Technical Thematic Report No. 18. - Inland colonial waterbird and marsh bird trends for Canada
Three questions were posed that ideally would have described the current status for waterbirds in each ecozone+. Unfortunately, the lack of trend data for many of the ecozones+ precluded this. However, the three questions have been addressed for the Great Lakes Ecozone+, which appears to have the greatest amount of data available. It should be noted, however, that although the Great Lakes basin is an important area for inland waterbirds, the information below only applies to that location and may not apply to the vast areas of the country where information is lacking.
What is happening to waterbirds?
On the Great Lakes, five of ten colonial waterbird species are declining, three are increasing, and two are stable with fluctuations. This illustrates the dynamic nature of this group of birds and the dynamic nature of biodiversity (see below). Some species are reacting positively to recent environmental changes others are reacting negatively. The Great Lakes food web has undergone extensive changes from pre-settlement times, e.g. invasive species and habitat changes. The current state of colonial waterbirds is probably a reflection of this.
For marsh bird populations, it is a different story. All focal marsh bird populations are declining Great Lakes basin-wide (excluding Lake Superior). Many secondary species are also declining. Many of these species are habitat specialists; as the area of their preferred habitat is diminished, so, too, are their populations. Outside forces are diminishing their biodiversity.
Why is the trend happening?
The three colonial waterbird species which are increasing (American White Pelican, Double-crested Cormorant, and Great Egret) are doing so probably because they are relatively recent new breeders on the Great Lakes or are making a come back after a serious population decline. In other words, they are occupying or re-occupying previously empty niches. Species which find themselves in that position often increase fairly quickly. Of the four species which are declining, the Great Black-backed Gull is doing so as a result of toxins (botulism type-E), the Common Tern is doing so probably because of competition with Ring-billed Gulls, and the two other gulls species, Herring and Ring-billed, might be declining as the result of populations being larger than the carrying capacity for that ecosystem during the 1980s as well as possible declines in food availability during the last 20 years. While it is difficult to determine the exact cause of declines in all marsh bird populations on the Great Lakes, some of the contributing factors include: habitat degradation within wetlands, watersheds, and riparian habitats; loss of habitat and ecosystem functioning through activities such as shoreline hardening; water levels; and invasive species.
Why is it important to biodiversity?
Long-term trends in population numbers and other vital metrics are the forces which shape biodiversity. Biodiversity is dynamic. Species decline and others increase, and/or new ones appear. This is what is happening with colonial waterbirds, and their populations are considered to be relatively healthy (except for Common Terns). On the Canadian Great Lakes, there are no colonial waterbirds listed as Species at Risk. However, this is not what is being observed with marsh birds. Population trends in the various species of marsh birds are almost all negative. Several species are listed in one or another Species at Risk category. The dynamics of their guild are no longer give and take, positive and negative. At that rate, species could be lost.
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