Biotic vs. Abiotic
Organisms with similar needs may compete with one another for resources, including food, space, water, air, and shelter. In any particular environment, the growth and survival of organisms depend on the physical conditions including light intensity, temperature range, mineral availability, soil type, and pH.   Physical or non-living factors such as these which influence living things are called abiotic factors.   Living factors which influence living things are called biotic factors.   Some examples of biotic factors include disease and predation.

Energy Flow
Energy flows through ecosystems in one direction, typically from the Sun, through photosynthetic organisms or producers, to herbivores to carnivores and decomposers.   The chemical elements that make up the molecules of living things pass through food webs and are combined and recombined in different ways. At each link in a food web, some energy is stored in newly made structures but much energy is lost into the environment as heat. Continual input of energy from sunlight is required to keep this process going.   Energy pyramids are often used to show the flow of energy in ecosystems.

Material Cycles
The atoms and molecules on the Earth cycle among the living and nonliving components of the biosphere. Carbon dioxide and water molecules used in photosynthesis to form energy-rich organic compounds are returned to the environment when the energy in these compounds is eventually released by cells through the processes of cell respiration and other life activities.   The number of organisms any environment can support is called its carrying capacity.  The carrying capacity of an environment is limited by the available energy, water, oxygen, and minerals, and by the ability of ecosystems to recycle the remains of dead organisms through the activities of bacteria and fungi.  Living organisms have the capacity to produce populations of unlimited size, but available resources in their environments are finite. This restricts the growth of populations and produces competition between organisms.

Organism Relationships
Organisms interactions may be competitive or beneficial.  Organisms may interact with one another in several ways.  Some of these relationships include producer/consumer, predator/prey, or parasite/host relationships. Other organisms interactions include those in which one organism may cause disease in, scavenge, or decompose another.

Biodiversity
Due to evolution, there is a great number of different organisms which fill many different roles in ecosystems.  The number of different organisms in an ecosystem is called biodiversity.  Increased biodiversity increases the stability of the ecosystem.   Biodiversity also ensures the availability of diverse genetic material that may lead to future discoveries with significant value to humans.  As diversity is lost, potential sources of these materials for these discoveries may be lost with it.  A great diversity of species provides for variations which increase the chance that at least some living things will survive in the face of large changes in the environment.

Ecological Succession
The environment may be changed greatly through the activities of organisms, including humans, or when climate changes. Although sometimes these changes occur quickly, in most cases species gradually replace others, resulting in long term changes in ecosystems.    These changes in an ecosystem over time are called ecological succession.   Ecosystems may reach a point of stability that can last for hundreds or thousands of years.  If a disaster occurs, the damaged ecosystem is likely to recover in stages that eventually result in a stable system similar to the original one.

This page and the links at the left are designed to aid students in reviewing the following topics pertaining to ecology which will appear on the New York State Living Environment Regents Examination; biotic vs. abiotic factors, energy flow, material cycles, organism relationships, biodiversity, and ecological succession.   In addition, students may test their knowledge of the material presented here by accessing multiple-choice questions from past Regents Exams.