Principal Ecological Question Addressed

What physiological, physical, or behavioral characteristics enable individual prey to avoid being eaten by a predator? What evolutionary forces might have caused these predator-avoidance traits to emerge and become common in prey populations? Can predator-avoidance adaptations actually contribute to long-term survival of a predator population, as well as persistence of prey and other species in the ecosystem? How might habitat features of an ecosystem (e.g., availability of structurally-complex habitat) influence predation rates and predator-avoidance adaptations of prey? Can human-generated habitat destruction influence predator-prey interactions, and if so, what are the likely effects on prey and predator populations and other living and nonliving ecosystem components?


What Happens

Through a lecture and class discussion, students are provided basic biological and ecological background information pertaining to characteristics of common predators (fish) and prey (snails) of aquatic ecosystems. Additionally, I introduce students to basic elements of experimental design (e.g., hypotheses, replication, standardization, use of statistics to make objective conclusions). Students are then challenged to design an experiment that answers the following questions:

  1. Can snails use chemical cues to detect threats to their survival in the form of shell-crushing predators?
  2. Do snails then move to habitats that provide refuges from these large predators?

Students are made aware of available resources and time-limitations, and then divide into small groups to design an experiment. Each group describes their proposal to the entire class. Using a guided-inquiry approach, I attempt to incorporate at least one component of each groupís design into a common experimental plan that the entire class will follow. A typical scenario (and one that I describe here in detail) would include students observing and quantifying changes in habitat use of snails exposed to chemical cues that signal the immediate presence of predatory fishes. Using aquaria containing equal quantities of dechlorinated water, structurally-complex habitat (e.g., stones, ceramic tiles), and snails, students first record numbers of snails that are visible in underwater habitats, including on aquarium walls and upper surfaces of stones and tiles where they would be vulnerable to fish predation. Aquaria are then randomly assigned to predator-free or predator-cue treatments (n = 5 replicates or aquaria per treatment). Water in aquaria of the predator-cue treatment is replaced with water containing chemical cues produced by crushed snails or fish. Water in predator-free treatment aquaria is replaced by dechlorinated water lacking these cues. Snails that detect and respond to cues will increase movement rates until finding a possible refuge from predation. Once snail activity in the predator-cue treatment slows or ceases, students again record numbers of "vulnerable" snails (i.e., snails visible on aquarium walls and upper surfaces of stones and tiles). Simple statistical tests are used to determine if chemical cues stimulated increased use of habitats that provide refuges from predators, including undersides of tiles, interstitial spaces between stones, or aquarium walls above the water line.


Experiment Objectives

  1. Use live animals to illustrate important ecological/evolutionary topics, including natural selection, predator-prey interactions, animal behavior, and habitat heterogeneity,
  2. Demonstrate the complex nature of species interactions in ecosystems, including rarely observed mechanisms that enable populations of small organisms to coexist with predators,
  3. Provide an example of a study that includes essential experimental design components (independent and dependent variables, experimental and control treatments, replication, standardization),
  4. Enhance skills and confidence in using statistical analysis techniques to answer research questions objectively,
  5. Generate increased student interest in ecology, as well as appreciation of snails and other small organisms that are ecologically important yet poorly understood.

Equipment/Logistics Required

Equipment needed to collect animals and maintain them in captivity

Equipment needed for the experiment (for a class of 10-25 students)

Logistic Requirements


Summary of What is Due

Biology Majors

Each student writes a short (approximately five double-spaced pages) paper based on the experiment. The paper is written in standard scientific journal style. Students are also graded on completeness of data tables and worksheets and accuracy of calculations.

Non-Science Majors

Each student must complete data tables and worksheets. Additionally, students must submit answers to questions that are intended to evaluate understanding of objectives, results, and applications of the experiment.


Keyword Descriptors

Ecological Topic Keywords:

abiotic factors, biotic factors, chemical ecology, community ecology, ecosystems, environmental adaptation, experimental design, feeding strategies, life history, multispecies interactions, population ecology, predation, predator-prey relations, species diversity, species interactions, trophic dynamics

Science Methodological Skills Developed:

collecting and presenting data, data analysis, designing experiments, evaluating alternative hypotheses, experimental design, hypothesis generation and testing, identify biotic and abiotic interactions, natural history, quantitative data analysis, scientific writing, statistics, use of primary literature, writing primary research paper

Pedagogical Methods Keywords:

background knowledge, cognitive skill levels, guided inquiry, problem-based learning, project-based teaching