Fighting their last stand? A global analysis of the distribution and conservation status of gymnosperms

Aim: Gymnosperms are often described as a marginal and threatened group, members of which tend to be out-competed by angiosperms and which therefore preferentially persist at higher latitudes and elevations. The aim of our synthesis was to test these statements by investigating the global latitudinal and elevational distribution of gymnosperms, as well as their conservation status, using all extant gymnosperm groups (cycads, gnetophytes, ginkgophytes and conifers).Location: Worldwide.Methods: We developed a database of 1014 species of gymnosperms containing latitudinal and elevational distribution data, as well as their global conservation status, as described in the literature. The 1014 species comprised 305 cycads, 101 gnetophytes, the only living representative of ginkgophytes, and 607 conifers. Generalized additive models, frequency histograms, kernel density estimations and distribution maps based on Takhtajan's floristic regions were used.Results: Although the diversity of gymnosperms decreases at equatorial latitudes, approximately 50% of the extant species occur primarily between the tropics. More than 43% of gymnosperms can occur at very low elevations (≤ 200 m a.s.l.). Gymnosperms, considering all species together as well as their main taxonomic groups separately, do not exhibit a latitudinal diversity gradient as commonly observed for many other taxa. Gymnosperms, and especially conifers, are on average less threatened at higher and equatorial latitudes.Main conclusions: Gymnosperms display an unusual latitudinal diversity gradient, which we suggest cannot fully be accounted for by angiosperm dominance and competitive superiority. We hypothesize that other factors explain their present distribution, such as the development of centres of endemism in several regions and the adaptation of certain taxa to cold and arid climates

Impacts of human-induced environmental disturbances on hybridization between two ecologically differentiated Californian oak species

Natural hybridization, which can be involved in local adaptation and in speciation processes, has been linked to different sources of anthropogenic disturbance. Here, we use genotypic data to study range-wide patterns of genetic admixture between the serpentine-soil specialist leather oak (Quercus durata) and the widespread Californian scrub oak (Quercus berberidifolia). First, we estimated hybridization rates and the direction of gene flow. Second, we tested the hypothesis that genetic admixture increases with different sources of environmental disturbance, namely anthropogenic destruction of natural habitats and wildfire frequency estimated from long-term records of fire occurrence. Our analyses indicate considerable rates of hybridization (> 25%), asymmetric gene flow from Q. durata into Q. berberidifolia, and a higher occurrence of hybrids in areas where both species live in close parapatry. In accordance with the environmental disturbance hypothesis, we found that genetic admixture increases with wildfire frequency, but we did not find a significant effect of other sources of human-induced habitat alteration (urbanization, land clearing for agriculture) or a suite of ecological factors (climate, elevation, soil type). Our findings highlight that wildfires constitute an important source of environmental disturbance, promoting hybridization between two ecologically well-differentiated native species.Peer reviewe