7.4

• Ecosystems, communities and populations

  Population: a group of organisms of the same species, living in the same place (habitat) at the same time

  Community: all the populations of different species living in the same place (habitat) at the same time

  • Ecosystem: a community, plus all the non-living (abiotic) conditions of their environment.

  Ecosystems are dynamic systems and can range in size from the very small to the very large.
• Within a habitat, a species occupies a different niche, governed by adaptation to abiotic and biotic conditions

  Niche: the role of a species within its habitat, e.g. what it eats, where and when it feeds. Each species has its own unique niche; if two species try to occupy the same niche, one will outcompete the other

  Abiotic conditions: the non-living features of the ecosystem

  e.g. temperature, pH

  Biotic conditions: the living features of the ecosystem

  • e.g. predation, food supply, disease
• Carrying capacity

  Carrying capacity: maximum stable population size an ecosystem can support

  Population size can vary as a result of the effect of abiotic factors…

  E.g. light, water, temperature

  • Ideal conditions for species à organisms grow fast, reproduce successfully

  Population size can vary as a result of interactions between organisms…
• Interspecific competition = competition between species

  Reduces resources available to both à limits both populations

  If one species is better adapted, it will out-compete the others

  Intraspecific competition = competition within a species

  • Smaller population = less competition à better for growth and reproduction à larger population = more competition à less growth and reproduction à smaller population…
• Predation

  Population sizes of predators and prey are interlinked

  • Increased predator population size à more prey eaten à prey population falls à less food for predators à predator population size falls à prey population rises à predator population rises again…
• Estimating the size of a population: quadrats

  For slow-moving or non-motile organisms…

  Randomly placed quadrats
• Use a grid / split area into squares

  Obtain random coordinates using a random number generator / calculator – place quadrats here

  Deciding the number of quadrats to use to collect representative data:

  Calculate running mean

  When enough quadrats, this shows little change

  Count frequency of species in quadrat

  Calculate mean per quadrat

  • Calculate population size by multiplying the mean by the number of quadrats that would cover the habitat
• Quadrats along a belt transect

  Used to look at changes in distribution

  Lay tape measure from one place to another

  Place quadrats at regular intervals along the line

  • Count frequency / percentage cover in quadrats
• Estimating the size of a population: mark-release-recapture

  The mark-release-recapture technique can be used for motile organisms:

  1. Capture sample of species, mark (harmlessly) and release

  2. Leave enough time for organisms to randomly distribute before collecting second sample

  𝒑 𝒖𝒍𝒂𝒕𝒊𝒐𝒏

  3. 𝑷𝒐

  𝒔𝒊𝒛𝒆 =

  𝒏𝒖𝒎𝒃𝒆𝒓 𝒊𝒏 𝒔𝒂𝒎

  𝒑 𝒍𝒆

  𝟏 × 𝒏𝒖𝒎𝒃𝒆𝒓 𝒊𝒏 𝒔𝒂𝒎

  𝒑 𝒍𝒆

  𝟐

  𝑵𝒖𝒎𝒃𝒆𝒓

  𝒎𝒂𝒓𝒌𝒆𝒅 𝒊𝒏 𝒔𝒂𝒎 𝒑 𝒍𝒆

  𝟐
• It’s easier to remember this equation if you actually understand it:

  Based on the principle that the proportion of marked individuals in the second sample would be

  the same as was marked initially in the total population. That is, 89:;<= :>=?<@ AB C>: D E< F = 89:;<= :>=?<@ ABAIA>EE J GHI>E B9:;<= AB C>: D E< F GHI>E D H D 9E>IAHB CAK<

  • This can be rearranged to form the equation for population size
• Assumptions made when using the mark-release-recapture method:

  1. Sufficient time for marked individuals to mix within the population

  2. Marking not removed and doesn’t affect chances of survival e.g. more visible to predators

  3. No immigration/emigration

  4. No births/deaths/breeding
• Succession

  Succession: process by which an ecosystem changes over time

  1. Colonisation by pioneer species (pioneer species = first species to colonise)

  2. Pioneer species (or generally just certain species at each stage in succession) change the abiotic conditions and make the environment less hostile e.g. die and decompose, forming soil which retains water
• 3. Environment becomes more suitable for other species with different adaptations, and may be less suitable for the previous species – better adapted species outcompetes previous species

  4. As succession goes on, biodiversity increases

  5. Climax community: final, complex, stable community
• Conservation of habitats frequently involves management of succession

  Human activities can prevent succession and stop a climax community forming: plagioclimax

  Conservation sometimes involves preventing succession in order to preserve an ecosystem in its current stage of succession