--> A group of living and non-living things occurring together, and the interrelationships between them
Habitat --> the place where an organism lives
Population --> All of the organisms of one species that live in the same habitat at the same time, and can interbreed
Community --> all the populations of different species that live in the same place at the same time, and can interact
Trophic level --> The level at which an organism feeds
Pyramids of numbers don't provide an accurate representation of how much living tissue exists at each level. Instead, pyramids of biomass can be drawn - area = dry mass of all organisms at that level. However different species may release different amounts of energy per unit mass - so a pyramid of energy is drawn. The organisms are burnt and a calorimeter works out the energy released per gram. These too have limitations - they only take a snapshot of the ecosystem at one point in time, and this may distort the figures for the efficiency of energy transfer.
--> The rate at which energy passes through each trophic level in a food chain
Primary productivity --> The total amount of energy fixed by photosynthesis. The net flux of carbon from the atmosphere to plants - MJ/m^2/year.
Net primary productivity --> The rate at which carbohydrate accumulates in the tissue of plants of an ecosystem. NPP = PP - respiratory heat loss
- Light levels are raised with light banks or planting crops early
- Drought resistant crops
- Greenhouses provide an appropriate temperature
- Crop rotation replenishes nitrates in the soil
- Pesticides reduce the loss of yield. Fungicides help plants resist fungal infection
- Young animals are harvested
- Selective breeding
- Antibiotics avoid unneccessary waste of energy to pathogens/parasites
- Zero grazing maximises energy allocated to muscle (meat)
--> A directional change in a community of organisms over time
Development of a community from bare ground
1. Algae and lichens begin to live on the bare rock. They're the pioneer community.
2. Erosion of the rock, and a buildup of dead/rotting organisms, provides soil for larger plants like mosses and ferns to grow. They succeed the algae and lichens.
3. In a similar way, larger plants succeed smaller plants until a final, stable climax community is reached.
Bacteria and fungi are decomposers - they break down dead and waste organic material.
They feed saprotrophically - by excreting enzymes:
1. Enzymes are secreted onto dead/waste material.
2. The enzymes digest the material into small molecules, which are then absorbed into the organism's body.
3. The molecules are stored or respired to release energy.
If bacteria and fungi did not do this, energy and nutrients would remain trapped in the dead organisms and would be wasted. They recycle the trapped nutrients.
The Nitrogen Cycle
Nitrogen-fixing bacteria (e.g Rhizobium) live inside the root nodules of leguminous plants.
They have a mutualistic relationship - the bacteria provide fixed nitrogen and the plant provides carbon compounds (e.g glucose). Proteins absorb oxygen, keeping the conditions aerobic so the bacteria can reduce nitrogen to ammonium.
Chemoautotrophic bacteria in the soil absorb ammonium ions. They oxidise them to nitrites or nitrates (in well aerated soils - this needs oxygen). Nitrates can be absorbed from the soil by plants and used to make nucleotide bases and amino acids.
Other bacteria convert nitrates back to nitrogen gas. When the bacteria are growing anaerobically, they use nitrates as a source of oxygen for respiration, producing nitrogen gas and nitrous oxide (dinitrogen oxide).
Competition happens when resources are not present in adequate amounts to satisfy the needs of all the individuals depending on them.
Between individuals of the same species. As factors become limiting, individuals have to compete. The best adapted individuals will survive and reproduce, slowing down population growth and moving into the stationary phase of the growth curve.
If population size drops, competition reduces, and the population increases.
Between individuals of different species. The more overlap there is between two species' niches, the more competition there is between the species. If there are two species whose niches are the same, one species will outcompete the other. This is known as the competitive exclusion principle.
Timber Production and Sustainability
Coppicing involves cutting a tree trunk close to the ground to encourage new growth. Once cut, several shoots grow, and eventually mature into stems. Wood can be harvested while keeping the tree alive.
Pollarding is like coppicing, but the tree is cut further up. This is used when there are a lot of deer, as they eat the emerging shoots in coppice trees.
Clear-felling of woody areas reduces soil mineral levels and causes soil run-off, polluting waterways.
Modern sustainable forestry relies on the following principles:
- Any harvested tree is replaced.
- The forest must maintain its ecological function
- Local people should benefit from the forest
Selective cutting removes only the largest, most valuable trees, so the habitat is still stable.
Foresters control pests and pathogens, only plant tree species where they will grow well, and position trees an optimal distance apart to ensure each tree supplies as much wood as possible.
The Galapagos Islands
Population size has placed huge demands on water, energy and sanitation services. More waste and pollution have been produced. An oil spill in 2001 damaged marine/coastal ecosystems, Scalesia trees and shrubs have been almost eradicated.
Over-exploitation of resources
Whales and seals were harvested by whaling boats faster than they could replenish themselves. Giant tortoises were taken to eat. Depletion of sea cucumbers has damaged underwater ecology, and many sharks are harvested for their fins.
The goat has eaten Galapagos rock purslane, which was native. They outcompete the giant tortoise for food, and change the habitat to reduce tortoise nesting sites. They also lead to soil erosion. Cats hunt the lava lizard and young iguanas.