[Pollinator] Front Page of NYT.com: Breezy Love, or the Sacking of the Bees

[Jennifer Tsang]

http://opinionator.blogs.nytimes.com/2010/03/09/breezy-love-or-the-sacking-o
f-the-bees/?hp

 




Breezy Love, or the Sacking of the Bees

By OLIVIA JUDSON
<http://opinionator.blogs.nytimes.com/author/olivia-judson/> 
 

Birds do it. Bees do it. Beetles, bats and light summer breezes do it.

I refer, of course, to that raunchiest of sex acts: the pollination of
flowers.

When it comes to sex, plants have more headaches than the rest of us. One
problem is that they can’t travel about to find a mate — they are, after
all, rooted to the spot — so they have to depend on intermediaries to bring
egg and sperm cells together. 

For mosses and ferns, the intermediary is water. For conifers like pine
trees and cypresses, the intermediary is wind. But for most flowering
plants, the intermediaries are animals.

Flowering plants are the largest, most successful group of plants on the
planet today. There are thought to be more than quarter of a million
different species — nearly 10 times more than all the other types of plants
added together. (To put things in perspective, the number of living species
of fish, amphibians, reptiles, birds and mammals combined is less than
58,000.) The flowering plants include roses and waterlilies, grasses and oak
trees, tulips and orchids. They include, in short, most of the plants that
come to mind when one thinks of vegetation. 

It was not always thus. Before the mid-Cretaceous, 100 million years ago or
so, flowering plants were scarce: conifers and their relations ruled the
landscape. But then, for reasons that are not well understood, flowering
plants upstaged all others, and the Earth came into bloom. 

Flowering plants were not the first to seduce animals into spreading their
pollen for them. Fossils suggest that some earlier groups of plants, now
extinct, had evolved a dependency on insects like scorpionflies.
Nonetheless, the earliest flowers appear to have been pollinated by insects,
and the full-scale blossoming of flowering plants coincides with the rise of
animals as go-betweens. Bees, for example, buzzed onto the scene with
flowering plants; the evolutionary history, and success, of both groups is
intimately linked. 

The appearance of flowering plants brought a new flamboyance to the planet.
Flowers pollinated by animals tend to be big and colorful; they often smell.
(To a human, flowers pollinated by bees typically smell pleasant; flowers
pollinated by flies tend to smell foul, like rotting meat.) Often, flowers
offer something for the animal to eat — a sip of nectar, perhaps. Sometimes,
they provide heat. 

(One plant that heats its flower is Philodendron solimoesense, an Arum from
the South American tropics. In doing so, it turns itself into an assignation
hotel for scarab beetles. The beetles arrive in the evening, spend the night
feeding and mating, spend the morning recuperating and head off to a new
flower later on — complete with pollen from their host. Sure enough, the
heat saves the beetles energy. Beetles in a heated flower don’t have to use
as many calories to keep warm as they would if they spent the night
outdoors.)

Yet, from time to time, flowering plants abandon their animals, evolving
instead to throw pollen to the wind. Wind-pollination — if you’re a
vocabulary fiend, the technical term is “anemophily,” meaning lover of wind
— has evolved at least 65 times in flowering plants, and around 10 percent
of the species do it. Indeed, as
<http://opinionator.blogs.nytimes.com/2010/03/02/evolution-by-the-grassroots
/>  I mentioned last week, many grasses are pollinated by the wind.

It’s not clear what causes this transition, though there are several ideas.
One is that it happens in plants that, although generally pollinated by
insects, already have a small capacity for wind pollination — small, light
pollen grains, and flowers that can, in principle, catch pollen if it floats
past on a breeze. Then, the balance between insects and wind can easily
shift. In a tropical forest, for example, the advantages of insects are
great: they provide highly targeted pollen-delivery in a complex milieu. But
in big open spaces, the wind may do a better job — especially if the climate
is inhospitable, and insects are few. Such circumstances may cause a shift
away from traits that lure insects, and enhance those that seduce the wind.

A plant that has sacked bees or other insects can make its flowers smaller,
less colorful and more aerodynamic. Liberated from the expense of making
nectar, it can make more pollen instead. A bee, after all, can only carry so
much pollen at once. The wind is not so limited. 

And wind-pollinated plants tend to produce huge quantities of pollen.
Whereas animal-pollinated plants produce a median of 3,450 pollen grains for
every ovule, wind-pollinated plants produce almost 10 times as much. No
wonder wind-pollinated plants are the chief causes of eye-itching,
nose-tickling human misery. (It’s not just the anemophilous flowering plants
that are to blame, though. Wind-blown cypress pollen is a major cause of
allergies in some parts of the world.)

This massive production of pollen is usually put down to the inability of
wind to make reliable deliveries. 

Charles Darwin himself suspected the wind of being a fickle and inefficient
messenger, and that view has largely held until this day. But there is
little actual evidence that wind-pollinated plants have more difficulty
getting themselves fertilized than other plants do. (Indeed, plants seem
adept at plucking pollen of the right species out of the breeze. How they do
this isn’t known.) Moreover, in animals, large numbers of sperm tend to
evolve when competition between different males to fertilize a female’s eggs
is fierce. In many wind-pollinated species, plants flower all together, and
for a brief time. Perhaps wind-pollinated plants face greater competition
from their rivals.

But whatever the causes, I’m glad that most plants have not sacked their
bees. In a world pollinated only by gusts and breezes, spring would be less
beautiful. And, for many of us, it would also be more tortured.

Notes:

I have taken species numbers, for flowering plants and all other plants, and
for the number of vertebrate species, from table 2.1 of Baillie, J. E. M.,
Hilton-Taylor, C. and Stuart, S. N. 2004. “A Global Species Assessment.”
IUCN Publications. It can be downloaded here
<http://www.iucn.org/knowledge/publications_doc/publications/> .

The reasons for the surge to dominance of flowering plants have been
extensively debated. A nice summary of the hypotheses is provided by
Berendse, F. and Scheffer, M. 2009. “The angiosperm radiation revisited, an
ecological explanation for Darwin’s abominable mystery.” Ecology Letters 12:
865-872. 

For scorpionflies serving as pollinators for now-extinct groups of plants,
see Ren, D. et al. 2009. “A probable pollination mode before angiosperms:
Eurasian long-proboscid scorpionflies.” Science 326: 840-847 and Ollerton,
J. and Coulthard, E. 2009. “Evolution of animal pollination.” Science 326:
808-809. For early flowering plants being dependent on insects, see, for
example, the discussion in Crane, P. R., Frils, E. M. and Pedersen, K. R.
1995. “The origin and diversification of angiosperms.” Nature 374: 27-33.
See also Hu, S. et al. 2008. “Early steps of angiosperm-pollinator
coevolution.” Proceedings of the National Academy of Sciences USA 105:
240-245. 

For the evolution of bees and their relationship to the evolution of
flowering plants, see Danforth, B. N. et al. 2006. “The history of early bee
diversification based on five genes plus morphology.” Proceedings of the
National Academy of Sciences USA 103: 15118-15123. The association between
pollinators and showy flowers is well known. For foul smells being produced
by flowers pollinated by flies, see Jürgens, A., Dötterl, S. and Meve, U.
2006. “The chemical nature of fetid floral odours in stapeliads
(Apocynaceae-Asclepiadoideae-Ceropegieae).” New Phytologist 172: 452-468.
For beetles and assignation hotels in the form of Philodendron solimoesense,
see Seymour, R. S., White, C. R. and Gibernau, M. 2003. “Heat reward for
insect pollinators.” Nature 426: 243-244.

The number of transitions from animal pollination to wind pollination is
given in Friedman, J. and Barrett, S. C. H. 2009. “Wind of change: new
insights on the ecology and evolution of pollination and mating in
wind-pollinated plants.” Annals of Botany 103: 1515-1527. This paper also
discusses the general shrinkage of flower features in wind-pollinated plants
as opposed to animal-pollinated plants, and provides an overview and
discussion of the hypotheses for what could cause such transitions. 

For the idea that the transition is most likely in insect-pollinated plants
that already allow occasional wind pollination, see Culley, T. M., Weller,
S. G. and Sakai, A. K. 2002. “The evolution of wind pollination in
angiosperms.” Trends in Ecology and Evolution 17: 361-369. See also
Ackerman, J. D. 2000. “Abiotic pollen and pollination: ecological,
functional, and evolutionary perspectives.” Plant Systematics and Evolution
222: 167-185; the author discusses many of the features of wind-pollinated
plants, and presents evidence that wind pollination is less haphazard than
had previously been thought. For the argument that wind pollination takes
off when insects are scarce or in some other way unreliable, see Cox, P. A.
1991. “Abiotic pollination: an evolutionary escape for animal-pollinated
angiosperms.” Philosophical Transactions of the Royal Society of London B
333: 217-224.

For the large numbers of pollen grains produced by wind-pollinated plants as
compared to insect-pollinated plants, see Cruden, R. W. 2000. “Pollen
grains: why so many?” Plant Systematics and Evolution 222: 143-165. For wind
pollination and allergic reactions see, for example, D’Amato, G. et al.
2007. “Allergenic pollen and pollen allergy in Europe.” Allergy 62: 976-990.


For Darwin’s skepticism about the efficiency of wind pollination, see
chapter 10 of Darwin, C. 1876. “The effects of cross and self fertilization
in the vegetable kingdom.” John Murray. This is available from Project
Gutenberg <http://gutenberg.net> . For wind-pollinated plants being able to
pluck the “right” pollen from the air, see Linder, H. P. and Midgley, J.
1996. “Anemophilous plants select pollen from their own species from the
air.” Oecologia 108: 85-87. The relationship between high levels of sperm
competition and high sperm numbers in animals is well documented. See, for
example, Parker, G. A. 1990. “Sperm competition games: raffles and roles.”
Proceedings of the Royal Society of London B 242: 120-126. For the
possibility, and some evidence, that large pollen numbers are due to
competition between males, not to the inefficiency of wind as a means of
delivery, see the Friedman and Barrett paper cited above. This paper also
provides evidence that wind-pollinated plants often flower together, a
mechanism which could both increase the efficiency of wind-pollination and
increase the chance of competition from rivals. 

Many thanks to Jonathan Swire for insights, comments and suggestions.

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