Tiny hitchhikers: An analysis of stowaway ants

We’ve all seen news coverage of the terrifying and destructive creatures that make their way across our borders by hitchhiking on shipments from around the world—Asian long-horned beetles via wood pallets, for example, or Brazilian wandering spiders in boxes of tropical fruit. These are only two of many examples worldwide, and the cumulative cost of these invasions is in the hundreds of billions of dollars as individuals, organizations, and governments race to contain outbreaks that can damage property, reduce ecosystem function, and threaten human health.

While some invaders are easy to spot and identify, others, such as ants, may go unnoticed thanks to their diminutive size and furtive nature. In fact, five different types of ant are listed among the world’s top invaders, and over a hundred other ant species are known to have expanded their ranges thanks to global trade activities. One recent study suggested that 147 varieties of ant have been transported—and transplanted—to date, but this is likely to be an underestimate given than more than 200 species have been intercepted at the borders of the USA alone.

Asian longhorned beetle
Asian longhorned beetle, which has caused the destruction of tens of thousands of trees across North America. Image courtesy of the College Green Magazine.

The uncertainty surrounding this issue prompted a team of Spanish and American researchers to undertake a global survey in order to more accurately estimate the number of ant species arriving, and establishing viable populations in, temperate habitats. Further, they wanted to explore whether these patterns were influenced by the geographical origins of the exotic ants—were they, for example, more likely to do well in habitats like those from which they came, or did they thrive in entirely different types of ecosystem?

The research team focused on data from the USA, New Zealand, and Holland—three countries characterized by high levels of shipping activity and excellent records of invasive species sightings. By employing a range of species richness equations, the researchers were able to use invasive ant presence/absence data to estimate the total number of introduced and established species in each of the three countries.

Estimates varied depending on which equations were used; for example, the total number of introduced species worldwide ranged from a low of 366 to all the way up to a high of 2027. The variation is a result of differences in how each equation deals with species that had very low detection levels. To deal with this uncertainty, the researchers calculated a mean across all the estimates—a figure that was often towards the lower to middle portion of the scale, meaning that the resulting analyses would likely be a conservative estimate of ant invasions in the focal regions.

The tawny crazy ant, a species that has invaded the Houston, Texas area of the USA. Image courtesy of TAMU.

It turned out that many of the invasive species had been spotted only once—a pattern that was particularly obvious in the USA. This suggests that there is a large diversity of exotic ants being transplanted around the world. Further, the richness estimates revealed a surprisingly high rate of introductions. Nearly 900 species are likely to have been transplanted globally, even though we’ve only observed and confirmed about 300. In North America, which is home to approximately 1000 types of ant, the number of invasives could almost equal the number of natives.

Not all of the exotics are likely to become established residents of their new ecosystems; the study estimated that just under 600 species found a way to embed themselves and set up colonies. This number is nearly 5 times higher than what has actually been observed during surveys, and represents nearly two-thirds of all introductions.

Most of the invasive species are Neotropical, with fewer coming from the Palearctic, Indo-Malay, and Australasian regions. The US was particularly hard hit by Neotropical ants, which made up the majority of both introductions (58%) and establishments (51%). The other two countries surveyed were most inundated from species from their own regions: most of Holland’s stowaway ants came from the Palearctic, while most of New Zealand’s came from Australasia. Interestingly, there were no significant biogeographical differences between the introduced and established ants within each study region, indicating that all species were equally likely to thrive once they had reached their new home.

Overall, the analyses suggest that approximately 1/14th (or 7%) of the world’s estimated 13,000 species of ants have been shipped to new habitats at some point. The authors admit that these estimates may be slightly inflated by the high rates of importation into the US, which offers more ant habitat than the other two countries examined here—both because it is physically larger and because it contains a greater variety of ecosystem types. However, results were similar even when additional countries were added to the sample, suggesting that the reported patterns are likely accurate reflections of global ant introductions.

Range of the red imported fire ant, a global pest. Image courtesy of the USDA.

There are probably two main reasons why stowaway ants are most successful within their own biogeographical regions. First, they are more likely to benefit from “climate matching”, or similarities between climatic conditions in their native and adopted countries. Second, there is greater “propagule pressure”—a larger number of migration opportunities thanks to simple physical proximity and repeated exposure.

That’s not exactly a surprising result, but probably a useful one nonetheless if we want to improve our ability to locate, identify, and prevent impending invasions. After all, ants can play a critical role in ecosystem function, and an imbalance in local myrmecofauna could have serious implications for the health of local habitats—or, at the very least, could prove to be an expensive and sometimes painful annoyance, as anyone who has dealt with a red imported fire ant could tell you.

Given the non-random nature of ant introductions, the authors suggest that we should develop targeted strategies for monitoring shipping routes (especially from the Neotropics). Now that we know where the ants end up, it would also be helpful to have a better understanding of where they originate. This will require further studies of introduced and established ant populations in other destination habitats, since the stowaways may hop on board at any number of ports—not just those in their homelands.

Miravete, V., N. Roura-Pascual, R.R. Dunn, and C. Gomez. 2014. How many and which ant species are being accidentally moved around the world? Biology Letters 10: 20140518.

NOTE: Since the publication of this blog post, the paper has been retracted. The authors’ public statement is as follows:

Herewith, we retract our paper ‘How many and which ant species are being accidentally moved around the world?’ by Verónica Miravete et al., published online on 23 October 2013 (Biol. Lett. 9, 20130540; doi:10.1098/rsbl.2013.0540). After careful examination of the original data on introduced and established ants on regions worldwide, we realized that we used a wrong list of species and omitted to include a reference (Sarnat E. (2012) North America checklist. Antkey <http://antkey.org>. Extracted 3 June 2014) in the paper. Although the main arguments and conclusions remain the same after correcting these errors, the use of the wrong version of the data affected the magnitude of the analyses conducted at the country level (in the electronic supplementary material) and, to a lesser extent, when all countries were considered together (in the main text). Therefore, we wish to retract the article. We deeply apologize for any inconvenience this publication might have caused to the readers of Biology Letters.

Guidelines for reintroducing resurrected species

If you read National Geographic or Salon or any of a growing number of popular press publications, you have probably heard about DeExtinction–the resurrection of extinct species via the kind of molecular techniques that seemed impossibly futuristic and fictional when they were shown in Jurassic Park 20 years ago. The concept first gained widespread attention thanks to the TEDx program, and its application has since been discussed in relation to everything from passenger pigeons to woolly mammoths.

A recent National Geographic article asked whether woolly mammoths might eventually roam the Earth again. Image courtesy of the NHM.

Unsurprisingly, opinions on DeExtinction vary widely; the idea of bringing back long-lost species seemingly inspires equal measures of fear, excitement, and curiosity. Another characteristic, evident in a recent Trends in Ecology and Evolution paper written by an international trio of zoologists, is caution. The researchers point out that DeExtinction is, most likely, an inevitability. Given this, they suggest that humans proceed only after doing a bit of soul-searching and asking themselves “whether DeExtinction can assist conservation efforts, and what might be the relative risk and benefits of species resurrections.”

At the heart of their discussion is the idea that the whole point of DeExtinction efforts is to return lost species back into the wild; as a result, each resurrection event should ultimately be accompanied by at least one subsequent translocation and/or reintroduction–management techniques for which the International Union for the Conservation of Nature (IUCN) has provided detailed guidelines. The aim of the current paper is to highlight how those guidelines can be re-framed as a series of ten questions about the past, present, and future of each resurrected species and its habitat. The resulting answers can act as a “filter” for selecting the organisms for which DeExtinction is likely to be most relevant and successful.

The thylacine, an extinct Tasmanian marsupial, was featured on the cover of this month’s Trends in Ecology and Evolution as the face of DeExtinction. Image courtesy of Wikipedia.

All ten questions can be answered with either a “yes” or a “no,” making decisions relatively clear-cut. As the authors point out, a negative response might indicate a doomed project or, alternatively, simply highlight the need to do additional research before proceeding. Importantly, the questions focus not only on biological and ecological issues associated with the reintroduction efforts, but also on potential social, economic, and legislative implications.

So what are the questions? It is, perhaps, most interesting to consider them in relation to one of the three case studies that the researchers include to illustrate the usefulness of the filtering process: that of the Xerces blue butterfly. This species, native to a small portion of California, was declared extinct in 1941. Judging by the answers to the authors’ proposed questionnaire, the butterfly is a prime candidate for DeExtinction.

  • Q1: Can the past cause(s) of decline and extinction be identified and addressed? Yes. The butterfly went extinct because of habitat loss and over-harvesting by collectors. Dune restoration, and banning of additional collection efforts, could prevent a repeat extinction event.
  • Q2: Can potential current and future cause(s) of decline and extinction be identified and addressed? Yes. Assuming that no new threats emerge–and that climate change does not have a negative impact on either the butterflies or their habitat–the techniques proposed in response to Q1 should be sufficient to protect the butterfly post-reintroduction.
  • Q3: Are the biotic and abiotic needs of the candidate species sufficiently well understood to determine critical dependencies and to provide a basis for release area selection? Yes. Species composition, soil characteristics, and light conditions within the butterfly’s habitat have been well documented.
  • Q4: Is there a sufficient area of suitable and appropriately managed habitat available now and in the future? Yes. The dunes originally inhabited by the species no longer exist, but alternative habitats can likely be created in a nearby park–however, the suitability of these may be compromised by climate change.
  • Q5: Is the proposed translocation compatible with existing policy and legislation? Yes. There are no known relevant policies.
  • Q6: Are the socioeconomic circumstances, community attitudes, values, motivations, expectations, and anticipated benefits and costs of the translocation likely to be acceptable for human communities in and around the release area? Yes. Public support is likely to be widespread, as evidenced by the fact that a nonprofit conservation group is named after the butterfly.
  • Q7: Is there an acceptable risk of the translocated species having a negative impact on species, communities, or the ecosystem of the recipient area? Yes. No negative impacts anticipated.
  • Q8: Is there an acceptable risk of pathogen-related negative impacts to the resurrected species and the recipient system? Yes. No harmful impacts anticipated.
  • Q9: Is there an acceptable risk of direct harmful impacts on humans and livelihoods, and indirect impacts on ecosystem services? Yes. No harmful impacts anticipated.
  • Q10: Will it be possible to remove or destroy translocated individuals and their offspring from the release site or any wider area in the event of unacceptable ecological or sociological impacts? Yes. Because the butterflies are dependent on a particular type of habitat that is not widespread, and because the insects emerge en masse in a single brood each year, it would not be difficult to collect them if needed.
The Xerces blue butterfly, shown here in a specimen collection housed in a museum, went extinct because of habitat loss and over-harvesting by collectors. Image courtesy of Wikipedia.

As evidenced by the case study evaluation of Yangtze River Dolphins, it is not even necessary to answer every single question on the list; where the first few questions elicit a negative response, it is immediately clear that there is too much uncertainty associated with the reintroduction to make it worthwhile. This may be a permanent state or may simply indicate that additional research, management, and/or policy goals need to be met before proceeding with DeExtinction efforts.

The authors conclude by acknowledging that there are many reasons why DeExtinction is appealing: In addition to giving people an opportunity to “[correct] past human wrongs” and to view fascinating wildlife in the flesh, the resurrected species might also provide any number of ecosystem services. Given these benefits–real or perceived–it seems highly likely that it is only a matter of time before DeExtinction is attempted. Recognizing this, the researchers advocate the early use of their planning criteria in the hopes that this simple question-and-answer technique “might eliminate several high-profile candidate species and, thus, avoid time, expense, animal welfare concerns, and the raising of false public expectations.”

Seddon, P.J., Moehrenschlager, A., and Ewen, J. 2014. Reintroducing resurrected species: selecting DeExtinction candidates. Trends in Ecology and Evolution 29(3):140-147.

A century of anthropogenic influences on black bear diets in Yosemite National Park

One of the main reasons people visit natural areas such as national parks is to have close encounters with free-living animals. In many such places, humans have developed a bad habit of using food scraps to either draw particular animals closer, or to create places where easy access to appealing foods ensures a reliable stream of animal visitors. Sometimes–as in the case of unattended trash bins in parking lots and behind hotels–there is no intention to feed the animals, but it happens anyway. Regardless of the exact circumstances, this sharing of food can be detrimental both to individual organisms and entire ecosystems; as a result, parks often devote a great deal of time and money to management practices designed to minimize wildlife access to anthropogenic food items.

Whether or not these management schemes work is another question–one at the heart of a recent research project conducted by an international team of scientists working in California’s Yosemite National Park. The researchers used both museum specimens and samples collected from living animals to explore changes in the diets of American black bears (Ursus americanus) in Yosemite between 1890 and 2007. This 117-year period encompasses four major anthropogenic disturbance regimes during which bears had access to varying levels of artificially introduced fish and food scraps; during these regimes, there were also differences in the degree to which bears were either encouraged to eat, or actively prevented from eating, these food items.

An American black bear (Ursus americanus). Image courtesy of Wikimedia Commons.

Because the researchers had no way of observing all the feeding behaviors of every bear included in the study (especially those that lived in the 19th century) they relied on stable isotope analysis to provide information on which types of foods comprised the bears’ diets. Stable isotopes are versions of atoms–in this case carbon and nitrogen–that have extra neutrons but do not undergo radioactive decay. Relationships between “normal” and “heavy” isotopes (12C vs. 13C, 14N vs. 15N) vary among different food sources in different regions, and therefore can be used as a sort of food fingerprint. This information can be extracted from a variety of animal products, including fecal samples, blood plasma, and–as in the current study–bone and hair.

Unsurprisingly, the scientists found that the isotopic composition of Yosemite bear tissues has changed over time, and that this pattern is associated with variations in the consumption of anthropogenic foods. Between the first and second focal periods (1890-1922 and 1923-1971), for example, they saw an increase in 15N associated with the availability of non-native fish in Yosemite-based hatcheries; closure of the last hatchery in 1956 resulted in a subsequent decrease of 15N.

Black bear eating anthropogenic food out of a(n unsecure) food locker at a Yosemite campsite. Image courtesy of the U.S. National Park Service.

Carbon isotopes, on the other hand, were fairly stable early on, but rose significantly during the second and third (1972-1998) study periods. This is predominantly associated with the closure of bear-feeding platforms. Bears that had developed a taste for anthropogenic foods went searching for them at their source, in concession areas and campgrounds. This habit not only increased consumption of 13C, but also led to a variety of human-bear conflicts that sometimes resulting in the killing of “problem” animals.

At the beginning of the fourth focal period (1999-2007), the U.S. government initiated an annual funding scheme designed to improve human-bear relations. Bear-proof trash and storage receptacles were installed throughout Yosemite, outreach programs were designed to teach visitors about the hazards of feeding bears, and particularly aggressive bears have been hazed, relocated, and even occasionally killed. The isotope analysis suggests that these efforts have been effective: Both 15N and 13C levels decreased between the third and fourth period, and are fairly similar to the (more or less) pre-anthropogenic-disturbance values measured at the very beginning of the first focal period.

Yosemite National Park. Image courtesy of Travel Top.

Indeed, plants and animals–black bears’ natural food sources–currently make up the majority (64-92%) of most Yosemite bears’ diets. However, a few sneaky individuals are still finding ways to dine out on anthropogenic foods, as evidenced by the fact that 8-36% of some animals’ meals are coming from human sources.

Preventing these indulgences (not to mention the cravings that drive them) would be beneficial to humans and bears alike. Particularly aggressive bears do occasionally hurt humans and their belongings: Over the past 20 years, there have been 12,000 reported conflicts, 50 injuries, and approximately $3.7 million in property damage. Bears, like other wildlife consuming anthropogenic foods, may also be susceptible to long-term health problems associated with high cholesterol and fat intake–though there is also evidence that high-calorie, high-protein anthropogenic foods increases reproductive success over the short term.

Yosemite has a long history of human-bear interactions, as shown by this archival photo. Image courtesy of De Anza College.

Perhaps even more worrying is our lack of knowledge about the potential ecosystem-level effects of bears’ dietary fluctuations. Changes in feeding preferences can destabilize food webs and disrupt vital ecosystem processes such as seed dispersal and nutrient cycling. Although modern bears may be returning to “normal” eating practices, their behaviors over the past several decades may have had significant long-term impacts on Yosemite and the wildlife that dwell within it. The researchers urge further study, both here and in other anthropogenically impacted systems, to improve our understanding of whether, and how, human nutrients may shape even those landscapes that we often regard as being more or less “undisturbed” wilderness.

Hopkins III, J.B., Koch, P.L., Ferguson, J.M., and Kalinowski, S.T. 2014. The changing anthropogenic diets of American black bears over the past century in Yosemite National Park. Frontiers in Ecology and the Environment 12(2):107-114.