Recovery Strategy for the Common Hoptree in Canada [Final] 2012: Threats
COSEWIC (2002) designated Common Hoptree as Threatened based on its restricted range, small population size, and the impacts of land development for cottages, nesting Double-crested Cormorants, and a twig-boring beetle. In the report leading to the designation, Ambrose (2002) also identified beach grooming, deer browse, invasive exotics, and storm erosion as threats to particular populations. Threats to the species were reassessed in December 2008 at a recovery strategy writing workshop. Habitat succession, recreational activities, livestock and vole browse, garbage dumping, and climate change were recognized as additional threats to the species (see individual sections below for discussion and justification). In addition, the term “Landscape Management” was used to encompass the impacts of both land development and beach grooming, while the impacts of storm erosion were included under the heading of "Altered Coastal Processes". More recently, Harris (pers. comm. 2011) has identified a second twig-boring insect (a moth) and a leaf-roller moth as potential threats. The impacts of the three insects have been combined under "Insect Herbivory". Table 2 provides additional detail surrounding each of these threats, including the overall level of concern (high, medium, or low), extent (range wide or local), occurrence (current or anticipated), frequency (continuous or unknown), severity (high, medium, or low), causal certainty (high, medium, or low), and ranked priority for action for each.
? = unknown
The greatest threat to most Common Hoptree populations today (of medium severity range wide, with a high causal certainty and medium to high overall level of concern) is from plant (Common Hoptree or other) or whole habitat removal, and, in some cases, replacement. Shoreline development is intensive along much of Lake Erie's north shore and habitat loss is occurring as land is developed for cottages. Many shoreline owners desire easier access to, and/or a view of, the water, or a manicured look to their beachfront property and therefore groom their private beaches or convert much of the area to lawn. Incompatible development has been observed at Fox Creek, Lypps, Linden, and Erie beaches and Thamesville, with extirpation of the Linden Beach population. Maintenance of roadsides and ditches (Thamesville, Walpole Island First Nation Snye Road at Old Ferry Road, Hillman Marsh Conservation Area, Lighthouse Point Provincial Nature Reserve, Point Pelee National Park, and around Pelee Island's west shore pumping station), as well as the grooming of beaches (Cedar Beach; Holiday Beach; Colchester, Seacliff, Erieau, Erie, and Crescent Beach to Windmill Point) has also led to the damage or loss of individuals and/or habitat.
Removal of Common Hoptrees also occurs due to the species' superficial resemblance to Poison Ivy (Rhus radicans). (Ambrose 2002). Historical landscape management impacts to Common Hoptree habitat include logging of Eastern Red Cedar (Juniperus virginiana) on Pelee Island and at Point Pelee National Park.
Hyperabundant, nesting Double-crested Cormorants impact the second largest Canadian Common Hoptree population, located on Middle Island. Since 2000, an average of 5 000 nests have been recorded on the island (Dobbie 2008). Research has shown that guano deposition can affect photosynthesis, resulting in damage and death to foliage and stems, as well as effecting soil chemistry changes (Hobara et al. 2001, Hebert et al. 2005). Areas under nesting trees are typically devoid of vegetation. A 2007 Common Hoptree inventory revealed that almost 20% of the population was severely damaged (50% or less of the plant in leaf) and another 19% was moderately damaged (51% to 90% of the plant living and in leaf). Thirteen percent had live stems clustered around a dead, central/main stem (Jalava et al. 2008). Although this is a localized threat, its high severity, established causal certainty, and potential, without management intervention, to extirpate this large population make it a threat of medium overall concern.
Altered coastal processes pose a medium level threat on Common Hoptree and its habitat. Extensive shoreline protection, stabilization, and alteration have led to disruption of the natural coastal processes that shape the dynamic shorelines and sand spits of Lake Erie's north shore. The western shoreline of Point Pelee National Park has historically been an accreting or expanding shoreline. Between 2004 and 2006, this shoreline eroded 11 m. The Colchester to Southeast Shoal Beach Nourishment Study (Baird 2010a) determined that, without erosion mitigation measures and sand replenishment in the littoral cell, 126 ha could be lost from the western shore within the next 50 years. The eastern shoreline of the Point Pelee peninsula, from Port Alma to the tip of Point Pelee National Park, is a naturally eroding shoreline, but erosion rates have increased to an average of 4 m per year north of the national park due to shoreline development (Baird 2010b). Similar erosion issues exist along the south shore of Rondeau Provincial Park due to sand entrapment by the harbour breakwaters at Erieau. Decreased sand delivery and increased erosion means that less suitable habitat is available for germination and that established trees are closer to the shore and therefore more susceptible to the uprooting action of storm waves and ice. At a minimum, population level effects are expected at Point Pelee National Park. Habitat loss along the south shore of Rondeau Provincial Park may be offset by gains at Laverne Kelly Memorial Park where much of the sand destined for the provincial park is captured behind by the Erieau breakwater.
The threat of habitat succession is also of a medium level of concern. Much of the alvar and savanna habitats of Pelee Island (Stone Road Alvar, Brown's Road, Lighthouse Point and Fish Point Provincial Nature Reserves, and Red Cedar Savanna) and the Lake Erie Sand Spit Savannas of Point Pelee National Park are moderately to severely threatened by habitat succession (Nature Conservancy of Canada 2008, Dougan & Associates and McKay 2009). Habitat succession is considered the main threat to alvar habitats on Pelee Island. Habitat succession, woodland canopy shading, and vine cover is also affecting both of the Walpole Island First Nation Common Hoptree populations (Jacobs pers. comm. 2010). Population level effects are expected at these locations as canopy cover progresses beyond optimal conditions for Common Hoptree. The presence of this threat and its impact at other sites remains to be determined. The prevention of disturbance regimes such as fire, as well as the alteration of others like coastal processes, has allowed native species that would normally be excluded from these habitats to move in. Without intervention, these habitats may convert to thickets of Rough-leaved/Drummond's Dogwood (Cornus drummondii), Common Prickly Ash (Aralia spinosa), Red Ash (Fraxinus pennsylvanica), Fragrant Sumac (Rhus aromatica), Staghorn Sumac (R. typhina), hawthorns (Crataegus spp.), and Dog Rose (Rosa canina), woodlands, or forests dominated by Eastern Red Cedar, oak (Quercus spp.), and maple (Acer spp.). Common Prickly Ash is successful as it is avoided by browsing species due to its prickles. Many pioneering tree and shrub species have allelopathic qualities (Staghorn and Fragrant Sumac, Red Cedar, Common Juniper [Juniperus communis], Common Hackberry [Celtis occidentalis], and the Threatened Dwarf Hackberry [Celtis tenuifolia]), or can crowd out Common Hoptree through their aggressive clonal growth habits (Poison Ivy [Toxicodendron radicans], Rough-leaved Dogwood, Fragrant Sumac, and Common Juniper). The impacts of allelopathic inhibition by other species on Common Hoptree and its habitat are unknown. Development of a thick mat of grasses can also inhibit seed germination.
A twig-boring bark beetle, identified by Steve Marshall (University of Guelph) as Phloeotribus scabricollis, is considered "a potential threat of unknown impact" (COSEWIC 2002). This species was found feeding on the flowering parts of Common Hoptrees at Fish Point and Lighthouse Point Provincial Nature Reserves, areas north of Point Pelee National Park, and at Hillman Marsh Conservation Area during surveys conducted for the 2002 COSEWIC report (Ambrose pers. comm. 2010). Large parts of the afflicted trees were affected, with decreased flowering and reduced tree crown resulting (Ambrose 2002). This beetle is likely native to Canada, given how specific beetles in this group are associated to their host plant. However, based on the fact that the species has only recently been located in the most southern areas of Canada, it may be adventive, in this case expanding its range northward from its native United States (Sutherland pers. comm. 2010).
A small moth, the Hop-tree Borer (Yponomeuta atomocella), is much rarer, with only four confirmed Canadian records. Further evidence of about 60 larvae at Point Pelee National Park and about 20 on Pelee Island was found in 2010. The larvae bore into the tips of young twigs, killing them. (Harris pers. comm. 2011). While recent occurrence levels suggest that this species may not affect Common Hoptree to any great extent, its impact in combination with other insect herbivores or if it periodically irrupts in larger numbers may be much greater, and therefore of concern.
The Hop-tree Leaf-roller Moth (Agonopterix pteleae), which is abundant at Point Pelee National Park, poses another possible threat (Harris pers. comm. 2011). Its larvae live in and feed on the rolled up edges of Common Hoptree leaves. Intensive and extensive defoliation (25% to 75% in2005/6 [Scarr et al. 2007] and similar levels in 2009/10 [Harris pers. comm. 2011]) has been observed. At this time, however, it is unknown if the tens of thousands of insects observed in 2009/10 represent an irruption of a species that has long been present and that will eventually die back or if the moth is a recent arrival that may decrease Common Hoptree growth or survivorship (Harris pers. comm. 2011).
Based on the potential of these insects to cause significant population level effects, there is moderate concern regarding the threat that they pose to Common Hoptree. Further investigation is required to determine their basic life history, along with their full extent and frequency of occurrence, and the severity of the impacts of each range wide.
Exotic and/or invasive plants and allelopathic tree species are a threat of medium to low level concern as they compete with Common Hoptree for water, nutrient, and light resources range-wide. Invasive, exotic plants are of concern at Fish Point Provincial Nature Reserve, Stone Road Alvar, Port Burwell Provincial Park (Ambrose 2002), and Erieau (McKay pers. obs. 2010). Of particular concern are Common Reed (Phragmites australis), White/Silver Poplar (Populus alba), Black Locust (Robinia pseudoacacia), Norway Maple (Acer platanoides), Scots Pine (Pinus sylvestris), Silver Birch (Betula pendula), White Mulberry (Morus alba), Black/European/Common Alder (Alnus glutinosa), Kentucky Bluegrass (Poa pratensis), Canada Bluegrass (Poa compressa), Quackgrass (Agropyron repens), Spotted Knapweed (Centaurea maculosa), Bouncing Bet (Saponaria officinalis), Japanese Barberry (Berberis thunbergii), and White Sweet Clover (Melilotus alba). Nitrogen-fixing species, like White Sweet Clover, improve soil conditions for other species that normally could not establish themselves in the nutrient depleted environments that Common Hoptree can, and therefore increase competition and shading by other species. On the alvars of Pelee Island, Eurasian pasture grasses are forming mats over bedrock, likely facilitating succession by shrubs (McFarlane pers. comm. 2010). Overall population level effects are considered moderate.
Other low level, localized threats to Common Hoptree populations are believed to include the impacts of recreational activities that cause damage and/or breakage of all age classes (e.g. use of off-road vehicles [Stone Road Alvar], bicycling, firewood collection, camping, and trampling [Stone Road Alvar]), mammal herbivory (historically by White-tailed Deer at Point Pelee National Park and Rondeau Provincial Park prior to species management, by livestock browsing on foliage and flowering parts, and vole girdling), and garbage dumping (Stone Road Alvar, Lighthouse Point Provincial Nature Reserve, Erieau). The range-wide, potential threat of climate change may increase the frequency, severity, and timing of severe storm events and resultant wind and wave driven shoreline erosion, while reducing the amount of ice cover and associated scour. This can accelerate habitat succession and shoreline stabilization. Seedlings in already harsh environments may not be able to survive more severe temperature extremes of dryness and heat. In addition to these current threats, historical impacts to Common Hoptree habitat include soil compaction caused by cattle grazing on the Pelee Island alvars.
The current Threatened designation of the species based on a slight decline in area of occupancy (Ambrose 2002), less than 2 500 mature individuals in Canada, and less than 1 000 in any population (COSEWIC 2002) is likely no longer valid. Given the small distribution of the species in Canada and the large gaps that occur naturally among the core areas, it is neither realistic nor appropriate to expand its extent of occurrence and area of occupancy beyond 20 000 km2 (COSEWIC's Threatened category threshold) and 500 km2 (COSEWIC's Endangered category threshold) respectively, or to focus on reducing population fragmentation. It is also not warranted to manage the population so that the number of locations will not decline from the 34 currently documented in the COSEWIC report (Ambrose 2002), for two reasons. First, a decline in the number of populations may actually occur if a more comprehensive survey of the Lake Erie shoreline locates the species in new areas close to a previously known population(s) resulting in the merging of populations formerly considered to be separate. Second, Common Hoptree is short-lived, and 40% of its populations have less than five mature individuals, making them very susceptible to extirpation from natural disturbances caused by severe weather events. Given the natural rarity of the species' habitat, as well as the gaps that still occur in knowledge relating to population size across the species' range, population targets for recovery are difficult to establish at this time. As such, maintenance of the current, known state of the Common Hoptree population in Canada is the short-term focus with aspirations to improve both knowledge and sustainability of individual populations and core areas over the longer term. As such, the population and distribution objectives for the Common Hoptree are as follows:
- to maintain Common Hoptree populations in seven core areas: Middle Island, Pelee Island, Essex County (including Point Pelee National Park), Walpole Island First Nation, Rondeau Provincial Park/Erieau, Port Burwell Provincial Park, and the Regional Municipality of Niagara, within suitable habitat types (sandy shoreline and alvar),
- to ensure that the number of mature individuals does not decline below 1000, and
- where feasible, to increase the size and reproductive capability of the smaller Common Hoptree populations that are currently considered unviable.
Key to the achievement of these population and distribution objectives over the long term is the restoration and maintenance of the natural disturbance mechanisms that provide for long term persistence of Common Hoptree critical habitat.
Efforts to update the population size and distribution data for the Common Hoptree in Canada have been initiated. Recent surveys have been completed in Point Pelee National Park (Jalava et al. 2008) and Rondeau Provincial Park (Dobbyn 2005) and are on-going in the Niagara region (OMNR unpubl. data). Population enhancement efforts have been ongoing at the Walpole Island First Nation since 2007 (Jacobs pers. comm. 2011) and an ecosystem protection plan is currently being developed based on the Community's Traditional Ecological Knowledge (Macbeth pers. comm. 2011). Communication products include a Common Hoptree identification card (OMNR 2009a), poster (OMNR 2009b), sticker (OMNR 2009c), and magnet (OMNR 2009d) being used primarily with property owners in the Niagara area and messaging incorporated into programming and communications at Point Pelee National Park and Rondeau and Port Burwell Provincial Parks. Little research has been conducted across the species range. Table 3 outlines and prioritizes the broad strategies and approaches needed to address the threats (Section 4) and meet the population and distribution objectives (Section 5).
In order to attain the Population and Distribution Objectives, the recovery planning approaches are summarized and ranked in Table 3 by their degree of urgency. Threats to the populations and information requirements are addressed. These approaches will be planned with due regard for negative impacts on other species at risk.
Much of the survey data for Common Hoptree were gathered eight to ten years ago. More recent surveys at Point Pelee National Park inventoried the number of stems rather than the number of individuals, thus making population estimates and trends through time difficult to determine. A thorough investigation of extant, historic, and extirpated sites, as well as areas of suitable habitat (e.g. the Lake Erie shore between Essex County and Regional Municipality of Niagara and the shores and islands of the Niagara River) is needed to update information regarding each population.
There are many activities that can be undertaken by landowners and land managers independently or jointly to promote Common Hoptree recovery. Communication of appropriate activities and promotion and support of stewardship is key to Common Hoptree recovery. Naturally occurring coastal processes, fire, wind throw, insect infestation, disease, and other disturbances would have maintained the open gaps and edges for Common Hoptree colonization and persistence in the past. To the extent possible, such natural processes should be allowed to continue, or should be restored or mimicked. It should be noted that habitat restoration activities required to address the threats of exotic and/or invasive species and/or habitat succession will result in some reduction of canopy cover. These actions, in locations where these issues have been identified as a threat(s), are deemed necessary and are not considered destruction of critical habitat, provided that the alterations promote greater use of the habitat by Common Hoptree.
First Nation communities have maintained local ecosystems for generations through the use of community Traditional Ecological Knowledge. It is important to work with Knowledge Holders as a means for species and ecosystem protection and recovery. Traditional Ecological Knowledge and western science can, together, better inform assessment, monitoring, and recovery of the ecosystems that support specific species at risk.
There has been relatively little research on the Common Hoptree across its range, let alone more locally in Canada. As such, Table 3 identifies research needed to complete critical habitat identification and achieve the recovery strategy objectives for Common Hoptree. This includes research to gain basic biological and ecological facts about the species, as well as key information regarding genetics, finalizing threat assessments on the insect herbivores and cormorant impacts, and gathering the information necessary to allow for adaptive management.
Basic, local, life history information is lacking. Research into limiting resources/conditions (light, nutrients, erosion, and deposition patterns); interspecific interactions (pollinators, competitors, predators, and parasites); sex ratios; the means, distance, and frequency of seed dispersal; seed bank viability; seed setting rate; germination success in the wild; seedling survivorship; longevity; and Common Hoptree's specific role in and impacts of succession is needed to inform species' recovery.
Genetic analysis is needed to guide site-based restoration to determine the native or introduced origin of trees at inland sites, the genetic differences within and between populations, the species' ability to clone (which could change the current understanding regarding the true number of individuals in Canada), site-specific diversity that may contribute to long-term retention or loss from certain sites, and the genetic significance of each site. This information will inform any restoration and augmentation activities deemed feasible.
8 Allelopathic plants suppress or inhibit the growth of other plants through the release of chemical toxins.
9 An adventive species is not native or fully established in a new area.
10 Repatriation restores a species to a location where it was formerly found, but is not currently present.
11 Augmentation adds individuals of a species to an existing population in order to increase population size.
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