Ecological succession – the gradual change in species composition of a given area
2 types of ecological succession: 1) Primary succession – starting with rock
2) Secondary succession – starting with soil
Primary succession will ALWAYS be on areas where there is NO SOIL – a) bare rock from a retreating glacier, b) newly
cooled lava; c) abandoned lot with an impervious surface (parking lot)
Primary succession is broken into 4 stages: pioneer, early, mid, and late succession
In primary succession the first organisms (lichens) to attach to the rock are called pioneer species. Mosses are generally
included as pioneer species because they require almost no soil to grow. Both of these produce mild acids that will slowly start decomposing the rock. This, in combination with chemical weathering (oxidation of minerals on the rock surface and dissolving polar molecules by rainwater) and physical weathering (wind, water expansion and contraction, etc) will ultimately change the rock to soil. It takes several HUNDRED to a THOUSAND years just to make 1 INCH of soil!
The next group is the early successional species, which 1) grow low to the ground, 2) can establish themselves quickly, 3)
produce many offspring, and 4) have short lives (so that they can be succeeded by other plant species) The species are typically what people have termed ‘weeds’!
The next group is the midsuccessional species, which are characterized by the need for lots of sunlight and limited
amounts of nutrients. These would include grasses, shrubs and herbs first and then replaced by pine trees and other small trees that require abundant sunshine.
The last stage of succession is the late successional species, which is characterized by plants that are shade tolerant and
require many nutrients. These would include many of the deciduous trees (those producing leaves) and those understory trees and shrubs that don’t require much sunlight.
Lichens & mosses --; weeds ---; grasses & shrubs --; pine trees ---; deciduous trees
Pioneer species Early successional Midsuccessional Late successional
Secondary succession begins when something has arrested the development of an area in primary succession. Usually all that remains is the soil. Scenarios that would begin secondary succession would be: 1) farmland/dwellings that have been abandoned, 2) a forest fire or clear cutting forests, 3)air or water pollution that has denuded the area, or 4) flooding that deposited excess soil that smothered the other vegetation or removed the vegetation down stream.
The only difference in the sequence of successional stages is that Secondary Succession does NOT have a pioneer species. All other stages will be as they were for Primary Succession.
Changes in succession bring about changes in the amount of species diversity, the trophic structure, the number of niches, the amount of nutrients available, and the amount of energy flow through the system.
ththLEARN Table 8-1 (13 ed), Table 6-1 (10 ed)
3 Factors that affect the rate of succession:
1) Facilitation – where one species makes the area suitable for the next species to move in and occupy new niches 2) Inhibition – where established species hinder the establishment of new species by various methods (toxic chemicals
that interfere with the growth of new species, the established species are taller and block sunlight for newer species
trying to get established, etc.)
3) Tolerance – late successional species are unaffected by earlier successional species that move in and occupy the
Ecosystems are CONSTANTLY undergoing change. It is rare for one ecosystem to make it to the late successional stage without being interrupted and start over with a secondary succession. These interruptions can be catastrophic, changing the ecosystem in one drastic movement or gradual, changing them over time. Those factors that can bring about catastrophic or ththgradual change are listed on Table 8-2 (13 ed) or Table 6-2 (10 ed). LOOK over these TABLES!
Intermediate Disturbance Hypothesis – communities that undergo frequent but moderate disturbances have the greatest
species diversity. The disturbances create large enough openings for new colonizing species but the disturbances are minor enough that some existing species still survive in the surrounding area. Ex: a large tree falls (like in the Opal Creek area of
Oregon) letting sunlight into that portion of the forest. Vegetation that needs more sunlight than what the standing trees can give will grow in the area but will be surrounded by many of the large trees. A variety of niches will have been created by this small disturbance.
Climax Community – late successional stage where the ecosystem is dominated by a few tall canopy trees with some open sunlight areas with other vegetation. There is equilibrium between the amount of shade tolerant and sun tolerant species of flora and fauna.
Ecosystems are in a state of change and really represent a patchwork that contains the different successional stages. One ecosystem does not just progress through to the climax community and stay there. Change will always come about!
3 aspects of stability or sustainability in living systems:
1) Inertia (Persistence) – the ability of a living system to resist being disturbed or altered
Ex: a heath bald – a high meadow found on mountains that resists tree growth and remains a meadow
2) Constancy – the ability of a system to keep its numbers within the limits imposed by available resources
Ex: some species of frogs have the ability to convert their sex (from female to male) if the numbers of males
becomes to small to sustain the population thus keeping the population fairly constant
3) Resilience – the ability of a system to bounce back after an external disturbance that is not too drastic
Ex: a forest fire in a pine forest releases the seeds from the pine cones and thus starts the regrowth of trees almost
Recent research has found that the old theory of –‘the more diverse the food web, the greater stability of the ecosystem’
may not be valid. Ecosystems with more species tend to have a higher Net Primary Productivity and can be more resilient
but at the same time the populations of individual species can fluctuate more widely in diverse ecosystems than in simpler ones. So some level of biodiversity provides insurance against catastrophe, but how much biodiversity is needed in various ecosystems remains uncertain.
Be able to explain the EXAMPLES about tropical rainforests’ stability vs the stability of a grassland
When evidence indicates that an activity can harm human health or the environment, we Precautionary Principle –
should take precautionary measures to prevent harm even if some of the cause-and-effect relationships have not been fully established scientifically. Ex: In 1972 Rowland & Molina discovered that there was an ozone hole over
Antarctica. They suggested in 1974 that CFC’s were the culprit for this depletion. In 1978 the EPA banned the use of CFC’s in aerosol products. There was no conclusive proof that CFCs were the culprit to ozone loss until 1987. The talks to
lower CFC production was already in the works with the Montreal Protocol. It was signed by many countries (including the US) in the latter part of 1987, which only called for the reduction of CFC production by 50% by the year 2000. Then in 1994 NASA reported an ozone hole over the Arctic that extended over parts of Europe, Canada, and the Northeast US which prompted the US President to sign into law that there would be no more CFC production starting January 1996.