Evaluating alternative explanations for an association of extinction risk and evolutionary uniqueness in multiple insular lineages

Abstract

Studies in insular environments have often documented a positive association of extinction risk and evolutionary uniqueness (i.e., how distant a species is from its closest living relative). However, the cause of this association is unclear. One explanation is that species threatened with extinction are evolutionarily unique because they are old, implying that extinction risk increases with time since speciation (age-dependent extinction). An alternative explanation is that such threatened species are last survivors of clades that have undergone an elevated extinction rate, and that their uniqueness results from the extinction of their close relatives. Distinguishing between these explanations is difficult but important, since they imply different biological processes determining extinction patterns. Here, we designed a simulation approach to distinguish between these alternatives using living species, and applied it to 12 insular radiations that show a positive association between extinction risk and evolutionary uniqueness. We also tested the sensitivity of results to underlying assumptions and variable extinction rates. Despite differences among the radiations considered, age-dependent extinction was supported as best explaining the majority of the empirical cases. Biological processes driving characteristic changes in abundance with species duration (age-dependency) may merit further investigation.

Reconciling supertramps, great speciators and relict species with the taxon cycle stages of a large island radiation (Aves: Campephagidae)

“…Species ages on the x ‐axes in Figures , and thereby illustrate relative timelines of species evolution, but do not imply causal relationships. The correlates of species age thus form parts of a predictable ‘taxon senescence’ (Žliobaitė et al., ) or ‘taxon ageing’ (Warren et al., ) syndrome. Ricklefs and Cox () speculated that dynamic co‐evolutionary interactions between pathogens and their hosts could be a potential underlying frequency‐dependent driver for population expansions and contractions through ecological release and the apparent competition in species abilities to escape pathogens, thereby reflecting a Janzen–Connell effect (Connell, ; Janzen, ), but at large evolutionary (temporal) and biogeographical (spatial) scales (Ricklefs, ).…”

“…We define ‘species age’ as the time since the most recent divergence of extant species, which is a proxy for evolutionary persistence and distinctiveness of a taxon, but can be biased by taxonomic delimitations and incomplete phylogenies (e.g. Warren et al., ). We used the phylogenetic generalized least‐squares regression method in the R package ‘caper’ (Orme et al., ) to account for phylogenetic non‐independence between species.…”

gen3sis: the general engine for eco-evolutionary simulations on the origins of biodiversity

“…Further exploration possibilities may include: (iv) revealing the mechanisms behind agedependent speciation and extinction patterns [102,108,156]; (v) contrasts between terrestrial and aquatic ecosystems [16]; and (vi) calculations of uncertainty resulting from climatic and geological dynamics [e.g. 24,25,26,41,42].…”

“…Besides the LDG, frontiers on the origins of biodiversity involve [16]: (i) quantifying speciation, extinction and dispersal events [114]; (ii) exploring adaptive niche evolution [26, 44]; and (iii) investigating multiple diversity-dependence and carrying capacity mechanisms [21, 111, 112]. Further exploration possibilities may include: (iv) revealing the mechanisms behind age-dependent speciation and extinction patterns [102, 108, 156]; (v) contrasts between terrestrial and aquatic ecosystems [16]; and (vi) calculations of uncertainty resulting from climatic and geological dynamics [e.g. 24, 25, 26, 41, 42].…”

On islands, evolutionary but not functional originality is rare

“…Third, rare and original insular species may also be highly vulnerable to external threats such as climate or land-use changes because they have low dispersal abilities and no land at proximity to escape . Finally, evolutionary original species may be at higher extinction risks due to increased extinction risks with age (Warren et al, 2018). The most functionally original species were not rarer (geographic rarity) than other species, but they were distributed in a low number of islands.…”