Scotch Bonnet Island and Mohawk Island National Wildlife Areas Management Plan: chapter 2

2 Ecological resources

2.1 Terrestrial and aquatic habitats

Scotch Bonnet Island is approximately 60% exposed limestone bedrock outcropping around the perimeter, and 40% barren ground in the interior. There is a shallow underwater limestone shelf visible around the island (Figure 9a). Historically, vegetation was limited to the higher area contained by the concrete breakwall. It was composed of an herbaceous cover including mustard species, Common Nightshade (Solanum dulcamara), grasses and a few low shrubs such as dogwoods (Cornus spp.) In recent years, much of the vegetation on the island has died, largely due to increased cormorant use and the resulting highly acidic excrement. Scotch Bonnet Island is now virtually devoid of any living vegetation (Figure 9b).

Mohawk Island is also effectively a low, flat, exposed limestone shoal, with approximately three-quarters of the island barely exceeding average lake levels. The surface consists mainly of bare limestone, but a thin layer of soil is found on some higher areas and in crevices. A sand and gravel beach and mound has been deposited over a southern and eastern portion of the rock shoal (Figure 10). A beach ridge, composed of Zebra Mussel (Dreissena polymorpha) and Quagga Mussel (Dreissena rostriformis bugensis) shells, has accumulated on the southeast side of the island.

Vegetation on Mohawk Island is also very sparse, and the diversity is quite low. Currently, there are only small patches of mixed herbaceous growth and grasses found in rock crevices and depressions (Figure 11). The species present include Old-field Cinquefoil (Potentilla simplex), Lady's Thumb (Polygonum persicaria), Common Dandelion (Taraxacum officinale), dock (Rumex sp.), Purple Loosestrife (Lythrum salicaria) and goldenrod (Solidago spp.). Mosses and lichens are present on the rocks. Sedges and rushes are among the common emergent aquatic species growing from a few underwater crevices.

Like Scotch Bonnet Island, Mohawk Island also once had more vegetation, with grassy flats, low shrubs and several trees. By 1956, much of the vegetation had been destroyed, probably as a result of gull activity and wave scour (Beardslee and Mitchell, 1965). Willows (Salix spp.) and Eastern Cottonwood (Populus deltoides) saplings observed on the island in the 1980s are no longer present.

Because these barren limestone islands now lack the buffering effect of vegetation and are exposed to frequent storm events, the potential for natural regeneration at both sites is very low, and new vegetation is unlikely to persist.

2.2 Wildlife

2.2.1 Birds

Birds at Scotch Bonnet Island NWA

As stated, both islands are important nesting, loafing and roosting sites for colonial waterbirds. They provide a safe place for adults to breed and raise their young and a safe area for loafing and roosting by non-breeding birds. Both islands are safe refuges from predators and an easy access point for foraging in nearby waters. The two islands contain a similar assemblage of species, with key differences. These are outlined below.

The primary nesting species of colonial waterbirds found on Scotch Bonnet Island NWA are Double-crested Cormorants (Phalacrocorax auritus) and Herring Gulls (Larus argentatus). In addition to these two species, Black-crowned Night Herons (Nycticorax nycticorax) and Great Black-backed Gulls (Larus marinus) also have nested on Scotch Bonnet Island but do not do so at present. Nesting data for colonial waterbirds on Scotch Bonnet Island NWA can be found in Appendix 1.

The Double-crested Cormorant was the first colonial waterbird documented nesting on Scotch Bonnet Island; in 1938, 6 pairs were noted nesting (Appendix 1). Over the next 17 years, this number ranged up to 200 nests (Appendix 1). From 1952 to 1992, the number of nesting pairs declined dramatically and ranged from 0 to 29. During the late 1950s to the early 1970s, the species was almost wiped out across the Great Lakes due to the effects of toxic chemicals and human persecution (Price and Weseloh, 1986; Weseloh et al., 1995, 2002). In 1993, cormorants showed a dramatic comeback and growth spurt on Scotch Bonnet Island, when 260 nests were counted there. Since then, numbers have fluctuated, peaking at 985 nests in 2006; there were 759 nests in 2010 (Appendix 1, Weseloh et al., 2002, 2003, unpublished data). In 2010, the cormorant colony on Scotch Bonnet Island was the 11th largest on Lake Ontario, and contained 2.4% of cormorant nests within both U.S. and Canadian waters of Lake Ontario (Weseloh et al., 2010, unpublished data). The cormorants have always nested in the raised centre portion of the island and on the ruins of the stone lighthouse and keeper’s house. Non-breeding birds also use the site for roosting, and large numbers can be found there in the autumn.

Herring Gulls were first recorded nesting on Scotch Bonnet Island in 1950, when 550 nests were noted (Appendix 1). This is the largest number of Herring Gull nests that has ever been recorded on the island, by more than a factor of two, and may have been an overestimate. Since then the numbers have fluctuated up to a peak of 246 nests in 1995 and have averaged 133 nests per year (N=25 years). Numbers have not exceeded 200 pairs since 1998; there were 128 nests in 2010 (Appendix 1). During the last complete census of nesting colonial waterbirds in the Canadian waters of Lake Ontario (2008), Scotch Bonnet Island ranked as the third largest of 24 Herring Gull nesting sites, behind Gull Island and Pigeon Island. It held 11.5% of the total nests on Lake Ontario in 2008. Herring Gulls nest throughout the island on both the raised central portion and on the exposed limestone. Immature Herring Gulls use the island as an overnight roosting area throughout the summer. In the early 1970s, Herring Gull eggs from Scotch Bonnet Island were monitored annually for contaminant concentrations (Gilbertson, 1974, 1975; Gilbertson and Hale, 1974a, 1974b; Gilbertson et al., 1976). The island is not part of the annual Great Lakes Herring Gull Monitoring Program (Mineau et al., 1984; Pekarik and Weseloh, 1998; Hebert et al., 1999), but it is a site used for the monitoring of Herring Gulls as part the colonial waterbird monitoring program (Fox et al., 2007a, 2007b).

Two other species of colonial waterbirds are known to have nested on Scotch Bonnet Island in the recent past, but only for short periods of time. Great Black-backed Gulls were noted nesting on 5 occasions between 1986 and 2001, with a maximum of 5 nesting pairs in 1999 and 2001 (Appendix 1). It is uncertain when this species ceased to nest on Scotch Bonnet Island. The island is not visited every year; even when it is, nesting Great Black-backed Gulls can be hard to confirm as they often flush from their nest, and the island, at the first sign of an approaching boat. In 2004, and every year since then, there have been re-occurring outbreaks of Type E botulism in Lake Ontario. This species has proven to be very sensitive to this disease and has died off at all nesting colonies on Lake Ontario, i.e., Little Galloo, Pigeon and Snake Islands as well as Scotch Bonnet Island. Their numbers have declined from over 40 nesting pairs to 0, and botulism has been found in all dead birds that have been necropsied (Campbell et al., 2009; Shutt et al., 2010). When Great Black-backed Gulls nested on Scotch Bonnet Island, they nested on the east end of the raised central portion of the island.

The other species of colonial waterbird that is known to have nested on Scotch Bonnet Island is the Black-crowned Night Heron. It was recorded nesting on the island in 3 years between 1990 and 1994, in numbers ranging from 27 to 17 pairs (nests) (Appendix 1). The herons nested in the small trees/bushes immediately west of the lighthouse and keeper’s house. When the herons started nesting there, there were no cormorants nesting on the island (Appendix 1). By 1994, when there were nearly 600 pairs of cormorants nesting there, some of which had started nesting in the same small trees as the herons, the number of heron nests declined by more than 35%. Black-crowned Night Herons were not observed nesting again on Scotch Bonnet Island after 1994 and, presumably, were driven away by the cormorants, who eventually took over all heron nests in the small trees.

The role of Scotch Bonnet Island, at least with respect to cormorants, has switched from being the primary colony nucleus in the 1940s to being an alternative nesting habitat for the growing population; other nearby islands have greater populations now (Price and Weseloh, 1986; Weseloh et al., 1995, 2002, 2003).

Other bird species reported nesting on the island include Canada Goose (Branta canadensis) (1984, one nest) and Rock Doves (Columba livia). Non-breeding Common Terns (Sterna hirundo) and Caspian Terns (Hydroprogne caspia) are observed regularly in the waters around the island. Ring-billed Gulls (Larus delawarensis) are seen often near Scotch Bonnet Island but have never been reported breeding there.

In spring and fall, the island is a stopover site for migratory birds, especially waterfowl and shorebirds. Several species of ducks, e.g., scoter species and Common Merganser (Mergus merganser), use the surrounding nearshore water during migration and winter, with only a few dabbling ducks using the island itself. During migration, it is not uncommon to see thousands of scaup species and Long-tailed Ducks (Clangula hyemalis) using the waters around Scotch Bonnet Island NWA and the embayments along the Prince Edward County shoreline. Consequently, this area is considered to be an important area in eastern Ontario for staging waterfowl (Dennis et al., 1984; OEHJV, 2007) (Appendix 3).

Birds at Mohawk Island NWA

At Mohawk Island NWA, the primary nesting species are Herring Gulls, Ring-billed Gulls, Double-crested Cormorants and Caspian Terns. The Herring Gull and Double-crested Cormorant colonies are the largest colonies in eastern Lake Erie, and the Caspian Tern colony is the only one in Lake Erie (Hebert et al., 2008; Weseloh, 2010a). Common Terns have nested intermittently and Great Black-backed Gulls have nested on fewer than five occasions (Peck and James, 1994; Moore et al., 2007, 2008; Weseloh and Moore, 2009). Colonial waterbird nest data for Mohawk Island NWA can be found in Appendix 2.

Herring Gulls were first recorded nesting on the island in 1943 (Appendix 2). Their numbers built up slowly, and by 1966, 200 nests were recorded. Since then, nest numbers have fluctuated mostly between 200 and 250 nests. Their peak of 259 nests was reached in 1980 (Appendix 2). The 253 nests recorded on Mohawk Island in 2007 represent 10.1% of the Herring Gull nests on Lake Erie (Weseloh et al., 2010, unpublished data).

Ring-billed Gulls were first recorded nesting on Mohawk Island in 1943, when 26 nests were recorded (Appendix 2). Their population grew much more quickly than the Herring Gull population; 300 nests were reported in 1950, and the population increased until it peaked at 6300 nests in 1964 (Beardslee and Mitchell, 1965; Ludwig, 1974; Haymes, 1977). Since then, the population has fluctuated in the range of 1500-2400 nests annually (Appendix 2, Morris, 2010; Weseloh, 2010b). The 2201 Ring-billed Gull nests recorded on Mohawk Island in 2010 accounted for 6.1% of the nests in Lake Erie (Weseloh et al., 2010, unpublished data).

Double-crested Cormorants are a more recent arrival on Mohawk Island. In 1983, they established a colony with 16 nests (Clark et al., 1983). Census data show that the nesting population has increased steadily, reaching a peak of 1586 nests in 2008 (Appendix 2). In 2009, there were approximately 800 pairs (de Solla, 2009, unpublished data). The overall dramatic increase in the Double-crested Cormorant population on Mohawk Island is consistent with population increases throughout the Great Lakes (Weseloh et al., 1995, 2002, 2003, 2009; Hebert et al., 2008). By 2009, Mohawk Island contained 9% of the cormorant nests recorded in all of Lake Erie (Weseloh et al., 2010, unpublished data).

Caspian Terns have nested on Mohawk Island annually since 1996, when 40 nests were discovered by Dr. Laird Shutt and his crew in an area of Zebra Mussel and Quagga Mussel shell deposition, on the southeast shore of the island (Weseloh, 2010a). Their nest numbers have fluctuated but grown steadily, and Mohawk Island now represents a regionally important nesting area for Caspian Terns; the 300 nests recorded in 2007 represented all the nests in Lake Erie and 10.3% of the nests in the Great Lakes (Weseloh et al., 2010, unpublished data). Caspian Tern nests are found exclusively in the area where Zebra and Quagga Mussel shells have been deposited (Weseloh, 2010a) (Figure 12,13).

Common Terns were formerly the predominant species nesting on Mohawk Island, with an estimated high of 1400-1800 pairs in 1946 (Beardslee and Mitchell, 1965). By 1960, tern numbers had dropped and gull numbers had increased, coinciding with a decrease in vegetation on the island. The Common Tern population on the island has fluctuated since that time, but no Common Terns have been observed nesting on the island since 2004 (Morris, 2010; Appendix 2).

The first Great Black-backed Gull nest record on Lake Erie was recorded on Mohawk Island NWA in 1991 (Peck and James, 1994). Since 1991, Great Black-backed Gulls nest very intermittently on the island, with single nests reported in 1993 and 1996 (Moore et al., 2007, 2008).

Although the numbers of colonial waterbird species present vary annually, each bird species usually nests in the same approximate location on Mohawk Island (Weseloh and Moore, 2009). General nesting areas for each species have been mapped and are shown in Figure 11. Nesting colonial waterbirds on Mohawk Island are particularly vulnerable to wave action. For example, in early June 2010, virtually all the Caspian Tern nests on Mohawk Island were destroyed when a large standing wave (seiche) caused the water level in eastern Lake Erie to rise by almost one metre, washing over the island (King and de Solla, 2010).

A number of bird species use Mohawk Island NWA as a loafing or stopover site, or pass through as migrants. This includes many gulls and terns, various waterfowl species, and a few shorebirds (Appendix 3).

At least 20 species of waterfowl have been recorded on the NWA or in the nearshore waters of the island (i.e. within 1.5 km of the shoreline) since 1970 (CWS, unpublished data; Dennis and Chandler, 1974; Dennis et al., 1984) (Appendix 3). Waterfowl such as Mallard Ducks (Anas platyrhynchos), Canada Geese (Branta canadensis), Common Goldeneyes (Bucephala clangula), scaup (Aythya spp.) and shorebirds such as sandpipers (Calidris spp.) use the Mohawk Island area when crossing Lake Erie during migration. Since the 1980s, the use of the area by Mallards, American Black Ducks and Canada Geese has increased, while the numbers of diving and sea ducks have declined slightly. For example, the number of Mallards and American Black Ducks using the area increased two-fold while Canada Geese increased thirteen-fold between the surveys in 1970-1980 and 1990-2000, respectively. Mohawk Island NWA and its nearshore waters are considered to be of secondary importance for migrating waterfowl, especially in comparison with areas further west on Lake Erie, such as Long Point (Dennis et al., 1984; OEHJV, 2007).

Some rare sightings that have been recorded over the years include two Glaucous Gull (Larus hyperboreus) specimens from dead or injured birds found on the island in 1975 and 1976 that are in the Buffalo Museum of Science. A Brown Pelican (Pelecanus occidentalis) was recorded on the island in 1991 by Ralph Morris’s Brock University students (Moore, personal communication, 2009). A variety of herons, egrets and shorebirds have been seen along Rock Point, the mainland shore closest to the island, and some of these species may also be present at or near Mohawk Island NWA.

2.2.2 Other wildlife

No other wildlife species have been recorded on Scotch Bonnet Island NWA. On Mohawk Island NWA, the only vertebrate species observed is the Eastern Garter Snake (Thamnophis sirtalis sirtalis), which lives among the foundations of the lighthouse. The sparse terrestrial vegetation on these islands offers little cover or food for vertebrate residents.

2.3 Species at Risk

No species at risk have been recorded on Scotch Bonnet Island NWA or on Mohawk Island NWA.