One of ecology’s fundamental challenges is to understand how interacting species coexist. Be they predator and prey, herbivore and plant, or a pair of competitors, interacting species appear to accommodate each other somehow such that they not only persist, but coexist in repeatable patterns of population densities and ecological niches. A rich vein of theory has laid out the conditions for coexistence and shown that those conditions might be more easily met in some interactions than others. Yet despite decades of effort, we still have paradoxical situations in which a pair of species coexists when all of the available theory predicts they ought not.
Nowhere is this paradox more striking than the case of intraguild predation. Intraguild predation (IGP) occurs when two species compete for resources with each other and one preys upon the other. IGPs can be unidirectional, in which only one of the species acts as a predator, or bidirectional, in which each species preys upon the other. These interactions are often separated in the life cycle of the animals. For example, two species may compete as adults but neonates and juveniles of one species are preyed upon by adults of the other. They can also be determined by body sizes; two individuals of similar size might be competing but larger individuals of one species might prey upon smaller individuals of the other. Theory predicts that there is a narrow range of conditions in which species engaged in an IGP interaction should be able to coexist. Yet, IGP interactions are widespread among many animal taxa.
One path to resolving this paradox might be the action of rapid evolutionary feedbacks, in which strong selection from a species interaction affects both numerical dynamics and the dynamics of allele frequencies. Rapid evolutionary feedbacks can stabilize pairwise interactions that would be unstable in the absence of such feedbacks. For an IGP, in which the nature of the interaction might depend largely on the numbers and body size distributions of the interacting species, rapid evolutionary feedbacks might be the key to understanding why theory and observation are so far apart. IGPs represent a very strong species interaction, one likely to generate strong natural selection on its participants, and so rapid evolution seems a very likely occurrence when two species collide in an IGP.
Guppies and Hart’s Killifish comprise a well-recognized IGP and our project offers an ideal opportunity to test the importance of rapid evolution for resolving the paradox of coexistence. We know that these two species coexist in the higher tributaries of the rivers of the Northern Range. Obviously this coexistence represents another challenge to the theory for coexistence in IGPs. But in addition, a variety of observational and experimental evidence has suggested that coexistence is not easily achieved. So has rapid evolution brought these species into an accommodation that leads to coexistence? As it happens, our experimental introductions of guppies into habitats with killifish offer us the opportunity to see if this happens and test ecological and evolutionary theories for coexistence.
This is the topic of our current award from the National Science Foundation. We are taking a diversity of approaches to addressing this big question. First, we are performing mesocosm experiments testing combinations of killifish and guppies with different genetic backgrounds. Second, we are performing a variety of short term experiments to understand better the nature of the interaction between these species when interacting individuals vary in body size. These include manipulations of entire pools as well as small experiments in aquaria on competitive ability. Third, we are attempting to estimate the trophic relations of these species within their communities where they coexist and where only one occurs. Fourth, we are continuing our long-term censuses of guppies and killifish. With this work we can quantify the numerical dynamics of these both species in our introduction sites and watch as an interaction that appeared very one-sided in the early years - guppies were reducing the size of the killifish populations - appears to change such that killifish numbers stabilize and, perhaps, rebound.
Nowhere is this paradox more striking than the case of intraguild predation. Intraguild predation (IGP) occurs when two species compete for resources with each other and one preys upon the other. IGPs can be unidirectional, in which only one of the species acts as a predator, or bidirectional, in which each species preys upon the other. These interactions are often separated in the life cycle of the animals. For example, two species may compete as adults but neonates and juveniles of one species are preyed upon by adults of the other. They can also be determined by body sizes; two individuals of similar size might be competing but larger individuals of one species might prey upon smaller individuals of the other. Theory predicts that there is a narrow range of conditions in which species engaged in an IGP interaction should be able to coexist. Yet, IGP interactions are widespread among many animal taxa.
One path to resolving this paradox might be the action of rapid evolutionary feedbacks, in which strong selection from a species interaction affects both numerical dynamics and the dynamics of allele frequencies. Rapid evolutionary feedbacks can stabilize pairwise interactions that would be unstable in the absence of such feedbacks. For an IGP, in which the nature of the interaction might depend largely on the numbers and body size distributions of the interacting species, rapid evolutionary feedbacks might be the key to understanding why theory and observation are so far apart. IGPs represent a very strong species interaction, one likely to generate strong natural selection on its participants, and so rapid evolution seems a very likely occurrence when two species collide in an IGP.
Guppies and Hart’s Killifish comprise a well-recognized IGP and our project offers an ideal opportunity to test the importance of rapid evolution for resolving the paradox of coexistence. We know that these two species coexist in the higher tributaries of the rivers of the Northern Range. Obviously this coexistence represents another challenge to the theory for coexistence in IGPs. But in addition, a variety of observational and experimental evidence has suggested that coexistence is not easily achieved. So has rapid evolution brought these species into an accommodation that leads to coexistence? As it happens, our experimental introductions of guppies into habitats with killifish offer us the opportunity to see if this happens and test ecological and evolutionary theories for coexistence.
This is the topic of our current award from the National Science Foundation. We are taking a diversity of approaches to addressing this big question. First, we are performing mesocosm experiments testing combinations of killifish and guppies with different genetic backgrounds. Second, we are performing a variety of short term experiments to understand better the nature of the interaction between these species when interacting individuals vary in body size. These include manipulations of entire pools as well as small experiments in aquaria on competitive ability. Third, we are attempting to estimate the trophic relations of these species within their communities where they coexist and where only one occurs. Fourth, we are continuing our long-term censuses of guppies and killifish. With this work we can quantify the numerical dynamics of these both species in our introduction sites and watch as an interaction that appeared very one-sided in the early years - guppies were reducing the size of the killifish populations - appears to change such that killifish numbers stabilize and, perhaps, rebound.
