Martorell, C. & R.P. Freckleton. 2014. Testing the roles of competition, facilitation and stochasticity on community structure in a species-rich assemblage. Journal of Ecology doi:10.1111/1365-2745.12173
At a given location in nature, why are some species present and others absent? Why do some species thrive and have lots of individuals and others are barely eeking out an existence? What determines how many species can live together there? These questions have fascinated (some might say obsessed) community ecologists for an almost embarrassing number of decades. They have proven difficult questions to answer and everyone has their favorite process they like to use to answer those questions. Competition for limiting resources is perennially a favorite process used to explain who gets into a community and who does well once they’re in it. But there are also a number of other processes that clearly play important roles. Theory and data are showing that the movement of species from location to location can alter what species exist where and how many individuals they have at a site. The role of facilitation (positive interactions among species) has increasingly been getting play as well, especially in stressful environments. There can also be a random component to the order that species arrive at a particular location. Because it can be difficult for very similar species to coexist, who is already at a location can influence who can then get into that location (this is sometimes referred to as historical or priority effects). I’m sure I missed some processes and I’m equally sure that someone out there right now is upset I didn’t include theirs. Others might (and by might I mean probably will) disagree with what I’m about to say, but most of the time it seems to me that we spend most of our time arguing about which process is most important. It’s competition! No it’s dispersal limitation! Niches! No niches! I have come to find this binary approach to studying communities wearisome. And here’s why. Does competition influence who exists at a particular location? Yes. Does dispersal? Yes. Does facilitation? Yes. Do stochastic processes? Yes. Do priority effects? Yes. We are at a point in ecology where I think we can feel confident that these various processes both exist and that they affect what we see in nature. Instead, we need to figure out how these processes work together to create the communities we observe. Does the role of a process stay constant through time? Or does it change depending on whether a community has been recently disturbed or is more established? Can we weave together these processes to predict how a community will look through time?
Right about now, you’re wondering if I will ever actually mention the Martorell & Freckleton paper. Here you go. Martorell & Freckleton (2014) take data from a long-term study of plants in Mexico and analyze all the pair-wise interactions among species in order to “document the intensity and demographic importance of interactions and stochasticity in terms of per capita effects, and to set them in a community context”. In effect, they used population models and the spatio-temporal data on plants to assess for each species observed how its presence and population growth/abundance was impacted by interactions with other species, interactions with individuals of the same species, variability in the environment, dispersal, and population stochasticity. If you want to know how they did this, you’ll need to read the paper. They found that both competition and facilitation between species played an important role in determining whether a new species could colonize a particular site. Once established, competition and facilitation played less important roles in explaining the abundance of species. Most of the variation in abundance between species can be explained by interactions with other members of the same species and by stochastic events influencing dynamics at a location.*
So why do I like this paper? Because it’s a step towards that integration of processes that I think we need to start doing. Their end message isn’t: process x affects ‘thing I’m interested in’ y. Their end message is about how these processes are working together and when they play a more (or less) important role for determining what species are present and how well they are doing at a site. Their results suggest a model of communities where interactions among species influences who establishes at a particular location (i.e. the species composition in community ecology lingo). However, stochastic events and interactions among members of the same species become important for understanding differences among species in abundances and population growth rates. Only time will tell if this particular integration of processes holds across different types of ecosystems. But right now it allows us to start talking about more sophisticated models of how species come together to create the diversity of species and abundances in a community.
And what does this paper say about predicting the species in a community and their abundances? My interpretation is that it says what I think a growing number of us have suspected for a while. For a specific location there is not a single expected configuration of a community. There are many possible configurations. This means that precisely predicting the species composition of a community will be difficult. But it also makes me wonder whether it might be possible to predict the space of possibilities and how probable those possibilities are. Given this disturbance rate and this pool of possible species, there’s a 60% chance of this configuration of species, but only a 10% chance for this one. I suspect many of my colleagues think that even this level of prediction or forecasting is pure science fiction thinking on my part. But like some of my other blogging colleagues (hi, Brian! hi, Peter!) I believe that pushing our field from one focused on ‘understanding’ to one focused on ‘forecasting’ or ‘predicting’ is one of the greatest challenges our science faces**. Figuring out how and when different processes operate and what aspects of community structure they are controlling is the first step towards forecasting. And that is exactly why I like this article.
* Disclaimer: I’ve distilled the paper down to the core message of what I found interesting and why. To understand what Martorell & Freckleton did, all of their results, and what they thought made their results interesting, you should really read the paper.
**Acknowledgments: Sadly, I can’t also link to the long and awesome conversations that Ethan, Allen Hurlbert and I have been having on this topic while on sabbatical. Trust me, they’ve been revolutionary experiences that you wish you were there for.