New sources for the emergence of new invaders

Biological invasions continue to occur worldwide, resulting in substantial impacts on ecosystems as well as human health and economies (1, 2). One of the fundamental tasks associated with managing these invasions is to predict which species are most likely to be future invaders—a task that has become increasingly challenging due to climate change and shifts in the source of potential invaders (3, 4). Scientists have undertaken considerable efforts to manage the potential risk of future invasions by focusing on species that have been introduced previously in other areas, often with considerable impact. Many elegant studies of these repeat offenders have used various combinations of phylogenetic, morphological, and life history characteristics to develop predictive models that predict which species are most likely to invade (5, 6). Most of these studies have relied on the data from the investigations of serial invaders to predict future invasions, and many management programs have placed their emphasis on high-profile invaders that have caused demonstrable damage in other regions (7). In short, scientists and managers have generally relied on information from the track records of previous invasions to predict future invasions. However, a new study in PNAS (8) calls this thinking into question by demonstrating that emerging invaders, species that have never been found outside of their native range, continue to be a significant proportion of new invasions. They also demonstrate that the continuing presence of newly emerging invaders is the result of new source pools contributing to the ongoing stream of invasions.

First Records and New Source Pools

Before the publication of this study, invasion biologists and ecologists would generally expect that the proportion of emerging invaders in the overall number of new invasions would likely decline as the source pool of potential new invaders became depleted over time (9). There is certainly a persistent belief among many nonscientists that most of the invasions that could take place have already done so. This would logically mean that the pool of potential new invaders in a given region would be on the decline. In fact, there are suggestions for some taxa like birds that this might have been the case (10). However, this article puts the final nail in the coffin for this belief and demonstrates that the proportion of newly emerging “first timers” shows no sign of decline.

The authors of the new study used an extensive database involving >45,000 records of first introductions involving >16,000 established invaders over a period of five centuries from 1500 onward. They found that the proportion of newly emerging invaders did not decline as expected. For some taxa, the proportion of emerging invaders stayed constant or even increased during the past century even as the numbers of invasions increased overall. This result puts to rest the idea that source pools are becoming depleted of newly emerging invaders. The authors reason that if invasion was simply the result of increased numbers of species introduced via the same pathways, the number of newly emerging invaders would decrease over time. They make the argument that their unexpected results are due to the inclusion of new pools of invaders. These new invader pools may come about for several reasons including changing climatic conditions, increasing access to these pools, and the presence of new pathways including the ornamental and horticultural trades.

Drivers of Source Pools

The authors move quickly from their basic contentions that the source pools are changing to examining the data supporting some of the potential drivers of these patterns. Using a generalized additive mixed-model analysis, they found that factors such as changing land use and export value of particular regions contribute to the changes in species pool. Environmental variables appeared to be less important, but the inherent limits of their model highlight this area as one ripe for future work. They also note that land use changes may have also contributed to increases in successful establishment of invaders.

These results are an important advance in our understanding of what allows a species to first become established. By examining newly emerging invasions over a long period of time and analyzing the factors responsible for these patterns, this study provides not only a unique look into past and present invasions, but also a look toward future invasions. Other studies have found that a range of other factors can contribute to the spread of introduced species following the establishment process. These studies have documented the subsequent spread of invaders once becoming established or by examining the cumulative of number of invaders by taxa within a region (11, 12). The real key to the insights of this study is the ability to look critically at the establishment of newly emerging invaders. This is a category of invaders that we know least about and have a minimal capacity to predict their spread and impacts. Thus, this study can uniquely draw conclusions regarding the factors that contribute most to the establishment of newly emerging “first-time” invaders.

Management Implications

The results that these authors present tell us a great deal about the drivers behind current invasion patterns, and in doing so create important messages for managing current and future invasions. The first is that we need to increase our vigilance regarding new introductions. It is not sufficient to simply look primarily for the species expected to show up based on their past invasion history. Also, our intuition about which species will invade must also be checked at the door as new species that have never before moved beyond their native range suddenly make their way past the current gatekeepers. Most importantly, we need to increase the size of the geographic horizons that we are scanning for these new invaders. Their work makes a strong case for changing source pools as a major contributor to newly emerging invaders. Factors such as climate change can have complex effects in different regions and can change the degree of “climate matching” among regions. As regional climates become more or less similar, the risk of invasion from one region to the other will increase or decrease, respectively (13, 14). We know considerably more about the invasion risk and the ability of those regions to respond to new invasions, and even the pathways likely to carry those species to new regions (Fig. 1). However, we still struggle to understand how the source pools that provide the invaders of the future are shifting as well. By highlighting the importance of new source pools and the processes that influence their contributions to future invasions, this study makes it clear that we will need to answer these questions quickly to mitigate the risk posed by future invasions.

Fig. 1.

Global invasion threat for the 21st century. Airport and seaport capacity, as well as animal, plant, and total imports between 2000 and 2009, is combined into global introduction risk. Projected biome shifts and increase in agricultural intensity and fire frequency between 2000 and 2100 (emissions scenario A2) are combined into global establishment threat. Introduction and establishment axes are combined into overall invasion threat. Reprinted from ref. 3.