Dynamics of Natural Populations Population growth curves Biotic potential versus environmental resistance Density dependence and critical number

Population Equilibrium Births Deaths A dynamic balance between births and deaths.

Population Growth Curves

Reproductive strategies: Many offspring with low parental care Few offspring with high parental care J-shaped growth curve S-shaped growth curve

Population Dynamics Environmental resistance: combination of biotic and abiotic factors that may limit population increase. Biotic potential: combination of biotic and abiotic factors that enhance population increase.

Density Dependence and Critical Numbers Factors of environmental resistance are either: –density-independent: effect does not vary with population density; e.g., adverse weather –density-dependent: effect varies with population density; e.g., infectious disease Critical number: the lowest population level for survival and recovery

Mechanisms of Population Equilibrium Predator-prey dynamics Competition –Interspecific –Intraspecific Introduced species

Predator-prey Balance: Wolves and Moose

Lessons to Be Learned About Predator-Prey Balance Absence of natural enemies allows an herbivore population to exceed carrying capacity which results in overgrazing of the habitat. The herbivore population subsequently crashes. The size of the herbivore population is maintained so that overgrazing or other overuse does not occur.

Plant-Herbivore Dynamics No regulatory control (predation) on herbivores Went into exponential growth pattern Overgrazed habitat Massive die-off of herbivores Reindeer on St. Matthew Island

Mechanisms of Population Equilibrium: Plant-Herbivore Compare the predator-prey with plant-herbivore methods of controlling the size of the herbivore population. How would the herbivore population growth curve look if diseases or predators were used as the control mechanism?

Keystone Species A single species that maintains biotic structure of the ecosystem Pisaster ochraeceus - a starfish that feeds on mussels, keeping them from blanketing the rocks.

Competition: Intraspecific Territoriality: defense of a resource against individuals of the same species –Examples of wolves and songbirds –Results in priority access and use of resources How do wolves and songbirds establish territory?

Competition: Interspecific Grassland contains plants with both fibrous and tap roots Coexist by accessing resources from different soil levels

Introduced Species Rabbits in Australia and on Phillip Island (next slide) Chestnut blight in United States Japanese beetles, fire ants, gypsy moths in United States Water hyacinth, kudzu, spotted knapweed, purple loosestrife (see Fig in text) in United States

Introduced Species Why have introductions of nonnative and exotic species resulted in a degradation of ecosystems? (Think in terms of environmental resistance and biotic potential.) An example of the answer to this question is given in the next slide.

Introduced Species: Rabbits in Australia Introduced into Australia from England in 1859 No natural enemies – rabbit population exploded Overabundant herbivore population devastated natural vegetation (see Fig in text). Using disease as control measure – why will this procedure fail in the long-term?

Rabbits Overgrazing in Australia

Phillip Island 1978 Largely devoid of vegetation

Phillip Island 1988 After Rabbit Removal

Vulnerability of different organisms to environmental changes.

Which Species Will Survive in a Rapidly Changing Environment? Panda Narrow distribution Small population Low genetic variation Large size Small # of offspring Limited migration Mouse Wide distribution Large population High genetic variation Small size Large # of offspring High migration

Pressures on Ecosystems What is the carrying capacity for the human population on Earth? How will the human ecological footprint impact on nature’s goods and services?