Howell’s triteleia (Triteleia howellii) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

The most direct and immediate threat to Triteleia howellii is habitat destruction. Consequently, dispersal into new sites is very limited. This is of particular concern in the grass-dominated meadows often associated with the Quercus garryana communities that are limited to the southeastern side of Vancouver Island and some of the Gulf Islands. This type of vegetation was much more common before colonization by European settlers. This destruction has continued to the present resulting in the elimination of almost all sites occurring outside parks or ecological reserves. Historically, Q. garryana communities and grass-dominated meadows have always been heavily influenced by human activity. Roemer (1972) believes that without human interference some of these stands would have eventually been replaced by Pseudotsuga menziesii forests.

The suppression of fire within the past century may also have contributed to the decrease of Triteleia howellii populations. Most of the sites in which T. howellii has been collected were likely maintained in the past as a result of periodic fires, both natural and unnatural. In the past, aboriginal peoples probably set fire to these stands to maintain them as an important habitat for wildlife (Roemer 1972). Since that time, these sites have experienced little disturbance, resulting in the invasion and expansion of many other species, especially introductions.

The introduction of European species has resulted in substantial changes, not only to the grass-dominated meadows associated with Quercus garryana, but also to the rocky xeric sites north and west of Victoria where Triteleia howellii has been collected in the past. One of the most devastating species is Cytisus scoparius (Scotch broom), which has become a dominant shrub on xeric, exposed sites throughout much of southeastern Vancouver Island and the Gulf Islands. Much of the vegetation is now dominated by introduced grasses. These species include Aira praecox (early hairgrass),Anthoxanthum odoratum (sweet vernalgrass), Cynosurus echinatus and Dactylis glomerata.

There are only nine known populations and some of these contain very few plants. Once a population becomes small, it becomes more vulnerable to demographic and environmental variation and loss of genetic variability. In some cases, small populations are at risk of inbreeding depression, genetic drift and loss of fitness (Primack 1998).