Diversity of floral visitors to sympatric Emphasi

Authors: Mariana Cuautle John N Thompson

Publish Date: 2009/08/11

Volume: 162, Issue: 1, Pages: 71-80

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Abstract

Most coevolving relationships between pairs of species are embedded in a broader multispecific interaction network The mutualistic interaction between Lithophragma parviflorum Saxifragaceae and its pollinating floral parasite Greya politella Lepidoptera Prodoxidae occurs in some communities as a pairwise set apart from most other interactions in those communities In other communities however this pair of species occurs with congeners and with other floral visitors to Lithophragma We analyzed local and geographic differences in the network formed by interactions between Lithophragma plants and Greya moths in communities containing two Lithophragma species two Greya species and floral visitors other than Greya that visit Lithophragma flowers Our goal was to evaluate if nonGreya visitors were common if visitor assembly differs between Lithophragma species and populations and if these visitors act as effective pollinators Sympatric populations of L heterophyllum and L parviflorum differ in floral traits that may affect assemblies of floral visitors Visitation rates by nonGreya floral visitors were low and the asymptotic number of visitor species was less than 20 species in all populations Lithophragma species shared some of the visitors with visitor assemblages differing between sites more for L heterophyllum than for L parviflorum Pollination efficacy experiments showed that most visitors were poor pollinators Single visits to flowers by this assemblage of species resulted in significantly higher seed set in Lithophragma heterophyllum 306 ± 39 SE than in L parviflorum 47 ± 34 SE This difference was consistent between sites suggesting that these visitors provide a better fit to the floral morphology of L heterophyllum Overall none of the nonGreya visitors appears to be either sufficiently common or efficient as a pollinator to impose strong selection on any of these four Lithophragma populations in comparison with Greya which occurs within almost all populations of these species throughout their geographic rangesAs evidence of coevolution has accumulated it has become clear that most coevolving relationships between pairs of species are shaped by interactions with yet other taxa Even the most commonly cited examples of extreme coevolution between pairs of species are known to be molded by a broader network of interactions Examples include nematodes that attack pollinating fig wasps Herre 1993 copollinators that alter onetoone mutualism between particular pollinators and plants Thompson and Cunningham 2002 and squirrels that can locally disrupt coevolution between lodgepole pines and crossbills Benkman et al 2001 The effect of other species on pairwise interactions often varies among populations creating the potential for a multispecific geographic mosaic of coevolution in many interactions Thompson 1994 2005In some interactions such as pollination mutualisms these geographically varying networks of species develop through a combination of diversification of closely related taxa and convergence of traits of unrelated species eg “pollination syndromes” In fact recent analyses of pollination and other mutualistic networks have shown that such mutualisms among freeliving species often result in much less compartmentalization than occurs in antagonistic interactions such as those between predators and prey Bascompte et al 2003 Jordano et al 2003 Guimarães et al 2006 Lewinsohn et al 2006 There is however always some compartmentalization in mutualistic networks Many of these networks include modules composed of a closely related group of species eg two or more congeneric plant species that interacts with another group of closely related species eg two or more congeneric pollinator species The number of species involved in these mutualistic modules often varies geographically ranging from a simple pairwise interaction in some communities to trios quartets or larger sets of interacting species in other communities As these interactions continue to diversify through speciation and secondary contact they could either continue to coevolve as a tight mutualistic network or change fundamentally in ways that allow incorporation of other unrelated species into the interactionHere we evaluate if a pairwise pollination mutualism has diversified to incorporate unrelated species as the mutualistic pair has undergone speciation and then reassembled in local communities as small sets of coexisting congeners Specifically we analyze local and geographic differences in the floral visitor network formed by two species of woodland star Lithophragma heterophyllum and L parviflorum Saxifragaceae and their assemblages of floral visitors The two Lithophragma species differ in multiple floral traits and they cooccur in communities that differ in overall floristic and faunistic composition providing an opportunity to evaluate how divergence in traits and community context may have reshaped the role of copollinators in these interactionsWe assessed visitation patterns to Lithophragma species in two widely separated communities where L heterophyllum and L parviflorum are sympatric In both communities Greya moths visit the flowers of both species and then lay their eggs in the plant tissues They spend much of their adult lives either nectaring resting or ovipositing on their hostplants In the year of this study 77–90 of L parviflorum plants received Greya eggs and 16–21 of L heterophyllum plants received eggs K Rich and J N Thompson unpublished data Our goal was to evaluate whether nonGreya floral visitors are also common visitors to these plants whether their visitation patterns differ between the two Lithophragma species whether the patterns were similar in the two communities and whether the most common of these visitors could act as effective pollinators as has been shown in some northern regions where only L parviflorum occursLithophragma Saxifragaceae is endemic to western North America comprising seven to nine species Taylor 1965 Kuzoff et al 1999 Lithophragma parviflorum which is the most derived species within the genus Kuzoff et al 1999 has the widest geographic distribution and encompasses the ranges of most of the other Lithophragma species It occurs in steppe grasslands savannas and woodlands from southern British Columbia to California and eastward into the Rocky Mountains Several subspecies have been recognized and these have sometimes been considered as separated species but molecular studies Kuzoff et al 1999 and crossing studies S Dwiggins and J N Thompson unpublished data have indicated that these populations form a single biological species In contrast Lithophragma heterophyllum which is a more basal species within the genus Kuzoff et al 1999 is restricted mostly to woodlands in the Coast Ranges of California It usually grows in partly or fully shaded welldrained slopes of oak or mixed coniferousoak woodland extending up to 1370 m elevation

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