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Populations in Ecosystems • Def’n: A population is the sum of the members of a particular species that lives in a particular ecosystem. (so what’s an ecosystem?) • An ecosystem is made of communities of living animals and plants that coexist, influence and depend on each other within their environment. The ecosystem includes living (biotic) and non-living (abiotic) parts. • To study ecosystems, we need to study the individual populations that exist within them. • We look at population size, density and distribution. Population Size • By monitoring the size of a population (how many individuals are in it), we can determine if it is endangered or is endangering other species. (For example, moose were introduced to Newfoundland a hundred years ago and are now over-running the province, destroying the vegetation and endangering motorists. A cull has just been announced.) Populations are influenced by; • • • • Births Deaths Immigration – individuals entering the area Emigration – individuals leaving the area • A balance of these four factors determines whether a population increases, decreases or stays the same. To measure population size (three ways) 1 Count every individual (census). This is tough to do in a big population or area. …but you can do it with elephants (aerial count). 2 Sample. Count within a sample area (quadrat), then extrapolate (good for plants or slow animals). For example, count the flowers in 1 m2 of a field, then multiply by the area of the field. Ex. 2 – turtles are counted in five quadrats of 1 km2 each in a wildlife preserve that measures 100 km by 300 km. Give an estimate to the population size. quadrat 1 – 37 turtles quadrat 2 – 59 turtles quadrat 3 – 1 turtles quadrat 4 – 233 turtles quadrat 5 – 76 turtles turtles counted sample area = pop size total area what’s the population size? solution 406 turtles = population size 5 km2 30 000 km2 Population size = 406 * 30000 = 2 436 000 turtles 5 Mark and recapture Good for fast moving animals with a large range. The idea here is to tag a certain number of individuals, then release them. The proportion of tagged: untagged animals observed can give a population estimate. Example – 1000 robins from a population are tagged and released. Three months later, 500 individuals are captured, of which 135 had been tagged. Estimate the population size. (Hint: X-multiply) solution # tagged birds in capture = Population size 1000 Pop size = # tagged in recapture # recaptured 135 500 Pop size = 500*1000 = 3703 robins 135 Population Density is.. The number of individuals per unit area (or volume) of habitat. (what’s habitat? – the specific environment that a population lives in). It is simply determined by dividing the number of individuals by the area (or volume) of habitat. Example: 300 000 sunfish live in 120 000 000 m3 of water in a pond. What’s the population density? solution • Sunfish population density = = 300 000 fish 120 000 000 m3 .0025 sunfish/m3 of water The population density can give useful information about the health of a population. Population Distribution • Populations of animals or plants can be uniformly distributed (spread out evenly), grouped in clumps (ex., flocks of birds, schools of fish), or randomly distributed (many plants) Ecological Factors • Within an ecosystem, each species has a habitat, the environment in which it lives. The habitat includes living (biotic) and non-living (abiotic) factors. Examples of each are below; • Biotic factors – predators, prey, competitors for space and food, ‘cooperative’ species, etc. • Abiotic factors – sunlight, rain, soil quality, pH, etc. Limiting Factors • These are factors that, if there is not enough, the population density will decrease. ex., the density of plants at the floor of a tropical forest is low because sunlight, a limiting factor, is very low there. ex. 2, Many plants need to be pollinated by bees to reproduce. This means that bees are a limiting factor. Almond trees are threatened by the collapse of the bee population due to environmental conditions Biological Cycles • The population size of a species can fluctuate over time. This is common in seasonal species like mosquitoes or in predator-prey relationships like wolves and deers, or lynx and snowshoe hares. • When lynx are few, the snowshoe hare increase in number. This causes lynx to become more numerous. The lynx then reduce the hare population by predation until the lynx population runs out of food and crashes, thus starting the cycle again.