Toronto Ravines, Invasive Plants, and the Limits of Garden Responsibility

Toronto Ravines, Invasive Plants, and the Limits of Garden Responsibility

(Dorothy and Patrick Smyth, 2025 – Toronto Master Gardeners)
A review of evidence and implications for inner-city home gardeners

 

Introduction

Toronto’s ravine system is one of the city’s defining features. Extending through multiple watersheds and neighbourhoods, the ravines provide flood mitigation, urban cooling, recreation, and important green space in an otherwise dense city. They are rightly valued and widely loved.

In recent years, however, ravines have increasingly been described as ecosystems in crisis. Invasive plants are often identified as a primary cause of ecological decline, and responsibility is sometimes extended—explicitly or implicitly—to individual residents, particularly inner-city home gardeners and those living near ravines. Gardeners are encouraged to view their planting choices as a major factor influencing ravine health.

This paper steps back from that narrative and asks a simpler, evidence-based question: what do we actually know about the drivers of ecological change in Toronto’s ravines, and what does the available evidence support regarding the role of inner-city home gardeners? Drawing on long-term ravine studies, wildlife research, and historical analyses, we argue that Toronto’s ravines have been shaped primarily by long-standing disturbance, infrastructure, hydrology, fragmentation, and historical land use. While invasive plants are present and can cause localized problems, existing evidence does not support placing the primary ecological burden for ravine condition on home gardeners.

1. Ravines as Historically Altered Landscapes

Historical research makes it clear that Toronto’s ravines have not been untouched natural systems since European settlement. As documented by urban environmental historians such as Jennifer Bonnell (2019), ravines were heavily logged, farmed, filled, used for dumping, and later reshaped by infrastructure projects throughout the nineteenth and twentieth centuries.

Most of the built form below Eglinton Avenue was constructed prior to 1946. From the mid-1800s through the mid-20th century, many ravine sections were extensively clear-cut and stripped of tree cover—native and non-native alike. For sustained periods they were viewed as hazards or wastelands rather than ecological assets. Streams were channelized, slopes destabilized, soils repeatedly disturbed, and dumping was widespread. By the time organized conservation efforts emerged in the mid-twentieth century, ravine ecosystems had already undergone profound and largely irreversible change.

This history matters because it challenges the idea that ravine degradation is a recent phenomenon driven primarily by modern invasive species or contemporary gardening practices. Instead, ravines should be understood as long-modified urban landscapes, with ecological trajectories shaped well before current debates about native and non-native plants.

2. Evidence from Long-Term Ravine Studies

The most frequently cited scientific work on ravine change is the Toronto Ravines Study: 1977–2017 (Davies et al., 2018). This study brings together resurveys of selected ravine plots originally studied in 1977, along with a series of component studies examining plants, birds, mammals, and other indicators.

The study provides valuable insight into site-specific change over four decades, including increased presence of some non-native species at particular locations. However, it is important to understand what the study does—and does not—do.

The Toronto Ravines Study was not designed as a statistically representative, city-wide survey of the entire ravine system. The total area resurveyed is approximately 5 hectares, compared with estimates of total ravine and valley lands ranging from roughly 11,000 to 18,000 hectares, depending on how ravines are defined. The study relies on a limited number of resurveyed plots and component studies conducted at different sites and times, using different methods. It does not provide comprehensive, abundance-based measurements of canopy composition or groundcover across the ravine system as a whole.

As a result, while the study is often cited to support broad claims about ravine-wide ecological decline or invasive dominance, those claims extend beyond what the data can reliably support. Localized change should not be conflated with system-wide patterns.

3. Faunal Studies, Birds, Ravines, and Urban Ecological Limits

Stepniak’s (2017) assessment of mammal and bird communities in selected Toronto ravines provides additional context for understanding ecological limits in urban ravines. By comparing surveys conducted in 1977 and 2017 at four small ravine sites, the study documents limited representation of area-sensitive forest birds and a small assemblage of small mammals.

Birds, along with small mammals and butterflies, are often used as visible and emotionally compelling indicators of ecological health in Toronto’s ravines. Concerns about bird populations are understandable and well founded. Here, birds serve as useful exemplars. However, it is important to be clear about which factors most strongly influence birds in an urban ravine system, and which factors are often overstated.

3.1 Breeding birds in Toronto ravines

Research consistently shows that only a limited set of bird species can breed successfully in narrow, fragmented ravine forests—such as those along Rosedale Valley Road—surrounded by dense urban development. These species tend to be habitat generalists and edge-tolerant birds rather than forest-interior specialists.

Stepniak’s resurvey found that the same small group of area-sensitive forest birds was present in both 1977 and 2017, with only minor change over forty years. This is a critical point: the constraints on ravine bird communities were already firmly established decades ago. There is no evidence in this study that recent changes in plant composition—native or non-native—were associated with detectable changes in breeding bird communities.

In practical terms, most ravines have not supported a full forest bird assemblage for a very long time, regardless of planting choices.

3.2 Historically absent species and realistic expectations

Many bird species that people hope to see in ravines—particularly deep-forest specialists requiring large, continuous tracts of mature canopy—have been largely absent from Toronto’s urban landscape for over a century. Their loss coincided with early deforestation, agricultural clearing, habitat fragmentation, loss of connectivity, and urban expansion, not with recent home-gardening practices.

This sets important limits on what ravine restoration can realistically achieve. Even with extensive native planting, ravines remain narrow, isolated, noisy, and heavily influenced by surrounding urban activity. Expecting plant choice alone to restore historical bird communities risks creating unrealistic expectations and misplaced frustration.

3.3 One of the biggest threats to birds: glass

One of the most significant—and best-documented—sources of bird mortality in Toronto has nothing to do with plant origin: collisions with glass.

Toronto is internationally recognized as a hotspot for bird–building collisions. According to FLAP Canada (Fatal Light Awareness Program), millions of birds are killed annually in Canada by flying into glass, with Toronto among the most affected cities. Ravines can increase collision risk, as birds use them as movement corridors and then encounter reflective or transparent glass at ravine edges.

This mortality affects both migratory and breeding birds and occurs regardless of whether surrounding vegetation is native or non-native. It represents a direct mortality pressure on bird populations that far exceeds the influence of home-garden-scale plant choices. Other well-documented urban pressures, including predation by domestic cats, further compound these effects. More information is available at https://flap.org.

3.4 What this means for home gardeners

Bird populations in Toronto ravines are shaped primarily by habitat fragmentation and limited forest size, long-standing historical losses, urban disturbance (noise, lighting, human activity), and major mortality sources such as window collisions and predation.

While vegetation structure can matter locally, there is no evidence that invasive plants—or the actions of inner-city home gardeners—are the primary driver of bird declines in ravines.

4. Invasive Plants and the Question of Scale

Invasive plants can influence ecological processes, particularly in disturbed environments. However, their impacts are highly context-dependent and interact with other drivers such as altered hydrology, soil compaction, canopy disturbance (including widespread ash mortality), and climate stress.

A key limitation in Toronto is the absence of a city-wide dataset measuring plant abundance, cover, or biomass across ravines. Most available data consist of species-occurrence records or localized surveys. Presence alone does not equal dominance or ecological impact, yet presence-based information is often used to support strong claims.

Without abundance-based, system-wide measurements, assertions that invasive plants are the primary drivers of ravine degradation remain speculative.

5. Pathways of Introduction and the Role of Gardeners

The claim that invasive ravine plants primarily originate from contemporary home gardens is often asserted but rarely demonstrated at the scale of the ravine system. Historical evidence shows that many widespread non-native species arrived through early horticulture, agriculture, transportation corridors, fill material, and watershed dispersal—often many decades ago.

Municipal planting practices also played a role. For much of the mid-20th century, Norway maple was widely planted by the City of Toronto in streetscapes, parks, and along ravine edges, contributing to its present distribution independent of contemporary home gardening.

Many of the most disruptive species are no longer widely sold, and existing ravine populations are largely self-sustaining. While gardeners near ravines should act responsibly, attributing ravine-scale ecological outcomes to individual planting decisions overlooks the dominant influence of historical land use and ongoing urban disturbance.

6. Implications for Management and Messaging

Taken together, the literature suggests that Toronto’s ravines function as novel urban ecosystems, shaped by centuries of alteration and maintained under persistent urban pressures. Invasive plants are one factor among many, but current evidence does not support narratives that place primary responsibility on inner-city home gardeners.

Effective ravine stewardship should emphasize watershed-scale management, infrastructure design, canopy resilience, erosion control, bird-safe building practices, and realistic ecological goals—rather than moralizing individual gardening choices.

Conclusion

Toronto’s ravines are ecologically valuable, but they are neither pristine nor primarily shaped by recent gardening practices. Historical transformation, urbanization, and structural disturbance have played dominant roles in shaping present conditions. While invasive plants warrant thoughtful, site-specific management, assigning disproportionate ecological burden to inner-city home gardeners is not supported by the available evidence and risks undermining constructive engagement with urban nature.

References (selected)

Bonnell, J. (2019). Remembering and Forgetting in Toronto’s Ravines.

Davies, Z. G. et al. (2018). Toronto Ravines Study: 1977–2017.

Stepniak, A. (2017). Ecological Integrity of Mammals and Birds in Toronto’s Ravines.

FLAP Canada (Fatal Light Awareness Program). https://flap.org

Methodological Note on Abundance

Abundance values are derived from a scaled frequency and persistence index (Combined_abund_scaled) based on repeated species records across multiple sites and years within the Toronto region. This metric does not represent measured cover, density, or biomass. Instead, it reflects how consistently a species is recorded across space and time, providing a proxy for relative persistence. Such indices are commonly used in urban and historical ecology where comprehensive plot-based inventories are unavailable. Values are used here for comparative purposes only and are not intended to infer dominance at individual sites.

Appendices

Appendices A–C provide ranked lists of the most abundant plant species in a Toronto-wide dataset. Although these data are not specific to ravine vegetation, they offer broader ecological and historical context for this paper by distinguishing species that are merely present from those that are widespread, persistent, and long-established across the urban landscape. Together, the three lists (all plants, non-native plants, and native plants) help ground discussion of invasive impact, introduction pathways, and gardener responsibility in observable Toronto-wide patterns rather than anecdote. The Toronto-wide plant dataset used for these appendices draws on aggregated occurrence records and abundance proxies compiled for urban ecological analysis; methodological details and validation of this approach are described in Cadotte et al. (2021), available at https://besjournals.onlinelibrary.wiley.com/doi/full/10.1002/2688-8319.12036

Appendix A: Top 25 Most Abundant Plants (Toronto-wide Dataset)

This appendix lists the 25 most abundant plant species in the Toronto-wide dataset, ranked by a scaled abundance metric. While the dataset is not ravine-specific, these species represent those most persistent and frequently recorded across the broader urban landscape and provide context for interpretations of ravine vegetation.

SCIENTIFIC_NAME	ENGLISH_COMMON_NAME	Combined_abund_scaled
Symphyotrichum cordifolium	Heart-leaved Aster	100.0
Rhus typhina	Staghorn Sumac	96.0
Trillium grandiflorum	White Trillium	92.0
Symphyotrichum novae-angliae	New England Aster	83.0
Anemone canadensis	Canada anemone	81.0
Equisetum arvense	Field Horsetail	81.0
Potentilla anserina	Silverweed	81.0
Acer negundo	Manitoba Maple	75.0
Cornus sericea	Red-osier Dogwood	73.0
Vincetoxicum rossicum	European Swallow-wort	70.0
Daucus carota	Wild Carrot	68.0
Rhamnus cathartica	Common Buckthorn	67.0
Potentilla supina	Bushy Cinquefoil	64.0
Epipactis helleborine	Eastern Helleborine	64.0
Tussilago farfara	Colt’s-foot	63.0
Linaria vulgaris	Butter-and-eggs	62.0
Vicia cracca	Tufted Vetch	62.0
Cichorium intybus	Chicory	61.0
Symphyotrichum ericoides	White Heath Aster	60.0
Solanum dulcamara	Bittersweet Nightshade	60.0
Salsola kali	Common Saltwort	59.0
Viburnum opulus	Highbush Cranberry	59.0
Sanguinaria canadensis	Bloodroot	57.0
Maianthemum stellatum	Star-flowered False Solomon’s Seal	57.0
Ageratina altissima	Common White Snakeroot	57.0

Appendix B: Top 25 Most Abundant Non-native Plants (Toronto-wide Dataset)

This appendix presents the 25 most abundant non-native plant species in the Toronto-wide dataset. Recorded year of introduction and cultivation status are included where available, helping distinguish long-established species from more recent or actively cultivated introductions.

SCIENTIFIC_NAME	ENGLISH_COMMON_NAME	Year_first_record_exotic	Cultivated	Combined_abund_scaled
Vincetoxicum rossicum	European Swallow-wort			70
Daucus carota	Wild Carrot			68
Rhamnus cathartica	Common Buckthorn			67
Epipactis helleborine	Eastern Helleborine			64
Tussilago farfara	Colt’s-foot			63
Vicia cracca	Tufted Vetch	1893		62
Linaria vulgaris	Butter-and-eggs			62
Cichorium intybus	Chicory	1885		61
Solanum dulcamara	Bittersweet Nightshade	1895		60
Salsola kali	Common Saltwort			59
Acer platanoides	Norway Maple			56
Ranunculus acris	Tall Buttercup			55
Hesperis matronalis	Dame’s Rocket	1906		52
Alliaria petiolata	Garlic Mustard			49
Trifolium pratense	Red Clover			49
Lythrum salicaria	Purple Loosestrife	1928		48
Saponaria officinalis	Bouncing-bet			48
Hypericum perforatum	Common St. John’s-wort			48
Echium vulgare	Common Viper’s Bugloss			48
Taraxacum officinale	Common Dandelion			48
Cypripedium calceolus	Lady’s slipper orchid		U	47
Reynoutria japonica	Japanese knotweed			46
Elymus repens	Creeping Wildrye			45
Cirsium arvense	Creeping Thistle	1892		43
Plantago major	Common Plantain			42

Appendix C: Top 25 Most Abundant Native Plants (Toronto-wide Dataset)

This appendix lists the 25 most abundant native plant species in the Toronto-wide dataset. These species form the core of the region’s persistent native flora and provide a comparator for evaluating claims about displacement by non-native plants.

SCIENTIFIC_NAME	ENGLISH_COMMON_NAME	Combined_abund_scaled
Symphyotrichum cordifolium	Heart-leaved Aster	100
Rhus typhina	Staghorn Sumac	96
Trillium grandiflorum	White Trillium	92
Symphyotrichum novae-angliae	New England Aster	83
Equisetum arvense	Field Horsetail	81
Anemone canadensis	Canada anemone	81
Potentilla anserina	Silverweed	81
Acer negundo	Manitoba Maple	75
Cornus sericea	Red-osier Dogwood	73
Potentilla supina	Bushy Cinquefoil	64
Symphyotrichum ericoides	White Heath Aster	60
Viburnum opulus	Highbush Cranberry	59
Maianthemum stellatum	Star-flowered False Solomon’s Seal	57
Sanguinaria canadensis	Bloodroot	57
Ageratina altissima	Common White Snakeroot	57
Symphyotrichum ciliatum	Rayless Alkali Aster	56
Prunus virginiana	Choke Cherry	56
Asclepias syriaca	Common Milkweed	55
Viburnum acerifolium	Maple-leaved Viburnum	55
Geranium maculatum	Spotted Geranium	55
Prunus serotina	Black Cherry	55
Hamamelis virginiana	American Witch-hazel	54
Arisaema triphyllum	Jack-in-the-pulpit	54
Impatiens capensis	Spotted Jewelweed	54
Acer saccharum	Sugar Maple	54