Dissertation Research

What mechanisms drive temperature effects on herbivore abundance?

Climate is a key driver of species abundances and distributions, affecting species both directly and indirectly by altering the complex multi-trophic food webs in which they are embedded. However, experimental manipulations of multi-trophic interactions along climatic gradients are rare, limiting our understanding of the exact mechanisms driving climate effects on species abundances.

In three separate studies (Nelson et al. 2018 Oikos, Mooney et al. 2016 Ecology Letters, and Nelson et al. In revision), we experimentally manipulated multi-trophic interactions among plants, aphids (plant-feeding insects), predators, and mutualist ants to assess the mechanisms driving temperature effects on the abundance of three aphid herbivore species. We found that aphids are more abundant in warmer sites, both at lower elevations and in sunnier meadows. Temperature does not affect aphids directly or indirectly through changes in plant quality. Rather, predator effects increase with temperature, but the benefits provided by ants increase even more strongly, thus enhancing aphid performance and abundance. This work highlights that in order to predict how climate change will affect species abundances and distributions, it is important to assess how climate alters species interactions, particularly at higher trophic levels.
Annika censusing aphid colonies.
The ant Formica podzolica tending the aphid Aphis asclepiadis feeding on osha (Ligusticum porteri).
How do mutualist identity and diversity affect herbivore populations?

Mutualisms, interactions between two species where both benefit, play a central role in structuring populations and maintaining biodiversity. Most mutualisms consist of networks of interacting species, but because few studies have documented the demographic consequences of associating with multiple mutualist partners, our understanding of the importance of species identity and diversity in mutualisms is limited.

To investigate the consequences of associating with multiple mutualist partners, I am using observational demographic studies to compare the effectiveness of three ant species as mutualists for aphids and also examining competitive interactions among these ants. I hypothesized that aphids would receive the strongest benefits when tended by (1) aggressive, competitively dominant ants that are effective at deterring aphid predators and (2) multiple ant species if they provide different, complementary benefits to aphids.

Preliminary results indicate that not all mutualist partners are equally effective, and aphids receive stronger benefits from more dominant, aggressive ant species. However, when aphids associated with multiple ant species simultaneously, they received fewer benefits compared to any of the ant species alone, indicating that increased mutualist partner diversity is not always beneficial. In summary, this work illustrates that in order to predict the outcomes of mutualisms, we must study their demographic consequences in a multi-species context.

Additional Projects

How do plant chemicals affect ant foraging and discovery of mutualists?

Ants are ecologically dominant members of most terrestrial ecosystems, where they play many roles at different trophic levels (e.g., as herbivores, predators, and mutualists). When foraging in complex habitats, ants rely on chemicals that they detect in the environment to locate resources. Because ant foraging is often associated with plants, plant chemicals can affect ant foraging (reviewed in Nelson et al. 2018, Current Opinion in Insect Science). However, the extent to which ants rely on plant chemicals to locate insect mutualists, and whether ant responses are innate or learned, is unknown.

I advised two undergraduate students on projects that investigated how plant odors affect ant foraging behavior. We trained ants to forage from scented baits to assess whether ants learn plant odors as a mechanism of locating mutualist rewards associated with those plants in the future. We found that ants associatively learn plant odors, being more likely to discover resources associated with odors on which they are previously trained (Nelson et al. In review).
Experimental setup for ant associative learning experiment. First, we trained ants to forage from baits associated with odors from particular plant species (bait at the top of the single stick, center). We subsequently tested whether ants would be more likely to discover baits with the same odor (versus odor from another plant species) (two baits at the top of the Y-maze, right). We filmed ant visitation to the baits (camera attached to the brick, left).