Gonzales and Arcese: Herbivory more limiting than competition on early and established native plants in an invaded meadow

Here is it, the big question! Which is more important: Competition or Predation? According to Gonzales and Arcese...in this case...predation wins! (kinda)

Here's a short review of what happened in this study. This study was conducted in a fragmented oak meadows in Canada. In these meadows, there was a native grass that is declining in abundance compared to an invasive grass that is increasing. On its face, this appears to be an open-and-shut case of competitive exclusion of the native by the invasive. However, the researchers also mention that this habitat is fragmented due to human activities, which along with general disturbance often have the effect of decreasing the local carnivores that would keep the herbivores in check. This has lead to an explosion of the deer population in these areas, which could have increased the effect of herbivory (predation on plants). And so with all this in mind, Gonzales and Arcese propose 3 hypotheses: 1) The invasives are out-competing the natives 2) Deer are consuming too many of the natives compared to the invasives or 3) some combination of both. They then conclude by saying they think that herbivory has the greatest effect

(completely unrelated to ComEco, this sort of reminded me what was asked in Stats last night about just starting with multiple hypotheses and then following the one that's the most right....does anyone else see that?)

In the experimental design, there were several. First with the grasses, they had a 2x2 factorial where they have transplanted native grasses and native grass seed addition that varied by + or - herbivory and + or - competition (either by clearing or severing the roots of neighbors). And this design was repeated for the flowering plant seed addition experiment. In a 3rd experiment, they did a bulb addition experiment of lilies, but how they set this up was a little hard for me to follow. And just to quickly summarize their results; all of the graphs for all of the experiments show that when you remove herbivory, the biomass for all examined species increased.

So what does it all mean? Well, after re-examining their hypothesis, they decide that the dominant effect on the decrease of the native plants is due to herbivory. When they first approached the problem, the usual suspect to blame in these cases seems to be competitive exclusion. However, after this experiment, this seems to be a case of "apparent competition" where because the natives where being preferentially fed on by the deer, we could only see that the natives were declining and the invasives were increasing.

And discovering that herbivory (plant predation) was having a larger effect on the natives than competition seems to be a complete 180 degrees from Errington's Hypothesis. Remember that Errington thought that predation would only have a small effect on organism abundance. Yes, the full hypothesis is meant for larger, motile organisms, but if you squint and turn your head to the side, you can see how part of it could be shown to apply (and then fail) for this situation. For example, Errington says that the survivors would have increased fecundity, presumable because their kin are not there taking up resources. However, this leads into the second part of Gonzales and Arcese's finding that herbivory isn't the whole story. There is also a competition effect! Sure the native grasses should be able to re-fill the spaces where their kin had been consumed from, but the invasive is just better at filling those empty spots.

So, in conclusion, Gonzales and Arcese show that herbivory can have dramatic effects on the welfare of native plant species in disturbed ecosystems that can lead to giving the invasive a competitive advantage. And another theme that we keep seeming to come back to; deer are evil and the cause of a lot of ecological damage. And if you needed more proof that deer are evil, I submit the following picture as my evidence. See you in class tonight!


Figure 1: Aren't they cute with that maniacal twinkle in their eyes?You have to click on the picture to see the whole thing. The blog has cropped it so you can't see all of it. I did a google image search for "evil deer" and this was the best shot. Seems like we keep coming back to how evil deer are with their tendency to mess up plant communities when their numbers get too high.