Plants with extra-floral nectaries

I have a special interest in Macaranga and Cecropia which leads to an interest in ant-plant relationships.

Many people know that many flowers have nectaries, i.e., glands that secrete sugary solutions, which attract pollinators. Not many people know, however, that many plant species also have such glands outside of the flower tube, such as on leaves or along newly developed twigs. Obviously these will have little use in pollination. (Sometimes these glands secrete oils instead; not sure if these are still called “nectaries”?)

Ants tend to be attracted to plants with extra-floral nectaries, therefore some plants appear to be always covered with ants. Such plants are called “myrmecophiles”. When the ants actually live and form colonies within the plant itself, the plant is called a “myrmecophyte”, or a true ant-plant. Contrary to common misuse of the term, not all Macaranga and Cecropia species are myrmecophytes; many Macaranga are actually myrmecophytes. A telling sign of myrmeco-phily and not -phytism is that the twigs are solid and not hollow.

And anyone with some painful field experience in the tropics will tell you that these ants bite. So this leads to the hypothesis that the ants defend the plants from herbivores, or perhaps even cut away climbers that may otherwise smother the young plant over time. It’s an attractive hypothesis because ant-plants tend to be fast-growing, light-demanding species, like Macaranga and Cecropia. So the benefits of getting rid of free-loaders of such a fast-growth, high-productivity strategy is high relative to the energetic costs of giving away some sweets.

If so, the plants with extra-floral nectaries should have higher fitness, e.g., higher growth rates and lower mortality.

An article just out by Muehleisen et al. (2016; Biotropica 48: 321) tried to test this, but found, after correcting for the phylogenetic conservatism of the extra-floral nectary trait, that there was no evidence for higher growth and survival rates.

Actually, I was more drawn to Figure S2 in the Supporting Information:

muehleisanetal2016_biotropica_figs2_families_extrafloranectaries_yasuni-pasoh-bci

At Pasoh, which is nearby in Negri Sembilan, Peninsular Malaysia, the species-rich families with the highest proportion of species with extra-floral nectaries are, in decreasing order: Euphorbiaceae, Dipterocarpaceae, and Ebenaceae. The Pasoh data comes from Fiala & Linsenmair (1995; Biodiversity & Conservation 4: 165).

At the two Neotropical sites (Yasuni in Ecuador and the Barro-Colorado Island in Panama), the legumes (Fabaceae) have the highest proportion of species with extra-floral nectaries.

Legumes here in tropical Asia also tend to have extra-floral nectaries… But we’re not quite as species rich in legumes as in the Neotropics. Other notable families here with extra-floral nectaries are the Chrysobalanaceae and the Salicaceae.

Many Euphorbiaceae (which include the Macaranga but also MallotusClaoxylon, Croton, etc.) in tropical Asia also tend to be disturbance-adapted species, i.e., abundant in forest gaps and along edges. On the other hand, the Dipterocarpaceae are the flagship of climax species here… But actually even among the dipterocarps, there is a gradient from the faster-growing and more shade intolerant to the slower-growing and more shade tolerant. May be worth looking into whether it is the faster-growing, light-loving dips that tend to have extra-flora nectaries?

Which leads me to a point about the approach in the paper. To be honest, I only glanced through the methods and the graphs, but I suspect the fitness advantage of extra-flora nectaries will not be evident in a simple test using mean growth or mortality rates, because it is confounded by the trade-offs in specific plant strategies. The Pasoh, Yasuni, and BCI multi-hectare plots that were used in the analysis are more-or-less intact forests. In such communities, you can expect that coexisting plant species is, on the average, already occupying close to the optimal fitness conditions that its ecological strategy is suited for. Naturally, those that don’t need extra-flora nectaries where they are growing will be doing well without them, and vice-versa; otherwise, if there is a residual fitness advantage, you would have expected species with extra-floral necatries to displace those without, over time. It’s a basic paradox of species coexistence and dynamic community equilibrium at the ecological time scale.

A better approach, I think, would be to break it up into two hypotheses:

(1) Species with extra-floral nectaries tend to have more ants (or some other “defenders”) on them.

(2) Plants with more ants on them tend to grow faster and survival better, all else constant.

The first may sound a bit duh, and is probably already documented somewhere. I am guessing that the second is, too. The crux is all else constant: the basis of comparison must be the same. For example, you could take both categories of plant species out of their comfort zone, i.e., put plants without extra-flora nectaries in the places where plants with extra-flora nectaries occur, and vice-versa. Or you could exclude ants from plants with extra-flora nectaries (e.g., taping sticky traps around the base of the stem and trimming off any plants that might serve as bridges for ants) and add fake nectaries to plants without (e.g., sweets??).

Other improvements could be to further break down (2) into two steps that reflect the hypothesized link to better fitness, which in this case could be herbivore attack or climber infestation. I think boldly stating these mechanistic links is the way to go. Also, in ant exclusion/sugar addition experiments, one may need to account for the ontogenetic shift in growth/mortality rates over time/size classes, which are obscured by averaging growth rates. Pioneers tend to show a distinct peak in maximal growth rates at small-to-intermediate size classes, while climax species show a more flat growth-size relationship. Ants may be more necessary in the early stages of rapid growth, and not so much when the pioneer tree is already shading out the undergrowth below but slowing down in growth. Likewise, mortality is U-shaped with respect to age/size class: highest for the youngest, and then gradually increasing again to claim the old.

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