James Hagan

Premise Numerous studies have found a positive association between dioecy and polyploidy; however, this association presents a theoretical conflict: While polyploids are predicted to benefit from self‐reproduction for successful establishment, dioecious species cannot self‐reproduce. We propose a theoretical framework to resolve this apparent conflict. We hypothesize that the inability of dioecious species to self‐reproduce hinders their establishment as polyploids. We therefore expect that genera with many dioecious species have fewer polyploids, leading to a negative association between polyploidy and dioecy across genera. Methods We used three publicly available databases to determine ploidy and sexual systems for 131 genera and 546 species. We quantified (1) the relationship between the frequency of polyploid species and the frequency of dioecious species across genera, and (2) the proportion of polyploids with hermaphroditism and dioecy across species, adjusting for phylogenetic history. Results Across genera, we found a negative relationship between the proportion of polyploids and the proportion of dioecious species, a consistent trend across clades. Across all species, we found that sexual system (dioecious or not) was not associated with polyploidy. Conclusions Polyploids are rare in genera in which the majority of species are dioecious, consistent with the theory that self‐reproduction favors polyploid establishment. The low frequency of polyploidy among dioecious species indicates the association is not as widespread as previously suggested. Our findings are consistent with previous studies identifying a positive relationship between the two traits, but only if polyploidy promotes a transition to dioecy, and not the reverse.

The spatial insurance hypothesis states that at landscape scales with environmental variability between different places, biodiversity increases ecosystem functioning if species respond asynchronously to environmental variation, and they are highest functioning in places where they dominate the relative abundance. Under this hypothesis, observed species turnover between places in a landscape with environmental variation might suggest that species diversity is an important driver of landscape‐scale ecosystem functioning. However, the spatial insurance implicitly assumes that species found in one place in the landscape will not be able to maintain high functioning in other places. Given this assumption of the spatial insurance hypothesis, we would predict that species' functioning in monoculture should decline if transplanted to a different place on an environmental gradient, away from where they naturally dominate. If this is the case, we would expect that the loss of one species in one place of the environmental gradient could not be compensated through the establishment of another species in that place. We tested this prediction using a model system of marine macroalgae on intertidal rocky shores on the Swedish west coast. We performed a reciprocal transplant experiment with adult individuals of four fucoid seaweeds that dominate the standing stock biomass at different depths on these shores and monitored their relative growth rate over 2 months. Counter to the assumptions made by the spatial insurance hypothesis, growth rates for three of the four species showed limited responses to being transplanted to different depth zones. Spatial insurance may, hence, play a minor role in sustaining landscape ecosystem functioning in this and other systems. Synthesis . The functional consequences of species loss at landscape scales may not be as obvious as observational studies and ecological models suggest. Consequently, natural patterns of species turnover should not be used directly to argue for the role of biodiversity at landscape scales. Instead, how species may or may not be able to compensate for the loss of other species is a critical aspect if we are to understand how changes in biodiversity at the landscape scale affect ecosystem functioning.