Populations In Ecosystems (A Level Only)
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課程筆記
Ecosystems and Key Ecological Terms
- An ecosystem is a community and its interactions with the non-living (abiotic) factors in the environment.
- Within an ecosystem, there is a flow of energy and nutrients are recycled (e.g. in the carbon, nitrogen and phosphorus cycles).
- A population is a group of organisms of the same species living in a particular space at a particular time that can potentially interbreed.
- A community is multiple populations living and interacting in the same area; species depend on each other for food, shelter and pollination, so removing one species can affect the whole community (interdependence).
- A habitat is the local environment in which a species normally lives.
- A niche is the role an organism plays in its ecosystem, including its use of resources, responses to abiotic factors and interactions with biotic factors; each species has a unique niche.
- If the niches of two species overlap, they compete; one may outcompete the other (which may adapt to a new niche or become locally extinct), or both may survive with smaller population sizes.
Biotic and Abiotic Factors
- Biotic factors are the living components of an ecosystem that affect the survival and reproduction of organisms, such as predation, competition, disease and food availability.
- Abiotic factors are the non-living components of an ecosystem that affect living organisms, such as temperature, light intensity, pH, water availability and mineral ions.
- Abiotic factors can limit population size; for example, if environmental temperature is far from a mammal's optimum, more energy is used for homeostasis, leaving less for growth and reproduction, so fewer reach reproductive age.
- Biotic factors include interspecific competition, intraspecific competition and predation.
- Interspecific competition occurs when different species compete for the same resources; if one species is better adapted it may outcompete the other (e.g. grey squirrels outcompete red squirrels in the UK because they eat a wider range of food and have better fat storage in winter).
- Intraspecific competition is competition for the same resources between individuals of the same species; when resources become limiting, the population stabilises at the carrying capacity.
- In a stable community, predator-prey populations cycle: prey numbers rise, predator numbers rise, prey numbers fall, predator numbers fall, and the cycle repeats.
Carrying Capacity
- Carrying capacity is the maximum stable population size of a species that an ecosystem can support.
- Abiotic and biotic factors limit survival and reproduction, so population size is limited at some point (the carrying capacity is reached).
- Most species remain below or at carrying capacity due to these limiting factors; humans are a possible exception as we have overcome many natural limitations.
- On a population growth graph, the point at which the graph flattens out (plateau) represents the carrying capacity.
- At carrying capacity, environmental factors prevent all individuals from surviving and reproducing, so the population can no longer grow in size.
Estimating Population Size: Sampling Methods
- Sampling is a method of investigating the abundance and distribution of species and populations.
- Random sampling uses sampling points chosen completely at random, avoiding bias by the person carrying out the sampling.
- Systematic sampling uses sampling points chosen by the person, which may introduce bias (e.g. placing quadrats in areas with fewer species because they are easier to count).
- Random sampling is best when the area is reasonably uniform or species distribution has no clear pattern; systematic sampling is more appropriate when there are changes in physical conditions (e.g. altitude, soil pH, light intensity).
- Frame quadrats are used for non-motile or slow-moving species; quadrats must be laid randomly, often by converting the area into a grid and using a random number generator to pick sample points.
- Species frequency is the probability of finding a species within any quadrat: (number of quadrats containing the species ÷ total number of quadrats) × 100.
- Percentage cover can be estimated by dividing the quadrat into 100 smaller squares and counting how many squares the species is found in.
- Belt transects involve placing quadrats at regular intervals along a tape measure to show how distribution or abundance changes with physical conditions; this produces quantitative data.
Estimating Population Size: Mark-Release-Recapture
- The mark-release-recapture method is used for motile animals.
- Technique: capture a large sample, mark individuals in a way that does not affect survival (e.g. non-toxic paint on a beetle's carapace), release them to mix randomly, then capture a second sample and count marked and unmarked individuals.
- Population estimate is calculated using: N = (n₁ × n₂) ÷ m, where N = population estimate, n₁ = number of marked individuals released, n₂ = number in the second sample, and m = number of marked individuals in the second sample.
- Assumptions: marked individuals disperse and mix fully; marking does not affect survival (e.g. does not make them more visible to predators); marking remains visible; and the population size stays the same during the study (no significant births, deaths or migration).
Succession
- Succession is the gradual change in an ecosystem over time, from a simple to a more complex structure, involving shifts in both biotic and abiotic conditions.
- Succession makes the environment less hostile and more suitable for new species, while becoming less suitable for previous species, so biodiversity changes continually.
- Primary succession occurs on newly formed or exposed land with no initial life (e.g. bare rock from cooled lava or dried-up lakebeds).
- The first species to colonise are pioneer species (often moss and lichens); their death and decay form basic soil (humus).
- Seeds of small plants and grasses then grow, further increasing soil depth and nutrient content; their roots hold soil in place and prevent it being washed away.
- Larger plants, shrubs and small trees can then grow in the less hostile conditions (deeper soil, more nutrients, more water), and finally large trees can be supported.
- The final, dominant species form a climax community – a stable, complex ecosystem with a variety of plant and animal species.
Conservation and Management of Succession
- Human activities such as mowing and grazing interrupt succession, maintaining ecosystems in earlier stages.
- Mowing prevents shrubs and trees from establishing, so only grasses persist; grazing livestock eat new shoots, halting succession and maintaining grass-dominated areas.
- Conservation often involves halting succession to protect species diversity, especially where intermediate stages (e.g. grassland, heathland) support many species not found in climax communities.
- Dominant species in climax communities can outcompete other species, or changes in abiotic conditions can make the environment unsuitable for some species.
- Intermediate habitats can be important for rare or threatened species, including pollinators like bees.
- Methods to prevent succession include grazing (introducing animals to eat tree/shrub shoots) and managed burning (controlled fires remove woody plants, allowing species like heather to regrow and resetting succession).
- For example, Scottish moorlands would naturally progress to spruce forest via succession, but management maintains both climax forests and earlier moorland to maximise species diversity.
Growth Rate of Microorganisms
- The population growth rate of microorganisms (e.g. bacteria or yeast) can be investigated by growing them in a broth culture and measuring the turbidity (cloudiness) of the suspension.
- As microorganisms reproduce and their population grows, the suspension becomes progressively more turbid, so less light passes through.
- Turbidity can be monitored using a turbidity meter, light sensor or colorimeter connected to a datalogger, taking measurements at fixed time intervals after inoculation.
- Results can be used to plot a population growth curve showing how the population grew over time.
- Bacterial colonies can grow rapidly, producing very large numbers within hours, which makes traditional linear scales difficult to use on graphs.
- Logarithmic scales increase by powers of 10 (e.g. 10² = 100, 10³ = 1000), allowing large changes in data to be shown on a compressed axis.
- On a log scale, intervals on the y-axis are not evenly spaced; to read values, identify powers of 10, estimate between them using known log₁₀ values (e.g. log₁₀(320) ≈ 2.5), or use a calculator.
- The pH scale is also logarithmic, as hydrogen ion concentration varies massively between each pH level.
Conservation and Human Need
- Humans use Earth's resources (land, water, wood, fossil fuels) for buildings, agriculture, fuel and electricity; as population and economic development increase, demand for these resources increases.
- This leads to environmental damage affecting ecosystems, climate and biodiversity, creating a conflict between human needs and conservation.
- Conservation involves managing species and habitats sustainably, meeting present needs without compromising the future.
- Conservation of habitats frequently involves management of succession; some oppose this due to short-term economic impacts, but careful resource management is essential to balance current use with long-term sustainability.
- Methods of conservation include national and marine parks (legal restrictions on access, development and hunting), public engagement (tourism income, local jobs, community support), zoos (captive breeding and research), botanic gardens (conserving rare plants, reintroduction, research and education), frozen zoos (preserving genetic material) and seed banks (storing plant diversity for future restoration).
- When evaluating data on conservation issues, consider whether a control is present, whether sampling was random (to avoid bias), the length and size of the study, and whether results from multiple studies agree before drawing firm conclusions.
- For example, studies on signal crayfish and white-clawed crayfish show that removal of an invasive species may or may not help native species; conflicting evidence and methodological limitations must be considered.
Required Practical: Investigating Species Distribution
- The distribution of a species depends on abiotic factors (e.g. light intensity, nutrient concentration, temperature) and biotic factors (e.g. competition, predators, pathogens).
- Frame quadrats are used for random sampling when there are no clear changes in physical conditions; belt transects are used for systematic sampling when there are obvious changes (e.g. altitude, light intensity).
- Apparatus includes a quadrat, two tape measures (at least 5 m), apparatus to measure the abiotic factor (e.g. digital thermometer, pH meter, photometer) and a random number generator.
- Random sampling method: set up two tape measures as x and y axes to form a 5 m × 5 m grid, generate 10 sets of random coordinates, place the quadrat with its bottom left corner on each coordinate, record percentage cover and measure the abiotic factor.
- Systematic sampling method: create a transect with a tape measure, place quadrats at regular intervals (e.g. every 1 m), record percentage cover and measure the abiotic factor.
- Plot a graph of percentage cover (dependent variable) against the abiotic factor (independent variable); a correlation indicates the effect of the abiotic factor on distribution.
- Statistical tests such as Spearman's Rank Correlation can assess how strong the correlation is.
- Correlation does not always mean causation; other abiotic or biotic factors (e.g. soil pH, pollinators) may influence results, so further investigation may be needed before drawing conclusions.
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練習題
免費預覽——58 題中的 8 題。註冊即可查看全部。
1.Which of the following best defines a community?
Easy- AA group of organisms of the same species living in a particular space at a particular time that can potentially interbreed
- BMultiple populations living and interacting in the same area
- CA community and its interactions with the non-living factors in the environment
- DThe role an organism plays in its ecosystem, including its use of resources and interactions with biotic and abiotic factors
2.Which of the following are examples of abiotic factors that can influence living organisms? (select all that apply)
Easy- ATemperature
- BPredation
- CSoil pH
- DCompetition
- ELight intensity
3.It is not possible for two species to occupy the same niche within an ecosystem. Which statement best explains why?
Easy- AOne species will always be better adapted and will outcompete the other, leading to competitive exclusion or local extinction of the less successful species.
- BThe two species will interbreed and produce infertile offspring, reducing both populations.
- CThe carrying capacity of the ecosystem will increase to accommodate both species indefinitely.
- DThe two species will develop identical adaptations and become a single species over time.
4.A niche includes an organism's use of resources, its responses to abiotic factors, and its interactions with biotic factors.
EasyTrue or false?
5.Match each ecological term with its correct definition.
Medium- Population
- Community
- Ecosystem
- Habitat
- A group of organisms of the same species living in a particular space at a particular time that can potentially interbreed
- Multiple populations living and interacting in the same area
- A community and its interactions with the non-living factors in the environment
- The local environment in which a species normally lives
6.Which of the following is the best definition of carrying capacity?
Medium- AThe maximum stable population size of a species that an ecosystem can support
- BThe total number of individuals of all species that an ecosystem can support
- CThe rate at which a population grows when resources are unlimited
- DThe number of individuals that migrate into a population each year
7.In a mark-release-recapture study, 187 insects were caught, marked, and released. Later, 230 insects were caught, of which 84 were marked. Use the formula N = (n1 × n2) ÷ m to estimate the population size. Which value is closest to your estimate?
Medium- A512
- B612
- C412
- D712
8.Which of the following are assumptions that must be met for a mark-release-recapture estimate to be valid? (select all that apply)
Medium- AMarked individuals mix randomly back into the population
- BThe marking does not affect the survival of the marked individuals
- CThe population size changes significantly during the study period due to births and deaths
- DThe marking remains visible throughout the sampling period
- EThe marked individuals are all caught again in the second sample
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