Can models of presence-absence be used to scale abundance? Two case studies considering extremes in life history

Nielsen, S. E., Johnson, C. J., Heard, D. C. and Boyce, M. S. 2005. Can models of
presence
/absence be used to scale abundance? Two case studies considering extremes
in life history.
/ Ecography 28: 197
/208.

Understanding patterns of species occurrence and abundance is a central theme of
ecology, natural resource management, and conservation. Although occurrence models
have been widely used for describing species distribution, particularly for rare species,
abundance models are less common, despite greater information for conservation and
management. Because presence-absence data are easier and less expensive to collect,
predictions of abundance from patterns of occurrence could prove useful. We examined
the relationship between occurrence and abundance for two species with very different
life histories: bracken fern Pteridium aquilinum and moose Alces alces. We predicted
that if occurrence and abundance were functionally related we should observe: 1)
correlation between predicted probability of occurrence and observed abundance; 2)
similar environmental covariates and estimated coefficients for occurrence models
developed separately for low-density, high-density, and global data sites; and 3) parallel
coefficients for the occurrence and abundance components of zero-inflated count
models. Probability of occurrence was not correlated with abundance-when-present for
bracken fern, while evidence for a relationship for moose was apparent at densities of
animals below 7 individuals per cutblock. Coefficients for models at different levels of
density did not vary significantly. However, once occurrence was accounted for,
measured environmental data appeared less important in describing abundance. For
bracken, covariates of zero-inflated count models differed in their expression of
occurrence and abundance. Differences were less extreme for moose; however, results
from the two-process models suggest that distribution and abundance may be a
function of different processes. Environmental factors influencing abundance may
differ from those limiting distribution. Life history, scale, site history, and sociocompetitive
processes further help shape patterns of abundance. Two-stage modeling
provides a powerful tool for describing animal and plant distribution where the
processes of occurrence and abundance are influenced by different factors.

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