January 7 , 2014
Biology 3SS3: Population Ecology
Ecology vs. Environmentalism, Science vs. Policy
Treehuggers vs. Hummer drivers
Ecology ≠ environmental science ≠environmentalism
Ecologist (in their professional capacity) should say what will happen, not what
you should do
Opinions about Advocacy
Ecologists should not make decisions
What is population ecology? What other kinds of ecology are there?
• Behavioural, physiological
• Community (micro vs macro)
Taxon specific: plant, animal, microbial, insect, human…
Goals of Ecology
• Understanding (basic science)
What population are you interested in?
• Human, crops, trees, fish, game, pests, invasive species, endangered
species, microbes, infected people, infected cells
Population Ecology: Questions
What processes affect organisms’ population sizes?
How do population sizes respond to these effects
• Why are some populations large, and others small?
• Why are some cyclical and others stable?
• When can similar species coexist in the same area? Math and Models
Population ecology uses models, and math
Math is a critical tool for linking processes to outcomes; it will play a central role
in the course
We will keep it simple (not necessarily easy)
Logic is more important than math, but math is still important
• A nasty disease spread by mosquitoes
• In some places (e.g., the southeasterm US) it has been eradicated almost
• They still have mosquitoes there
• In other places it persists at high levels despite concerted efforts at
• What are the risk factors for malaria spread, and what determines when it
can be controlled?
• Native to the UK
• Red squirrels are rapidly disappearing from England
Loss of suitable habitat?
Competition (for food/habitats) from gray squirrels (invasive
species) introduced form North America?
Diseases carried by gray squirrels?
More than one of the above? Synergy?
• Introduced to North America from Eurasia
• Gypsy moth defoliation
• Irregular population pattern
• Reduce growth rate of the trees
• Dramatic population cycles: can devastate forests at peak population sizes
• What causes gypsy moth population cycles, and (how) can these cycles be
Resource availability (food quality)
Predators (mice, birds) cannot respond fast enough
Parasitoids (wasp that lay their eggs and paralyze the moths)
Disease (virus, fungus)
Does the information we have for this factor big enough to be included in the
January 9 , 2014
Population Dynamics Why are there no fish in my favourite mountain lake?
• Not suitable
• Fish never arrived
• Fish thrived for a while and then something happened
• Disease, new predator, environmental catastrophe
What factors will determine what will happen if I introduce some fish: depends on
the reason for why they are not there
• Suitable environment (temperature, oxygen,…)
• Resources (mostly food)
• Enemies – disease predators
• Start with one dandelion; it produces 500 seeds, of which 1% survive to
• How many dandelions will be there after 3 years?
• Imagine we have a population of rabbits
• Baby rabbits become adults after one month
• Each pair of adult rabbits produces one pair of baby rabbits each month
• Rabbits never die
• What happens to this population: similar to exponential growth
• These are the Fibonacci numbers
What is a Model?
A model is a set of assumptions that can be used to make predictions
For example, we make assumptions about rabbit lifespan and reproduction, and
then evaluate the consequences of these assumptions
Ecological models always have many unstated assumptions
What is an unstated assumption about the dandelion model? Models are Useful
• Linking scales
• Individuallevel actions and populationlevel effects