Food Webs

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Photo: coyote © iStock.com/4loops

Declines in terrestrial predators

Most large native carnivores, including wolverine, have severely declined in abundance or have been extirpated from much of their ranges in the more populated regions of North America. Remaining ranges and larger populations are generally in the north and west of the continent.3

North American range of the wolverine
Map: North American range of the wolverine. Click for graphic description (new window).
Long Description for North American range of the wolverine

This map of North America shows the historical and current distributions of the wolverine. Historically the wolverine range extended much further south than the current range. In western North America the historical range extended down the coast as far as northern California and inland to the Sierra Nevada and the American Rockies. The current range in western North America extends south just as far as Vancouver Island; inland the range extends south through the Canadian Rockies into the American Rockies, but not as far as it did in the past. Historically the wolverine range followed the Canada-U.S. border, dipping further south in places and rising north before reaching the eastern coast of the U.S., coming up to the Atlantic Maritime coast and Labrador. Currently the distribution has receded north, though it still includes areas in the following ecozones+: Pacific Maritime, Boreal Cordillera, Taiga Cordillera, Taiga Plains, and Hudson Plains. Current range also includes Alaska, and most of the Arctic Ecozone+ with only a few islands showing no historical or current distribution.

Source: adapted from Hummel and Ray, 20083

In the Newfoundland Boreal Ecozone+, the wolf, a native top predator, was extirpated in the 1920s.4 Eastern coyotes, first sighted in the ecozone+ in 1987, have become a major predator, feeding on a variety of species and competing with native predators such as bear, lynx, and red fox.5

In the Mixedwood Plains Ecozone+, changes in predators and hunting, combined with milder winters and increased forage on lands altered by forestry and agricultural activities, have meant that populations of whitetailed deer have grown rapidly in recent decades.6, 7 Foraging by high numbers of deer has altered forest plant communities,8, 9 thereby affecting habitat for other species, including insects, birds, and small mammals.6

In the Prairies, the decline of the gray wolf began with the extirpation of the plains bison in the late 1800s and continued due to overharvest of ungulates and predator control.10 The loss of the wolf has changed predator-prey dynamics. In southeastern Alberta, western coyote abundance increased 135% between the periods 1977 to 1989 and 1995 to 1996.11

The change in top predators from wolves, which mainly hunted ungulates, to western coyotes, which eat a wider range of foods11, 12 and are not major ungulate predators,13 has shifted the abundance and distribution of prey species.

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Decline of the amphipod Diporeia

Decline of the amphipod Diporeia in Lake Huron

Diporeia density (thousands per m2), 2000 to 2007
Three graphs: decline of the amphipod Diporeia in Lake Huron. Click for graphic description (new window).
Long Description for Decline of the amphipod Diporeia in Lake Huron

This graphic consists of three maps of Lake Huron which display the distribution of densities of the amphipod Diporeia in 2000, 2003, and 2007. The year with the highest density was 2000, with densities decreasing in each subsequent study year. In 2000 Diporeia was found in the majority of the lake with the highest concentrations in the central west portion of the lake (reaching densities of 3,000 to 3,500 amphipods per square metre) and decreasing in density incrementally towards the edges of the lake (where densities were from 0 to 500 per square metre). In 2003 low densities of Diporeia were widely dispersed across a large area of the north and central lake (reaching densities from 0 to 1,500 per square metre). In 2007 a large area of low density (0 to 500 per square metre) was dispersed across the northern and central lake, with four patches of higher density (500 to 1000 Diporeia amphipods per square metre) present on the western side of the lake.

Source: Environment Canada and U.S. Environmental Protection Agency, 20092
Photo: adult diporeia © Tomas Hook
Adult Diporeia (size of a rice grain)
 
Photo: Lake Huron © iStock.com/janeff

Small invertebrates are important in Great Lakes food webs as they provide a link between the base of the web (algae, bacteria, and bits of dead organic matter) which they eat, and fish, which eat them. Since 1995, populations of Diporeia amphipods, historically abundant, widespread, and dominant in deep-water food webs, have declined drastically in all lakes except Lake Superior.2 These declines coincide with the introduction of invasive zebra and quagga mussels, but the continuing downward trend is more complex, likely with several interacting causes.

Declines in Diporeia have had major impacts on Great Lakes food webs, with both forage fish and commercial species negatively affected. For example, when Diporeia declined, growth and body condition of lake whitefish declined significantly in areas of lakes Huron, Ontario, and Michigan.2

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Trends in population cycles

Population cycles are especially important features in boreal forest and tundra,1 Canada’s largest terrestrial ecosystems. Herbivores are at the heart of these systems. The 10-year snowshoe hare cycle drives the cycles of many bird and mammal predators in the boreal forest,19 particularly lynx and coyote. The hare cycle itself is a result of interaction between predation and the vegetation that forms the hares’ food supply.20 In Arctic tundra, lemmings and other small rodents drive population dynamics of many predators.21

Snowshoe hare and lynx cycles, boreal forest, Kluane, Yukon

Density of hares and lynx, 1976 to 2009
Graph: snowshoe hare and lynx cycles, boreal forest, Kluane, Yukon. Click for graphic description (new window).
Long Description for Snowshoe hare and lynx cycles, boreal forest, Kluane, Yukon

This line graph displays the population rise and fall of two species in a predator-prey relationship from 1976 to 2009. Snowshoe hare populations showed cycles of steep growth and decline throughout the study period. Peaks in snowshoe hare population were: 4.4 hares per hectare in 1981, 2.5 hares per hectare in 1988, 2.7 hares per hectare in 1998, and a much lower density of 1.2 hares per hectare in 2006. The measure of lynx populations shown is the number of lynx tracks per 100 kilometres counted in transects, with data beginning in 1976. Lynx populations peaked shortly after hare population peaks. In 1990 to 1991 the mean number of tracks per 100 kilometres peaked at 53.9, in 1998 to 1999 tracks peaked at 82.4, and in 2006 to 2007 tracks peaked 39.4. The overall pattern is that the timing and the magnitude of animal population cycles of lynx followed that of snowshoe hares, and that the last peak of this cycle was smaller and shorter for both animals.

Source: data from Krebs, 201014

Population density peaks in 2006 in Yukon were smaller and shorter than previous peaks. Similar dampening of hare cycles is emerging in the Northwest Territories.15 Continued monitoring is needed to see if this is a change in the cycles or part of natural fluctuations.

Arctic small mammal population cycles

Long datasets are needed to detect and understand ecosystem change, especially when populations may be cyclic.16 Small- mammal monitoring programs in the Northwest Territories and Nunavut, have not been in place long enough to detect trends. Lemming cycles at Bylot Island, Nunavut, showed signs of weakening in the mid-2000s17 but high densities of lemmings in 2008 and 2010 returned the long-term trend to stable.18

Globe

Global Trends

In northern Europe, population cycles in lemmings, voles, grouse, and insects have been weakening over large areas since the early 1990s. Some studies show linkages to climate change, especially to the effects of warmer winters.22, 23

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