http://amborella.net/2010PlantSystematics/DATA/3-24-APG%202003%20Bot%20J%20Linn%20Soc%20APGII.pdf

An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II

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.

/pdf/the-nested-assembly-of-plant-animal-mutualistic-networks-55huvwamt7.pdf

The nested assembly of plant-animal mutualistic networks

▪ Abstract Floral evolution has often been associated with differences in pollination syndromes. Recently, this conceptual structure has been criticized on the grounds that flowers attract a broader spectrum of visitors than one might expect based on their syndromes and that flowers often diverge without excluding one type of pollinator in favor of another. Despite these criticisms, we show that pollination syndromes provide great utility in understanding the mechanisms of floral diversification. Our conclusions are based on the importance of organizing pollinators into functional groups according to presumed similarities in the selection pressures they exert. Furthermore, functional groups vary widely in their effectiveness as pollinators for particular plant species. Thus, although a plant may be visited by several functional groups, the relative selective pressures they exert will likely be very different. We discuss various methods of documenting selection on floral traits. Our review of the literatur...

Pollination Syndromes and Floral Specialization

▪ Abstract The pollination of flowering plants by animals represents a critical ecosystem service of great value to humanity, both monetary and otherwise. However, the need for active conservation of pollination interactions is only now being appreciated. Pollination systems are under increasing threat from anthropogenic sources, including fragmentation of habitat, changes in land use, modern agricultural practices, use of chemicals such as pesticides and herbicides, and invasions of non-native plants and animals. Honeybees, which themselves are non-native pollinators on most continents, and which may harm native bees and other pollinators, are nonetheless critically important for crop pollination. Recent declines in honeybee numbers in the United States and Europe bring home the importance of healthy pollination systems, and the need to further develop native bees and other animals as crop pollinators. The “pollination crisis” that is evident in declines of honeybees and native bees, and in damage to web...

https://www.jstor.org/stable/pdfplus/221703.pdf

ENDANGERED MUTUALISMS: The Conservation of Plant-Pollinator Interactions

The mutually beneficial interactions between plants and their animal pollinators and seed dispersers have been paramount in the generation of Earth's biodiversity. These mutualistic interactions often involve dozens or even hundreds of species that form complex networks of interdependences. Understanding how coevolution proceeds in these highly diversified mutualisms among free-living species presents a conceptual challenge. Recent work has led to the unambiguous conclusion that mutualistic networks are very heterogeneous (the bulk of the species have a few interactions, but a few species are much more connected than expected by chance), nested (specialists interact with subsets of the species with which generalists interact), and built on weak and asymmetric links among species. Both ecological variables (e.g., phenology, local abundance, and geographic range) and past evolutionary history may explain such network patterns. Network structure has important implications for the coexistence and stability of...

/pdf/plant-animal-mutualistic-networks-the-architecture-of-413odbi3jn.pdf

Plant-Animal Mutualistic Networks: The Architecture of Biodiversity

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: (1) 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.

/pdf/generalization-versus-specialization-in-plant-pollination-1wb21n3rxj.pdf

Generalization versus specialization in plant pollination systems

Field studies in South Africa showed that floral spur length in the Disa draconis complex (Orchidaceae) varies enormously between populations in the southern mountains (means = 32-38 mm), lowland sandplain (mean = 48 mm), and northern mountains (means = 57-72 mm) We tested the hypothesis that divergence in spur length has resulted from selection exerted through pollinator proboscis length Short-spurred plants in several southern mountain populations, as well as long-spurred plants in one northern mountain population, were pollinated by a horsefly, Philoliche rostrata (Tabanidae), with a proboscis length that varied from 22 to 35 mm among sites Long-spurred plants on the sandplain were pollinated by the tanglewing fly, Moegistorynchus longirostris (Nemestrinidae), which has a very long proboscis (mean = 57 mm) Selection apparently favors long spurs in sandplain plants, as artificial shortening of spurs resulted in a significant decline in pollen receipt and fruit set, although pollinaria removal was not significantly affected Fruit set in the study populations was limited by pollen availability, which further suggests that selection on spur length occurs mainly through the female component of reproductive success

https://www.jstor.org/stable/pdfplus/2410959

Long-tongued fly pollination and evolution of floral spur length in the Disa draconis complex (Orchidaceae)

We studied the patterns of adaptive radiation in Disa, a large orchid genus in southern Africa. A cladogram for 27 species was constructed using 44 morphological characters. Pollination systems were then mapped onto the phylogeny in order to analyze pathways of floral evolution. Shifts from one pollination system to another have been a major feature of the evolutionary diversification of Disa. Unlike many plant genera that are pollinated mainly by a single group of insects, radiation in Disa has encompassed nearly all major groups of pollinating insects; in all, 19 different specialized pollination systems have been found in the 27 species included in this analysis. Another striking pattern is the repeated evolution of broadly similar pollination systems in unrelated clades. For example, butterfly-pollinated flowers have evolved twice; showy deceptive flowers pollinated by carpenter bees, twice; long-spurred flowers pollinated by long-tongued flies, four times; night-scented flowers pollinated by moths, three times; and self-pollination, three times. This suggests that a few dominant pollinator species in a region may be sufficient to generate diversification in plants through repeated floral shifts that never retrace the same pathways.

Phylogeny and radiation of pollination systems in DISA (Orchidaceae).

The parasitic nutritional mode is a frequently evolved adaptation in animals (Price, 1980), as well as in flowering plants (Kuijt, 1969). Heterotrophic angiosperms can be classified as either mycotrophs or as haustorial parasites. The former derive nutrients via a symbiotic relationship with mycorrhizal fungi. Haustorial parasites, in contrast, directly penetrate host tissues via a modified root called a haustorium and thereby obtain water and nutrients. Although such categories are often a matter of semantics, we use the term parasite in a strict sense to refer to haustorial parasites. Angiosperm parasites are restricted to the dicot subclasses Magnoliidae, Rosidae, and Asteridae; have evolved approximately 11 times; and represent approximately 22 families, 265 genera, and 4,000 species, that is, about 1% of all angiosperms (Fig. 8.1). Owing to their unique adaptations, parasitic plants have long been the focus of anatomical, morphological, biochemical, systematic, and ecological research (Kuijt, 1969; Press and Graves, 1995). For the vast majority of parasitic plants, negative effects upon the host are difficult to detect, yet others (e.g., Striga, Orobanche) are serious weeds of economically important crops (Kuijt, 1969; Musselman, 1980; Eplee, 1981; Stewart and Press, 1990; Press and Graves, 1995).

Kim E. Steiner

Papers

Generalization versus specialization in plant pollination systems

Long-tongued fly pollination and evolution of floral spur length in the Disa draconis complex (Orchidaceae)

Phylogeny and radiation of pollination systems in DISA (Orchidaceae).

Molecular Phylogenetic and Evolutionary Studies of Parasitic Plants

The evolution of parasitism in Scrophulariaceae/Orobanchaceae: plastid gene sequences refute an evolutionary transition series.