Ecology: the study of theinter-relationships between organisms and their environment
Biosphere: the life supporting layer of land, air and water that surrounds the Earth
Ecosystem: more or less self-contained functional unit comprising of all the biotic and abiotic features in a specific area
Population: a group ofinterbreeding organisms of one species in a habitat
Community: all the populations of different organisms living and interacting in a particular place at the same time
Habitat: where a community of organisms lives
Ecological niche: the role an organism has in itsenvironment. No two species occupy exactly the same niche
To study a habitat, it is often necessary to measure the abundance of a species in a given space. Random and systematic sampling techniques are used to obtain a representative sample, thereforeconclusions will be valid.
There are three factors to consider when using quadrats:
- Size of the quadrat
This will depend upon the size of the plants or animals being counted and how they are distributed within the area.
2. The number of quadrats to record
The larger the number of sample quadrats, the more reliable the results will be. However, as the recording of a specieswithin a quadrat is time consuming, a balance must be struck between thevalidity of the results and the time available.
3. The position of each quadrat
To produce statistically significant results, random sampling must be used in order to avoid bias.
How to carry out random sampling?
1. Lay out two long tape measuresat right angles along two sides of the study area
2. Obtain a series of coordinates by using random numbers taken from a table or generated by a computer
3. Place a quadrat in the intersection of each pair of coordinates and record the species within it
Pay attention: Issues may arise where organisms are only partly within the quadrat. It must be decided beforehand how these will becounted and the decision reached must be carried out consistently throughoutthe fieldwork.
- It is sometimes more logical to measure abundance and distribution using a systematic method, particularly where someform of transition in the community takes place.
- A line transect comprises a string or tape stretched across the ground in a straight line.
- Any organ is mover which the line passes is recorded, or alternatively, the species present are recorded at set intervals on the line.
- Belt transects may also be used. It is a strip, usually a metre wide. Species within the belt are recorded or quadratsare placed within it at set intervals (interrupted belt transect). This method is more time consuming than the line transect but is more representative.
- The measure of abundance within these two sampling techniques can either be measured by finding the frequency (more time consuming)or the percentage cover (subjective).
3. Mark-release- recapture
This method is used since most animals are mobile, it can be difficult to estimate their abundance using the above methods.
A known number of animals are caught, marked in some way, and hen released back into the community. A set amount of time later (around a day), the same number of individuals are collected and the proportion of those that are marked is recorded.
The size of the population is then calculated as follows:
Estimated population size = number of individuals in 1st sample x number of individuals in 2nd sample
number of marked individuals recaptured
The assumptions requires for this technique
- The proportion of marked to unmarked individuals in the second sample is the
- same as the proportion of marked to unmarked individuals in thepopulation as a whole
- The marked individuals released from the first sample distribute themselves evenly among the population and have sufficient time to do so
- No immigration or emigration into and out of the population
- There are few, if any, births and deaths within the population
- The method of marking is not toxic, nor does it compromise camouflage and make the individual more liable to predation
- The mark is not lost or rubbed off
- Where possible, the organisms should be studies in situ. If it is necessary to remove them, the numbers taken should be kept to a minimum.
- Any organisms removed should be returned to their original habitat, even if they’re dead
- A sufficient period of time should elapse before the site is used for fieldwork again
- Disturbance or damage should be avoided
Variation in population size
1. A period of slow growth as the initially small number of individuals reproduce to slowly build-up their numbers
2. A period of rapid growth where the ever-increasing number of individuals continue to reproduce
3. Population growth declines until its size remains more or less stable. Food supply or increased predation may limit numbers
Factors that affect population size
Each species has a different optimum temperature at which it is able to survive. If temperatures fall below the optimum, enzymes will work more slowly so organisms’ metabolic rate will be reduced,however if temperatures rise too far above the optimum, the enzymes will be denatured.
Light is a basic necessity of life and is the ultimate source of energy for ecosystems. As light intensity increases, so does the rate of photosynthesis, and growth rate of plants is increased and in turn, their population size increases. The population of herbivores will therefore also increase.
This affects the action of enzymes.Each enzyme has an optimum pH at which it operates most effectively. A population will be larger where the appropriate pH exists.
4. Water and Humidity 💧
Where water isscarce, populations are smaller and consist only of species which are well adapted to living in such conditions. Humidity affects the transpiration rate of plants and the evaporation of water from the bodies of animals.
1. Intraspecific competition:
Competition for resources between two individuals of the same species.Intraspecific competition is the driving force behind natural selection sinceindividuals with the best characteristics are more likely to win thecompetition and go on to breed and pass on their genes.
2. Interspecific competition:
Competition for resources between two individuals of different species.Where populations of a species initially occupy the same niche, one will normally have a competitive advantage over the other. The population of thisspecies will gradually increase in size while the other will diminish.
Predation occurs when one organism is consumed by another. The predator-prey relationship shows cyclical changes:
- Predators eat their prey, reducing the population of prey
- With fewer prey available, there is greater
- intraspecific competition
- between predators for the prey that are left
- The predator population is reduced as some individuals are unable to obtain enough prey for their survival
- Fewer prey are eaten so their population increases
- With more prey now available, the predator population in turn increases
- The human population has for most of our history been kept in check by food availability, disease and climate.
- However, thedevelopment of agriculture and manufacturing has led to exponential growth. Wars,disease and famine have caused only temporary reversals in this upward trend.
Population growth = (births + immigration)– (deaths + emigration)
Percentage population growth rate = population change during the period x 100
population at the start of the period
Factor affecting birth rate
- Economic conditions: Countries with low GDP per capita tend to have higher birth rates
- Culture and religion: Some cultures encourage larger families while many religions areopposed to the use of contraception
- Social pressures andconditions: In some Countries, a large familyimproves social standing.
- Contraception: The use of birth control is banned in some countries while inothers it is expensive/difficult to acquire
- Political factors: Education, taxation, policies and incentives influence birth rates
Factor affecting death rate
- Age profile:Ageing populations tend to have a higher death rate
- Life expectancy:MEDCs have longer life expectancies than LEDCs
- Food supply: An adequate, balanced diet reduces the death rate
- Clean water and sanitation: Water-borne diseases such as cholera are spread through contaminated water and poor sanitation
- Medical and educationalprovision: Access to sufficient health care andeducation reduces the death rate
- Natural disasters: Areas prone to flooding, earthquakes, volcanic eruptions etc willhave a higher death rate
- War: Deaths during wars produce an immediate drop in the population anda longer term fall as a result of fewer fertile adults
The population of countries go through changes where the birth and death rates may rise and fall, altering the levelsof natural increase/decrease.
This is known as demographic transition and it leads to a levelling-off of the population and the re-establishment of the typical sigmoid population growth curve. It is illustrated in the Demographic Transition Model (DTM).
Further information about birth and death rates, as well as life expectancy and infant mortality, can be obtained through analysis of population pyramids. They can show whether a population is stable and the birth and death rates are balanced, if it is increasing and a there is a high birth rate shownby a wide base, or if the population is decreasing, shown by a narrow base.
Survival curves show the percentage of all individuals born in a population that're still alive at anygiven age. They allow us to calculate life expectancy by simply reading off the graph the age at which 50% survive.
👧Type I – Long life expectancy with lowinfant mortality and most of the cohort dying in old age. Type I curves are shown by large mammals and human societies where families are small and there is high investment in parental care.
🦜Type II – There is an intermediate life expectancy and a roughly constantdeath rate regardless of age. Type II curves are shown by animals that areequally susceptible to predation or disease at any age, such as small mammalsand birds. They are also shown by human societies facing a serious epidemic such as the AIDS epidemic in many African countries.
🐸Type III – Short life expectancy, with most of the cohort dying in infancy and few surviving to old age. Type III curves are shown by animals that do little to no parenting and produce large numbers of offspring to compensate,such as insects and fish.
- How to investigate populations
- Factors that affect population size
- Population structrure
- Survial Curves
That is the end of the topic
Draft by Eva (Biology)