Linking Lynx and Lodgepole Pines - SJMA
Lynx and Snowshoe Hare Cycle population cycling of the lynx and the snowshoe hare. Tech Tip: Students select the right or left arrows (¢ or ¡) to. The spectacular cycles of snowshoe hares and their predators have .. In particular, the snowshoe hare cycle is not strictly a lynx–hare cycle. Kerry Murphy for sparking my curiosity in Canada lynx and snowshoe hares in the first place. Additionally, I want . Relationship between horizontal cover and snowshoe hare density for (a) all transects, and mapping technical guide .
These shortages of so-called rabbits, which apparently occurred approximately every 10 years, are regularly mentioned in Canadian historical documents from the 18th and 19th centuries.
Those rabbits were in fact snowshoe hares Lepus americanusand their year cycle is one of the most intriguing features of the ecology of the boreal forest.
BBC - GCSE Bitesize: Predators and their prey
Ten-year cycles were first analyzed quantitatively when wildlife biologists began to plot the fur trading records of Hudson's Bay Company during the early s. The Hudson's Bay Company, established inkept meticulous records of the numbers of furs traded from different posts spread across Canada. The most famous time series drawn together from those records was that of Canada lynx Elton and Nicholson ; Figure 1. The lynx is a specialist predator of snowshoe hares, and the rise and fall in lynx numbers mirrors, with a slight time lag, the rise and fall of snowshoe hare populations across the boreal region.
The spectacular cycles of snowshoe hares and their predators have captured the attention of biologists as well as historians. These cycles are highlighted in virtually all ecology texts and are often cited as one of the few examples of Lotka-Volterra predator—prey equations, a simple model which shows never-ending oscillations in the numbers of predators and their prey.
Cycles seem to violate the implicit assumption of many ecologists that there is a balance in nature, and anyone living in the boreal forest would be hard pressed to recognize a balance among the boom and bust in nature's economy.
The challenge to biologists has been to understand the mechanisms behind these cycles, which has not been easy. Fortunately, over the last 40 years ecologists working in Alberta, the Yukon Territory, and Alaska have put together an array of studies that have resolved most, but not all, of the enigmas behind these cycles KeithBoutin et al.
To understand any fluctuating population, one must first know in detail the mechanisms of changes in births, deaths, and movements that are the proximate causes of the changes in numbers. Before we describe these details, we should note that these year hare cycles tend to occur in synchrony across broad regions. Indeed, hares across most of Canada and Alaska reached a peak in — during the most recent cycle. We explain the reasons behind this synchrony below, but let us note here that movements of hares cannot explain these population changes via immigration or emigration.
Movements on a local level might be important, but at the regional level all populations rise and fall in unison. Population changes must be driven by changes in births and deaths. Reproductive and survival changes Snowshoe hares can have three or four litters over a summer, with five leverets on average in each litter.
All hares begin to breed in spring when they are 1 year old, so age at sexual maturity is a constant. If reproductive rates are to vary, only litter size or the number of litters can change. Lloyd Keith and his students at the University of Wisconsin, working in central Alberta, supplied the first detailed description of the way in which hares change their reproductive rate over a year cycle Cary and Keith Reproductive output reaches its peak very early in the phase of population increase, when females are producing 16—18 young per summer.
It then begins to fall rapidly while numbers are still rising; it reaches a nadir in the year of the density peak or 1—2 years thereafter, during the decline phase of the cycle. The response of reproductive output to hare density seems to lag 1—2 years, so that, as shown in Figure 2for example, the lowest reproductive output occurred in2 years after the density peak of We found a similar but not identical pattern of response in hares in the southwestern Yukon.
The reproductive rate at the cyclic peak in the Yukon was about two-thirds of the maximum, compared with one-half of the maximum in the Alberta peak.
But reproduction in both areas was minimal in the decline phase about one-third of maximum reproduction and highest in the early increase phase of the year cycle StefanHodges. Changes in mortality rates are the other driver of changes in hare numbers over the cycle. At Kluane Lake in the Yukon we measured survival rates of hares with radio collars during two population cycles. Additional data on survival of hares over two cycles in central Alberta was obtained through mark-and-recapture methods and radiotelemetry Keith The pattern of change for adult hare survival is shown in Figure 3.
Adult survival rates begin to drop slowly as the population increases to a peak but then drops dramatically for 1 to 2 years, part of the cause of the collapse in population numbers Krebs et al. Once low numbers are reached, adult survival rates improve slowly but do not reach the maximum until 4 to 5 years after the peak. Juvenile survival can be broken down into preweaning survival for the first 30 days of life, and postweaning survival from 30 days to 1 year of age the following April.
Postweaning survival follows the pattern already illustrated in Figure 3 for adult hares. Preweaning survival is more difficult to measure, and we have data only from the Yukon for this stage of the life cycle.
We caged pregnant hares caught in the wild when they were near term and then radio-tagged the leverets immediately after birth when the cage was removed. Figure 4 shows that survival is very poor in this early stage. Relatively high survival occurs only in the first 2 to 3 years of the increase phase, and survival is already low before peak hare density is reached.
Preweaning survival remains low in the peak and at least 2 years into the decline phase, and the lowest survival occurs near the end of the decline when hare density is already very low Stefan The demographic pattern of the hare cycle is remarkably clear and consistent.
Lloyd Keith and colleagues found the same changes in central Alberta cycles that we found in southwestern Yukon cycles Keith and Windberg The key finding is that both reproduction and survival rates begin to decay in the increase phase of the cycle, 2 years before peak densities are reached.
Maximal reproduction and highest survival rates occur early in the increase phase of the cycle. By the late increase phase, reproduction has already slowed and survival rates of both adults and juveniles are falling. Both reproduction and survival rates continue to fall for 2 to 3 years after the peak of the cycle, and over the low phase they start to recover to high values.
Causes of the cycle What causes these changes in reproduction and survival?
- Predators and their prey
There are three main factors that seem most likely to cause hare cycles: In addition to these single-factor explanations, two multifactor explanations have been suggested, one involving food and predation, and the other—the most complex hypothesis—involving all three factors. Many other factors might affect snowshoe hare cycles, but these seem more likely to be modifying influences than primary causes.
Relationship Between Rabbits & Lynx
Disease and parasitism are two ecological factors that might affect hare populations but do not seem to be an essential cause of cycles. Parasite loads, for example, might cause hares to be in poor condition and therefore more susceptible to predators. Lloyd Keith and his students surveyed hare parasite loads for many years in Alberta and concluded that none of the many parasites of the hares caused much direct mortality Keith et al.
Experimental work with antihelminthics in field populations of hares either had no measurable impact on reproduction and survival Sovell and Holmes or produced minimal effects Murray et al. The conclusion is that disease and parasites may affect some hare populations sporadically see reports in Chitty, for examplebut they cannot be an essential cause of cycles.
The food hypothesis is attractive because it can explain both why reproduction changes over the cycle and why survival might change as well. There are two variants of the food hypothesis. First, hares may run out of food and starve, or, second, the quality of the food may decline.
Because hares eat a variety of green plants in the summer, no one has considered food shortage in summer to be an important factor. Winter food plants are the small terminal twigs of willow, birch, and small trees, as well as other shrubs; most studies have concentrated on the possibility that winter foods are limiting to hares Keith et al.
Like all herbivores, snowshoe hares have preferred winter foods and may browse a large fraction of these preferred plants at the peak of the cycle. There is little evidence from our Yukon studies that overall food quantity is limiting at any time.
We measured food abundance over the cycle by quantifying edible forage and we assessed consumption rates of marked twigs. Consumption increased markedly during the peak Smith et al. Alternatively, food quality could change over the cycle. John Bryant at the University of Alaska suggested one attractive qualitative food hypothesis based on secondary chemicals: Shrubs and small trees can fight back against browsers by increasing their content of secondary chemicals such as tannins and resins, which deter digestion in herbivores Bryant Indeed, experimental browsing of shrubs in Alaska has shown that the plants can respond to damage by increasing their secondary chemical defenses Bryant et al.
The key question is whether these plant changes can influence the hare cycle. To answer this question, we conducted five food-addition experiments during two hare cycles in the southwestern Yukon.
In four we provided high-quality rabbit chow without limit to hares; in the fifth experiment we added high-quality natural food to a declining hare population Krebs et al. The response of hares to rabbit chow is classic: Hares move into the food-addition areas and their density increases approximately two- to threefold in comparison with control areas. But once the density increases on the food-addition areas, the hare cycle continues unchanged.
Hares decline in number at the same time and at the same rate on the food areas as on unmanipulated controls. Artificial food-addition experiments have been criticized because the added food is high quality and not natural.
Linking Lynx and Lodgepole Pines
We tried to address this criticism by supplying natural food to one declining hare population. Tony Sinclair and Jamie Smith had shown that snowshoe hares largely avoided small white spruce trees because the needles contain camphor Sinclair and SmithRodgers and Sinclair Consequently, small spruce seedlings were the least preferred food in cafeteria trials with hares.
But foliage from large white spruce trees with branches beyond the reach of hares contain no camphor, and these branches become highly preferred food when supplied in a cafeteria trial. This observation was dramatically verified when a large white spruce tree was blown over by a windstorm: Hares devoured the fallen branches.
We therefore decided to feed a population of hares through a decline by cutting down white spruce trees and thus providing natural, highly preferred food to a collapsing hare population.Hunting Snowshoe Hares with 22s
Stan Boutin and Scott Gilbert did this experiment over three winters, with the results shown in Figure 5. The extra natural food produced no detectable effect on the rate of population collapse. The failure of this extra food to affect the hare population decline was shown clearly on five areas in two cycles in the southwest Yukon. Such results imply that food shortage by itself is not the explanation for the hare cycle. Whatever secondary chemical changes occur in winter food plants, they are at most a contributing factor, not the primary cause of the hare collapse.
The animals begin to starve and, in their weakened state, lynx catch them more easily. Undernourished hares also produce fewer offspring, with newborns less robust and therefore less likely to survive. A study of pregnant wild hares published in the November "Journal of Animal Ecology" found that any kind of stress, not just fear of predators, caused a similar decrease in reproductive efficiency.
Lynx Cycles For the lynx, feast rapidly turns into famine.
After a year or two of decline, the hare population bottoms out and starvation starts stalking the cats. At first, they make do with rodents, squirrels and wild birds; but in terms of both quantity and quality, this prey is inadequate. When their nutritional needs aren't being met, lynx can't maintain the reserves of body fat necessary to survive in the cold climate.
They're more vulnerable to predation and starvation, and the females can't provide their kittens with enough nourishment during gestation and afterward. According to the government of the Northwest Territories, such declines in lynx populations typically last three to five years.
When food plants grow back unimpeded, and the hare population accordingly recovers, the lynx population follows suit.
The Global Warming Factor Lynx expert Stan Boutin of the University of Alberta is concerned about the impact global warming will have on lynx-snowshoe hare cycles. Among three genetically distinct lynx subspecies isolated from each other by the Rocky Mountains or long distances, some pursue hare over crusty snow, which gives the cats the advantage, while others chase their quarry over fluffy snow, which gives the hares the advantage.
Rising temperatures will result in fluffy snow turning crusty in some regions, thereby increasing the hunting success of lynx -- and influencing the cycle in ways that might have a negative impact on both animals, he says.