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Lecture 7

BIOL 3170 Lecture 7: Lecture 7 - Population cycles

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BIOL 3170
Mark Vicari

Population cycles Populations often rise and fall predictably in cycles What causes these cycles? • It is not always food availability o Predator/parasite regulation (kind of related to food availability) o Group selection (predicted  proven wrong) o Climate o Intrinsic factors (time delay) o Disturbances o Migration Food availability and predator-prey relationship: • Example: Hare and lynx population cycles • Lynx populations increase and decrease after hare populations increase and decrease o This indicates that the lynx population is responding to the hare population ▪ Therefore, it must be regulated by the availability of hares • What regulated hare population? o It is predicted that the hare population is somewhat limited by the lynx population, but mainly by plant availability • Thus the cycles are the result of a three-way, carnivore-herbivore-plant interaction Group selection and parasite regulation: • Group selection = Idea that alleles can become prevalent in a population because they provide a benefit to the group regardless of the effect the alleles for the individual’s fitness o i.e. Natural selection acts on the group, not the individual • Widely been rejected Group selection idea: At peaks (near K), natural selection selects for individuals with low reproductive success so population growth slows down and popln declines?? e.g. individuals which produce less offspring are selected for because it will benefit the group Wynne-Edwards proposed that the red grouse population cycles (below) can only be explained by group selection:  why though?? • However, it was discovered was that the heather plant (grouse’s food) was a host for a nematode parasite which also affect the grouse o Further study showed that grouse populations were lowest when nematode populations were highest ▪ Affected birds had smaller brood sizes and were preyed upon more often ▪ When grouses were treated for nematodes, the populations didn’t crash o Thus, it wasn’t group selection that caused the cycles, it was the nematodes Climate: • Example: Soay sheep on St Kilda archipelago, Scotland • K is exceeded every 3-5 years which leads to crashes o After each crash, population requires usually 2 years to return to K • These crashes seem to be in response to pressure changes, NAO North Atlantic oscillation (NAO) • Pressure changes which affect climate i
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