Oecologia (2002) 132:369–373 DOI 10.1007/s00442-002-0998-1 PLANT ANIMAL INTERACTIONS B. Anderson · J. J. Midgley It takes two to tango but three is a tangle: mutualists and cheaters on the carnivorous plant Roridula Received: 5 September 2001 / Accepted: 10 June 2002 / Published online: 3 July 2002 © Springer-Verlag 2002 Abstract Roridula dentata is associated with hemipter- wards (Soberon and Martinez del Rio 1995). The result ans, which facilitate nitrogen assimmilation from insects. may be mutualism collapse, which may trigger the ex- R. dentata is also associated with spiders and their role tinction of obligate partners (Boucher et al. 1982). This in digestion is unknown. We quantify approximately fluctuation of reward quality (e.g. nectar availability) has how much nitrogen Roridula assimilates from insects been identified as one of the major factors constraining through “indirect digestion.” Using δ15N we then deter- the evolution of specialist or one-on-one, obligate mutu- mine whether nitrogen absorption from hemipteran in- alisms (Thompson 1994). Howe (1984) suggests that di- sects differs with varying spider densities. In this way, verse faunas frequently affect reward quality negatively we are able to determine their nutritional role. At low by competing with mutualists. As a result, diverse faunas spider densities, indirect digestion of prey accounts for are sometimes viewed as the source of unpredictable re- approximately 70% of plant nitrogen. These values are ward quality. comparable to methods of direct prey digestion found in Carnivorous plants have a plethora of closely associ- other carnivorous plants. However spiders decrease the ated invertebrate fauna with both facultative and obli- numbers of hemipteran individuals inhabiting Roridula gate relationships that range from parasitisms to mutual- plants and also decrease efficiency of indirect prey di- isms (Beaver 1983, 1985; Juniper et al. 1989). Because gestion by up to 30%. We deduce that spiders are cheat- of their diversity, these systems may be excellent for ers as they exploit plant rewards without offering any re- exploring the evolution and breakdown of mutualisms wards in return. However, indirect carnivory is still effi- and the role of cheating. However, very few studies cient enough when hemipteran densities are at their low- have examined carnivorous plant relationships in detail: est, ensuring that the mutualism does not break down. Zamora (1990) observed that ants, which are kleptopar- asites of Pinguicula, steal a large proportion of prey. Keywords Mutualism · Evolution · Cheating · Context Clarke and Kitching (1995) noted that Nepenthes pitch- dependency · Carnivorous plant ers may capture an excess of prey, which often causes putrification of the pitcher if not removed. But specialist ants living on the Nepenthes plants remove excess prey, Introduction a process that reduces the incidence of putrification by up to 12-fold. Bradshaw and Creelman (1984) showed Thieves or cheaters are an inevitable part of almost every that pitcher fauna may increase the rate of digestion in mutualistic system (Springer and Smith-Vaniz 1972; Jan- Sarracenia purpurea and thus could be considered mu- zen 1975; Thompson 1982). The invasion of mutualisms tualists. by cheaters may cause variation in the reward quality of The plant Roridula gorgonias Planch. catches insects mutualisms (e.g. Pellmyr et al. 1996). For example, using sticky traps but has no digestive enzymes to utilise cheaters such as nectar thieves may leave little rewards the prey (Marloth 1925). Using an isotopic labelling for mutualistic pollinators. As a result, these pollinators technique, Ellis and Midgley (1996) determined that R. may be forced to seek flowers with more reliable re- gorgonias has a digestive mutualism with a specialist he- mipteran (Dolling and Palmer 1991) that consumes the captured prey. These hemipterans defecate on Roridulas’ B. Anderson (✉) · J.J. Midgley leaves and the plants absorb nitrogen directly through Botany Department, University Of Cape Town, P Bag Rondebosch, 7701 South Africa their cuticle (Ellis and Midgley 1996). However, the e-mail: [email protected] amount of nitrogen gained in this manner remains to be Fax: +27-21-6504041 quantified. 370 Using isotopic methods, we aim to examine and quan- rhizal (VAM) association, reference plants were chosen that also tify the effects of two different relationships with had this association (based on Allsopp and Stock 1993). Pamer- idea (n=5 from each population) and live prey items were also col- Roridula. We examine an unstudied species of Roridula lected from each population (prey items were pooled by grinding (R. dentata L.) with a unique species of associated he- all insects together). Prey consisted of four Hymenoptera, three mipteran (Pameridea marlothi Poppius). A monophylet- Coleoptera, three Diptera and one Hemipteran. At these sites we ic group of hemipterans (one genus, two species) is asso- felt it unnecessary to control for rooting depth as R. dentata has a shallower rooting depth than all the other plants sampled. Since ciated with every known Roridula population and each δ15N typically increases with rooting depth (Gebauer and Schulze of the two Roridula species is associated with a different 1991; Hogberg 1997; Schmidt and Stewart 1997) any differences species of Pameridea (Dolling and Palmer 1991). In ad- between Roridula and reference plant δ15N values will thus be dition, a specialist spider (Synaema marlothi Dahl., conservative. Thomsidae) is also associated exclusively with R. denta- In a separate experiment, two more sites were used to compare levels of insect-derived nitrogen in R. dentata with and without ta. These spiders occur on most R. dentata populations spiders. These sites are geographically distant from Pop6, Pop7 in the southern and central part of their range. Where and Pop8. The first population (Pop5) is a small R. dentata popu- Synaema occurs, it is usually very common and builds its lation with no spider inhabitants and is in the Tulbagh region nests in the axils of Roridula’s branches and dead leaves (33°21′ S, 19°05′ E). The closest known R. dentata population to this is situated approximately 17 km away in the Ceres area (Marloth 1925; personal observation). The spiders are (33°22′ S, 19°15′ E). This site (Pop9) is dry with sandy soil and carnivores and presumably compete with hemipterans has several plant species (e.g. Leucadendron rubrum Burm.f., and for resources (Marloth 1925). In addition it has been re- Protea nana (P.J. Bergius) Thunb., both Proteaceae) in common corded that Synaema also consumes Pameridea (Lloyd with Pop5. In contrast to Pop5, the spider Synaema marlothi is abundant at Pop9. Leaves were bagged before sampling as above. 1934; personal observation). Spiders may thus disrupt At the time of sampling, five Roridula and five P. nana (reference the mutualism between Roridula and Pameridea if they plants) were sampled per population. decrease the numbers of hemipterans but do not supply plants with nitrogen. Alternatively spiders may also fa- cilitate nitrogen absorption in a similar way to Pamer- Mass spectrometer methods idea. Until now, no studies have been performed on the The relative contribution (%NCI) by insects to the nitrogen content spiders and their role in Roridula nutrition. Spiders are of a carnivorous plant was calculated by Schulze et al. (1991) us- also very commonly associated with other carnivorous ing methods modified from Peterson and Fry (1987). Where δ15 δ15 δ15 plants but their roles in other systems also remain un- NCAR is the N value for the carnivorous plant, NI is the δ15 δ15 δ15 studied. In this study, we quantify the nutritional benefits N value for insect prey and NREF is the N value for refer- ence plants (see Eq.1). of the hemipteran mutualist and determine the nutritional role played by spiders. (1) The equation used to determine the percentage of nitrogen that Roridula gains from hemipteran faeces is derived from Schulze et Materials and methods al. (1991) (see Eq. 2). The densities of hemipterans and spiders may be related to nitro- (2) gen uptake in plants; therefore, these were the first data collected. δ15 In addition to the five sites mentioned below, density data were Two methods were used to calculate NFAECES. The first is an in- collected from another four sites. These sites were Pop1 (32°07′ S, direct method while the second is direct. 19°00′ E), Pop2 (32°40′ S, 19°11′ E), Pop3 (32°50′ S, 19°12′ E) and Pop4 (32°49′ S, 19°13′ E). Synaema marlothi occurred at all except three study sites (Pop1, Pop8, Pop3) used in this study. At Method 1 every R. dentata population, 30 rosettes (approximately 20 leaves each) from random plants were rapidly bagged and the number of Since insects discriminate against 14N during excretion (Peterson adult hemipterans and spiders were counted. This enabled us to and Fry 1987) and retain 15N, their faeces will be depleted of 15N, observe the densities of hemipterans and spiders relative to each relative to their food source. The value of δ15N retained by an in- other. It also enabled us to relate levels of insect-derived nitrogen sect is: in Roridula to hemipteran or spider density. Three geographically close sites (Pop6: 32°39′ S, 19°11′ E, (3) Pop7: 32°40′ S, 19°11′ E, and Pop9: 32°40′ S, 19°11′ E) were used to investigate the nutritional impacts of hemipterans and the The δ15N value for faeces will be δ15N of the prey minus the δ15N role played by spiders. These sites were chosen because they retained by the insects: shared very similar vegetation, are geographically close and dif- fered in the density of spiders and Pameridea. Young leaf samples (4) (n=5 per species) were collected from Roridula and reference plants at each site for isotopic analysis. Approximately 1 week be- fore leaf harvesting, all leaves were bagged in transparent nylon Method 2 bags while still in bud so as to exclude prey items and Pameridea.