Organisms and Populations
Organisms and Their Environment
Ecology is the study of how organisms interact with one another and with their surroundings. The place where an organism lives is its habitat, and everything around it makes up its environment. The environment has two kinds of components: abiotic (non-living) factors — such as temperature, water, light and soil — and biotic (living) factors — other organisms like predators, prey, competitors and parasites.
Organisms must cope with their abiotic conditions, and over time they develop adaptations — special features that help them survive and reproduce in their habitat. Some examples:
- Desert plants (like cactus) have thick stems to store water, spines instead of leaves, and a thick waxy surface to reduce water loss.
- Desert animals (like the kangaroo rat) can survive on very little water and stay in burrows during the day.
- Polar animals have thick fur or fat to keep warm; small ears and limbs reduce heat loss.
- Some animals migrate or go into a resting state to escape harsh seasons — hibernation (winter sleep) and aestivation (summer dormancy).
Where an organism cannot change its surroundings, it adjusts its own body (physiology) or behaviour to manage. These adaptations show how living things are wonderfully matched to their environment.
One is non-living, one is living.
- Abiotic: non-living factors (temperature, water, light, soil).
- Biotic: living factors (predators, prey, competitors).
Deserts are hot and dry.
- Thick stem to store water; spines instead of leaves.
- Thick waxy surface to reduce water loss.
It helps escape harsh seasons.
- Hibernation is a resting (dormant) state in which some animals pass the cold winter.
Key Points
- Ecology = study of organisms and their interactions with surroundings; habitat + environment.
- Environment: abiotic (temperature, water, light, soil) + biotic (predators, prey, competitors).
- Adaptations: desert plants (water-storing stem, spines, waxy), polar animals (fur/fat), hibernation/aestivation.
Population Attributes and Growth
Organisms of the same species living together in a particular area form a population. Studying populations helps us understand how species survive, grow and decline. A population has several measurable attributes that an individual does not:
- Population density — the number of individuals per unit area (or volume).
- Birth rate (natality) and death rate (mortality) — the rate at which individuals are born and die.
- Sex ratio — the proportion of males to females.
- Age structure — the proportion of individuals of different ages (young, adult, old), often shown as an age pyramid, which can predict whether a population will grow, stay stable or decline.
Populations change in size through births and immigration (which add individuals) and deaths and emigration (which remove them). When food and space are unlimited, a population can grow very fast in a J-shaped (exponential) curve. But in nature, resources are limited; the largest population an environment can support is its carrying capacity (K). As the population nears K, growth slows and levels off, giving an S-shaped (sigmoid / logistic) curve. The S-shaped curve is more realistic because no environment has unlimited resources.
It is a group of one species.
- A population is a group of organisms of the same species living together in a particular area.
These apply to groups, not individuals.
- Population density and birth/death rates (also sex ratio, age structure).
It is a limit set by resources.
- Carrying capacity (K) is the largest population an environment can support with its resources.
Key Points
- Population = same species in an area; attributes: density, natality/mortality, sex ratio, age structure (age pyramid).
- Size changes by births + immigration (add) vs deaths + emigration (remove).
- Unlimited resources → J-shaped (exponential); limited → S-shaped (logistic) levelling at carrying capacity (K).
Population Interactions
No species lives alone; populations interact with one another in many ways. These population interactions can help or harm the species involved:
- Predation — one organism (the predator) kills and eats another (the prey), e.g. a lion eating a deer. Predators control prey numbers and keep ecosystems balanced. (Herbivores eating plants are like predators of plants.)
- Competition — two organisms struggle for the same limited resource (food, space, light), e.g. two plants competing for sunlight. Both are harmed.
- Parasitism — one organism (the parasite) lives on or in another (the host) and harms it, e.g. ticks on a dog, or tapeworm in humans. The parasite benefits, the host is harmed.
- Mutualism — both species benefit, e.g. bees and flowers (bees get nectar, flowers get pollinated), or lichens (an alga and a fungus living together).
- Commensalism — one benefits and the other is unaffected, e.g. an orchid growing on a tree branch for support.
A useful way to summarise these is by who is helped (+), harmed (−) or unaffected (0): predation and parasitism are (+ −), competition is (− −), mutualism is (+ +), and commensalism is (+ 0). These interactions weave organisms into the web of life and keep nature in balance.
One organism eats another.
- Predation is when a predator kills and eats prey.
- Example: a lion eating a deer.
Both partners benefit.
- Bees and flowers: bees get nectar, flowers get pollinated.
- Both benefit, so it is mutualism.
One gains at the other's cost.
- The parasite benefits; the host is harmed.
Key Points
- Predation (+−): predator kills prey (lion–deer); controls numbers.
- Competition (−−): struggle for the same resource.
- Parasitism (+−): parasite benefits, host harmed (tick, tapeworm).
- Mutualism (++): both benefit (bee–flower, lichen); commensalism (+0): one benefits, other unaffected (orchid on tree).