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Book Review – Pollination and Floral Ecology

Pollination and Floral Ecology

Pollination and Floral Ecology

Pat Willmer. 2011. Princeton University Press. £65. pp. 832.

Any text book that tries to assess and summarise the whole of a multidisciplinary research field such as pollination ecology and floral biology is required to be four things:  (1) comprehensive in its scope; (2) up to date in its coverage of the literature; (3) accurate in its assessment of the current state of the field; and (4) authoritative in the conclusions it presents.

This volume by Professor Pat Willmer of the University of St Andrews certainly ticks the first box.  It’s a huge book, and covers everything relating to the evolution of flower attraction and reward systems, ecological interactions with pollinators, biochemistry, physiology, agriculture and conservation; all in 29 chapters split into three sections, with 87 pages of references.  The literature extends to 2010, which is impressive for a book published in 2011 (though see my comments below about completeness of the literature).   Specialist terms are highlighted in bold to direct the reader to the glossary at the back, a useful device even if there are a few inaccuracies, which I’ll mention later.

So far so good, and the author is to be congratulated on putting together such a comprehensive, not to mention timely, single-author book.  It’s clearly the summation of a career devoted to studying pollinators and flowers, and the author’s passion for her subject is apparent throughout.

However when we come to points 3 and 4, things are less straightforward.  There are some issues with accuracy that are troubling in a book aimed at newcomers to the field as well as established researchers.  To give just a few examples:

- on p.18 we are told that asclepiads have “one stamen” (they have five); on p.169 and in the glossary that asclepiad pollinia are the pollen grains from one anther (they are the contents of half an anther); and on p.170 that the pollinaria are “glued” to pollinators (they actually clip on).

- in the glossary, tree ferns are referred to as “cycads”, an error that is repeated on p.89.

- on p.88 there is a statement suggesting that tree fern spores were dispersed by “animal fur” 300 million years ago, long before the evolution of mammals, and that this (and dispersal of spores of fungi and mosses) is the equivalent of pollination: it is not, it equates to seed dispersal.

These are troubling errors of basic botany that are forgivable in an early draft of the book (everyone makes mistakes) but not in the final published version, after it’s been read, reviewed, checked and edited.  If the book goes to a second edition I hope that these (and other) mistakes will be fixed.  But they do hint at a fundamental problem with a book (and a field) as large and complex as this: a single author is arguably unlikely to be able to do justice to all of the subject matter.

There are parts of the book where it is unclear (to me at least) what the author is actually saying.  For example, on p.96 there is a graph which, it is suggested, demonstrates that pollination by animals is “technically uncommon when assessed in terms of the numbers of broad taxonomic groups that use it”, though the legend to the figure claims that “most orders of plants have no families” that possess wind pollination.  This is confusing: what is to be concluded by someone new to the field?  Is animal pollination common or rare?  Likewise, on p.91 we are told that the “first angiosperms… would probably have had their pollen moved mainly by wind…”, but then on p.92 that “an element of insect pollination could be regarded as almost ancestral”.  Which is correct?

There are other aspects to the book that are simply out of date; for example the linear, rather deterministic schemes set out in Figures 4.6 and 4.8 showing that Cretaceous flowers were open and radially symmetrical, and only later evolved into complex, bilateral flowers in the Tertiary, ignores fossil discoveries showing that orchids evolved in the Cretaceous (Ramírez et al., 2007).  Likewise, discussion of “counterproductive” crypsis in flowers (p.124) neglects recent findings of cryptic, wasp-pollinated plants in South Africa (e.g. Shuttleworth & Johnson, 2009).

There is a theme emerging here: some of the botany that the book presents is inaccurate, confused or out-dated.  Fortunately the zoological aspects of the book are much better, as one might hope from a Professor of Zoology.

The final criterion, that the book should be “authoritative in the conclusions it presents”, is however, in my view, the main weakness of this volume.  The author is unhappy with recent developments in the field, particularly as they relate to community-scale assessments of plant–pollinator interactions, in terms of network analyses and predictive utility of pollination syndromes.  Clearly Professor Willmer is on a mission to rebalance what she perceives as failings within some of the current trends in studying pollination.  A book review is not the place for a technical dissection of the author’s arguments, which is best left to the peer-reviewed literature (though I would argue that that’s also the place to present some of the criticisms the author introduces, rather than into a text book such as this).  I could focus the whole of this review on these topics because: (a) they take up a large proportion of the book, about one-third of the text pages; and (b) they are highlighted on the cover as being one of the main contributions of the book; specifically, that the author provides a critique of previous work that does not distinguish between “casual visitors and true pollinators” that can in turn result in “misleading conclusions about flower evolution and animal-flower mutualism”. Unfortunately her targets are straw men, and one – I believe quite telling – example will suffice.

On p.447 there is a criticism of the use by Waser et al. (1996) of Charles Robertson’s historical data set, and specifically that the analyses they present “…did not distinguish visitors from pollinators even though Robertson’s database did include information on this”.  However Waser et al. clearly state (p.1045 of their paper) that only pollinators were included in the analyses, not all flower visitors, and that “visitation is not a synonym for pollination… non-pollinating visitors are excluded (as in Robertson 1928)” (p.1048).

Why should Professor Willmer make a statement to the contrary?  Evidently she wishes to impress upon her readers that (in her opinion) there are fundamental problems in current approaches to studying pollination at a community level.  But even if that were the case (and I don’t believe it is) misrepresenting previous studies to suit an argument is poor scholarship at best.

Regardless of whether some of her criticism is well founded, the author does not seem to appreciate that plant–flower visitor interaction networks are ecologically important regardless of whether or not a flower visitor acts as a pollinator.  More fundamentally, true pollination networks possess similar attributes to flower visitor networks, for example a nested pattern of interactions, and arguments about level of generalisation of species are a matter of scale, not category (Ollerton et al., 2003).

At the end of her Preface, Professor Willmer reveals to us quite a lot about her personal attitude to research when she states that some readers might find her approach “too traditional” in an “era where ecological modelers [might be claimed to] have more to tell us than old-style field workers”.  What the author fails to appreciate is that this is a grossly false dichotomy and that most of the pollination ecologists who have embraced new analytical methodologies for understanding plant–pollinator interactions are also “old-style field workers” with considerable experience of studying the ecology of flowers and their pollinators beyond the computer screen.

In summary this is a book that, for all its good qualities of comprehensiveness and (mostly) up to date coverage, should be read with caution: parts of it are neither as accurate nor as authorative as the field of pollination and floral ecology deserves.

 

Jeff Ollerton

Email jeff.ollerton@northampton.ac.uk

LITERATURE CITED

Ollerton J, Johnson SD, Cranmer L, Kellie, S. 2003. The pollination ecology of an assemblage of grassland asclepiads in South Africa. Annals of Botany 92: 807-834.

Ramírez SR, Gravendeel B, Singer RB, Marshall CR,  Pierce NE. 2007. Dating the origin of the Orchidaceae from a fossil orchid with its pollinator. Nature 448: 1042-1045.

Shuttleworth A, Johnson SD. 2009. The importance of scent and nectar filters in a specialized wasp-pollination system. Functional Ecology 23: 931-940.

Waser NM, Chittka L, Price MV, Williams N, Ollerton J. 1996. Generalization in pollination systems, and why it matters. Ecology 77: 1043-1060.

 

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Avoidance of interspecific pollen transfer in Pedicularis

Avoidance of interspecific pollen transfer in Pedicularis

Avoidance of interspecific pollen transfer in Pedicularis

Plants surrounded by individuals of other co-flowering species may attract more pollinators but can suffer a reproductive cost from interspecific pollen transfer. Yang et al. compare pollination and reproduction in Pedicularis densispica (lousewort) when occurring alone or together with co-flowering Astragalus pastorius. They find that mixed populations attract many more nectar-seeking bumble-bees, which move frequently between the species. However, differences in floral architecture mean that P. densispica is pollinated via the dorsum of the bees whilst A. pastorius receives pollen via the abdomen, thus avoiding interspecific transfer. The overall result is that co-flowering yields more seeds that are heavier and have higher germinability than in pure populations of P. densispica.

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Hydrochoric gene flow in invasive riparian Impatiens

Hydrochoric gene flow in invasive riparian Impatiens

Hydrochoric gene flow in invasive riparian Impatiens

Riparian systems are prone to invasion by alien plant species, which may be facilitated by hydrochory, the transport of seeds by water. Love et al. study gene flow associated with hydrochoric dispersal of the invasive riparian plant Impatiens glandulifera (Himalayan balsam) in two contrasting river systems and find a significant increase in levels of genetic diversity downstream, consistent with the accumulation of propagules from upstream source populations. There is strong evidence for organisation of this diversity between different tributaries, reflecting the dendritic organisation of the river systems studied. The results indicate that hydrochory, rather than anthropogenic dispersal, is primarily responsible for the spread of I. glandulifera in these river systems.

 

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Evolutionary history of Afro-Madagascan Ixora

Ixora regalis

Ixora regalis

The pantropical genus Ixora is one of the largest genera in Rubiaceae, with approximately 530 species. Tosh et al. conduct phylogenetic analyses based on four plastid and two nuclear ribosomal markers to infer the historical biogeography of Afro-Madagascan Ixora species. They find that Madagascan Ixora do not form a monophyletic group, but are represented by two separate lineages of different ages, with at least one dispersal event occurring from East Africa into Madagascar in the late Pliocene. Both lineages of Madagascan Ixora exhibit morphological characters that are rare throughout the rest of the genus, including pauciflorous inflorescences and extreme corolla tube length.

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Abscisic acid underlies genotypic variation in stomatal responses

Abscisic acid underlies genotypic variation in stomatal responses

Abscisic acid underlies genotypic variation in stomatal responses

Stomata formed at high relative humidity (RH) are less responsive to abscisic acid (ABA), an effect that varies widely between genotypes. Giday et al. study four rose cultivars (Rosa hybrida) grown at 60% and 90% RH and find stomatal responsiveness to desiccation and ABA feeding to be attenuated in two of them at high RH. [ABA] is lower in plants grown at high RH, an effect that is more pronounced in these sensitive cultivars. They determine that the sensitive cultivars undergo a larger decrease in [ABA], rather than having a higher [ABA] threshold for inducing stomatal functioning. However, the cultivar differences in stomatal closure following ABA feeding are not apparent in response to H2O2 and downstream elements, indicating that signalling events prior to H2O2 generation are involved in the observed genotypic variation.

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Corolla morphology and diversification rates in toadflaxes

Corolla morphology and diversification rates in toadflaxes

Corolla morphology and diversification rates in toadflaxes

The role of flower specialization in plant speciation and evolution remains controversial. Fernández-Mazuecos et al. use a time-calibrated phylogeny in conjunction with morphometric analysis to study bifid toadflaxes (Linaria sect. Versicolores), which have highly specialized corollas. They determine that a restrictive character state (narrow corolla tube) is reconstructed in the most-recent common ancestor. After its early loss in the most species-rich clade, this character state has been convergently reacquired in multiple lineages of this clade in recent times, yet it seems to have exerted a negative influence on diversification rates. The results suggest that opposing individual-level and species-level selection pressures may have driven the evolution of pollinator-restrictive traits in the bifid toadflaxes.

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Stabilization of yield in plant genotype mixtures

Stabilization of yield in plant genotype mixtures

Stabilization of yield in plant genotype mixtures

Sowing crops as mixtures of varieties instead of a monoculture can result in more stable yields, especially in variable environments. Creissen et al. grow dissimilar Arabidopsis thaliana genotypes in monocultures and mixtures under strong competition and abiotic stress, and find that mixtures achieve more stable seed production through compensatory interactions. Competitive ability and performance in mixtures can be predicted from above-ground traits, even though below-ground competition appears to be more intense. The results suggest that phenotype screens of varieties could improve the choice of mixtures for agriculture in unpredictable environmental conditions.

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Control of quiescent centre formation in adventitious roots

Control of quiescent centre formation in adventitious roots

Control of quiescent centre formation in adventitious roots

Adventitious roots (ARs) are part of the root system in numerous plants, and are required for successful micropropagation. Della Rovere et al. study the quiescent centre (QC) in ARs of Arabidopsis thaliana and find that accumulation of auxin and the expression of the QC marker WOX5 characterize the early derivatives of the AR founder cells both in planta and in explants. An auxin maximum is determined to occur at the AR tip, to which WOX5 expression is restricted, establishing the positioning of the QC. Cytokinin causes a restriction of LAX3 and PIN1 expression domains, and concomitantly the auxin biosynthesis YUCCA6 gene is expressed in the apex. They conclude that establishment of the QC requires the co-ordinated action of auxin and cytokinin.

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Pollen receipt and quality in endemic-rich communities

Pollen receipt and quality in endemic-rich communities

Pollen receipt and quality in endemic-rich communities

The magnitude of pollen limitation is highly variable among habitats, species and differing plant communities. Alonso et al. analyse natural variability in pollen receipt and tube formation, and compare pollen quality and quantity between co-flowering endemics and non-endemics at three biodiversity hotspots in Andalusia, California and Yucatan. They find that only a combination of abundant and good quality pollen and a low number of ovules per flower can confer relief from pre-zygotic pollen limitation. Endemics are not always disadvantaged: the relative pollination success of endemic and non-endemic species, and its quantity and quality components, are community dependent.

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Cluster roots, P and growth rate in Lupinus

Cluster roots, P and growth rate in Lupinus

Cluster roots, P and growth rate in Lupinus

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. Wang et al. measure relative growth rate (RGR) among three lupin species, L. albus, L. pilosus and L. atlanticus, with similar shoot P status and find that cluster-root formation is suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among these species cannot be explained by species-specific variation in RGR or leaf P concentration. This goes against the expectation that, even when P-uptake rates are high, plants with fast growth rates might not accumulate shoot P and hence a correlation between RGR and cluster-root formation could be anticipated.

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