally in a state of dynamic response to changing
conditions.
Under extreme conditions, such as frequent fire (in any habitat which is occasionally dry enough to produce fire and never wet enough
to accumulate a thick biomass of detritus is fire-prone), climax communities often tend towards species which are resistant to these
changes. Fire-resistant tress will often grow rapidly after a fire wipes out other competition (some of them even have seed-release
mechanisms only activated by fire). Some well-defined edges in community composition can be defined by fire (climatic climax vs.
fire climax in forest vs. prairie).
Grazing can also have a dramatic effect on community composure as some plants are tolerant to it while others are not. The movement
of different animals creates its own succession of plants, which vary at different times by which plants are preferred or avoided by
which animals.
Transient climaxes: extreme seasonal changes can destroy established communities (small ponds which dry up in summer and freeze
in winter). Cyclic climax: when one species relies on conditions created by a previous species (A requires conditions from B required
conditions from C creates conditions for A); generally driven by harsh environmental conditions and can be quite regular.
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