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1. 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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2. The invasion paradox dissolves when using phylogenetic and temporal perspectives

   https://doi.org/10.1111/1365-2745.13812  

   Ernst, Adrienne R. ; Barak, Rebecca S. ; Hipp, Andrew L. ; Kramer, Andrea T. ; Marx, Hannah E. ; Larkin, Daniel J. ( November 2021 , Journal of Ecology)

   The prediction that higher biodiversity leads to denser niche packing and thus higher community resistance to invasion has long been studied, with species richness as the predominant measure of diversity. However, few studies have explored how phylogenetic and functional diversity, which should represent niche space more faithfully than taxonomic diversity, influence community invasibility, especially across longer time frames and over larger spatial extents.

   We used a 15‐year, 150‐site grassland dataset to assess relationships between invasive plant abundance and phylogenetic, functional and taxonomic diversity of recipient native plant communities. We analysed the dataset both pooled across all surveys and longitudinally, leveraging time‐series data to compare observed patterns in invasion with those predicted by two community assembly processes: biotic resistance and competitive exclusion. We expected more phylogenetically and functionally diverse communities to exhibit greater resistance to invasion.

   With the pooled dataset, we found support for the long‐standing observation that communities with more native species have lower abundance of invasive species, and a more novel finding that more phylogenetically diverse communities had higher abundance of invasive species. We found no influence of aggregate (multivariate) functional diversity on invasion, but assemblages with taller plants, lower variability in plant height and lower seed mass were less invaded. Viewed longitudinally, the phylogenetic diversity relationship was reversed: the most phylogenetically diverse communities were most resistant to invasion. This apparent discrepancy suggests invasion dynamics are influenced by both site attributes and biotic resistance and emphasizes the value in studying invasion across time.

   Synthesis. Our results provide insight into the nuances of the diversity–invasibility relationship: invasion dynamics differed for different dimensions of diversity and depending on whether the relationship was evaluated longitudinally. Our findings highlight the limitations of using single time‐point ‘snapshots’ of community composition to infer invasion mechanisms.

    

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3. Expanding the invasion footprint: Ventenata dubia and relationships to wildfire, environment, and plant communities in the Blue Mountains of the Inland Northwest, USA

   https://doi.org/10.1111/avsc.12511  

   Tortorelli, Claire M. ; Krawchuk, Meg A. ; Kerns, Becky K. ; Fraser, ed., Lauchlan ( August 2020 , Applied Vegetation Science)

   A recently introduced non‐native annual grass,Ventenata dubia, is challenging previous conceptions of community resistance in forest mosaic communities in the Inland Northwest. However, little is known of the drivers and potential ecological impacts of this rapidly expanding species. Here we (1) identify abiotic and biotic habitat characteristics associated with theV. dubiainvasion and examine how these differ betweenV. dubiaand other problematic non‐native annual grasses,Bromus tectorumandTaeniatherum caput‐medusae; and (2) determine how burning influences relationships betweenV. dubiaand plant community composition and structure to address potential impacts on Inland Northwest forest mosaic communities.

   Blue Mountains of the Inland Northwest, USA.

   We measured environmental and plant community characteristics in 110 recently burned and nearby unburned plots. Plots were stratified to capture a range ofV. dubiacover, elevations, biophysical classes, and fire severities. We investigated relationships betweenV. dubia, wildfire, environmental, and plant community characteristics using non‐metric multidimensional scaling and linear regressions.

   Ventenata dubiawas most abundant in sparsely vegetated, basalt‐derived rocky scablands interspersed throughout the forested landscape. Plant communities most heavily invaded byV. dubiawere largely uninvaded by other non‐native annual grasses.Ventenata dubiawas abundant in both unburned and burned areas, but negative relationships betweenV. dubiacover and community diversity were stronger in burned plots, where keystone sagebrush species were largely absent after fire.

   Ventenata dubiais expanding the overall invasion footprint into previously uninvaded communities. Burning may exacerbate negative relationships betweenV. dubiaand species richness, evenness, and functional diversity, including in communities that historically rarely burned. Understanding the drivers and impacts of theV. dubiainvasion and recognizing how these differ from other annual grass invasions may provide insight into mechanisms of community invasibility, grass‐fire feedbacks, and aid the development of species‐specific management plans.

    

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4. 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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5. Plant invasion impacts on fungal community structure and function depend on soil warming and nitrogen enrichment

   https://doi.org/10.1007/s00442-020-04797-4  

   Anthony, M. A. ; Stinson, K. A. ; Moore, J. A. ; Frey, S. D. ( December 2020 , Oecologia)

   null (Ed.)

   Abstract The impacts of invasive species on biodiversity may be mitigated or exacerbated by abiotic environmental changes. Invasive plants can restructure soil fungal communities with important implications for native biodiversity and nutrient cycling, yet fungal responses to invasion may depend on numerous anthropogenic stressors. In this study, we experimentally invaded a long-term soil warming and simulated nitrogen deposition experiment with the widespread invasive plant Alliaria petiolata (garlic mustard) and tested the responses of soil fungal communities to invasion, abiotic factors, and their interaction. We focused on the phytotoxic garlic mustard because it suppresses native mycorrhizae across forests of North America. We found that invasion in combination with warming, but not under ambient conditions or elevated nitrogen, significantly reduced soil fungal biomass and ectomycorrhizal relative abundances and increased relative abundances of general soil saprotrophs and fungal genes encoding for hydrolytic enzymes. These results suggest that warming potentially exacerbates fungal responses to plant invasion. Soils collected from uninvaded and invaded plots across eight forests spanning a 4 °C temperature gradient further demonstrated that the magnitude of fungal responses to invasion was positively correlated with mean annual temperature. Our study is one of the first empirical tests to show that the impacts of invasion on fungal communities depends on additional anthropogenic pressures and were greater in concert with warming than under elevated nitrogen or ambient conditions. 

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