If you feed them, they will come: the effects of nitrogen fertilization on community composition in a salt marsh
Eutrophication has gained a pretty bad reputation considering that it is a natural process. The word itself comes from the Greek “eutrophia” which means “healthy” and simply means the addition of nutrients into an ecosystem encouraging plant growth. Of course, there’s good reason why eutrophication has such negative connotations these days. Since the industrial revolution, the amount of fertilizing nutrients, particularly nitrogen and phosphorus, entering ecosystems from runoff has doubled. Just like your mother told you, too much of a good thing can be bad: too much nitrogen cause unlimited plant, algal, or phytoplanktonic growth. When these organisms die, their decomposition uses up an incredible amount of oxygen, creating areas where these is little (hypoxic) or no (anoxic) oxygen. One of the most famous areas for this overgrowth leading to hypoxia from eutrophication is the Gulf of Mexico dead zone, which is 6000-7000 square miles!
But this is not necessarily how the story always ends. A little bit of fertilizer runoff can actually fertilize natural ecosystems. This typically increases the growth of plants or other photosynthetic organisms, whose growth is normally limited by the scarcity of nitrogen and phosphorus. When there are more plants, there is more energy available to herbivores and, in turn, omnivores and carnivores, altering the food web.
So – as you can guess – we are very interested in studying how these food webs are changed, and how the fertilizer runoff from human activities alters the structure of communities. One effect frequently seen is that the increase in nutrients allows rarer plant species the opportunity to grow. This increase in plant diversity leads to a greater diversity of herbivores, since herbivores that only feed on one type of plant are drawn in. Thus this increase in diversity of species can ripple throughout the system.
One question that ecologists have been asking is how a pure plant biomass increase would effect the diversity of herbivores, omnivores and carnivores in an ecosystem. That is – not an increase in plant diversity, but a simple increase in the amount of food available. This is a harder question to study than you might think. Most ecosystems are diverse to start, and the rarer species with greater nutrient requirements are always looking for a way to wiggle their way in to grow and reproduce.
A recent study in Ecology features some beautiful experiments that do a great job addressing this question. Gina Wimp and her crew studied a salt marsh on the coast of Tuckerton, NJ. Salt marshes are usually a monoculture: they only have one species of plant growing, a grass called Spartina alterniflora. Not only is it a monoculture, but just one plant! This grass species has a rhizomal root system, which can spread its roots underneath the soil and shoot up new stalks where it can find the resources and space. So in miles of salt marsh, you will often find a single organism. Pretty amazing!
The researchers used this monoculture to their advantage to see whether the species composition of the community changed when there was simply MORE Spartina available as a food source. The researchers marked off three types of plots in the salt marsh. One was an unaltered control plot, and the other two had “low” and “high” nitrogen additions 5 times throughout the 2-month growing period. The “high” level was the maximum amount of nitrogen you could add without killing the Spartina. They samples the insect communities 4 times during the season, and also took measurements of the changes in plant biomass and grass height when they sampled these insects with a D-vac suction sampler. (Yes, a vacuum for bugs.)
What did they find? Well, to start off, they did find significantly greater growth of Spartina in the fertilized plots, and it remained a monoculture, confirming what we already thought we knew. (Phew!) In the fertilized plots, they found not only more insects living on the grass, but a greater diversity of species, as you can see in the figure below. The asterisks above the sampling date show that there was a significant difference between each nitrogen treatment – that is, the different in number of insects was large and consistent enough that it was not due to chance. AND! On top of that, the actual communities of insects were also significantly different.
In summary: the increase in the amount of plant biomass alone (through fertilization) not only increased the number of insects and the number of different species sequentially, but also the relative abundances of each of those species to one another. WHOAA!
What does this mean really? Most of the focus on overfertilization from runoff has been on these dead zones and areas of hypereutrophication that I discussed earlier. But this study shows that, even when plant diversity is unaffected, the addition of nitrogen and phosphorus affects not only plant growth, but what species are living in an area and how many of each are around. That is: we are changing entire ecological communities.
On one hand, this seems like a good thing. When a little bit of fertilizer reaches ecosystems, there is an increase in production and an overall increase in biodiversity, which we generally agree is a good thing. But the problem with changing the structure of a community and even altering its food web is the potentially for causing a fundamental change in an ecosystem itself. Seems like a strange concept – but what if the increase in growth brings in an organisms that somehow disrupts the growth of Spartina in the first place? Then, while we temporarily have a buzzing, diverse community of organisms, the initial community could be lost, decreasing overall biodiversity.
I’m not trying to scare you or be a fearmongerer: but these are the extreme questions we have to ask when contemplating how our actions are influencing the typically balanced communities of organisms that surround us. This truly beautifully-designed study gives us the information that communities can change just from more food resources being available. And we can use this information to help regulate agriculture and prevent vast ecological changes from occurring that could be detrimental to the organisms, and us, in the end.
Wimp, G., Murphy, S., Finke, D., Huberty, A., & Denno, R. (2010). Increased primary production shifts the structure and composition of a terrestrial arthropod community Ecology, 91 (11), 3303-3311 DOI: 10.1890/09-1291.1