Flooded jellyskin (Leptogium rivulare) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Given its restriction to seasonally inundated habitats, the most obvious threat to the survival of populations of Leptogium rivulare is alteration of the water levels or the periodicity of their fluctuations. Changes of these kinds commonly occur when swamps are drained for agricultural uses or to make land areas available for urban development or road building. The damming of rivers greatly affects periodic flooding downstream, and would thus affect the potential habitat of L. rivulare. Hardly a river system anywhere within the range of this species is not regulated to some extent by damming for flood control or hydroelectric power generation.

Across much of its range, lakes were also dammed at their outlets for early timber operations. If the lichen existed on their shoreline rocks, as is now known to be the case at a single Manitoba site, permanent flooding may have eliminated it.

Seasonal ponds that fill with meltwater should be unaffected by alterations to waterways or lake systems. But in unusually dry years, most of the ponds where the lichen occurs do not fill to normal levels in spring. As the species typically occurs in the upper 50 cm of the flood zone, a shortfall of 25 cm, such as has been observed in two recent years, may deprive the top half of the lichen population of the condition that is an absolute requirement for its persistence. If spore dispersal is by water, then that part of the population left stranded will not contribute to reproduction, either.

Another possible threat is the loss of the trees that are almost the only substrate the lichen grows on. Commercial logging is unlikely, because of the usually low quality of the tree species involved. But in some ponds, the species has been found on as few as two trees. Hence, even removal or destruction of trees on a small scale, such as a surveyor cutting a sight line, could have a significant impact. In the past four years at the Darling Township site, such trees have occasionally been removed for firewood (two properties), surveying (one property), recreational trail cutting (two properties) and private, personal-use, sandpit operations (one other property).

Increasingly, too, trees are subject to potentially devastating alien diseases and pests. One of this lichen’s substrate trees, American Elm, has already been decimated by an introduced fungus, which we call Dutch elm disease. Two other species, Black Ash and Red Ash, face a new threat. In 2002, an alien beetle, the Emerald Ash Borer, appeared in North America. It not only killed great numbers of ash trees in southeastern Michigan, but prompted the clearing of these trees from quarantine zones (Roberts 2003). The beetle has been found in Canada, in southwestern Ontario (Ash Rescue Coalition 2003).

At the Ottawa site, the potential for damage to the trees by vandalism is to be expected. Recreational users from massive new subdivisions nearby have recently created a new, unauthorized footpath immediately beside the two heaviest populations of the lichen. Also, as the nature of this location changes from freely rural to a large urban park, declining air quality may ultimately become a survival issue for Leptogium rivulare there.

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