One of these tulips is not like the others. Photo by Jørund Myhre.
On the last day of April, two blog carnivals—collections of links to posts on a given topic—are freshy posted, and both are worth some of your surfing time.
First, over at Seeds Aside, is a double March/April edition of Berry-go-Round, which rounds up online writing about all things botanical, with everything from peppers to savannah treetops to electrical signalling within the tissues of carnivorous plants.
And then over at her blog on Scientopia, Scicurious is hosting an edition of the Diversity in Science Carnival devoted to “imposter syndrome,” the nagging fear of secret inferiority that almost everyone seems to feel at some point in a scientific career. In science, imposter syndrome can be especially troublesome for women and members of minority groups, who may not see many folks that look like them amongst their colleagues.◼
One way plants can gain an advantage in their dealings with pollinators, seed dispersers, or herbivores is to act collectively. For instance, when oak trees husband their resources for an extra-big crop of acorns every few years instead of spreading them out, acorn-eating rodents are overwhelmed by the bumper crop, and more likely to miss some, or even forget some of the nuts they cache. These benefits of synchronized mass seed production, or “masting,” are straightforward, but how it happens is less clear. A paper in the latest issue of Ecology Letters has an answer — synchronization happens accidentally [$-a].
Bumper acorn crops ensure that squirrels miss a few. Photo by douglas.earl.
Resource limitation works in concert with pollen limitation by catching plants at two stages of the seed-production process. First, if the resources required for seed production are more than can be accumulated in a single year, or if the availability of resources varies from year to year, then some years will be spent building up reserves instead of producing flowers. When reserves are built up, seed production is limited by the availability of pollen to fertilize flowers. Plants that flower when most of the rest of the population doesn’t will fail to set much seed, so they’ll have reserves to make seeds in the next year. This doesn’t require Janzen’s “internal physiological system” for the plants to synchronize, although such a system might evolve to reduce the likelihood of wasting resources by flowering out of synch.
The new paper tests this model in populations of a western U.S. wildflower, Astralagus scaphoides, which flowers at high frequency every alternate year. The authors prevented seed production in the plants by removing their flowers, either in a “press” of three years in a row or in a single “pulse” during one high-flowering year. The plants’ response to these treatments would reveal the role of resource and pollen limitation in synchronizing seed production.
If resource depletion after fruit set prevents reproduction in successive years, we predicted that ‘press’ plants would ﬂower more than control plants every year, as they were never allowed to set fruit. We predicted that ‘pulse’ plants would ﬂower again in 2006, but not set fruit due to density-dependent pollen limitation in a low-ﬂowering year.
The authors also measured the sugars stored in the roots of plants collected before and after flowering in a high-flowering year.
The resource-budget hypothesis worked. Plants prevented from setting seed were forced out of synch with the rest of the population. “Pulse” plants flowered the year after treatment, but because few other plants did, they received little pollen and set little seed. They then had resources to flower yet another year, with the rest of the population this time, and set much more seed, depleting their reserves and bringing them back into synch. “Press” plants continued to flower at high rates each year, as long as they were prevented from setting any seed. Sugar levels built up in the tested roots during non-flowering years, and dropped after high-flowering years.
So masting arises as an emergent result of two limitations acting on plants — the resources needed to make seed, and good access to pollen. A couple of simple rules lead, undirected, to an ordered system that affects entire natural communities.
Crone, E., Miller, E., & Sala, A. (2009). How do plants know when other plants are flowering? Resource depletion, pollen limitation and mast-seeding in a perennial wildflower. Ecology Letters, 12 (11), 1119-26 DOI: 10.1111/j.1461-0248.2009.01365.x
In the natural world, cooperative interactions evolve not as expressions of altruism, but as careful “negotiations” between interacting species. Each player may benefit from the relationship, but each stands to benefit from trying to cheat the other. In this month’s issue of The American Naturalist, we see a prime example: mutualistic ants sterilize their host plants [$-a] to get the most out of the interaction.
The ant species Allomerus octoarticulatus is part of a classic protection mutualism with the tropical tree Cordia nodosa, in which the plant grows structures called domatia that provide shelter for a colony of ants, and nutrient rich “food bodies” for the ants to feed on. The ants, in turn, patrol the plant and drive off herbivores. This mutually beneficial relationship also sets up a conflict of interest. The tree must divide its resources between providing food and shelter for its resident ant colony — growing new domatia and fruiting bodies — and its own reproductive efforts — growing flowers and fruit. The ants, naturally, would prefer for the host tree to spend as much energy as possible on them.
Indeed, Allomerus octoarticulatus has been observed killing the flowers of its host trees. This is what led the new paper’s author, Megan Frederickson, to conduct a simple experiment on C. nodosa, asking whether such pruning prompts the tree to grow more domatia. She experimentally removed flowers from trees occupied by a species of ants that don’t engage in flower pruning to see if pruned trees grew more domatia — and pruned trees grew more domatia over the course of four months than trees that were allowed to flower and produce fruit.
Ant-hosting plants need not be totally subject to the whims of their protectors, however — this kind of regulation works both ways. A study published last year in Science found that ant-hosting Acacia trees cut back on support for their resident ant colonies [$-a] when herbivores are removed and ant protection is no longer needed. (I wrote about this study back when it was released.) It seems likely that flower-pruning ants are exerting strong selection on Cordia nodosa to circumvent this behavior — a new tree variant that can overcome pruning, or make life uncomfortable for pruning ants, should have a large selective advantage.
In the absence of such a mutation, as Frederickson points out, Allomerus octoarticulatus is creating a tragedy of the commons by reducing the long-term viability of its host tree’s populations in exchange for the short-term benefit of more living space. As it stands, Cordia nodosa can only reproduce when it hosts non-pruning ant species, which are a minority in the populations Frederickson studied. Only time, and further study, can determine whether this mutualism might break down altogether.
Frederickson, M. (2009). Conflict over reproduction in an ant-plant symbiosis: Why Allomerus octoarticulatus ants sterilize Cordia nodosa trees. The American Naturalist, 173 (5), 675-81 DOI: 10.1086/597608
Palmer, T., Stanton, M., Young, T., Goheen, J., Pringle, R., & Karban, R. (2008). Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna Science, 319 (5860), 192-5 DOI: 10.1126/science.1151579