Crops may benefit from being close to natural areas

Well, I’m back after a long semester teaching about plant ecology and evolution!.  I’d like to share with you an article I just read from Ecology Letters, authored  by Lucas Garibaldi and colleagues.

Because effective pollination is so important to crop output and many natural areas have high pollinator abundance and richness, people have wondered if declines in natural areas may have negative effects on agricultural areas. These negative effects may range from decreases in pollinator richness to decreases in pollinator stability over time and space.  Pollinator stability is generally thought to be a good thing because it provides for consistent and predictable pollinator service for crops.  Unfortunately, there is not a lot of information about pollinator stability out there.  This is where this study comes in.

The main question of this study was whether proximity to natural areas A) had an effect on pollinator richness or visitation rate or B) had any effect on the stability of pollinator richness or visitation rate.  The authors (there are many!) collected many studies that individually looked at pollination service and distance to natural areas and analyzed them all together, which is termed a meta-analysis.

The upshot is that the scientists found that both pollinator richness and visitation to crop plants declined with increasing distance from natural areas.   Stability in pollinator richness and visitation also declined with distance from natural areas.  These results held even when honeybees, which are introduced pollinators from Europe, were excluded from the meta-analysis, suggesting that the benefits on crop plants were due to native bees! So, just ‘bee’ aware out there that pollinators originating from natural areas could be helping put fruits and vegetables on your table….have a good holiday season!

Fieldwork in Arizona: Searching for the lost pollinators!

A female honeybee approaching a Datura flower

Well, I’ve been in the field with my student assistant Skyler Jordan (DU ’13) for about two weeks now and we are finding some interesting things.

Datura bud, about to open in the evening

Our aim on this mission was to first discover if honeybees were visiting Datura wrightii flowers (see picture), and if they were collecting pollen, collecting nectar, and/or pollinating the plant. This is interesting because most folks think that Datura is mostly hawkmoth-pollinated, as a flower blooms at sunset, emits a strong sweet odor, and then shrivels up in the heat of the next day.

So far, we have found that the majority of visits to D. wrightii are by honeybees at this point in the season (this probably changes as time goes by), and that the bees are carrying major loads of D. wrightii pollen, both on their abdomens and also in their pollen baskets. So far so good, but we still don’t know if they are actually bringing pollen to the female part of the plant, the stigma. To do that, we are going to let single bees visit flowers to see if that one visit is sufficient to set seed. Wish us luck! More updates from the field in later posts…

What inbreeding does to flowers

Datura wrightii flower

Hi all, it has been some time since I have posted a story, I apologize.  I have been traveling the country, visiting awesome plants and ending up in Tucson, AZ.  I’m studying  Datura wrightii flowers (seen at left) and their pollinators and herbivores with Dr. Judie Bronstein at the University of Arizona.

In any case, I last threatened to discuss the implications of having sex with yourself.  Now, the answer may be intuitive.  For example, humans normally do not wish to mate with close relatives because a) it seems abhorrent to us, and b) it probably seems abhorrent because the children of these relationships often have severe physical or mental problems.

Why does this happen?  Let’s say there is a gene mutation out there that causes severe physical problems, and let’s say it is recessive, or that it is masked by the normal version of the gene when they are together in a cell (called the heterozygous state).  Now this normally doesn’t pose a problem because of the masking effect.  If, however, two people who are both heterozygous (likely if they are related to each other) mate, there is a 25% chance, on average, that their offspring will have both forms of the mutant gene (called the homozygous recessive state).  Then, there is no masking at all,  and the mutation causes the negative changes in that offspring.

The same thing happens in plants, except that they can actually have sex with themselves, and don’t need another separate plant.  So, inbreeding in plants can cause serious problems in the offspring.  ‘Like what?’ you might ask?  Well, read on!

In a seminal work using our old friend Mimulus guttatus, Ivey and Carr (2005) manipulated plants so that one set was produced from inbreeding parents and one set from non-inbreeding, or outcrossing parents.  They then measured important floral traits like the diameter of the petals (together they are called the corolla), the length of the petals, and the distance between the stigma (the female bit) and the anthers (the male bits).  They also counted how many flowers were produced and seed set.  Finally, they counted how many pollinators visited the plants and how long each pollinator took to forage on the flower.

Figure showing effect of inbreeding on seed set

Not surprisingly, inbreeding significantly decreased corolla diameter, corolla length, the number of flowers produced, and the number of seeds produced (see figure above).  This might be expected if mutant genes are being expressed in the homozygous state through inbreeding.  Perhaps through the effects on corolla size, inbred plants also received fewer initial pollinator visits

The implications of these results are that once inbreeding starts, plants may get fewer pollinator visits and thus may start to self-pollinate, leading to even more inbreeding.  One could imagine that this cycle could continue until the population goes extinct or the population evolves to be completely self-pollinating, which we sometimes see in nature.  It might be more likely that a few outcross pollen grains landing and fertilizing eggs in the population bring enough genetic diversity to avoid such  cycles.  A little genetic diversity can go a long way!

With that, I’ll sign off — I hope everyone has a bloomin’ Fourth of July!

Reference:

Ivey, C. T. and D. E. Carr.  2005. Effects of herbivory and inbreeding on the pollinators and mating system of Mimulus guttatus (Phrymaceae).American Journal of Botany 92: 1641-1649

Invasive plants can alter pollination of native plants

Foxglove, Digitalis purpurea

Exotic plants are species that those that have been moved, usually by humans, from their native habitat to another area where they do not occur naturally.  Invasive exotic plants are those that significantly impact the ecosystem in the introduced area.  These types of plants spell big trouble for many areas in the United States, and some are even known to the general public like kudzu and purple loosestrife.

We have known for decades that invasives can negatively impact natural species by competing for resources such as soil nutrients and light.  Unfortunately, we do not have a good grasp on how invasive might affect the pollination of naturally-occurring species.  There are two main hypotheses about the effects.  First, invasives that occur in high densities may actually attract lots of pollinators to other plants in a given area, thus benefitting the co-occurring natives (the ‘magnet species effect’), or the invasives could ‘steal’ pollinators that would normally visit the native species.  Few studies have tested these hypotheses over multiple populations, which brings me to this week’s topic experiment.

Dietzsch et al. (2011) were interested in how the invasive plant Rhododendron ponticum affected pollination of the native Digitalis purpurea, or foxglove, in Ireland.   As an aside, Foxglove is famous because it produces digoxin (digitalis), which is used to treat some heart disorders.   It is worth noting that foxglove can be found in the U.S. too, but here it is considered an exotic because it is native to Europe.  The authors surveyed six areas that contained foxglove, but differed in the amount of invasive Rhododendron present.  At each site, they     counted how many times pollinators visited patches of foxglove, how much foxglove pollen was found on foxglove stigmas (the female part), and how many seeds, on average, were produced per fruit.  Simply put, if the ‘magnet species effect’ was occurring, then pollinator visitation, the amount of Rhododendron pollen found on Rhododendron stigmas, and Rhododendron seed set would increase with higher amounts of Rhododendron, whereas you would expect the opposite relationship if Rhododendron was taking pollinators from foxglove.

The researchers found that as the amount of Rhododendron increased, pollination visitation rates to foxglove decreased, thus supporting the idea that the invasive plant was stealing pollinators from the native plant.  They found that conspecific pollen (pollen from foxglove) found on foxglove stigmas also decreased with increasing amounts of Rhododendron,  supporting the hypothesis that invasives were stealing pollinators from foxglove.   Interestingly, the authors did not find that seed or fruit set declined with increasing amounts of Rhododendron, even though pollination did decline.  One explanation for this puzzle is that foxglove can produce seeds from self pollen that is deposited by a visitor, even if the visitor just bumbles around the flower one time without depositing any pollen from another plant.  So, like Mimulus guttatus from last week, foxglove may be able to assure some sort of reproduction by self-pollination.  Again, thatleads us to the problem of inbreeding depression, which is where I will take us next week…  Until then, take a close look around you and enjoy the plants!

Reference:

Deitzsch A.C., Stanley D.A., and J. C. Stout.   2011.  Relative abundance of an invasive alien plat affects native pollination processes.  Oecologia, published online April 12 2011

Can plants evolve if pollinators disappear?

Mimulus guttatus flowers

Recently, beekeepers have noticed drastic declines in honeybee populations (termed ‘Colony Collapse Disorder’), with the latest culprit thought to be a combination of a fungus and a virus.  These declines could have big impacts on the price of certain fruits and vegetables dependent on bee pollination.  Other types of bees, like bumblebees, are also under threat in their native habitats, not by viruses but by other imported bumblebee species.  Because many native flowers in the U.S. are dependent on bumblebees for fertilization, researchers are also investigating how these plants might evolve in face of pollinator loss.

To look for this type of evolution, Roels and Kelly (2011) devised an experiment that compared evolution in two experimental environments: one environment consisted of Mimulus guttatus plants in a greenhouse without any pollinators; the second environment was plants in a greenhouse with a colony of bumblebees.  The researchers let the bumblebees pollinate freely among the plants in this environment while the flowers were open and receptive.  After one generation, they collected and replanted a subset of the resulting seeds, and re-did the experiment.  They did this for five generations, collecting seeds each generation, mimicking the action of evolution.

Not too surprisingly, they first found that the plants without pollinators set fewer seeds than the plants with pollinators.  After this initial decrease, though, the plants without bees started setting more and more seed, even with the continued absence of pollinators.  The researchers found that this was, in part, due to the plants’ increased ability to self-pollinate, with pollen from a single flower able to fertilize the same flower.  What’s cool is that the self-pollination was facilitated by the evolution of a smaller stigma (female part) – anther (male part) distance in the pollinator-free treatment.  The smaller the distance, the easier it is for pollen to land on the stigma.  So, it seems possible that some plants can evolve to have sex with themselves rather than wait around for a bumblebee to do their dirty work for them.  There are problems, of course, with having sex with yourself, but that is another story….stay tuned!

Reference:

Bobdyl-Roels, S.A. and J. K. Kelly.  2011.  Rapid evolution caused by pollinator loss in Mimulus guttatus.  Evolution, accepted article

Flowers are caught looking downslope, for good reason!

Viola blanda

As Spring progresses here in central Ohio, I’ve had the chance to stroll the hillsides, looking at the spring ephemerals that have flowered in April or will flower shortly.  Some of these hillsides are quite steep and studded with plants like Spring Beauties (Claytonia virginica) and the Common Blue Violet (Viola sororia).  I’ve always been interested in how these plants can get fertilized in the short time between when they emerge and when the understory becomes shaded by the canopy trees.  In addition to confronting the short window of light, these plants may also have to deal with the nasty cold weather of April.

It turns out that one way that such plants might improve their chances is to orient their flowers where the most open space exists – that is, the side facing away from the hill slope.  Now, dear reader, when I first heard of this hypothesis I thought it was rubbish, as I thought that any old direction was OK, as long as the flower was open.  But, it turns out that some scientists in Japan have actually looked to see if hill-loving plants actually turned away from the hill.

First, they measured the Angular Distance between the direction of the Slope and the Flower direction (ADSF, see Figure 1) on flowers from ten different species of plants.  They simply wanted to see if the ADSF was closer to 0 degrees than one would expect by chance alone.  They also wanted to see if flowers were more likely to point downslope as the slope of the hill increased because the steeper you get, the closer your flower could be against the wall, so to speak.  Finally, Ushimaru et al. (2006) did a clever thing – they actually changed the ADSF of 30 flowers to face the slope while leaving 30 flowers unchanged.  If the ADSF was important, then you might see decreased fitness in the changed flowers.

Non-random distributions of ADSFs

They found that for all ten species, the ADSF was significantly lower than one wouldexpect by change – it certainly did seem that the flowers pointed downslope rather than any other direction (see Figure 2).  Furthermore, as the slope got steeper, the ADSF got smaller – exactly what you might predict if steeper slopes afforded less area for pollinators.  But as these are observations and there might be other, unmeasured factors that were causing the pattern, the manipulative experimentwas key. Here they found something really cool – flowers that had their ADSFs altered exported significantly less pollen than flowers pointing downslope, thus showing that floral orientation certainly mattered for the male component of fitness.  All in all, a pretty short and sweet paper that certainly pointed my attention in another direction this spring.

Reference:

Ushimaru, A., D. Kawase, and A. Imamura. 2006. Flowers adaptively face down-slope in 10 forest-floor herbs. Functional Ecology 20: 585-591.

Why pseudocopulation is as bad as it sounds

Most folks know that bees visit flowers to collect nectar, and that the bees also transmit pollen from one plant to another.  It’s  a trade that has been shaped by evolution: the bees get food, the plant gets sex, and all is well in the world.  But, of course, there are exceptions to this romance.  Some orchids have shaped by selection to drop the nectar reward, probably to save on resources, so how do they get bees to visit?

Well, pseudocopulation of course!  This is exactly what it sounds like – there’s a lot of action, but the deed is not actually done.  The thrifty orchids have been selected to produce a bottom petal that looks and smells like the fattest, most egg-heavy and receptive female the male bee has ever seen.  The male bee then goes crazy and tries to copulate with the plant, repeatedly and with much frustration.  There are some pretty funny videos of this.  Pretty soon the male gets the point and tries to find another female-looking object  – which is usually just another orchid petal – so he again gets busy with another plant, thus moving pollen from one plant to another.

This is bad news for the male – he wastes time and sometimes ejaculate on a plant instead of a lady bee.  But female bees in the vicinity also suffer – it turns out that the more orchids there are in the area, the less likely a male bee is likely to try copulating with anything – it appears that the male bees learn that things that like females are bad news.  What’s more, the females in many species cannot fly and thus can’t go looking for males!  So, what is a female bee to do?

Graph showing increased attractiveness of females with distance from an orchid patch (Wong et al. 2004)

A work by Wong et al. (2004) suggests that the females may actually just pack up and walk to areas with less orchids.  When the experimenters placed female decoys that looked like and smelled like real females at ever-increasing distances from an orchid patch, males attempted more copulations the farther away the decoys were from the patch (see graph at left).  So, it appears that the females may actually increase their odds of mating by crawling away from areas with orchids, which is just damn cool.

Reference:

Wong B. B. M., C. Salzmann, and F. P. Schiestl. 2004. Pollinator attractiveness increases with distance from flowering orchids. Proceedings of the Royal Society of London Series B-Biological Sciences 271:S212-S214.

Why petals are more than eyecandy to pollinators

As a flower-lover, I have had many occasions to observe plants over days, weeks,and even years.  One phenomenon that has always intrigued me is petal movement in plants – sometimes I see flowers that can open and close during the day (e.g. Baby blue eyes, Nemophila menziesii). What is particularly puzzling about the flowers that close during the day is that this action makes it very difficult for pollinators to do their business, so that reproduction could be hindered.

Van Hase et al. (2006) have recently published a paper looking at the function of flower closure in the South African region of Namaqualand, an area of stunted forests and shrubs that receives frequent winter rain.  Many of the plants in this area also have flowers that close during the day, and the authors hypothesized that floral closure may protect pollen from water damage.  To test this, they measured a bunch of environmental variables to see if they were associated with floral opening and closure.  Then they actually manipulated flowers in eight species by folding back the petals with a rubber band to get at the function of petals as protection devices.

They found that the single most important variable in determining floral closure was ambient temperature – models with this factor explained 60% of the variation in closure times.  The relationship between temperature and closure was sigmoid, meaning that there was a threshold of temperature after which lots of flowers began to close.  They also found that in half of the manipulated plants, leaving petals open significantly decreased the amount of damaged or unviable pollen – suggesting that at least in some plants floral closure may serve to protect pollen and thus male sperm.

What is really interesting about this study is that the authors show that petals, which are normally thought to serve as pollinator attractors, can also serve to protect pollen even before it is moved by the pollinators.  We also know that rain can disrupt pollination events even after the pollen reaches the female part, so petal closure could also protect the sperm as they move towards the plant ovule.  All in all, I think that we’ll find that petals and other reproductive structures can do a lot more than just attract mutualists.  So, I hope you’ll appreciate both the beauty and function of petals this spring as you tiptoe through the tulips!

Reference:

Von Hase A., R. M. Cowling, and A. G. Ellis. 2006. Petal movement in cape wildflowers protects pollen from exposure to moisture. Plant Ecology 184:75-87.

DOI 10.1007/s11258-005-9053-8