Old fashions are often recycled by new generations of young people attempting to stand out and be unique (just like everyone else!). Bell bottomed jeans, the paragon of 1970s fashion, saw a (thankfully brief) resurgence in the late 1980s. More recently, the leg warmers and popped collars of the 1980s were suddenly cool again. Strangely enough, ideas in ecology often reappear on a similar 15-20 year cycle. In the case of ecology, however, new life is breathed into old ideas in order to bring order to the chaos that spawned by new areas of research. Thanks to peer review, old ideas are integrated into modern theory in a substantive way, and don’t burn out quickly like the latest fad.
With his seminal book The Ecology of Invasions by Animals and Plants, Elton (1958) introduced the concept of biotic resistance (way cooler than bell bottoms). Specifically, he posited that diverse resident plant and animal communities are better able to compete with invading species and are more stable. The rise in concern over the negative impacts of exotic invasive species has led to renewed interest in defining the properties of ecological communities that can resist invasion. While Elton’s age-old idea has proved a useful theoretical construct its validity is far from resolved (Levine et al. 2004). Specifically, in order to respond to the threat of invasive species, ecologists must be able to detail the mechanisms by which certain ecological communities are able to resist invasion. Spoiler alert! Simply having more species is not enough. It is necessary to know something about the species (i.e., their traits), not just how many there are. MacArthur and Levins (1967) proposed the concept of limiting similarity (again, way more useful than leg warmers). Their niche-based model hypothesized that a potential invader is unable to invade if one or more resident species has the same resource-acquisition traits. Recently, the concept of limiting similarity has been extended to suggest that a plant community that possesses a wide range of traits should be able to ‘filter out’ a wider range of invaders (Funk et al. 2008). By extension, land managers should be able to use a trait-based approach to restore plant communities with species that have the same traits as potential invaders. This is an elegant idea, comforting in its simplicity.
But does that really work? In an important move beyond theory, dating back to MacArthur and Levins (1967), and an accumulating number of case studies dating back to 2000, Price and Partel (2013) conducted a meta-analysis of 39 comparisons from 18 studies of limiting similarity in plant communities. Their results draw a number of valuable conclusions about the biological mechanisms behind limiting similarity and about the design of experiments that test this compelling concept.
What they found:
1. Limiting similarity was more effective at reducing the performance of established invaders, as opposed to resisting the establishment of invaders in the first place. Basically, species with similar traits to those of invaders will not keep those invaders from getting a foothold in communities, but apparently are pretty good at competing with them. This result mirrors the results of an earlier meta-analysis examining biotic resistance by a broad range of ‘resistors’ including fungal pathogens, plant competitors and insect herbivores (Levine et al. 2004).
2. Limiting similarity was effective among forbs, but not among grasses. Interestingly, this result may reflect problems with the way plant traits are usually generalized in the form of plant functional groups – C3 grasses, C4 grasses, leguminous forbs, and non-leguminous forbs. In short, this classification may not capture the relevant nuances of the temporal or spatial distribution of resource acquisition in plant communities and how this affects invasion. In a few studies, in fact, it was early-growing forbs that most effectively resisted invasive early-growing grasses.
3. Limiting similarity was demonstrated in de novo synthetic communities but not in manipulated natural communities. Experiments that created synthetic plant communities composed of species with particular functional traits, and then added invaders with the same traits, demonstrated limiting similarity. However, experiments in natural communities where the species composition (and therefore trait composition) was manipulated by selectively removing species, limiting similarity was NOT demonstrated. This is the perennial curse of applied ecology – predicted relationships are supported in experiments, but do not hold across the uncontrolled variation of natural communities. This might be because synthetic communities are too early in their development and therefore not comparable to natural communities, or the effects of limiting similarity vary across unmeasured environmental gradients.
Of course, to any ecologist worth his/her muster, this last point is the clarion call, and it is hard to resist. We need to characterize how these experimentally demonstrated patterns vary across observable ranges of environmental variation. To do that, maybe all we need to do is check what was in fashion two decades ago. Thankfully, most ecologists already have a closet full of flannel. Smells like teen spirit . . .