A stepwise, multi-scale, viability analysis of the disjunct population of Cirsium scariosum at Mingan Archipelago National Park Reserve of Canada.
Nantel, Patrick*,1, Dénommée, Nancy2, Cantin, Danielle3, 1 Ecological Integrity Branch, Gatineau, Québec, Canada2 Mingan Archipelago National Park Reserve of Canada, Havre-St-Pierre, Québec, Canada3 Biologist - consultant, Ottawa, Ontario, Canada
ABSTRACT- Cirsium scariosum is a monocarpic perennial whose main geographic range is centered in Idaho. Only one population occurs in Eastern North America, on the Mingan archipelago, a group of predominantly limestone islands on the north shore of the Gulf of St. Lawrence. This disjunct population is composed of groups of plants (colonies) dispersed on four islands over short stretches of sand and gravel beaches, at the edge of the forest. We computed population growth rates () and cumulative extinction probabilities (CEPs) using data from 1995-2004 obtained at different scales, from simple counts to stage-based vital rates. Counts of five colonies yielded ranging from 0.976 to 1.227, with high variances over the sampled period. Stage-based counts (of seedlings, small rosettes, large rosettes, and flowering plants) input into quadratic programming yielded one transition matrix for each colony, with systematically lower than those from counts (0.914 - 1.061). Finally, we built two series of yearly transition matrices for each colony using monitoring data from tagged plants, one series without and one with seed dormancy. Transition matrices without seed dormancy gave stochastic ranging from 0.876 to 1.050, with confidence intervals overlapping those obtained with quadratic programming for two of the five colonies surveyed. Stochastic were 5% higher when computed from matrices including seed dormancy, and closer to computed from simple counts. This suggests that seed dormancy can play a key role in population maintenance. However, CEPs after 50 years range from 0.07 to 0.32 when based on simple counts, and from 0.30 to 0.91 when based on stochastic projections with yearly matrices that include seed dormancy. The counts from one of the colonies were highly auto-correlated, and those from two others had major outliers. These conditions violate the assumptions underlying the diffusion approximation used to compute CEPs. Overall, results indicate that the studied population has a high probability of extinction in the medium term, because of the small sizes of its component colonies, their low growth rates and highly variable vital rates. It needs to be monitored using detailed methods, because the conditions under which it evolves make the simpler methods unreliable.
Key words: thistle, population projection, rare plants, transition matrices
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