Topic 27 -Community Dynamics Flashcards
succession
change in community structure (at a specific location) through time
change in species composition and population dynamics
b and d of species change in response to environmental conditions
leads to shifting patterns of species dominance and diversity
succession involves a pattern of species…
colonization and extirpation from a specific region -with species displacing other species from a community over time
early successional species (“pioneer” or “colonizing”) are the
first to colonize an area
r-selected life history traits -high growth rate, smaller size, short-lived, high rates of population growth, high degree of dispersal (colonization)
can often survive under extreme abiotic conditions but are poor competitors
Late successional species (“climax” species) replace
early successional species in a community over time
k-selected life history traits -slower growth rate, larger size, long-lived, low rates of population growth, low degree of dispersal (colonization)
generally better competitors (outcompete early species, extirpation) but could not colonize because they cannot survive under extreme abiotic conditions
our understanding of succession is mainly based on
plant communities
Primary succession
process occurs at a site not previously occupied by a community ex. rock cliffs, sand dunes
Secondary succession
process occurs on a site previously occupied by a community after a disturbance
disturbance
any process that results in the removal (either partial or complete) of the existing community ex natural fire, windstorm, flood, cold, drought etc
Disturbance event
relatively discrete event that disrupts community structure and function
Disturbance regime
pattern of disturbance events that characterize a landscape over a longer period of time
Disturbance intensity
proportion of a species population that the disturbance kills or eliminates
Disturbance scale
spatial extent of the impact of the disturbance relative to the size of the affected landscape
disturbance frequency
mean number of disturbances that occur within a particular time interval
small-scale disturbance creates
a gap -provides access to physical space for colonization by new individuals (often enhances diversity by increasing environmental heterogeneity)
ex wind storm leading to death of tree
Large-scale disturbance reduces or eliminates local populations which
significantly modifies physical environment leading to a secondary succession ex fire
succession is driven by the processes of species colonization and extirpation, explain
colonization increases species richness
extirpation decreases species richness
diversity peaks during middle stages -after arrival of later species but before replacement of early species
without disturbances
later species will replace early species -diversity declines
with high disturbances
succession is kept at early stages -low diversity
with intermediate disturbances
colonization is maintained and competitive replacement (extirpation) is minimal
intermediate diversity hypothesis
pattern of high diversity at intermediate frequencies of disturbance
Human disturbances have
a more profound and long lasting impact on communities than natural disturbances
human activities involve
continuous management of communities
completely remove natural communities and replace with cropland, pastures, paved highways/cities etc.
Traditional view of communities
communities are in a state of equilibrium or a state of balance (climax state) -unless seriously disturbed
reality of communities
most communities often have a certain level of disturbance occurring normally all the time
current view of communities
communities are generally in a state of recovery from disturbance (not usually in climax state)
