Chapter 13 Population Dynamics over Space and Time Populations Fluctuate Naturally over Time Vocab Overshoot Die off Fig 13 1Population fluctuations in red deer on the Isle of Rum Red deer have a stable population fluctuation btwn 200 400 Fig 13 2 Population fluctuations in the algae of Lake Erie Has wild fluctuations high oscillation Fig 13 3 Whitefish Age Structure The whitefish population in Lake Erie experienced an unusually high amount of reproduction in 1944 However young fish are not captured by fishing nets until they are 2 years old so the large 1944 cohort was not detected until 1946 This large reproduction event led to age structures that were dominated by that cohort of fish in subsequent years Fig 13 6 The population dynamics of wolves and moose on Isle Royale Population estimates from 1959 to 2011 indicated that moose and wolf populations both experience wide fluctuations Fig 13 7 The overshoot and die off of reindeer population A herd of 25 reindeer was introduced to St Paul Island in Alaska in 1911 The population experienced a rapid increase in size that approximates an exponential J shaped growth curve shown by the dashed black line After growing to nearly 2 000 animals in 1938 the population crashed probably because the animals exhausted the food supply Government attempted to cull the population in 1940 and 1941 but by 1950 only 8 individuals remained In 1951 31 new reindeer brought to the island and since then the population has been managed Fig 13 8 Cyclic fluctuation in gyrfalcons From 1730 to 1770 falconry was popular in Denmark and the number of falcons captured and exported from Iceland showed regular cycles that occurred approximately every 10 years After 1770 the popularity of falconry declined and the low demand for falcons could be met even in low population years Fig 13 9 Cyclical fluctuations of grouse species in two different provinces of Finland The three species of grouse experience cycles every 6 to 7 years and appear to fluctuate in synchrony with each other and across and across the provinces Density dependence with time delays can cause populations to be inherently cyclic Vocab Delayed density dependence When density dependence occurs based on a population density at some time in the past Vocab Damped oscillations A pattern of population growth in which the population initially oscillates but the magnitude of the oscillations decline over time Density dependence with time delays can cause populations to be inherently cyclic Delayed Density Dependence Equation rN 1 N t K dN dT Vocab Delayed density dependence Damped oscillations Stable limit cycles Fig 13 10 Population modelers have determined that the amount of cycling in a population experiencing delayed density dependence depends on the product of r and When this product is a low value r 37 the population approaches the carrying capacity without any oscillations If this product is an intermediate value 37 r 1 57 the population initially oscillates but the magnitude of the oscillations declines over time this is known as damped oscillations When the product is a high value r 1 57 the population continues to exhibit large oscillations over time which is known as a stable limit cycle Delayed Density Dependence in the Flixweed Flix weed Descurania Sophia was introduced to North America from Europe It has K 100 r 1 1 and 1 There were 10 plants per m2 and 20 plants per m2 in year 2 Using this data we can calculate the expected change in population size in year 3 The flixweed will add 20 individuals in year 3 dN dt N 2 1 r N 2 1 dN dt K 1 1 20 1 10 100 dN dt 20 r N 3 1 N 3 1 100 35 1 1 40 1 20 K dN dt In year 4 35 new flixweed will be added to the population making a total population of 75 individuals Fig 13 11 The importance of energy reserves in causing population cycling a Daphnia geleata water fleas can store high amounts of energy which allows them to survive and reproduce even after reaching carrying capacity When energy reserves run out the population crashes to very low numbers and then rebounds and continues to oscillate b Bosmina longirostris water fleas can only store a low amount of energy so as the population nears carrying capacity they experience reduced survival and reproduction As a result the population remains near its carrying capacity and oscillates much less Fig 13 12 A J Nicholson s experiment on Population cycling in sheep bowlies a When researchers limited food for larvae but not adults they observed a delay between the time that the adults produced a large number of eggs and the time these eggs hatched into larvae died from high larval competition and failed to produce new adults As the adult population increased the unlimited food upply allowed them to continue laying eggs The large number of eggs hatched but they all died and did not reach adulthood due to limited food supply Before the last few adults died they laid a small number of eggs When these eggs hatched the fixed food supply provided an abundance of food for the low number of larvae As a result the larvae experienced a high rate of survival and growth As a result the adult population experienced regular cycles b When adults were initially raised with unlimited food but then given limited food halfway through the experiment they began to experience density dependence without a time delay As a result the adult population still fluctuated but no longer experienced regular cycles THIS PROVED that time delays between life stages caused population cycles Chance events can cause small populations to go extinct Vocab Deterministic model predicts a result without accounting for random variation in population growth rate Stochastic model incorporates random variation in population growth rate Demographic stochasticity Variation in birth rates and death rates due to random differences among individuals Environmental stochasticity Variation in birth and death rates due to random changes in the environmental conditions Fig 13 13 Smaller populations and the likelihood of extinction Bird populations of the Channel Islands off the coast of California were measured in terms of the number of breeding pairs Islands with larger population sizes had a reduced probability of going extinct over a period of 80 years Fig 13 14 The probability of plant populations going extinct Reserachers surveryed 359 plant populations in Germany in 1986 and then again in 1996 They then placed the populations into six size categories and averaged