Habitat partitioning through reproductive interference in Geranium

We’re familiar with seeing insects flit from one flower to the next, carrying pollen as they do so. What happens when they carry pollen to the wrong flower species? Sachiko Nishida and colleagues have been testing some predictions on how reproductive interference in Geranium species could explain how clumps of specific species form.

The idea that Nishida and colleagues tested was how pollen landing on the wrong plant can alter reproductive success, and what plants form the next generation. If there is no problem with heterospecific pollen, then accidental deliveries don’t hamper your reproductive chances. In this situation, there’s no problem mixing with other plants. If there’s weak interference, than a couple of other species as neighbours aren’t a problem, but you wouldn’t want to be surrounded by them. At this scale you’ll see some local partitioning. If the interference is strong, then so too will be the separation.

“With these considerations in mind, we argue that reproductive interference has sometimes been the driving force behind the present observed distributions of not only native/alien species pairs but also native/native species pairs,” write Nishida and colleagues. “[W]e focus on interactions between two species of Geranium, G. thunbergii and G. wilfordii, native to Japan. At large scale, the distributions of the two species appear to overlap…, but our preliminary field survey indicated that they likely occupy separate habitats.”

“We conducted a field survey to investigate the effects of reproductive interference between G. thunbergii and G. wilfordii on seed set in the two species, together with hand pollination experiments to examine the effect of heterospecific pollen deposition on seed set. We also examined whether genotypic variations in the putative hybrid and in its postulated parent species support its identification as a hybrid. Then, referring to the genotype results, we estimated the frequency of hybridization in offspring resulting from artificial pollination with mixed (conspecific and heterospecific) pollen of G. thunbergii and G. wilfordii. In addition, we compared seed set and the proportion of pollen grains with apertural protrusions between the pure species and the putative hybrids to evaluate the relative fertility of the putative hybrids.”

“Finally, at one study site, we measured the exact locations of individual plants along a transect to examine the degree of spatial partitioning between the two Geranium species.”

The team found that weak interference did indeed explain some of the habitat partitioning in Geranium. Another factor was the existence of a hybrid zone, with the hybrids being less fit than the parent species, which also worked to separate the species.“Our findings suggest that ongoing reproductive interference can be recognized even between native wild plant species if its intensity is not very strong and their distributional relationships are examined at an appropriate scale. However, when reproductive interference is intensive or unidirectional, it would still be difficult to identify interference as the key driving force behind spatial partitioning observable at present in the wild, because it would rapidly lead to exclusive distributions. We do not yet know how to detect the ‘ghost of reproductive interference past’ (analogous to the ‘ghost of competition past’) between wild plant species or how to prove that past reproductive interference was responsible for exclusive distributions observed in the present. Nevertheless, we have some hope that it may be possible to discover some trace of the ‘ghost’, especially if hybrids still exist.”