Summary of topic 2.6
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Limiting factors constrain population growth and determine a population's carrying capacity. Limiting factors include resources like nutrients, water, and temperature. Population growth follows an S-curve as it reaches carrying capacity, leveling off when limiting factors balance birth and death rates. Succession is the long-term change in a community as it colonizes bare areas. Primary succession starts from bare substrate while secondary succession follows a disturbance. Succession proceeds through pioneer, intermediate, and climax stages as species diversity and soil/habitat develop over time.
Summary of topic 2.6
- 2. Limiting Factors and Carrying Capacity • Limiting Factors – – – – Variables which constrain the growth of populations These include temperature, light, water and nutrients Wide range of environmental factors = stenoecious species Narrow range of environmental factors = euryoecious species • Carrying Capacity – The maximum number of organisms of a particular species that a given area or ecosystem can sustainably support population size carrying capacity time
- 3. Limiting Factors and Carrying Capacity oligotypic species mesotypic species polytypic species
- 4. Temperature • Plants have different niches with regard to temperature • Different temperatures may be limiting at different stages in a plant’s lifecycle • e.g. The South African Protea flower only germinates after bush fires activate its seeds, kill off competitors and provide ash to enrich the soil with nutrients
- 5. Water • Too little water may cause water stress in plants: – Germination may fail – Seedlings may die – Seed yield may be reduced • Plants have developed adaptations to survive in different levels of water stress: – Hydrophytes – water tolerant plants – Mesophytes –plants that survive in moist environments – Xerophytes – plants that survive in dry environments
- 7. Population Growth • When a species is introduced into a new environment its population grows in a characteristic way. • The curve is generally called a sigmoid growth curve or S-curve
- 8. Population Growth S-Curve • The S-curve has three phases (1,2,3) – 1. Exponential growth phase Resources are plentiful. Birth rate (natality) is greater than death rate (mortality) – 2. Transitional phase Natility falling, mortality rising. But natality is still greater than mortality – 3.Plateau phase natality = mortality. Something is now limiting the birth rate. Eg predation, competition for resources, food shortage, disease. In reality the population wobbles around k, with overshoot and die-back k k = carrying capacity The s-curve is typical of longlived species that are able to reach their carrying capacity – e.g. top predators
- 9. Population Growth J-Curve • The J-curve only shows exponential growth • The species does not reach the transitional or plateau phases, therefore growth does not slow down The J-curve is typical of shortlived species that exploit a ‘boom and bust’ strategy – e.g. rodents, pest insects and microbes
- 10. Density-dependent Limiting Factors • These factors increase mortality rate as population density increases – e.g. competition for resources, space, disease, predation, parasitism – Predators are attracted to more dense populations, disease spreads more easily • They act as negative feedback mechanisms and lead to stability around the carrying capacity • They also act as natural selection factors which help to increase the fitness of the population • They include internal factors such as densitydependent fertility or size of available breeding territory and external factors such as predation or disease
- 11. Density-independent Limiting Factors • These factors increase mortality rate but are not related to population density – they are generally abiotic factors. They may also reduce birth rate – e.g. extremes of weather (drought, hurricanes), longterm climate change (natural and anthropogenic), geophysical events such as tsunamis and volcanic eruptions • They may act in conjunction with densitydependent factors, but do not act to stabilise populations
- 12. Human Influences on Limiting Factors • Human activities may cause population increases by: – – – – Increasing availability of resources (e.g. fertiliser use) Reducing competition (e.g. pesticide use) Reducing predation (e.g. by over-hunting) Introducing animals to new areas • Human activities may cause population decreases (or even extinction) by: – Habitat destruction – Introduction of alien species – Over-hunting
- 13. r- and K-Strategists • Slow growing, long-lived species which exploit S-curve growth are called K-strategists • Fast growing, short-lived species which exploit J-curve (boom and bust) growth are called r-strategists • Species which employ a strategy somewhere between these extremes are called C-strategists • K-strategists tend to be large, have a small number of offspring and high level of parental care (e.g. mammals) • r-strategists tend to be small, have large number of offspring and very little parental care (e.g. insects)
- 14. r and K Survivorship Curves K-strategists Number of individuals surviving 1000 100 C-strategists 10 r-strategists 1 0 50 Percentage of lifespan 100
- 15. r and K Survivorship Curves • K-strategists tend to survive for their entire potential lifespan and then die over a short time period (e.g. humans, salmon) • r-strategists tend to die at a young age, and those that don’t are likely to live a long time (e.g. turtles, oysters) • Limiting factors that affect the shape of the curve include: – Competition for resources – Adverse environmental conditions – Predator-prey relationships
- 16. Ecological Succession • The long-term change in a community after it starts to colonise bare substrate • Pioneer community climax community • Bare rock lithosere • Freshwater hydrosere • Dry habitat xerosere http://www.bbc.co.uk/programmes/p0038s03
- 17. Succession Model (Lithosere) Bare uncolonised rock exposed Colonisation by lichens, weathering of rock and build up of dead organic material Growth of moss, further weathering; soil begins to form Growth of small plant such as grasses, leading to further improvement of soil Larger plants can grow in the deeper and more nutrient rich soil Climax Community of shrubs and trees
- 18. Succession • Primary Succession – Occurs on a previously uncolonised substrate (e.g. a newly formed bit of bare igneous rock, newly exposed surfaces such as landslips, newly quarried rock face, newly exposed sanbanks or sanddunes) • Secondary Succesion – Occurs in places where a previously existing community has been destroyed (e.g. by a forest fire, newly exposed sediment at the edge of a lake) – This occurs faster since soil and seed already exist
- 19. Succession • Succession occurs in stages • The first colonisers are pioneer species – Fast growing species such as lichens and mosses which don’t require thick soil • Secondary species (especially grasses) then take over – These are slower growing and are able to outcompete the pioneer species for light and nutrients • Tertiary species (especially herbaceous plants) then take over – These take advantage of thicker soil appearing as litter breaks down • Quaternary species then take over – Largely shrubs appear which are able to outcompete the herbaceous plants • Finally a climax community is produced (e.g. oak forest) – Thick soil has been produced, trapping more moisture and nutrients – A community of soil organisms develops – An animal community develops to exploit the available niches http://edroness.blogspot.mx/2014/02/ecologicalsuccession.html Private Life of Plants – episode 4
- 20. Succession Feature Start of Succession End of Succession Amount of organic matter Small Large Amount of Nutrients Small Large Nutrient conservation Poor Good Amount of detritus Small Large Niches Few but wide Many but small Size of colonisers Small Large Life cycles Simple Complex Growth patterns r-strategists dominate K-strategists dominate Diversity Low High Stability Low High Productivity Low and unstable High and stable pH High Low Biodiversity Low High
- 21. Productivity • In the early stages of succession, GPP is low due to the low density of producers. However, NPP is fairly high due to the low respiration rates in the whole community • As succession proceeds, GPP increases as soils become more structured and the density of organisms increases. NPP stabalises as niches become occupied • The ratio of GPP/NPP is called the P/R ratio. In the climax community, overall productivity is balanced by overall respiration – therefore as succession proceeds, the P/R ratio approaches a value of 1
- 22. Productivity 1 P/R Ratio Climax Forest Agricultural Crop 0 time
- 23. Plagioclimax • An interruption to succession is termed plagioclimax • Humans are often the source of this • Sometimes it is accidental and sometimes on purpose (e.g. farmers do not want forests to become established on their land) • The greatest impact is through direct deforestation • Humans also affect succession by: – – – – – Erosion of footpaths Trampling of plants Hunting Land clearance Introduction of grazing animals
- 24. Questions 1. Briefly describe the stages of primary succession 2. What is P/R ration and how does it change as succession proceeds 3. List the characteristics of a typical climax community 4. What is plagioclimax. Give examples of how humans may cause it