The Mystery of the Salt Marsh: An Anatomy of a Scientific Investigation

The work of science is to develop explanations of natural events. For students of science, this means learning to understand scientific explanations.

This is a bit tricky because scientific explanations are different from the usual kinds of explanations that students are asked to understand in other subjects.
A scientific explanation is really a kind of argument that is based on the construction of a model and the testing of the model with a carefully designed investigation that yields a body of data that can examined and used to make a case to support or refute the model.
In today’s blog, I have used a published investigation to follow the steps used by the scientists to develop a scientific explanation.
Salt Marsh Decline
Salt marshes are very important environments and many of them are dying because the plants such as cordgrass whose roots hold the soil together are rapidly and mysteriously disappearing. Because of the importance of salt marsh environments, finding the answer to why the cordgrasses and other vegetation are disappearing really matters. Instead of environments with abundant vegetation, providing support for lots of different forms of life, the shoreline is reduced to a lifeless mud flat.
This kind of die off has been observed all over the Western Atlantic region and investigators have posed a number of different possible reasons: eutrophication (that is, an overabundance of nitrates and phosphates that encourage algae blooms), pollution that changes the acid-basic balance, diseases that attack the vegetation, and even the mechanical wave action caused by outboard motors.
It is important to note that the work of any investigator is influenced by the work of all of the others investigating the given phenomenon. So for example, it has been suggested that in the specific area of Cape Cod, sport fishing and crabbing has reduced the top level predators (stripped bass, blue, and green crabs). The loss of the top level predators results in an increase in the numbers of the crabs that are herbivores. The now more numerous plant eating crabs destroy the cordgrasses and other plants that hold together the shoreline.
But so far, while this explanation is appealing, it has not been subjected to a test.
In a report published in Ecology Letters (March 2014), investigators conducted an experiment to see what would happen to the shoreline vegetation if top level predators (such as blue and green crabs) are excluded and herbivorous crabs are allowed to flourish.
The experiment was conducted by creating special predator exclusion pens that would be placed in eighteen randomly chosen locations around a particular salt marsh. A control set of cages that permitted entrance to the top level predator crabs was also established.
The investigators were able to quantify the amount of cordgrass in both the experimental and the control settings, the density of the herbivorous crab borrows, as well as the nutrients in the water and the water movement available at the beginning of the experiment. At the end of the experimental period (one growing season for the cordgrass), the amount of cordgrass was collected from the experimental and control settings, it was dried and weighed.
The experiment would then measure the what happened within each of these protected environments as compared with the control environments. The experimenters would know how much cordgrass was present when the experiment began as well as the size of the population of the herbivorous crabs at the outset and end of the experimental period.
At the end of the experimental period, it was found that in the predator exclusion pens, there was a great loss of cordgrass, and increase in the density of the herbivorous crab borrows.
The amount of cordgrass in the predator exclusion pens was reduced by greater that 60% when compared with the pens that permitted predator access.
The experimenters conclude that the experiment shows that removal of the top level predators can result in the rapid (that is, in a single season) destruction of the cordgrass population.
The experiment is part of a larger argument. The larger argument is that of the whole body of research findings about the relationships among organisms in an ecosystem. Another way to name the argument is to call it “scientific consensus” or “consilience.”
Thus, the experimenters point to other examples of what happens when top level predators are eliminated, allowing for the consumers (that is, in this case the herbivorous crabs) to run wild in the arctic and in the Southeastern United States Gulf coast.
The experimenters also argue that the solution to the “problem” is not simply one of preventing recreational fishing for stripped bass and blue crabs. The model of the ecosystem includes an understanding that impacts on one part of the will have predictable or non-predicable impacts elsewhere in the system.
Therefore, the experimenters argue that
While our results indicate that predator depletion can cause rapid, dramatic shifts in the biotic and abiotic condition of New England salt marshes, increasing population density and human activity in coastal areas suggests that multiple interacting threats are likely to become increasingly common, with the potential to fundamentally alter ecosystems worldwide.

With this conclusion they place their particular argument into the context of a larger argument about the interrelationships among organisms (including human beings, and their activities).

Source Material:

Carl Zimmer, When Predators Disappear So Does the Ecosystem. http://www.nytimes.com/2014/05/15/science/when-predators-vanish-so-does-the-ecosystem.html?_r=0

You can read about the complete experiment at
http://onlinelibrary.wiley.com/doi/10.1111/ele.12287/abstract

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