ABSTRACT
Abiotic environmental variables strongly affect the outcomes of species interactions. For example, mutualistic interactions between species are often stronger when resources are limited. The effect might be indirect: water stress on plants can lead to carbon stress, which could alter carbon-mediated plant mutualisms. In mutualistic ant-plant symbioses, plants host ant colonies that defend them against herbivores. Here we show that the partners' investments in a widespread ant-plant symbiosis increase with water stress across 26 sites along a Mesoamerican precipitation gradient. At lower precipitation levels, Cordia alliodora trees invest more carbon in Azteca ants via phloem-feeding scale insects that provide the ants with sugars, and the ants provide better defense of the carbon-producing leaves. Under water stress, the trees have smaller carbon pools. A model of the carbon trade-offs for the mutualistic partners shows that the observed strategies can arise from the carbon costs of rare but extreme events of herbivory in the rainy season. Thus, water limitation, together with the risk of herbivory, increases the strength of a carbon-based mutualism.
Subject(s)
Ants/physiology , Cordia/physiology , Dehydration , Hemiptera/physiology , Animals , Carbohydrate Metabolism , Cordia/parasitology , Costa Rica , Herbivory , Host-Parasite Interactions , Mexico , Models, Biological , Nicaragua , Rain , SymbiosisABSTRACT
Theory suggests that spatial structuring should select for intermediate levels of virulence in parasites, but empirical tests are rare and have never been conducted with castration (sterilizing) parasites. To test this theory in a natural landscape, we construct a spatially explicit model of the symbiosis between the ant-plant Cordia nodosa and its two, protecting ant symbionts, Allomerus and Azteca. Allomerus is also a castration parasite, preventing fruiting to increase colony fecundity. Limiting the dispersal of Allomerus and host plant selects for intermediate castration virulence. Increasing the frequency of the mutualist, Azteca, selects for higher castration virulence in Allomerus, because seeds from Azteca-inhabited plants are a public good that Allomerus exploits. These results are consistent with field observations and, to our knowledge, provide the first empirical evidence supporting the hypothesis that spatial structure can reduce castration virulence and the first such evidence in a natural landscape for either mortality or castration virulence.
Subject(s)
Ants/physiology , Biological Evolution , Cordia/parasitology , Host-Parasite Interactions , Models, Biological , Symbiosis , Animals , Cordia/growth & development , Ecosystem , Female , Flowers/growth & development , Fruit/growth & development , Geography , Peru , Population Density , VirulenceABSTRACT
Cordia nodosa Lamark (Boraginaceae) is a myrmecophyte (i.e., plants housing ants in hollow structures) that provisions associated ants with food bodies (FBs) produced 24 h a day. Distributed over all the young parts of the plants, they induce ants to forage continually and so to protect the plants. Metabolites are stored in the inner cells of C. nodosa FBs as they form. In addition the peripheral cells have an extrafloral nectary-like function and secrete a substance that covers the FBs. The amalgam of these two functions, distinct in other known cases, is discussed taking into account the origin of FBs and extrafloral nectaries.
