Introduced Diseases and the Transformation of Our Ravines

Below and to the south of the dwelling [Castle Frank] was a deep ravine down which between hog-back formations ran a stream named Castle Frank brook, which flowed into the Don, just above a small island on the west side. The marshes gave way on the right at this point to good land covered with elm, butternut and basswood trees (From Robertson’s Landmarks of Toronto, Vol. 1).

Texts like Robertson’s and Lady Simcoe’s diary provide us with important glimpses of what the Toronto ravine system looked like when it was ecologically integral.

The above passage lists three tree species: American elm (Ulmus americana), butternut (Juglans cinerea), and basswood (Tilia americana). Two of these species, butternut (see our entry “Butternut bridges over the Don”) and elm, have had their populations decimated by diseases introduced in the last century: butternut canker and dutch elm.

This story is too familiar. From American chestnut to American elm and continuing with butternut and all our species of ash, a huge part of the transformation that has occurred in our ravine system and other natural areas is due to the widespread loss of native tree species to recently introduced diseases.

There is important work to be done finding and developing resistant strains of these species, but for those species that cannot at the moment be regained, the question becomes what has their loss meant for the ecological functioning of the environment? To what extent are these services provided by the species that have moved in? How has the change impacted other flora and fauna, and is it possible to increase the presence of other species to mitigate these effects? Some of this work has already been done (including by Eric Davies, a member of our team), but we still lack clear data on the scope of the transformation in Toronto’s ravines. It is useful then to have some quantitative measurement of these changes to couple with our historical vignettes. In addition, a survey of the ravine system is an excellent way to locate resistant trees. This is what our project, beginning by revisiting of the Taylor and Scrivener 1975 Park Drive ravine survey, strives to achieve for the city.

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