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Biotic Potential Limits to Growth Biotic Potential • Population vs. Individuals – Individuals are born; they grow, and they die. – Populations have a birth rate, a growth rate and a death rate. – Populations can evolve and the show a dispersion pattern. • Recruitment – survival of the young through the early growth stages of a species. • Plants generally reproduce through seeds and spores. • Animals reproduce by laying eggs or bearing live young. Biotic Potential • The biotic potential is the maximum rate at which members of a species can reproduce given unlimited resources and ideal environmental conditions. – The number of offspring that a species may produce under ideal conditions. • This kind of growth starts slowly and then increases rapidly producing an exponential or J-shaped curve. Exponential Growth J-curve • Bacterial growth shows this type of growth. • With plenty of food and the right temperature, bacteria can divide into two about every 20 minutes. • After 24 hours, you would see about 1,000,000,000,000,000,000,00 0,000 (1024), or one million billion billion bacteria! Exponential Growth Exponential or logarithmic growth leads to a much faster population growth. Human Population Growth Over Time Limits to Growth • If bacteria are transferred onto a growth medium, the population increases exponentially initially but then stops increasing. • The reason for this is that the environment can no longer support the vast number of new cells being produced. • A food shortage may result. • Bacteria also produce waste products which may be toxic to themselves. Limits to Growth •A decrease in births and an increase in deaths lead to a population decline. •The J-curve has become an S-shaped curve representing sigmoidal or logistic growth. •Changes in the organism’s environment have prevented the population from realizing its biotic potential. Population Growth • If migration is ignored (to make a simpler model) and population change is monitored based on births and deaths only: • Rate of Change in Population Size = (Births – Deaths) x Population number – If births > deaths => population increases – If deaths > births => population decreases – If births >>> deaths population increases exponentially or logarithmically Causes of Population Fluctuations • The number of individuals in a population changes according to fluctuations in the birth and death rate. • Birth Rate = number of births = # adults in population Natality • Death Rate = Number of Deaths = # adults in population Mortality Population Changes Carrying Capacity • The maximum population size a particular environment can support. • The number of individuals of a species that can survive, in an area over time. • For most populations, four factors interact to set the carrying capacity: – – – – Availability of raw materials – food, water. Availability of energy Accumulation of waste products and their disposal. Interactions among organisms – competition, predation, parasitism. • All of these forces acting together to limit population size is known as environmental resistance. Carrying Capacity •Certain factors have a primary role in limiting the size of a population. •These are called limiting factors. •Some are easy to identify whilst others are less obvious. Limiting Factors • Grass plants need both nitrogen and magnesium in their soil for chlorophyll production. – These are limiting factors as without them the grass population will not increase. – Adding fertilizer containing the two minerals makes the grass grow and reproduce increasing the population. • The carrying capacity has increased because a limiting factor was removed. – The carrying capacity has not been removed because other factors such as water, light, etc. will be a limiting factor. Environmental Resistance • Environmental resistance is – all the limiting factors that tend to reduce population growth rates and set the maximum allowable population size or carrying capacity of an ecosystem. • Accumulation of waste products is often a limiting factor for animals especially when they exist in a small space like goldfish in a bowl. If there is no way to dispose of waste products they can become toxic and kill the organism. • This decline in population size is known as the death or decline phase. • A rapid decrease in population size is known as a crash or bust. Environmental Resistance • Environmental resistance lowers the biotic potential of a population converting the Jcurve to an S-curve. • In this diagram, a fish population could be subjected to the environmental resistance factors of: – – – – – Decreased oxygen supply Low food supply Disease Predators Limited space Changes in Population Size • Both biotic and abiotic factors affect population size. • Some populations fluctuate each year but have a fairly obvious pattern whereas other populations can fluctuate wildly from year to year not producing an obvious trend. – Boom and bust – Colonizers – Random cycles Population Changes • The number of algae in a lake may increase rapidly, producing a choking algal bloom. – This bloom may produce toxins which enter the food chain or can actually make the algae itself die off in an effect called “boom and bust”. • When new species are introduced, the population can fluctuate for many years until it settles down. • Sometimes a population fluctuates based on climate or another abiotic factor. Boom and Bust • In 1911, 25 reindeer were introduced onto an island off Alaska. No predator existed on the island. • The herd grew exponentially until it reached 2000 in 1938. • Overgrazing resulted and the reindeer population declined significantly. • By 1950 only 8 reindeer survived. Colonizing Species • Sheep were introduced into Tasmania by Europeans. The population showed exponential increase initially but once the environmental resistance kicked in, it leveled off to show an Scurve. Wild Fluctuations in Population • Mule Deer in Texas • Fluctuations in mule deer populations are normal. • Possible factors: – – – – Drought Habitat degradation Low reproduction Water due to shift in agricultural practices – Predators