Long‐term persistence of a Neotropical ant‐plant population in the absence of obligate plant‐ants

Abstract

Interactions between ants and ant-plants are considered classic examples of obligate mutualisms. Previous studies have indicated that for many ant-plants the loss of ant colonies results in severe defoliation or mortality. Although individual plants can persist for some period of time without their mutualistic partners, to date populations of ant-free plants have only been recorded at high altitudes or on remote islands where herbivores are also scarce. We studied the interaction between ants, herbivores, and the ant-plant Tococa guianensis in the Cerrado region of central Brazil. Using a survey conducted across a large geographic region, we show that there is interpopulation variation in ant occupancy across sites and habitats. At most sites surveyed, plants were inhabited by Allomerus octoarticulatus, an obligate plant-ant. Plants with obligate ants had significantly lower standing levels of herbivore damage than plants with opportunistic ants and plants with no ant occupants. Furthermore, experimental removal of A. octoarticulatus resulted in increased levels of damage in both young and mature leaves. Despite the protection provided by obligate ants, populations of T. guianensis were found to persist without these ants in some areas. Plants without A. octoarticulatus had significantly greater leaf toughness and trichome density than those with A. octoarticulatus. Furthermore, trichome density in plants with A. octoarticulatus increased after ants were removed, probably as a response induced by increased levels of herbivore damage. To our knowledge, this is the first record of the occurrence of native myrmecophyte populations without their mutualistic ants in mainland low-elevation sites. Several factors may help to explain the long-term persistence of T. guianensis populations without plant-ants in some areas of the Brazilian Cerrado, including its potential for induced morphological defenses against insect herbivores and selection for increased levels of constitutive defenses. In addition, T. guianensis may be under lower herbivore pressure in the sites we studied than in other parts of its range because existing populations were small, isolated, and some were very close to the edge of this species' distribution.

Do extrafloral nectaries present a defensive role against herbivores in two species of the family Bignoniaceae in a Neotropical savannas?

“…Within the context of our study, GMT would predict local differences among populations (and species in expanded temporal scale), which might lead to variation in the ant-plant interaction (e.g., Alonso 1998;Manzaneda and Rey 2008), with some populations not being protected by the ants. The geographic variation of ant-plant interactions on savannas were observed among populations of the ant-plant Tococa guianensis, in which some populations were found to persist in some areas without these obligate ants on the plants ( Moraes and Vasconcelos 2009). The lack of a defensive role of EFNs in both species of Anemopaegma studied suggests that the ants available in our study sites were not sufficiently aggressive to protect the visited plants against herbivores (very efficient herbivores).…”

The Curious Case of the Camelthorn: Competition, Coexistence, and Nest-Site Limitation in a Multispecies Mutualism

“…Most myrmecophytes associate with a guild of ants, via either (i) an individual plant associating with multiple ant species over its lifetime or (ii) different individual plants in a population associating with different ant species. Ant partners may also vary across geographic scales (Longino 1989), and some myrmecophytes lose their mutualist ants altogether (Moraes and Vasconcelos 2009). This highlights the important issues of scale and specificity in studying species interactions (Thompson 2005).…”

Mutualisms in a changing world: an evolutionary perspective

“…While these changes are mediated over ecological time scales, the potential for evolutionary shifts over longer time scales is clear. In protective mutualisms that have evolved in the context of natural enemies and in which investment in mutualist traits is costly, the loss of those enemies may favour genotypes that invest less in the mutualism (Moraes & Vasconcelos 2009). Extreme differences in life spans of the interacting parties (e.g., ants and trees) create imbalances in the potential for each player to respond evolutionarily to anthropogenically altered environments.…”