we have repeatedly discovered a sort of "balance" in vertebrate populations -- a balance between mortality and reproduction

this balance has appeared in comparisons of life-history strategies, in evidence for density-dependent population regulation, and in evidence that natural selection shapes life-history strategies ... we have seen that many (or all) of these balances result from trade-offs between reproduction and survival

there are also "balances" in the relationships between species ... in particular, between populations of predators and their prey

the wolf (called Gray Wolf by mammalogists but also known as "timber wolf" or just "wolf") and its prey (mostly species of deer, including moose, elk, and caribou) provide an example

on Isle Royale in Lake Superior, moose increased to 1000-3000 animals in early 1930s, decreased drastically by disease and starvation a few years later, built up again and decreased by starvation again in late 1940s -- after 1949 when wolves arrived (apparently on their own across the ice during winter), fluctuations in numbers of moose diminished (total slowly increased to 1200 in 1975 then stabilized with minor fluctuations)

in Denali National Park in Alaska, Dall sheep increased steadily to 1928 and then suffered widespread mortality in following few winters -- subsequently wolf numbers increased in the park -- sheep numbers have since remained relatively stable -- wolves in this area also prey on moose and caribou (the latter only during their seasonal migration)

this evidence (admittedly circumstantial) suggests that wolves control the numbers of their prey and even that wolves and their prey reach an equilibrium

to show that predators and prey are in equilibrium, we must demonstrate that . . .

• increasing or decreasing the numbers of either species results in corresponding changes in the numbers of the other species
• numbers of both species eventually return to original levels

no controlled and replicated experiments on vertebrate predators have confirmed both of these conditions -- but moose and wolves have been studied many places in North America in the last four decades -- the results indicate that . . .

wolf populations often affect moose populations

(1) wolves account for most of the mortality of moose on Isle Royale

during 1958-62 there were on average 23 wolves, 600 moose in late winter, 225 moose calves each spring -- wolves killed an estimated 140 calves and 83 adults/year -- 85 calves survived their first year (225 minus 140) -- annual production of the moose herd and the annual kill by wolves was about the same -- typical for moose-wolf systems in Canada

(2) comparison of areas with and without removal of predators confirm that moose numbers are higher where predators are removed

in Alaska and Yukon, predators (wolves and grizzly bears) were systematically poisoned from 1948-1960 -- moose populations soared -- after poisoning stopped both moose and predators returned to their original levels by 1976 -- now a major conservation issue in Alaska

(3) moose populations average twice as dense in areas with alternative prey for wolves in areas where deer or elk are abundant, these smaller species of deer are easier prey for wolves

but the effect of wolves on moose depends on weather and habitat

(4) deep snow makes calves more vulnerable to wolves

in several studies, severe winters have resulted in increased predation and lower moose numbers the following summer

(5) moose with calves tend to seek safe places in the habitat (often called refuges)

when moose numbers decrease a greater proportion of mothers and calves have safe refuges and thus escape predation -- limited refuges for prey create density-dependent population control (higher proportion of moose are killed when moose numbers are high)

(6) numbers of moose on Isle Royale doubled during 1960-1970

numbers of calves increased only slightly -- increased numbers and less nutritious food (as a result of forest succession following fires) resulted in greater vulnerability of undernourished calves to wolves -- wolf population doubled in following decade 1970-1980

conversely, moose populations affect wolf populations

(7) wolves spend a lot of time and energy in killing enough prey to feed themselves

on Isle Royale, of 131 moose detected by hunting packs of wolves, only 66 are tested, 7 wounded, and 6 killed -- most kills are young, old, or diseased moose

(8) wolf populations in Canada, when not controlled by humans, tend to increase or decrease when moose populations increase or decrease

on Isle Royale wolves increased during 1960-70 when moose increased -- wolves decreased during 1980-2000 when moose decreased (because suppression of forest fires resulted in less regenerating forest that provides optimal food for moose)

note some general properties of predator-prey interactions . . .

• refuges for prey tend to protect prey from extinction
• alternative prey for predators tend to protect predators from extinction
• availability of food and intensity of predation have interacting influences on prey
  (the effect of a change in one depends on the level of the other)

predators sometimes seem to kill just enough prey -- and prey seem just vulnerable enough

wolves even produce a population of healthier moose

they kill primarily young, diseased, and old moose and leave mostly healthy middle-aged animals in their most productive years -- when moose populations were high in 1929-30 only 6% of births were twins -- when populations were lower in 1959-63 38% of births were twins (recall the changes in fecundity of Red Deer as their density changed on Rhum)

what do we make of this observation?

• do populations evolve to avoid over-exploiting their food?
• do populations evolve to maximize the sustained yield of their food?
• do populations evolve to maximize the general well-being?

notice that we are now talking about an evolutionary balance between populations of predators and herbivores -- in other words we are interested in changes in the alleles in populations -- an ultimate explanation for predator-prey interactions

in contrast, earlier we discussed an ecological balance between predators and prey -- we focused on the behavioral and physiological responses of individual predators and prey -- a proximate explanation for predator-prey interactions