Evolutionary Biology Online Journal Club

Forister et al 2011 Proc B – Paper summary

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We talked about adding more content to the blog so I thought I’d start by adding my summary of the paper we read today (Oct 16th 2012). If you have comments or questions please feel free to add them below in the comment thread.

Forister, M.L., Gompert, Z., Nice, C.C., Forister, G.W. and Fordyce, J.A. (2011) Ant association facilitates the evolution of diet breadth in a lycaenid butterfly. Proc Roy Soc B 278: 1539-1547

  • Note that the R code for the simulations can be found in an earlier post HERE

Context:

  • Lycaenidae are a large family of butterflies, more than half of them engage in ant interactions that are mutualistic, commensalistic, or parasitic
  • From an ecological perspective, these interactions are of interest in the context of enemy-free space, b/c ant-association provides caterpillars with protection
  • Evolutionary implications less studied, but it has been predicted that these mutualistic interactions with ants could affect  host-range evolution
  • Generally observed that lycaenid butterflies that have mutualistic relationships with ants have broader host-range
  • Two complimentary mechanisms have been proposed to explain this:
  1. Presence of ants influences adult oviposition decisions such that adults are more likely to lay eggs on plants with ants that are likely to tend/protect their offspring
  2. The presence of ants creates sufficient reduction in predator pressure to facilitate survival on novel hosts.

Question:

  • Can ant protection facilitate larval survival on a novel host such that it facilitates the evolution of diet breadth in Lycaenidae butterfly caterpillars?

System:

Approach:

  • Field study – Goal: Describe and compare abundance and richness of natural enemies, abundance of ants, and abundance of ant-tended Hemiptera
  • Field experiments(2)
    • Goal Exp. 1: Compare caterpillar survival on native and novel host plants with and without ant-interaction permitted
    • Goal Exp. 2: Compare caterpillar survival only on novel host, with and without ant-interaction permitted
  • Simulation modellingusing a stage-structured demographic model.
    • Goal: Explore the role of ant protection in persistence of Lycaeides melissa populations

Main results:

  • Field study:
    • Natural enemies more abundant on native host
    • Enemy richness greater on native host
    • Dominant predators included Geocorus bugs and crab spiders
    • Number of ant individuals higher on novel host
    • Number of Hemiptera higher on novel host
  • Experiment 1:
    • 199/338 caterpillars survived (58.9%)
    • Significant effect of plant species (higher on native host)
    • No effect of ant exclusion
      • But did see an increase in variance of survival for ant-permitted novel host treatment
  • Experiment 2:
    • Presence of ants increased survival significantly
    • Mean survival roughly equivalent to results from Exp. 1 on novel host
    • Protection likely indirect for the early instar caterpillars used in the exp.
  • Simulation:
    • When the proportion of novel host was very high (~1), butterfly population only persists when ant tending also high (i.e., results in a population that is dependent on the presence of ants)
      • This is a consequence of low survival on caterpillars on novel host when ants are lacking
    • Lower prop. of novel host (<0.6) causes persistence of ants to be insensitive to presence of ants
    • When minimum fraction of plants flowering is lowered, there is a greater chance of population extinction (technically, a greater parameter space where populations go extinct)
    • A result of the fact that without flowers the novel host is poor
    • When both hosts lack flower the native host is preferred, but when both hosts flower adults have no preference b/w native and novel host
      • Consequently, when more plants are flowering prop. more eggs are distributed to lower-quality novel host.

Implications:

  • Mutualistic ant association can facilitate use of a novel host by reducing predation – not just by ant-associated oviposition.
  • Quality of novel host is so poor that without buffering against predation populations would not be able to persist solely on novel host
  • Complimentary to previous work on enemy-free space affecting diet breadth
    • Diet breadth may be affected by novel host which provide an escape from predators associated with novel host.
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Author: hossiet

PhD student at Carleton University. I study predator-prey interactions. My current project examines the ecology and evolution of caterpillar eyespots.

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