May 8, 2009

Native animals: Are they a help or hindrance to exotic plants?


Article reviewed: Effects of long-term consumer manipulations on invasion in oak savanna communities


by E.W. Seabloom, E.T. Borer, B.A. Martin, J.L. Orrock. Published in the journal Ecology.


The plot line: The authors used some fenced off areas that excluded native (mostly) animals in a central California oak forest. The plots are valuable because some are very old and they have been kept intact since installation (it is a lot of work to maintain them for long periods). They asked these questions:

  1. Do animals help or hinder the spread of exotic plant species?
  2. Do animals help or hinder the regeneration of oaks?
  3. Do large oak trees help or hinder exotic plant species?

Inside of the animal exclusion plots, they found fewer exotic plants. So the answer to question #1 seemed to be that animals helped exotic plants spread. They didn’t find any difference in oak seedling densities inside versus outside of the exclusion plots, but they found what they felt was a dense population of oak seedlings in general (both inside and outside plots). So the answer to question #2 seemed to be that animals neither helped nor hindered oak regeneration. They found fewer exotic plants when the cover of mature oak trees was greater. So the answer to # 3 seemed to be that large oak trees hindered exotic plants.


Relevant quote: removal of exotic plants and exotic consumers alone may be insufficient to reverse invasions.”

Relevance to landowners:

Oak woodlands in California are under a broad spotlight of interest for lots of reasons. There is commercial value (e.g. cattle grazing), development pressure (they are near major California urban centers), and conservation worries (e.g. “Sudden Oak Death”). And they’re easy on the eyes, certainly to people who like trees, but also to people who like verdant, open vistas.

This study points out the importance for landowners to consider the complex ecological interactions between plant and animal communities. While we tend to think of habitat (i.e. the plants) influencing animal populations, the reverse is also true. 


Relevance to managers:

The interesting thing that happened here was that native animals actually increased the number and prevalence of exotic plants. They seemed to have facilitated exotic plant invasions, and it was most likely the smaller mammals that did the facilitating. We tend to think of large exotic animals as being Johnny Appleseeds for exotics, but not in this case. So, if control or reduction of exotics is an objective, one might conclude that the appropriate management action is to shoot all of the animals, especially the small ones. But of course this would lead to a whole new set of more severe problems.

The authors also studied the role of mature oak trees on the presence of exotic plants. Exotic plants were less common wherever adult oaks were present. The implication is that a declining oak population could result in an increasing exotic plant population. They therefore suggest that restoration of oaks will be beneficial to oak woodland ecosystems. While I generally agree with anyone who advocates for more trees, it’s also important to realize there is lot of uncertainty in the degree to which denser forests will inherently keep out exotics, as discussed in last week’s entry. I do agree, however, that a sustainable population of oaks in oak woodlands is a certainly a reasonable management objective.


Critique (I always have one, no matter how good the article is) for the pedants:

They used plots that were originally set up for a variety of different studies, so the experimental design is unavoidably messy. Plots are of different size, age, and number per treatment. They tested for these factors in the analysis and found that they weren’t a big deal, but power of detection was low because some plots had very few reps (two of the plots had unique treatments so replication was 1). So the treatments probably were far from perfect at filtering out exact groups of animals. Nonetheless, their basic message- that animals in general influence oaks and plant exotic plants, seems to be supported by the design.

I don’t see how they excluded mule deer with 2-meter fencing. Anyone with a garden knows that deer can easily jump a 2-meter fence. They could have looked for deer or boar scat to confirm the effectiveness of large herbivore exclusions.

They argue that oaks are “replacing themselves” because they found lots of oak seedlings and saplings. I think this discussion of oak sustainability is a bit misplaced. They didn’t track oaks over time or look at the population profile of ages or sizes, so they can’t really say one way or another whether the population is sustaining itself.

Oaks were called seedlings or saplings if they had a stem diameter of less than 5 meters wide. Now THAT's a big seedling (I'm sure they meant 5 centimeters). I shouldn't complain about type-o's. Anyone who has read this far knows that.