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Summary Notes on Ecosystems
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A -levels notes on ecosystems
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EcosystemsHabitat – the place where an organism lives
Population – all of the organisms of one species, who live in the same place at the same time, and who can
breed together
Community – all of the populations of different species, who live in the same place at the same time, and who
can interact with each other
Ecosystem – a community of animals, plants and bacteria interrelated with the physical and chemical
environment
e.g. large scale (African grassland), medium scale (playing field), small scale (rock pool, large tree)
The role of each species in an ecosystem is its niche.
how and what it feeds on, what it excretes and how it reproduces
it is impossible for 2 species to occupy exactly the same niche in the same ecosystem
Factors affecting ecosystems:
Biotic factor: environmental factors associated with living organisms in an ecosystem that affect each other
Producers: plants (and some photosynthetic bacteria), which supply chemical energy to all other
organisms
Consumers: primary consumers are herbivores, which feed on plants, and which are eten by
carnivorous secondary consumers. These are then eaten by carnivorous tertiary consumers
Decomposers: bacteria, fungi and some animals feed on waste material or dead organisms
Abiotic factors: non-living components of an ecosystem that affects other living organisms
pH, relative humidity, temperature, concentration of pollutants
Ecosystems are dynamic
Cyclic changes: repeat themselves in a rhythm
e.g. movement of tides and changes in a day length ; predator and prey population interactions
Directional change: go in one direction and tend to last longer than the lifetime of organisms within the
ecosystem
e.g. the deposition of silt in an estuary, or the erosion of coastline
Unpredictable/erratic changes
e.g. lightning or hurricanes
, Population size and carrying capacity
-the balance between the death rate and the rate of reproduction determines the size of a population
Lag phase – only a few individuals acclimatizing to their habitat; the rate of reproduction is low and the growth
of the population size is slow
Log phase – resources are plentiful with good environmental conditions; reproduction can happen quickly, with
the rate of reproduction > mortality; the population size increase rapidly
Stationary phase – the population size has levelled out at the carrying capacity of the habitat; the rates of
reproduction and mortality are equal; the population size is stable or fluctuates slightly due to small variations
in environmental conditions each year
Limiting factors:
-density independent: act irrespective of size of the population e.g. low temperature
-density dependent: act more strongly as population size increases e.g. availability of food, water, light, O 2,
shelter, levels of parasitism and predation, intensity of competition for resources
r-Strategies and k-Strategies represent 2 ends of a continuum of strategies adopted by living thing:
k–Strategists: species whose population is determined by the carrying capacity
e.g. birds, larger mammals, larger plants
limiting factors exert an increasingly as the populations get closer to the carrying capacity, causing the
population size to get closer to the carrying capacity
low reproductive rate, slow development, late reproductive age, long lifespan, large body mass
r–Strategist: species whose population is determined by the physical rate (r) at which individuals can reproduce
e.g. bacteria, pioneer species
the population size increase so quickly that it can exceed the carrying capacity of the habitat before the
limiting factors start to have an effect
high reproductive rate, quick development, young reproductive age, short life span, small body mass
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