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The Environment, Levels of Ecology, and Ecosystems

Organismal Ecology

Ecology is the study of the interactions between the organisms and the environment they live in

Environmental factors are the set of conditions that surround an organism
Such as: temperature, light, water, air, soil, nutrients

Organisms are individual living things that are capable of responding to stimuli, growing, reproducing, and maintaining homeostasis

Population Ecology

Populations are organisms in a particular geographic area that belong to the same species

Community Ecology

Communities are all organisms of all species living in a geographic area.

Ecosystem Ecology

Ecosystems are the community and all abiotic factors like weather, water, soil, and elevation within a geographic location.

Biosphere Ecology

Biospheres are all of the ecosystems on the planet.

Ecosystems, Habitats, and Ecological Niches


Niches are the roles that are played by a particular species within a particular ecosystem

Population Density

Population density is measured by the number of individuals per unit of area.

Density-dependent factors are the factors that determine the density of a population within an ecosystem, and include the amount of habitat, food, water, and shelter that are available, as well as the rate of predation, disease, and reproduction that are occurring as the population increases.

Density-independent factors are population-limiting factors that are not affected by population size, including predation, many types of disease, and weather.

Food Chains, Trophic Levels, and Energy Flow in an Ecosystem

Food Chains

Food chains are a sequence of organisms that feed on each other.

Producers are organisms that get their energy from an abiotic source (such as the sun).

Consumers are organisms that get energy from other organisms.

Trophic Levels

Trophic levels are the levels of a food chain where organisms obtain their energy.

The order in which trophic levels ascend is producer, primary consumer, secondary consumer, tertiary consumer, quaternary consumer.

Food Webs

Food webs are a combination of food chains that are interconnected to create a network of feeding relationships.

Herbivores are plant-eating animals.

Carnivores are animals that consume other animals.

Omnivores are animals that eat both animals and plants.

Detritivores are organisms that feed on waste products and dead organic material.

Energy Flow Through a Food Web

Only 10% of energy is passed on through each trophic level, and the biomass transfer efficiency is the reason that the trophic levels rarely advance past four levels.

Interspecific Competition, Competitive Exclusion, and Niche Differentiation

Interspecific Competition

Interspecific competition is when two or more species in a community are competing for resources.

Competitive Exclusion

Competitive exclusion is when one species outcompetes another in a part of its habitat so well that the second species is excluded from that part. Competitive exclusion is one possible outcome of strong interspecific competition.

Local Extinction

Local extinction is when one species is outcompeted by another so effectively throughout the entire local habitat that it becomes extinct in that area.

Niche Differentiation

Niche differentiation is when similar species with similar niches become specialists in specific areas and create more than one specific niche, which allows both species to coexist.

Fundamental niches are the niches that contain all of the resources that a population is theoretically capable of using.

Realized niches are the niches that contain all the actual resources a population uses.

Predator/Prey Interactions, Camouflage, Mimicry, and Warning Coloration

Predator/Prey Interactions

Predators are animals that hunt and kill other animals for food.

Prey are animals that are hunted and killed by predators.


Camouflage is the ability of an organism to blend in with its surroundings.

Chemical Warfare

Batesian mimicry is a type of mimicry where a harmless animal mimics a dangerous or unpalatable animal.

Mullerian mimicry is when two or more dangerous/unpalatable species all resemble one another.


Coevolution is when two species evolve in a coordinated fashion by adapting to changes in each other.

Symbiotic Relationships: Mutualism, Commensalism, and Parasitism


Symbiosis means a close, long-term interaction between two different species.

Hosts are the larger organisms in a symbiotic relationship upon or inside of which the smaller organism lives.

Symbionts are the smaller organisms in a symbiotic relationship that lives in or on the host.


Parasitism is an association between two different species where the symbiont benefits and the host is hammed.

Vectors are parasites that transmit disease-causing pathogens to other species of animals.


Commensalism is an association between two different species where one species enjoys a benefit, and the other is not significantly affected.


Amensalism is an association between two organisms of different species where on species is inhibited or killed and the other is unaffected.

Populations: Density, Survivorship, and Life History

Population Density

Population density is the number of organisms per unit of volume or area.

Different ways to measure population density includes:

  • direct counting
  • estimation by indirect indicators
  • sampling
  • mark-recapture method

Life Histories

Life history is the sequence of events in an organism’s life that relates to its survival and reproduction.

Life histories include factors like the number of offspring produced, frequency of reproduction, amount of care and resources dedicated to offspring, and the kind of survivorship curve the organism exhibits.

Survivorship Curves

Survivorship curves is a graph of the number of individuals still alive at each age.

Type I: if the death rate of a species is highest when it reaches old age

Type II: if the death rate is relatively constant over all age groups (linear)

Type III: if the death rate is highest before individuals reach maturity

r/K selection theory

r/K selection theory attempts to explain the differences in life history based on the type of habitat an organism lives in

r-selected species are short-lived species with a high growth rate that produce a large number of offspring, each of which has a low probability of survival to adulthood

K-selected species are long-lived species with a slower growth rate that produce a small number of offspring into which the parents invest a lot of resources to ensure a high probability of survival to adulthood

This theory did not hold up to scientific scrutiny, so it is now thought that each species has a specific life history custom-tailored to maximize the fitness of individual organisms based on the factors that play the biggest roles in the survival and reproduction of those individuals.

Carrying Capacity, Migration, and Dispersion

Carrying Capacity

Carrying capacity is the maximum stable population size that can be sustained over a long period of time

Logistic growth is when a natural population approaches its carrying capacity, its growth rate slows, and eventually, the population stops growing when the carrying capacity is reached. It is shaped like an S.

Exponential growth is when populations experience rapid growth, a growth in which the number of individuals multiplies with each successive generation. It is shaped like a J.


Range is the geographic limits within which a population or individual lives.

The larger the population’s range is, the more resources are available to that population and the greater the carrying capacity will be if all other factors stay the same.


Dispersion is the pattern of spacing of individuals within a population.

The patterns of dispersion are uniform, clumped, or random.

Uniform dispersion patterns are often a result of partitioning of resources among individuals resulting from interspecific competition.


Migration is the seasonal movement of organisms over long distances.

Migrations allow populations to increase their range and resource pool and therefore they have the ability to increase a population’s carrying capacity.

Dispersal, Colonization, and Island Biogeography

Dispersal Mechanisms

Dispersal is the spread of organisms to new areas. Not to be confused with dispersion, which is the pattern of spacing of individuals within a population.

Habitat Fragmentation

Immigration is a one-time migration of an organism or group of organisms into an area.

Colonization is when an organism or group of organisms starts a population in a habitat where the species was not already present.

Island Biogeography

Island biogeography is an ecological theory that estimates the number of species that live on an island based on its size and proximity to a mainland source of species.

Conservation Biology, Habitat Fragmentation, and Metapopulations

National Parks and Conservation Biology

Conservation biology is when a biologist tries to conserve an environment’s natural state, and that has resulted in national parks.

Habitat Fragmentation

Human civilization has fragmented many natural habitats

Metapopulation Theory

Metapopulations are groups of local populations that are connected by immigration.

Metapopulation theory states that if there is a patchy environment, you can have lots of small populations of a single species, and if one small population goes extinct in a certain area, then the other populations can repopulate that patch of the environment through wildlife corridors.

Wildlife corridors are routes that animals can use to migrate between different patches of natural habitat.

Importance of Metapopulation Theory

It helps a biologist to figure out which areas are important to save for certain species, and which areas are beyond saving or not worth saving.

Ecological Succession: From Pioneer to Climax Communities

Ecological Succession

Ecological succession is the process by which the species structure of an ecological community changes over time.

Primary Succession

Primary succession is succession that begins in an area where the soil has not yet formed, such as the formation of a new island by volcanic eruption or the recession of a glacier.

Secondary Succession

Secondary succession is succession that begins after an event clears the community but leaves the soil intact.

How Introduced and Invasive Species Alter Ecological Balance

Introduced Species

Introduced species are species that are living outside of their natural environments and were brought there by human activity.

Alteration of Ecological Balance

Ecological Balance is the condition of equilibrium among the different species of an ecosystem

Local Extinction

Local extinction is when a species ceases to exist in a local area.

Invasive Species

Invasive species are non-native species whose introduction into an area has caused economic or ecological harm.

Biological Cycling

Composition of Living Organisms

Oxygen, carbon, hydrogen, and nitrogen make up about 96% of the total weight of all living organisms.

Macronutrients are other elements that are required for living organisms

Biogeochemical Cycling

Biochemical cycles are how an element cycles through organisms and the environment

Phosphorus Cycle

Phosphorus cycle is the biochemical cycle that phosphorus follows through organisms and the environment.

  1. Phosphorus is found in many types of rock as phosphate ions.
  2. Weathering of the rock releases phosphate into the soil, where it can be absorbed by plants.
  3. Once in plants, the phosphate becomes available to consumers and eventually detritivores,, which can recycle it back into the soil or back into consumers.
  4. Some of the phosphate ends up in the local water table, and envetually it becomes part of new sedimentary rocks and unavailable for biological use until the rock is brought back to the surface by geological forces and weather releases phosphate into the soil.

Nitrogen Cycle

Nitrogen fixation is the process of creating ammonia from atmospheric nitrogen

Plants an most other producers cannot utilize atmospheric nitrogen so they must rely on nitrogen fixing bacteria to turn nitrogen gas into ammonia that they can use.

Nitrification is when bacteria convert ammonia to nitrite and then to nitrate in a process

Eutrophication is the ecological process that occurs when excess nutrients are added to a body of water. It can lead to algal blooms that lead to mass kill-offs.

Carbon Cycle

Carbon is used by living organisms, not only as a structural component, but also as the primary means of energy storage, transfer, and usage.

Photosynthesis is the process by which autotrophs convert light energy into chemical energy

Cellular respiration is the process by which oxygen is used to convert organic molecules to carbon dioxide and water and provide energy for the cell. It returns carbon dioxide back to the atmosphere.

Acid Rain

Acid rain occurs when fossil fuels and other organic materials are burned, they release carbon dioxide, sulfur dioxide, and nitrogen oxides into the atmosphere

Fossil Fuels

Acid rain occurs when fossil fuels and other organic materials are burned, they release carbon dioxide, sulfur dioxide, and nitrogen oxides into the atmosphere

Fossil Fuels, Greenhouse Gases, and Global Warming

The Effect of Humans on the Carbon Cycle

Over the past 200 years, humans have started to have a measurable effect on the carbon cycle by burning fossil fuels and releasing more CO2 into the atmosphere.

Greenhouse Gases

Greenhouse gas is a gas in the atmosphere that absorbs infrared radiation from the Earth and slows the rate of heat loss.

The three major greenhouse gases in the Earth’s atmosphere are water vapor, carbon dioxide, and methane.

Global Warming

Global warming is an increase in the average worldwide temperature caused by higher concentrations of greenhouse gases in the atmosphere