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Word Wednesday: Top down or bottom up?

Wednesday, November 9th, 2016

A few weeks ago we discussed food webs and introduced the topic of top-down and bottom-up control. Today we are going to dig a little deeper into those processes and look at some examples in the marine environment.

Biological communities are influenced by all of their parts, which include the animals, plants, bacteria and fungi that live in them. These interacting species are placed into ecological trophic levels depending on their feeding relationships. A species at the top of the food chain would be considered a consumer and a species at the bottom would be considered a producer. Changes to one of the levels in the food chain can result in changes in other levels which scientists call a trophic cascade. These cascades can affect levels below that altered level or levels above the altered level. We categorize these directional cascades as top-down or bottom-up control, which simply means the changes to the food chain occurred at either the top trophic level or the bottom trophic level and then impacted the other levels up or down the food chain.

A top-down controlled system focuses on how top consumers influence lower trophic levels. This could be thought of as a predator driven system. One of the classic examples of a top-down trophic cascade is the sea otter story from the Pacific coast. Key players in the food chain are sea otters, sea urchins and kelp. Decreases in the sea otter population, whether due to human hunting (historically) or climate change (recently), led to increases in the urchin population. As urchins graze on kelp, more sea urchins resulted in less kelp cover. These changes can have indirect effects too. In the sea otter example, loss of kelp forests affected the fish populations that used the kelp as a habitat.

Top-down, kelp habitat
Food webs in kelp forests with and without sea otters showing the impacts of a trophic cascade. Source: Brumbaugh AMNH-CBC,

In contrast, a bottom-up system focuses on the lower trophic levels and the factors that drive interactions at the base of the food chain. This could be thought of as a resource driven (resources being food or habitat availability) system. An example of bottom-up control would be nutrient driven changes to seagrass ecosystems. In the northern Gulf of Mexico increases in nutrient levels to seagrass meadows from land runoff can cause increased growth of the epiphytes that grow on seagrass blades. This increased abundance of epiphytes changes the population sizes of herbivores that graze on this algae (mesograzers) and this increase results in increases in their predators.

Overgrowth of epiphytes on seagrass blades resulting from nutrient input. Photo credit: Jonathan Lefcheck.

But does this mean that a system may only exhibit one type of control? Yes, no and sometimes. The answer is not straightforward, as control mechanisms seem to change with the type of ecosystem, over time and sometimes on conditions prior to a change. Coastal wetlands are a good example of this. Traditionally salt marshes were thought to be exclusively controlled by bottom-up processes; food webs were dependent on the amount of marsh grass and algal primary production and a correlation could be drawn between the amount of nutrients flowing into the system and its productivity. However, more recent research on consumer populations in salt marshes has shown that geese and snail herbivory can cause extensive loss of the habitat and trigger a top-down trophic cascade. ACER scientists Drs. Valentine and Heck have spent much of their research career looking at the relative importance of these controls in seagrass ecosystems.

ACER researchers are looking at how the Deepwater Horizon oil spill (DWH) affected the diversity and resilience of coastal ecosystems. How the oil affected inshore and offshore food chains is one aspect of this research. Did the oil cause any trophic cascades? Did the cessation in commercial fishing in the northern Gulf after the spill and the potential increased abundance of top level consumers drive the observed changes in other fish populations? Did the emigration of some large consumers such as sharks from the area result in increases in their prey populations? Did oil coating salt marshes grasses result in decreased marsh primary production that affected the abundance of herbivores? Given the nursery function of salt marshes, did oil settling onto mudflats result in decreased abundance of infaunal species and therefore fewer shrimp, fish and crabs making it to adulthood? The answers to these questions will help us to understand the impact of the DWH and other disturbances on coastal food webs.