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Effects of climate change on mutualisms and multi-trophic interactions Climate is a key driver of species abundances and distributions. While climate change can have strong direct effects on species abundances, it can also indirectly affect species by influencing the complex multi-trophic food webs in which they are embedded. However, because few studies have documented the relative importance of these direct and indirect effects, we have a limited understanding of the exact mechanisms driving species responses to climate change. My work includes experimental manipulations of plant-insect multi-trophic interactions along multiple subalpine climatic gradients at the Rocky Mountain Biological Laboratory in Gothic, Colorado, both at the ecosysem (e.g., elevational gradient) and patch (e.g., adjacent sunny vs. shaded meadows) levels. I have found that herbivore abundance (of three aphid species, each feeding on a different host plant) increases with aridity. Yet, aridity does not affect herbivore abundance directly. Instead, herbivore abundance increases with aridity because although natural enemy (i.e., predators and parasitoids) abundance increases, ant activity shows a parallel but even stronger increase with aridity, shifting the interaction from neutral (no benefit) to mutualism (positive effect on herbivore abundance). Relevant publications: Nelson & Mooney 2022, Annual Review of Ecology, Evolution, and Systematics Nelson et al. 2019, Journal of Animal Ecology Nelson et al. 2019, Oikos Mooney et al. 2016, Ecology Letters |
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Maintenance of biodiversity in mutualisms
Mutualisms play a central role in structuring populations and maintaining biodiversity and ecosystem function. While most mutualisms are comprised of diverse networks of interacting species, it is not well understood how diversity is maintained within mutualisms, as classic mutualism theory predicts that specialization (rather than generalization) should be favored. I examine how variation in multiple ant traits (e.g., activity, diet, thermal tolerance, resource discovery ability, and competitive dominance), both within and across species, influence coexistence in ecological communities and mutualistic networks. I have found evidence that dominance-discovery tradeoffs among ant species, in which ants that discover mutualist aphids the fastest are later competitively excluded from associating with the highest quality mutualist partners, promote coexistence among species. To determine the consequences of ant species diversity for partner fitness, I am currently constructing demographic models to project aphid population dynamics under varying mutualist ant community compositions. I have found that aphid populations grow fastest in the most biodiverse ant communities, demonstrating the importance of biodiversity for maintaining mutualism function. Two first-author manuscripts based on this work are currently in preparation and expected to be published within the next year. Relevant publications: Sheard et al. 2020, Journal of Biogeography Nelson & Mooney 2021, Annals of the Entomological Society of America |
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Trade-offs in seed dispersal and fruit defense
Plants face contrasting selective pressures to attract mutualists but deter antagonists. Although many plants produce rewards (e.g., fleshy fruits or elaiosomes) to attract animals to disperse their seeds, fruits are also vulnerable to attack by pre- and post-dispersal seed predators and pathogens. Currently, I am investigating the causes and consequences of variation in plant allocation to seed dispersal and fruit defense, both in eastern U.S. deciduous forests and in the tropics. I have found that the effectiveness of seed dispersal services provided by ants, which disperse >11,000 plant species globally, is strongly reduced by defensive secondary metabolites in fruits. Secondary metabolites not only reduce seed dispersal quantity, but they also alter seed dispersal quality through changes to how ants handle and where they deposit seeds on the forest floor. Relevant publications: Nelson & Whitehead 2021, TREE Nelson et al. 2019, Current Opinion in Insect Science Nelson et al. 2019, Ecological Entomology Whitehead et al. 2021, Oikos |
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