Abstract
Mounting evidence indicates that we are witnessing the sixth mass extinction period. Given the important goods and services biodiversity delivers to humans, there is a need for a continued commitment to investigate what pre-disposes some taxa to greater risk of extinction. Here, we investigate this question using a phylogenetic comparative method and fitting a cumulative link mixed effect model on biological, ecological and evolutionary data of cycads, the most threatened lineage in the plant kingdom. We identified nine groups of threats to cycads, with habitat loss, over-collection, fire and reproduction failure being the most prominent, but only four of these threats (habitat loss, over-collection, medicinal uses and reproduction failure) clustered on the cycad tree of life. This clustering suggests that closely related species may be exposed to similar threats, perhaps because of geographic regionalization of cycad genera. Nonetheless, the diversity of threats and several variables linked to the biology and ecology of cycads correlate with extinction risk (e.g. altitude, height, diameter, geographic range), and different variables seem to be linked to different IUCN status of cycads. Although their predictive power is generally < 50 %, geographic range and maximum diameter stood out as the best predictors particularly for the Vulnerable (VU) category, with a predictive power of 87 % and 69 %, respectively. Using our best model for VU, we predicted all five Data Deficient (DD) species of cycads to be in the VU category. Collectively, our results elucidate the pattern of extinction risk in cycads and, since most threats that we identified as drivers of extinction risk of cycads are anthropogenically mediated, we recommend stronger legislation to regulate human-cycad interactions and the commitment of all governments globally to implement this regulation.
Highlights
A better understanding of the drivers of extinction risk is necessary to inform conservation decisions, and predicting future risk could be informed of the historical extinction events
The diversity of threats and several variables linked to the biology and ecology of cycads correlate with extinction risk, and different variables seem to be linked to different IUCN status of cycads
At IUCN category level, we found that all models yielded their best prediction for VU category, we found that different variables are good predictors of different IUCN categories
Summary
A better understanding of the drivers of extinction risk is necessary to inform conservation decisions, and predicting future risk could be informed of the historical extinction events. We are comparatively well-informed of the pre-disposition of vertebrates and, to a lesser extent, angiosperms, to extinction risk as well as how their phylogenetic trees would be affected by species loss (Davies et al 2011; Mooers et al 2012; Davies and Yessoufou 2013). Such knowledge is yet to be well-demonstrated for gymnosperms, the latter group is more threatened than angiosperm Such knowledge is yet to be well-demonstrated for gymnosperms, the latter group is more threatened than angiosperm (e.g. $70 % of cycads are threatened, IUCN 2010; Yessoufou et al 2017)
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