Ecology of Predator-Prey Dynamics: Hare & Fox

• Exploitation enhances fitness for one individual, reduces fitness for the exploited individual.
• Predators kill and consume other organisms.
• Parasites live on host tissue and reduce host fitness, generally not killing the host.
• Parasitoid refers to an insect larva consuming the host.
• Pathogens induce disease.

Exploitation and Abundance

• Predators, parasites, and pathogens influence the distribution, abundance, and structure of prey populations.
• Exploitative interactions can influence prey and host populations.

A Herbivorous Stream Insect and Its Algal Food

• Lamberti and Resh (1983) studied the influence of caddisfly (Helicopsyche borealis) larvae on algal and bacterial populations on which it feeds.
• These larvae can make up 25% of the biomass of benthic animals.
• Experiments showed that larvae reduce the abundance of their food supply.

Bats, Birds, and Herbivory in a Tropical Forest

• Kalka et al. (2008) examined how birds and bats affected tropical forest arthropods.
• Treatments included controls, daytime bird exclusion, and nighttime bat exclusion.
• Compared to controls, bird exclusion increased arthropods by 65%.
• Compared to controls, bat exclusion increased arthropods by 150%.

A Pathogenic Parasite, a Predator, and Its Prey

• Lindstrom et al. (1994) studied the spread of mange mites (Sarcoptes scabiei) on foxes in Sweden and the indirect effects on foxes' prey.
• Mange mites cause hair loss, deterioration, and death in foxes.
• Fox populations declined by 70%.
• The number of mountain hares (Lepus timidus), a prey species, increased 2 to 4 times after fox population declined.

Dynamics

• Snowshoe hares (Lepus americanus) and lynx (Lynx canadensis) have well-documented population cycles.
• Elton proposed abundance cycles are driven by variation in solar radiation.
• Keith suggested that the cycles are do to "overpopulation theories."

Keith's "Overpopulation Theories"

• Decimation by disease and parasitism
• Physiological stress at high density
• Starvation due to reduced food

Alternative Theory

• Hare cycles are driven by predators

The Role of Food Supply

• Hares live in conifer-dominated boreal forests, where there is dense growth of understory shrubs.
• Hare density can reach 1,100 to 2,300/km².
• In winter, they browse on buds and stems of shrubs and saplings.
• One population reduced food biomass from 530 to 160 kg/ha over 4 months.
• Shoots produced after heavy browsing can increase levels of plant chemical defenses.
• This process reduces usable food supplies during population declines.

The Role of Predators

• Lynx are one predator of snowshoe hares, but other predators also play a large role.
• Predation can account for 60 to 90% of hare mortality during peak densities.
• Predators exhibit functional and numerical responses to increased hare density.
• Both predation and food contribute to hare population cycles; they are complementary.
• Hares increase, reducing quantity and quality of food while predation increases.

Experimental Test of Food and Predation Impacts

• Krebs et al. (1995) conducted a field experiment to test the impacts of food and predators on snowshoe hares.
• They found increased hare numbers with increased food availability, reduced predation, and a combination of the two.
• The hare population cycle resulted from the interaction among 3 trophic levels: hares, their plant food supply, and their predators.

Population Cycles in Mathematical and Laboratory Models

• Lotka-Volterra assumes the host (prey) population grows exponentially, limited by parasites, pathogens, and predators.
• rₕNₕ= exponential growth by host population, is opposed by P, the rate of parasitism/predation.
• Nₕ = Number of hosts
• Nₚ = Number of parasites/predators.

Growth of Predator Population

• Lotka-Volterra assumes the parasite/predator growth rate is determined by the rate of conversion of food into offspring minus the mortality rate of the parasite/predator population.
• cpNₕNₚ = conversion rate of hosts into parasite/predator offspring (where c = a conversion factor and p = rate of parasitism/predation).
• dₚNₚ = parasite/predator deaths.

Model Behavior

• Host exponential growth is opposed by exploitation.
• Host reproduction is translated into immediate destruction by the predator.
• Increased predation leads to more predators.
• More predators give a higher exploitation rate.
• A larger predator population reduces the host population, in turn reducing the predator population.
• Produces oscillations in both populations.

Unrealistic Assumptions of the Lotka-Volterra Predator-Prey Model

• Predictions of eternal oscillations along a narrow path are not necessarily realistic.
• Neither population is subject to carrying capacities.
• Changes in one population result in immediate responses by the other.
• Lotka-Volterra models made valuable contributions by demonstrating that predator-prey interactions can produce population cycles.

Laboratory Models

• Utida observed reciprocal interactions in adzuki bean weevils Callosobruchus chinensis and a parasitoid wasp, Heterospilus prosopidis, over 112 generations.
• Gause reported similar patterns in P. aurelia preying on yeast in a shorter experiment.
• Most laboratory experiments have failed to produce Lotka-Volterra oscillations, leading to the extinction of one population within a short period.

Refuges

• Hosts and prey need refuges to persist in the face of exploitation.
• In laboratory and mathematical models, Gause attempted to produce population cycles with P. caudatum and Didinium nasutum.
• Didinium consumed all Paramecium, and both went extinct.
• However, with sediment (Paramecium refuge), Didinium went extinct, and a few Paramecium survived.

Refuges and Mites

• Huffaker studied a prey species, the six-spotted mite (Eotetranychus sexmaculatus) and a predator mite Typhlodromus occidentalis.
• Habitats were oranges and rubber balls with partial barriers to mite dispersal.
• Typhlodromus crawls while Eotetranychus crawls or balloons.
• Both species maintained population oscillations spanning 6 months (3 cycles).

Exploited Organisms and Variety of Refuges

• Refuges can take: flight for a bird, large sizes, etc.
• Spatial refuges include burrows, trees, etc.
• Small isolated populations of invasive Opuntia stricta cactus act as spatial refuges because herbivorous insects cannot always find them.
• St. John's wort also persists in small populations and is thereby protected from beetle predators.

Protection in Numbers

• Living in a large group provides a refuge.

Predator's response to increased prey density:

• (Prey Consumed/Predator) x (Predators/Area) = (Prey consumed/Area)

• Many species use predator satiation defense.

• Prey can reduce the individual probability of being eaten by living in dense populations.

• Beyond a threshold, increases in prey density do not lead to increases in predator density or feeding rates.

The Ecology of Fear and Refuges

• Reintroduction of gray wolves (Canis lupus) to Yellowstone National Park resulted in elk (Cervus elaphus) avoiding riparian areas.
• Riparian trees, especially willow, may be on the increase as a result of the reduced foraging by elk.

Complex Interactions

• Exploitative interactions weave populations into a web of relationships that defy easy generalization.
• Conservatively there are 10 million species.
• The number of exploitative interactions is far greater.
• Lake Okeechobee, Florida, contains approximately 500 known species, linked by approximately 25,000 exploitative interactions (50 times the number of species).

Parasites and Pathogens That Manipulate Host Behavior

• Spiny-headed worms (Acanthocephalans) change the behavior of amphipods, making it more likely that infected amphipods will be eaten by a suitable vertebrate host.
• Infected amphipods exhibit positive phototaxis, bringing them closer to surface predators.
• Uninfected amphipods exhibit negative phototaxis, keep them away from the suitable vertebrate hosts.

A Plant Pathogen That Mimics Flowers

• Rust fungus Puccinia monoica manipulates the growth of host mustard plants (Arabis spp.).
• Puccinia infects Arabis rosettes and invades actively dividing meristemic tissue.
• Rosettes rapidly elongate and are topped by a cluster of bright yellow leaves.
• These pseudoflowers are fungal structures, including reproductive structures.
• The structure then secrete sugary fluids, attracting pollinators that assist the fungus in outcrossing.

Entangling Exploitation with Competition

• Park found that the presence or absence of a protozoan parasite (Adelina tribolii) influences competition in flour beetles (Tribolium).
• Effects of parasites become entangled with predation among beetles and cannibalism.
• T. castaneum is the most cannibalistic.
• Adelina reduces the density of T. castaneum but has little effect on T. confusum.
• Without Adelina, T. castaneum is usually the strongest competitor; with Adelina, T. confusum becomes the strongest competitor.

Explore predator-prey dynamics, focusing on mange mites' impact on fox populations and subsequent effects on mountain hare populations. Examine Elton and Keith's theories on snowshoe hare cycles. Investigate the impact of hare browsing on plant defenses and the role of predation in hare mortality.