Different pitcher shapes and trapping syndromes explain resource partitioning in Nepenthes species

Abstract

Nepenthes pitcher plants display interspecific diversity in pitcher form and diets. This species‐rich genus might be a conspicuous candidate for an adaptive radiation. However, the pitcher traits of different species have never been quantified in a comparative study, nor have their possible adaptations to the resources they exploit been tested. In this study, we compare the pitcher features and prey composition of the seven Nepenthes taxa that grow in the heath forest of Brunei (Borneo) and investigate whether these species display different trapping syndromes that target different prey. The Nepenthes species are shown to display species‐specific combinations of pitcher shapes, volumes, rewards, attraction and capture traits, and different degrees of ontogenetic pitcher dimorphism. The prey spectra also differ among plant species and between ontogenetic morphotypes in their combinations of ants, flying insects, termites, and noninsect guilds. According to a discriminant analysis, the Nepenthes species collected at the same site differ significantly in prey abundance and composition at the level of order, showing niche segregation but with varying degrees of niche overlap according to pairwise species comparisons. Weakly carnivorous species are first characterized by an absence of attractive traits. Generalist carnivorous species have a sweet odor, a wide pitcher aperture, and an acidic pitcher fluid. Guild specializations are explained by different combinations of morpho‐functional traits. Ant captures increase with extrafloral nectar, fluid acidity, and slippery waxy walls. Termite captures increase with narrowness of pitchers, presence of a rim of edible trichomes, and symbiotic association with ants. The abundance of flying insects is primarily correlated with pitcher conicity, pitcher aperture diameter, and odor presence. Such species‐specific syndromes favoring resource partitioning may result from local character displacement by competition and/or previous adaptations to geographically distinct environments.

Ecological outsourcing: a pitcher plant benefits from transferring pre‐digestion of prey to a bat mutualist

“…Some empirical evidence to support this hypothesis exists for other Nepenthes species (Chin, Chung & Clarke ; Gaume et al . ), but N. hemsleyana and N. rafflesiana do not commonly co‐occur in mixed populations. In sites where levels of anthropogenic disturbance are minor or absent, N. hemsleyana tends to occur in closed forests, whereas N. rafflesiana is generally found in open sites (Clarke ).…”

“…By outsourcing its prey capture mechanism to bats, N. hemsleyana may avoid indirect competition, for example with the closely related N. rafflesiana for the same arthropod prey. Some empirical evidence to support this hypothesis exists for other Nepenthes species (Chin, Chung & Clarke 2014;Gaume et al 2016), but N. hemsleyana and N. rafflesiana do not commonly co-occur in mixed populations. In sites where levels of anthropogenic disturbance are minor or absent, N. hemsleyana tends to occur in closed forests, whereas N. rafflesiana is generally found in open sites (Clarke 2006).…”

A classification of functional pitcher types in Nepenthes (Nepenthaceae)

“…It is as though in the funnelshaped pitchers the development of the upper cylindrical waxy zone is "switched off", the ovoid basal non-waxy detentive zone instead expanding to fill the gap, and directly support the peristome instead. Gaume et al (2016) report that in N. rafflesiana not only does the shape and waxiness of the pitcher switch from lower to upper pitcher, but that lower pitchers have non-viscid fluid, while upper pitchers have viscid fluid. In addition, the upper pitchers (type 2) of this species capture more flying insects than do their type 1 lower pitchers.…”

“…However, the number of prey individuals is dominated by ants: >80 % of prey in both upper and lower pitchers. More recent analysis of trapping syndromes in six species of Nepenthes in Brunei concluded that cylindrical pitchers with waxy walls (type 1) are particularly efficient at trapping and retaining ants, compared with other syndromes (Gaume et al 2016). Earlier, Bonhomme et al (2011) concluded that "wax only appears to be efficient for ants".…”

“…obs 2019). Gaume et al (2016) record viscid fluid in N.hemsleyana Macfarl. (which we confirmed) which is also waxy, and we confirmed this from observations of live plants cultivated at Kew.…”