Direct and indirect effects of biological factors on extinction risk in fossil bivalves

Abstract

Biological factors, such as abundance and body size, may contribute directly to extinction risk and indirectly through their influence on other biological characteristics, such as geographic range size. Paleontological data can be used to explicitly test many of these hypothesized relationships, and general patterns revealed through analysis of the fossil record can help refine predictive models of extinction risk developed for extant species. Here, I use structural equation modeling to tease apart the contributions of three canonical predictors of extinction--abundance, body size, and geographic range size--to the duration of bivalve species in the early Cenozoic marine fossil record of the eastern United States. I find that geographic range size has a strong direct effect on extinction risk and that an apparent direct effect of abundance can be explained entirely by its covariation with geographic range. The influence of geographic range on extinction risk is manifest across three ecologically disparate bivalve clades. Body size also has strong direct effects on extinction risk but operates in opposing directions in different clades, and thus, it seems to be decoupled from extinction risk in bivalves as a whole. Although abundance does not directly predict extinction risk, I reveal weak indirect effects of both abundance and body size through their positive influence on geographic range size. Multivariate models that account for the pervasive covariation between biological factors and extinction are necessary for assessing causality in evolutionary processes and making informed predictions in applied conservation efforts.

Fig. 1.

Models of the direct and indirect effects of biological factors on extinction risk in Cenozoic bivalves depending on whether covariation between factors is accounted. A, abundance; B, body size; G, geographic range size; D, species duration. Values are model coefficients. Solid lines denote significant effects at α = 0.05. Dashed lines are hypothesized but nonsignificant effects. Positive and negative effects are indicated by arrows and filled circles, respectively. Clade-independent coefficient estimates are black, and clade-specific coefficient estimates are colored blue (Carditoidea), orange (Pectinoidea), and green (Veneroidea). (A) When biological factors are assumed to be independent in their effects on extinction risk, geographic range is the strongest predictor, with abundance also contributing. (B) When covariation is taken into account, geographic range seems to be the only factor that contributes directly to extinction risk. (C) A multigroup model that includes both direct and indirect effects has the greatest support and identifies weak indirect effects of abundance and body size on extinction risk through the influence on geographic range size and opposing direct effects of body size on extinction risk among clades.

Fig. 2.

Relationship between sampling quality and the observed distribution of species’ first (A) and last (B) stratigraphic occurrences. If variation in sampling strongly affects the observed durations of species, a negative correlation is expected between the frequency of first occurrences in time bin t and the number of sampled occurrences in the preceding bin (t − 1). Conversely, poor sampling in interval t + 1 is expected to generate an excess of last occurrences in time bin t. No correlation exists between quality of sampling and the temporal distribution of observed stratigraphic range endpoints.