A species’ response to climate change is due to the direct effect of climate change as well as the indirect effect of other factors such as predator-prey interactions. Gilg et al. (2009) chose to observe the terrestrial vertebrate predator-prey community of the high Arctic to see if it exemplifies these effects. This community is made of one prey (the collard lemming) and four predators (the snowy owl, the Arctic fox, the long tailed skua, and the stoat). The authors found that climate change will indirectly reduce the predator’s reproductive success and population densities, and may ultimately lead to local extinction of some of the predator species. — Clara Lyashevsky
Gilg, O., Sittler, B., and Hanski, I., 2009. Climate change and cyclic predator—prey population dynamics in the high Arctic. Global Change Biology 15, 2634–2652.
The average temperature in the Arctic is expected to increase by 4–7oC in the next 100 years. This increase in temperature will strongly influence the snow regime, duration of snow cover, snow depth, and snow quality. Because snow is a key environmental factor in high Arctic ecosystems these changes will influence the terrestrial Arctic populations and communities. Previous studies have predicted changes in phylogenies, geographical ranges, and population sizes due to climate change. The authors interpret the results of a study that assesses the impact of climate change on high Arctic terrestrial vertebrate community and they found that climate change affects changes in snow depth and duration as well as increased ice crusting in winter and spring. These instances impact the dynamics of terrestrial Arctic vertebrates.
Using data collected since 1988 in the Karupelv valley in eastern Greenland, the authors constructed a model to run likely scenarios of how climate change would influence the phenology and demography of the species involved, in particular the dynamics of the lemming-predator community under altered environmental conditions.
In northeast Greenland, the collard lemming reproduces mostly in winter and exhibits regular cyclic dynamics. During this time, only the stoat remains a significant predator of the lemming. In the summers, when the lemmings are abundant, the Arctic fox, the long-tailed skua, and the snowy owl are all common and feed on lemming. Although the different predators prey on the same animal, they differ in their functional and numerical responses. For example, Arctic foxes and long-tailed skuas maintain a stable density of adults, fox litters can be large while skuas only lay a maximum of two eggs a year.
The Karupelv valley is a large Tundra area. The Zackenberg valley is a smaller one and was established as an ecological research and monitoring site in 1995 using the same methods as at Karupelv. In order to create a logical model, the authors took into account the fact that the lemming winter nests were counted within a small area at Zackenberg where the habitat is more favorable for the lemming than in the entire Zackenberg study area used for monitoring the predator populations. The authors adjusted this by making the reasonable assumption that the numerical response of young skaus to lemming density is the same at both sites. As a consequence, they used a maximum likelihood function to force the Zackenberg response to fit the response previously published for Karupelv.
The model created by the authors is defined by two differential equations for the lemming and the stoat and by nondynamic equations that give the numerical responses of the remaining predators. They used N to denote the size of the lemming populations, P the size of the stoat population, and Pi and Piyoung the numbers of adult and young individuals in the remaining predators (i = 1–3, for snowy owl, long-tailed skua, and Arctic fox). All of the predators have type III functional responses, with W being the maximum predation rate (in lemmings per year) and D the slope of the functional response.
Snow conditions affect the ecology of the collard lemmings in the high Arctic because they reproduce primarily in the winter and spend most of the year under the cover of snow. Any change in snow conditions in the spring will influence the lemming dynamics directly. There are three main changes that are expected to occur in the snow regime in eastern Greenland. First, as warming climate increases, the length of the snow-free period will increase. Second, increase in temperature and precipitation effects snow quality. Third, warmer climates will increase the average snow depth, which could lead to more solid winter precipitation. The authors suggest that this is the first sign of a severe impact of climate change on the lemming-predator communities in northeast Greenland.
The authors came up with four plausible scenarios of how climate change may influence the dynamics of the lemming-predator community. The first scenario (A) is the control for the rest. The second scenario (B) predicts what would happen if there was an increase in availability of alternative food sources for the stoat and the fox. The third scenario (C) changes the numerical responses of the two mammalian predators, reducing the maximum rate of mortality of the stoat and increasing the minimum density of adult foxes. The fourth scenario (D) changed both the functional and numerical responses of the stoat and leaves the responses of the Arctic fox unchanged.
Scenario A, increasing the length of the snow-free period, greatly increases cycle length and reduces the amplitude and peak density. Scenario B leads to very contrasting results depending on which predator species is affected. Changing the functional response of the Arctic fox has practically no impact on lemming dynamics while increasing the half-saturation constant of the stoat reduces lemming peak density and amplitude. However, cycle length is not affected. Scenario C, increasing fox density, does not significantly change the lemming dynamics; it increases the cycle length slightly. Improving stoat survival slightly increases the amplitude and cycle length and reduces peak densities. Scenario D resulted in all the major expected changes: increase in the length of the snow-free period with related changes in the phonologies of the four predators, increase in stochastic lemming mortality in the spring due to altered snow quality, and improved functional and numerical responses of the stoat due to additional food sources. In conclusion, the authors predict that climate change can lead to smaller cycles and peak densities of lemmings.
From the data calculated by the authors’ model, the effect of climate change is much greater for Zackenberg than for Karupelv. The lemmings in Karupelv valley seem to decrease in density as a result of climate change but the effect is greater on those in Zackenberg valley. There is a large contrast between the two study sites, Zackenberg and Karupelv, suggesting that some seemingly minor environmental differences may lead to substantially different population dynamic consequences.
Reduced maximum density of lemmings is detrimental to the populations of the predators. From the experimental data, the authors conclude that climate change will indirectly induce a decline in the predators’ reproductive success and population densities and may ultimately lead to local extinctions of some predator species.