Declining relict plants: Climate effect or seed dispersal disruption? A landscape-scale approach

Biodiversity loss, including the decrease of diversity of ecological interactions, is known to reduce the capacity of ecosystems to cope with the effects of global change. Here we assessed whether the distribution and survival of two declining relict plant species, Daphne rodriguezii and Cneorum tricoccon, were affected by the mutualism disruption with their only seed-disperser and by climatic and habitat conditions. Due to the lack of data on demographic rates, we used an indirect approach to test these hypotheses. We used presence–absence data as response variables and took two distinct sets of predictors (i.e. habitat and topoclimatic conditions), which were hypothesized to be the main determinants of the demographic rates under question. With these two datasets we fitted species distribution models by means of MaxEnt. Such models were later used to build Combined Species Distributions Models (CM). For each plant species, these CM allowed evaluating the role of both climatic and non-climatic factors, such as the mutualism disruption. Results showed that both climate and habitat conditions determined the current distribution of the two species at a landscape scale. Additionally, the mutualism disruption between C. tricoccon and its native seed-disperser affected plant distribution, moving it to areas where a new alien disperser was present. This alien disperser modified the pattern of habitat selection by plants in terms of habitat quality, reducing their presence in suboptimal areas, which might be the determinant for their survival. Our findings highlight the need of a better understanding of the role of mutualisms within natural communities in order to undertake appropriate conservation actions on threatened plant populations. Furthermore, acting on key factors affecting plant–disperser disruptions (e.g. controlling invasive species) might help to mitigate the effects of global change on declining relict plants.