CSEC Biology notes on mutualism, parasitism, commensalism, adaptations, nutrient cycles, population growth, limiting factors, and carrying capacity.
This page brings together three parts of ecology: relationships among organisms, recycling of nutrients, and changes in population size.
Organisms interact in many ways. Three important relationships are mutualism, parasitism, and commensalism.
| Relationship | Species 1 | Species 2 | Example |
|---|---|---|---|
| Mutualism | benefits | benefits | Rhizobium and legume |
| Parasitism | benefits | harmed | tick and dog |
| Commensalism | benefits | unaffected | orchid and tree |
Mutualism is a relationship in which both organisms benefit.
Examples include Rhizobium bacteria in legume root nodules, termites and Trichonympha, and clownfish with sea anemones. In each case, both organisms gain something from the relationship.
Parasitism is a relationship in which one organism benefits and the other is harmed.
The parasite benefits. The host is harmed.
Examples include ticks feeding on mammals, tapeworms living in the intestine, Plasmodium causing malaria, and dodder growing on another plant.
Commensalism is a relationship in which one organism benefits and the other is neither helped nor harmed.
Orchids growing on trees show commensalism: the orchid gains support and better access to light, while the tree is not harmed.
Predators and prey often have features that help them survive.
Predators may have sharp claws, sharp teeth or beaks, forward-facing eyes for judging distance, speed, camouflage, venom, or stings. Prey may have camouflage, side-facing eyes for a wider field of view, speed, group behaviour, poisons, shells, spines, or other protective structures.
A hawk has forward-facing eyes, which helps it judge distance accurately when catching prey.
Nutrients are reused in ecosystems. This cycling helps maintain balance.
The main cycles for this section are:
Carbon moves between the atmosphere, organisms, dead matter, oceans, and fossil fuels.
Key processes:
The water cycle moves water between the atmosphere, land, and living organisms.
Key processes:
Nitrogen is needed to make proteins and DNA. Most organisms cannot use nitrogen gas directly, so bacteria are essential.
Main stages:
Decomposers link feeding relationships to nutrient cycles by returning minerals to the soil so producers can keep growing.
A population is affected mainly by birth rate, death rate, immigration, and emigration.
Population size increases when birth rate or immigration increases, or when death rate or emigration decreases. Population size decreases when death rate or emigration increases, or when birth rate or immigration decreases.
Carrying capacity is the maximum population size that an environment can support over time.
As a population grows, limiting factors become stronger.
Examples:
If a population graph levels off, it has likely reached carrying capacity. The population is no longer growing rapidly because limiting factors are controlling it.
Humans are affected by the same basic limits as other organisms, but technology allows humans to reduce some limiting factors.
Humans have increased food supply through agriculture, reduced disease through medicine and sanitation, and created more living space through building and land clearing.
However, human population growth still increases pressure on:
Population questions may use graphs. For example, a population may fall sharply after a hurricane, drought, disease outbreak, pest invasion, or introduction of a new predator.
Organisms interact with each other, nutrients cycle through ecosystems, and population size changes when birth, death, movement, and limiting factors change.