April 12, 2019 at 3:48 pm #322July 2, 2019 at 2:45 pm #234
CPC recommends collecting no more than 10% of an individual or population seed production in one season and no more than 10 out of 90 years. Frequent low-intensity harvests are associated with a lower extinction risk than infrequent high-intensity harvests.
The research that supports this recommendation is derived from Menges et al. (2004). Note that it is important to know some aspects of the species population demography, life history, and the initial population size to determine whether your population could be impacted by harvesting 10% of its seed crop in multiple years. If you have enough data, it is possible to generate models to examine the sensitivity of population growth to reduction in fecundity caused by seed harvest. In the absence of this data, realize that generally a population with fewer than 50 individuals will have a higher extinction risk than larger populations. Species that depend on annual fecundity would be most sensitive to harvest. These would be short-lived species (especially annuals) that don’t store seeds in a persistent seed bank (Figure 1B.1). Menges et al. (2004) used theoretical modeling in which they categorized 22 species (25 populations with published demographic data) into three types: Extinction Prone, Sensitive I (high initial extinction risk), Sensitive II (low initial extinction risk), and Insensitive. Insensitive species, nine species with populations with 50 or more individuals, could withstand any intensity of harvest over 100 years and had no extinction risk. The insensitive species they modeled were: Ardisia escallonioides, Calochortus obispoensis, Erythronium elegans, Neodypsis decaryi, Pedicularis furbishiae at Hamlin, Primula vulgaris, Themeda triandra, and Thrinax radiata. Note that these species are trees, shrubs, and iteroparous herbaceous perennials. Species categorized as extinction prone had 100% extinction probability with or without seed harvests. They included: Arabis fecunda, Ariseaema triphyllum, Eupatorium perfoliatum, and Pedicularis furbishiae at St. Francis. The Sensitive I species (Danthonia sericea and Eupatorium resinosum) were iteroparous herbs with clonal growth that had high extinction risk >40% without seed harvest and increased extinction risk with seed harvest above 10% in 50% of the years, while Sensitive II species (Arabis fecunda, Astragalus scaphoides, Calathea ovandensis, Dipsacus sylvestris, Fumana procumbens, Heteropogon contortus, (The 10 Percent Rule cont.) Horkelia congesta, Pana quinquefolium, Pedicularis furbishiae, and Silene regia) had initially low extinction risk that increased with seed harvest frequency and intensity at levels above 10% harvest in over 10% of years.
Works Cited Menges, E. S., E.O. Guerrant, Jr, and S. Hamze. 2004. Effects of seed collection on the extinction risk of perennial plants. In E. O.Guerrant, Jr., K. Havens, and M. Maunder, editors.eds. Ex situ plant conservation: supporting species survival in the wild. Island Press, Washington, DC.
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