Competition for resources and predation are important causes of natural selection, and evolutionary responses can modify the strength of species interactions and the structure of food webs.
The threespine stickleback (Gasterosteus aculeatus) is well known for its repeated speciation into benthic and limnetic specialists in post-glacial lakes in British Columbia. Resource competition between stickleback is an important driver of diversification, whereas interactions with other species sometimes seems to inhibit or reverse speciation. Prickly sculpin (Cottus asper) is an omnivorous benthic fish that both eats stickleback and competes with them for benthic invertebrate prey. This interaction - intraguild predation - is interesting both because the combination of competition and predation should place strong selective pressures on the intraguild prey (stickleback), and because evolution of either species can change the trophic position of sculpin and thus the structure of the food web.
Our collaboration combined a comparative analysis of body shape with a pond mesocosm experiment to study the evolutionary and ecological consequences of the sculpin-stickleback interaction. The presence of sculpin is consistently associated with body shapes associated with antipredator defense and zooplanktivory. Our experiment confirmed that stickleback that have evolved in lakes with sculpin are less vulnerable to predation and more planktivorous (pdf). These evolutionary shifts have the potential to change both the trophic position of sculpin and the distribution of invertebrate biomass in the food web. Our current research is addressing the potential role of similar dynamics when fish species are introduced into New Zealand freshwater food webs.
Travis has also used theoretical approaches to investigate the evolutionary dynamics of entire food webs. Early in his degree, he worked on a model that simulates food web assembly through speciation and the evolution of traits (body size and niche width) that determine ecological interactions. Across a range of parameter space corresponding to variation in trade-off strength, the model produced an unexpected emergent positive relationship between omnivory and instability (pdf). This pattern suggests that food webs with extensive omnivory are more invasible by new species relative to food webs with clearly defined trophic levels. A follow-up analysis of trait evolution suggested that only adaptive radiations into food webs with stable trophic levels are likely to produce a persistent ‘early burst’ pattern of phenotypic evolution (pdf).
The ecological and evolutionary consequences of food web interactions between species