Estimating Abundances of Interacting Species Using Morphological Traits, Foraging Guilds, and Habitat

<div><p>We developed a statistical model to estimate the abundances of potentially interacting species encountered while conducting point-count surveys at a set of ecologically relevant locations – as in a metacommunity of species. In the model we assume that abundances of species with similar traits (e.g., body size) are potentially correlated and that these correlations, when present, may exist among all species or only among functionally related species (such as members of the same foraging guild). We also assume that species-specific abundances vary among locations owing to systematic and stochastic sources of heterogeneity. For example, if abundances differ among locations due to differences in habitat, then measures of habitat may be included in the model as covariates. Naturally, the quantitative effects of these covariates are assumed to differ among species. Our model also accounts for the effects of detectability on the observed counts of each species. This aspect of the model is especially important for rare or uncommon species that may be difficult to detect in community-level surveys. Estimating the detectability of each species requires sampling locations to be surveyed repeatedly using different observers or different visits of a single observer. As an illustration, we fitted models to species-specific counts of birds obtained while sampling an avian community during the breeding season. In the analysis we examined whether species abundances appeared to be correlated due to similarities in morphological measures (body mass, beak length, tarsus length, wing length, tail length) and whether these correlations existed among all species or only among species of the same foraging guild. We also used the model to estimate the effects of forested area on species abundances and the effects of sound power output (as measured by body size) on species detection probabilities.</p></div

Maximum observed count of forest edge species, forest interior species, and forest edge and interior species and estimates of their abundance (posterior means and 95% credible intervals) at sample locations.

<p>Dashed line indicates the estimated relationship between mean abundance of birds and forested area.</p

Estimated probability of detection (on logit scale with 95% credible interval) vs. sound power output (mW) of each species.

<p>Estimated probability of detection (on logit scale with 95% credible interval) vs. sound power output (mW) of each species.</p

Dissimilarity in morphological traits between species of same foraging guild (upper panel) and between species of different foraging guilds (lower panel).

<p>Dissimilarity in morphological traits between species of same foraging guild (upper panel) and between species of different foraging guilds (lower panel).</p

Maximum observed count of ovenbirds and estimates of their abundance (posterior means and 95% credible intervals) at sample locations.

<p>Dashed line indicates the estimated relationship between mean abundance of ovenbirds and forested area.</p

Estimated correlations in abundance between species (posterior medians and 95% credible intervals) as a function of dissimilarity in their morphological traits.

<p>Black indicates species of same foraging guild; red indicates species of different foraging guilds. Upper panel corresponds to estimates obtained by assuming correlated abundances among all species; lower panel correponds to estimates obtained by assuming correlated abundances among species of same foraging guild. Estimated correlations are equal to zero for species with trait dissimilarities greater than 0.5.</p

Posterior means and 95% credible intervals for the parameters of three models.

<p>Model 1 assumes uncorrelated abundances. Model 2 assumes correlated abundances among species of the same foraging guild. Model 3 assumes correlated abundances among all species. Monte Carlo standard errors are given in parentheses.</p

Appendix E. Congeneric and within-guild pairs of species identified as true complete checkerboards (TCC) or true partial checkerboards.

Congeneric and within-guild pairs of species identified as true complete checkerboards (TCC) or true partial checkerboards

Appendix C. Observed and expected range overlap of exclusive congeneric and within-guild pairs.

Observed and expected range overlap of exclusive congeneric and within-guild pairs

Appendix D. Observed numbers of TCC, TPC, and significance tests.

Observed numbers of TCC, TPC, and significance tests