Posts Tagged ‘invasive species’
Published in Open Lab 2010, a print compilation of the best science blog posts of the year.
“Overfishing” is a term associated with resource depletion, extinction, and human greed. While the definition of overfishing is technically a subjective measure (How much fishing is too much?), it has been widely accepted to mean catching more of an aquatic resource than can be replenished naturally by the system. The idea of depleting a marine resource is ubiquitous and familiar these days, with the bluefin tuna even featured as the cover article of the New York Times Magazine this past June.
The idea may be commonplace now, but this was not always so. A 2003 paper by Nicholas Dulvy and others enumerates the reasons why it was long believed that marine populations were more resilient than terrestrial species, and less likely to go extinct due to overfishing, habitat loss, invasive species, disease, and other causes. Jean Baptiste de Lamark himself was a proponent of the “paradigm of ocean inexhaustibility” due to the high fecundity of fish. He (and others) argued that because fish lay so many eggs and have excessive offspring (with little care put into each), we could never actually catch enough of a population to cause any damage. One problem with this argument is that fecundity often increases with size of an individual. Since we selectively catch larger fish, we’re catching the most reproductively able of a population and causing a large impact per fish caught. Other arguments about the impossibility of aquatic extinction include broad geographic range and dispersal, and that economic extinction of a fishery would precede biological extinction of a species (all of which have counter-arguments).
In all the discussion of overfishing, it is always humans that are doing the fishing to the detriment of non-human species, either through depletion of a fished species itself, or by reducing resources for other species that rely on it for prey. It is we humans who must reduce our impacts and allot resources for other species on our fair planet.
Last month (August 2010), an article from ICES Journal of Marine Science asks whether humans are the only species capable of overfishing. More interesting than the research itself is the questions it raises about our own relationship with “nature.”
The story of cormorants in the Baltic Sea
The Great Cormorant (Phalacrocorax carbo) is a seabird that lives in the Baltic Sea, along with many other locales. According to the Helsinki Commission, in the 1950s and 1960s the bird was overhunted to near-extinction locally, at which point they were put under government protection. Over the rest of the 20th century, the bird population improved dramatically, recolonizing old haunts with great success. They were so successful that they began expanding their original range, initially colonizing Estonia in 1983. In 2005, there were 20 great cormorant colonies in Estonia with an estimated 10,000 nesting pairs.
Over the course of this period, fishing decreased in Estonia waters, in part to conserve the estuarine wetlands that are important for bird migration and fish spawning. Despite this, many commercially valuable fish stocks plummeted. Though working with a limited data set (fish were sampled only in 1995 and 2005), in the ICES paper, the scientists satisfactorily concluded that this loss of fish species was due to overexploitation, not by humans, but by these great cormorant colonies. The cormorants were fishing 10-20 times more than the commercial catch of fish species such as perch Perca fluviatilis and roach Rutilus rutilus, decreasing the fishes’ ability to recover year after year.
How this questions our typical relationship with “nature”
This is an interesting story for several reasons. The birds were able to spread their range as far as they did and, in the end, compete with humans for food resources because we were trying to protect them. Their near-extinction in the 1950s probably led the government to be hesitant to lift protection because the birds were no longer birds, but a symbol of species recovery. After such a great success, how could we take their resources away and potentially lead them to extinction once more?
The fact alone that they are seabirds also makes their presence hard to define. Some cases of “invasive species” are very clear cut. For example, brown tree snakes are not from Guam, but were brought there and are now wreaking havoc on native animal populations. But seabirds toe the line. They are able to fly anywhere, and simply live on colonies at sea. Who are we to determine where geographically those colonies exist? The authors of the paper do not even use the word “invasive” to describe the expansion of great cormorants into Estonia until the end of the paper.
Are these birds invasive? It depends on your definition of the term. Some would argue that, yes, they did not live there before but do now, and are affecting the ecosystem to the detriment of other species. But it’s all relative: invasive species are defined by an anthropocentric view of the world, in which what is “natural” is the distribution of organisms we initially encountered and recorded. But who are we to decide that a species belongs or does not belong in a certain place? Who are we to tell the cormorants that they cannot live on that rock near an ample food supply? We’re the only species that sets these sorts of boundaries; all the other species are just trying to utilize resources and survive.
The idea that humans are the only species able to overexploit a resource is also anthropocentric. It makes Homo sapiens the center of the world, the ones who determine the fate of all other organisms, who can harvest them for ourselves or choose to spare them. This case of the cormorants places us back in our role as a competitive species: we have to decide whether or not we are willing to take back our resource, even if it means losing some of these big, aesthetically-valuable cormorants. We are no longer the masters of nature, but rather are inserted back into it.
I hope I manage to keep up with this case and find out what happens in Estonia. At this point, “taking back our resource” would not mean going in and competing by fishing; there are too many cormorants, so we would simply deplete the resources further. Instead, the Estonian government would have to enter the colonies and manage the population through oiling or pricking eggs to kill the developing birds (the Helsinki Commission estimates that this is done to 18% of nests in Denmark). Already 10,000-20,000 birds are shot in the Baltic Sea area each year, but public protests limit the amount of population control that is performed.
We may have simply lost control of the situation at this point. There may just be too many cormorants to keep them from overfishing, for our own sake or to preserve the fish as an ecosystem resource.
Dulvy, N., Sadovy, Y., & Reynolds, J. (2003). Extinction vulnerability in marine populations Fish and Fisheries, 4 (1), 25-64 DOI: 10.1046/j.1467-2979.2003.00105.x
Vetemaa, M., Eschbaum, R., Albert, A., Saks, L., Verliin, A., Jurgens, K., Kesler, M., Hubel, K., Hannesson, R., & Saat, T. (2010). Changes in fish stocks in an Estonian estuary: overfishing by cormorants? ICES Journal of Marine Science DOI: 10.1093/icesjms/fsq113
I rarely think about how invasive species affect genetics. It’s always in terms of ecosystems or species: invasive brown tree snakes gobbling up birds and lizards in Guam, or zebra mussels overwhelming and altering the environment of the Great Lakes. How one species outcompetes and replaces another, changing the natural system. This is partly because many of the common examples are of predator-prey relationships, where the two species are very distantly related and could never breed, thus keeping genetics out of the picture. But what about situations where the introduced animal and native animal are similar?
This gets us into the muddy waters of what defines a species. For sexually reproducing organisms, a species is the group of animals with whom one can exchange genetic material via reproduction, or, in other words, can produce fertile offspring. To distinguish one species from another under this definition, a scientist would need a pretty wide worldview. How else could he know that a squirrel from England could not mate with a US squirrel if it tried? And the honest answer is: he can’t. (Unless he collected squirrels from around the world and tried to mate them all with one another… but that’s a lot of work.) Thus, species are often also defined based on location or geography, despite the fact that maybe they could mate if they had access to one another. But what are the chances that a squirrel will swim across the Atlantic for a new girlfriend?
And there’s where invasive species fit in. In a paper published this week in PNAS out of Knoxville, TN, Lexington, KY, and UC Davis, scientists studied the Salinas Valley in central California, where salamanders from Texas and New Mexico had been introduced in the 1950s for use as bait by fisherman. These salamanders, the Barred Tiger Salamanders (Ambystoma tigrinum mavortium) had been defined as a separate species from the threatened native California Tiger Salamanders (Ambystoma californiense), as their populations had been living apart for 3-10 million years, and thus it was unlikely that they were still genetically similar enough to mate. But – alas – this assumption was wrong. The invasive salamanders have been mating with the native species for the last 50 years, producing hybrids which are able to mate with either species and one another. The question: is this hybridization significantly changing the DNA of the native species?
To investigate this question, the researchers identified an introduction site at a pond in central California, and took samples of over 200 salamanders (by clipping the end of their tail and immediately releasing them) at this site and others within a region 200 km north. Using salamanders of each species from non-invaded ranges, they determined the baseline genetic makeup of each species.
They scanned the genomes of the sampled salamanders (say it 10 times fast) for 68 genetic markers to see if any of the invasive genes had “taken over” the native genes. They saw no real difference in 65 of these species — that is, the salamanders retained their native genes. However, they saw a drastic increase in 3 of the genes. In the figure below, taken from their paper (click on image for larger size), the little “thermometers” measure the DNA differences at different sites, native in white and invasive in black, with the introduction site indicated by the red arrow. The upper left (A) shows the big picture: of the 68 gene markers studied total, invasive genes are only apparent at the introduction site. The other 3 boxes (B, C, D) show the three genes that have spread — and as you can see, they have spread far and deep, despite their invisibility overall (A). The authors were thorough: they tested whether this pattern was due to either sampling error or random genetic drift without natural selection, and neither of these biases accounted for the pattern of these 3 genes.
The function of these genes is unknown. However, by studying the behavior of the animals, it seems they are related to reproduction. The hybrids have larger larvae with greater survival and develop more quickly, ever hastening their dispersal. This raises a few questions:
1. If these invasive genes are helping survival, then who cares if they invaded? It is easy to look at this as actually beneficial to the threatened native salamanders. However, it has unknown impacts on the surrounding ecosystem. These bigger larvae eat a lot more, impacting the populations of their prey species through indirect effects of the invasion. A change in the abundance of one species affects all others – what seems to be an immediate benefit can be incredibly harmful in the long run.
2. How do we define a species? The native salamanders are a threatened species. If they have received genes that increase their numbers through hybridization, is this a comeback? Are they still A. californiense? Do these 3 genes alone make them A. mavortium? Are they an entirely new species? Is it possible to stop the invasion of these genes throughout the state without killing off a threatened species?
I don’t have the answers to these questions. We human beings are drawn to classification: we want to put all of the animals in neat little piles and call it fin. But the truth is that species are eternally evolving — as Peter and Rosemary Grant have shown with their Galapagos finches, most recently in November 2009. The monkeys that live on one side of a jungle can have a different genetic makeup than the ones on the other side even if they can still mate.
Clearly the introduction of these salamanders, which was just an innocent attempt to raise some bait locally, has had unforeseen impacts on the ecosystem. Humans’ ability to travel has meant that we are bringing animals together that have not evolved to live together, or have evolved apart millions of years ago. In some ways it feels like what is done is done — and I am not enough of an expert on habitat restoration to tell you otherwise. But try little things: wash the mud off of your boots before you go hiking in another state or country, don’t release your foreign pets locally (as my roommate Erinrose and I have been tempted to do with our pet turtle, Nicolas Cage), volunteer at your local wildlife refuge. Biodiversity is important.
How can we save our planet?
Fitzpatrick, B., Johnson, J., Kump, D., Smith, J., Voss, S., & Shaffer, H. (2010). Rapid spread of invasive genes into a threatened native species Proceedings of the National Academy of Sciences, 107 (8), 3606-3610 DOI: 10.1073/pnas.0911802107