AgriPop Friday Talks

Friday's Agripop Talk

05 April 2013 - 11:30 - Salle Ragondin

Musings on community ecology by a lonely plant ecologist stuck in the forest of Chizé: Can we infer process from pattern?

Nicolas Gross

 

Image from www.stuckincustoms.com/

Stuck in the forest. Can you imagine this happening to a guy who's spent most of his (research) life so far studying nice open grasslands? Well, our research centre is indeed in the middle (sort of) of a forest but Nico is surviving, has been making escapes to study grasslands in Spain (more on that on Friday) and even started to explore the grasslands in our study area outside the forest.

So, why does Nico study grasslands? Well, plants do not escape or hide from researchers and in general do not move (except as seeds or pollen). Hence, a quite convenient study system to investigate what drives the strcture of ecological communities - the main interest of Nico's research. 

OK, but what is that 'pattern vs. process' bit? Well, that's actually a highly contentious one - here an example (hey, I know, it's a paper from 1989, so what! Still interesting to read, isn't it?). A pattern is what we see, like the configuration or traits of a system, but what we actually want to understand is which processes did generate these patterns. Unfortunately, it is often the case that a multitude of contrasting processes could have created the same patterns, which has led to decade-long debates. In general this is related to the problem of distinguishing correlation from causation (here a recent paper on that). One key process that has been often debated is the role of biotic interactions and this is what Nico will talk about this Friday. To exemplify his points he will use a grassland study system in New Zealand, similar to this:

from: http://bit.ly/SSmJZb

Well, nearly -- just 500km more south of that. And the title will actually be slightly less informal. So, here's the official abstract:

Importance of biotic interactions in structuring plant communities: Moving from patterns to processes

How biotic interactions determine the structure of plant communities is a central question in ecology. Different approaches have been used to address this question. These can be broadly separated in two categories: (i) the individual-centered approach; (ii) the community scale approach. The first approach has mainly focused on the species response to local biotic interactions (competition, facilitation) along large ecological gradients. This approach has led to a better understanding on how the intensity of biotic interactions changes depending on local environmental conditions. However, it is difficult from this approach to evaluate how biotic interactions observed at the individual scale translate at the community level and determine the properties of the whole system. 

The second approach has focused on observed pattern of species similarities or dissimilarities within communities in order to evaluate the community scale impact of biotic interactions. In this context, the use of plant functional traits has been proposed as a key metric to quantify species convergence and divergence across communities, the central assumption being that trait divergence reflects competition while trait convergence reflects the effect of environmental filtering. 

However, inferring the impact of biotic interactions from community scale patterns remains challenging because many alternative processes can lead to similar trait distributions at the community scale. For instance, while competition has been primarily hypothesized to promote trait divergence at the community level, superior competitors can also have a disproportionally large effect on local resources and thus act as an environmental filter. Facilitation has also been shown to increase trait divergence at the community level by promoting the coexistence of functionally contrasted species. Finally, abiotic factors are usually considered to cause community trait convergence. They may also act as a disruptive force when different functional strategies exhibit equal fitness in response to similar abiotic constraints (e.g. stress-avoidance vs. tolerance strategies). 

Here, I present some results from a study conducted in highly invaded grasslands (New-Zealand) where we have evaluated how biotic interactions between alien and native species determine the success of aliens in their new environment. I will show how we have tried to merge species and community scale approaches to explicitly test the importance of biotic interactions at the whole community scale.

Figure 1. Highly invaded grasslands in New-Zealand. View from Mt Herbert, 920 m (Banks Peninsula, South Island). Semi-natural grasslands are constituted by a mix of native and alien species (tussock species are Poa cita and Festuca Novae-Zelandiae). Small alien plant species dominate inter-tussock area (e.g. Poa pratense; Lolium perenne; Agrostis capillaris), while tall alien species are more abundant close to the tussocks (e.g. Holcus Lanatus; Dactylis glomerata; Arrhenatherum elatius).

So, see you all this Friday, usual time (11:30) and place.