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Relationships Between Organisms and the Environment

Relationships Between Organisms and the Environment

The teacher understands the relationships between organisms and the environment.

Introduction to Ecosystems
Ecology is the study of the interactions between organisms and their physical and biotic environment. Deanne Erdmann, MS, reviews some general ecological terms, the varying levels of organization in an ecosystem, energy flow through an ecosystem, and some of the biogeochemical cycles which occur in nature.

The Concept of the Ecosystem
This overview from the University of Michigans provides good descriptions of the components and functions of ecosystems.

Explanations of each of the subtopics related to ecology competencies are given in each of the following sections.


Abiotic vs. Biotic Components

The beginning teacher identifies the abiotic and biotic components of an ecosystem.

Key Concepts:

  • Abiotic components of ecosystems are those that are non-living. These include:
    • amount of water present,
    • nutrient and mineral availability,
    • soil structure and substrate,
    • climate,
    • amount of sunlight available.
  • Biotic components of ecosystems are those that are living or were previously living.
  • The biotic and abiotic components shape how species have adapted over time, and what species can exist in any given ecosystem.
  • Matter can alternate between the abiotic and biotic environment in cycles. For example, carbon can be incorporated as sugar in a plant (biotic), but can later be released into the atmosphere after it is consumed in the form of carbon dioxide (abiotic).


Abiotic and Biotic Factors
Mr. G’s Environmental Systems further details the biotic and abiotic factors that affect life in woodland ecosystems.

Ecosystem Roles and Energy Flow through Ecosystems

The beginning teacher analyzes the interrelationships among producers, consumers, and decomposers in an ecosystem.

Key Concepts:

  • The three main ecosystem roles an organism can occupy are producers, consumers, and decomposers.
  • Producers can synthesize sugars for energy from an abiotic source in processes such as the following.
    • Photosynthesis in plants and algae, where the energy in sunlight is absorbed and transformed into the chemical bonds of sugar.
    • Chemosynthesis in deep ocean bacteria, where the oxidation of inorganic compounds exuded from hydrothermal vents act as an energy source. This is done in the absence of light.
  • Consumers rely on the consumption of other organisms as a source of organic compounds, other nutrients, and energy
    • Herbivores, organisms which consume only plants, are known as primary consumers.
    • Carnivores, organisms which predate on (“eat”) other animals, are known as higher level consumers (secondary, tertiary, quaternary, etc).
  • Decomposers obtain energy and organic compounds from decaying dead organisms, recycling nutrients (mainly nitrogen and phosphorus) back to producers in the process.
  • Food chains can be used to visualize the relationship of these three groups.
  • However, ecosystems are more accurately represented by multiple combinations of food chains called a food web.
  • Roughly 90% of energy is lost through each connection in a food web due to heat and waste loss in organisms. Therefore, producers tend to be more abundant (in biomass) than herbivores, herbivores more abundant than carnivores, etc., in order for each group to have enough energy to sustain their populations. This produces what is known as an ecological pyramid.


Energy Ecosystems and the Atmosphere
Nancy Moreno, Ph.D., discusses the flow of energy through life systems and ecological pyramids.

Energy Flow though Ecosystems
The resources below provide examples for energy flow through a variety of ecosystems.


Population Dynamics

The beginning teacher identifies factors that influence the size and growth of populations in an ecosystem.

Key Concepts:

  • The size and growth of a population depends on the rates of births, deaths, immigration and emigration. If a population’s size is constant, these rates are in a state of dynamic equilibrium.
  • Factors that affect birth and death rates (and therefore population growth) can be dependent on or independent of population density (the number individuals in an amount of space).
  • Density dependent factors lead to repeating cycles in population size.
  • Principles of population ecology are used extensively in the management of wildlife. Hunting seasons, catch limits, size restrictions, and quotas used for fish, seafood, and game are all ways in which governments of the world promote healthy and sustainable population sizes for these organisms.


Population Growth
Learn how populations grow and are limited in this resource from Mr. G’s Environmental Systems.


The beginning teacher analyzes adaptive characteristics that result in a population’s or species’s unique niche in an ecosystem.

Key Concepts:

  • Niche represents the sum total of all the ways it utilizes resources in its environment: its habitat, diet, time of activity, method and time of reproduction, space utiization, and other factors.
  • If two species share the same or a similar niche, they will both compete for the same resources and the worst competitor will be driven to extinction (in that area). This is called competitive exclusion.
  • Habitats that are more complex (in food sources, prey refuges, soil substrates, etc.) have more potentially available niches, and therefore tend to have higher diversity of species of organisms.


Ecological Niches
This section of The Habitable Planet discusses discussing fundamental vs. realized niches, competitive exclusion, and the niches of generalists and specialists.

Energy Flow

The beginning teacher describes and analyzes energy flow through various types of ecosystems.

Key Concepts:

  • Ecosystems include autotrophs (organisms, such as plants, that manufacture their own food from external sources of energy) and heterotrophs (consumers, such as animals, fungi and many protists).
  • Once energy enters an ecosystem, it is passed from one organism to another by ingestion (as food) or decomposition.
  • Primary producers convert light energy or, rarely energy from chemosynthesis, into chemical bonds.
  • Consumers rely on producers for their energy sources.
  • All food chains begin with producers, followed by primary consumers, secondary consumers and tertiary consumers.


Energy Flow through Ecosystems
Energy flow in marine and terrestrial ecosystems is discussed in this resource from The Habitable Planet.

Model Ecosystems
Use this interactive module developed by McGraw Hill to learn about energy flow in forest ecosystems.

The Effect of Populations on Ecosystems

The beginning teacher knows how populations and species modify and affect ecosystems.

Key Concepts:

  • Species can affect one another and ecosystems in a variety of ways.
  • Communities tend to become more complex over time. This process, known as succession, leads to changes in soil, and the populations of organisms that are present.
  • Primary succession takes place when organisms gradually inhabit a bare substrate (such as rock), leading to the development of soil and gradual increases in the numbers of kinds and species. Over time, as conditions change, different groups of organisms become prevalent.
  • Secondary succession occurs in an area where a disturbance, such as fire, has occurred.
  • In general, early stages of succession are characterized by fast-growing (or weedy) species that tolerate extreme conditions, known as r-selected species. Gradually, these early invaders are replaced by other species (K-selected species) that compete more effectively in the environment that has been colonized (and changed) by the weedy colonizers.
  • Invasive species are those that are introduced into a new habitat, where they out compete native species that share similar niches. Invasive species can have drastic effects on biodiversity and energy flow in communities.


Ecological Succession
Ecological succession, pioneer species, and climax communities are discussed in great detail in this resource by SparkNotes.

Science Daily provides a succinct overview of ecological succession and R/K selection theory.

An Invasive Species
The asian carp, an herbivorous species of fish introduced to the Mississippi River, threatens the ecosystems and fishing industry of the Great Lakes, as reported by WEWSTV.

Algal Blooms
Algal blooms can drastically impact ecosystems by poisioning waterways or by removing dissolved oxygen from water when the algae is decomposed.

HISD Project REACH U.S. Department of Education