# Abstracts

### Confidence Intervals for Expected Abundance of Rare Species

**David Fletcher**

University of Otago

**Malcolm Faddy**

Queensland University of Technology

In many ecological research studies, abundance data are skewed and contain more zeros than might be expected.
Often, the aim is to model abundance in terms of covariates, and to estimate expected abundance for a given set of
covariate values. Welsh et al. (1996) have advocated use of a conditional-model approach for this purpose. This
allows one to separately model presence and abundance given presence, which should lead to a more complete
understanding as to how the covariates influence abundance. The focus of this talk is on the calculation of
confidence intervals for expected abundance given particular values of the covariates. The Wald confidence
interval used by Welsh et al. (1996) is symmetric, and therefore unlikely to be of much use for skewed data,
where confidence intervals for abundance measures are likely to be asymmetric. We show how to calculate a
profile likelihood confidence interval for expected abundance using a conditional model.

References

- Welsh, A.H., Cunningham, R.B., Donnelly, C.F. and Lindenmayer, D.B. 1996. Modelling the abundance of rare
species: statistical models for counts with extra zeros. Ecological Modelling 88: 297-308.

**Session 3c**, Environmetrics: 14:15 — 14:35, Room 445

Presentation Program