Experimental test of hybrid fitness change as a cause of species collapse following species invasion

Recent years have seen increased incidences of hybridization between previously stable sympatric species, sometimes leading to collapse. The causes, and particularly the role of improved hybrid fitness in a changing environment, are poorly known. A recent example is the sympatric stickleback species pair of Enos Lake, Vancouver Island, which collapsed into a hybrid swarm following the invasion of the lake by American signal crayfish. Environmental changes can increase hybridization through increased inter-species mating or enhanced hybrid fitness, though few mechanisms have been experimentally tested. Using mesocosms, we tested whether crayfish addition alters the prey community and changes F1 hybrid fitness relative to pure limnetic and benthic threespine stickleback. Crayfish addition depleted benthic invertebrate biomass and negatively impacted survival of all three stickleback cross types. Crayfish had little effect on relative survival but led to a higher growth rate of hybrids relative to pure species. This improvement in hybrid fitness is unlikely to be the sole reason for the Enos Lake collapse, as breakdowns in premating isolation might have also been crucial. Nevertheless, this work provides a rare experimental demonstration of a causal link to changes in hybrid fitness, providing evidence that reduced selection against hybrids has contributed to the collapse.