Climate and capture mechanism in pitcher plants

Nepenthes (Nepenthaceae, approx. 120 species) are carnivorous pitcher plants with a centre of diversity comprising the Philippines, Borneo, Sumatra and Sulawesi. They rely on captured prey to augment nutrition from nutrient-poor substrates. Nepenthes pitchers use three main mechanisms for capturing prey: epicuticular waxes inside the pitcher; a wettable peristome (a collar-shaped structure around the opening); and viscoelastic fluid. It has been suggested that the Nepenthaceae provide an example of adaptive radiation, based on pitcher specialization for nutrient capture.

Previous studies have provided evidence suggesting that the first mechanism may be more suited to seasonal climates, whereas the latter two might be more suited to perhumid (very wet) environments. A recent paper in Annals of Botany tests this idea using climate envelope modelling. A total of 94 species, comprising 1978 populations, were grouped by prey capture mechanism (large peristome, small peristome, waxy, waxless, viscoelastic, non-viscoelastic, ‘wet’ syndrome and ‘dry’ syndrome). Nineteen bioclimatic variables were used to model habitat suitability.

Prey capture groups putatively associated with perhumid conditions (large peristome, waxless, viscoelastic and ‘wet’ syndrome) had more restricted areas of probable habitat suitability than those associated putatively with less humid conditions (small peristome, waxy, non-viscoelastic and ‘dry’ syndrome). Overall, the viscoelastic group showed the most restricted area of modelled suitable habitat.

This is the first study to demonstrate that the prey capture mechanism in a carnivorous plant is constrained by climate. Nepenthes species employing peristome-based and viscoelastic fluid-based capture are largely restricted to perhumid regions; in contrast, a wax-based mechanism allows successful capture in both perhumid and more seasonal areas. With a geographical range extending from Madagascar to New Caledonia, Nepenthaceae is a successful and diverse carnivorous family. Nepenthes can be found from sea level to >3000 m elevation, on a variety of substrates. They occur in both perhumid and seasonal tropical environments, and the range of pitcher morphologies is striking, even to the casual observer. This diversity of pitcher morphology is mirrored by the range of nitrogen sources exploited, including bat excreta, leaf litter and arthropods. Yet, despite this wide range of pitcher structure and function, this paper demonstrates a consistent, large-scale pattern defining the relationship between bioclimatic variables and prey capture mechanism.