WK7 - INTERACTIONS: Predation: Part 1: The impacts of predation

Predation is an exploitative interaction where one organism (the predator) consumes another (the prey).
Differs from competition: predation has a positive impact on the predator and a negative impact on the prey.

Direct Effects: Predator gains food, prey is injured or killed.
Indirect Effects: Changes in population densities.
Intraspecific Predation (Cannibalism): Predation within the same species, common in spiders and praying mantises.
Interspecific Predation: Predation between different species.

Predation: Causes immediate death of the prey.
Parasitism: Parasite consumes nutrients from a host, decreasing host fitness but not necessarily causing death.
Parasitoids: An exception where the parasite eventually kills the host (e.g., parasitoid wasps laying eggs in caterpillars).

Categorized by lethality and specificity.
Herbivores: Low lethality, general prey choice (e.g., eating grass without killing the plant).
Parasites: Rarely lethal, highly specific (e.g., deer ticks preferentially feeding on deer).
Carnivores: High lethality, generalist prey choice (e.g., eating a range of herbivores).
Parasitoids: Kill one prey during a prolonged attack, highly specific interaction (e.g., parasitoid wasp).

Specialists (Monophagous): Eat only one type of food (e.g., anteater eating only ants).
Generalists (Polyphagous): Eat a broad diet (e.g., yabby eating almost anything).
Species exist along a gradient from narrow to broad diets.

Burrowing Bettong (Bui) in Australia:
- Once widespread, populations disappeared from mainland Australia due to fox predation by 1942. Remnant populations were only found on offshore islands.
- Example of range contraction due to predation.
- Prairie-proof fences are now used to reintroduce bettongs to the mainland.

Introduction of a species to a new area without its natural enemies allows successful establishment and population growth.
Foxes in Australia: Introduced without natural enemies, leading to rapid population expansion.
Plants in their native range are affected by many pathogens and viral species. Once you take that plant out of its native range and put it into a new environment, we find that there are far fewer pathogens.

Caddisfly Larvae and Algal Blooms:
- Caterpillar larvae graze on algae and build protective sand houses.
- Experiment with elevated tiles showed that larvae couldn't access algae on elevated tiles, leading to greater algal biomass.
- Demonstrates the effect of caddisfly larvae on algae biomass in rivers.

Dingoes Predation on Feral Pigs:
- When dingoes are present, few young piglets survive. Population is mostly older pigs (2-5 years old).
- When dingoes are absent, many young piglets are present, and there are fewer older pigs.
- Predation shifts the population structure towards older individuals.

Fox Predation on Rock Wallabies:
- Foxes were introduced to Victoria and spread across Australia, decreasing rock wallaby abundance.
- Removal of foxes led to a quick increase in rock wallaby abundance.
- Release from predation results in a significant population increase.

Starfish Predation on Barnacles and Mussels:
- Starfish predation promotes biodiversity by countering competition between barnacles and mussels.
- When starfish are present, both barnacles and mussels coexist.
- When starfish are removed, mussels become dominant, leading to competitive exclusion of barnacles.
- Starfish are needed to maintain diversity in the community.

Numerical Response: How predator abundance changes relative to prey abundance.
Functional Response: Changes in predator behavior related to how many prey are eaten per predator.
- Mediated by search time, handling time, and satiation.
Total response is the combined effect of numerical and functional responses.

Functional Response: Number of larvae eaten increases with larvae density, but plateaus at a satiation point.
Numerical Response: Number of nesting pairs increases as the larvae population grows.
Combined Response: Percentage mortality due to predation increases with larvae numbers, but the effect is small at high prey densities.
Predators have a greater effect when prey numbers are low.

Insects emerging simultaneously to overwhelm predators.
Masting in trees, where all trees release seeds at the same time to satiate predators.

Models predator-prey interactions and population dynamics.
Rate of change in prey population: $\frac{dN}{dt} = rN - pNP$
- r: exponential rate of increase
- N: size of the prey population
- p: rate of predation
- P: number of predators
Effect of predation is subtracted from prey population growth.
Hare and Lynx Example: Shows nine to eleven-year cycles of abundance.
- Initial increase in hare numbers, followed by an increase in lynx numbers.
- As lynx eat more hares, hare numbers decrease, followed by a decrease in lynx numbers (and vice versa).
- Oscillation between the two species.
Growth of predator populations: $\frac{dP}{dt} = cP<em>rN</em>hN<em>p - dP</em>n$
- c: conversion rate of prey into predator offspring
- Pr : predation rate
- Nh : population of the prey
- Np : population of predators
- dP: death rate of predators.

Prey exponential growth is often contained by predator responses to prey population growth.
Increased predation equals more predators, which leads to a higher exploitation rate.
Larger prey population eventually reduces prey population, in turn reducing predator population (oscillations).
Exceptions: Prey can overwhelm predator reaction through strategies like masting or swarming.