Even in the places where we expect to find ecologically simple systems -- caves, deserts, polar regions -- ecological communities are enormously complex. One framework for resolving this complexity is to view the community as an integrated a food web. When we look at the community as a set of interacting components, we can see that no species lives in isolation, and that a change to one species affects all the species in that community.
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For example, if this community were to lose wolves, caribou would be released from the pressures of predation; and if this allowed the caribou population to grow, the plant community would experience increased herbivory. My research has focused on just one tiny corner of this food web: the interactions between plants, insects and wolf spiders. In this little bubble, the spiders are the wolves; the insects are the caribou. It turns out that even in this tiny corner of the food web, there is a whole lot to study and puzzle apart. A bird‘s eye view of the food web that I put together with Amanda Koltz really drives home that point. You could imagine that if you tug on just one piece of this food web, like the green bubble that represents plants in the upper right-hand corner, the rest of the food web would have to respond in turn.
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Humans are having profound effects on the environment. Some geologists have even argued that we are in our own geological epoch: the Anthropocene. In a myriad of ways, humans are changing, and have already changed, the way that food webs work. Climate change and land use change are reshaping the ecosystems in which we live. At the same time, species losses of important predators, like wolves, and introductions of predators, like these asian carp, may affect the top of the food web, but they ripple down to affect every member of the community – including, of course, us humans!
One aspect of global change that we may not think about a lot is the human impact on global nutrient cycles. Without humans, the amount of nitrogen available for plants to take up is primarily limited by the amount that microbes can fix from the atmosphere, and the amount of phosphorus is primarily limited by the slow process of weather wearing down rocks. But, by mining phosphorus, synthesizing fertilizers, and combusting fossil fuels that contain reactive nitrogen, we have changed this equation. All these fertilizers don‘t stay in one spot, but rather move around the environment in the air and through the world‘s waterways. As a result of our activities, we have doubled the amount of biologically available Nitrogen in the world, and increased the amount of P flowing to the oceans by 8 times. |
Nitrogen (N), phosphorus (P) and other elements are key for plant growth. In the images below, they are represented by a big green N. Plants take up N, store it in their tissues, and compete with each other. Those competitive interactions are shown as arrows between plant species in the images below. The plants feed the next trophic level (herbivores), who negatively affect plants through herbivory. These negative effects are represented by dashed arrows. Herbivores in turn are the food for predators, which negatively affect herbivores through predation (more dashed arrows).
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Ecological theory and evidence from nutrient addition experiments in ecology give us some idea of what will happen under altered nutrient cycling.
Usually, extra N gets taken up by the plants, which in turn support more insect biomass – this can move up the food chain, even allowing for extra trophic levels, like this little cartoon parasitoid of the wolf spiders. Increased N may also promote the amount of herbivory performed by consumers like insects. Consumers benefit not just from the extra amount of food, but also from the ways fertilizer affects plant tissue quality. |
But, these are all fairly near-term effects. In the long term, the change in the relationship between competing plant species can gradually lead to changes in plant community composition, and even plant species loss. These losses can cascade up the food chain. My research has tended to focus on three main questions about longer-term effects of nutrient pollution, which are not as well-researched or understood.
- Will increased plant production and improved plant quality lead to more herbivory……or will predators keep up with herbivores?
- Will greater production support more complex food web…or will plant species loss lead to simplified food webs?
- Can we predict which species will thrive, and which will be lost?