Adaptive radiation in food webs and the reality of trophic levels on macroevolutionary timescales

Background/Question/Methods

Modern food webs are the result of the interplay between ecological interactions and evolutionary processes. Evolutionary radiations can fill trophic niche space over long timescales, and phylogenetic comparative methods provide an opportunity to investigate the dynamics of this diversification. Early stages of adaptive radiations are often characterized by rapid trait diversification, which may be followed by slower evolution if lineages become subject to different regimes of stabilizing selection. In a food web context, discrete trophic niches, particularly integer trophic levels, may function as stable evolutionary attractors. Quantitative reviews of food web structure suggest that relatively few species unambiguously occupy integer trophic levels above the primary producer and herbivore levels. However, present-day food web structure does not necessarily tell us whether integer trophic levels, or discrete trophic niches in general, tend to represent adaptive peaks that are discovered by one or multiple lineages during an adaptive radiation. We address this question by analyzing the distribution of trophic position – estimated from nitrogen and carbon stable isotopes – on the phylogenetic tree of the diverse rockfish genus Sebastes.

Results/Conclusions

Trophic positions of adult rockfish in the northeast Pacific varied from approximately 3 to 4.5. Trophic position diversity does not show any signature of a slowdown in the rate of evolution following an early burst, instead suggesting a model of recurrent evolution within a constrained trophic niche space. This is consistent with a model prediction that early bursts of trophic position are only expected in clades that show much lower levels of omnivory than rockfish. We used a recently introduced method to search for evolutionary shifts between selective regimes, asking whether rockfish trophic niche data suggests attraction to discrete optimum values corresponding to trophic levels. While some lineages of Sebastes appear to have undergone evolutionary shifts in trophic niche space, there is limited evidence that shifts to integer trophic levels are common. This is largely consistent with isotopic niche data in other fish clades, suggesting that discrete higher trophic levels do not generally serve as evolutionary attractors in aquatic food webs. The prevalence of omnivorous networks rather than discrete trophic levels has important implications for the dynamics of food webs at both long and short timescales.