Molina-Venegas et al. (2017) mapped relative phylogenetic endemism (RPE) of 728 endemic vascular plants across 48 ecoregions of the Baetic–Rifan biodiversity hotspot (southern Iberian Peninsula and northern Morocco) and found centres of palaeoendemism clustered around the Strait of Gibraltar, positively correlated with present-day total annual precipitation, while neoendemism concentrated in the high eastern Baetic–Rifan mountains tracked topographical relief. Because only contemporary climatic variables were used, the study explicitly left open the extent to which the persistence of palaeoendemic lineages — and the formation of neoendemic species — is explained by climatic stability through geological time (palaeoclimatic trajectories and climate-change velocity spanning the Neogene and Quaternary) rather than by current precipitation patterns. A decade of macroecological work has since confirmed a general association between long-term climatic stability and palaeoendemism, but only with Quaternary, temperature-based stability metrics and at global or coarse regional grains; no analysis has quantified the stability-versus-contemporary-climate partition for this hotspot at its regional scale, and no precipitation-based stability metric or Neogene climate trajectory has been incorporated anywhere for the region. The problem is to determine, quantitatively and at the ecoregion scale of the Baetic–Rifan hotspot, the relative contribution of long-term climatic stability versus present-day precipitation to the spatial distribution of palaeo- and neoendemism.

Background

Biodiversity hotspots concentrate large numbers of species found nowhere else (endemics), and a central question of biogeography is whether such concentrations are evolutionarily young (neoendemism, produced by recent in situ speciation) or old (palaeoendemism, the long-term persistence of relict lineages). Phylogenetic endemism (PE) is a measure that combines geographic ranges with evolutionary-tree branch lengths: for a given area it sums the branch lengths of the phylogeny that are restricted to that area, so it is high where evolutionarily distinctive lineages have small ranges (Rosauer et al., 2009). Relative phylogenetic endemism (RPE) divides PE computed on the actual branch-length tree by PE computed on the same tree topology with all branches made equal in length; high RPE indicates that the range-restricted evolutionary history sits on long terminal branches (old lineages, palaeoendemism), whereas low RPE indicates concentration on short branches (young lineages, neoendemism) (Mishler et al., 2014). In practice RPE is used as a standardized effect size (SESRPE) against a null model. Climate-change velocity — the speed at which a species would need to shift its range to keep climatic conditions constant as climate changes through time — is the standard metric of how locally stable climate has been; low-velocity areas acted as climatic refugia where endemism accumulated globally (Sandel et al., 2011) and in the Iberian Peninsula specifically (Abellán and Svenning, 2014). The Baetic–Rifan hotspot (southern Iberian Peninsula and northern Morocco, the westernmost part of the Mediterranean Basin hotspot) has a complex Cenozoic history: rapid uplift of the Baetic–Rifan mountain ranges beginning roughly 8 million years ago, the Messinian salinity crisis (about 5.9–5.3 million years ago, when the Mediterranean nearly dried out), establishment of the seasonal Mediterranean precipitation rhythm at least since about 3.6 million years ago, and the Quaternary glacial–interglacial cycles. Molina-Venegas et al. (2017) compiled all 728 native vascular plant taxa endemic to the region (including 30 geographically isolated relict 'isolated lineages'), estimated PE and SESRPE across 48 eco-geographical regions, and regressed these against present-day precipitation and temperature variables and elevation range. Palaeoendemism clustered around the Strait of Gibraltar and correlated positively with total annual precipitation; neoendemism concentrated in the high, cold eastern ranges; endemic species richness, unlike RPE, tracked elevation range and mean annual temperature. The authors interpreted the wet, oceanically influenced Strait of Gibraltar area as a long-term refugium for Tertiary relict lineages, but emphasized that their analysis rested on contemporary variables, not historical ones. Palynological and palaeobotanical evidence indicates that the region's climatic gradient has been broadly similar since the Pliocene (Rodríguez-Sánchez and Arroyo, 2008), which makes present-day precipitation a plausible proxy for long-term conditions — but that proxy has never been tested against actual palaeoclimatic trajectories or stability metrics. The study therefore explicitly left open 'the extent to which the persistence of palaeoendemic lineages and/or the formation of neoendemic species can be explained in terms of climatic stability through geological time'.

Problem Statement

Determine the extent to which the persistence of palaeoendemic plant lineages — and the formation of neoendemic species — across the ecoregions of the Baetic–Rifan biodiversity hotspot is explained by long-term climatic stability through geological time, as compared with present-day precipitation patterns. Concretely, the task is to quantify, across the 48 ecoregions at their grain and over the Neogene and Quaternary, how much of the spatial variation in relative phylogenetic endemism (and endemic species richness) is attributable to long-term climatic stability versus contemporary precipitation, temperature and elevation range, in a hotspot whose climate is water-limited and whose documented palaeoendemism signal is tied to humid habitats. The palaeoclimate data, stability metrics and modelling approach that can resolve this partition remain open. The answer must establish whether long-term climatic stability carries explanatory power beyond present-day precipitation for the palaeoendemism concentrated around the Strait of Gibraltar (and correspondingly for the neoendemism of the eastern Baetic–Rifan ranges), whether it does not, or whether the two cannot be separated with available palaeoclimatic data — and, in the latter case, what palaeoclimate evidence would be required to decide the question.

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