It takes two to tango but three is a tangle: mutualists and cheaters on the carnivorous plant Roridula

Abstract.

Roridula dentata is associated with hemipterans, which facilitate nitrogen assimmilation from insects. R. dentata is also associated with spiders and their role in digestion is unknown. We quantify approximately how much nitrogen Roridula assimilates from insects through "indirect digestion." Using δ¹⁵N we then determine whether nitrogen absorption from hemipteran insects differs with varying spider densities. In this way, we are able to determine their nutritional role. At low spider densities, indirect digestion of prey accounts for approximately 70% of plant nitrogen. These values are comparable to methods of direct prey digestion found in other carnivorous plants. However spiders decrease the numbers of hemipteran individuals inhabiting Roridula plants and also decrease efficiency of indirect prey digestion by up to 30%. We deduce that spiders are cheaters as they exploit plant rewards without offering any rewards in return. However, indirect carnivory is still efficient enough when hemipteran densities are at their lowest, ensuring that the mutualism does not break down.