Predation in Introductory Ecology: Study Notes

INTRODUCTORY ECOLOGY: PREDATION NOTES

Course Title: Introductory Ecology (BIOL 2060)
Instructor: Rajesh Rajaselvam
Affiliation: Dalhousie University
Publisher: McGraw-Hill Ryerson Ltd, Edwin Giesbers

COPYRIGHT AND USAGE

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  • Copying this material for distribution or sharing outside of the class may violate copyright law.

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OUTLINE OF CONTENT

  • Predation & Parasitism

    • Introduction

    • Impacts of Predators on Prey Populations

    • Predator-Prey Dynamics

    • Predator Avoidance Ecology: Concepts & Applications (Sher & Molles)

INTRODUCTION TO PREDATION

  • Exploitation:

    • An interaction between populations that enhances the fitness of one individual while reducing the fitness of the exploited individual.

    • Herbivores consume live plant material but usually do not kill plants.

    • Predators kill and consume other organisms.

CYCLES OF ABUNDANCE IN SNOWSHOE HARES AND THEIR PREDATORS

  • Snowshoe Hare (Lepus americanus) and Lynx (Lynx canadensis):

    • One of the best-studied animal population cycles is between snowshoe hares and their predator, the lynx.

    • Population cycles are well documented with extensive records of trapping data from the Hudson Bay Company (18th, 19th, and 20th centuries).

    • These interactions serve as a model for studying the impacts of predation on population dynamics.

    • Historical records allowed ecologists to estimate the relative abundances of Canada lynx and snowshoe hare over approximately 200 years.

HISTORICAL FLUCTUATIONS IN POPULATIONS

  • Documented fluctuations in lynx and snowshoe hare populations are based on pelts purchased by the Hudson Bay Company.

  • The role of predation was explored, demonstrating a remarkable match in the cycles of both populations.

HYPOTHESES EXPLAINING POPULATION CYCLES

  • Several hypotheses have emerged to explain the regularity of cycling patterns:

    • Sunspot Hypothesis (Charles Elton, 1924):

    • Abundance cycles driven by plant growth; variation in solar radiation alters plant growth, subsequently affecting hare population sizes.

    • Lynx populations respond to changes in snowshoe hare abundance.

    • However, sunspot cycles do not match prey population cycles, leading to its rejection.

    • Sunspot cycles refer to recurring increases and decreases in sunspots over an 11-year average period.

    • Lloyd Keith’s Overpopulation Theories (1963):

    • Stated that high population growth phases are followed by decimation through disease, parasitism, physiological stress due to high density, and starvation from reduced food availability.

    • Alternative Hypothesis:

    • Suggests predators increase in response to rising prey availability, ultimately reducing prey populations.

    • Population cycles in predators and prey result from multiple co-occurring mechanisms.

ROLE OF FOOD SUPPLY IN POPULATION DYNAMICS

  • Habitat of Snowshoe Hares:

    • Hares inhabit boreal forests characterized by dense understory shrub growth.

    • They exhibit high potential rates of increase, capable of doubling their population size each generation during growth phases.

    • During peak hare population densities, food shortages arise as hares browse heavily on shrubs and trees in winter, causing plant chemical defenses to hinder food quality and reduce edible supplies.

ROLE OF PREDATORS

  • While lynx are specialized predators of snowshoe hares, other predators also impact hare populations, including:

    • Goshawks, great horned owls, mink, long-tailed weasels, red foxes, and coyotes.

    • Predation can account for 60-90% of mortality during peak hare densities.

    • Functional Response of Lynx:

    • Peaks at intermediate hare densities while coyotes show increased feeding rates with higher hare densities.

    • Functional Response:

      • Refers to how predator populations increase relative to increases in prey populations, influenced by:

      1. Movement of additional predators into prey areas (immigration).

      2. Increased reproductive rates of predators due to heightened food availability.

EXPERIMENTAL TEST OF FOOD AND PREDATION IMPACTS

  • An extensive field experiment lasting 8 years utilized nine 1 km² plots led by Krebs at the University of British Columbia (UBC).

  • Food Availability:

    • Some plots were supplemented with rabbit chow.

  • Predation:

    • Electric fences excluded predators from certain plots.

  • Combined Treatment:

    • Extra food added while predation was excluded to evaluate the impact of food and predation on prey populations.

NON-CONSUMPTIVE EFFECTS OF PREDATORS (THE ECOLOGY OF FEAR)

  • Consumptive Effects of Predation:

    • Direct impacts on prey populations via capture and consumption.

  • Non-Consumptive Effects of Predation:

    • Changes in prey due to predator presence, even without direct predation, including:

    • Shifts in morphological traits.

    • Stress responses in physiology.

    • Altered behavior patterns.

  • A study measuring blood cortisol levels (stress indicator) in snowshoe hares indicated higher cortisol levels in years with increased predation rates and in plots where predators were present compared to those where predators were excluded.

  • Predators also influence reproductive rates in hares, leading to both increased mortality rates and decreased birth rates in hare populations.

  • Prey populations are influenced by food availibility consuption by predators, and non -consuptive effects of predators

INFLUENCE OF FOOD AVAILABILITY, PREDATION, AND NON-CONSUMPTIVE EFFECTS ON POPULATIONS

  • Conclusion: Prey populations are shaped by three factors: food availability, consumption by predators, and non-consumptive effects of predators.

PREDATOR-PREY DYNAMICS IN A MATHEMATICAL MODEL

  • The Lotka-Volterra Predator-Prey Model is introduced to incorporate predator population growth into exponential population growth models.

  • Use parameters:

    • h for predator species (herbivore or host)

    • p for predator species (predator or parasite).

LOTKA-VOLTERRA EQUATIONS AND GRAPHICAL REPRESENTATION

  • The Lotka-Volterra equations outline the relationships governing predator-prey dynamics and population growth.

  • Zero-Growth Isoclines:

    • Graphical analysis demonstrates combinations of prey and predator populations where each remains constant.

    • The model predicts cyclic patterns:

    • Prey decrease / Predators decrease

    • Prey increase / Predators decrease

    • Prey decrease / Predators increase

    • Prey increase / Predators increase

PREDATOR-AVOIDANCE MECHANISMS

  • Prey populations do survive despite the presence of predators due to various mechanisms:

    • Animal Displays

    • Refuge Use

ANTIPREDATOR ADAPTATIONS

  • Species employ various defense strategies against predation:

    • Cryptic Coloration:

    • Camouflage that allows an organism to blend into its environment.

    • Mimicry:

    • Resemblance of one species (mimic) to another (model).

    • Batesian Mimicry:

      • When a palatable species mimics an unpalatable one.

    • Müllerian Mimicry:

      • Unpalatable species share similar warning coloration to reinforce the deterrent effect.

    • Physical Defenses:

    • Armor, claws, stingers, horns.

    • Predation Satiation:

    • A defense tactic where prey occurs in very high densities.

DEFENSE STRATEGY: PREDATOR SATIATION BY PERIODICAL CICADAS

  • Periodical Cicadas (Magicicada spp.):

    • These insects emerge as adults synchronously every 13 or 17 years.

    • While living as nymphs underground, they synchronize their emergence, leading to millions emerging within days.

    • Densities can reach approximately 4x10^6 individuals per hectare.

SIZE AS A REFUGE FROM PREDATORS

  • Large size may offer protection from predation.

    • Example: Adult elephants can kill adult lions, providing refuge for mature elephants.

    • Smaller young African elephants remain vulnerable to lion predation.

EXAMPLES OF REFUGE SIZE AND SURVIVORSHIP

  • In studies excluding sea stars, mussels achieved greater survivorship by growing larger, making them unsuitable as prey when the sea stars returned.

  • This experiment illustrates the potential for size to offer a refuge against predation.

MIDTERM EXAM INFORMATION

  • Date: October 27, 2025 (Monday)

  • Time: 11:30 AM

  • Location: McCain Arts & Social Sciences Auditorium-1

  • Format: 34 (+2) Multiple-Choice Questions

  • Duration: 50 Minutes

  • Coverage: All topics discussed up to and including competition.

  • Policies: Mandatory exam, no makeup unless justified with evidence.

  • Contact for Questions: Rajesh Rajaselvam & Joanna Zigouris

SAMPLE QUESTIONS

  • Questions were likely discussed but not listed in the provided transcript; further examination preparation is recommended.

This comprehensive guide captures a wealth of information on predation, its implications on prey populations, and the intricate dynamics present between predators and prey. Students should familiarize themselves with these concepts in preparation for exams and practical applications in ecology.