Association With Ants and Pitcher Infauna
Nepenthes bicalcarata plays host to an unusual species of ant that makes its nest in the plant's hollow tendrils. Described as Camponotus schmitzi in 1933, it is a member of the extremely populous and widespread genus of carpenter ants.
This unique animal–plant interaction was noted by Frederick William Burbidge as early as 1880. In 1904, Odoardo Beccari suggested that the ants feed on insects found on and around the plant, but may fall prey to it themselves. In 1990, B. Hölldobler and E.O. Wilson proposed that N. bicalcarata and C. schmitzi form a mutually beneficial association. At the time, however, no experimental data existed to support such a hypothesis. A series of observations and experiments carried out in Brunei by Charles Clarke (published in 1992 and 1998), and by Clarke and Roger Kitching (1993 and 1995), strongly support the mutualism theory.
Nepenthes bicalcarata is a myrmecotroph (ant-fed plant), obtaining nutrients from C. schmitzi in the form of egesta and, occasionally, ant remains. It has been estimated that this input accounts for 42% of the plant's total foliar nitrogen (76% in plants with ant occupancy rates above 75%). Camponotus schmitzi has also been observed to attack newly caught insects and therefore prevent prey escape. At other times, the ants are very passive, remaining hidden under the inner peristome fold, presumably so as not to dissuade visitation by potential prey species. This behaviour is in stark contrast to other myrmecophytic ants, which are typically highly territorial. John Thompson has suggested that N. bicalcarata may be the only plant species that obtains nutrients through both insect capture and ant-hosting habits.
Camponotus schmitzi is able to swim in the pitcher fluid using tripod-like leg coordination similar to that of terrestrial locomotion and can remain submerged for up to 30 seconds. When feeding, it appears to target large prey items only, cooperatively retrieving them from the fluid. Hauling food from the pitcher fluid to the peristome—a distance of no more than 5 cm—may take up to 12 hours. In this way the contents of N. bicalcarata pitchers is controlled such that organic matter does not accumulate to the point of putrefaction, which could lead to the demise of pitcher infauna (which also appear to benefit the plant) and sometimes the pitcher itself.
The ants have been observed to clean the peristome of fungal hyphae and other contaminants, thereby maintaining high trapping efficiency over the pitcher's lifespan. Research conducted by Dennis and Marlis Merbach has shown that C. schmitzi also benefits N. bicalcarata by protecting it from pitcher-destroying weevils of the genus Alcidodes. In order to create a favourable environment for its pitcher inhabitants, it appears that N. bicalcarata actively maintains the pH of its pitcher fluid at a less acidic level than that found in most other Nepenthes species (this might explain the occasional presence of tree frog eggs in its pitchers). In doing so, however, the plant reduces its ability to digest and assimilate nutrients from captured prey. The pitcher fluid of N. bicalcarata is also less viscoelastic than that of most Nepenthes species, and appears to lack functional digestive enzymes. Nepenthes bicalcarata is therefore highly reliant on its ant symbiont. Indeed, plants not inhabited by C. schmitzi do not appear to benefit significantly from carnivory, with any gains from prey digestion being offset by the high costs of pitcher construction. Conversely, ant-inhabited plants have more leaves and a greater total leaf area, and ant presence is associated with lower pitcher abortion rates and more voluminous pitchers (and consequently greater prey biomass).
Camponotus schmitzi nests solely in the tendrils of N. bicalcarata and rarely ventures onto other plants. The species is completely dependent on N. bicalcarata for food and domicile. Nepenthes bicalcarata, on the other hand, is able to survive and reproduce without the presence of the ants; it is a facultative mutualist. This being the case, there appear to be few mature plants over 2 metres in height not colonised by C. schmitzi. The ants seem to favour upper pitchers and rarely colonise lower pitchers. This is likely due to the fact that terrestrial traps are periodically submerged in water during heavy rains. Flooding of the ants' nest chamber could result in the death of the developing eggs, larvae, and pupae.
A species of mite, Naiadacarus nepenthicola, appears to be restricted to the pitchers of N. bicalcarata. It is thought to feed on decomposing leaves and insects that are caught in the pitchers. Deutonymphs of this mite are dispersed through phoresy on C. schmitzi.
Left centre: Effect of C. schmitzi occupancy on leaf apex abortion and pitcher production rates. In the first chart (A), cases where the tendril was found to be cut are grouped under unknown fate (denoted with a question mark) and "pitcher" encompasses both living and dead traps (in non-pitchering plants, the latter).
Right centre: Prey biomass accumulated over a pitcher's entire lifespan as a function of pitcher volume in ant-occupied and unoccupied lower pitchers.
Right: A: Isotopic signatures (δ15N) of C. schmitzi, ant-occupied plants (PA), and unoccupied plants with no evidence of previous colonisation (PnoA-no hole). B: Relationship between C. schmitzi occupation rate and plant foliar δ15N.
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