Literature
A selection of papers about plant - pollinator interactions. In progress!
Bascompte, J., P. Jordano, et al. (2003). "The nested assembly of plant-animal mutualistic networks." Proceedings of the National Academy of Sciences of the United States of America100(16): 9383-9387.
Most studies of plant-animal mutualisms involve a small number of species. There is almost no information on the structural organization of species-rich mutualistic networks despite its potential importance for the maintenance of diversity. Here we analyze 52 mutualistic networks and show that they are highly nested; that is, the more specialist species interact only with proper subsets of those species interacting with the more generalists. This assembly pattern generates highly asymmetrical interactions and organizes the community cohesively around a central core of interactions. Thus, mutualistic networks are neither randomly assembled nor organized in compartments arising from tight, parallel specialization. Furthermore, nestedness increases with the complexity (number of interactions) of the network: for a given number of species, communities with more interactions are significantly more nested. Our results indicate a nonrandom pattern of community organization that may be relevant for our understanding of the organization and persistence of biodiversity.
Johnson, S. D. and K. E. Steiner (2000). "Generalization versus specialization in plant pollination systems." Trends in Ecology & Evolution15(4): 140-143.
The long-standing notion that most angiosperm flowers are specialized for pollination by particular animal types, such as birds or bees, has been challenged recently on the basis of apparent widespread generalization in pollination systems. At the same time, biologists working mainly in the tropics and the species-rich temperate floras of the Southern hemisphere are documenting pollination systems that are remarkably specialized, often involving a single pollinator species. Current studies are aimed at understanding: (I)the ecological forces that have favoured either generalization or specialization in particular lineages and regions; (2) the implications for selection on floral traits and divergence of populations; and (3) the risk of collapse in plant-pollinator mutualisms of varying specificity.
Jordano, P., J. Bascompte, et al. (2003). "Invariant properties in coevolutionary networks of plant - animal interactions." Ecology Letters6(1): 69-81.
Plant-animal mutualistic networks are interaction webs consisting of two sets of entities, plant and animal species, whose evolutionary dynamics are deeply influenced by the outcomes of the interactions, yielding a diverse array of coevolutionary processes. These networks are two-mode networks sharing many common properties with others such as food webs, social, and abiotic networks. Here we describe generalized patterns in the topology of 29 plant-pollinator and 24 plant-frugivore networks in natural communities. Scale-free properties have been described for a number of biological, social, and abiotic networks; in contrast, most of the plant-animal mutualistic networks (65.6%) show species connectivity distributions (number of links per species) with a power-law regime but decaying as a marked cut-off, i.e. truncated power-law or broad-scale networks and few (22.2%) show scale-invariance. We hypothesize that plant-animal mutualistic networks follow a build-up process similar to complex abiotic nets, based on the preferential attachment of species. However, constraints in the addition of links such as morphological mismatching or phenological uncoupling between mutualistic partners, restrict the number of interactions established, causing deviations from scale-invariance. This reveals generalized topological patterns characteristic of self-organized complex systems. Relative to scale-invariant networks, such constraints may confer higher robustness to the loss of keystone species that are the backbone of these webs.
Lewinsohn, T. M., P. I. Prado, et al. (2006). "Structure in plant-animal interaction assemblages." Oikos113(1): 174-184.
We present a comprehensive approach to detect pattern in assemblages of plant and animal species linked by interactions such as pollination, frugivory or herbivory. Simple structural models produce gradient, compartmented or nested patterns of interaction; intermediate patterns between a gradient and compartments are possible, and nesting within compartments produces a combined model. Interaction patterns can be visualized and analyzed either as matrices, as bipartite networks or as multivariate sets through correspondence analysis. We argue that differences among patterns represent outcomes of distinct evolutionary and ecological processes in these highly diversified assemblages. Instead of choosing one model a priori, assemblages should be probed for a suite of patterns. A plant-pollinator assemblage exemplifies a simple nested pattern, whereas a plant-herbivore assemblage illustrates a compound pattern with nested structures within compartments. Compartmentation should reflect coevolutionary histories and constraints, whereas differences in species abundance or dispersal may generate nestedness.
Memmott, J. (1999). "The structure of a plant-pollinator food web." Ecology Letters2(5): 276-280.
The pollination biology literature is dominated by examples of specialization between plants and their pollinators. However, a recent review shows that it is generalization that prevails in the held, with most plants having a number of pollinators and most pollinators visiting a number of plants. Consequently, the vast majority of plant-pollinator interactions are embedded in a complex web of plant-pollinator interactions. These plant-pollinator webs can be studied in the manner of conventional food webs and the aim of this paper is to illustrate how contemporary methods of web construction and analysis can be applied to plant-pollinator communities.
Olesen, J. M. and P. Jordano (2002). "Geographic patterns in plant-pollinator mutualistic networks." Ecology83(9): 2416-2424.
Recent reviews of plant-pollinator mutualistic networks showed that generalization is a common pattern in this type of interaction. Here we examine the ecological correlates of generalization patterns in plant-pollinator networks. especially how interaction patterns covary with latitude, elevation, and insularity. We review the few published analyses of whole networks and include unpublished material, analyzing 29 complete plant-pollinator networks that encompass arctic, alpine, temperate, Mediterranean, and subtropical-tropical areas. The number of interactions observed (1) was a linear function of network size (M) the maximum number of interactions: In I = 0.575 + 0.61 In M; R-2 = 0.946. The connectance (C), the fraction of observed interactions relative to the total possible. decreased exponentially with species richness, the SLIM of animal and plant species in each community (A + P): C = 13.83 exp[-0.003(A + P)]. After controlling for species richness, the residual connectance was significantly lower in highland (>1500 in elevation) than in lowland networks and differed marginally among biogeographic regions, with both alpine and tropical networks showing a trend for lower residual connectance. The two Mediterranean networks showed the highest residual connectance. After correcting for variation in network size, plant species were shown to be more generalized at higher latitude and lowland habitats, but showed increased specialization on islands. Oceanic island networks showed an impoverishment of potential animal pollinators (lower ratio of animal to plant species. A : P. compared to mainland networks) associated with this trend of increased specialization. Plants, but not their flower-visiting animals, supported the often-repeated statements about higher specificity in the tropics than at higher latitudes. The pattern of interaction buildup as diversity increases in pollination networks does not differ appreciably from other mutualisms, such as plant-seed disperser networks or more complex food webs.
Ollerton, J., S. D. Johnson, et al. (2003). "The pollination ecology of an assemblage of grassland asclepiads in South Africa." Annals of Botany92(6): 807-834.
The KwaZulu-Natal region of South Africa hosts a large diversity of asclepiads (Apocynaceae: Asclepiadoideae), many of which are endemic to the area. The asclepiads are of particular interest because of their characteristically highly evolved floral morphology. During 3 months of fieldwork (November 2000 to January 2001) the flower visitors and pollinators to an assemblage of nine asclepiads at an upland grassland site were studied. These observations were augmented by laboratory studies of flower morphology (including scanning electron microscopy) and flower colour (using a spectrometer). Two of the specialized pollination systems that were documented are new to the asclepiads: fruit chafer pollination and pompilid wasp pollination. The latter is almost unique in the angiosperms. Taxa possessing these specific pollination systems cluster together in multidimensional phenotype space, suggesting that there has been convergent evolution in response to similar selection to attract identical pollinators. Pollination niche breadth varied from the very specialized species, with only one pollinator, to the more generalized, with up to ten pollinators. Pollinator sharing by the specialized taxa does not appear to have resulted in niche differentiation in terms of the temporal or spatial dimensions, or with regards to placement of pollinaria. Nestedness analysis of the data set showed that there was predictability and structure to the pattern of plant-pollinator interactions, with generalist insects visiting specialized plants and vice versa. The research has shown that there is still much to be learned about plant-pollinator interactions in areas of high plant diversity such as South Africa. (C) 2003 Annals of Botany Company.
Santamaria, L. and M. A. Rodrigues-Gironés (2007). "Linkage rules for plant-pollinator networks: trait complementarity or exploitation barriers." Plos Biology5(2): 354-362.
Vázquez, D. P. (2005). "Degree distribution in plant - animal mutualistic networks: forbidden links or random interactions?" Oikos108(2): 421-426.
Vázquez, D. P. and M. A. Aizen (2003). "Null model analyses of specialization in plant-pollinator interactions." Ecology84(9): 2493-2501.
Recent studies have suggested that plant-pollinator interactions may be less specialized than previously thought. We contrasted patterns of specialization observed in five plant-pollinator interaction webs with predictions based on null models. In the five data sets, the observed number of extreme specialists and extreme generalists was significantly higher than the null expectation. This pattern was mostly due to a positive correlation between species frequency of interaction (f) and their estimated degree of generalization (s). After accounting for this association, the expected frequency distribution of degree of specialization generated by the null model closely matched the observed frequency distribution in the five data sets. A second null model, which explicitly incorporated the correlation between f and s, also generated expected frequency distributions of specialization that closely resemble those observed in the data sets. To make progress in understanding the distribution of degree of specialization in pollination systems it will be necessary not only to improve the quality of the data and to refine methods used to quantify specialization, but also to answer the question of why more frequently interacting species appear to be more generalized.
Vázquez, D. P. and M. A. Aizen (2004). "Asymmetric specialization: a pervasive feature of plant - pollinator interactions." Ecology85(5): 1251-1257.
Although specialization in species interactions has usually been equated to reciprocal specialization, asymmetric specialization (i.e., a specialist interacting with a generalist) is also likely. Recent studies have suggested that asymmetric specialization in species interactions could be more common than previously thought. We contrasted patterns of asymmetric specialization observed in 18 plant-pollinator interaction webs with predictions based on null models. We found that asymmetric specialization is common in plant-pollinator interactions, and that its occurrence is more frequent than expected under a simple null model that assumed random interactions among species; furthermore, large assemblages with many pairs of,interacting species tend to have more asymmetric interactions than smaller assemblages. A second null model, which incorporated a correlation between species frequency of interaction and degree of specialization observed in most data sets produced patterns that were generally closer to those present in the data. At least three kinds of explanations could account for the observed asymmetric specialization, including random interactions among individuals (rather than species), adaptive consequences of specialization, and artifacts, such as data aggregation and sampling biases. Future studies should be aimed at understanding the relative importance of each of these alternative explanations in generating asymmetric specialization in species interactions.
Waser, N. M., L. Chittka, et al. (1996). "Generalization in pollination systems, and why it matters." Ecology77(4): 1043-1060.
One view of pollination systems is that they tend toward specialization. This view is implicit in many discussions of angiosperm evolution and plant-pollinator coevolution and in the long-standing concept of ''pollination syndromes.'' But actual pollination systems often are more generalized and dynamic than these traditions might suggest, To illustrate the range of specialization and generalization in pollinators' use of plants and vice versa, we draw on studies of two floras in the United States, and of members of several plant families and solitary bee genera, We also summarize a recent study of one local flora which suggests that, although the colors of flowers are aggregated in ''phenotype space,'' there is no strong association with pollinator types as pollination syndromes would predict. That moderate to substantial generalization often occurs is not surprising on theoretical grounds. Plant generalization is predicted by a simple model as long as temporal and spatial variance in pollinator quality is appreciable, different pollinator species do not fluctuate in unison, and they are similar in their pollination effectiveness. Pollinator generalization is predicted when floral rewards are similar across plant species, travel is costly, constraints of behavior acid morphology are minor, and/or pollinator lifespan is long relative to flowering of individual plant species. Recognizing that pollination systems often are generalized has important implications. In ecological predictions of plant reproductive success and population dynamics it is useful to widen the focus beyond flower visitors within the ''correct'' pollination syndrome, and to recognize temporal and spatial fluidity of interactions. Behavioral studies of pollinator foraging choices and information-processing abilities will benefit from understanding the selective advantages of generalization. In studies of floral adaptation, microevolution, and plant speciation one should recognize that selection and gene flow vary in time and space and that the contribution of pollinators to reproductive isolation of plant species may be overstated. In conservation biology, generalized pollination systems imply resilience to linked extinctions, but also the possibility for introduced generalists to displace natives with a net loss of diversity.