Plant-pollinator networks: Adding the pollinator's perspective

Bosch J., Martín González A.M., Rodrigo A., Navarro D. (2009) Plant-pollinator networks: Adding the pollinator's perspective. Ecology Letters. 12: 409-419.
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Doi: 10.1111/j.1461-0248.2009.01296.x

Resum:

Pollination network studies are based on pollinator surveys conducted on focal plants. This plant-centred approach provides insufficient information on flower visitation habits of rare pollinator species, which are the majority in pollinator communities. As a result, pollination networks contain very high proportions of pollinator species linked to a single plant species (extreme specialists), a pattern that contrasts with the widely accepted view that plant-pollinator interactions are mostly generalized. In this study of a Mediterranean scrubland community in NE Spain we supplement data from an intensive field survey with the analysis of pollen loads carried by pollinators. We observed 4265 contacts corresponding to 19 plant and 122 pollinator species. The addition of pollen data unveiled a very significant number of interactions, resulting in important network structural changes. Connectance increased 1.43-fold, mean plant connectivity went from 18.5 to 26.4, and mean pollinator connectivity from 2.9 to 4.1. Extreme specialist pollinator species decreased 0.6-fold, suggesting that ecological specialization is often overestimated in plant-pollinator networks. We expected a greater connectivity increase in rare species, and consequently a decrease in the level of asymmetric specialization. However, new links preferentially attached to already highly connected nodes and, as a result, both nestedness and centralization increased. The addition of pollen data revealed the existence of four clearly defined modules that were not apparent when only field survey data were used. Three of these modules had a strong phenological component. In comparison to other pollination webs, our network had a high proportion of connector links and species. That is, although significant, the four modules were far from isolated. © 2009 Blackwell Publishing LtdCNRS.

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Relationship between body size and homing ability in the genus Osmia (Hymenoptera; Megachilidae)

Guédot C., Bosch J., Kemp W.P. (2009) Relationship between body size and homing ability in the genus Osmia (Hymenoptera; Megachilidae). Ecological Entomology. 34: 158-161.
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Doi: 10.1111/j.1365-2311.2008.01054.x

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1. The maximum homing ability of female bees, that is, their capacity to return to the nest after being displaced a certain distance, is considered to be an estimate of their maximum foraging distance. 2. The present study provides data on homing ability and body weight for Osmia lignaria and combines them with data for five other congenerics, O. rufa, O. cornuta, O. pedicornis, O. cornifrons, and O. emarginata for analysis. These species are important pollinators of spring-flowering plants, and some have been developed as commercial crop pollinators. 3. It is shown that homing ability is positively and linearly related to body weight (r2 = 0.81; P = 0.01). 4. These results should be of use in selecting Osmia species as potential crop pollinators and establishing adequate buffer distances around genetically modified crops.

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A geographic selection mosaic in a generalized plant-pollinator-herbivore system

Gómez J.M., Perfectti F., Bosch J., Camacho J.P.M. (2009) A geographic selection mosaic in a generalized plant-pollinator-herbivore system. Ecological Monographs. 79: 245-263.
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Doi: 10.1890/08-0511.1

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The concept of Selection Mosaic is central to the Geographic Mosaic Theory of Coevolution. Most information on coevolving interactions, however, comes from specialized organisms. In contrast, an accurate understanding of the effect of geographically varying evolutionary dynamics on the evolution of generalist organisms is lacking, although these kinds of organisms are the most frequent in nature. In flowering plants, pollinators and herbivores are important selective agents. In this study we investigate whether a geographic selection mosaic for floral traits in a generalist plant, Erysimum mediohispanicum (Brassicaceae), can be mediated by the interplay of mutualistic and antagonistic interacting organisms. In eight populations we quantified the selection exerted by these organisms on several plant traits. We found significant spatial variation in pollinator assemblage. In different populations, the main pollinators belonged to different functional groups (beeflies, large bees, small bees, and beetles). Damage by ungulates also varied among populations. Consequently, we found that different populations were under different selective regimes, and the traits affected by selection depended on the local interaction intensity with pollinators and mammal herbivores. Some traits, such as flower number and stalk height, were selected similarly in most populations. Other traits, such as corolla diameter and tube length, were selected only in some populations. Finally, we found divergent selection for some traits, such as corolla tube width and corolla shape, which were selected in contrasting directions in different localities. This spatial variation in selective scenarios results in populations with strong selective regimes (hot spots) intermingled with populations with weak selective regimes (cold spots). Four important outcomes emerge from the E. mediohispanicum selection mosaic. (1) Interactions with generalist organisms may produce strong selection. (2) Spatial changes in main pollinators result in divergent selection across populations. (3) Geographic mosaics depend on a balance between mutualistic and antagonistic selection. (4) Selection mosaics operate at fairly small spatial scales. These findings will surely contribute to expanding the conceptual framework of the Geographic Mosaic Theory of Coevolution. © 2009 by the Ecological Society of America.

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The functional consequences of diversity in plant-pollinator interactions

Perfectti F., Gómez J.M., Bosch J. (2009) The functional consequences of diversity in plant-pollinator interactions. Oikos. 118: 1430-1440.
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Doi: 10.1111/j.1600-0706.2009.17491.x

Resum:

The role of biological diversity in maintaining ecosystem functioning is a central issue in ecology. Most studies on diversity-functioning relationships have focused on ecosystem and community levels, leaving the extension of those relationships to other organization levels, such as populations, as a challenging and unsolved issue. Empirical studies have shown links between pollinator diversity and plant fecundity, suggesting that a diversity- functioning relationship at the population level may occur in pollination systems. We theoretically explored the effect of pollinator diversity on plant reproduction. We found that low pollinator diversity is beneficial when the most abundant pollinators are the most effective. In contrast, when the most effective pollinators are not the most abundant, we found an optimal value of pollinator diversity at which plant fecundity is maximized. When we parametrized our model with real data, we obtained that an increase in pollinator diversity was beneficial for the reproduction of some plants whereas it was harmful for other plants, the outcome depending exclusively on the differences in effectiveness among pollinators. Consequently, our theoretical approach suggests that in pollination systems the diversity-function relationship may be explained as the consequence of the interaction between among-pollinator differences in effectiveness and frequency of interaction, without the need to invoke additional ecological mechanisms.

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