Wetland Area, Shape and Diversity

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Extensive, heterogeneous wetlands as well as less extensive, isolated wetlands both make significant contributions to supporting biodiversity at the local and watershed scales.

The presence of larger, contiguous swamps and marshes (e.g., more than 30 hectares) are important for area-sensitive species such as Prothonatory Warbler and Black Tern. However, the presence of complexes of smaller, more isolated wetlands in the landscape are also important in that they provide habitat for many wetland-dependent amphibians and reptiles.

Swamps have the potential to support area-sensitive wildlife species (i.e., those that require larger areas of continuous habitat in which to be productive) or edge-intolerant species (i.e., those that prefer to use habitat away from the influence of habitat edges, also sometimes referred to as “interior” habitat species). In some watersheds with many land use pressures, treed swamps may be the only remaining significant contributors to interior-forest habitat, and the discussion on forest size and species that may be expected (see Section 2.3) applies here as well.

However, treed swamps provide interior habitat for a different suite of specialist area-sensitive forest species compared to large patches of upland forest.

Larger marshes also have the ability to support area-sensitive wildlife species (Smith and Chow-Fraser 2010). Area-sensitive birds may include species such as Marsh Wren, Black Tern and Forster’s Tern. Black Tern will nest in smaller marshes if larger feeding areas are located nearby. Some other species, such as Least Bittern and King Rail, occasionally occur in smaller marshes, but long-term viable populations are associated with extensive marshes.

Extensive swamps and marshes also tend to have greater habitat heterogeneity (i.e., the habitat is more varied within them), which in turn tends to support more species of wildlife (e.g., Golet et al. 2001). In marshes, this is called

“interspersion” or the juxtaposition of different marsh communities. High levels of habitat interspersion (e.g., the presence of open water/submerged vegetation, emergent vegetation and in some cases shrubs) within a marsh provide higher-quality habitat for a wider variety of species than, for example, a narrow band of cattails around the shoreline.

For example, some species require extensive stands of emergent plants with few or no openings (e.g., Northern Harrier), while others seem to prefer areas dominated by emergent plants but with small, isolated openings (e.g., Least Bittern). The ratio of open water/submerged vegetation to emergent vegetation and the interspersion pattern may

Capture the full range of wetland types, areas and hydroperiods that occurred historically

within the watershed. Swamps and marshes of sufficient size to support habitat heterogeneity

are particularly important, as are extensive swamps with minimum edge and maximum

interior habitat to support area-sensitive species.

vary considerably from year to year because marshes are dynamic systems. However, area remains a key factor, and more extensive marshes are more likely to be used as

productive habitat by more species of wildlife (e.g., Attum et al. 2007; Webb et al. 2010).

Relatively isolated (i.e., not coastal or riparian), smaller wetlands can also be important for local or regional biodiversity. For example, amphibians such as Wood Frog and Spotted Salamander have been documented in wetlands ranging in size from 0.1 to 5.2 hectares (Babbitt 2005; Lehtinen and Galatowitsch 2001). These wetlands can have variable hydroperiods, may be permanently wet (i.e., year-round) or only seasonally wet (typically in the spring and part of the summer), and can be “hot spots” for some groups of amphibians, particularly when they do not support predatory or competing fish (Snodgrass et al. 2000; Werner et al. 2007).

Interestingly, current research indicates that hydroperiod may be a more important factor than wetland size in determining the diversity that can be supported by isolated wetlands (Baldwin et al. 2006b; Hermann et al. 2005;

Paton and Crouch 2002; Snodgrass et al. 2000).

Complexes of relatively isolated wetlands also tend to be more supportive of biodiversity than single, isolated ponds. Amphibians and reptiles are known to use multiple local ponds,

sometimes over the same season (Joyal et al.

2001; Semlitsch 2008). Waterbirds are also known to use complexes of small wetlands, especially for springtime pairing and feeding, and have been documented using isolated wetlands in urban and peri-urban landscapes in London, Ontario (Pearce et al. 2007). Some birds have specifically adapted to use wetland complexes in the landscape and will readily move between them to forage (e.g., Northern

reason that the Ontario Wetland Evaluation System recognizes the concept of wetland complexes (OMNR 1994).

The presence of coarse woody debris in many wetland types—particularly swamps, but also riparian areas and terrestrial areas associated with wetland pockets—is important to many species. The functions of this debris include providing cover and nutrients for fish and other aquatic organisms, and providing important cover and overwintering habitat for pond-breeding amphibians that spend the bulk of their life cycle in associated uplands (Keddy 2010).

Maintenance of the full range of wetland vegetation community types that occur in a watershed is also key to sustaining biodiversity.

Meyer et al. (2010) found that at Long Point on Lake Erie, while the overall abundance and diversity of birds was greater in Common Reed habitat, the abundance of marsh-nesting birds was greater in meadow marsh habitat, supporting the need to protect these types of more specialized habitats.

The role of wetland shape in supporting habitat and species diversity is difficult to discuss independently because it is so closely related to, and in the literature is often confounded with, habitat area and fragmentation in the landscape (Ewers and Didham 2006). The limited available data indicate that the optimum shape of a wetland varies by wetland type. Swamps, which are a type of forest, are better able to support area-sensitive and edge-intolerant species when they are relatively compact and regularly shaped (e.g., circular or squarish) (see Figure 13, on forest shape determining amount of core habitat, in the Forest Habitat Guidelines).

However, some other wetland-dependent

transitional areas between open water and adjacent uplands) and thrive where there is more “edge” (Attum et al. 2007; Stevens et al.

2002). Long, narrow marshes may also provide more water quality benefits since they maximize water contact with vegetation that is responsible for the uptake and transformation of many nutrients and other contaminants. The link between wildlife species diversity and

abundance, and the presence of wetlands, has been made repeatedly for amphibians. Specific research on hydroperiods of seasonally inundated wetlands in forests has shown that wetlands with longer hydroperiods (but not permanent water) support a higher diversity of amphibians, irrespective of wetland size (Babbitt 2005; Baldwin et al. 2006b; Herrmann et al. 2005).