Annals of Botany 2009 103(9):i; doi:10.1093/aob/mcp114
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Ecology and evolution of plant–pollinator interactions (Viewpoint)
Some of the most
exciting advances in pollination biology have resulted from
interdisciplinary research combining ecological and evolutionary
perspectives.
Mitchell et al. (pp. 1355–1363) highlight
application of these approaches to two themes: (1) the link
between pollinator behaviour and plant mating patterns, and
(2) generalization and specialization in pollination systems.
The authors conclude with several suggestions for future investigations
that will further unite research in evolutionary and ecological
pollination biology.
Plant mating mediated by traplining pollinators
Rather than wandering
randomly, some pollinators make repeated, sequential visits
to sets of plants.
Ohashi and Thomson (pp. 1365–1378) first review published work – computer simulations, laboratory
experiments and field observations – on the consequences
of such traplining for bumble-bees. New simulations then explore
the effect of traplining on plant mating patterns and floral
evolution.
Pollinators influence among-flower variation in selfing
Adjacent flowers
on
Mimulus ringens floral displays often vary markedly in selfing
rate. Using paternity analysis,
Karron et al. (pp. 1379–1383) experimentally demonstrate that this striking among-flower variation
results from increased deposition of geitonogamous (among-flower,
within-display) self pollen as bumble-bees probe consecutive
flowers on each floral display.
Seasonal variation in pollination success of alpine plants
Seed production
of an alpine-snowbed plant,
Phyllodoce aleutica, is strongly
influenced by flowering time, which varies due to the effects
of snowmelt regime.
Kameyama and Kudo (pp. 1385–1394) find that the outcrossing rate of this species varies greatly
(10–88 %) among populations, and reflects both the timing
of pollinator availability during the season and the cryptic
self-incompatibility of this species.
Honey-bees limit outcrossing in Grevillea
Honey-bees have
invaded Australian systems that evolved with vertebrate pollinators.
Whelan et al. (pp. 1395–1401) explore whether differences
in pollinators could explain variation in outcrossing among
populations of
Grevillea macleayana. Bird-exclosure reveals
that honey-bees are effective in removing pollen from flowers,
but birds are needed for high levels of outcross pollen deposition.
New frontiers in competition for pollination (Review)
Plants that share
pollinators may compete for pollination.
Mitchell et al. (pp. 1403–1413) present a conceptual framework for studies of competition for
pollination that involves both the quantity and quality of pollination
services and considers both male and female sex functions of
flowers. Using this framework, they explore how competition
affects plant mating systems and multispecies' interactions.
They also examine the effects of invasive species, climate change
and pollinator declines on patterns of competition for pollination.
Yeasts in floral nectar
A peculiarity of
floral nectar that remains relatively unexplored is its role
as a natural habitat for micro-organisms. A quantitative survey
conducted by
Herrera et al. (pp. 1415–1423) on 130 species
of insect-pollinated plants from Spain and Mexico reveals that
yeasts occur very frequently in floral nectar, where they sometimes
reach densities of up to 4
x 10
5 cells mm
–3. Incorporating
nectar yeasts into the scenario of plant–pollinator interactions
opens a number of intriguing avenues for research.
Realized tolerance to nectar robbing
Irwin (pp. 1425–1433) uses an experimental common garden to find that
Ipomopsis aggregata exhibits variation in realized tolerance to nectar robbing and
that increased flower production is associated with tolerance.
By linking concepts and techniques from studies of plant–pollinator
and plant–herbivore interactions, this work provides insight
into the role of floral traits in pollinator attraction and
plant defence.
Pollination biology of Datura wrightii
Datura wrightii is associated with the hawkmoth
Manduca sexta at two stages:
adults feed on nectar, then deposit their herbivorous offspring.
Bronstein et al. (pp. 1435–1443) show that adult moths
are highly effective pollinators of
Datura even though much
of their foraging activity is devoted to the bat-pollinated
Agave palmeri. Agave thus has the potential to alter both the
costs and benefits of this interaction.
Patterns and processes in mutualistic networks (Review)
Ecologists and
evolutionary biologists are becoming increasingly interested
in the study of networks of interacting plants and animal mutualists.
Vázquez et al. (pp. 1445–1457) offer an overview
of the mechanisms influencing the structure of these networks
and show that substantial progress has been made in our understanding
of the processes behind the patterns observed.
Nectar depth and proboscis length distributions
Plant–pollinator
interaction webs exhibit consistent structural features, such
as long-tailed distributions of generalization degrees and nestedness
of interactions. The recognition of these shared features has
led to a variety of explanations. By using the degree of size
matching between nectar depth and proboscis length as a new
network parameter,
Stang et al. (pp. 1459–1469) show that
beside size thresholds and species' abundances, size distributions
are important in understanding observed patterns of interaction.
Global test of pollination syndromes
‘Pollination
syndromes’ are suites of phenotypic traits hypothesized
to reflect convergent adaptations of flowers for pollination
by specific types of animals.
Ollerton et al. (pp. 1471–1480) test whether pollination syndromes successfully capture patterns
of covariance of floral traits in communities surveyed on three
continents. They conclude that syndromes do not successfully
describe the diversity of floral phenotypes or predict the pollinators
of most plant species. For example, although hummingbirds are
the most frequent visitors to
Ipomopsis aggregata, this species
is also visited by
Bombus appositus.
A generalized pollination system in the tropics
Aphelandra acanthus has an unusual mix of floral traits that corresponds to a pollination
system generalized to bats and hummingbirds.
Muchhala et al. (pp. 1481–1487) find that bats are more effective than hummingbirds in transferring
conspecific pollen. However, bats also transfer large amounts
of heterospecific pollen. Such interspecific pollen transfer
probably decreases bat effectiveness and may select for generalized
floral traits that attract both bats and hummingbirds.
Extreme floral variation and pollinator attraction
Flowers of
Echinopsis ancistrophora vary dramatically among populations, in length,
nectar production and timing of anthesis.
Schlumpberger et al. (pp. 1489–1500) find that these traits are associated with pollination by bees
or hawkmoths, suggesting incipient differentiation at the population
level. While hawkmoths only visit the longest flowers with rich
nectar supplies, bees are more flexible, accounting for pollination
not only in short, but also in intermediate flowers.
Fly pollination in Ceropegia
Fly pollination
is widespread within the flowering plants but is relatively
poorly researched compared with pollination systems involving
larger, more charismatic taxa such as bees and birds.
Ollerton et al. (pp. 1501–1514) study the pollinator diversity of
Ceropegia, which temporarily
traps the flies that visit its flowers, in relation to the phylogeny
and the biogeography of this asclepiad genus.
Ecology and evolution of wind pollination (Review)
Friedman and Barrett (pp. 1515–1527) provide a comprehensive review of the ecology and evolution
of pollination and mating in wind-pollinated plants. They review
the literature and provide their own new experimental results
on pollen limitation, pollen capture and pollen-transfer efficiencies.
They discuss the effects of floral and inflorescence architecture
for pollination and mating, and suggest that in many wind-pollinated
hermaphroditic plants with unisexual flowers geitonogamous selfing
is likely to provide reproductive assurance.
How do flowers achieve adaptive accuracy?
Adaptive accuracy
is a conceptual approach that explores the causes of individual
and population phenotypic departure from adaptive optima. For
flowers to achieve high fitness, they must have precise and
accurate pollen placement on pollinators and precise and accurate
stigma contact with pollinators, relative to the position of
pollen placement.
Armbruster et al. (pp. 1529–1545) show
that floral integration and fusion of floral parts can improve
floral precision, but that selection for herkogamy (outcrossing)
often operates in conflict with pollination accuracy, leading
to departure of the population mean from the pollination optimum.
Testing adaptiveness of anther position (Review)
Anther position
within flowers may affect pollen removal and subsequent male
seed siring success.
Conner et al. (pp. 1547–1556) show
that this is true for anther exsertion (the degree to which
anthers protrude from the corolla tube) in wild radish. Whether
the four-high and two-low anther arrangement common to most
mustards is also adaptive is less clear.
Phenotypic manipulations and selection of floral associations (Viewpoint)
A basic theme in
pollination ecology is that pollinators select for suites of
floral traits.
Campbell (pp. 1557–1566) argues for using
phenotypic manipulations to evaluate the separate and combined
effects of traits. Methods for studying multivariate selection
are critically reviewed, and the experimental approach is illustrated
with western USA and New Zealand alpine plants.
Pollinators and floral traits in Rocky Mountain columbine
Brunet (pp. 1567–1578) observes wide variation in pollinator abundance and composition
over time and space at sites in the western USA, but one species
of bumble-bee or hawkmoth predominates in each population each
year. Floral characteristics vary widely among populations;
however, whiter flowers are associated with the annual presence
of hawkmoths and spur length is influenced by the presence of
a specific hawkmoth species.
How much does agriculture depend on pollinators?
Productivity of
many crops benefits from the presence of pollinating insects,
so a decline in pollinator abundance should compromise global
agricultural production.
Aizen et al. (pp. 1579–1588) consider variation among individual crops in their degree of
pollinator dependency and estimate that the disappearance of
pollinators would have only minor effects on total agriculture
production and diversity. However, they predict that a pollinator
shortage would substantially increase demand for agricultural
land, particularly in the developing world, which could contribute
significantly to global environmental change.
Modelling pollination services
Lonsdorf et al. (pp. 1589–1600) describe and test the first quantitative model that predicts
pollinator abundance on a landscape. On two of three test sites,
the model explained up to 80 % of variance among farms for pollinator
abundance. The approach provides a foundation to evaluate and
develop land-use plans that promote pollinator conservation
and service delivery.
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