Rapid Changes and Thresholds
Lessons and examples from this report
Slow, incremental change may not seem important until thresholds are taken into account.
Ocean acidification, caused by uptake of carbon dioxide from the atmosphere, occurs in some Canadian marine ecosystems and is an emerging issue in others; the rate of change is slow. Research and global change models provide good evidence that acidification will continue to increase as a result of climate change. Some ocean acidity thresholds are well known because they are chemical and physiological and are relatively easy to define – when the water becomes too acidic, calcium carbonate shells and skeletons cannot form properly, affecting shellfish, corals, and other sea creatures. (See Marine Biome.)
The historical distribution of native grasslands, the most endangered of Canada’s biomes, has been greatly reduced, mainly through conversion for agriculture. There are several types of grasslands, each supporting a distinct mix of species, including many species at risk. The natural processes that maintained grasslands in the past, like fire and grazing by free-roaming bison herds, are now absent or modified. Development and recreation continue to convert and fragment the land in some areas and the spread of invasive non-native species and changes in grazing practices continue to alter the composition and structure of the vegetation. Each type of grassland will have its own threshold beyond which it will no longer be able to support its unique mix of species. (See Grasslands Biome.)
Stressors may interact in unexpected ways to produce surprises.
Nutrient loading to the Great Lakes was a problem that led to collaborative action between the United States and Canada, starting in the 1970s, to reduce nutrient inputs and clean up the lakes. These measures were successful – water quality improved, harmful algal blooms and oxygen depletion problems decreased, and diversity of native algal species increased. However, as lakeshore areas continued to be modified, human populations surrounding the lakes continued increasing and invasive non-native species have become more prevalent, altering many of the lakes’ characteristics. Although regulation continues to limit nutrient inputs, some combination of the changes that are taking place in the lakes has resulted in reappearance of harmful algal blooms in some near-shore areas. (See Nutrient Loading.)
Change in one ecosystem component brings with it a suite of widespread consequences.
Summer sea-ice extent is shrinking, a rapid change that is now well established. The decline of multi-year ice may have reached or crossed a threshold. Ecological consequences are emerging, especially in Hudson Bay, where the ice-free season has increased the most. Examples include a reduction in Arctic cod, a fish that is associated with ice; an increase in capelin, a fish more tolerant of warmer water; reduced body condition of polar bears; and range expansion of a new top predator, the killer whale, into the bay. (See Marine Biome and Ice Across Biomes.)
Large predators, including wolves, have declined or have been extirpated from much of their original ranges in the more populated areas of Canada. Smaller predators, like western coyotes and raccoons, have in turn expanded their ranges and increased in numbers. These more adaptable predators eat a wide range of food items, altering abundance of other species. In the Mixedwood Plains, with fewer predators, white-tailed deer have become more abundant, leading to major changes in forest vegetation. (See Food Webs.)
Damage to ecosystems may speed up because of interactions of stressors.
Coastal erosion in the Atlantic Maritime Ecozone+ is increasing, threatening wetlands, beach, and dune ecosystems. Development and hardening of the foreshore have made coastal ecosystems more susceptible to erosion. Rise in sea level, reduced sea ice, and more tropical storms in the Atlantic, all related to climate change, accelerate the rate of erosion. (See Coastal Biome.)
Thresholds are influenced by both environmental sensitivity and the severity of the threat.
Some lands and waters, due to their underlying geology, have greater capacity to buffer acid deposition than others, so the threshold beyond which ecosystem damage occurs varies from place to place, even with the same levels of acid deposition. Once the threshold is crossed, high levels of impacts occur rapidly. For example, certain salmon rivers in Nova Scotia have been particularly affected because of their lack of capacity to buffer acid. (See Acid Deposition.)
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