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1. Mutualism disruption by an invasive ant reduces carbon fixation for a foundational East African ant‐plant

   https://doi.org/10.1111/ele.13725  

   Milligan, Patrick D. ; Martin, Timothy A. ; John, Grace P. ; Riginos, Corinna ; Goheen, Jacob R. ; Carpenter, Scott M. ; Palmer, Todd M. ; Irwin, ed., Rebecca ( March 2021 , Ecology Letters)

   Invasive ants shape assemblages and interactions of native species, but their effect on fundamental ecological processes is poorly understood. In East Africa,Pheidole megacephalaants have invaded monodominant stands of the ant‐treeAcacia drepanolobium, extirpating native ant defenders and rendering trees vulnerable to canopy damage by vertebrate herbivores. We used experiments and observations to quantify direct and interactive effects of invasive ants and large herbivores onA. drepanolobiumphotosynthesis over a 2‐year period. Trees that had been invaded for ≥ 5 years exhibited 69% lower whole‐tree photosynthesis during key growing seasons, resulting from interaction between invasive ants and vertebrate herbivores that caused leaf‐ and canopy‐level photosynthesis declines. We also surveyed trees shortly before and after invasion, finding that recent invasion induced only minor changes in leaf physiology. Our results from individual trees likely scale up, highlighting the potential of invasive species to alter ecosystem‐level carbon fixation and other biogeochemical cycles.

    

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2. Frenemy at the gate: Invasion by Pheidole megacephala facilitates a competitively subordinate plant ant in Kenya

   https://doi.org/10.1002/ecy.3230  

   Palmer, Todd M. ; Riginos, Corinna ; Milligan, Patrick D. ; Hays, Brandon R. ; Pietrek, Alejandro G. ; Maiyo, Nelly J. ; Mutisya, Samuel ; Gituku, Benard ; Musila, Simon ; Carpenter, Scott ; et al ( December 2020 , Ecology)

   Biological invasions can lead to the reassembly of communities and understanding and predicting the impacts of exotic species on community structure and functioning are a key challenge in ecology. We investigated the impact of a predatory species of invasive ant,Pheidole megacephala, on the structure and function of a foundational mutualism betweenAcacia drepanolobiumand its associated acacia‐ant community in an East African savanna. Invasion byP. megacephalawas associated with the extirpation of three extrafloral nectar‐dependentCrematogasteracacia ant species and strong increases in the abundance of a competitively subordinate and locally rare acacia ant species,Tetraponera penzigi, which does not depend on host plant nectar. Using a combination of long‐term monitoring of invasion dynamics, observations and experiments, we demonstrate thatP. megacephaladirectly and indirectly facilitatesT. penzigiby reducing the abundance ofT. penzigi’s competitors (Crematogasterspp.), imposing recruitment limitation on these competitors, and generating a landscape of low‐reward host plants that favor colonization and establishment by the strongly dispersingT. penzigi. Seasonal variation in use of host plants byP. megacephalamay further increase the persistence ofT. penzigicolonies in invaded habitat. The persistence of theT. penzigi–A. drepanolobiumsymbiosis in invaded areas afforded host plants some protection against herbivory by elephants (Loxodonta africana), a key browser that reduces tree cover. However, elephant damage onT. penzigi‐occupied trees was higher in invaded than in uninvaded areas, likely owing to reducedT. penzigicolony size in invaded habitats. Our results reveal the mechanisms underlying the disruption of this mutualism and suggest thatP. megacephalainvasion may drive long‐term declines in tree cover, despite the partial persistence of the ant–acacia symbiosis in invaded areas.

    

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3. Tree resprout dynamics following fire depend on herbivory by wild ungulate herbivores

   https://doi.org/10.1111/1365-2745.13186  

   LaMalfa, Eric M. ; Kimuyu, Duncan M. ; Sensenig, Ryan L. ; Young, Truman P. ; Riginos, Corinna ; Veblen, Kari E. ; Sankaran, ed., Mahesh ( May 2019 , Journal of Ecology)

   Savanna tree cover is dynamic due to disturbances such as fire and herbivory. Frequent fires can limit a key demographic transition from sapling to adult height classes in savanna trees. Saplings may be caught in a ‘fire trap’, wherein individuals repeatedly resprout following fire top‐kill events. Saplings only rarely escape the cycle by attaining a fire‐resistant height (e.g. taller than the minimum scorch height) during fire‐free intervals.

   Large mammalian herbivores also may trap trees in shorter size classes. Browsing herbivores directly limit sapling height, while grazing herbivores such as cattle facilitate sapling growth indirectly via grass removal. Experimental studies investigating how meso‐wildlife, megaherbivores and domestic livestock affect height of resprouts following fire are rare, but necessary for fully understanding how herbivory may reinforce (or counteract) the fire trap. In our study system, interactive fire–herbivore effects on transitions from sapling (<1 m) to adult tree (>1 m) height classes may be further influenced by plant defences, such as symbiotic ants.

   We used the Kenya Long‐term Exclosure Experiment (KLEE) to investigate how post‐fire resprout size of a widespread monodominant East African tree,Acacia drepanolobiumwas influenced by (a) herbivory by different combinations of cattle, meso‐wildlife (15–1,000 kg) and megaherbivores (>1,000 kg) and (b) the presence of acacia–ant mutualists that confer tree defences. We sampled height, stem length and ant occupancy of resprouts exposed to different herbivore combinations before and after controlled burns.

   Resprout height of saplings that were short prior to fire (<1 m) was reduced primarily by meso‐wildlife. Negative effects of elephants on post‐fire resprout height increased with pre‐fire tree size, suggesting that resprouts of the tallest trees (with the greatest potential to escape the fire trap cycle) were preferentially browsed and reduced in height by elephants. There were no significant cattle effects.

   Synthesis. We provide experimental evidence for two potential pathways through which large herbivores exert control over sapling escape from the fire trap: (a) post‐fire meso‐wildlife browsing of short (<1 m) resprouts and (b) elephant browsing of the largest size class of resprouts, which would otherwise be most likely to escape the fire trap.

    

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

   https://doi.org/10.1002/ecy.3880  

   Milligan, Patrick D. ; Martin, Timothy A. ; Pringle, Elizabeth G. ; Prior, Kirsten M. ; Palmer, Todd M. ( November 2022 , Ecology)

   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 forAcacia drepanolobiumhost‐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 mimosaeandTetraponera penzigi, increased tree carbon fixation. Of these two species, only the nectivoreC. mimosaeincreased 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.

    

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5. Mutualistic acacia ants exhibit reduced aggression and more frequent off‐tree movements near termite mounds

   https://doi.org/10.1111/btp.12572  

   Henry, Lucas P. ; Tokita, Christopher K. ; Misra, Mayank ; Forrow, Avery B. ; Rubenstein, Daniel I. ( June 2018 , Biotropica)

   In many ant–plant mutualisms, ants establish colonies in hollow thorns, leaf pouches, or other specialized structures on their host plants, which they then defend from herbivores. Resource heterogeneity could affect the maintenance of these mutualisms if it leads to one or both partners altering their investment in the interaction. Such a phenomenon may be especially pertinent to theAcacia–antmutualism found in East African savannas, where termite mounds have a profound effect on the spatial structuring of resources used by both plants and ants. Here, we examined whether the proximity to termite mounds ofAcacia drepanolobiumtrees is associated with variation in the behavior of one of their ant associates,Crematogaster nigriceps. We found that ant colonies near termite mounds had decreased aggressive responses to simulated herbivory as well as increased off‐tree movement. We hypothesize that these changes are the result of resident ant colonies near termite mounds shifting investment from defense of their host plant to foraging for nearby resources.

    

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