Hybridization following environmental change: Experimentally exploring the impacts of eutrophication on stickleback species maintenance

Background/Question/Methods

Increased eutrophication in lakes and ponds is an escalating environmental threat worldwide. Eutrophication has been shown to disrupt the delicate balance between selection and gene flow that maintains separation of young species in many natural systems, sometimes resulting in species collapse into hybrid swarms. This has been observed in whitefish, where decreased deepwater habitat forced close proximity of different breeding species, and in African cichlids, where turbidity reduced visual cues necessary for mate selection. However, cases of eutrophication leading to species collapse have yet to be tested experimentally. In this study, I examine the impact of experimental eutrophication on species maintenance of threespine stickleback (Gasterosteus acculeutus) through a large-scale pond experiment. In this experiment, limnetic and benthic species of threespine stickleback were added to large experimental ponds, half of which have been eutrophied using fertilizer. These individuals are left to breed freely and produce offspring for 1.5 years, with fin clips for genomic sampling being taken mid-way and at the end of the experiment. In this experiment, we predicted that 1) the increase in water turbidity resulting from eutrophication will affect mate choice, increasing hybridization rates and 2) that eutrophication will differentially impact overwinter survival of hybrid and pure species.

Results/Conclusions

We found evidence of an increased hybridization rate compared to wild populations in both the control and treatment conditions of this experiment. Preliminary results suggest a small but insignificant increase in hybridization when benthic and limnetic individuals are breeding under the fertilizer treatment. However, we found no increase in hybrid survival over winter compared to benthic or limnetic individuals. These results are consistent with research showing that small disruptions to habitat can impact fidelity of mate choice in young species, but that this does not necessarily translate into higher hybrid fitness.