Everyone loves a paradox. Invasion biology is full of them (ok… all biology). One of the more irritating ones is the problem of scale (Fridley et al. 2007).
When invasion biologists conduct large scale observational studies (such as by scouring state/regional/country floras and herbariums and counting up species) they tend to find a positive relationship between native and exotic species richness (that is, areas with lots of native species also have lots of invasive species). But when they conduct small scale experimental studies (such as setting up replicated microcosms, like ponds or plots, and then counting what shows up) they tend to find a negative relationship between native and exotic species (areas with lots of native species have very few invasives). What the heck, you say? There are probably several reasons for this paradox.
Classifying things can be a problem, for one thing. Say you are asking the question, do exotic plants cause damage to native ecosystems? To answer it, you look at the exotic plants in the area. Some of these will be invasive and currently causing major problems. Some of them will be very problematic in the future, but maybe they just got to this new location, and are biding their time, not spreading or causing any particular problems. And some of them, though exotic, will never spread beyond their current small population, and never be a problematic invasive. If you average across all of these exotics as a whole, you’d see that only one third seem to do any damage. On average, you’d conclude, exotic plants do not damage native ecosystems. But this is only because you are grouping things together too broadly, and washing out the signal (and also, you can’t read the future).
And that’s the type of thing that’s been happening in some of these large observational studies, as Guo et al, 2012, point out. When you count as ‘native’ anything that is native somewhere on the continent your estimates of native richness increase and your estimates of exotic richness decrease. And yet, it is just as weird to see an eastern Blue Jay on the west coast of North America as it is to see a European Starling, really. Using a dataset that identifies these “domestic exotics”, and looking at the state level within the lower 48, Guo et al found that the previous positive relationship between native and exotic richness went away. Instead they found that native richness was related to environmental variables (land area in a state, temperature, land productivity), while exotic richness was predominately influenced by human factors (human population size, years since the state joined the Union, proximity to a border or port). Where people are and where we’ve been the longest, that’s where you’ll find most of the exotics.
Fridley JD, Stachowicz JJ, Naeem S, Sax DF, Seabloom EW, Smith MD, Stohlgren TJ, Tilman D, & Von Holle B (2007). The invasion paradox: reconciling pattern and process in species invasions. Ecology, 88 (1), 3-17 PMID: 17489447
Guo, Q., Rejmanek, M., & Wen, J. (2012). Geographical, socioeconomic, and ecological determinants of exotic plant naturalization in the United States: insights and updates from improved data NeoBiota, 12 DOI: 10.3897/neobiota.12.2419