Predator-prey interactions

Course: Marine Biology (BR22620)
Focus on marine ecology: interactions of organisms with each other and their environments
Lecture by Prof Paul Shaw

Definition: linear sequences showing how energy is transferred between organisms in an ecosystem.
- Typical length: 2-3 steps
Example: Rocky shore food chain includes:
- Producers: Fucus serratus
- Primary consumers: Littorina mariae / obtusata
- Secondary consumers: Carcinus maenas
- Tertiary consumers: Dicentrarchus labrax

Definition: complex networks of interacting food chains.
Metrics for analyzing food webs:
- Link number (N links, L)
- Linkage density (L/S)
- Connectance (L/S²)
- Generality (n resource/taxa)
- Web complexity and interaction strength.
North-West Atlantic Food Web: illustrates complex interactions among all macrophyte spp. and phytoplankton spp.

Structure:
- Tertiary consumer
- Secondary consumer
- Primary consumer
- Producer
Controls:
- Bottom-up control: bottom resource levels dictate population size.
- Top-down control: predators limit prey populations, thus influencing lower trophic levels.

Organism Focus:
- Mytilus californianus (mussels) create mono-specific stands.
- Upper limit: set by desiccation tolerance.
- Lower limit: influenced by predation from Pisaster ochraceus (starfish).
Observations:
- Absence of Mytilus leads to increased diversity in algae and invertebrates.
Keystone Predator hypothesis
- impact of spp. disproportionally large relative to its abundance/biomass. removal of spp. causes a large shift in the structure of the community and the extinction of some species.

Key Species:
- Baleen whales, smaller toothed whales, sperm whales, various seals, penguins, and krill.
Importance of Krill:
- Euphausia superba: foundational species affecting multiple predator species.
- Sustainable fishery statistics and production of krill.

Importance of Krill:
- Density is heterogeneous in space and time and recruitment correlates with available resources.
- Krill as a crucial food source for many predators (e.g., blue whales consuming up to 8 tonnes daily).
- important to land-based predators and pelagic fish. (70% of higher order predators diet made up of krill dependence).
- Evidence of competition dynamics affecting predator populations based on krill abundance.
- high krill- low competition
- low krill - comp.
- whale reduction hypothesis - commercial whaling = more krill available
- Trends noted in krill density related to climate change indicate potential future impacts on predator species.
- when krill abundant- less efficient food web - fewer top predators
- when krill abundant - more top predators - fewer steps in food web.
- The copepod and amphipod food pathways cannot support the same level of predator demand.
- = food shortages for predators when krill are sparse.
- Macaroni and crabeater penguins experience reduced breeding performance when krill is limited.
- kill density decreases with the reduction of sea ice (as they consume sea ice algae)

Definition: Daily migration patterns exhibited by zooplankton between surface and deeper waters.
movement of zooplankton in the water column.
Characteristics: Largest animal migration in biomass globally.
Types of DVM:
- Nocturnal migration: single ascent/descent.
- Twilight migration: two ascents/descent during the night.
- Reverse migration: surface during the day; depth at night.

Proximate Cause: Light intensity variations (dawn, dusk, environmental conditions).
Ultimate Causes:
1. Potential metabolic advantages in warmer waters at night.
2. Predator evasion to minimize predation risks.
Facilitated transport to new feeding areas influenced by currents.
Predator Evasion Hypothesis:
Supporting Evidence:
- Daily vertical migration (DVM) of fish is more pronounced when planktivorous fish are abundant and is greater in larger, pigmented species compared to smaller, transparent ones.
Using the Predator Evasion hypothesis, we can predict that:
- DVM may vary among individuals of the same species based on nutritional state, with those having high lipid reserves spending less time at the surface.
- DVM may also reverse in the presence of many tactile predators.

DVM crucial for nutrient cycling and energy transfer within marine ecosystems.
Impact on global biogeochemical cycles, including carbon and nitrogen transport.

Complexity of marine food webs includes human impacts as top predators.
Major trophic levels typically include 3-4 levels, governed by both top-down and bottom-up controls.
Keystone species (like Pisaster ochraceus) significantly influence community structure beyond their biomass.
Diel vertical migration represents a massive ecological event with ramifications for nutrient transport and food web dynamics.