Population Growth 2 Ecology BIOL3005 PDF

Population growth 2 Antoine OHC Leduc Ecology BIOL3005 SQU  The “Intrinsic rate of increase” under ideal conditions of unlimited space and food supply  Gives a geometric increase  These these conditions are completely unnatural  In the lab  Food and space are fixed...

Population growth 2 Antoine OHC Leduc Ecology BIOL3005 SQU  The “Intrinsic rate of increase” under ideal conditions of unlimited space and food supply  Gives a geometric increase  These these conditions are completely unnatural  In the lab  Food and space are fixed  The curve levels-off to become an “S” shaped (= logistic) curve  The carrying capacity:  The maximum population that can live in these fixed conditions  b) Logistic Curves in Lab Experiments Carrying capacity The fly Drosophila feeds on yeast but yeast constantly reproduces at a certain rate The Drosophila abundance is limited by the amount of new yeast being produced  b) Logistic Curves in Lab Experiments Yeast also follows a logistic curve as its abundance increases Yeast feeds on the abundant sugar Food not limiting # Yeast cells in 1/250 mm3 Alcohol but it produces Concentration ethanol during mg alcohol per cm3 anaerobic Population respiration Growth High concentrations of ethanol is poisonous to young yeast So only old yeast survive Time (hours)  b) Logistic Curves in Lab Experiments Simple logistic curves only work with organisms such as Paramecium that have no larval/young stage Adult divides (into two) to form another adult The entire population is identical and feed on the same food  Logistic curve: ii) problem of “overshoot” Overshoot Population briefly rises above carrying capacity is due to larvae/young feeding on a different food to adult Young are born at a time when food supply is adequate By the time they grow and start feeding on adult food… The food is depleted and unable to support the increased population Some must die Carrying capacity Daphnia magna  E.g. Cows produce calfs For months they feeds on milk from their mother They do not compete with the adults to feed on grass By the time they are old enough to feed on grass… Compete with the adults So the population is now too large for the available grass So some die from starvation (the weakest/least competitive ones) Carrying capacity is the maximum number of individuals of population that may be supported by the environmental conditions Which factor of the environment is the most critical one for the carrying capacity? (last week’s lecture) The limiting factor  Lab vs. Wild Populations A stable carrying capacity occurs in the lab In the wild is unnatural It assumes that the only mortality is starvation and that starvation can be controlled  Logistic curve: iii) No carrying capacity due to starvation Field experiments on wild populations tend to result in mass starvation An example is the release of reindeer onto two islands Both populations became extinct as they ate all the food available No predators was present If the population survives the crash it will recover to give a series of population explosions each followed by mass starvation An example is shown by Daphnia in a Canadian lake: Logistic curves are often not occurring in wild populations Some population abundances tend to oscillate - not stabilize Starvation is unusual in wild populations Wild populations are controlled by other factors: Predation Parasitism Diseases  Logistic curve: iv) No carrying capacity due to extreme climate Ibex in the mountains of Switzerland showed rapidly recovered after hunting had been banned… But the population never stabilized The population is controlled by winter food (plants) In cold winters plants have little growth so the ibex population falls from lack of food Capra ibex  Logistic curve: iv) No carrying capacity due to extreme climate Many bird populations fluctuate from yearly depending on winter severity Cold winters kill many more birds Birds have a small body mass and so cannot easily stay warm Number of females Passer domesticus Time (years) Populations fluctuating depending on the severity of the winter Common in many Temperate plants and animals Wild populations do not usually show a stable carrying capacity Many different factors are acting on the population abundance Climate Logistic curve v) No carrying capacity due to lack of nesting sites Cormorants on the Great Lakes (USA) all feed on the same food supply However their abundance is different on different islands Their abundance is not controlled by the size of the island But by the number of nesting sites available each year on each island Logistic curve v) No carrying capacity due to lack of nesting sites With high rainfall… The lake level rises and low-lying islands lose most of their nesting sites while more rocky islands are little affected Annual populations depend on rainfall Water level affect different locations at different extents by the same weather