Community Dynamics (also known as Biological Succession)

Community Dynamics is the process of change and development in communities of all living organisms—including plants, microorganisms, and small and large creatures of every sort.  

Populations of an organism will appear in an environment as its requirements for establishment are met.  As the successional level of a community as a whole advances, some species will begin to find that conditions become less suitable for its reproduction.  When this occurs, its population declines in number, and the species may even disappear from the local environment.

The Rule of Succession
There is a fundamental rule of succession that drives all thinking and decision-making when dealing with the natural environment:

A species will move into an environment when the conditions are suitable for its establishment,
and will move out of that environment when the conditions become unsuitable for its reproduction

Photos here Tig 1 and 2

This land was bare soil 180 days earlier.  There initial germination was datura spp. (castor oil plant - or Jimson weed in the USA).  Shortly after some low successional but edible grasses established.  The paddock was grazed 90 days after initial germination, and is now ready for the 2nd graze.  It is nearly mid winter, and you can see dead datura hanging on the electric fence, and in the background.  The seed bank of datura that was left behind to reestablish in the spring was the second largest I have ever seen! (the largest seeding I ever saw was on a research station when scientists deliberately seeded datura prior to testing a new chemical before its release).

This is the land immediately after the 180 day (mid winter) graze.  Dung is breaking down, litter is beginning to cover the soil surface, and many species are starting to establish.  Succession is tending upwards.  What happened next was extraordinary: during the spring just a few datura established.  None of them was healthy, and all of them aborted their seed heads.  The rule of succession always works.

Succession and Stability
In a low successional community there are comparatively few species present.  Amongst those present, it is usual for their populations to fluctuate widely and often very rapidly—weed and insect outbreaks for instance.  These communities of organisms are usually very unstable and out of balance, as depicted to the left of the image below.  

On the other hand, in high successionally conditions there are many species, and whilst number of individuals within each species tends to be lower, their relationships with each other lead to very stable populations.  There is balance, as the right hand side of the image below shows.

Within nature, succession to generally advance.  You can liken it to a coiled spring.  When compressed, succession is retarded, but when the compressive force is released the spring rapidly moves upwards towards a higher level.  It is likely you will see rapid changes in succession when you deliberately manage for them.

Outbreaks of problem organisms
If you discover an outbreak of a problem insect or plant, you can be fairly certain that you have done something that moved succession down.  Spend some time to diagnose what you might have done, and make some new decisions to change things around.

The extent of succession
The ultimate or climax level of vegetative succession in any environment is governed by both rainfall and the location’s position on the brittleness scale, but in any location it is likely that under appropriate management more species can be induced to establish than now exist.

In a brittle-tending environment non disturbance  or rest tends to decrease succession, leading to ecological instability.  On the other hand, Rest applied in a non-brittle tending environment may lead to increased ecological complexity!

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