A test of ecological selection against young-of-the-year hybrids of sympatric sticklebacks

Ecological selection against hybrids, the reduction in hybrid fitness attributed solely to environmental factors, was tested by introducing young-of-the-year benthic, limnetic and F₁ hybrid sticklebacks Gasterosteus aculeatus to divided experimental ponds and lake enclosures. The frequency of hybrids in samples taken at the end was significantly lower than their frequency at introduction. Hybrid survival was significantly lower in pond-sides in which they were initially the most common cross type than in pond-sides in which they were initially rare, suggesting that hybrid survival may be frequency-dependent. Growth rate of F₁ hybrids was marginally lower than benthic growth rates, being significantly lower than in ponds and not different in lake enclosures. The diet of hybrids overlapped with both parent species in ponds and with benthic diets in lake enclosures. The results suggest that ecological selection is acting against young-of-the-year hybrid sticklebacks.