Colonisation and competition dynamics can explain incomplete sterilisation parasitism in ant–plant symbioses

Symbiotic ant traits produce differential host-plant carbon and water dynamics in a multi-species mutualism

Cooperative interactions may frequently be reinforced by "partner fidelity feedback," in which high- or low-quality partners drive positive feedbacks with high or low benefits for the host, respectively. Benefits of plant-animal mutualisms for plants have been quantified almost universally in terms of growth or reproduction, but these are only two of many sinks to which a host-plant allocates its resources. By investigating how partners to host-plants impact two fundamental plant resources, carbon and water, we can better characterize plant-partner fidelity and understand how plant-partner mutualisms may be modulated by resource dynamics. In Laikipia, Kenya, four ant species compete for Acacia drepanolobium host-plants. These ants differ in multiple traits, from nectar consumption to host-plant protection. Using a 5-year ant removal experiment, we compared carbon fixation, leaf water status, and stem non-structural carbohydrate concentrations for adult ant-plants with and without ant partners. Removal treatments showed that the ants differentially mediate tree carbon and/or water resources. All three ant species known to be aggressive against herbivores were linked to benefits for host-plant resources, but only the two species that defend but do not prune the host, Crematogaster mimosae and Tetraponera penzigi, increased tree carbon fixation. Of these two species, only the nectivore C. mimosae increased tree simple sugars. Crematogaster nigriceps, which defends the tree but also castrates flowers and prunes meristems, was linked only to lower tree water stress approximated by pre-dawn leaf water potential. In contrast to those defensive ants, Crematogaster sjostedti, a poor defender that displaces other ants, was linked to lower tree carbon fixation. Comparing the effects of the four ant species across control trees suggests that differential ant occupancy drives substantial differences in carbon and water supply among host trees. Our results highlight that ant partners can positively or negatively impact carbon and/or water relations for their host-plant, and we discuss the likelihood that carbon- and water-related partner fidelity feedback loops occur across ant-plant mutualisms.

More Than a Functional Group: Diversity within the Legume–Rhizobia Mutualism and Its Relationship with Ecosystem Function

Studies of biodiversity and ecosystem function (BEF) have long focused on the role of nitrogen (N)-fixing legumes as a functional group that occupies a distinct and important niche relative to other plants. Because of their relationship with N-fixing rhizobial bacteria, these legumes access a different pool of N than other plants and therefore directly contribute to increases in productivity and N-cycling. Despite their recognized importance in the BEF literature, the field has not moved far beyond investigating the presence/absence of the legume functional group in species mixtures. Here, we synthesize existing information on how the diversity (species richness and functional diversity) of both legumes and the rhizobia that they host impact ecosystem functions, such as nitrogen fixation and primary productivity. We also discuss the often-overlooked reciprocal direction of the BEF relationship, whereby ecosystem function can influence legume and rhizobial diversity. We focus on BEF mechanisms of selection, complementarity, facilitation, competitive interference, and dilution effects to explain how diversity in the legume–rhizobia mutualism can have either positive or negative effects on ecosystem function—mechanisms that can operate at scales from rhizobial communities affecting individual legume functions to legume communities affecting landscape-scale ecosystem functions. To fully understand the relationship between biodiversity and ecosystem function, we must incorporate the full diversity of this mutualism and its reciprocal relationship with ecosystem function into our evolving BEF framework.

Synergistic effects of fire and elephants on arboreal animals in an African savanna

Disturbance is a crucial determinant of animal abundance, distribution and community structure in many ecosystems, but the ways in which multiple disturbance types interact remain poorly understood. The effects of multiple-disturbance interactions can be additive, subadditive or super-additive (synergistic). Synergistic effects in particular can accelerate ecological change; thus, characterizing such synergies, the conditions under which they arise, and how long they persist has been identified as a major goal of ecology. We factorially manipulated two principal sources of disturbance in African savannas, fire and elephants, and measured their independent and interactive effects on the numerically dominant vertebrate (the arboreal gekkonid lizard Lygodactylus keniensis) and invertebrate (a guild of symbiotic Acacia ants) animal species in a semi-arid Kenyan savanna. Elephant exclusion alone (minus fire) had negligible effects on gecko density. Fire alone (minus elephants) had negligible effects on gecko density after 4 months, but increased gecko density twofold after 16 months, likely because the decay of fire-damaged woody biomass created refuges and nest sites for geckos. In the presence of elephants, fire increased gecko density nearly threefold within 4 months of the experimental burn; this occurred because fire increased the incidence of elephant damage to trees, which in turn improved microhabitat quality for geckos. However, this synergistic positive effect of fire and elephants attenuated over the ensuing year, such that only the main effect of fire was evident after 16 months. Fire also altered the structure of symbiotic plant-ant assemblages occupying the dominant tree species (Acacia drepanolobium); this influenced gecko habitat selection but did not explain the synergistic effect of fire and elephants. However, fire-driven shifts in plant-ant occupancy may have indirectly mediated this effect by increasing trees' susceptibility to elephant damage. Our findings confirm the importance of fire × elephant interactions in structuring arboreal wildlife populations. Where habitat modification by megaherbivores facilitates co-occurring species, fire may amplify these effects in the short term by increasing the frequency or intensity of herbivory, leading to synergy. In the longer term, tree mortality due to both top kill by fire and toppling by large herbivores may reduce overall microhabitat availability, eliminating the synergy.