Trophic Interactions

Question 1

Are marine mammals just predators?

Answer 1

No! They can also be prey

Question 2

How do marine mammals adjust their foraging behavior in the presence of a predator (think of Bottlenose Dolphin case study)?

Answer 2

• Prey biomass of bottlenose dolphins is higher in shallow water
• Therefore, bottlenose dolphins will forage for prey in shallow waters
• Tiger sharks (predator) are also common in shallow warm waters
• To avoid predation, bottlenose dolphins forage in shallow cold waters (away from tiger sharks)
  Marine mammals do NOT just go where their prey is

Question 3

Define oligotrophic

Answer 3

Nutrient poor region
Typically tropical regions with warmer waters

Question 4

How can marine mammals (such as baleen whales) be considered providers?

Answer 4

Baleen whales provide significant energy into oligotrophic systems during winter months

Question 5

In what ways can baleen whales provide energy to their surroundings?

Answer 5

• Placentas
• Carcasses
• Sloughed skin

Question 6

What larger ecosystem effects do the presence of baleen whales have on their surroundings?

Answer 6

• Baleen whales provide a predictable and significant source of energy to their surroundings
• Attract predators (white sharks, tiger sharks, killer whales) who can take advantage of food resource
• Whale falls provide habitat and energy to deep sea organisms

Question 7

What effects can marine mammals have on predation (think of Seals and Northern Cod case study)?

Answer 7

• Depletion of fish stocks
• 1992, cod stock collapsed in New Finland
• Proposed solution was to kill seals in order to replenish cod stocks
  Is this a good solution?

Question 8

Define meso-predator

Answer 8

A predator in the middle of the food web

Question 9

What are some possible scenarios that could explain the depletion of the Northern Cod fish stock?

Answer 9

1. Seals eat commercially important fish
   Result: Killing seals replenishes cod stocks
2. Seals eat meso-predators (ex: squid) which are feeding on cod stock
   Result: Killing seals increases meso-predator population cod is further depleted
3. Multiple trophic interactions contributed to cod depletion (not just seals)
   Overall, all trophic interactions must be understood in order to figure out what led to the depletion of a particular resource

Question 10

Describe a bottom up trophic effect

Answer 10

Relative abundance of organisms at lower trophic levels drives the abundance of organisms in the middle and upper trophic levels

Question 11

Describe a top up trophic effect

Answer 11

Opposite of bottom up. Relative abundance of organisms at upper trophic levels drive abundance of organisms in the middle and lower trophic levels.

Question 12

Describe a trophic cascade

Answer 12

The indirect effect of predators on lower trophic status species through top-down forcing

Question 13

What is a Keystone species?

Answer 13

A species whose impact on is community or ecosystem is large relative to its abundance.

Question 14

What happens when a keystone species is removed?

Answer 14

A phase shift (trophic cascade) can occur which results in dramatic changes in ecosystem structure and function

Question 15

Describe the phase shifts associated with sea otter abundance in the North Pacific Kelp Forests

Answer 15

• Sea otters feed on sea urchins
• Sea urchins feed on kelp forests
• When sea otter abundance is high, sea urchin populations are kept in check and kelp forests thrive
• When sea otter populations are low, a phase shift occurs, sea urchin populations go unchecked and kelp forests cannot grow (urchin barren)

Question 16

How do killer whales effect the sea otter abundance, sea urchin abundance and growth of kelp in Northern Kelp Forests?

Answer 16

• When killer whale predation is high, sea otter abundance decreases
• Sea urchin abundance increases
• Kelp forests are depleted

Question 17

How is kelp growth affected by sea urchin predation?

Answer 17

• Because sea otter predation levels are higher closer to the surface, sea urchin populations are greater at deeper levels
• Therefore grazing intensity on kelp is highest at deeper levels
• Kelp species that are well defended against herbivores grow at deeper level than kelp that have poor defense against herbivores
  Entire system is driven by otter predation

Question 18

What is the Cleaver Equation?

Answer 18

Relates the metabolic rate and mass of an organism

Question 19

Is the Cleaver Equation effective for estimating the consumption of marine mammals?

Answer 19

Not really because it is hard to measure the metabolic rate of marine mammals

Question 20

What are some methods involving marine mammal behavior and prey data that can be utilized to estimate prey consumption in marine mammals?

Answer 20

• High resolution tag data can be used to track lunge feeding events
• Morphometric data: filtration rates of water (baleen whales)
• Fishery acoustics can measure prey density at different depth
  All of these data methods can be used to roughly estimate prey consumption per year

Question 21

How can drone data be used to estimate consumption in baleen whales?

Answer 21

• Can measure seasonal changes in body condition (how fat they are)
• Baleen whales accumulate energy reserves during the summer foraging season and utilize these energy reserves during the breeding season
• Changes in body condition can be utilized to estimate the annual energetic budget of whales

Question 22

How can marine mammal population sizes be used to estimate consumption?

Answer 22

Take individual consumption estimates and multiply them by population size estimates
This is a very challenging estimate

Question 23

Describe an El Nino event?

Answer 23

• Trade winds (east to west) weaken
• Warm water is pushed East
• Results in little to no upwelling of nutrient rich cold water

Question 24

Describe coastal upwelling

Answer 24

• Driven by the Coriolis effect
  -Nutrient rich cold water surfaces
• Allows the growth of organisms such as plankton and algae in the photic zone
• Results in increased productivity

Question 25

What marine mammals would be impacted the most during an El Nino event and why?

Answer 25

• Pinnipeds
• They are less mobile than other marine mammals due to them being tied to land during their breeding seasons (central place foragers)

Question 26

What are the bottom up effects of an El Nino event?

Answer 26

• Less coastal upwelling results in less nutrients
• Less prey for marine mammals (such as fur seals that feed on fish that feed on nutrients in the photic zone)
• Marine mammals that are unable to escape the effects of an El Nino event (ex: pinnipeds) have decreases in population size