F215 - Module 3 - Ecosystems

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Ecosystems

Population - all the organisms of the same species living in the same area at the same time and can interbreed together

Niche - the role of a species in a community - interactions and position in the food chain

Ecosystem - a self-contained community together with all the physical features that influence the community and the interactions between organisms and their environment

Producer - an organism that converts simple inorganic compounds into complex organic compounds

Consumer - an organism that gains energy from complex organic matter

Decomposer - an organism that feeds on waste from other organisms or on dead organisms

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Energy flow

Only a small part of energy entering a trophic level becomes available to the next trophic level.
Energy is lost in food chains because animals:

  • never eat all the available food
  • cannot digest all the food they eat
  • use energy in their respiration so they can move, hunt, chew, reproduce etc.
  • lose heat energy to their surroundings
  • lose energy in urine and faeces - this energy may pass to decomposers

Energy transfers can be calculated from samples of organisms. Each sample is dried and then burned in oxygen in a bomb calorimeter. The heat energy produced passes to a known mass of water and the temperature rise of the water is measured.

Human activities manipulate the flow of energy through an ecosystem:

  • replacing natural vegetation with crops and livestock
  • deflecting natural succession to maintain grassland
  • increasing productivity of producers through soil improvement, irrigation, fertilisers and removal of competing weeds and damaging pathogens and pests
  • increasing productivity of organisms through selective breeding or genetic engineering
  • sheltering organisms from damaging environmental factors
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Succession

Succession is a change in the structure and species composition of a community.

The changes that result when the starting point is bare, uncolonised land are called primary succession, which occurs in sand dunes, lava and ash from volcanic eruptions, landslides, and land and lakes left by retreating glaciers.

The first colonisers on bare rock are lichens and mosses (pioneer species), allowing soil to build up and plants to appear. In temperate climates, woodland is the endpoint of succession, and it is called the climax community.

When an established community is destroyed, a new community develops and this is called secondary succession.

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Studying ecosystems

Quadrats:

  • find the percentage cover
  • count the numbers of individuals present and finding the average number per quadrat or species density
  • find the species frequency by the proportion of quadrats with that species present

Point Quadrats - frames through which long pins are lowered and each species which touches a pin is recorded

Line transect - a line across a habitat. All species touching the line are identified and their position on the line recorded. Suitable for habitats with graduations in conditions.

Belt transect - quadrats placed at intervals along a line transect

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Decomposers and the nitrogen cycle

Decomposition - saprotrophs secrete enzymes onto dead and waste material which digest the material into smaller molecules which are then absorbed into the organism's body, where they are stored or respired to make energy.

Nitrogen fixation - nitrogen gas --> ammonia (Rhizobium)

Nitrification - ammonia --> nitrite (Nitrosomonas)
                     nitrite --> nitrate (Nitrobacter)

Denitrification - nitrate --> nitrogen gas

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Population size

Carrying capacity - the maximum population size that can be maintained over a period of time in a particular habitat.

Limiting factors - factors that limit the growth of population size including the availability of resources such as food, water, light, oxygen, nesting sites or shelter; the effects of other species such as parasites or predators; or the intensity of competition for resources.

The carrying capacity is the upper limit that limiting factors place on the population size.

Predation can act as a limiting factor on a prey's population size, which can in turn affect the predator's population size.

  • when the predator population grows bigger, more prey are eaten
  • the prey population then gets smaller, leaving less food for the predators
  • with less food, fewer predators can survive and their population size reduces
  • with fewer predators, fewer prey are eaten and their population size increases
  • with more prey, the predator population gets bigger and the cycle starts again
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Competition

If a resource in an ecosystem is in short supply, there will be competition between organisms for that resource.

Intraspecific competition - between individuals of the same species. Individuals best adapted to obtaining food will survive to reproduce while others will not. There are slight fluctuations in population size but overall it remains relatively stable; if the population size drops, competition reduces and the population size increases. If the population size increases then competition increases and the population size drops.

Interspecific competition - between individuals of different species, and can affect the population size of a species and its distribution within an ecosystem.

Competitive exclusion principle - if two species have exactly the same niche, one would be out-competed by the other and become extinct. But this isn't always true:

  • sometimes one population may just remain much smaller than the other
  • a wide range of variables may act as limiting factors for the growth of different populations.
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Sustainable management

Small-scale timber production:

  • Coppicing - involves cutting a tree trunk close to the ground to encourage new growth
  • Pollarding - cutting the trunk higher up. This is useful when the population of deer is high, as they like to eat the new shoots on a coppiced tree, if cut higher up, the deep cannot reach the shoots.
  • Rotational coppicing - dividing a wood into sections and cut one section each year until they've all been cut. By the time they want to coppice the first section again, the new stems have matured and are ready to be cut.

Large-scale timber production:

  • any tree which is harvested is replaced by another tree
  • if each tree supplies more wood, then fewer trees need to be harvested:
    - control pests and pathogens
    - only plant particular tree species where they know they will grow well
    - position trees at an optimal distance apart. If trees are too close they will compete for light and space and will grow tall and thin, producing low-quality timber
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Conservation

Conservation - the maintenance of biodiversity, including diversity between species, genetic diversity within species, and maintenance of a variety of habitats and ecosystems.

A steadily increasing human population can threaten biodiversity through:

  • over-exploitation of wild populations for food, sport, and commerce: species are harvested faster than they can reproduce
  • habitat disruption and fragmentation as a result of more intensive agricultural practices, increased pollution or widespread building
  • species introduced to an ecosystem by humans which may out-compete native species which may become extinct

Economic and social reasons for conservation:

  • many species provide a valuable food source and were originally domesticated from wild species. Genetic diversity in wild strains may be needed in the future to breed for disease resistance, drought tolerance or increased yield. New species may be domesticated for food use.
  • natural environments are a valuable source of potentially beneficial resources. Many of the drugs we use today were discovered in wild plant species
  • natural predators and pests can act as biological control agents

Preservation - protecting areas of land as yet unused by humans, in their 'untouched' form

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The Galapagos Islands

Habitat disturbance:

  • Conversion of land - forests of scalesia trees have been eradicated to make way for agricultural land
  • Demand for oil has increased - oil spills have an adverse effect on marine life
  • More waste and pollution has been produced from building and increased populations

Over-exploitation of resources:

  • sharks for their fins
  • giant tortoises were taken as they could survive with little food on ships before being killed and eaten - the last remaining tortoise, Lonesome George, died in 2012.
  • harvesting whales and seals to sell, faster than they could breed
  • fishing for exotic species

Introduced species (non-native):

  • those that come with humans such as cats, insects, fruit & veg and goats
    - goats eat native species, out-compete the giant tortoise for grazing, and transformed the forest into grassland leading to soil erosion
  • may out-compete native species or prey on them - bring diseases/hunt species
  • may destroy habitats such as nesting sites for birds
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