A synergistic effect puts rare, specialized species at greater risk of extinction

Theory and empirical evidence have long suggested that some species are extremely vulnerable to extinction because they have combinations of extinction promoting traits. However, ecologists have not considered whether the form of the relationship between traits is additive (not synergistic) or nonadditive (synergistic). We looked at how traits and their interactions were related to the difference in species' population growth rates between experimentally fragmented forest and continuous forest. Two traits acted synergistically; natural abundance and degree of specialization interacted so that beetle species that were rare and specialized had a greater reduction in their growth rates in fragments, compared to continuous forest, than the sum of the reductions in growth rates attributable to these traits. In other words, species that were both rare and specialized were especially vulnerable to extinction. From a conservation perspective, an implication of our findings is that making predictions about extinction risk from a single trait, like abundance or population variability, may be risky because traits may act synergistically rendering species more, or less, vulnerable than predicted by that single trait. There is currently a great deal of interest in which traits predict the sensitivity of species to a given threat, but if we are going to look at risks in this way, then we also need to consider how traits interact, because this can alter the vulnerability of species.