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Recovery Strategy for the Red Mulberry (Morus rubra) in Canada

4. Threats

Populations of Red Mulberry in Canada face four significant threats listed in order of importance: hybridization; habitat loss and fragmentation, impacts from nesting Double-crested Cormorants, and disease and the stress factors that make trees susceptible. Threats posed by other exotic species and grazing by White-tailed Deer and snails are of lesser concern. Table 1 classifies each threat.

Table 1: Threat Classification

ThreatExtentCausal CertaintyOccurrenceFrequencySeverityOverall Level of Concern
HybridizationWidespreadHighCurrentContinuousHighHigh
Habitat Loss and FragmentationWidespreadHighCurrentContinuousHighHigh
Nesting Double-crested CormorantsLocalizedHighCurrentContinuousHighHigh
Disease & Causative Stress FactorsWidespreadHighCurrentContinuousMediumMedium
Other Exotic SpeciesWidespreadLowCurrentContinuousUnknownLow
HerbivoryUnknownLowUnknownUnknownUnknownLow


4.1 Hybridization

Hybridization with White Mulberry is the most significant, population level threat to Red Mulberry in Canada. White Mulberry was introduced from eastern Asia for the silkworm industry. It has naturalized across eastern North America and freely crossbreeds with Red Mulberry (Farrar 1995, Waldron 2003). Almost all Red Mulberry populations in Canada occur in communities mixed with White Mulberry; with hybrids between the two species common (Ambrose 1999). Burgess (2004a, Burgess et al. 2005) found that 53.7 % of the Red Mulberry trees in six of the core populations (five or more individuals less than 1 km away from at least one other individual) in southern Ontario were hybrids. Of those hybrids, approximately 67% were genetically more similar to White Mulberry than Red. Based on an analysis of the pollen pool in two different locations, Red Mulberry pollen production per tree is similar to that of hybrid and White Mulberry trees. However, because White and hybrid Mulberry trees are more common than their native counterpart, only 8% of the total mulberry pollen rain comes from the native Red Mulberry (Burgess et al. 2008b). Selective removal of White and hybrid mulberry trees in a 50 m diameter around reproductive, female Red Mulberry trees resulted in a 14% increase in pure Red Mulberry seed produced by those individuals (Burgess et al. 2008b). This shows that Red Mulberry is experiencing a strong mating disadvantage associated with its low abundance. The reduction in Red Mulberry offspring was found to be largely attributable to crossbreeding with hybrid trees.

From observations at Fish Point Provincial Nature Reserve, on Pelee Island, where frequent tree blow-downs have occurred, it appears that White and hybrid Mulberry trees establish naturally at a high rate while Red Mulberry seedlings are rarely encountered (K. S. Burgess pers. comm.). Transplant experiments show that seedling and juvenile survival and fitness were much higher for White Mulberry and their hybrids than Red Mulberry in all environments and that no habitat differentiation occurred between Red, White, and hybrid Mulberry trees that could shelter Red Mulberry from the effects of hybridization (Burgess and Husband 2006). In addition, offspring from female White Mulberry trees were more likely to survive than those from female Red Mulberry trees (Burgess and Husband 2004).

The large number of White Mulberry trees and hybrids across the landscape, and the genetic makeup of the hybrids, suggest that the Red Mulberry is being genetically assimilated by White Mulberry. Given the negative effect that hybridization has on mating and establishment in Red Mulberry (Burgess 2004a), it is likely that, without recovery action, hybridization may result in the extirpation of pure Red Mulberry in Canada. Furthermore, habitat disturbance promotes hybridization with rare taxa (Wolf et al. 2001).

4.2 Habitat Loss and Fragmentation

Loss of suitable habitat poses a threat to Red Mulberry of only slightly lesser magnitude to that of hybridization. Land clearing for agriculture, industry, urban development, and utility and transportation corridors has greatly reduced the amount of natural wooded habitat in Carolinian Life Zone of southwestern Ontario. In some areas within the historical range of Red Mulberry, less than 3% forest cover remains, much of which is highly fragmented (Larson et al. 1999). The historical range of Red Mulberry in Canada once extended through eastern Toronto to Whitby, but these sites have disappeared (Figure 2), likely due to land clearing and habitat degradation (Ambrose 1987). Two populations in the Niagara Region have been lost to construction in the last 20 years (G. Meyers pers. comm. 1985), and others were likely impacted by valley infilling and development adjacent to what are now small populations. In addition, natural events, like the June 6, 2010 Harrow to Leamington tornado that passed near one of the Essex County woodlands containing a small Red Mulberry population, have the potential to eliminate populations. The resultant increased distances between populations, particularly the smaller ones, enhances their susceptibility to natural randomly occurring events and/or anthropogenic impacts that could lead to extirpations of additional species' occurrences. Beyond clear cutting, other high intensity forestry practices (high grading or diameter limit cuts) can damage vegetation, cause soil compaction which may result in reduced Red Mulberry establishment, cause soil disturbance which may promote increased establishment of exotic plants and increase evaporation resulting in decreased soil moisture levels thereby increasing drought-related stress on individual trees.

4.3 Nesting Double-crested Cormorants

Ontario's Double-crested Cormorant (Phalocrocorax auritus) population has increased dramatically over the past 30 years. Large colonies of nesting cormorants are threatening the long-term persistence of Red Mulberry populations and their habitat on Middle Island (10 trees in 2002/3 [North-South Environmental Inc. 2004]) and East Sister Island (five trees [S. Dobbyn unpub. data 2009, NHIC unpub. data 2010]) in the western basin of Lake Erie. Research has shown that cormorants impact trees in their breeding locations by physically breaking branches, stripping foliage for nesting material (Korfanty et al. 1999) and through the deposition of excrement on trees, leaves, and soil. The latter can affect photosynthesis as well as soil chemistry (Hobara et al. 2001, Hebert et al. 2005).

Since 2000, an average of 4 897 nests have been recorded on Middle Island, while an average of 4 752 have been recorded on East Sister Island during the same period (Parks Canada unpub. data). Double-crested Cormorant population estimates for the islands, 24 485 and 23 760 respectively, are based on an average of 2.5 adults (includes non-breeding individuals loafing around each island) and 2.5 chicks per nest (Hatch and Weseloh 1999; T. Dobbie pers. comm. 2010). On Middle Island, cormorant nests have been found in Red Mulberry trees as well as in adjacent trees, with all but one Red Mulberry tree being negatively impacted. One Middle Island tree appears to be dead and another nearly so (T. Dobbie pers. comm. 2010). This population, in particular, is threatened with extirpation. On East Sister Island, the population may be less impacted as three of five trees are in areas of low to moderate cormorant nesting while another, in an area of more extreme impacts, appears to be faring well due to its location in a patch of lower trees and shrubs not yet used for nesting by cormorants (S. Dobbyn pers. comm. 2010).

4.4 Disease and Causative Stress Factors

Red Mulberry is known to suffer from twig blight, twig dieback, cankers, and root rot (Ambrose et al. 1998). Health assessments of four populations of Red Mulberry indicate that some populations are in very poor health, suffering population-level declines described as a "gradual, general deterioration" (McLaughlin and Greifenhagen 2002; Spisani et al. 2004). The former study concluded that no single pathogen was responsible for the disease symptoms. Rather, several opportunistic, canker-causing pathogens and two opportunistic root disease pathogens affected the diseased trees. These pathogens are not known to infect healthy tissues, but can successfully cause damage to stressed and weakened hosts. Probable factors causing such stress include drought, low soil fertility and/or poor or suppressed canopy position. The Fish Point Provincial Nature Reserve and Point Pelee National Park populations were not found to be as healthy as the one at Rondeau Provincial Park due to a lower water table and less developed Red Mulberry tree canopies as a result of competition with neighbouring trees. The Royal Botanical Gardens population was found to have a broad range of health conditions based on more fertile and moist soils, but often suppressed canopy position (McLaughlin and Greifenhagen 2002).

Other research indicates that the species is highly sensitive to air pollution, with high levels likely making the species more susceptible to disease (Little 1995). In West Virginia, ozone damage to Red Mulberry leaves is believed to increase susceptibility to an opportunistic twig canker disease (Nectria cinnabarina) leading to the death of whole trees (O. Loucks pers. comm. 1996). Areas of reduced air quality may also be impacting populations through nitrogen enrichment, which has been identified to have a serious impact on natural grasslands (Wedin 1992). Similarly, soil enrichment from agricultural pollution may negatively impact soil microbes, which could make Red Mulberry habitats and populations more susceptible to White Mulberry colonization and hybridization. Studies of mycorrhizal6 functioning on other species have established a negative impact of nitrogen deposition, causing mycorrhizae to become more parasitic7 on plants rather than having the usual mutualistic8 relationships (Allen 1991). Given that many of these stress factors can, and do, occur together, they may have cumulative stress effects on Red Mulberry, increasing susceptibility to attack by opportunistic pathogens, leading to reductions in population size and potential extirpations.

4.5 Other Threats

The following are either unconfirmed threats or threats considered to be of low concern relative to the four primary threats listed above.

4.5.1 Other Exotic Species

Other invasive species, beyond White Mulberry, may negatively impact Red Mulberry or/and its habitat. Several introduced insect species are increasing their distribution across southern Ontario. Emerald Ash Borer (Agrilus planipennis) and Asian Long-horned Beetle (Anoplophora glabripennis) are two insects of high concern due to their invasive nature and ability to infest and kill healthy trees. The Emerald Ash Borer primarily targets ash species while the Asian Long-horned Beetle attacks a variety of tree species. The expansion of either or both insect ranges could alter forest composition and Red Mulberry habitat, with unknown impacts to Red Mulberry. Invasive plant species, such as European and Glossy Buckthorn (Rhamnus cathartica and R. frangula), Norway Maple (Acer platanoides), Tree of Heaven (Ailanthus altissima), European Alder (Alnus glutinosa), Garlic Mustard (Alliaria petiolata), and Dog-strangling Vine (Vincetoxicum nigrum), may pose a threat to mature Red Mulberry trees or their seedlings by aggressively competing for light, producing chemicals toxic to other plants or inhibiting mycorrhizal activity (Vaughn and Berhow 1999).

4.5.2 Herbivory

The fruit of Red Mulberry is an attractive food source of birds and small mammals, which, if eaten and dispersed before it is fully mature, may result in lower regeneration success (Johnson and Lyon 1976). High populations of gastropods can hinder seedling growth. Grazing by eight species of native snails and slugs was observed at Point Pelee National Park (T. Pearce pers. comm. 1992) to effectively eliminate seedlings (Ambrose 1991). Gastropod impacts at other sites are unknown. In areas of high deer populations, browsing of Red Mulberry has been observed and is a further hindrance to the establishment of new seedlings (Ambrose 1993, Thompson 2002b).

6 Mycorrhizal refers to a close, and mutually beneficial, association between a fungus and the roots of a tree in which the fungus is wrapped tightly around the tree rootlets or actually penetrates the cells of the tree roots.
7 A parasitic animal or plant lives in or on another plant or animal, obtaining the nourishment it needs from this individual without benefitting or harming the other plant or animal.
8 Mutualistic relationships refer to the way in which two different species interact in a way that benefits both.