Lecture 9 Predation and Predator Management

Question 1

Optimal foraging theory

Answer 1

Optimal foraging theory: maximize nutrition
return per energy invested
* eat the organs to stay optimal*
time is money

Question 1

Confusion effect

Answer 1

the effect arising when it
becomes difficult for predators to pick out individual prey from groups

Question 2

Describe how predators can have effects on a prey population’s demographics.

Answer 2

Predators can significantly affect the demographics of prey populations by targeting specific groups, such as the very old, young, or weak, thereby influencing population structure (e.g., age or sex distribution). For example, areas without wolves have shown higher cases of diseases like Swine Fever, suggesting that predation helps control disease spread by removing individuals that might otherwise live longer and spread illness. Additionally, predation can remove unfavourable phenotypes, favoring prey traits such as crypsis, vigilance, agility, speed, defensive structures, and herding behavior, which can lead to evolutionary changes in prey populations.

Question 3

Identify traits removed through natural selection by predators.

Answer 3

Predators remove unfavourable phenotypes from prey populations through natural selection, thereby favoring traits such as:

Crypsis (ability to avoid detection)
Vigilance (alertness)
Agility and speed
Defensive structures
Herding behavior
These traits are advantageous for survival in the presence of predators, indicating that natural selection driven by predation can significantly influence the evolutionary direction of prey species.

Question 4

Describe how parts of a captured prey item vary in nutritional quality, and explain how
different predators use those parts.

Answer 4

Parts of a captured prey item vary significantly in nutritional quality, which influences how different predators utilize these parts. For example:

Polar Bears: When seal availability is high, polar bears selectively consume only the organs and blubber of their prey. The blubber, comprising 30-40% of the seal’s total body mass and containing approximately 93% fat and 7% water, provides about 9.3 kcal/g. In contrast, the carcass, making up 60-70% of the total body mass with about 70% water, yields roughly 4.5 kcal/g. This selective feeding behavior is guided by the optimal foraging theory, which posits that animals aim to maximize their nutritional return per unit of energy expended in hunting and processing food.
Facilitated Feeding: The concept of facilitated feeding illustrates that the preference for different parts of prey varies across the food chain. Predators higher up the food chain, such as lions, may preferentially consume organs due to their higher nutritional value, while scavengers or animals lower on the food chain, like certain birds, might consume the remaining parts, such as the integuments (skin and associated tissues).
This differentiation in prey part utilization underscores the complexity of predator-prey interactions and the strategies developed by predators to optimize their energy intake, which can have significant implications for both the predator’s survival and the broader ecosystem dynamics.

Question 5

Solve problems related to optimal foraging theory. For example, use a graph to explain how
foraging decisions would change as environmental conditions change.

Answer 5

Optimal foraging theory suggests that animals will maximize their nutrition return per energy invested in hunting or foraging. This concept can be applied to understand how foraging decisions change with environmental conditions through the use of graphs that illustrate predator behavior in response to prey numbers. These responses can be categorized into three main types:

Type I – Linear Response: The predator’s consumption rate increases linearly with prey density. This implies a constant foraging efficiency, where each additional unit of effort yields a constant return in terms of prey captured. In this scenario, changes in environmental conditions that affect prey availability will directly affect the predator’s foraging effort and success rate in a linear manner.
Type II – Decelerating Response: Here, the consumption rate initially increases rapidly with prey density but then decelerates as prey density continues to increase, eventually reaching a plateau. This reflects a scenario where predators can increase their intake up to a certain point, beyond which additional prey does not lead to a proportionate increase in consumption. This could be due to satiation or the increased difficulty of handling additional prey. Environmental changes that increase prey density might lead to increased foraging success up to a certain point, after which the benefits to the predator do not increase significantly.
Type III – Saturation but also Acceleration at Low Densities: This response is characterized by a slow start at low prey densities, where predators might ignore or not efficiently exploit the prey due to low encounter rates or alternative food sources being more attractive. As prey density increases, the consumption rate accelerates, indicating an increased focus or specialization on the now abundant prey type, before eventually saturating at very high prey densities.
Environmental changes can shift the availability and density of prey, thereby influencing which of these response types a predator exhibits. For example, an increase in prey density due to favorable conditions (e.g., abundant food resources for the prey) might move a predator from a Type I to a Type II or III response, altering their foraging strategy to exploit the more abundant prey. Conversely, a decrease in prey density might force predators to adapt by broadening their diet or improving their hunting efficiency to maintain their energy intake.

Graphical representations of these foraging responses can help illustrate how predators might shift their strategies in response to changing environmental conditions, thereby optimizing their energy expenditure in relation to the expected nutritional return from their foraging efforts.

Question 6

Identify factors that influence predator-prey relationships, and explain why.

Answer 6

Factors that influence predator-prey relationships and their reasons include:

1. Predator vs. Prey Ratio:
   
   • Numerical Supply of Prey: The more prey available, the better the nutrition, survival, and reproduction rates of the predator.
   • Prey Diversity: A more diverse prey base can buffer predator numbers against population cycles in prey, possibly maintaining or even increasing the predator:prey ratio.
   • Biotic Potential and Longevity: Prey typically have a higher biotic potential than predators, which influences the predator:prey ratio.
   • Geographic Concentration of Predators: Predators are attracted to areas with high concentrations of prey.
   • Intrinsic Regulation of Predator Numbers: Some predator species are territorial, which may limit their population density and, consequently, their numerical response to prey abundance.
   • Competition Among Predators: Both intra- and interspecific competition can lead to lower nutritional health for predators.
2. Vulnerability of Prey:
   
   • Habitat Quality: Good cover for escape and visibility to detect predators are crucial for prey survival.
   • Decreased Animal Quality at Increased Prey Densities: High prey densities can lead to decreased ability to detect, escape from, or fight predators.
   • Dispersal of Subordinate Prey into Poor Habitats: In territorial species, subordinate individuals are forced into worse habitats.
   • Evolved Predator-Evasion Strategies: Prey have adaptations that limit the effectiveness of predators, such as crypsis or specific nesting areas.
   • Increased Predator Defense at Higher Prey Densities: More individuals mean more eyes for vigilance and a stronger confusion effect, making it harder for predators to target individual prey.
   • Learning Avoidance Behavior: Prey learn to avoid predators if predation becomes more common.
3. Predator Behavior in Response to Prey Numbers:
   
   • Type I – Linear Response: Predators consume prey in a linear relationship to prey density.
   • Type II – Decelerating Response: Predation rate initially increases with prey density but then plateaus.
   • Type III – S-shaped Response: Predation rate is low at very low prey densities but accelerates as prey density increases before eventually plateauing at high densities.

These factors collectively influence the dynamics of predator-prey interactions, shaping the behavior, population size, and evolutionary trajectories of both predators and prey within an ecosystem.