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News and Resources

Food Web Dynamics - October 19, 2014

     The ecosystems in the coastal bays of Maryland are very diverse and extremely productive. Whether it be terrestrial or aquatic, there are a plethora of different plants and animals that are not only vital for the health of the local environment, but also play a major role in the complex coastal bays food web.

     The coastal bays food web is large and complicated, however it can basically be thought of as a pyramid shape. At the bottom are the primary producers, or autotrophs. This level generally consists of plants or phytoplankton, organisms that can use photosynthesis to make their own “food”. The next level, which is slightly smaller on our pyramid, is the primary consumers or herbivores. These organisms are heterotrophs, which means they must feed other organisms to obtain necessary nutrients to survive, and can differ depending on the ecosystem. In aquatic ecosystems, primary consumers can be zooplankton or filter feeding fish while in terrestrial ecosystems, it can be insects, small rodents, or other herbivores. The next three levels are all heterotrophs and are either omnivores or carnivores. These levels, in ascending order up the pyramid and descending in population size, are secondary, tertiary and quaternary consumers.

     Quaternary consumers are considered to be on the “top” of the food web pyramid, and are severely impacted by changes in the food web below them. However, changes in the food web do not just effect higher level consumers though. For example, here in the Chesapeake and coastal bays of Maryland, Bay Anchovies and Atlantic Menhaden are extremely vital primary and secondary consumers. These smaller fish can gather in huge schools to feed on phyto and zooplankton and provide food to larger fish and animals, such as striped bass, bluefish, weakfish, osprey and eagles.

     Because of the large abundance of these fish, Menhaden and Bay Anchovy fisheries are some of the largest along on the east coast, bringing in tons of fish and millions of dollars each year. Due to availability and popularity of these fish for commercial use, the population of these two important fish have decreased steadily due to a variety of reasons, including overfishing and higher mortality rates of eggs and juveniles. As Menhaden and Bay Anchovy populations decline, it puts more pressure on their natural predators to find other favorable food or search larger areas for these dwindling fish schools. It also changes the structure of the pyramid at the bottom, allowing for larger populations of phyto and zooplankton due to less predation from these two filter feeding fish. An increase in plankton populations can be detrimental to the whole food web as it can decrease water quality and kill fish, shellfish and even other aquatic vegetation like eel grass.

     In ideal situations, populations of animals, like Menhaden, Bay Anchovy and their predators and prey, tend to stay around their carrying capacity. Carrying capacity is the population size that can be supported by available nutrients and prey abundance in the ecosystem. Most species never stay at their carrying capacity though; their population size tends to be in flux from year to year, and will migrate above and below the carrying capacity size as trends dictate.

     A common example of this are wolf and deer populations. When deer populations increase above their carrying capacity, due to food abundance or other reasons, then wolf populations will also increase due to more prey being available. Eventually the stress from wolf predation and other factors will decrease the deer population. As deer, or prey, populations decrease the wolf populations will also decrease due to famine, stress or diseases. The decrease in predator populations of wolves will allow the deer populations to increase again, which in turn will allow for wolf populations to increase again. This important predator-prey relationship is a cycle which, when broken, can lead to disastrous results in both populations.

     The predator-prey relationship can be divided up into bottom-up and top-down controls. Bottom-up control can be thought of as the bottom of the food web controlling the populations of the top of the food web. Bottom-up control means that the maximum healthy population size of predators higher up the food web is set by the available resources on the bottom of the food web. Top-down control is the force that predators exert on prey. It means that the predators at the top of the food web control prey population sizes through predation, which ensures that resources, such as primary consumers, do not get over exploited. These two controlling forces help to stabilize predator and prey populations and help them migrate back towards the ecosystem carrying capacity size.

     Food webs are extremely complicated and help to stabilize the populations of animals and plants within their ecosystems. Unfortunately, humans have created problems for many different ecosystems and food webs by killing off top level predators, over using natural biological resources, destroying natural habitat for development and pollution. These factors can change food web dynamics and put serious stress on the animals and plants that live there. Considering that humans are a big part of the global food web and our population is only increasing, problems that we created or are creating now will have serious negative repercussions, financially and environmentally, in the future.

Harrison Jackson is the Coastal Stewards Coordinator for the Maryland Coastal Bays Program.




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