Question

5. The study of marine food goes beyond that of following the fate of energy across levels (i.e., trophodynamics). Trophic relationships class we disscussed two ways in which trohic control change population abundance, the top- trophic can also control population abundance. In down and the bottom-up trophic control. Let's assume a food chain with four trophic levels: phyoplankton, krill, baleen whales, and killer whales. simplified Southern Ocean marine increasing their attack rate on baleen whales. Describe how top-down control changes the abundance of each trophic level below that of killer whales. For each trophic level, provide a plot showing the variation in abundance over time. (4 (a) Assume that killer whales are primary (b) Now assume that for the same marine food web there is a decrease in ocean productivity due to earlier melting of seasonal sea ice. Describe how bottom-up control changes the abundance of each trophic level above that of phytoplankton. For each trophic level, provide a plot showing the variation in abundance over time. (4 points)

Answer 1

Top-down control refers to the phenomenon of a top predator controlling the population dynamics of the ecosystem.

Bottom-up control is exercised by the limitation of resources like food source, in this case phytoplankton.

The Southern Ocean Marine Food Chain can be drawn as follows:

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KILLER WHALES BALEEN WHALES KRILL PHYTOPLANKTON

ANSWER OF PART A

Time Baleen Whale

A.

B.

Time Krill

Time Phytoplankton

C.

If the Killer whales increase their attack rate on the Baleen Whales, the population of Baleen Whales will decrease as shown in Fig. A. The Baleen Whales used to feed on the krills. In absence of predator, population of krills will increase as in B. Krills survive on phytoplanktons. Hence, the consumption rate of phytoplanktons will increase and its population size will decrease as in C. Eventually, however, the decreasing population size of phytoplankton will limit the growth of krills and thereby baleen whales, thus disturbing the food chain.

ANSWER OF PART B

Time

A.

Time Baleen Whale

B,

Time Killer Whale

C.

In the Bottom-up approach, if the amount of phytoplankton decreases, the population size of Krills will also decrease (A) in absence of food and thereby the population sizes of Baleen Whales (B) and subsequently killer whales (C) will also decrease in absence of sufficient food.