|Pollination in Natural and Fragmented Communities
W. Scott Armbruster, University of Portsmouth
The pollination of tropical flowers occurs in the context of the plant and animal communities in which flowers occur, and thus plant-pollinator relationships are increasingly vulnerable to the effects of human disturbance of those communities. Yet successful pollination is essential for plant reproduction and hence for the maintenance of ecosystem stability and persistence, as well as for the services ecosystems provide. One of the most pervasive forms of community disturbance is fragmentation of tropical habitats. In this context, it is important to understand the sources of sensitivity to disturbance. For example, are specialized pollination relationships are more vulnerable to disturbance? Is this trend related to asymmetries in pollination food webs? This symposium brings together researchers addressing the stability and community ecology of pollination systems in natural communities and those investigating the effects of habitat fragmentation and related disturbances on plant-pollinator relationships.
Character displacement among bat-pollinated flowers of the genus Burmeistera: analysis of mechanism, process, and pattern
Nathan Muchhala and Matthew D. Potts
University of Miami, Coral Gables
Coexisting plants that share pollinators can compete through interspecific pollen transfer. A long-standing idea holds that divergence in floral morphology may reduce this competition by placing pollen on different regions of the pollinators bodies. However, surprisingly little empirical support for this idea exists. Burmeistera is a speciose neotropical genus that exhibits wide interspecific variation in the degree to which the reproductive parts are exserted outside of the corolla. Coexisting Burmeistera share bats as their primary pollinators, and the degree of exsertion determines the site of pollen deposition on the bats heads. Here we study the mechanism, process, and pattern of floral character displacement for assemblages of coexisting Burmeistera. Flight cage experiments with bats and pairs of Burmeistera species demonstrate that the greater the divergence in exsertion length, the less pollen transferred interspecifically. Null model analyses of exsertion lengths for 19 species of Burmeistera across 18 sites (each containing 2-4 species) demonstrate that observed assemblage structure is significantly overdispersed relative to what would be expected by chance. Local evolution, rather than ecological sorting, appears to be the primary process driving this pattern of overdispersion because local adaptation of the nine widespread species accounts for a large portion of the observed pattern. Taken together, results of this study provide strong support for the idea that competition through interspecific pollen transfer can drive character displacement in plants.
Keywords: bat pollination, chiropterophily, floral morphology, competition, Ecuador
Vulnerability of the pollination systems of columnar cacti (tribe Pachycereeae) in arid and semiarid areas of Mexico
Instituto de Ecología, Departamento de Ecología de la Biodiversidad, Universidad Nacional Autónoma de México
Columnar cacti (tribe Pachycereeae) are one of the most conspicuous and important ecological groups of plants over vast areas of arid-semiarid regions of America. The group is distributed from Southwestern United States to Mexico, the West Indies and Northern South America, being south-central Mexico the area with the highest diversity. In this presentation, I synthesize the biogeographical, historical, and the ecological evidence concerning the pollination ecology of this group of plants in order to analyze the geographical patterns of specialization and therefore the degree of vulnerability to human disturbances. The results indicate that the early evolution of columnar cacti occurred during the Tertiary, linked with the evolution of nectar-feeding bats, the most important pollinators. Indeed, 72% of columnar cacti have a batflower syndrome, but the specialization to bat pollination seems to occur only within the tropics, whereas extratropical cacti are pollinated by a wide spectrum of animals including birds, bats and bees. This dichotomy found within and outside the tropics among columnar cacti with bat-pollinated flowers is explained as a consequence of the predictability of pollinators throughout the year. We hypothesize that geological events occurred at the end of the Tertiary period and the large climatic fluctuations occurred during the Quaternary were the most important scenarios that have contributed to the evolution of Pachycereeae and at the same time to the unpredictability of pollinator presence and the evolution of generalized pollination systems out of the tropics. The high degree of vulnerability of biotic interactions contributing to the maintenance of columnar cacti in tropical-dry communities is discussed in terms of cultural practices and human disturbances on nectarfeeding bats the most efficient pollinators and seed dispersers.
Keywords: Columnar cacti, nectar-feeding bats, tribe Pachycereeae, pollination biology, vulnerability to human disturbance
The ecology of “special” rewards in the tropics: plant-pollinator co-specialisation and web symmetry
Scott W. Armbruster
University of Portsmouth
Most plants reward pollinators with nectar or pollen. In the tropics and subtropics, however, a significant proportion of plants offer alternative rewards. These include nutritive rewards, such as oils used by bees for larval provisioning and brood sites for rearing larvae, and non-nutritive rewards, such as fragrances and resin or waxes used by bees for mate attraction or nest construction, respectively. Studies of pollination webs and specialization have focused on nectar and pollen reward systems, largely concluding that generalization is common and that pollination webs may often be asymmetrical (specialist flowers are pollinated by generalist animals and specialist animals visit generalist flowers). This may lead to plants with specialized pollination systems being at no less risk to disturbance-induced extinction than plants with generalized pollination systems. Here I ask: does this pattern also hold for plants with special rewards? The answer seems to be no. Special rewards involve fewer kinds of animals and may lead to specialization; e.g. brood-site systems attract only a few taxa of wasps, moths, or beetles; floral oils attract a few genera of apid bees in the New World and melittid bees in the Old and New Worlds; floral fragrances attract only male euglossine bees (Apidae); floral resins and waxes attract only a few genera of megachilid and/or apid bees. Use of these special rewards by small sets of insect species engenders a degree of specialization in plant-pollinator relationships not seen in most nectar- /pollen-reward systems. There is ample evidence that, despite pre-adaptive origins, most of these systems are co-evolved, at least diffusely. In contrast to nectar and pollen systems, special rewards may often generate symmetrical pollination webs, leading to mutual dependency and greater risk of secondary extinction as a result of disturbance.
Keywords: bees, floral rewards, pollination, pollination-webs
Pollination, plant reproduction, and population genetics in fragmented habitats: facing theory with evidence
Ramiro Aguilar, 1,2, Mauricio Quesada2, Yvonne Herrerias2, Lorena Ashworth1, Leonardo Galetto1, Jorge Lobo,3 and Marcelo Aizen4
3Escuela de Biología, Universidad de Costa Rica
The loss and fragmentation of natural habitats by human activities are pervasive phenomena in terrestrial ecosystems across the Earth and the main drivers of current biodiversity loss. Throughout the last two decades fragmentation studies on plant populations have mainly focused on plant fecundity and plant-pollinator interactions. More recently, population genetic parameters have also been investigated in fragmented and degraded habitats. We quantitatively reviewed the results from fragmentation studies, with the aim of testing (I) whether pollination and reproduction of plant species are differentially susceptible to habitat fragmentation depending on certain reproductive traits that typify the relationship with and the degree of dependence on their pollinators, and (II) whether fragmentation is eroding the genetic composition of fragmented populations. We found an overall large and negative effect of fragmentation on pollination and on plant reproduction. The compatibility system of plants, which reflects the degree of dependence on pollinator mutualism, was the only reproductive trait that explained the differences among the species effect sizes. Self-incompatible plants showed strong negative effects of habitat fragmentation on reproduction, whereas self-compatible plants were not significantly affected. Contrary to theoretical expectations, the effect sizes of plants with different degree of pollination specialization did not differ between them. Both, pollination specialist and generalist had large and negative effects of fragmentation. The overall genetic diversity of plants is negatively affected by fragmentation. No significant effects were found on inbreeding parameters in fragmented population. Allelic richness, in particular, decreased more strongly in self-incompatible species as a consequence of fragmentation. Surprisingly, most papers evaluating genetic effects of fragmentation have evaluated adult individuals in relatively new fragmented landscapes. We evaluate these results in the face of theory and propose possible explanations for the observed trends. Finally we discuss the conservation implications of these findings and give suggestions for future research on this area.
Keywords: Habitat fragmentation, Pollination, Plant reproduction, Conservation genetics, Plant-pollinatorinteraction
The effects of tropical forest fragmentation on euglossine bees in southern Costa Rica
Barry J. Brosi
Given the importance of beesthe most important taxon of pollinatorsto both the persistence of native plant communities and the success of the human agricultural enterprise, it is imperative that we better understand their responses to ongoing global changes, especially in light of the lack of scientific consensus on putative pollinator declines. The euglossine, or orchid, bees (Hymenoptera: Apidae: Euglossini) are a tropical pollinator group of particular ecological salience, especially for long-distance transport of pollen between widely spaced individuals of low-density tropical plants. Despite being well studied relative to other groups of tropical bees, there are only three studies of the response of orchid bees to forest fragmentation; all were in Brazil and considered seven or fewer forest patches. To expand inference from these studies, I sampled male orchid bees with chemical baits over three months in 23 forest fragments in southern Costa Rica, ranging in area from 0.25 ha to 230 ha. Forest fragment area, isolation, landscape context, and elevation all affected euglossine bee communities, though not in a uniform or consistent manner. Orchid bees are more abundant in larger and less isolated forests, and in sites with greater proportions of nearby surrounding forest cover. Estimated species richness of euglossine bees is greater in forest fragments more than one hectare in area. Community similarity of orchid bees was related only to site elevation, when colinearity between site proximity and elevation was considered. The extensive dispersal abilities of euglossine bees likely help to buffer the effects of forest fragmentation on their species diversity, and allow them to utilize even the very smallest forest fragments in the study area.
Keywords: bees, Euglossini, habitat fragmentation, isolation, dispersal
Multiple Allee effects in plant regeneration in a fragmented landscape
The tree Anacardium excelsum was, formerly, widely distributed across the landscape of Guanacaste in Costa Rica, but within this region is now mostly confined to forest fragments that range from a few trees to fragments that are hundreds of hectares in size. We investigated the impact of fragmentation on several stages of regeneration, including pollination, seed production, seed establishment and early seedling growth. We found evidence for multiple Allee effects (disproportionate decline in per capita growth rates at low densities) acting on different regeneration stages, with fragment size having both negative and positive effects at these different life history stages. Furthermore, the vulnerability of later regeneration stages may depend upon impacts at earlier stages, thus Allee effects may be exacerbated or alleviated depending on earlier events. These processes appear to be mediated by fragment size. Consequently, predicting the impact of fragmentation on regeneration of forest trees is likely to be complex and not easily generalisable. Differential and non-independent responses among several reproductive stages necessitate a more careful consideration of Allee effects in the context of fragmentation and life-stage interactions.
Keywords: Allee effect; Anacardium; fragmentation; pollination; regeneration