The fitness consequences of vertical migration behaviour: How important is spatial heterogeneity to maintaining multiple behavioural strategies in a natural population of Daphnia pulicaria?
Theory and experiments suggest that spatial heterogeneity in the environment can help maintain multiple behavioural strategies. Zooplankton undergo daily vertical migration behaviours which vary depending on the environmental gradients and predation pressure in a lake. Though typically only one migration behaviour is observed per lake, zooplankton in some lakes show multiple migration behaviours that appear to coexist for extended periods of time. In a lake with two distinct behaviours, we examined the importance of strong environmental gradients to the fitness of distinct Daphnia pulicaria phenotypes exhibiting either a shallow-water migration or a deep-water migration. Using specially designed migration robots to manipulate migration, we performed fitness bioassays where each phenotype underwent either their natural migration behaviour or that of their competitor for a period of two weeks. Population growth rates were estimated by measuring biomass at days 0, 7 and 14. We expected the fitness of populations undergoing their natural migration behaviour to be similar, since the phenotypes co-occur seasonally. We expected the fitness of shallow migrators undergoing the deep migration to suffer due to the low hypolimnetic oxygen availability. We expected deep migrators undergoing the shallow migration to have similar fitness to shallow migrators in their natural migration.
Deep migrators had higher fitness than shallow migrators across all treatments, with positive average growth rates. Shallow migrators undergoing their natural behaviour had an average growth rate close to zero, while shallow migrators in the deep migration had a negative average growth rate. These results demonstrate that in the absence of predation, there is no apparent fitness advantage to choosing one migration behaviour over the other for deep migrators as they sustain indistinguishable positive average growth rates across treatments. By contrast, shallow migrators were handicapped when forced to migrate into the food-rich but oxygen-poor hypolimnion. This indicates that in the absence of predation, deep migrators should overtake shallow migrators as the dominant phenotype since they display higher growth rates regardless of migration behaviour. This result supports previous work showing that the deep migrators slowly outcompete the shallow migrators from spring to fall. Thus, by experimentally forcing phenotypes to undergo the migration behaviour of their competitor, our experiments reveal that strong environmental gradients prevent shallow migrators from outcompeting the deep migrators, but do not impact fitness of the deep migrator, suggesting that other biotic interactions (e.g., predation) may be preventing deep migrators from excluding shallow migrators.