Biodiversity & Ecosystems

The biosphere is the part of Earth where life exists - from deep ocean vents to mountain peaks. Within the biosphere, organisms live in specific ecosystems.

An ecosystem = biotic community + abiotic environment. Examples include a woodland, a pond, a rocky seashore, or a hedgerow.

Biotic factors are living influences: competition, predation, parasitism, disease, symbiosis, and interdependence (e.g. bees pollinating flowers).

Abiotic factors are non-living influences: temperature, light intensity, pH, soil moisture, humidity, wind speed, mineral content, and dissolved oxygen.

Every ecosystem has a carrying capacity - the maximum population size an environment can sustain indefinitely. If a population exceeds carrying capacity, resources become scarce, leading to competition, disease, or emigration.

Key concept: Organisms do not exist in isolation. Every living thing depends on other organisms for food, shelter, reproduction, or nutrient cycling.

A food chain shows the flow of energy from one organism to another. Each step is a trophic level.

Trophic levels:

• Producers (autotrophs) - plants/algae that photosynthesise
• Primary consumers (herbivores) - eat producers
• Secondary consumers (carnivores) - eat primary consumers
• Tertiary consumers (top predators) - eat secondary consumers
• Decomposers - bacteria and fungi that break down dead matter

The 10% rule: Only about 10% of energy at one trophic level is available to the next. The rest is lost as heat through respiration, movement, and excretion.

There are two types of food chain:

• Grazing food chains begin when a herbivore eats a plant.
• Detrital food chains begin with decaying plant or animal material.

A food web is a set of interconnected food chains showing the complex feeding relationships in an ecosystem.

Pyramids of numbers illustrate the population at each trophic level. They can be inverted (e.g. one tree supporting many insects).

An adaptation is any inherited feature - structural, physiological or behavioural - that helps an organism survive and reproduce in its habitat. Adaptations arise gradually through natural selection.

• Structural - a hawk's hooked beak and sharp talons for gripping prey.
• Physiological - a camel's ability to concentrate urine to conserve water.
• Behavioural - swallows migrating south each autumn to avoid winter food shortages.

An organism's niche is the specific job it does in an ecosystem: what it eats, where it lives, when it is active, and how it interacts with other species. Two species with an identical niche cannot coexist for long - one will outcompete the other, or they will evolve to use slightly different resources (niche partitioning).

Carrying capacity is the maximum population an environment can sustain over time without damaging its resources. It is shaped by:

• Availability of food and water
• Physical space and shelter
• Climate and weather
• Predators and disease
• Reproductive rate of the species
• Human activity - habitat loss, pollution and agriculture can push carrying capacity down

When a population exceeds its carrying capacity, competition intensifies, disease spreads, and numbers crash back down. Populations therefore tend to fluctuate around a stable equilibrium rather than growing forever.

Every organism competes for limited resources: food, water, light, mates, space. There are two kinds of competition:

• Intraspecific - between members of the same species. Usually the most intense form because individuals need identical resources.
• Interspecific - between different species sharing a resource, e.g. red and grey squirrels competing for hazelnuts.

Predation is a feeding relationship in which one organism (the predator) kills and eats another (the prey). Predator and prey populations often rise and fall in coupled cycles: prey numbers grow → predator numbers grow → prey numbers fall → predator numbers fall.

Symbiosis means "living together". Three main types are recognised:

• Mutualism - both partners benefit, e.g. bees getting nectar while pollinating flowers, or Rhizobium bacteria fixing nitrogen in the root nodules of clover.
• Commensalism - one benefits, the other is unaffected, e.g. barnacles hitching a ride on a whale.
• Parasitism - one benefits, the other is harmed, e.g. a tapeworm in a mammal's gut, or mistletoe on an oak tree.

Interdependence describes the wider web of these relationships - remove one keystone species and the whole community can shift. Reintroducing wolves to Yellowstone in 1995 famously changed grazing patterns, allowed young trees to regrow, and even altered how rivers flowed.