Instruction 6-5

Biodiversity | Ecosystem Changes | Fluctuation in Population Size | Water, Carbon and Nitrogen Cycle | Stability in an Ecosystem | Energy Pyramid | Accommodation and Adaptation

Stability in an Ecosystem
CA GR.9-12 6.e.

A Steady State or Dynamic Equilibrium is where conditions are held more or less constant by negative feedback systems operating within the ecosystem. Most of this has been discussed in lessons 6-2 and 6-3 so this is just going to be somewhat of a review lesson, but we will go into more in-depth with concepts and vocabulary words.

The Principles of Ecosystem Stability are:

  • Ecosystems dispose of waste and replenish nutrients by recycling all elements.
  • Ecosystems use sunlight as their source of energy.
  • The size of a consumer population is maintained such that overgrazing and other forms of overuse do not occur.
  • Biodiversity is maintained.

Factors influencing ecosystem stability are biotic potential and environmental resistance. This could be in the form of: positive and negative factors of population growth either abiotic or biotic, species diversity that is highly correlated with stability, as well as climate.

Stability of an ecosystem also needs to have a resistance to change. This resistance to change has three forms: Inertia - the resistance to change, Resilience - the ability to recover from change and succession - the replacement of species by another.

Biotic Potential

Population vs. Individuals

  • Individuals are born; they grow, and they die.
  • Populations have a birth rate, a growth rate and a death rate.
  • Populations can evolve and the show a dispersion pattern.


Biotic Potential

  • Definition: The number of offspring that a species may produce under ideal conditions.
  • Recruitment - survival of the young through the early growth stages of a species.
  • Plants generally reproduce through seeds and spores.
  • Animals reproduce by laying eggs or bearing live young.

Critical Number

  • There is population number below which a species cannot survive.
  • The result is extinction.

Ecological factors which can affect dynamic change in a population or species in a given ecology or environment are usually divided into two groups: abiotic and biotic.

Abiotic factors are geological, geographical, hydrological and climatological parameters. A biotope is an environmentally uniform region characterized by a particular set of abiotic ecological factors. Specific abiotic factors include:

  • Water, which is at the same time an essential element to life and a milieu
  • Air, which provides oxygen, nitrogen, and carbon dioxide to living species and allows the dissemination of pollen and spores
  • Soil, at the same time source of nutriment and physical support
  • Soil pH, salinity, nitrogen and phosphorus content, ability to retain water, and density are all influential
  • Temperature, which should not exceed certain extremes, even if tolerance to heat is significant for some species
  • Light, which provides energy to the ecosystem through photosynthesis
  • Natural disasters can also be considered abiotic

Biotic ecological factors are considered as either intraspecific and interspecific relations.

Intraspecific relations are those, established between individuals of the same species, forming a population. They are relations of co-operation or competition, with division of the territory, and sometimes organization in hierarchical societies.

Interspecific relations are interactions between different species: are numerous, and usually described according to their beneficial, detrimental or neutral effect for example, symbiosis, mutualism or competition.

The most significant is the relation of predation (to eat or to be eaten), which leads to the concept of food chains (for example, the grass is consumed by the herbivore, itself consumed by a carnivore, itself consumed by a carnivore of larger size). A high predator to prey ratio can have a negative influence on both the predator and prey by decreasing the availability of food and high death rate prior to sexual maturity which can decrease (or prevent the increase of) populations of both the predator and the prey populations.

Selective hunting of species by humans, which leads to population decline, is one example of a high predator to prey ratio in action. Other interspecific relations include parasitism, infectious disease and competition for limiting resources, which can occur when two species share the same ecological niche.

The existing interactions between the various living beings go along with a permanent mixing of mineral and organic substances, absorbed by organisms for their growth, their maintenance and their reproduction, to be finally rejected as waste. These permanent recyclings of the elements (in particular carbon, oxygen and nitrogen) as well as the water are called biogeochemical cycles.

They guarantee a durable stability of the biosphere (at least when unchecked human influence and extreme weather or geological phenomena are left aside). This self-regulation, supported by negative feedback controls, ensures the ongoing of the ecosystems.

It is shown by the very stable concentrations of most elements of each compartment. This is referred to as homeostasis. The ecosystem also tends to evolve to a state of ideal balance, reached after a succession of events, the climax (for example, a pond can become a peat bog).

Environmental Resistance

Environmental resistance is the combination of all the abiotic and biotic factors that may limit the growth of a population.
The influence of an extrinsic factor comes from outside the population. Intrinsic factors are "built into" the populations themselves. Below is a partial list of factors that may limit the growth of a population.

List of factors:

Biotic factors

Abiotic factors

nutrients (plants) light
food (animals) pH
territory salinity
predation temperature
ability to defend chemical environment
suitable habitat  
water supply  
birth control  
mating behavior war  
social stress  
ability to migrate  
hierarchy disease  

Ecological Succession

Ecosystems are dynamic. They are constantly adapting to environmental changes. They have definite limits of tolerance (limiting factors). Among the most dominating of the abiotic factors are temperature and precipitation; which over an extended period of time represents climate.

A Comparison of Immature and Mature Ecosystems

Characteristic Immature Mature
Food chains Linear, mostly grazers Web-like, mostly detritus
Food webs Simple Complex
Populations Unstable Stable
Decomposers Few Many
Annual plant size Small Large
Net productivity High Small
Species diversity Low High
Niche specialization Broad Narrow
Nutrient cycles Open Closed

Primary succession on land (temperate zone)

Bare Rock Lichens Small annual plants, lichens Perennial herbs, grasses Grasses, shrubs, shade intolerant trees, shade tolerant trees
Pioneer Stages Intermediate Stages Climax Community
-----------------------------Time in hundreds of years------------------------------------>

Primary succession on bare rock. Rocks erode, sand accumulates, decomposition adds organic matter, and voila! soil. As soil continues to form, larger (and woody) plants appear. Volcanic eruption is primary succession. Following the eruption, there is no life in the blast zone. Eventually, however, pioneer species of plants do establish themselves, beginning the course of succession.
Primary succession can also occur from man-made structures.

Secondary succession on land (temperate zone)

Community Crabgrass Tall grass, herbaceous plants Pines come in Pine Forest Hardwoods come in Hardwood forest climax community
Years 0-1 1-3 3-10 10-30 30-70 70+

So what is secondary succession? Secondary succession occurs where communities used to exist, but were destroyed by natural or man-made causes. These could be in areas where forests are cut down or destroyed by fire, other natural disasters, and diseases. Abandoned farmlands also undergo secondary succession. A volcano or other catastrophe will also cause an area to undergo secondary succession. Usually, secondary succession re-establishes the original community much faster than primary succession because soil already exists; nevertheless, it may take a hundred years or more for the climax stage to return, if it ever does.

Aquatic succession (temperate zone)

Bare bottom Submerged vegetation Emerging vegetation Temporary pond and prairie Beech and maple forest
Pioneer Community  ----------------------------------------------------------------------------------------------------> Climax Community


1. Pioneer species - the first species to colonize an uninhabited area. For plants it begins with lichens and mosses, for animals generally insects.

2. Succession - the repeated replacement of one species by another over time. It is the result of interspecies competition and is sometimes referred to as serial succession. The entire progression from a pioneer to a climax community is called a sere.

3. Climax community - the final stage of succession. It is a long-lived, self-perpetuating community or ecosystem based on the limiting factors of the environment.

4. Primary succession (bare rock succession) - succession occurring on an area previously devoid of life. Lichens, a mutualistic relation between an algae and a fungi, are the pioneer plant community on land.

5. Aquatic succession - phrase used to describe primary succession in bodies of water. Except for the oceans, inland bodies of water are considered to be temporary. Primary succession involves the gradual filling-in of the water body until a "dry" terrestrial environment exists supporting a climax community.

6. Secondary succession - succession occurring in an area previously inhabited, but set back to an earlier stage. It begins with the destruction of disturbance of the existing ecosystem. This may be the result of natural disaster such as tornadoes, fires, hurricanes, volcanoes, etc. or the action of man in cases of agriculture, logging, filling in wetlands etc.

7. Geographical succession - an expression used to explain the relationship between altitude, latitude, and climax communities. If one was to climb a mountain at the equator, they would pass through the same sequence of communities as if they had walked toward the pole.

8. Population Explosion - the term for exponential growth of a population.

9. Replacement Level the term used when recruitment equals adult mortality rates.

Experiments for Home and Classroom

You can try these desert experiments at home. You can create a mini-pond at home and grow your own algae, see how cyanobacteria grow mostly in the warm, upper layers of a lake or see how warm temperatures speed up bacterial growth. Desert plants grow better in the heat than plants imported from colder places. See how plants require the right temperature in order to grow. A little bit of air or water, pollution goes a long way. Find out how.

Here are 12 ecology-related experiments for you to try:
Before starting any of the experiments listed be sure to read the instructions carefully.

Build a small ecosystem and make a mold terrarium, watch tiny blue, green and white plants grow on leftover food.

for Students, Parents and Teachers

Now let's do Practice Exercise 6-5(top).


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