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Invasive Species

When Aliens Invade: Disturbed Food Webs in the Mediterranean

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Growing up on the North Shore of Boston, I spent countless summers clambering over rocky coastline, exploring the hidden treasures of tide-pools.  New England was home to a rich and diverse web of organisms living in those craggy underwater worlds, including several large and unique native crab species.  I remember being about ten years old when I first saw a smaller, heavily banded crab scuttling across my favorite haunts.  This was the Asian Shore Crab (Fig 1), an invasive species that likely made its way into coastal New England waters by hitching a ride in ships’ ballast water.  It is much smaller than native crab species and reproduces much more quickly.  Over the past two decades, it has swept up and down the eastern seaboard.  As a child, I could literally watch the invasion progress, seeing more and more of these tiny crabs each year as they came to dominate the tide-pool landscape.

Invasive species are those that are not native to a specific location and which have a tendency to spread to a degree that may cause damage to the environment, human economy, or human health.  But really, what does it mean to be “not native to a specific location”?  Movement of species around the world is a hugely common theme in evolution.  Species’ ranges expand or contract based on a number of environmental factors.  Some species migrate halfway around the world to reproduce – think about the classic formation of Canadian geese flying across the sky.  Humans are technically native to Africa, and yet we have now spread across the entire globe.  It’s not an easy question to answer, but we tend to associate native species with those that have been in an area for hundreds of years.  Invasive species have been introduced more recently and have rapidly become a pest in their new area.

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Fig 1: An Asian Shore Crab with the distinctive light and dark banding pattern on its legs, shown with a human hand for size comparison. (Source: Invasive Asian Shore Crab website)

At their most destructive, invasive species can completely out-compete native species and come to dominate the landscape (Fig 2).  This lack of diversity in an ecosystem can have severe negative consequences to human health and the economy – for example, while Asian Shore Crabs are edible, they are not the prized delicacy that native crab species are.  And once an invasion begins, it’s almost impossible to completely eradicate the pest species from its new habitat.  It’s not all doom and gloom, though, since researchers can help us take steps to alleviate the consequences of an invasion.  For example, the invasive winter moth in Massachusetts has been well controlled using a parasite that is native to its home range and which depends on the moth to reproduce.  There are also major initiatives in New England to stop the spread of the Asian Longhorn Beetle by educating people on the risks of moving firewood around the area.

Fig 2: Kudzu covering trees and other vegetation at Piedmont Park in Atlanta. (Source: Scott Ehardt)

Fig 2: Kudzu covering trees and other vegetation at Piedmont Park in Atlanta. (Source: Scott Ehardt)

Goren and colleagues focused on a marine invasion in the Mediterranean.  Fish have been moving through the Suez Canal, entering the Mediterranean near Port Said in Egypt, and from there invading the rest of the Mediterranean, including the coast of Israel (Fig 3).  These researchers examined the impact that such an invasion may be having on the Mediterranean food-web.

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Fig 3: Map of the study site and surrounding area. (Source: Goren et al. 2016)

Methods

Goren and colleagues sampled fish off the coast of the Ashdod port in Israel, over 100 miles away from Port Said.  They used a fishing technique known as trawling, which involved pulling a net through the water behind a boat (Fig 4).  They trawled at depths of 20 and 40m, performing 60 tows at each depth.  Around half of the tows occurred at night and half occurred during the day.  The entire catch from each tow was refrigerated and brought to the ichthyology lab at Tel Aviv University, where it was sorted, identified, measured, and weighed.  Each species was assigned a trophic level based on previous data.  Trophic levels indicate a species’ place in the food web – photosynthetic plankton, for example, would have a very low trophic level, while a killer whale would have a much higher level.  The authors then calculated mean trophic levels for various groups, including non-native vs. native species.

Fig 4: Diagram of a trawling tow. (Source: Public Commons)

Fig 4: Diagram of a trawling tow. (Source: Public Commons)

Results

Over 200,000 individuals representing 111 species were caught during the course of this experiment.  The authors identified 33 non-native species, which comprised 54% of all the sampled individuals.  The mean trophic level among non-native species was higher than the mean trophic level among native species (Fig 5). Since the non-native species account for a disproportionate amount of the overall biomass, the high average trophic level among non-native species is likely driving the mean trophic level of the entire ecosystem.

Fig 5: Mean trophic levels of non-native vs. native species.  Non-native species show an elevated mean trophic level.  (Source: Goren et al. 2016)

Fig 5: Mean trophic levels of non-native vs. native species. Non-native species show an elevated mean trophic level. (Source: Goren et al. 2016)

Conclusions

It is very clear that the Mediterranean basin has undergone a significant invasion by fish coming through the Suez Canal.  This invasion has served to raise the mean trophic level of the ecosystem. Marine ecosystems with high mean trophic levels tend to be less efficient in terms of energy flow between trophic levels.  The higher mean trophic levels in the Mediterranean may suggest that there is a deficit of fish belonging to the critically important intermediate trophic levels which normally support top predators, help maintain ecosystem diversity, and are key components of the fishing industry in the Mediterranean. While the Mediterranean invasion may never be reversed, it will be important to continue monitoring ecosystem diversity in the area and begin working on strategies to intervene as necessary.

Fig 6: Alepes djedaba, one of the invasive species studied.  (Source: Public Commons)

Fig 6: Alepes djedaba, one of the invasive species studied. (Source: Public Commons)

Source

Goren, M., Galil, B. S., Diamant, A., & Stern, N. (2016). Invading up the food web? Invasive fish in the southeastern Mediterranean Sea. Marine Biology,163(8), 180.  doi:10.1007/s00227-016-2950-7

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