Plant-pollinator Associations Research Articles

MEASURING BEE DIVERSITY IN DIFFERENT EUROPEAN HABITATS AND BIOGEOGRAPHICAL REGIONS

Bee pollinators are currently recorded with many different sampling methods. However, the relative performances of these methods have not been systematically evaluated and compared. In response to the strong need to record ongoing shifts in pollinator diversity and abundance, global and regional pollinator initiatives must adopt standardized sampling protocols when developing large‐scale and long‐term monitoring schemes.We systematically evaluated the performance of six sampling methods (observation plots, pan traps, standardized and variable transect walks, trap nests with reed internodes or paper tubes) that are commonly used across a wide range of geographical regions in Europe and in two habitat types (agricultural and seminatural). We focused on bees since they represent the most important pollinator group worldwide. Several characteristics of the methods were considered in order to evaluate their performance in assessing bee diversity: sample coverage, observed species richness, species richness estimators, collector biases (identified by subunit‐based rarefaction curves), species composition of the samples, and the indication of overall bee species richness (estimated from combined total samples).The most efficient method in all geographical regions, in both the agricultural and seminatural habitats, was the pan trap method. It had the highest sample coverage, collected the highest number of species, showed negligible collector bias, detected similar species as the transect methods, and was the best indicator of overall bee species richness. The transect methods were also relatively efficient, but they had a significant collector bias. The observation plots showed poor performance. As trap nests are restricted to cavity‐nesting bee species, they had a naturally low sample coverage. However, both trap nest types detected additional species that were not recorded by any of the other methods.For large‐scale and long‐term monitoring schemes with surveyors with different experience levels, we recommend pan traps as the most efficient, unbiased, and cost‐effective method for sampling bee diversity. Trap nests with reed internodes could be used as a complementary sampling method to maximize the numbers of collected species. Transect walks are the principal method for detailed studies focusing on plant–pollinator associations. Moreover, they can be used in monitoring schemes after training the surveyors to standardize their collection skills.

Temporally persistent patterns of incidence and abundance in a pollinator guild at annual and decadal scales: the bees of Larrea tridentata

A recent alternative model to conventional coevolution, the geographic mosaic theory of coevolution, posits that reciprocal selection is episodic and local, rather than persistent and spatially extensive. However, little empirical evidence addresses this model's tenets, in particular the temporal stability of local plant–pollinator interactions. We evaluated this tenet using the richly diverse guild of bees at creosote bush (Larrea tridentata), a generalist plant that hosts many specialist bees. We systematically resampled over 2–5 years at 11 sites across the south-western USA. Incidence and abundance also were compared for survey sites sampled 20 ± 2 years earlier. Average Morisita-Horn faunal similarities of local bee guilds was 87% for sequential years and 36% after 20 years. Similarities in taxonomic composition of resampled local bee guilds could be statistically represented as a random assemblage drawn from the regional source pool of 54–68 bee species that could be expected at Larrea, weighted by regional abundance. At every site, only the minority of abundant bee species was typically persistent in local guilds, even after > 20 years. Most bee species in the Larrea guild were chronically uncommon, geographically sporadic and temporally unpredictable, attributes that render them numerically inconsequential as pollinators in their local guild. Persistence among abundant bee species in local pollinator assemblages satisfies one condition by which reciprocal selection could act locally. The well-being of these more abundant core species, and not bee diversity per se, may better characterize the health of such plant–pollinator associations. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85, 319–329.

Carbon allocation to volatiles and other reproductive components in male Ficus carica (Moraceae)

Volatile compounds are often mediators of plant-pollinator interactions. Their emission is presumed to be costly, but this cost has seldom been quantified. Figs of Ficus carica (a dioecious species) release volatile compounds when receptive, thus attracting the agaonid wasp Blastophaga psenes. In dioecious fig species, wasps lay eggs inside male figs and pollinate female ones. For a male tree, we estimated carbon allocation to reproduction using the annual growth module (AGM) as the unit of measurement. Over the growing season, leaf and fig carbon exchange and construction costs were measured, as well as carbon investment in stamens, provisioning pollinators, and biosynthesis and release of volatile compounds. Representativity of the tree studied was evaluated by measuring some of these parameters on seven other male fig trees. The results show that 7.6-16.4% of the carbon assimilated by leaves and figs was invested in reproduction. Of the carbon invested in reproduction, pollinator attraction and feeding represented only 0.08-0.12% and 1.84-2.33%, respectively, while pollinator sheltering (fig construction and respiration) represented 97.6-98.0%. In this strict and coevolved plant-pollinator association, the main male reproductive investment was thus in the structures sheltering the associated pollinators.

Competition Between Distantly Related Taxa In the Coevolution of Plants and Pollinators

Competition among distantly related plants for pollinators and among distantly related animals for pollen and nectar plays a potentially important role in the organization of ecological communities and the coevolution of plant-pollinator relationships. Plants which rely on animals to disperse their pollen potentially compete for pollinators by processes similar to interference and exploitative competition. Coexisting plant species may evolve to avoid or reduce such competition by character displacement in floral morphology and/or phenology. One important difference between competition for pollinators and most other kinds of competition is that pollinator resources are not used up and made absolutely unavailable to competitors. Consequently, plant species can potentially overlap completely in their utilization of pollinators. The disadvantages of competing apparently are sometimes outweighed by the advantages of sharing pollinators, because distantly related plant species frequently show evolutionary convergence in floral morphology, blooming time and nectar rewards to utilize the same pollinators. Distantly related animal taxa may compete for floral nectar and pollen by both interference and exploitation. The mechanisms of such competition depend primarily on the energetic costs and benefits of foraging and aggression. Exploitative competition is very important because nectar feeders of small body size and low energy requirements can forage economically and reduce nectar availability to levels that will not support larger animals. Thus small nectarivores often can exclude larger competitors from flowers to which both taxa have equal access. Plants may evolve to influence the outcotre of competition among animal visitors and favor species that provide the best pollination services. Thus flowers specialized for pollination by large animals often show morphological or phenological specializations which make rewards unava'lable to smaller animals. Interference is adaptive only when the benefits of exclusive use of a resource outweig.i the costs of defending it. Because distantly related kinds of flower visitors often differ in body size and energetic requirements, interference competition among them is probably rare although it is often important among closely related nectarivores. The community level consequences of competition in the ecology and evolution of plant-pollinator associations are still poorly understoood. Competition among distantly related pollinators for plant floral rewards appears to play a major role, but competition among plants for pollinator services may be only a weak force. Although the basic interaction between plant and pollinator usually is a mutualistic one, certain species of both plants and animals parasitize this interaction and compete with the mutualists for limited resources. Thus some animals rob nectar and pollen and compete with legitimate pollinators without providing pollination services. Similarly, some plants offer no floral rewards but obtain pollinator services by mimicing rewarding flowers of other species. The effects of these kinds of interactions on the organization of communities of plants and pollinators provide a fertile area for future research.