“Ontogeny and consistent individual differences mediate trophic interactions”

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Denon Start (Sep 2018)

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In the world of dragonflies, age and individual behavioral differences combine to determine who eats who

Anax junius dragonfly preparing to attack a smaller libellulid dragonfly larvae.
(Credit: Rosemary L. Martin, University of Toronto Mississauga)

Traits shape where species can occur, and consequently the assembly of ecological communities. However, species’ traits are not invariant—individuals of the same species are often markedly different, with probable consequences for entire communities. Individuals can differ in appearance or behavior because of (1) consistent individual differences or (2) because individual traits change across the lifetime of the organism (across ontogeny). Do consistent individual differences and ontogenetic shifts have similar impacts on community assembly? What is the joint effect of both types of individual variation? Start addresses these questions by developing a conceptual understanding of consistent individual differences and ontogenetic trait shifts. He goes on to demonstrate the utility of considering multiple sources of intraspecific variation using dragonfly larvae in an aquatic mesocosm experiment.

Start argues that the effect of a consistent individual trait difference will depend on the functional role of that individual in the community, itself a function of ontogeny. Using an aquatic mesocosm experiment at the Koffler Scientific Reserve (University of Toronto), he shows that young dragonfly larvae are often prey to larger dragonflies, demonstrating an ontogenetic shift in functional role from prey to predator. Individual dragonfly larvae also differ in activity rate, but the effect of activity rate depends on ontogenetic stage; young and active dragonfly larvae are more likely to be consumed by a predator, but older and active dragonfly larvae are themselves the most voracious predators. Start ultimately shows that the effects of ontogeny and consistent individual differences are interactive, jointly shaping species interactions and the assembly of ecological communities.

We are increasingly recognizing the importance of considering processes occurring across biological scales (e.g. individuals to communities). Start provides a road map for considering when ontogeny and consistent individual differences will shape ecological communities; given the large trait differences observed in many populations, and the ubiquity of changes across the lifecycle, the framework presented here is likely to have general utility for virtually all ecological systems.


Abstract

Ecologists use species traits to predict individual responses to environmental change, and ultimately to understand the composition of biological communities. However, this ignores known and substantial intraspecific variation that can have important consequences for species interactions and community composition. This within-species variation results from two distinct sources: ontogeny and consistent individual differences. Ontogeny and consistent differences interact to produce phenotypes, but the community-level consequences of this interaction have not been studied. Using larval dragonfly communities I investigate patterns of intraguild predation by manipulating (1) consistent individual differences in activity rate, and (2) the ontogeny of the focal and interacting species. I show that activity rate is a consistent individual trait, but that the effect of activity rate on intraguild predation depends on the functional role of an organism in the community (predator or prey). An organism’s functional role itself varies across ontogeny of both the focal and interacting individuals. I suggest that ontogeny and consistent individual differences interact to produce intraspecific variation, with consequences for species interactions, communities, and eco-evolutionary dynamics.