Populations in Ecosystems

• Ecology:

  • ecology is the scientific study of the factors which determine the distribution and abundance of organisms
  • this involves finding out what lives where, in what numbers and why
• Key terms:

  • species
  • habitat
  • population
  • community
  • ecosystem
  • biotic
  • abiotic
  • intraspecific competition
  • interspecific competition
  • carrying capacity
  • niche
• Niches and adaptations:

  • a niche refers to the way in which an organism fits into an ecological community or ecosystem
  • through the process of natural selection, a niche is the evolutionary result of a species' anatomical, physiological and behavioural adaptations to its surroundings
  • each organism occupies its own ecological niche within the community - no two species can have the same niche and successfully live alongside each other - one will always outcompete the other eventually
  • an ecological niche of an organism includes its feeding role, the abiotic conditions it lives in, its physiology, behaviour and reproduction methods, where it exactly lives
• Variation in population sizes:

  • populations within an ecosystem can only reach a certain size. The maximum size that a population can remain sustainable in a particular habitat is called the carrying capacity
  • for most non-human species, intraspecific competition for resources such as food increases as the size of the population increases and gets nearer to the maximum that can be supported sustainably in a particular habitat
  • if the carrying capacity is exceeded, the population declines because its environment can no longer support the excess numbers. In many situations this can happen very rapidly because excessive demand degrades the environment, and this may lead to a sudden fall in the size of the population

  the population size can vary as a result of;

  • the effect of abiotic factors
  • interactions between organisms e.g. competition and predation
• Competition:

  more individuals of each species are born than the environment can support;

  • plants compete for light, water and minerals
  • animals compete for food, territory, mates, breeding sites (e.g. nesting sites)

  there are two types of competition;

  • intraspecific competition
  • interspecific competition
• Intraspecific competition:

  • this occurs between individuals of the same species for the same resource
  • this type of competition drives natural selection and evolution of a species. Those best adapted to make use of the essential resources will be more likely to survive, reproduce more and pass on their alleles

  population increases when resources are plentiful;

  • more organisms are then competing for the same amount of space/territory and food, so resources become limiting
  • population then decreases
  • there is less competition which is better for growth and reproduction
  • population then increases again
• Interspecific competition:

  • this occurs between individuals of different species - e.g. weeds and crop plants in a field competing for light, minerals etc
  • no two species can occupy exactly the same niche as there would be too much interspecific competition between them for the same resources
  • the more similar the niches of the two competitors, the greater the competition will be between them
  • the effect of this competition is likely to cause some species to disappear from the area as they are competitively excluded by stronger competitors
• Predation:

  • as the number of predators increase, the prey population will decline
  • a decline in prey numbers will mean less food for predators, more intraspecific competition, less reproduction, increased mortality
  • in response to this fewer prey will be eaten, therefore more survive to breed, so the prey population increases
• Predation:

  • the overall effect is that both populations will fluctuate but only within narrow limits, each species preventing the other increasing beyond the size that the environment will support
  • the changes in the predator population 'lags behind' the changes in the prey population
  • in each species the best adapted individuals have the best chance of survival, so both species benefit
  • having a cycle as simple as this is rare as most predators have multiple prey species. They generally feed on the most common species, preventing severe declines in predator and prey numbers
  • predator-prey relationships can also be more complicated than they appear e.g. prey could decrease due to a change in other environmental conditions, such as abiotic factors (e.g. temperature) or other biotic factors (e.g. lack of food for the prey)
  • for a given example, you must state the frequency of the cycle (e.g. every 4 years)
• Investigating populations of organisms - sampling:

  • in order to investigate ecosystems, we may need to look at the abundance (numbers) of organisms in a particular area or how a species is distributed
  • it is rarely practical to count all the organisms in an area. Therefore, samples are taken
  • they need to be large to improve the reliability of the mean, to make it more representative, and avoid anomalies having too big an effect
  • samples are often taken randomly to ensure they are not biased
  • fieldwork is often only able to identify patterns in the ecosystem and does not prove why the organisms are found there. It is therefore often useful to gather data relating to the abiotic factors in an area
• Abundance and distribution:

  the size of a population (abundance) can be estimated using;

  • randomly placed quadrats for non-motile or slow-moving organisms
  • the mark-release-recapture method for motile organisms

  the distribution of a species can be measured using a belt transect (systematic sampling method)
• Non-motile organisms - measuring population size with random quadrats (RP12):

  quadrats can be used to measure density, frequency, or cover of non-motile or slow moving organisms e.g. plants

  • use 2 tape measures to set up a grid
  • use a random number generator to produce random coordinates (reduce bias)
  • place a quadrat down at the given coordinates

  you can then;

  • count the number of the organisms inside the quadrat (species density) - most common
  • or calculate the percentage cover - good for clumped species
  • or calculate the species frequency - whether it is present or not
• Non-motile organisms - measuring population size with random quadrats (RP12):

  • to standardise the counting method, use the North-East rule
  • repeat many times, to make the sample representative (10% of the area)
  • calculate a mean or take a running mean and finish sampling when this stays consistent

  to calculate the population size in the area;

  • for 1m$^2$ quadrat - mean number per quadrat x total area being investigated
  • for 0.25m$^2$ quadrat - mean number per quadrat x 4 (to get per m$^2$) x total area

  it is important to collect enough data (large sample size - at least 10% of whole area) to carry out statistical tests. To compare mean population sizes of 2 different areas, we should use a student's T-test
• Non-motile organisms - measuring distribution across an area with transects;

  if you are trying to find out how species distribution changes across an area you need to sample systematically. You need to sample along a line at regular intervals: this is a transect (sometimes called a belt transect);

  • lay a tape measure across an area to create a transect line
  • place quadrat at regular intervals along the transect and sample e.g. count the number of species inside
  • use several transects to get a representative sample of the area
  • you can also take measurements of relevant abiotic factors and the distribution of a species. This can give us information about possible causes of the distribution observed. However, correlation doesn't mean causation
• Representative samples:

  to make data more representative;

  • increase the number of quadrats used at each point along the transect
  • decrease the distance between the quadrats
  • lay multiple transects across an area
• Kite diagrams:

  • kite diagrams are drawn to show the distribution and abundance of the species
  • the width of each band represents the relative abundance of each organism (the widest sections there are a lot of the species, in the narrow section there are very few)
• Motile organisms - measuring population size using mark release recapture;

  motile organisms are organisms that are able to move, therefore using quadrats is not practical. The mark-release-recapture (MRR) method allows an estimation of population size to be calculated;

  • capture a large sample of the organisms from a given area. Random sampling methods can be used to identify where to place the nets or traps
  • they are then marked and counted. The method of marking must not harm the animal or affect their survival chances
  • release the sample back into the population. Give the animals sufficient time to remix/distribute back into the population
• Motile organisms - measuring population size using mark release recapture;

  • capture another large sample, using the same sampling technique as before
  • count the total number caught, and the number of these which are marked

  population size can be estimated using the equation;

  total population size = (no. caught in 1st sample x no. caught in 2nd sample) $\div$ no. marked in 2nd sample
• Assumptions of mark release recapture:

  • no birth / death
  • no immigration / emigration
  • the animals mix at random and have time to fully reintegrate once captured
  • the marks do not make the animals more vulnerable to predators or harm them in any way

  this limits how and when this technique can be used. For example, during breeding season there will be too many births, and during migration there'll be too much immigration and emigration. Completely sticking to these restrictions almost never happens, but it will still give a rough idea of numbers
• Succession:

  succession is the way in which the different species of organisms which make up a community change over a period of time

  there are two types of succession;

  • primary - this starts on bare ground e.g. bare rock, volcanic islands - the environment is incredibly hostile. There is a lot of exposure to weather conditions. Bare grounds leads to lots of temperature fluctuations, due to a lack of shade and shelter. No soil leads to very little water or nutrient retention
  • secondary - this starts on ground that has been populated with plant species, but these have since been destroyed e.g. by a fire. The conditions are less hostile than in primary succession, due to the presence of soil
• Primary succession:

  • pioneer species colonise the hostile area. Pioneer species are highly adapted to cope with the harsh conditions
  • as they colonise the area, some will die off which helps to improve the conditions of the area because dead organic matter (humus) is decomposed by microorganisms. This forms basic soil which therefore makes the conditions less hostile - the abiotic conditions of the area change
  • the conditions are now suitable for new plant species to colonise (grasses, encouraging insects) into the area and making the ecosystem more complex. This increases biodiversity. When these die, they decompose and increase the quality and depth of the soil. The conditions are now becoming more and more favourable
• Primary succession:

  • new organisms with different adaptations can move in and as this continues to happen these species change the environment, so it becomes less suitable for the pioneer species - they become outcompeted by newer species colonising the area
  • the cycle continues. Animals move in, increasing the number of niches
  • this continues until a climax community is reached
• Secondary succession:

  • this is similar to primary succession but starts with land that has been cleared of plant e.g. due to forest fires
  • this means that there is fertile soil, water can be retained, and it is slightly less hostile
  • therefore, it starts at a later stage than primary succession and pioneer species tend to be larger plants e.g. shrubs
• Climax community:

  • this is the final stage of succession and is very stable
  • it often has high biodiversity and complex food webs; however, this will depend on the location of the ecosystem
  • the climax community is determined by the main abiotic factor
  • for example, trees may not develop on very high mountains because it is too windy, or the soil is too thin
• Conservation and succession:

  • conservation is the protection and management of species and habitats in a sustainable way
  • an individual species can only survive if it has a suitable habitat to live, so many conservation projects involve management of succession
• Conservation and succession:

  • in Britain, much of the countryside such as grassland and moorland is managed to prevent the natural succession process and prevent a climax community from developing. The aim is to ensure there are a variety of different types of habitats in the UK
  • this can be done in a variety of ways. Grazing is a good way to prevent further succession (sheep, deer will eat young shrub/tree seedlings preventing their growth). Controlled burning is another way to prevent succession continuing
  • this will hold the ecosystem in a plagioclimax - an area or habitat in which the influences of humans have prevented the ecosystem from developing further
  • other examples of conservation include seedbanks, captive breeding, relocating species and having protected areas for species to thrive
• Arguments for conservation:

  • there are economic reasons e.g. conservation of rainforests which provide resources that can be turned into medicines and sold
  • some believe its the right thing to do as they believe humans caused the need for conservation to being with, and people enjoy visiting these places
  • there are ecological reasons such as preserving natural sinks (preventing carbon being released and increasing global warming) and not disrupting food chains
• Arguments against conservation:

  • may destroy someone's livelihood - i.e. increase in elephants may destroy more crops
  • it can interfere with non-protected species if others are given preferential treatment, e.g. peregrine falcons encouraged to nest in red knot bird areas
  • cost - is it worth it? Mainly charity work, could this be better spent?