Spoon-leaved moss (Bryoandersonia illecebra) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

All three known extant populations of Bryoandersonia illecebra fall very near to human development and activity. The ready access to adjacent sites afforded by these developments probably enhanced the discovery of the populations, and to argue that all Canadian populations are threatened by development may therefore appear circular and premature. However, human development in southern Ontario is very intense: the small region that provides the conditions required by B. illecebra is the most heavily developed in Canada. Most natural sites are threatened by development. Furthermore, this intense development has allowed relatively many botanists to access a large number of sites over a relatively long period of time. That B. illecebra has not been discovered in more sites despite this accessibility, and that the species has apparently disappeared from some sites at which it was formerly recorded suggests that it is indeed very rare.

In Essex County, populations are within a few metres of a community greenway (converted from a Railway line to a multi-use recreation trail). The plant community in which the population occurs is uninviting to passing hikers and cyclists, as it is dense with hawthorn and poison ivy. This site and the others in which Bryoandersonia illecebra was observed are at least seasonally flooded--wet enough to support healthy mosquito populations for at least part of the year. Although the site is contiguous with a relatively extensive occurrence of natural habitat, it is bordered closely by agricultural development. Successional habitat change in part of the site (Figure 8; Mike Oldham personal communication) may currently threaten that part of the Essex population that was originally recorded in 1982.

In Elgin, Bryoandersonia illecebra is near a popular hiking trail. It is separated from the trail by a thin strip of dense fallen and living vegetation. The trail is largely wooded, but agricultural and urban developments occur nearby.

The Welland population of Bryoandersonia illecebra, which is the smallest known extant population, occurs within twenty metres of a roadway. It is within a few feet of the strip of roadside debris that has penetrated the forest margin.

Sites where Bryoandersonia illecebra was not found in 2001-2002, but from which the species was collected since 1972 provide few clues as to what factors may have eliminated the species. Swamp land at the wet deciduous wood lot in Elgin County provides a diverse array of habitats that appear not to have been disturbed since the 1980s when the species was last collected there, although mechanical forest thinning was evident in some sections of the address. Agriculture and recreational vehicle traffic are evident at the Middlesex site. Members of the Elgin Hiking Club confirm that there has been no substantial human disturbance to the two sites along the Elgin hiking trail, yet the hillside where B. illecebra was reportedly collected in 1983 was virtually uninhabited by any moss in 2002 (Figure 9). No human activity was noted in Jolley’s Swamp, although the site represents a small fragment of land surrounded by roads, agricultural land and a gravel pit. These developments were in place when B. illecebra was collected there in the 1970s and 1980s. Vegetation in Jolley’s Swamp is thought to be more dense than it was when William Stewart collected the species there (Figure 9; E. Stewart, K. Bachner, personal communication), and the site appears to be drier.

As discussed in the preceding sections, several more general factors also have the potential to limit the species’ abundance and distribution. These factors may be summarized as follows:

1. Climate – Bryoandersonia illecebra is associated with the eastern deciduous forest, which is characteristic of warm climates found largely in the south-eastern United States. Only a very small, southern part of Canada supports eastern deciduous forest species, and even if this region were pristine, suitable habitats would not be as common as they are near the centre of the species’ range.
2. Habitat change – Forest destruction and fragmentation decrease the already small number of habitats available to Bryoandersonia illecebra, increasing the species’ overall vulnerability to disturbance at any one site. This situation is exacerbated by the small population sizes at some sites. Human disturbance also increases the distance between populations, thereby diminishing genetic exchange between them. Pollution associated with human activity is especially detrimental to pleurocarpous mosses such as B. illecebra. Natural community succession, on the other hand, may also be affecting habitat availability -- several sites are thought to have become more densely vegetated since the species was first collected.
3. Species’ biology – B. illecebra is dioicous, making it necessary for male and female plants to co-occur to ensure dispersal by spores. No sporophytes or male plants have been observed among Canadian collections of B. illecebra. B. illecebra does not possess alternate means of dispersal such as asexual propagules. In addition to dioicy, B. illecebra also features other adaptations to stable habitats, placing it at a disadvantage in disturbed regions where its survival depends on dispersal to new sites.