19.2 - Variation In Population Size Flashcards
The number of individuals in a population is the…
Population size
Populations are dynamic because…
they vary in size and composition over time.
Imagine a situation in which a single photosynthetic bacterial cell, capable of asexual reproduction, is placed in a newly created pond. It is summer and so there is plenty of light and the temperature of the water is around 12°C — mineral nutrients have been added to the water. In these circumstances the bacterial cell divides rapidly because all the factors needed for the growth of the population are present.
1) there is therefore….
2) how may things change over time?
1) no limiting factors
2)
- Mineral ions are consumed as the population becomes larger.
- The population becomes so large that the bacteria at the surface prevent light reaching those at deeper levels.
- Other species are introduced into the pond, carried by animals or the wind, and some of these species may use the bacteria as food or compete for light or minerals.
- Winter brings much lower temperatures and lower light intensity of shorter duration.
Changes in abiotic/biotic factors will influence the…
rate of growth and the size of the population.
In summary, no population continues to grow indefinitely because…
- certain factors limit growth,
- e.g. the availability of food, light, water, oxygen and shelter, and the accumulation of toxic waste, disease and predators.
What is carrying capacity
Each population has a certain size (the carrying capacity) that can be sustained over a relatively long period and this is determined by limiting factors.
What are some of the abiotic conditions that influence the size of a population
- temp
- light
- pH
- humidity/water
How does temp influence the size of populations (specifically cold-blooded animals + plants)
- Each species has a different optimum temperature at which it is best able to survive.
- The further away from this optimum, the fewer individuals in a population are able to survive and the smaller is the population that can be supported.
- In plants and cold-blooded animals, as temperatures fall below the optimum, the enzymes work more slowly and so their metabolic rate is reduced.
- Populations therefore have a smaller carrying capacity.
- At temperatures above the optimum, the enzymes work less efficiently because they gradually undergo denaturation.
- Again the population’s carrying capacity is reduced.
How does light influence the size of populations
- The rate of photosynthesis increases as light intensity increases.
- The greater the rate of photosynthesis, the faster plants grow and the more spores or seeds they produce.
- Their carrying capacity is therefore potentially greater.
- In turn, the carrying capacity of animals that feed on plants is potentially larger.
How does pH influence size of populations
- This affects the action of enzymes.
- Each enzyme has an optimum pH at which it operates most effectively.
- A population of organisms is larger where the appropriate pH exists and smaller, or non-existent, where the pH is different from the optimum.
How does temp influence size of populations (warm blooded animals)
- The further away from this optimum, the fewer individuals in a population are able to survive and the smaller is the population that can be supported.
- The warm-blooded animals, that is, birds and mammals, can maintain a relatively constant body temperature regardless of the external temperature. Therefore you might think that their carrying capacity would be unaffected by temperature.
- However, the further the temperature of the external environment gets from their optimum temperature, the more energy these organisms expend in trying to maintain their normal body temperature.
- This leaves less energy for individual growth and so they mature more slowly and their reproductive rate slows.
- The carrying capacity of the population is therefore reduced.
How does water and humidity influence population sizes
- Where water is scarce, populations are small and consist only of species that are well adapted to living in dry conditions.
- Humidity affects the transpiration rates in plants and the evaporation of water from the bodies of animals.
- Again, in dry air conditions, the populations of species adapted to tolerate low humidity will be larger than those with no such adaptations.
In general terms, when any abiotic factor is below the optimum for a population…
- fewer individuals are able to survive because their adaptations are not suited to the conditions.
- If no individuals have adaptations that allow survival, the population becomes extinct.
Explain why populations never grow indefinitely.
- Certain factors limit growth,
- e.g. food
Distinguish between biotic and abiotic factors.
Biotic factors involve the activities of living organisms. Abiotic factors involve the non-living part of the environment.
Explain the benefits of using a logarithmic scale for population numbers when plotting a graph of these data.
- Using a standard scale, most of the points plotted for the population of the world would be so close together as to be indistinguishable from each other on the graph.
- A logarithmic scale separates these points.
Calculate to three significant places the log values for the human population in each case.
Plot asuitable graph to show the growth of the human population over the past 12 000 years.
- As the time scale is back in time from the present day, the values can be treated as minus values and so the scale plotted from -12 000 years to 0 years (present).
- The points are joined by a series of straight lines rather than a line of best fit because we cannot be certain that the intervening values would fall on the curve plotted.
- This is because human populations can fluctuate over relatively short periods e.g. due to diseases, famine etc.
How would you calculate population growth and % of population growth
