Why do we see a species growing on one hillside and not another? Why are some fields dominated by native plants, and others exotics? These are questions addressing the establishment of plants in different habitats, a core concept in ecology with many theories to go along with it. These theories split into a mix of those looking at (1) characteristics of the environment (extrinsic factors, like soil moisture or propagule pressure), and those looking at (2) traits of the species (like competitive ability or growth rate).
(1) Perhaps a species is not found on a hillside because wind patterns have never carried its seeds in that direction, or maybe an exotic could only establish in a field if enough of its seeds were dispersed there (propagule pressure). These are extrinsic factors, qualities of the environment or conditions that make it more or less likely for a species to establish. We can think of these as the first filters determining which species can inhabit an area – the traits of a species will never come into play if there is no initial introduction.
(2) Next, a species can only grow on a particular site if its traits help it survive and reproduce in those conditions. The types of traits can be divided into two types: those that help the plant deal with abiotic features of the environment, and those that help the plant deal with other community members, or the biotic factors in the environment.
What they did: In this study, Kempel and collaborators performed a field experiment to determine the role of extrinsic factors and species characteristics in determining species establishment. They introduced 45 native and 48 exotic plant species into an existing plant community, and measured establishment over three years. To determine the role of extrinsic factors, they manipulated disturbance and propagule pressure, and measured existing biomass in each plot. To determine the role of species characteristics, they included a large number of species in the experiment, and measured their traits in the greenhouse – performing competition manipulations, herbivore feeding trials, and measuring a suite of other traits.
What they found: A lot of their findings support basic theory:
- No disturbance and high standing biomass decreased establishment of added species (biotic resistance)
- Establishment increased with increased propagule pressure
- Species that were better defended against generalist herbivores outperformed those that were not.
More interesting, though, is that the role of these factors change over time. Extrinsic factors are important in year one (yellow and pink in figure), while over time the relative importance of species traits increases, especially those related to biotic interactions (like herbivore resistance and competitive ability, purple in figure). They conclude that the role of a particular trait in a species’ establishment is context dependent, based on what the extrinsic factors are.
Why it’s cool: This paper is particularly cool because of the large effort needed to quantify traits for 93 plant species. Though some traits can be found in trait databases, traits that relate to biotic interactions, like competitive ability, typically cannot. Only with a huge effort like this could a lot of theory be tested.Kempel, Chrobock, Fischer, Rohr, and van Kleunen (2013). “Determinants of plant establishment success in a multispecies introduction experiment with native and alien species.” Proceedings of the National Academy of Sciences of the United States of America 110(31): 12727-12732.