Environmental stress influences Malesian Lamiaceae distributions

Abstract

Dual effects of spatial distance and environment shape archipelagic floras. In Malesia, there are multiple environmental stressors associated with increasing uplands, drought, and metal-rich ultramafic soils. Here, we examine the contrasting impacts of multifactorial environmental stress and spatial distance upon Lamiaceae species distributions. We used a phylogenetic generalized mixed effects model of species occurrence across Malesia's taxonomic database working group areas from Peninsular Malaysia to New Guinea. Predictor variables were environmental stress, spatial distance between areas and two trait principal component axes responsible for increasing fruit and leaf size and a negative correlation between flower size and plant height. We found that Lamiaceae species with smaller fruits and leaves are more likely to tolerate environmental stress and become widely distributed across megadiverse Malesian islands. How global species distribution and diversification are shaped by multifactorial environmental stress requires further examination.

Keywords: Malesia; Wallacea; biogeography; dispersal; macroecology; mints; stress tolerance.

FIGURE 1

(a) The contributions of ultramafic soils, minimum monthly rainfall, and lowland area to environmental stress – which is a principal component axis that accounts for 53% variation in these variables. (b) Spatial eigenvector scores and environmental stress across taxonomic database working group (tdwg) areas of Malesia and their global position (inset map). (c) Drivers of Lamiaceae species occurrence across Malesia according to phylogenetic generalized mixed effects models. We ran 25 separate models each with a randomly selected number of species equal to the tdwg area of least species richness – each point represents effect score and standard error from each model. Gray and black bars are the mean effect. Bar colors correspond to mean p < .05 (black) and mean p > .05 (gray). (d) Predicted occurrences of species with varying leaf and fruit size across the environmental stress gradient of Malesian tdwg areas.

FIGURE 2

Phylogenetic context of species occurrence probabilities in Borneo and Sulawesi. Two of nine taxonomic working group regions selected to highlight large environmental stress differences between spatially adjacent islands. This also means the probabilities are better visualized than if all data from nine areas were plotted. Species occurrence probabilities are their mean value resulting from the 25 model iterations. Lineages highlighted in dark gray are most diverse in temperate and subtropical regions.