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Insular plant turnover across a 22‐year interval: a critical retrospective of the roles of pseudoturnover and cryptoturnover

Morrison, Lloyd W.
Journal of biogeography 2017 v.44 no.5 pp. 1007-1017
adults, data collection, immigration, islands, reproduction, vascular plants, Bahamas
AIM: To document patterns of insular plant turnover spanning a 22‐year period, and to critically evaluate the potential influence of cryptoturnover and pseudoturnover. LOCATION: Small islands in the Exuma Cays, Andros and Abacos archipelagos of the Bahamas. METHODS: The vascular plants inhabiting 243 small islands in four archipelagos of the Bahamas were surveyed by the same investigator in multi‐year (4–5 years) intervals. Turnover rates were calculated, and the data record was examined for evidence of cryptoturnover and pseudoturnover. RESULTS: Observed rates of turnover on a per island basis were low (mean < 4% per year). Extinctions represented 68% of turnover events. Cryptoturnover could cause turnover estimates to be up to three times too low, although most cryptoturnover involved propagules that did not reach adult status and species that were not common on the small islands. Pseudoturnover due to the potential overlooking of species was very low (maximum rates of 5–9% of observed turnover). Yet if attainment of a reproductive state, rather than simple residency, is required, pseudoturnover would be higher. MAIN CONCLUSIONS: Although often not explicitly addressed or quantified, cryptoturnover and pseudoturnover have cast doubt on numerous turnover studies. In this Bahamas data set, the magnitude of such errors will depend on how an immigrant is defined (i.e. whether maturity or reproduction is required). More restrictive assumptions of immigration (i.e. maturity of the propagule to a reproductive state) would increase pseudoturnover, whereas more lenient assumptions (i.e. simple residency) would increase cryptoturnover. Given the robust patterns of extinctions outnumbering immigrations, neither source of error would greatly affect the overall patterns documented in this Bahamian system. These sources of error will continue to be important concerns in any study of temporal diversity, however, and should be considered seriously in the design of future studies.