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Community dynamics

I. Disturbance - any factor that alters a landscape/community, these events may be small scale or large scale, may be commonplace (communities have evolved to deal with them) or catastrophic.

A. Weather - winds, precipitation, temperature

1. Small scale - a single severe winter freeze or ice storm might be enough to reduce the population of insectivorous birds, a late freeze in the spring might kill or prevent your mums from blooming that year.

2. Medium scale - Clumps of trees felled by a wind storm (called a windthrow). This is important because it allows sunlight to the forest floor so species other than the trees in question can grow (succession).

3. Large scale - May be a single atypical event (hurricane) or a major, long-term weather shift (ice age - glaciation).

B. Fire - may be catastrophic in some habitats, but may be a regular part of the seasonal cycle in others. In these communities plants and animals have evolved to survive and even depend on fire (prairie & savanna, some pine forests). The seeds of several types of pine will not germinate unless exposed to fire.

C. Flooding
- may undercut or remove sections of forest, strip topsoil
+ may deposit rich soil, produce bars for the development of bottomland forest.

D. Alien/introduced species - when a species invades or is introduced to an area it previously did not inhabit, it may (a) go extinct, or (b) increase in population size very rapidly. An alien species has not local predators, parasites or competitors to keep it in check (it may also bring new diseases/parasites with it which might adversely effect local species)

1. European Starling - has caused decreased pop of other cavity dwelling birds like woodpeckers.

2. Gypsy moths

3. American Chestnut blight - removed dominant tree from many forests in US

4. Kudzu

E. Human activities - deforestation for lumber, fuel, farmland, construction, etc. Pollution is a secondary effect of humans.

F. Other factors - earthquakes, mudslides, volcanic eruptions, asteroid strikes, etc.

II. Succession - temporal variation in community structure
A sequence of changes initiated by a disturbance - primarily this process is described by the plants, though in marine habitats, where stationary animals exist, they may be used to describe a succession. Community changes in succession include increases in species diversity and changes in species composition. Also- mobile animals also change with the succession of flora or stationary animals

A. Sere - the successional series of plants which describes succession in a given habitat
e.g. dune: hardy perennial grasses, followed by numerous annuals, as detritus is added to soil by perennials, followed by tough shrubs, then pines
e.g. FW march in Maine: annual weeds (ruderals), herbacious perennials, shrubs, pines, hardwoods
Animals also change with the alteration in plant types

B. Primary succession - establishment and development of plant communities in newly formed habitats previously or currently lacking plants - no roots, seeds or nutrients
Colonizers and Stress tolerants, some ruderals
e.g. sand dunes, lava flows, receding glaciers, erosion, landslides

C. Secondary succession - following a disturbance, the return of vegetation

1. Disturbance - a discrete process which partially or completely removes the current vegetative community (and consequently the animal community) and may alter resource availability
   - leaves a gap where a new sere will begin.
   e.g. tree falling in forest, high winds, flooding, fire -sproutable roots, seeds, nutrients intact

2. Gap size of the disturbance in important to what species will colonize and begin to grow - what sere will follow
   a. competition wins small disturbances (saplings, not weeds) competitors
   b. colonization wins large disturbances (weeds, not saplings) ruderals

D. Climax community - result of succession, a more or less stable environment, described by the major plant types involved. Stable until disturbed. Concept considered passe, communities not considered to reach stability.

1. Approaching climax communities- once forest vegetation established itself, pattern of light intensity and soil moisture and nutrients do not change as markedly as earlier in the sere.
   a. At this point the replacement of species is a competitive exclusion process - what saplings can grow initially in the shade and can more efficiently use the other resources, so as to out-compete the current adult trees.
   b. The types of saplings one sees, will suggest if the community is at climax or still approaching.

2. Stable climax communities - until disturbance, relatively little change. Some people don't believe that they exist, a community is always changing. (Slowly) or always disturbed. "Climax" is just a slowing in the cycle of the sere.

3. Transient climaxes - often seasonal, pond life followed by freeze

E. Change over geological time (Fig. 21.21)

1. Climatic change - periods of glaciation can radically alter the distribution of communities

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