Acid Rain and Water Birds
Acid rain poses a serious threat to wildlife (most notably water-dependent
birds in eastern Canada) through a variety of processes that occur
at lower trophic levels. The ultimate effect of decreasing pH on
birds varies with the feeding habits of the species and the severity
of the acidification, but has clearly led to adverse reproductive
effects for some water birds arising from shifts in habitat selection
or diet (see diagram).
Several species of birds are likely affected by the long term effects
of acid rain on the quality and quantity of their food resources
in aquatic environments (see diagram).
Several common duck species (including Common Goldeneye, Hooded
Merganser, Ring-necked Duck and American Black Ducks), which utilize
small water bodies as preferred nesting and brood-rearing habitat,
are threatened by the vulnerability of these water bodies to acidification.
Fish-eating species, such as Common Loon and Common Mergansers,
prefer large lakes and river systems, but are equally at risk from
the effects of acid rain on fish prey. Other fish-eating species,
such as kingfishers, herons and bitterns, as well as riparian species
such as tree swallows, are affected as a result of alterations to
aquatic food webs.
Water Bird Habitats
Three main types of habitat are generally threatened by acid rain
- wetlands, small lakes and large lakes (see
habitat).
- Wetlands may be any size, but are usually
small (less than 5 hectares in size) and take the form of marshes,
swamps, bogs, fens, beaver ponds or forested swamps. These are
key water bird and wildlife habitats and often act as sinks for
sulphur, controlling the chemistry of downstream water bodies.
- Small, glaciated lakes are common on
the Canadian Shield, and are preferred by several types of water
birds. Many of these lakes are very sensitive to acid rain because
they have small watersheds that are situated on granitic bedrock
which offers little buffering. Hence, these lakes acidify quickly
and require many years to recover.
- Large, oligotrophic lakes are important
for sportfish (such as lake trout) and were the first lakes in
which the acid rain problem was recognized. These lakes are important
for several fish-eating species (also referred to as piscivores),
and have typically been the most studied by government agencies.
Water Bird Guilds
Three main groups of water-dependent birds are typically identified
as potentially threatened by the effects of acid rain on aquatic
ecosystems - piscivores, dabbling ducks and diving ducks (see
water bird guilds).
- Piscivores are fish-eating birds that
are typically found on large lakes and rivers. Lake acidity affects
piscivores (notably Common Loons and Common Mergansers) more directly
than any other bird group, because fish populations, the main
prey of piscivores, are either reduced or lost in acid-stressed
lakes.
- Dabbling Ducks are omnivorous (generalists)
birds that can be found on all types of habitats, but are typically
associated with wetlands. This group includes the Mallard, the
American Black Duck, the Wood Duck and the Green-winged Teal.
Across most of the acid-stressed regions of eastern Canada, the
Black Duck is the most common dabbling duck. During the breeding
season, female and young dabbling ducks forage heavily on aquatic
invertebrates. This guild is very adaptable and can use a wide
variety of food resources available in all types of habitat. Acid
rain reduces the quality and diversity of their invertebrate prey.
- Diving Ducks are insectivorous (insect-eating)
ducks that use all types of habitats, but are often associated
with small lakes. This group includes the Common Goldeneye, the
Hooded Merganser and the Ring-Necked Duck. These ducks also forage
primarily on aquatic invertebrates through the breeding season,
and their distribution and breeding success is influenced by the
availability and quality of invertebrate prey in acid-stressed
lakes.
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