“Anyone who has ever pulled weeds out of their lawn will understand,” says Dr. Joe Fargione, of Cedar Creek Natural History Area, about his paper published last July in the Proceedings of the National Academy of Science. What the study found was that in experimental plots where grassland plants were growing, new plants had a harder time invading and establishing themselves if there were already plants there that had similar characteristics and functions.

Invasion in the lawn!	Plant community assembly and invasion…

The experiment consisted of plots that had between 1 and 24 species of plants growing in them. When these plots were four years old, the experimenters chose seeds of 27 other plants and added them to portions of the plots. After giving them three years to grow, the experimenters then measured how well the introduced species had fared. The experimenters also measured the plant resources available in the plots – like the amount of bare ground that had been there at the beginning, the amount of nitrate and water in the soil, and the amount of light that made it through the foliage.
The results of the experiment suggested that different groups of plants took advantage of the available resources in different ways – for example, some grew in the spring while others grew later in the summer, or some had really shallow roots while others had really deep roots – and it appeared that if there were many types of plants in a plot, there was less of a chance that an invading plant would have the right characteristics for allowing it to get the resources it needed. “The simplest way to phrase it,” says Dr. Fargione, “is that there are fewer vacant niches.” Plus, the results indicated that an invader was the most inhibited from surviving when it tried to grow in a plot where there were already plants that had nearly the same strategies for getting nutrients as the invader.
These results make a lot of intuitive sense, especially when you transpose it into other areas of life. For example, imagine three neighborhoods: the first has an abundance of butcher shops up and down the streets, and they are all constantly vying for customers to come buy their wares; the second has bakers and candlestick makers on every corner but nowhere to buy meat; and the third has a butcher, a baker, and a candlestick maker, and a café, deli, flower shop and bookstore to boot! In the first neighborhood it would be really difficult for a butcher of any sort to set up shop because all the customers already have a place they regularly go. However, it would be very easy for any other types of businesses to enter the community and prosper because the market is open for them. In the second neighborhood a butcher would probably be able to establish himself since that corner of the market is available. Finally, in the third neighborhood – with such an abundance of useful businesses – it might be difficult for any new shop to attract a clientele.
Sure it’s a silly example, but if you replace the businesses with plants – grasses, flowers, trees, and so on – you have a very simplified version of what Dr. Fargione’s experiment suggests may be a way plant communities become established and how they respond to invading species. These ideas aren’t new. In fact, they were first introduced after World War II, when invasion was on everyone’s minds says Dr. Fargione. “It has long been suspected by ecologists,” he adds, “but there have not been many experiments on it.”
Actual experimental tests suddenly became a priority when, in 2001, Dr. Stephen Hubbell, of the University of Georgia, proposed a neutral theory for community assembly. The neutral theory is basically a very simple model that assumes that all species – if they belong to the same level of nutrient acquiring, like all trees or all large herbivores – are competitively equal. That would mean that there are no differences that matter between species and that once a species randomly arrives in an area, its chance of surviving there is equal to the chance of any other species that might arrive. This is a premise that Dr. Fargione says scientists know is not exactly true, but the theory, “works surprisingly well with few assumptions,” says Dr. Hubbell.
“Whether communities are assembled randomly [neutral theory] or with a repeatable process [niche theory] has broad implications for basic and applied ecology,” wrote Dr. Fargione and coauthors in their paper. A large part of community assembly is undeniably random, such as - in the case of plants - whether a seed will land in an area, whether it will germinate, whether it will get enough rain in its first weeks, and so on. “No ecologist I know questions the existence of stochastic forces in population and community dynamics,” explains Dr. Hubbell. “The debate is over how important they are relative to deterministic processes.”
Dr. Fargione says that he and the other researchers were able to detect patterns in how the plants actually influenced one another, rather than just the influence of random chance, because the experiment they ran was so large. He adds that, with the experiment they were looking for general principles to combine with neutral theory for a stronger understanding of plant community assembly and invasion and that “it is always dangerous to take basic science and try to make it applied right away.” But, he continues, the results of the experiment could act as a guide, especially in areas such as ecosystem management, preventing the invasion of harmful exotic species, and maintaining agricultural and roadside cultures. The experiment implies that “not just any combination [of species] will be most resistant to weeds,” Dr. Fargione says.

Is it random? Or predictable?	Or both?

It is interesting and important to look at what has been found so far with respect to neutral and niche processes in the way communities of species are established and maintain themselves because there are upcoming special forums in the journals of Ecology and Functional Ecology on just this topic. “I predict that this will be an ongoing discussion for many years to come,” says Dr. Hubbell. “Whatever the outcome of the debate,” he adds, “neutral theory will have served a useful purpose if only to cause a healthy debate in the field of ecology, by demanding more rigorous evidence of non neutral process.”