by Jaclyn Long
figures by Wei Wu
Grasslands are a type of ecosystem that make up over a quarter of the earth’s land. These habitats are often found in between deserts and forests, and are characterized by low levels of rainfall and regular fires. In North America, grasslands are usually called prairies. The rich soil held in place by grass roots makes them particularly useful for agriculture. However, grassland ecosystems are also critical sources of biodiversity and are thought to be important natural carbon sinks. Balancing the agricultural use and preservation of North American prairies is very challenging, and this ecosystem is one of the most threatened in the United States today.
Because of the numerous species that call prairies home, as well as the unique habitat, these ecosystems can be difficult to understand. However, it’s important that we develop a complete picture of the state of these prairies so that we can best protect and preserve them. Ecological networks, which consist of species linked by their interactions, provide a holistic way to understand the grassland ecosystem.
Ecological networks are made up of interacting species
The first step in building an ecological network is knowing all of the species present in a given ecosystem. When we think of ecological communities, we often think of the numerous animals, plants, fungi, and other organisms that are part of those communities. In the North American prairie, there are hundreds of species of grass alone, including littlebluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), switchgrass (Panicum virgatum), and western wheatgrass (Western wheatgrass). Some of the animals present in the prairie include bison, prairie dogs, jackrabbits, owls, rattlesnakes, and coyotes. Additionally, there are many fungi, such as mushrooms, bacteria, and insects, such as grasshoppers. Making a list of all of the species only provides us with so much information. The way that organisms interact with each other and with their environment is also necessary information to build an ecological network (Figure 1).
Many interactions between species are centered around resources. For example, competitive interactions occur when two organisms are both seeking a limited supply of the same resource. This resource could be food, water, or a specific habitat. Predation is when one organism kills and eats another organism. For example, coyotes in the prairie eat smaller animals such as jackrabbits and prairie dogs. Similarly, herbivory is a type of predation when one organism eats some or all of a photosynthetic organism, such as plants. Many members of the prairie ecosystem, including grasshoppers, prairie dogs, jackrabbits, and bison, feed on the grass.
Additionally, there are many types of symbiotic interactions. Mutualism occurs when both species in this interaction benefit. Large herbivores such as bison eat primarily grass, but aren’t able to digest all of the cellulose from the plants. Specific bacteria that live in the intestine of these animals help the bison break down their food, and in exchange the bison provides a warm and nutritious place for the bacteria to live. Commensalism means that one species benefits and another is neither helped nor harmed. When cattle graze on grasses, they disturb insects that live in the grass. This helps cattle egrets, prairie birds that feed on insects, access their food while not impacting the cattle. Finally, parasitism is characterized by one species benefiting at the expense of another. An herb called rattle lives on many grass plants, and feeds off of nutrients and water that flow through the grass’ roots. This prevents the grass from obtaining its own nutrients, causing it to grow more slowly.
Food webs are examples of ecological networks
Once we understand the interactions between the multiple species present in a community, we can begin to assemble networks of those interactions. One simple way to depict networks of feeding interactions are food webs (Figure 2).
Food webs are more realistic and informational than linear food chains because they allow us to understand and visualize the complexity in feeding relationships between multiple members of the ecological community. A food chain in the prairie ecosystem might include only some of the species. For example: grasshoppers eat grass, prairie dogs eat grasshoppers, and rattlesnakes eat prairie dogs. By constructing a food web, we can understand that both prairie dogs and owls eat grasshoppers, and that rattlesnakes eat both prairie dogs and jackrabbits.
There are many ways to add additional information to food webs to increase their complexity depending on the information you wish to depict. The arrows in food webs conventionally point towards the organism that is gaining energy. Flow webs add a quantitative aspect by measuring the amount of energy transferred between species. Different weights can be added to the arrows in the food web to describe the differences in energy transfer. Because bison eat more grass than prairie dogs do, the arrow pointing from the grass to the bison would be thicker than the arrow pointing from the grass to the prairie dogs. Functional webs include emphasis on how important a particular species’ functions are for the community. Adding this information helps us understand the relative contributions of each species to the strength of an ecological network. If a large portion of the energy flow is going through one species, removal of that species would be particularly disruptive to the community. In the prairie, grass species are particularly important as both sources of food to many animals as well as a physical structure to hold soil in place. This is evident from their central role in the functional web, and might guide farmers to limit how much grass they remove for agricultural purposes.
Features of ecological networks
There are many features that describe ecological networks, which are shaped by the species present in a network and their interactions. The biodiversity of an ecosystem describes both the number of different species present in a community and their relative abundance. A grassland ecosystem that has multiple different types of grasses, as well as many animals and microbes present, would be considered biologically diverse.
Complexity is the number of links between species in a network. Stability refers to the property of little change within an ecological network, even as the environmental conditions shift. For example, the amount of rainfall in the prairies has decreased in recent years. Prairie ecosystems with multiple types of grasses that can survive in different precipitation levels are able to maintain the species interactions that constitute the ecological network despite this change. Finally, resilience is the ability of an ecosystem to return to its original state when a large outside perturbation, such as a fire, occurs.
The effects of stability and complexity on resilience is a major topic of ecology research. Suppose there are two separate prairie ecosystems: Prairie A, where coyotes have only two options for prey and Prairie B, where they have five options (Figure 3). If there is an extinction event of one of the two species that the coyotes in Prairie A eat, this will be a major disruption to their food source and would negatively impact the coyote population. However, if this same event occurs in Prairie B, the coyotes would still have four other options and therefore the removal of one species would not have as large of an impact. There is a lot of evidence that highly diverse ecological communities have improved stability and resistance to disruptions. Understanding these features of ecological networks is especially useful for helping us predict the impact of future changes in the environment.
Climate change and human activity can disrupt ecological networks
Human activity can have tremendous and lasting impacts on ecosystems. The best-known example of this in the grasslands of the United States was the Dust Bowl, which occurred in the 1930s in the Midwest and Southern Plains. A rapid expansion of agriculture in the Great Plains led to the removal of much of the grass, which was critical for holding the soil in place. When a prolonged drought occurred, crops failed, and the soil blew away in dust storms. Perhaps if farmers had understood the critical role of the grass in the preservation of the land, and as a food source for the numerous species in the ecosystem, this ecological disaster could have been avoided.
Prairie ecosystems will also be impacted by climate change. The prairies in North America will likely experience hotter temperatures and drier conditions, as well as more frequent disruptive events such as floods, fires, and hurricanes. Scientists predict that grasslands with higher amounts of biodiversity will be more resilient to these changes, since there will be more species that can adapt to the changing conditions. Additional efforts to conserve grassland ecosystems in a changing climate include improving connectivity, so that species can migrate between different grasslands, as well as livestock management and control of invasive species.
The prairie ecosystem is an excellent example of how using ecological networks can help us understand how an ecosystem functions, the risks it faces, and what we can do to help preserve its health. By using networks to map how different species are interacting with each other, we can identify the most critical species in order to protect them. We can also use networks to understand how stable or resilient an ecosystem might be to changes in order to predict how human activity will impact it. As our planet changes, and as we seek to use different areas for human purposes such as agriculture, ecological networks are a valuable tool to limit damage to the environment.
Jaclyn Long is a first year Ph.D. student in the Immunology program at Harvard Medical School. You can find her on Twitter as @JaclynMLong.
Wei Wu is a second-year student in the Master of Design Studies program at Harvard Graduate School of Design. She explores simultaneously at the intersection of public art, design fiction, and emerging technologies.
For More Information:
- While this article focuses on North American prairies, grasslands can be found all over the globe. Learn more about grasslands in other parts of the world here.
- Read more Science in the News coverage of climate change and biodiversity
- Learn more about the history of the Dust Bowl here.
- Read about efforts to preserve the American Prairie here
This article is part of our special edition on networks. To read more, check out our special edition homepage!