What factors determine species range limits has long been an interesting question to biologists, which also has critical implications for understanding species range expansions under climate change. Both abiotic and biotic factors might determine species range limits, yet a comprehensive investigation of all relevant factors is in need. To fill this knowledge gap, we studied the effects of winter climate, herbivory, seed predation and soil microbes on oak species range limits using a combination of transplant experiment, field survey, and greenhouse experiment. 1) To test the effect of winter climate, we measured the germination rates of three oak species beyond their ranges, and compared against those of the other three sister species that are native to the experiment site. 2) To test the effect of herbivory, we transplanted seedlings of the same range-restricted species beyond their ranges, and compared herbivory rates against those of the other three native species. 3) To test the effect of seed predation, we compared the proportion of seed predation at the species range center vs. at the range edge. 4) To test the effect of soil microbes, we compared seedling survival rates when grown in same-species soil, sister-species soil, and control sterile soil.
Results/Conclusions
1) In the transplant experiment, we found that winter climate beyond species ranges did not suppress germination of the species transplanted beyond their ranges, as compared to the native species, which suggests that winter climate does not set species range limits. 2) We found that herbivory had the same strong negative effect at the seeding stage on species transplanted beyond their ranges, as compared to the native species, which indicates that herbivory does not set range limits. 3) In the field survey, we found that edge populations suffered a similar degree of seed predation as the central populations, suggesting that seed predation does not set species range limits. 4) Finally, in the greenhouse experiment, we found that when seedlings of range-restricted species were grown with soil inoculum from the widespread sister species, the seedlings suffered from lower survival rates than when grown with soil inoculum from its own species.
We conclude that soil microbes can set species range limits for range-restricted species through “apparent competition”, while winter climate, herbivory and seed predation do not seem to directly constrain species ranges. Our study sheds light on the importance of belowground biotic interactions in plant species distributions.