Food Webs

Status and Trends
some major changes; trends for many unknown
Concern, some getting worse at slow to moderate rate; for others, rate of change unknown
data only for some parts of food webs and some regions
Medium confidence in finding
early indications of changes in some population cycles
Red flag

KEY FINDING 20. Fundamental changes in relationships among species have been observed in marine, freshwater, and terrestrial environments. The loss or reduction of important components of food webs has greatly altered some ecosystems.

This key finding is divided into three sections:

Food webs are formed through linkages of the different organisms in a system, building on the primary producers (plants, algae, and microorganisms), and involving an array of consumers and decomposers.1 Population cycles are regular periodic peaks and lows in animal abundance that are driven largely by the dynamics of some food webs. Food webs and population cycles are important because they shape the structure and function of ecosystems. Changes in species diversity are often related to changes in food webs.

An example of the far-reaching effects of severe reductions in an important part of a food web is the decline of cod and other predatory fish off the Atlantic coast. This loss of fish predators led to further ecosystem shifts, with, for example, large increases in shrimp (see Marine Biome).



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Photo: coyote ©

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).
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).
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 ©

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).
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


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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