Experimental test of Darwin's naturalization hypothesis

Among the numerous and still informative ecological predictions made by Darwin, one posits that when species are introduced into regions where they were not formerly found, the most successful tend to not have close relatives already occupying the region. This is known as Darwin's Naturalization Hypothesis, and his logic was that among close relatives, where ecological requirements should be most similar, the struggle for existence is most severe. Thus the modern formulation is that invader success is influenced by the amount of time since two species shared a common ancestor (usually called phylogenetic distance). Tests of this hypothesis have been primarily done on large species inventories, with results from different studies either supporting or refuting it. In a new study by Lin Jiang and colleagues published in the American Naturalist, they cleverly use bacteria with known relatedness to test this hypothesis.

They used four species of bacteria: Bacillus pumilus, B. cereus, Frigoribacterium sp. and Serratia marcescens as residents in every possible 1, 2, 3 and 4-species communities and invaded them with a subspecies of S. marcescens. What they found was that the invader density was highly significantly related to phylogenetic distance, so that the invader reached its greatest density when communities contained only distantly-related species.

Though these types of laboratory experiments are simplistic (I too use these systems), they offer insights into particular mechanisms, which may otherwise be difficult to detect in noisier systems.

Jiang, L., Tan, J., & Pu, Z. (2010). An Experimental Test of Darwin’s Naturalization Hypothesis The American Naturalist, 175 (4), 415-423 DOI: 10.1086/650720

How to plan an experiment that could last 99 years

For a number of reasons, including the fact that most grants only allow for research over a time span of 1-3 years, ecologists and evolutionary biologists usually plan experiments that last few years (with notable exceptions, such as the LTER). A usual approach to study long term phenomena is to take advantage of “natural” experiments. This allows us to understand about processes over long time periods, but usually with limited control on the initial conditions.

In a recent paper by Thomas Bruns and collaborators I learned about another way. They study spores viability of an important genus of ectomycorrhizal fungi, symbiont of Pinaceae: Rhizopogon. Pinaceae (the family of pines and other conifers) need ectomycorrhizal fungi to survive and usually spores and seeds are dispersed independently. It was not known how long their spores can last, which has very important implications, for example for colonization of areas not previously colonized by Pinaceae, or colonization after large scale disturbances, since if seeds cannot find mycorrhizae they have really few chances of survival. Now we know, based on this research that spore banks can be build and last probably decades.

What they did is really interesting, and was inspired on a previous study on seeds. They planted known number of spores of several species of Rhizopogon in terracotta pots, that were later planted into the ground (to mimic natural conditions). They planted 16 replicates, and they plan to open and analyze them later in the century based on the spore viability (for example, if in a few years most spores seem to be not viable that may reduce the expected length of the experiment to increase resolution). This paper found that after 4 years the inoculum potential of these spores seems to be increasing with time. I found the approach used in this experiment really fascinating and I look forward to see what happens in the next years!

Thomas D. Bruns, Kabir G. Peay, Primrose J. Boynton, Lisa C. Grubisha, Nicole A. Hynson, Nhu H. Nguyen, Nicholas P. Rosenstock (2009). Inoculum potential of
spores increases with time over the first 4 yr of a 99-yr spore burial experiment
New Phytologist, 181 (2), 463-470 DOI: 10.1111/j.1469-8137.2008.02652.x