Performance of specialists and generalists across ecological and evolutionary scales

Background and aim: Current conservation biology suggests that in most regions, the species in most decline today are ecological specialists, i.e. species that occur across a narrow range of environments. Conversely, the speciation literature suggests that on an evolutionary time-scale specialists leave as many or more descendant lineages as generalists, i.e. they have high rates of global diversification. This begs the question: which of these two processes has more influence on the regional scale, i.e. do specialists leave more or fewer descendants than generalists within a region? In this presentation we give a few examples of the performance of specialists and eneralists. Methods: For the Netherlands we now have species-level information on species co-occurrences in small plots, habitat requirements, phylogeny, life-history traits and long-term trends. Using the Dutch Vegetation Database, we quantified niche volumes of Dutch plant species and used sister taxon comparisons to compare specialist and generalist sister taxa for the relative numbers of descendants across two temporal scales: ecological and macroevolutionary. Results: We show, first, that specialist species are more likely to be currently declining, i.e. to leave only few descendant populations. Second, most specialist clades left fewer descendant species within a region than their generalist sister clades. These results held after accounting for species life histories. Differences between specialist and generalist sister clades increased with clade age, suggesting that they reflect differences in rates at which specialists left descendants (rather than differences in ecological limits to the numbers of specialists and generalists). Conclusion: We conclude that specialists left only few descendants within a region (i.e. the Netherlands), both at ecological and macroevolutionary scales. While specialists may leave numerous evolutionary descendants at a global scale, these might be absent from most regions. Humans, by threatening specialist species, may hence further accelerate biotic homogenization with descendants of generalist lineages proliferating within regions while specialist lineages disappear.