Notice where the graph says “R2=0.53.” What does that mean? R2 is a statistic that can range from zero to one. The graph above suggests that these processes are important on Isle Royale. These two ideas can be combined by supposing that kill rate should be greatest during years when the ratio of moose per wolf in the population is greatest. Why? One reason is that when predators are more abundant, they might spend more time on activities like defending their territories from other predators. A more recent idea is that kill rate might also tend to be lower during years when predators are more abundant. One of the earliest theories, going back nearly a century, is that kill rate should be greatest during years when prey are most abundant, mainly because abundant prey should be easier to find when they are abundant. The challenge has been to understand why. Imagine an entire winter where you got twice the normal amount of food, or less than a third.Įcologists have long known that predator kill rates are variable. That kill rate was less than a third of a typical kill rate. The lowest kill rate we ever observed was in 1981. That kill rate was more than twice the average kill rate. The highest kill rate we ever observed was in 1992. The most remarkable observation is just how variable kill rate is. In this graph, the symbols represent the average monthly kill rate for each winter between 19. Consequently, the units on kill rate are kills per wolf per month. More specifically, the kill rate is the number of moose killed, divided by the number of wolves in the population making those kills, divided by the time we spent observing those kill events. The kill rate is calculated by observing how frequently wolves kill moose. The per capita kill rate (or sometimes just called kill rate) is a special statistic indicating how much food the average wolf in a population gets. The true explanation is quite a bit more complex. In other words, fluctuations of wolves and moose on Isle Royale are not explained by either simple top-down or bottom-up explanations. This graph shows that wolf and moose abundances are neither positively nor negatively related. This kind of relationship is what ecologists would refer to as the top-down control of a food chain. This kind of relationship is what ecologists refer to as the bottom-up control of a food chain.īy contrast, if moose abundance was determined primarily by wolf predation, and if wolf abundance was a good indication of predation pressure, then wolf and moose abundance would be negatively related. If, for example, wolf abundance was determined primarily by food availability and if total moose abundance indicated food availability, then wolf and moose abundances would be positively related. This graph tells a great deal about how wolf and moose populations are interconnected. Wolf and moose densities are the total number of wolves and moose on Isle Royale, divided by the size of Isle Royale, 544km 2. Expressing abundance in terms of density allows us to compare Isle Royale wolf-moose dynamics with those observed in other parts of the world. The year 1980 was an important tipping point in wolf-moose dynamics, triggered by canine parvovirus (Wilmers et al. The open circles represent years prior to 1980, and the black circles are years after 1980. Each symbol on this graph represents the density of moose (read from the horizontal axis) and the density of wolves (read from the vertical axis) for a particular year.
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