From: ASSESSMENT OF SPECIES DIVERSITY IN THE MIXEDWOOD PLAINS ECOZONE
DRAGONFLIES AND DAMSELFLIES
P. M. Catling, R. Hutchinson and B. Ménard
1. HISTORY CHANGES
At the time of European settlement large areas of the Mixedwood Plains were wetlands. The western half of ecoregion 135 and the southern portion of ecoregion 132 were 40-80 % wetland. In these areas wetlands have been reduced to approximately 5 % of their former area (Snell 1987). The reduction is largely a consequence of conversion of wetlands for agricultural purposes. The impact of this loss on wetland-dependent flora and fauna, including dragonflies must have been severe, at least in terms of reduction in species population numbers and sizes. However, the reduction to 5% does not take into account the changes that have taken place in the wetlands that have survived. Although many wetlands still exist, their once rich flora and fauna has been reduced to a common subset including the species adapted to man-made changes such as pollution, eutrophication, channelization, introduction of exotics, and water level stabilization. In some cases biodiversity losses are attributable to inappropriate management decisions. A few examples involving dragonflies have been documented. During the early 20th century the Swamp Spreadwing (Lestes vigilax) and the Skimming Bluet (Enallagma geminatum) were very abundant at Grenadier Pond on the west side of Toronto. By 1953 the Swamp Spreadwing had died out and and the Skimming Bluet had become very scarce (Walker 1953). More recently there has been no trace of either of these species at the drastically changed pond. Their disappearance has been associated with pollution, but a controlled outlet preventing natural water level fluctuation as well as dredging and destruction of extensive beds of emergent vegetation probably also played a role.
A few studies in adjacent regions have failed to document the amount of change in species composition that would be anticipated by wetland losses. For example, Kielb (1996) anticipated changes in composition of libellulid dragonfly fauna in southeastern Michigan and southwestern Ontario due to reduction in wetlands and effects of pesticides, but comparison with a list produced 37 years earlier by Kormondy (1958) did not suggest substantial changes. Similarly the abstract of Masteller's (1993) paper concerning Presque Isle State Park, Eric County, Pennsylvania. does not allude to changes, although it covers a period of 90 years.
Due to the inadequate recent field studies, it is not possible to say that any damselfly or dragonfly occurring in Mixedwood Plains over the past few hundred years has been extirpated, but nor can it be stated with certainty that it has not happened. Several species such as Stylurus plagiatus and Gomphus viridifrons have not been recorded in over 50 years (Pratt, pers. comm.).
Because some species have not been observed for many years, it would be a mistake to claim that recent discoveries have added to the total existing biodiversity. Since 1953 three damselflies have been found in Ontario that were not encountered during an earlier comprehensive inventory that was conducted over many years. These are Westfall's Slender Bluet (Enallagma traviatum westfalli), Smoky Rubyspot (Hetaerina titia) and Double-striped Bluet (Enallagma basidens). The first two may have been here all the time, since they are very localized and easily overlooked, but the last evidently moved in very recently from the south. The Double-striped Bluet was first reported in Canada from Pelee Island in 1985 (Cannings 1989) and is now common in the extreme southwestern Canadian portion of the Mixedwood Plains Ecozone (pers. obs.) in the western portion of the Lake Erie lowland (ecoregion 135). There have also been recent additions to the Ontario list of dragonflies from within and/or adjacent to the Mixedwood Plains, eg. Dromogomphus spoliatus, Macromia taeniolata, Gomphaeschna furcillata, and Progomphus obscurus (Pratt 1996, Burke 1996).
Perhaps even more interesting than the new records are some notable changes in the distribution and abundance of species long known to be present. Prior to 1959 the northern range limit of the native Familiar Bluet (Enallagma civile) included the Lake Erie region of Ontario and central New York north to approx. 43·. By the 1990s it had extended its range at least 200 km northward and had become abundant to north of 45· in Ontario, New York and southwestern Quebec (Hellebuyck 1993; Catling 1997, 1998b). The northward expansion has been attributed to climatic warming and secondarily to availability of new man-made habitats.
Another case involves the Azure Bluet (Enallagma aspersum). During the first half of the 20th century, this insect was thought to be restricted to bog lakes of the Canadian Shield within the Mixedwood Shield Ecozone. It is now known from much of the Mixedwood Plains Ecozone where it occurs in a variety of shallow ponds including calcareous and alkaline gravel pit ponds, ornamental ponds and sewage lagoons (Catling & Pratt 1997). It is possible that a race adapted to man-made habitats moved into the Mixedwood Plains recently, possibly from the south.
