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Population interactions and diversity


Limiting Factors

limiting factors

The total amount of available sunlight in any area is fixed – so there is a 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 organisms to grow is a limiting factor.

Herbivores (plant-eaters) are limited by the amount of plants.

Other limiting factors for herbivores are the amount of O2 gas, water


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.

Result: In any place, only one species lives in a niche:

Two different forms of life can live in the same place, but only if they have different roles

e.g. They hunt at different times of the day, or eat different foods.

Niche birds

Energy flows through ecosystems in one direction

Flows from the Sun,

through photosynthetic organisms (including green plants and algae)

to herbivores, then carnivores 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 Anup Shah, Global Issues,  Why Is Biodiversity Important? Who Cares?

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

  • Social benefits, such as

    • Research, education and monitoring

    • Recreation and tourism

    • Cultural, artistic or religious value


Human Influences on Biodiversity

From the Regents Exam Prep Center website, Oswego City School District.

Archived: Regentsprep.org Ecology Human Biosphere influence

Humans are a part of Earth’s biosphere.

Yet when humans alter ecosystems then serious consequences may result.

Humans can, deliberately or accidentally, change the equilibrium in ecosystems. Humans are destroying other species as a result of population growth, consumption, and technology.

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.

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.

This 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)

Capture solar energy to produce their own food (photosynthesis)

Examples: Plants and green algae; some bacteria; some protists.


Examples of autotrophs



An organism that can’t make it’s own food.

It needs to ingest some other organism for food.

Most life is heterotrophic.

Examples include: all mammals, reptiles, birds, insects. All fungi (including mushrooms) and many bacteria.



A type of heterotroph. Therefore they eat autotrophs.

These are plant eating animals AKA primary consumers.

Examples: Cows, deer, bees

Are there other organisms, besides animals, that get food from plants?

Bacteria and protists that feed on living plants are usually termed plant pathogens (plant diseases)

Microbes that feed on dead plants are saprotrophs.



A type of heterotroph. Therefore they eat autotrophs.

They are meat eating animals. AKA secondary consumers.

Examples: Bears, Lions, Wolves.



A type of heterotroph. Therefore they eat autotrophs.

They do equally well eating plants or animals.

Examples: Humans, dogs, crows, swallows, large ground finches



They eat already-dead bodies (of animals or plants)

Examples: Vultures,  burying beetles, blowflies

Some carnivores will engage in scavenging on occasion, e.g.  hyenas, jackals, African lions, leopards, polar bear, and wolves.



An organism that lives on another organism, taking nutrition and shelter from it. The parasite benefits while the host gets sick.

Parasitism is a non-mutual relationship between species: One species, the parasite, benefits at the expense of the other, the host.

Parasites typically do not kill their host.  Are generally much smaller than their host. Often live in/on their host for an extended period.

Large parasite animals (can be seen by the human eye)

Examples: roundworms, tapeworms, ticks, lice, fleas, parasitic worms (helminths), many wasps

Large parasite plants

Fungi – One could classify many fungi as a form of parasite.

Yes – some plants are parasites to other plants, like the Christmas-time favorite, mistletoe

Small parasites (pinpoint sized, or microscopic)

Examples: pathogenic (disease-causing) protozoa, bacteria or viruses.



The top of the food chain. They eat and decompose all dead forms of life.

Decomposers are the last stop on the food chain. They eat the things no one else wants to, to get their nutrients.

Examples: bacteria, worms, slugs, snails, and fungi-like mushrooms.

Symbiosis: Types of relationships

Any type of a close, long-term relationship between two different organisms.

In 1879, Heinrich Anton de Bary defined symbiosis as “the living together of unlike organisms”.


When two organisms of different species work together, each benefiting from the relationship.


Oxpecker (a kind of bird) and the rhinoceros or zebra. Oxpeckers land on rhinos or zebras and eat ticks and other parasites that live on their skin. The oxpeckers get food and the beasts get pest control. Also, when there is danger, the oxpeckers fly upward and scream a warning, which helps the symbiont (a name for the other partner in a relationship).”

Other examples: Bees and flowers, Dogs and humans, Spider crabs and algae, Human gut flora and humans.

from New England Complex Systems Institute, Evolution, by Shlomiya Bar-Yam.


A relationship between two living things from different species in which one of the organisms benefits.

While the other organism does not benefit, it is also not harmed by it.

Often, commensalism occurs between a larger species and a smaller one. The smaller organism is usually the beneficiary.


The remora fish rides attached to sharks…. The remora benefits by gaining a measure of protection, and it feeds off of the remains of the meals of the larger fish.

The cattle egret is a type of heron that will follow livestock herds. The cattle egret benefits because it eats insects that are stirred up when the livestock move through the grass.

Barnacles are a type of crustacean that attach to whales. Barnacles cannot move on their own, so they use the whale to move around and find locations with food.

from SoftSchools.com


Parasitism is a non-mutual relationship between species: One species, the parasite, benefits at the expense of the other, the host. See above section on this webpage for details.


Endosymbiosis is one species living inside another one.

Example: Microbes live inside termites and help them digest wood. The microbes get food & a safe place to live. The termite gets energy from the microbe digestion by-products.

Termite Gut and endosymbiosis

from Genomics:GTL Transforming Cellulosic Biomass,” U.S. Department of Energy Office of Science and Office of Energy Efficiency and Renewable Energy, June 2006, genomicscience.energy.gov/biofuels/ and U.S. DOE. 2006. “Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda,”

Endosymbiosis is also the name of a hypothesized process in evolution. It explains the origin of eukaryotic cells from prokaryotes.

According to this theory, mitochondria and chloroplasts are descendants of formerly free-living bacteria.


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 & webs

A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition.

FoodChain land and sea

Food webs

Food webs connect many different food chains.

Here is a fascinating food web.

Food Web A

Food Web 1




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.

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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