Environmental Problems, Their Causes, and Sustainability 1

Number of individuals(a) Stable(b) Irruptive(c) CyclicTime(d) IrregularFigure 9-7 General types of simplified population changecurves found in nature. (a) The population size of a species witha fairly stable population fluctuates slightly above and below itscarrying capacity. (b) The populations of some species mayoccasionally explode, or irrupt, to a high peak and then crash toa more stable lower level. (c) Other species undergo sharp increasesin their numbers, followed by crashes over fairly regulartime intervals. Predators sometimes are blamed, but the actualcauses of such boom-and-bust cycles are poorly understood.(d) The population sizes of some species change irregularly formostly unknown reasons.Do Predators Control Population Size? TheLynx–Hare CycleThe population sizes of some predators and theirprey change in cycles that appear to be caused byinteraction between the two species, but other factorsmay be involved.Some species that interact as predator and prey undergocyclic changes in their numbers: sharp increasesare followed by crashes (Figure 9-8).For decades, predation has been the explanationfor the 10-year population cycles of the snowshoe hareand its predator, the Canadian lynx. According to thistop-down control hypothesis, lynx preying on hares periodicallyreduce the hare population. The shortage ofhares then reduces the lynx population, which allowsthe hare population to build up again. At some pointthe lynx population increases to take advantage of theincreased supply of hares, starting the cycle again.Controlled laboratory experiments involving branconidwasp predators and bean weevil prey produce asimilar pattern of out-of-phase fluctuations in thepredator and prey populations.Some research has cast doubt on this appealing hypothesis.Researchers have found that some snowshoehare populations have similar 10-year boom-and-bustcycles on islands where lynx are absent. These scientistshypothesize that the periodic crashes in the harepopulation can also be influenced by their food supply.Large numbers of hares can die following a periodwhen they consume food plants faster than the plantscan be replenished, especially during winter.Once the hare population crashes, the plants recoverand the hare population begins rising again in ahare–plant cycle. If this bottom-up control hypothesis iscorrect, the lynx do not control hare populations. Instead,the changing hare population size may causefluctuations in the lynx population.The two hypotheses are not mutually exclusive.One problem is that the simple model of the predator–preyrelationship assumes that the lynx are the onlypredators of hares and that the lynx prey only on hares.Neither is true in many of the real-life communitieswhere these species are found. According to extensiveresearch by Charles J. Krebs, the 10-year population cycleof snowshoe hares in boreal forests is caused by aninteraction between predation (by lynx, coyotes, andother predators) and food supplies (especially in winter),and predation is the dominant process.9-2 REPRODUCTIVE PATTERNSAND SURVIVALHow Do Species Reproduce? Sexual PartnersAre Not Always NeededSome species reproduce without having sex andothers reproduce by having sex.Figure 9-8 Population cycles for the snowshoehare and Canadian lynx. At one time scientistsbelieved these curves provided circumstantialevidence that these predator and prey populationsregulated one another. More recent researchsuggests that the periodic swings in thehare population are caused by a combination ofpredation by lynx and other predators (top-downpopulation control) and changes in the availabilityof the food supply for hares. The rise and fallof the hare population apparently helps determinethe lynx population (bottom-up populationcontrol). (Data from D. A. MacLulich)Population size (thousands)16014012010080604020HareLynx01845 1855 1865 1875 1885 1895 1905 1915 1925 1935Year168 CHAPTER 9 Population Ecology