Unit 4 Chapter 1

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1.1 Populations and ecosystems

  • Ecology - Study of the inter-relationships between organisms and their environment
  • Abiotic - Non living componants of an environment (e.g. rocks and rain)
  • Biotic - Living components (e.g. predation and competition)
  • Biosphere - Life-supporting layer of land, air and water surrounding the earth
  • Ecosystem - All of the interacting biotic and abiotic factors 
  • Population - Group of interbreeding organisms of one species in a habitat
  • Community - Populations of different species living & interacting in one place at the same time
  • Habitat - The place where a community of organisms lives - there are many habitats within an ecosystem (microhabitats are smaller units within a habitat with their own microclimate)
  • Ecological niche - Describes how an organism fits into the environment, refers to where an organism lives and what it does there. Some species can appear very similar but will have slightly different diets or nesting needs, for example. 
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1.2 Quadrats

Three factors to consider when using quadrats:

  • The size of quadrat- Depends on the size of the species being counted. Larger species need larger Quadrats. Where a species occurs in groups rather than being evenly distributed, a lot of smaller quadrats will give more representative results. 
  • The number of  quadrats to use - The more quadrats used, the more reliable the results will be. As this can take a while, a balance needs to be struck between the validity of the results and the time available for the study. The more different species in the area being studied, the more quadrats needed to aquire valid results.
  • The position quadrats - To produce statistically significant results, random sampling is used

Random sampling 

1. Lay out 2 long tape measures at right angles, along two sides of the study area 

2. Obtain co-ordinates using random numbers taken from a computer or a table

3. Place a quadrat at the intersection of each pair of coordinates and record the species within it

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1.2 Systematic sampling along transects

Sometimes it is more useful to measure the abundance & distribution of a species in a systematic rather than a random manner - this is particularly important where transition in the communities takes place (e.g. the distribution of organisms on the sea shore is determined by the relative periods of time they spend under the water & exposed to the air, their vertical height up the shore.) 

Stages of zonation are especially well shown using transects.

  • A line transect comprises a string or tape stretched across the ground in a straight line. Any organism over which the line passes is recorded.
  • A belt transect is a strip, usually a metre wide, marked by putting a second line parallel to the first. The species occuring within the belt between the lines are recorded. 
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1.2 Measuring abundance

Random sampling is used to obtain measures of abundance (the number of individuals of a species within an area). It's measured in several ways, depending on the size of the species & the habitat. Examples include:

  • Frequency - The likelihood of a species occuring in a quadrat. If, for example, a species occurs in 15 out of 30 quadrats, the frequency of its occurence is 50%. This is useful where a species, such as grass, is hard to count. It gives a quick idea of the species present and their general distribution within an area. However, it doesn't give information on the density and detailed distribution of a species.
  • Percentage cover - An estimate of the area within a quadrat that a particular plant species covers. It's useful where a species is particularly abundant or hand to count. The advantages in these situations are that data can be collected quickly & individual plants don't need to be counted. It's less useful where organisms occur in overlapping layers (more likely to be plants)

To get reliable results, it's necessary to ensure the sample size is large, many quadrats are used & the mean of all the samples is obtained. The more samples, the more representative of the community the results will be. 

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1.2 Mark-release-recapture techniques

Estimated population size = size of 1st sample x size of 2nd sample/ number of marked individuals recaptured

This technique relies on several assumptions:

  • The proportion of marked to unmarked individuals in 2nd sample is the same as the proportion of marked to unmarked in the whole population
  • Marked individuals from 1st sample distribute themselves evenly amongst the remainder of the population and have time to do so
  • The population has a definate boundry so there is no immigration to or emigration from the population
  • There are few, if any deaths and 'births' within the population
  • The method of marking is not toxic nor does it make them more susceptible to predation
  • The mark is not lost or rubbed off during the study 
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1.3 population growth curves

The usual pattern of growth for a natural population has three distinct phases (there's a graph in the book):

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 population continues to reproduce. The population size doubles during each interval of time. The graph line becomes increasingly steep. 

3. A period when the population growth declines until its size remains more or less stable. The decline may be due to the food supply limiting numbers or due to increased predation. The graph therefore levels out with only cyclic fluctuations due to variations in factors such as food supply or the population size of predators. 

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1.3 Population size

Algal growth is quick in a fresh pond, with plenty of light & 12C temperatures. There are no limiting factors. In time, things change:

  • Mineral ions are used up as the population gets bigger
  • The population becomes so big that the algae at the surface blocks light to those below
  • Other species are introduced into the pond, carried by the wind, or animals, some of these species might use the algae as food or compete for light or minerals. 
  • Winter brings lower temperatures & low light intensity

The going gets tough! The population growth slows & may stop altogether - population reaches relatively constant size

Factors that limit population size are either:

  • Abiotic - (non living part of the environment e.g temperature, light pH & water/humidity)
  • Biotic - Activities of living organisms and include competition and predation. 
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1.4 Competition

  • Intraspecific - competition within one species (e.g. robins competing for breeding territories) it's the availability of resources that determines the size of a population. The greater the availability the larger the population. 
  • Interspecific - competition between different species. When populations of two species occupy the same niche, one will normally gain a competitive advantage. The species that has the advantage will grow, while the other will decrease. If the conditions don't change, one species will eventually be removed. This is the competitive exclusion principle. 
  • Competitive exclusion principle states that where 2 species compete for limited resources, the one that uses the resources most effectively will eventually eliminate the other. No 2 species can occupy the same niche indefinitely. 
  • Two species of seabird, cormarants & shags, appear to occupy the same niche, both living on cliffs & fishing in the sea. However, analysis shows shags eat mainly sand eels & herring, but cormorants eat more flat fish, gobies & shrimps, they occupy different niches. 
  • To show how a factor influences population size you have to link it to birth & death rate - more food may not increase the population, but the existing individuals may just get bigger. More food means more individuals survive & reproduce - this takes longer to cause population growth
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1.5 Effect of predator prey relationship

The relationship between predators & their prey & its efect on population size can be summarised as follows:

1. Predators eat their prey, thereby reducing the population of the prey organism

2. With fewer prey available, the predators are in greater competition for the prey that are left 

3. The predator population is reduced as some individuals are unable to obtain enough prey to survive

4. With fewer predators left, fewer prey are eaten + their population increases

5. With more prey now available, the predator population increases

However, in natural ecosystems, organisms eat a range of foods and therefore the fluctuations in populations size shown in the graph (page 18 of the book) are often less severe 

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1.6 Human population

The human population, like that of other organisms has for most of our history been kept in check by food availability, disease, predators and climate.

Factors that led to explosion in human population: Industrial revolution & development of agriculture

Wars, disease and famines only cause temporary reversals in the upward trend. 

The usual population growth curve is not followed by the human population

The exponential phase, (in which the population grows rapidly), continues rather than gives way to the stationary phase in which the population stabilises. 

Population growth = (births + immigration) - (deaths + emigration)

% population growth = Population change during period / population at start of period x 100

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1.6 Factors affecting birth rates

Birth rates are affected by:

  • Economic conditions - Countries with lower per capita income tend to have higher birth rates 
  • Cultural and religious backgrounds - Some cultures encourage larger families and some religions disagree with the idea of contraception
  • Social pressures & conditions - In some countries, a large family improves social standing
  • Birth control - The extent to which contraception and abortion are available and used greatly influences birth rate 
  • Political factors - Governments influence birth rates through education and taxation policies

Birth rate = No. of Births per year / total population in same year x 1000

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1.6 Factors affecting death rates

Death rates are affected by:

  • Age profile - The greater the proportion of elderly people in a population, the higher the death rate is likely to be 
  • Life expectancy at birth - The residents of economically developed countries live longer than those of economically less developed countries 
  • Food supply - An adequate and balanced diet reduces death rate 
  • Safe drinking water and effective sanitation - Reduce death rate by reducing the risk of contracting water-borne diseases such as cholera
  • Medical care - Access to healthcare and education reduces death rate
  • Natural disasters - The more prone a region is to drought, weather events and disease, the higher the death rate
  • War - Deaths during wars produce an immediate drop in population and a long term fall as a result of fewer fertile adults

Death rate = number of deaths per year / total population x 1000

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1.6 population structure

In more developed countries, such as the USA & those in Europe, there has been a big increase in life expectancy. This leads to a change in societies from where life expectancy is short & birth rate high to those where life expectancy is long & birth rates low. This is demographic transition. It leads to a levelling off of the population and the re-establishment of a typical sigmoid population growth curve. May not affect all countries. 

Age population pyramids: 

  • Stable population - Birth & death rates are balanced so there is no change in population size
  • Increasing population - There is a high birth rate (wider base to pyramid) and fewer older people, giving narrower apex. This type of population is typical of economically less developed countries
  • Decreasing population -  There is a lower birth rate (narrow base to pyramid) and a lower mortality rate, leader to more older people (wider apex). This type of population occurs in certain economically more developed countries, such as Japan.

Average life expectancy = Age at which 50% of the population is still alive

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