Topic 2.6 Changes


Assessment 2.6.1: Explain the concepts of limiting factors and carrying capacity in the context of population growth.
à Limiting factors: factors that limit the distribution or numbers of a particular population, they are environmental factors which slow down population growth.
E.g. Food and shelter could limit population's ability to grow rapidly.

à Carrying capacity: the maximum number of organisms that an area or ecosystem can sustainably support over a long period of time. This limit can only be exceeded on a temporary basis and then the population will go into decline.

Assessment 2.6.2: Describe and explain S and J population curves
S curves: at the start there are few limiting factors and the population increases exponentially. This continues until the population size approaches carrying capacity. Growth rate then slows and the population tends to stabilize at carrying capactiy.

external image s-curve.jpg?w=407&h=308

J curves: the population will continue to go beyond the carrying capacity for a long time before crashing back to a lower level

external image the-j-curve_blanksm.jpg?w=314&h=232

Assessment 2.6.3: Describe the role of density‑dependent and density‑independent factors, and internal and external factors, in the regulation of populations.

à Density-dependent factors: factors that relate to population density, they lower the birth rate or raise the death rate as a population grown in size

à Density-independent: factors that are not related to the population density, affect pollution irrespective of population density, e.g. environmental change

àInternal factor: include density-dependent fertility of size of breeding territory

àExternal factors: include predation or disease

  • Density-independent factors operate as negative feedback mechanisms leading to stability or regulation of the population


Assessment 2.6.4: Describe the principles associated with survivorship curves including, K‑ and r‑strategists.

à Survivorship curves
  • R-strategists: initial colonizers; large numbers of a few species; highly adaptable; rapid growth and development; early reproduction; short life; small size and very productive
    e.g. Insects and Crabs
  • K-strategists: dominant species; diverse range of species; generalists; slow development; delayed reproduction; longer living; larger size; less productive
    e.g. Human and Blue Whale

external image survivorcurve.gif?w=400&h=363

Factors that influence survivorship include
  • competition for resources
  • adverse environmental conditions
  • predator-prey relationship.

Assessment 2.6.5: Describe the concept and processes of succession in a named habitat.

Succession: the long-term change in the composition of a community

Primary succession-occur on a previously un-colonized substrate
  • -Occur on dry land=rerosere
  • Occur on water=hydrosere
  • Occur on land created by dune=psammosere
  • occur on land first raised out of an ocean=halosere


external image primary-success.jpg

Hydrosere succession
Hydrosere succession


Secondary succession- occur in places where a previous community has been destroyed


  • Early community= pioneer community
  • Final community=sera stage
  • Final Stage of succession=climax community
secondary success
secondary success





Zonation: the arrangement or patterning of plant communities or ecosystem into bands in response to change, over a distance, in some environmental factors.


http://sciencebitz.com/?page_id=45 (Further reading)


Assessment 2.6.6: Explain the changes in energy flow, gross and net productivity, diversity and mineral cycling in different stages of succession.

  • In early stages of succession, gross productivity is low due to the initial conditions and low density of producers. The proportion of energy lost though community respiration is relatively low too, so net productivity is high, that is, the system is growing and biomass is accumulating
  • Mid stage: GP high interest, increase in biomass as plants forms and biome bigger
  • In later stages of succession, with an increase consumer community, gross productivity may be high in a climax community. However, this is balanced by respiration, so net productivity approaches zero and the production: respiration (P: R) ratio approaches one.



Assessment 2.6.7: Describe factors affecting the nature of climax communities.

Factors affecting the nature of climax communities
  • Climatic: temperature, rainfall, winds and light availability
  • Edaphic: presence of soils and nutrient
  • Tropagraphic: attitude, slope, aspect and ground water
  • Biological: competition and species interaction, affection of human” fire, grazing, etc