A helping hand for inequality
Competition is a powerful force in biology. The image of ‘Nature red in tooth and claw’ calls to mind animals fighting over food or mates, and plants sucking up nutrients, or overshadowing their neighbours. But, particularly where the environment is harsh, it’s known that living things can actually give each other a boost. The most famous examples are symbioses, where both partners benefit from the interaction: ant plants, for instance, house a colony of ants, which defend their host. But in other cases, termed facilitation, help seems to be given more inadvertently.
The grass studied. Image credit: S.G. Reynolds, FAO.
In a hot, dry desert, the shade from one plant may be the best place for another to grow. One microbe breaking down a piece of dead wood may make it easier for another to live there. In the alpine meadow in Tibet that this study examined, the story is probably that plants slow down the evaporation of water from the soil, and possibly also shelter each other from particularly cold conditions.
Competition has long been known to promote inequality. If a group of plants start off about the same size, those that have a little advantage, for whatever reason, will grow quicker, and that gives them a bigger advantage. So their sizes spread out, as the successful competitors press home their advantage.
The new research finds that facilitation can also increase inequality. Is that a perverted act of charity, which gives most to those who’re already rich? Thankfully, it’s more a matter of chance: plants don’t grow evenly spaced, and those that happen to be near neighbours benefit more from facilitation than those that are isolated. The scientists do suggest, however, that larger plants may be more likely to get close enough to neighbours for the positive effect, which is more like ‘giving to the rich’.
Most of the paper concerns a computer model which they used to demonstrate this. But they did also an experiment with the grass pictured above, Elymus nutans (it doesn’t seem to have an English name). In line with the model, inequality increased at the lowest density (due to facilitation) as well as at higher densities (due to competition). Inequality is measured as the ‘coefficient of variation’, which is the standard deviation (how much measurements differ from the average) divided by the average (the mean).
Graph of inequality versus density
This was tested in experimental plots of 1m² each. That’s a good way to test for an effect at first, as it simplifies the situation. It will be interesting to see whether this sort of thing is important in natural communities, particularly as that involves more than one species interacting.
Reference:
Chu, C., Weiner, J., Maestre, F., Xiao, S., Wang, Y., Li, Q., Yuan, J., Zhao, L., Ren, Z., & Wang, G. (2009). Positive interactions can increase size inequality in plant populations Journal of Ecology, 97 (6), 1401-1407 DOI: 10.1111/j.1365-2745.2009.01562.x
