For real guys, bigger isn't always better

In my previous post I alluded to a study that framed a potential answer to the question: Why are there so many small plants? Well, here is that study-its results are fascinating.

As I mentioned before, traditionally people assume that bigger is better in terms of plant competition. I also mentioned that the majority of plants are small. So, if the majority of plants are small, how come most plants aren’t big if that is considered to be so advantageous? 
 

It is known that bigger plants generally produce more viable offspring- and this makes sense, right? They usually have more flowers and thus more fruits and more seeds. They are also less sensitive to some of the physical constraints of the environment. So, in the “traditional plant competition theory boxing ring” round 1 goes to the big guys. Let’s think about this though. We have a few big guys with a lot of offspring. But we have way more small guys than big guys. So does the collective offspring of the small guys trump the big ones? Round 2 here we come.

Jesse Chambers conducted a natural field experiment to test this idea. She sampled natural populations of 21 herbaceous angiosperm plant species throughout the Kingston area. Each population was harvested at reproductive maturity by placing a 1x1m plot in the area of highest density-it was assumed this would be where crowding was the most intense.  Within the 1x1m plot, reproductive plants were harvested, bagged, dried and weighed. Plant size frequency distributions were created by dividing the range of individual plant sizes for each population sample into 10 equal deciles of plant size. Relative reproductive output per size decile was calculated as the relative total mass per decile relative to the grand mass total for the entire population. Estimated reproductive output was generated under the assumption that total seed production is proportional to plant mass. 

Figure 1: Size and fecundity relationships for Cardamine parviflora

Here is what Jesse found- it’s pretty cool, but expected. All species had size distributions that were strongly right-skewed. The number of seeds per plant was counted for the entire population of the species Cardamine parviflora and it was found that the mean seed mass did not differ between the 10 largest and smallest plants and fecundity was directly proportional to plant size (See Figure 1). This confirmed the estimate of relative reproductive output. 

Here’s the kicker: For each of the 21 herbaceous study species, the vast majority of the offspring production within the population was contributed by the three, four, or five smallest deciles of plant size. And in 7 of those species the majority was from the 2 smallest deciles of plant size distribution. 

Wait so the smallest plants are collectively producing the most offspring in crowded vegetation? Yeah! They are!

So what does this all mean then? Well, as it says in Chambers and Aarssen “A renowned chemical evolutionist, Leslie Orgel is credited with saying, “Evolution is cleverer than you are””. It seems, well, it seemed intuitive that larger plants would have the ability to produce large and highly fecund offspring, a huge component of plant fitness and that this made bigger better. Chamber’s study is interesting in that is opens the eyes of plant ecologists to a new potentially important component of plant fitness- that being the ability to produce offspring that will survive AND reproduce before death despite having to live as small, suppressed weaklings. This is evidence for the concept of plants having reproductive economy. Reproductive economy is a product of natural selection in plants as a result of the inability of most plants to escape being small as a result of crowding. 

So, bigger isn’t necessarily better. Natural selection does favour large plant size, just not most of the time. Most of the time, plant communities and populations are crowded where most plants are relatively the same, suppressed size. 

I think it’s a knock-out. Round 2 goes to the small plants!





Source: Chamber, J and Aarssen, L.W. 2008. Offspring for the next generation: most are produced by small plants within herbaceous populations. Evolutionary Ecology. 23:737-751.