A field comparison of two capture-mark-recapture estimators of small mammal populations

Abstract

ABSTRACT. The results obtained by two estimators of population sizes, MNKA and Mh, were compared for four species of sma ll mammmais -Didelphis ol/rito Wied, 1826, Philollder /relloto (O ilers, 1818), NectolllYs sql/olllipes (Brants, 1827) and Akodon cursor (Wi nge, 1887) -during a long-te rm population study. The MNKA estimator consistently underestimated the population sizes in relation to Mh. On the other, the probabilistic estimator Mh, which reduces bias through the jackknife technique, cOllld not be used in all cases as its assumptions were not always met. Correction factors between the estimates obtained by the two methods were calculated for the last three species, for which catchability did not valY signiticantly in time and that presented positive correlation between the estimates by the two models. In order to combine the adavantages of both methods for sma ll mammal population st udies, is suggested the use of probabilistic closed popUlation models and to calculate a correction factor based in another model which al low estimates in all cases, and which provides correlated estimates. This correction factors should be used in those cases where the probabilistic model cannot be used. KEY WORDS. Capture-recapture,jackknile estimator, minimun number known alive, population size Capture-mark -reeapture population estimation models have been widely described in the li terature. For closed populations, several models have been developed whieh do not make the restrictive assumption of equa l catchability; this asssumption is a problem when estimating population size of animals of complex behaviour s uch as mammal s. There are models wh ich deal with heterogeneity of capture probabilities among individuals (BURNHAM & OVERTON 1979;CHAO 1988), a long time (DARROCIl 1958), or due to behav ioural response to traps (OTIS el af. 1978); the first and third problems can also be dealt with altogether (Mode l Mbh , OTIS el al. 1978). However, no model has been proposed which allows to deal with the three so urces of heterogeneity simultaneously; the source(s) not dealt with remain as assumptions to be fulfilled (SEBER 1986).The main dirficulty in the application of s uch probabilistic model s is to collect enough data to satisfy all the assumptions . Data co ll ected with emphasis in

Evaluation of abundance indexes in open population studies: a comparison in populations of small mammals in southern Brazil

“…On the other hand, there are difficulties in the application of probabilistic models for closed populations because they require more data and cannot be used in many cases (Gentile and Fernandez, 1999). Open population models are then preferred, especially the deterministic ones (e.g.…”

A comparison of abundance estimators for small mammal populations

“…The precise estimation of abundance, incorporating variable probabilities of capture, requires large amounts of data to satisfy the assumptions of probabilistic methods (GENTILE & FERNANDEZ 1999, SLADE & BLAIR 2000. Because obtaining the recommended sample size (50-200 individuals per taxon, depending on the capture probabilities; OTIS et al 1978) from capture-mark-recapture data for mammals is difficult, primarily due to low local abundances and/or low capture/recapture rates, enumeration estimators of population size, such as the MNA, are still widely used in small mammal population studies, especially in the tropics (CROUZEILLES et al 2010, GENTILE et al 2004, LIMA et al 2006, MESERVE et al 2001, YUNGER et al 2002.…”

“…Studies comparing probabilistic estimators with the MNA have shown that the enumeration estimator produces biased estimates when heterogeneity among individuals and behavioral responses to traps (trap shyness or trap happiness) are present and when capture probabilities are low (BOULANGER & KREBS 1994, EFFORD 1992, GENTILE & FERNANDEZ 1999, HILBORN et al 1976, NICHOLS & POLLOCK 1983. Simple counts are sometimes selected over probabilistic estimators because they are easier to obtain, even though they may underestimate the actual abundance.…”

Pequenos mamíferos não-voadores do Planalto Atlântico de São Paulo: identificação, história natural e ameaças

“…Além disso, aumentou de abundância após fogo, que extinguiu várias espécies localmente (Pires et al 2004, Figueiredo e Fernandez 2004, o que indica sucesso em áreas perturbadas. Foi mais freqüente em ambientes com alta densidade de arbustos próximos ao chão e alta densidade de serrapilheira (Gentile e Fernandez 1999), foi capturada em área de solo encharcado coberta por vegetação rasteira espaçada no Espírito Santo (Paresque et al 2004), e em bordas de fragmentos e áreas cobertas por bambus no Distrito Federal (Nitikman e Mares 1987). Existem vários outros registros em ambientes alterados (Lessa et al1999.…”

“…Em outras regiões da Mata Atlântica foi capturada em fragmentos florestais (D'Andrea et al 1999, Gentile e Fernandez 1999, Pires et al 2002 …”