We tend to think of national parks as pristine areas, protected from outside influences by their boundaries. The reality is very different. Parks are affected by previous and current land management practices, such as forest harvest, insect control, dams and fire control. Even remote areas are influenced by pollutants and climate change. The fact is, parks are part of larger ecosystems, subject to support and stresses from various sources, and must be managed accordingly. The management plan provides the overall context for the managment of national parks.

Stressors are events, actions, factors or long-term change in natural process that prevent ecosystems from recovering their integrity. By their very nature, stressors affect the state of ecological integrity in national parks.

Stresses originate from both inside and outside the parks. Inside the parks, the presence of alien species, the putting out of natural fires, high levels of visitor use, transportation corridors, non-conforming activities, and inappropriate infrastructure all affect ecological integrity . Stresses from outside also cause problems, ranging from regional to global in nature. An example of a regional stress is change in the landscape in lands adjacent to national parks, from factors such as urban development, logging, mining, agriculture and transportation. Stresses of a global nature, such as long-range movement of air pollutants and climate change, are also affecting ecological integrity within parks. Parks are part of larger ecosystems and very much reflect the state of the larger regions where they are located.

A sample of the broad internal and external issues facing Canada's parks includes:

Habitat loss
In Canada, over 90 per cent of Carolinian forests have been converted to farmland or towns. On the prairies, 99 per cent of the native tall-grass communities and 75 per cent of mixed grass communities have disappeared. In Atlantic Canada, 65 per cent of the coastal marshes have been drained of filled. Across northern Canada, only 35 per cent of the boreal forest remains undisturbed. Largely as a result of this habitat loss, many Canadian species are currently threatened;
Habitat fragmentation
Fragmentation of remaining habitat is as serious a problem as habitat loss. Many species, from grizzly bears to flying squirrels and salamanders have difficulty surviving in habitats that are broken into isolated fragments. Even within parks, fragmentation occurs as a result of developments such as communities, facilities, trails, roads and railways. Roads and railways also cause direct wildlife mortality. Hundreds of large mammals and thousands of birds, amphibians and other creatures are killed on park roads each year;
Losses of large carnivores
Across Canada and especially in the south, large carnivores are disappearing or are absent, upsetting natural predator-prey relationships and systems. Even though large carnivores are protected within national parks, these predators are threatened by stresses such as human use and development inside parks, as well as hunting, land development, and other pressures that occur outside park boundaries. From Ontario eastward, wolves are gone from all national parks except Pukaskwa and La Mauricie. In the west, wolves have disappeared from Elk Island and Grasslands national parks. In several national parks, wolf populations are low and have a low probability of persistence;
Air pollution
Airborne pollutants, such as those causing acid rain, continue to harm many parks. Atlantic Canada and southern Quebec have been called the "tailpipe of North America" because this area lies downwind from the major urban and industrial regions of the continent. More than two decades of research at Kejimkujik National Park show that low pH levels in the park's water are associated with decreased reproductive success of brook trout. Georgian Bay Islands and La Mauricie national parks continue to face the risk of acid deposition in excess of the ability of landscapes within these parks to buffer sulphate and other acidic compounds;
Pesticides used outside of parks are being detected within parks. For example, the pesticide toxaphene was widely used (outside national parks) until two decades ago. It can disrupt endocrine systems, damage lungs, livers and kidneys, and cause problems with reproductive and immune systems, developmental disorders and cancer. Research at Bow Lake in Banff National Park has found toxaphene in some zooplancton, while trout in Bow Lake have toxaphene concentrations up to 20 times greater than other fish in the lake and up to 1000 times greater than trout from other lakes in the park. A study in La Mauricie National Park showed high mercury levels in the blood and feathers of the park's loons; mercury in their feathers is higher than any other studied site in North America. Mercury levels in loons from Kejimkujik National Park are also high, leading to reduced nesting and hatching success. The pesticide DDT has been found at significant levels in lake sediments and in fox snakes at Point Pelee National Park. High DDT levels have been correlated with reduced frog populations and species loss in several other parks and wildlife reserves along the northern edge of Lake Erie;
Alien species
Invading non-native species, both plants and animals, cause problems for parks across Canada. In Point Pelee National Park, garlic mustard is invading Carolinian forests and out-competing native species. In Riding Mountain National Park the high number of alien plant species in the native rough fescue grasslands is a cause for concern as native plants are out-competed by the invaders. In Gros-Morne National Park, moose and snowshoe hares introduced to Newfoundland several decades ago are altering habitat and vegetation regimes inside the park;
Growing level of human use within most national parks have created crowding, overuse of facilities and infrastructure such as sewage treatment systems, over-development and a myriad of other problems that in turn degrade water and air quality, cause erosion and damage wildlife habitat. In Waterton Lakes National Park, every valley has either a road or a hiking trail - or both. Only the most northerly parks have not yet been subject to high use demands.

Unfortunately, this list of stressors affecting national parks is not complete; a number of other activities are also threatening the natural state of parks' ecosystems.