Presentation on theme: "Changes 1- Populations Topic 2.6. Factors that increase population size: Natality (Birth rate) is recruitment to a population through reproduction Immigration."— Presentation transcript:
Factors that increase population size: Natality (Birth rate) is recruitment to a population through reproduction Immigration from external populations e.g. Bird migration Factor reducing population size: Mortality (death rate) which is the death rate from any source e.g. predation Emigration, where individuals leave the population for another habitat Population Immigration Emigration Death Birth
For a stable population: Immigration Emigration + Natality + Mortality =
Lag phase: Population growth begins slowly from a few individuals. Exponential Phase: Exponential growth occurs, the conditions are ideal and maximum growth rate is reached. Transitional Phase: Growth rate begins to slow down as factors such as food, water and space become limiting. Plateau phase: Carrying capacity for the population has been reached and the population number becomes stable. The carrying capacity is the population size that can be supported by a particular environment. Decline phase: If there is a sudden change in the environment meaning that the environment can no longer support the population, such as a drought causing food shortage, the population will crash and the whole process begins again. A Sigmoid (s - shaped) Population Curve Population size Time Plateau phase Decline phaseTransitional Phase Exponential Phase Lag phase
Carrying Capacity. An ecosystem can only support a certain size of a population – its carrying capacity. Carrying capacity is affected by the amount of energy available to the organism – more energy available means a higher population!
Population size can be affected by the following : Density dependent factors: These are any factors, dependent on the density of the population in question. Generally their effects increase as population increases. Some examples of these are predation, disease and competition. They tend to form a negative feedback mechanism that controls population. Density independent factors: These are any factors, not dependent upon the density of the population in question. Some examples of these are climate or weather and catastrophe.
Internal and external factors affecting populations Internal – factors that act within a species. Eg. limited food supply, availability of territory, density dependent fertility. (Take a look at this interesting article about overcrowding by Aidan Sammons – link )link External – factors that act between different species. Eg. Predation and disease.
Case studies Rabbits The Lynx and the Snowshoe Hare.
In 1859, Thomas Austin brought out 24 rabbits, 5 hares and 72 partridges and released them on his property, in Victoria. In 1887, Rabbits were first sighted in Northern Territory.
The gestation period for a rabbit averages 31 days. A rabbit can live as long as 10 years Rabbits can start breeding as early as 3-4 months of age European Rabbits have an average litter size of 7 – 9 but may be as high as 15! 8 Rabbits eat as much grass as 1 sheep!
By 1907 Rabbit numbers had reached plague proportions (up to an estimated 600million) causing untold environmental damage:
It didn’t work….. So in 1950, after research carried out by Frank Fenner, Myxomatosis was deliberately released into the rabbit population, causing it to drop from an estimated 600 million to around 100 million.Frank Fenner Myxomatosis Genetic resistance in the remaining rabbits allowed the population to recover to 200-300 million by 1991.
To combat this trend, Commonwealth Scientific and Industrial Research Organisation (CSIRO) scientists developed and accidentally released calicivirus (also known as Rabbit Haemorrhagic Disease or RHD) in 1996.Commonwealth Scientific and Industrial Research Organisation (CSIRO) calicivirus The Australian Government refuses to legalise a vaccine to protect pet rabbits against Myxomatosis, for fear the rabbit may escape and transmit resistance to the disease. A legal vaccine exists in Australia for RHD. There is no cure for either Myxomatosis or RHD, and many affected pets are euthanised.
Activity: Using the information in this power-point: Draw a sketch of the population curve for Rabbits in Australia from 150 to present day. Label relevant parts of the curve and include approximate dates and population sizes. Discuss with a friend whether the methods of control were ethically or environmentally justifiable.
Predator Prey Relationships The lynx and the Snow Hare
Activity: How will increasing numbers of Snowshoe hare affect the numbers of lynx? How will increasing numbers of Lynx affect the numbers of Snowshoe hare? How will decreasing numbers of Snowshoe hare affect the numbers of lynx? Does this explain the population graph?
J – population curves Show a ‘Boom and Bust’. Exponential growth followed by ‘Dieback’. Growth is not limited by density dependent factors. Often ‘overshoots’ the carrying capacity of the ecosystem. Examples: Bacteria, algae, invertebrates, many fish species and some small mammals. Population Time Population Time
There are 2 groups of species with differing reproductive strategies. Those that go for rapid growth (r-strategists) and those that go for Karrying Kapacity (K-strategists). These are extremes – most species fall somewhere in the middle. r-strategistsk-strategists Life spanShortLong Growth rateRapidSlow Age of maturityEarlyLate How many offspring?LoadsFew How large offspring?SmallLarge Parental careLittle or noneLots Stability of environment it lives in UnstableStable Internal or external population regulation? ExternalInternal Trophic levelsLowerHigher ExampleWeeds, planktonOak trees, elephants
Survivorship curves. This is the fate of groups of individuals through their life span. Age % Population of survivors K- strategists – Few young, lots of parental care, so mortality at young ages is low. Most organisms live to average lifespan. r- strategists – many young, limited parental care, so high infant mortality. Few organisms live to full lifespan. Most organisms come somewhere in between.
What is the lifespan of these birds? What is the potential population size? How many survivors are there after the 1 st year? What is the percentage mortality after the1st year? What is the percentage mortality after the 2 nd year? What is the percentage mortality after the 3 rd year? What would the survivorship curve for this population look like? Is this a K or an r strategist? Activity: 200 eggs laid 100 50 25 12 Deaths: 100 young and eggs Deaths: 100 birds Age in years Population pool- birds of breeding age. Question form Page 164 – Environmental Systems and Societies Course companion – Jill Rutherford – Oxford University press 2009 - 12340 A survival model for a species of bird