Estimating rates of population change for a neotropical parrot with ratio, mark- recapture and matrix methods

  • Sandercock B
  • Beissenger S
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Abstract

Robust methods for estimating rates of population change (k ) are necessary for applied and theoretical goals in conservation and evolutionary biology. Traditionally, k has been calculated from either ratios of population counts (obser ved k or k obs ), or population models based on projection matr ices (asymptotic k or k asy ,). New markrecapture methods permit calculation of k from mark- resighting information alone (realized k or k rea ), but empir ical compar isons with other methods are rare. In this paper, rates of population change were calculated for a population of green-rumped par rotlets (Forpus passer inus) that have been studied for more than a decade in central Venezuela. First, a ratio method based on counts of detected birds was used to calculate kà obs . Next, a temporal symmetry method based on mark- recapture data (i.e. the k - parameterization introduced by Pradel, 1996) was used to calculate kà rea . Finally, a stage-structured matr ix model based on state-speci® c estimates of fecundity, immigration, local survival, and transition rates was used to calculate kà asy . Analyses were conducted separately for females and males. Overall values of kà from the three methods were consistent and all indicated that the ® nite rate of population change was not signi® cantly diþ erent from 1. Annual values of kà from the three methods were also in general agreement for a major ity of years. However, kà rea from the temporal symmetry method had the g reatest precision, and apparently better accuracy than kà asy . Unrealistic annual values of kà asy could have been due to poor estimates of the transitional probability of becoming a breeder (c à ) or to a mismatch between the actual and the asymptotic stable stage distr ibution. In this study, the trade-oþ between biological realism and accuracy was better met by the temporal symmetry than the matr ix method. Our results suggest that the temporal symmetry models can be applied with con® dence to populations where less information may be available.

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Authors

  • B K Sandercock

  • S R Beissenger

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