Generalized mutualisms promote range expansion in both plant and ant partners

Mutualism improves organismal fitness, but strong dependence on another species can also limit a species' ability to thrive in a new range if its partner is absent. We assembled a large, global dataset on mutualistic traits and species ranges to investigate how multiple plant-animal and plant-microbe mutualisms affect the spread of legumes and ants to novel ranges. We found that generalized mutualisms increase the likelihood that a species establishes and thrives beyond its native range, whereas specialized mutualisms either do not affect or reduce non-native spread. This pattern held in both legumes and ants, indicating that specificity between mutualistic partners is a key determinant of ecological success in a new habitat. Our global analysis shows that mutualism plays an important, if often overlooked, role in plant and insect invasions.

Scaling of indirect defences in Central American swollen-thorn acacias

Myrmecophytes may adjust the investment on ant rewards, depending on tree size and ant defence level. In swollen-thorn acacias (Vachellia collinsii), we tested whether the level of protection provided by the resident ants (defending vs. non-defending) influenced the relation between tree size and ant rewards, or between types of ant rewards (housing and food). We quantified ant rewards in trees occupied by defending and by non-defending ants. We predicted: (1) a positive relation between plant diameter and ant reward investment, with a steeper slope for defending than for non-defending ant species; and (2) that if there is any tradeoff between ant rewards, it should be aggravated (steeper slope) when inhabited by non-defending ants. We found that most structures for ants grew according to plant diameter, but contrary to our first prediction it was independent of the level of ant defence. Most ant rewards did not show a tradeoff between them, besides a weak negative relation between spine length and number of pinnules, which contrary to the prediction occurred when occupied by defending ants. The evidence shows that the interacting ants had a weaker influence on the scaling of defence structures in myrmecophytes than the habitat (location).

FIRST RECORD OF Crematogaster carinata (FORMICIDAE) NESTING IN THE FRUITS OF Macropsychanthus (LEGUMINOSAE: PAPILIONOIDEAE)

Most of the ants that are associated with plants usually nests in vegetative organs such as stems, leaves o petioles. Aiming to contribute to the plant-insect interactions, we report the first record of Crematogaster carinata nesting inside the fruits of Macropsychanthus comosus. We did fieldwork in two localities of department of Antioquia and one of the Bolivar department, we opened 32 pods and determined the insects inside them. We found Crematogaster carinata and a beetle (Coleoptera: Chrysomelidae: Bruchinae) inside. 15 of the 32 pods had ants (47 %), but only seven of them had queens (22 %). The ants left the falling pods. The beetle larvae drilled 15 seeds (14 %). This report points the importance of the research in legume-insect interactions, ant-behavior, trophic webs, and trade-off relationships research.

Nutrient provisioning of its host myrmecophytic tree by a temporary social parasite of a plant-ant

One of the most advanced ant–plant mutualisms is represented by myrmecophytes sheltering colonies of some plant-ant species in hollow structures called domatia. In turn, the myrmecophytes benefit from biotic protection and sometimes nutrient provisioning (myrmecotrophy). Furthermore, over the course of evolution, some ant species have become social parasites of others. In this general context, we studied the relationship between its host trees and Azteca andreae (Dolichoderinae), a temporary social parasite of the plant-ant Azteca ovaticeps, and, as such, obligatorily associated with myrmecophytic Cecropia obtusa trees (Urticaceae). A first experiment showed that the δ15N values of the young leaves of Cecropia sheltering a mature A. andreae colony were very similar to those for trees sheltering Azteca alfari or A. ovaticeps, two typical Cecropia mutualists for which myrmecotrophy is known. In a second experiment, by injecting a 15N-labelled glycine solution into locusts given as prey to A. andreae colonies, we triggered an increase in δ15N in the young leaves of their host Cecropia. Thus, 15N passed from the prey to the host trees, explaining the outcomes of the first experiment. We discuss these results in light of the notion of ‘by-product benefits’.

Highly modular pattern in ant-plant interactions involving specialized and non-specialized myrmecophytes

Because Tachia guianensis (Gentianaceae) is a "non-specialized myrmecophyte" associated with 37 ant species, we aimed to determine if its presence alters the ant guild associated with sympatric "specialized myrmecophytes" (i.e., plants sheltering a few ant species in hollow structures). The study was conducted in a hilly zone of a neotropical rainforest where two specialized myrmecophytes grow at the bottom of the slopes, another at mid-slope, and a fourth on the hilltops. Tachia guianensis, which occurred everywhere, had its own guild of associated ant species. A network analysis showed that its connections with the four other myrmecophytes were rare and weak, the whole resulting in a highly modular pattern of interactions with one module (i.e., subnetwork) per myrmecophyte. Three ant species parasitized three out of the four specialized myrmecophytes (low nestedness noted), but were not or barely associated with T. guianensis that therefore did not influence the parasitism of specialized myrmecophytes.

Hollow Internodes Permit a Neotropical Understory Plant to Shelter Multiple Mutualistic Ant Species, Obtaining Protection and Nutrient Provisioning (Myrmecotrophy)

The Neotropical understory plant Tachia guianensis (Gentianaceae)-known to shelter the colonies of several ant species in its hollow trunks and branches-does not provide them with food rewards (e.g., extrafloral nectar). We tested whether these ants are opportunistic nesters or whether mutualistic relationships exist as for myrmecophytes or plants sheltering ant colonies in specialized hollow structures in exchange for protection from enemies and/or nutrient provisioning (myrmecotrophy). We noted 37 ant species sheltering inside T. guianensis internodes, three of them accounting for 43.5% of the cases. They protect their host plants from leaf-cutting ant defoliation and termite damage because individuals devoid of associated ants suffered significantly more attacks. Using the stable isotope ¹⁵N, we experimentally showed that the tested ant species furnish their host plants with nutrients. Therefore, a mutualism exists. However, because it is associated with numerous ant species, T. guianensis can be considered a nonspecialized myrmecophyte.

Context dependency of rewards and services in an Indian ant–plant interaction: southern sites favour the mutualism between plants and ants

Protection-based ant–plant mutualisms may vary in strength due to differences in ant rewards, abundance of protective ants and herbivory pressure. We investigated geographical and temporal variation in host plant traits and herbivory pressure at five sites spanning the distribution range of the myrmecophyteHumboldtia brunonis(Fabaceae) in the Indian Western Ghats. Southern sites had, on average, 2.4 times greater abundance of domatia-bearing individuals, 1.6 times greater extrafloral nectary numbers per leaf, 1.2 times larger extrafloral nectary sizes, 2.2 times greater extrafloral nectar (EFN) volumes and a two-fold increase in total amino acid and total sugar concentrations in EFN compared with northern sites. A strong protection-based mutualism with ants occurred at only one southern site where herbivory was highest, suggesting that investments in attracting ants correlate with anti-herbivore benefits gained from the presence of protective ants. Our results confirm a temporally stable north–south gradient in myrmecophytic traits in this ant-plant as several of these traits were re-sampled after a 5-y interval. However, the chemical composition of EFN varied at both spatial and short-term temporal scales suggesting that only repeated measurements of rewards such as EFN can reveal the real spectrum of trait variation in an ant–plant mutualistic system.