Valentí Rull

The concept of taxon cycle involves successive range expansions and contractions over time through which a species can maintain indefinitely its core distribution. Otherwise, it becomes extinct. A typical taxon cycle can be subdivided into four stages: (I) expansion, (II) population differentiation, (III) local extinction and incipient speciation (eventually initiating a new cycle), and (IV) relictualization and extinction. Taxon cycles have been defined mostly for tropical island faunas. Examples from continental areas are scare and similar case studies for plants remain unknown. Most taxon cycles have been identified on the basis of phylogenetic and phylogeographic studies, straightforward empirical evidence of expansion and contraction phases based on the fossil record are still lacking. Here empirical fossil evidence is provided for recurrent Eocene to present expansion and contraction cycles in a mangrove taxon (Pelliciera), after a Neotropical-wide study of the available pollen records (~80 sites). This recurrent behavior is compatible with the concept of taxon cycle from biogeographical, chronological and ecological perspectives. Evolutionary predictions are more difficult to evaluate with the available evidence. The biotic (dispersal, competition, facilitation, niche segregation) and abiotic (environmental shifts) drivers potentially involved in the initiation and maintenance of the Pelliciera expansion/contraction cycles are analyzed, and the ecological and evolutionary implications for Neotropical mangroves are discussed. Two main expansion/contraction Pelliciera cycles have been identified, the first during the Eocene and the second between the Oligocene and the present. It is suggested that the Eocene cycle would have led to the extinction of Pelliciera but the Late-Eocene arrival of a gerenalist long-distance disperser that dominates the extant Neotropical mangroves (Rhizophora) provided the conditions for its survival and further Oligo-Miocene expansion. After a drastic Mio-Pliocene contraction, Pelliciera is restricted today to a small relict patch similar, in extension and location, to its original Eocene range. Whether this could be a trend toward extinction is considered, under the predictions of the taxon cycle theory. The recurrent expansion and contraction cycles identified for Pelliciera from the Eocene to the present have strong potential for being the first empirically and unequivocally documented taxon cycles, and likely the only taxon cycles documented to date for plants.