Muhlenberg's centaury (Centaurium muehlenbergii COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Given our current limited understanding of Centaurium muehlenbergii habitat requirements and population dynamics, it is unclear what factors have contributed to the species’ current restricted abundance and distribution in Canada. Its rarity may be the result of intrinsic factors such as low competitive ability, limited dispersal ability, high habitat specificity, or inherently low recruitment and survival; extrinsic factors such as geographic barriers or past climatic changes; recent land-use history (e.g., habitat conversion, grazing, fire suppression); or a combination of any or all of these.

None of the seven threat factors identified below has been shown empirically to be responsible for declines (either current or historical) in the abundance or range of Centaurium muehlenbergii within Canada. This may simply be because the species has not been monitored long enough in its native Canadian habitat for deleterious impacts stemming from such threats. However, in the absence of any direct evidence to the contrary, the threats are considered here to be clearly imminent but with the level of impact uncertain.

1. Invasive species encroachment

The impact of invasive species is second only to that of habitat loss as a cause of species declines throughout the world (D’Antonio and Vitousek 1992, Myers and Bazely 2003). On eastern Vancouver Island, Garry oak and associated ecosystems have been invaded by non-native plants to such a degree that exotic species now comprise 59-82% of the total herbaceous cover (Roemer 1995 in Fuchs 2001, Erickson 1996). Furthermore, the proportion of introduced species in Garry oak meadows increased from an estimated 25% of the total in1972 to40-76 % of the total in 1995 (Roemer 1995 in Fuchs 2001), suggesting that the rate of invasion is accelerating rather than slowing. Some of these species represent accidental introductions but a large number have been intentionally introduced into the area for livestock forage, erosion control, or ornamental purposes.

Several authors have identified the types of processes that may be modified by non-indigenous plant species. In general, it is thought that invasive plants have the ability to pre-empt safe sites (places where seeds can germinate) and otherwise suppress recruitment of native plants; alter vegetation stand structure; increase soil moisture deficits; alter soils and micro-climates through litter deposition; increase the probability, extent and severity of fires through increased fuel loading; alter soil nutrient levels and distribution; and compete directly with native species for space, water, light, and nutrients (Bergelson 1990, Facelli and Pickett 1991, D’Antonio and Vitousek 1992, Smith 1994, Gordon 1998, Brown and Rice 2000, MacDougall 2002, Myers and Bazely 2003). In cases where introduced species have higher evapotranspiration rates than those of the native flora, hydrologic regimes may be permanently altered and water tables lowered, thereby altering the distribution of native species (Gordon 1998).

Oak Bay

The Garry oak meadow that supports this population has been heavily invaded by alien plant species, most notably perennial grasses such as orchard grass (Dactylis glomerata), sweet vernalgrass (Anthoxanthum odoratum), kentucky bluegrass (Poa pratensis), velvet-grass (Holcus lanatus), English ryegrass (Lolium perenne), creeping bentgrass (Agrostis stolonifera), and hedgehog dogtail grass (Cynosurusechinatus). Each of these grassescompetes aggressively for water andnutrients and can form dense litter layers that block light and suppress the regeneration of native plants. Litter accumulation from these species also creates conditions for high-intensity fires (Garry Oak Ecosystems Recovery Team 2002). Orchard grass, which forms particularly extensive root systems and requires high nitrogen inputs, may pose the greatest imminent threat to C. muehlenbergii persistence. Originally introduced to coastal B.C. as a meadow forage crop, this species is still grown for hay and used in grass-seed mix to stabilize clearings and road cuts.

European ash (Fraxinus excelsior) is an introduced, exotic tree dominant in a 0.5 hectare section of the park not far from the C. muehlenbergii site. If not controlled, this species will likely continue to spread into adjacent areas, competing with native vegetation for moisture, soil nutrients, and light (Collier et al. 2004). Other invasive shrubs posing a potential threat to C. muehlenbergii at this site include English hawthorn (Crataegus monogyna), Scotch broom (Cytisus scoparius), Himalayan blackberry (Rubus discolor), leather-leaved daphne (Daphne laureola), and gorse (Ulex europaeus). Due to the wetness of the habitat, however, these species are not likely to establish in the microsites occupied by C.muehlenbergii, and thus do not represent an immediate threat.

Orchard grass, velvet-grass, sweet vernalgrass, hedgehog dogtail grass, Scotch broom, gorse, leather-leaved daphne, English hawthorn, and Himalayan blackberry have all been ranked among the top ten invasive plants on Vancouver Island in terms of the significance of their impact on Garry oak and associated ecosystems, their resistance to control or management, and the urgency associated with their control or management (Murray 2004).

Gulf Islands

Introduced species do not appear to pose such a serious threat at this site, although the alien weeds brass buttons (Cotula coronopifolia) and hairy cat's-ear (Hypochaeris radicata) have both become established here and may be competing with Centaurium muehlenbergii for essential resources.

Nanaimo

The highly invasive Scotch broom has established on the margins of the vernal seep at this site, posing an imminent threat toCentaurium muehlenbergii and its habitat. Introduced to the Pacific Northwest as a garden ornamental in 1850, Scotch broom has since become a dominant component of the plant community on eastern Vancouver Island. In many areas this leguminous shrub forms monospecific stands that have completely overtaken the native vegetation (Roemer 1972, Fuchs 2001). A nitrogen-fixer, Scotch broom has the potential to increase soil nitrogen levels, thereby changing the supply of this resource to the ecosystem (Parker and Haubensak 2004). It also generates large amounts of woody fuel that can support high intensity fires and in this way alter the natural disturbance regime.

2. Trampling

Oak Bay

The municipal park that supports the Oak Bay population is situated in a residential area within a few km of downtown Victoria. Due to its accessibility, this location receives high volumes of pedestrian, bicycle, and dog traffic throughout the year, but particularly in the spring and summer months when the life cycle of Centaurium muehlenbergii is in its most critical stages. Light to moderate use by people on foot likely does not harm this species, and the trampling may actually help to suppress weeds (M. Fairbarns, pers. comm. 2003). On the other hand, traffic from bicycles and people can result in the creation of new trails, change the micro-topography of vernal pool bottoms, truncate swale connectivity, crush vernal pool vegetation, and facilitate the spread of invasive plants.Although bicycling is prohibited within the park, this rule is rarely enforced, and soil compaction and heavy traffic have eliminated vegetation cover in many areas (Collier et al. 2004). Mountain bikers have in the past constructed unauthorized bike jumps close to theCentaurium muehlenbergii site, posing an additional threat to this population.

Dogs in the park are required to be on leashes throughout the spring months (April-June), but this rule is also rarely enforced, and dogs are regularly permitted to run, dig, and chase balls in the wet meadow containing the single known colony of Centaurium muehlenbergii. A recent park use survey conducted during the spring months found that almost 50% of visitors to the main meadow were accompanied by dogs, whereas only 2% of the dogs were observed to be on leash at a given time (Collier et al. 2004). The diminutive C. muehlenbergii plants are easily trampled or ripped up by dogs running through the site, and may also be vulnerable to being buried or crushed by dog feces. Commercial dog-walking services have recently begun to make use of the park on a regular basis, further exacerbating these impacts. A recently completed draft stewardship plan for the park notes: “The frequent presence of commercial dog walking activity in the Park will be highly damaging to soil and vegetation under most conditions. Particularly because plant species at risk are concentrated in open meadows where dogs are brought for exercise there is an obvious conflict between the preservation of park values and the uncontrolled use of the Park by some dogs” (Collier et al. 2004).

Gulf Islands

The Gulf Island site is a relatively remote, uninhabited island where direct threats from recreational activities are likely not a major factor.

Nanaimo

This site is adjacent to a public walking and mountain biking trail and is also prone to trampling by people, bicycles and dogs, especially in late spring after the ground has begun to dry but is still not yet hard.

3. Development

Oak Bay

Early land survey maps of Victoria and Oak Bay (from around 1860) indicate the presence of extensive Garry oak meadows and seasonal wetlands in areas now dominated by urban and residential developments. Many of these low-lying, vernally wet areas may have provided suitable habitat for Centaurium muehlenbergii at one time. It is now estimated that < 5% of this habitat type remains (T. Lea, pers. comm. 2003). The park containing the single extant C. muehlenbergii population is bounded on three sides by single-family residences and suburban streets, and on the fourth side by a thoroughfare. However, the park itself is being managed by the municipality as a natural area (Collier 2003), with no further developments planned at this time.

Gulf Islands

Long-term plans for this site (a First Nations Reserve) are unknown. However, development does not appear imminent.

Nanaimo

The vacant property supporting the Nanaimo population is owned by a private forestry company and is up for sale at the time of completion of this report. This ecologically unique region has been identified as a top-ten priority for protection and securement under the Recovery Strategy for Garry Oak and Associated Ecosystems (GOERT 2002). In recognition ofthis, the City of Nanaimo is now taking steps to develop a conservation/stewardship plan for the area in an attempt to protect both the habitat and resident species at risk (R. Lawrance, pers. comm. 2005).The land on which theCentaurium population occurs had been designated an Environmental Development Permit Area (EDPA), and as such is subject to restrictions on land use to ensure the viability of the sensitive ecosystems and rare plant species found there. The City of Nanaimo instructed the developer to set aside 7 hectares for conservation,encompassing grassland and upland ecosystems as well as the Centaurium population. The lot in question is still owned by the developer, but the plan is to transfer title of this piece to City Parks (J. Kirby, pers. comm. 2008).

4. Off-road vehicles

Traffic from off-road vehicles, which compacts the soil, facilitates the spread of invasive species, alters hydrology, dislodges and crushes vegetation, and fragments the habitat, is frequently cited as posing a threat to species of vernal pools and other ephemeral wetlands (Clark et al. 1998, U.S. Fish and Wildlife Service 1998, Douglas and Illingworth 1998, Donovan 2004, Parks Canada Agency 2006).

Oak Bay

Over the years, the occasional passage of trucks and other motorized vehicles through this park has left deep, long-lasting ruts through many of the vernal pools and low-lying meadows, including areas occupied by Centaurium muehlenbergii. The park’s close proximity to residential neighbourhoods dictates that the municipality maintain an aggressive fire-suppression policy, such that fire trucks and other maintenance vehicles (e.g., mowers) continue to access the park on a periodic basis. Recently, a fire department truck became stuck in the central meadow containing C. muehlenbergii while servicing fire hydrants during the wet winter season. A load of gravel was subsequently brought in to fill the hole left by the fire truck, as a result of which a segment of the remaining C. muehlenbergii population was lost (A. Ceska, pers. comm. 2004). In the event a wild fire does break out in the park, fire trucks and other heavy vehicles would likely have to be brought into the central meadow to help contain it, with potentially disastrous consequences for the C. muehlenbergiipopulation there.

Nanaimo

Although not authorized for such use, this property is accessed on a periodic basis by ATV (all-terrain vehicle) enthusiasts, 4x4s and (to a lesser extent) dirt bikes. Off-road traffic has carved deep ruts down to bedrock through the seepage area occupied by Centaurium muehlenbergii, fragmenting the habitat, altering the watercourse, and directly endangering the survival of plants through crushing. Recreational off-roading has also disturbed and compacted the soil in surrounding upland meadows, reducing the long-term likelihood of C. muehlenbergii establishing in new areas.

5. Hydrologic alterations

Populations of annual plants are often naturally subject to strong variations in density over time and space (Venable and Lawlor 1980). However, high population turnover does not appear to be a fundamental characteristic of Centaurium muehlenbergii ecology at the northern limit of its range in British Columbia. This may be due to the relatively predictable nature of its environment (i.e., regular winter flooding alternating with summer drought conditions at two locations, periodic tidal flooding at another). At the same time, the extreme conditions posed by this fluctuating environment may have served historically to limit competition from less stress-tolerant woody plants and mesophytic herbs, thus facilitating the local persistence of C. muehlenbergii. If so, any major changes to the current hydrological regime, either as a result of local habitat perturbations or larger scale events such as global warming, could have a significant negative impact on the viability of C. muehlenbergii populations in Canada.

Oak Bay

This park has had an extensive history of draining and ditching dating back over a century, with significant consequences for the vegetation communities in the area (Collier et al. 2004). Most of the early water diversions were carried out to improve pasture for grazing, but constructed drainage systems for roads and residential properties adjacent to the park have also had permanent impacts on the local hydrologic regime (Collier et al. 2004). A busy suburban thoroughfare bisects the park at one end, effectively splitting the park in two. In addition, fire suppression has facilitated encroachment of woody species (both native and non-native) that may in turn be affecting light availability as well as patterns of surface and subsurface water flow through the site. At the same time, some area residents have called for increased ditching to reduce the amount of standing water on walking trails (R. Collier, pers. comm. 2005). Ditching has also been discussed as a way to eliminate mosquito breeding habitat in response to increased public concern over West Nile virus.

Nanaimo

As noted above, tire ruts left by off-road vehicles have affected the drainage patterns at this site, possibly to the detriment of the Centaurium muehlenbergii population. A transmission corridor bisects the property immediately adjacent to the population, further affecting the flow of water through the area. Past clear-cutting of adjacent forestland, and associated road construction, have likely had a considerable impact on the local hydrologic regime and may also have reduced the quality of Centaurium muehlenbergii habitat.

6. Altered fire regimes

Prior to European settlement of Vancouver Island, natural and human-initiated fires played an important role in the maintenance of the region’s dry Douglas-fir forests and Garry Oak savannas (Turner and Bell 1971, Roemer 1972, MacDougall et al. 2004). The suppression of fire within the past century may have contributed to the demise of Centaurium muehlenbergiipopulations. Two of the three sites in which this species has been recorded (Oak Bay and Nanaimo) were probably maintained in the past by periodic burning. Fires would have suppressed much of the competing vegetation, resulting in open microsites where C. muehlenbergii might have become established. Fires may have also played an important role in the development and fertility of soils, by ensuring the steady release of organic nutrients into the upper soil horizon.

At the Oak Bay site, aggressive fire suppression management has led to a substantial reduction in the amount of Centaurium muehlenbergii habitat available locally, due to widespread encroachment of Douglas-fir (Pseudotsuga menziesii) and a dramatic increase over historical times in the cover of invasive native shrubs such as snowberry (Symphoricarpos albus), Nootka rose (Rosa nutkana), and Indian plum (Oemleria cerasiformis) (Collier et al. 2004). In open wooded areas where C. muehlenbergii may have once survived, snowberry and various species of exotic shrubs noted above now dominate the understory. The density of these species severely limits light penetration and thus growth of the herbaceous layer. Furthermore, the incursion of shrubs and trees into adjacent open habitats may have begun to affect local hydrologic and light regimes through alteration of drainage patterns, increased competition for water, increased shading, and thatch buildup. If unchecked, this process could result in feedback loops that accelerate the overall rate of secondary succession.

7. Canada geese

Canada geese (Branta canadensis) have been undergoing exponential population growth on southeastern Vancouver Island and adjacent islands since the 1960s, a growth spurred partly by intentional introductions (McGrenere 1990, Campbell et al.1990, Carsen 2000). These birds are active grazers on vegetation, and in the process often churn up large areas of soil with their beaks. They have become abundant on the small Gulf Island occupied by Centaurium muehlenbergii. Here the steady grazing activity, along with associated contamination from guano deposits, may be having negative impacts on C. muehlenbergii growth and survival. However, this has not been investigated or confirmed.

Page details

Date modified: