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Population interactions and Diversity


The total amount of available sunlight in any area is fixed – so there must be some maximum amount of plants that can grow in this area.

The amount of sunlight is a limiting factor for how many plants can grow. Also, the space needed for plants to grow is a limiting factor.

Herbivores (plant-eaters) are limited by the amount of plants.  Plants are their limiting factor.
Other limiting factors for herbivores are the amount of O2 gas, water, and space to live.

If two forms of life live in the same area, and have similar needs, then they will compete for resources.

When two or more species compete, eventually some of these species will be driven away, or go extinct.

What is left is that in any place, only one species lives in a niche:

A niche is the place that an organism lives in, and the role it plays.

Two different forms of life can live in the same place, but only if they have different roles
(for instance, hunt at different times of the day, or eat different foods.

Niche birds

Energy flows through ecosystems in one direction

From the Sun, through photosynthetic organisms (including green plants and algae)
to herbivores, to carnivores and then to decomposers.

Energy flow in ecosystem

Benefits of biodiversity

Biodiversity benefits both the (non-human) species in the ecosystem, as well as humans.

“At least 40 per cent of the world’s economy and 80 per cent of the needs of the poor are derived from biological resources. In addition, the richer the diversity of life, the greater the opportunity for medical discoveries, economic development, and adaptive responses to such new challenges as climate change.”

— The Convention about Life on Earth, Convention on Biodiversity,  http://www.cbd.int/

The following is from http://www.globalissues.org/article/170/why-is-biodiversity-important-who-cares


Why Is Biodiversity Important?

Biodiversity boosts ecosystem productivity where each species, no matter how small, all have an important role to play.

For example,

And so, while we dominate this planet, we still need to preserve the diversity in wildlife.

A Healthy Biodiversity Offers Many Natural Services

Ecosystems such as the Amazon rainforest are rich in diversity. Deforestation threatens many species such as the giant leaf frog, shown here. (Images source:Wikipedia)

A healthy biodiversity provides a number of natural services for everyone:

  • Ecosystem services, such as
    • Protection of water resources
    • Soils formation and protection
    • Nutrient storage and recycling
    • Pollution breakdown and absorption
    • Contribution to climate stability
    • Maintenance of ecosystems
    • Recovery from unpredictable events
  • Biological resources, such as
    • Food
    • Medicinal resources and pharmaceutical drugs
    • Wood products
    • Ornamental plants
    • Breeding stocks, population reservoirs
    • Future resources
    • Diversity in genes, species and ecosystems
  • Social benefits, such as
    • Research, education and monitoring
    • Recreation and tourism
    • Cultural values


from the RegentsPrep website


Human Influences on Biodiversity

When humans alter ecosystems either by removing specific organisms, serious consequences may result. Human beings are part of the Earth’s ecosystems. Human activities can, deliberately or accidentally, change the equilibrium in ecosystems. Humans are destroying other species as a result of population growth, consumption, and technology. Human destruction of habitats through direct harvesting, pollution, atmospheric changes, and other factors is especially threatening current global biodiversity.

An example of a human activity which has decreased biodiversity is the use of monoculture in modern agricultural practices. Monoculture involves planting one variety of a species over a huge area. This leaves this area more vulnerable to predation or disease and the loss of many or all species.

Uses of Biodiversity

In addition to the aesthetic beauty added to the world by many different organisms, biodiversity also ensures the availability of a rich variety of genetic material that may lead to future agricultural or medical discoveries with significant value to humankind. As diversity is lost, potential sources of these materials may be lost with it.

Roles of organisms

Autotrophs (producers)

Green plants and alage
They capture solar energy to produce their own food (photosynthesis)

Herbivores (plant eaters! also called primary consumers. Heterotrophs)

They eat autotrophs.

Carnivores (meat eaters! also called secondary consumers, heterotrophs)

Eat primary consumers as their main source of food.

Omnivores (heterotrophs)

They do equally well eating plants or animals.


An organism that can not make its own food, and so must consume other organisms.


They eat already dead bodies.


They live off of other animals or plants.


The top of the food chain, because decomposers eat and decompose all dead forms of life.










Endosymbiosis is one species living inside another one.

Example: Protozoans that live inside termites and help them digest wood.


Ectosymbiosis is one species living on the surface of the other species.
Example: Lice that feed on the skin, blood, or oil secretions of the host.

Food chains

FoodChain land and sea

Food webs

Here is a fascinating food web 🙂

Food Web A

Food Web 1

Textbook: Holt Environmental Science

Ecosystems, organism, population, community, habitat

Evolution, Darwin’s finches, artificial selection,

The diversity of living things, Bacteria, fungi, protists, plants, animals

Energy flow in ecosystems, food chains, food webs, food pyramids

The carbon cycle, the nitrogen cycle, The Phosphorus Cycle

Ecological Succession, communities managed by fire, Maps in Action: Doppler radar tracking of bats and insects in Texas

Niches, competition, predation, parasitism, mutualism, commensalism, symbiosis


Learning Standards

Massachusetts Science and Technology/Engineering Curriculum Framework

7.MS-LS2-6(MA). Explain how changes to the biodiversity of an ecosystem—the variety of
species found in the ecosystem—may limit the availability of resources humans use.

7.MS-LS2-2. Describe how relationships among and between organisms in an ecosystem can be competitive, predatory, parasitic, and mutually beneficial and that these interactions are found across multiple ecosystems

7.MS-LS2-3. Develop a model to describe that matter and energy are transferred among living and nonliving parts of an ecosystem and that both matter and energy are conserved through these processes.
Clarification Statements:
• Cycling of matter should include the role of photosynthesis, cellular respiration, and decomposition, as well as transfer among producers, consumers (primary, secondary, and tertiary), and decomposers. Models may include food webs and food chains.

HS-LS2-2. Use mathematical representations to support explanations that biotic and abiotic factors affect biodiversity, including genetic diversity within a population and species diversity within an ecosystem.
Clarification Statements:
• Examples of biotic factors could include relationships among individuals (feeding relationships, symbiosis, competition) and disease.
• Examples of abiotic factors could include climate and weather conditions, natural disasters, and availability of resources.
• Examples of mathematical representations include finding the average, determining trends, and using graphical comparisons of multiple sets of data.

HS-LS2-6. Analyze data to show ecosystems tend to maintain relatively consistent numbers and types of organisms even when small changes in conditions occur but that extreme fluctuations in conditions may result in a new ecosystem. Construct an argument supported by evidence that ecosystems with greater biodiversity tend to have greater resistance to change and resilience.
Clarification Statement:
• Examples of changes in ecosystem conditions could include modest biological or physical changes, such as moderate hunting or a seasonal flood; and extreme changes, such as volcanic eruption, fires, the decline or loss of a keystone species, climate changes, ocean acidification, or sea level rise

Benchmarks, American Association for the Advancement of Science

In all environments, organisms with similar needs may compete with one another for limited resources, including food, space, water, air, and shelter. 5D/M1a*
The world contains a wide diversity of physical conditions, which creates a wide variety of environments: freshwater, marine, forest, desert, grassland, mountain, and others. In any particular environment, the growth and survival of organisms depend on the physical conditions. 5D/M1b*

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