Non-native (exotic or alien) plant and animal species, introduced into an ecosystem where they did not previously live, are considered invasive when they spread aggressively and impact native species. Whether introduced by well-intentioned gardeners, or accidentally imported, biological invaders are a serious ecological problem.
Invasive species in Garry oak ecosystems include plants and animals. At least 173 invasive species have been identified including 4 trees, 15 shrubs, 147 herbs, 1 reptile, 6 birds and 7 mammals.
A few of them include:
For control of invasive species, please see the following resources produced by GOERT’s Invasive Species Steering Committee:
The prevalence of invasive exotic plants in Garry oak and associated ecosystems in British Columbia is well documented. Roemer (1972) found that 25% of the species within the core of the Garry oak range in British Columbia were introductions from the Mediterranean and other parts of Europe. He noted that these species are so fully integrated into the ecosystem that they comprise a part of the characteristic plant association. Subsequent sampling at different sites in 1995 revealed that 40–76% of the herbaceous species in camas meadows of the core area were exotic, and that exotic species comprised 59–82% of the herbaceous cover, suggesting an increased presence in exotic species over the 2 decades (Roemer 1995).
An assessment of regional parks within the Capital Regional District concluded that 100% of the parks are threatened by the invasion of non-native plant species, 36% of them severely so (Fleming 1998). Out of 546 species of vascular plants of Garry oak and associated ecosystems, 29% of them are introduced.
Although relatively fewer shrub species (22%) are introduced compared to the proportion of introduced herbaceous species, introduced shrubs pose some of the most serious threats to Garry oak ecosystems in British Columbia.
Scotch broom, introduced to the region as a garden ornamental in 1850, has since spread to become a dominant component of much of the Garry oak landscape. It has also invaded other regions of British Columbia and Canada, as well as the USA, Europe and other parts of the world. Scotch broom has dramatically altered the vegetation structure of these ecosystems by forming a dense shrub layer where shrubs were formerly absent, sparse, or at most patchy. This shrub layer shades out native understorey species adapted to open conditions. These changes can negatively affect habitat suitability for animal species, including many birds and butterflies, that require open vegetation structure.
Furthermore, Scotch broom is a nitrogen-fixer, and thus has the potential to change ecosystem-wide resource supply. It also generates large amounts of woody fuel that can support high intensity fires and in this way alter the natural disturbance regime.
Gorse shares similar characteristics and destructive potential with Scotch broom but to date is not as widespread in British Columbia.
Other invasive shrubs of great concern include English ivy, Himalayan blackberry, cut-leaved blackberry and laurel-leaved daphne.
Exotic grasses are a ubiquitous presence in Garry oak ecosystems, and dominate the herbaceous flora at many if not most sites. They include perennial grasses, primarily the result of past and current intentional seeding for livestock forage and erosion control, and annual grasses that were for the most part accidental introductions. Common species include orchardgrass, common velvet-grass, Kentucky bluegrass, barren brome, sweet vernalgrass, hedgehog dogtail, perennial ryegrass, and others. Many exotic grasses can directly out compete native species by reducing light at the ground level and aggressively capturing water and nutrients. They also have the potential to alter ecosystem processes by producing nitrogen-enhanced litter, changing ground-level microclimates from extensive litter, altering fire regimes as a result of their high flammability and enhancement of soil moisture deficits, and other characteristics.
The potential for these ecosystem-level changes is particularly great when exotic grasses invade ecosystems that were not previously grass-dominated. The historical composition of Garry oak plant communities in British Columbia, especially those from the core area, is largely unknown. Consequently, the previous relative abundance of grasses and forbs is not clear, and to what extent these grass invasions have caused ecosystem changes has not been directly investigated. Introduced forbs are also numerous in British Columbia, and include dovefoot geranium, hairy vetch, common vetch, cleavers, chickweed, sheep sorrel, and hairy cat’s ear. Invasive forbs are not generally described as being as destructive as invasive grasses in Garry oak ecosystems.
Gypsy moths (Lymantria dispar) evolved in Europe and Asia. They were accidentally introduced to North America, near Boston, in 1868 or 1869. Since then the range of gypsy moths has expanded and continues to increase. Occasionally isolated populations of the moths are found in British Columbia’s Garry oak trees.
While gypsy moths feed on the leaves of hundreds of species of plants, their most common North American hosts are aspen and oak trees. In fact, gypsy moth caterpillars have a voracious appetite for oak leaves. They can completely defoliate trees! If all of the leaves are eaten in successive years, the infested trees may die, especially if they are weakened by diseases and other stressors. This could have a devastating effect on the species that depend upon the trees, and the adjacent habitats.
The Garry Oak Ecosystems Recovery Team (GOERT) believes that gypsy moths pose a critical threat to the integrity of Garry oak ecosystems. We support the objective of a gypsy moth free British Columbia and the 2004 approach to gypsy moth management (as presented to GOERT by the BC Ministry of Forests). However, this support comes with the following provisos:
For more information about gypsy moths and the 2004 control program, visit the BC Ministry of Forests website.
Since 1995, large numbers (up to 80% in some areas) of tanoaks (Lithocarpus densiflorus), coast live oaks (Quercus agrifolia) and black oaks (Quercus kelloggii) have been dying in California’s coastal counties. The epidemic, referred to as Sudden Oak Death (SOD), was first seen on tanoak in Mill Valley (Marin County) in 1995. Since then, it has spread north into ten central coastal counties of California and Curry County, Oregon. In 2003 the disease was also found for the first time in nurseries in British Columbia, California, Oregon, and Washington.
In June 2000, University of California researchers isolated a previously unknown species of Phytophthora (“Phy-TOFF-thoruh”), a fungus-like organism, from dying oak trees. Relatives of this “fungus” caused the Irish potato famine, Port-Orford cedar root disease in the Pacific Northwest and are causing oak dieback in many parts of the world. In January 2001 researchers reported that a new species of Phytophthora isolated as early as 1993 from ornamental rhododendrons in Germany and The Netherlands matches the newly discovered species found in California. This new species has since been officially named: Phytophthora ramorum. The name refers to the pathogen’s tendency to cause infection on branches.
Important research discoveries have continued since then. So far, Garry oaks haven’t been affected by SOD. Researchers also found that the plants most prone to the disease include several other oak species, blueberries, honeysuckle, huckleberry, rhododendrons, azaleas, viburnum, pieris, and camellia. Some species of Douglas-fir, arbutus, sumac, and maple are also at risk. Scientists determined that Phytophthora ramorum may be spread through infected wood, soil and rainwater. However, probably the most important way in which humans spread the pathogen around is by moving infected plants and plant parts. The leaves of hosts such as bays, madrones and rhododendrons contain large amounts of spores, which may be dispersed through the air under moist and windy conditions.
For more information on sudden oak death, visit the following web sites: