Chapter 3

• Ecology: studies interactions among organisms
• And their environment
• Ecology and evolution are tightly intertwined
• Biosphere: the total living things on
Earth
• And the areas they inhabit
• Ecologists: study relationships at higher levels

• Organismal ecology examines relationships between individuals and their environment
• Population ecology: investigates population changes
• The distribution and abundance of individuals
• Why some populations increase and others decrease
• Community ecology: focuses on patterns of species diversity and interactions
• Ecosystem ecology studies living and nonliving components of systems to reveal patterns
• Nutrient and energy flows

• Habitat: the environment where an organism lives
• It includes living and nonliving elements
• Habitat use: nonrandom patterns where organisms live
• Habitat selection: the process by which organisms actively select habitats in which to live
• Species use different criteria to select habitat
• Soil, topography, vegetation, other species
• Species have different habitat needs
• Depending on body size, season, etc.
• Species survival depends on having suitable habitats

• Niche: an organism’s use of resources
• Along with its functional role in a community
• Habitat use, food selection, role in energy and matter flow, interactions with other individuals
• Specialists: have narrow niches and specific needs
• Extremely good at what they do
• But vulnerable when conditions change
• Generalists: species with broad niches
• They use a wide array of habitats and resources
• Survive in many different places

All populations show characteristics that affect their future dynamics
Humans drove passenger pigeons, North America
’ s most
• Population size: the number of individuals present at a given time
• Can increase, decrease, cycle, or remain the same abundant bird, to extinction

• Population density: the number of individuals in a population per unit area
• Large organisms usually have low densities
• They need many resources and a large area to survive
• High densities make it easier to find mates
• But increase competition and vulnerability to predation
• Also increase transmission of diseases
• Low densities make it harder to find mates
• But individuals enjoy more space and resources

• Population distribution (dispersion): spatial arrangement of organisms
• Random: haphazardly located individuals, with no pattern
• Resources are widespread
• Uniform: evenly spaced individuals
• Territoriality, competition
• Clumped: most common in nature
• Arranged according to resources

• Sex ratio: proportion of males to females
• In monogamous species, a 1:1 sex ratio maximizes population growth
• Age structure (distribution): the relative numbers of organisms of each age in a population
• Helps predict population growth or decline
• In species that continue growing as they age
• Older individuals reproduce more (e.g., a tree)
• Experience makes older individuals better breeders

• Natality: births within the population
• Mortality: deaths within the population
• Immigration: arrival of individuals from outside the population
• Births and immigration add individuals to a population
• Emigration: departure of individuals from the population
• Deaths and emigration remove individuals

• Growth rate: rate of change in a population ’ s size per unit time
• Equals (birth rate + immigration rate) – (death rate + emigration rate)
• Tells us the net changes in a population ’ s size per 1000 individuals per year
• Growth rate is expressed as a percent:
• Population growth rate * 100%
• Populations of different sizes can be compared

• Exponential growth: a population increases by a fixed percent
• Graphed as a J-shaped curve
• It occurs in nature with:
• Small populations
• Low competition
• Ideal conditions

• Exponential growth rarely lasts
• Limiting factors: physical, chemical, and biological attributes of the environment limiting population growth
• Environmental resistance: all limiting factors together
• Stabilizes the population size at its carrying capacity
• Terrestrial animals: space, food, water, mates, shelter, breeding sites, temperature, disease, predators
• Plants: sunlight, moisture, soil chemistry
• Aquatic systems: salinity, sunlight, temperature, etc.

• Carrying capacity: the maximum population size the environment can sustain
• Determined by limiting factors
• Limiting factors slow and stop exponential growth
• Forms an S-shaped logistic growth curve

• Density-dependent factors: limiting factors whose influence is affected by population density
• Increased density increases the risk of predation, competition for mates, and disease
• Results in the logistic growth curve
• Environmental resistance has a stronger effect on larger populations
• Density-independent factors: limiting factors whose influence is not affected by population density
• Temperature extremes, floods, fires, and landslides

• Environments are complex and ever-changing
• The carrying capacity can change
• Humans lower environmental resistance for ourselves
• Increasing our carrying capacity
• Technologies have overcome limiting factors
• We have appropriated immense amounts of resources
• But by increasing the carrying capacity for humans
• We have reduced the carrying capacity for countless other organisms
• Calling into question our own long-term survival

• Biotic potential: an organism ’ s capacity to produce offspring
• K-selected species: species with long gestation periods and few offspring
(i.e., a low biotic potential)
• Offspring have a high likelihood of survival
• The population stabilizes at or near carrying capacity
• Good competitors
• r-selected species: species that reproduce quickly
• Have a high biotic potential
• Little parental care, populations fluctuate greatly

• Scientists have noticed troubling changes in the environment
• As Monteverde dried out, species have disappeared
• Golden toads, harlequin frogs, and more had been pushed from their cloudforest habitat into extinction
• Species from lower, drier habitats moved into the cloud forest
• Population sizes of cloud-forest bird species declined
• Changing climate and disease are causing population fluctuations and changing the makeup of communities

• Human development, resource use, and population pressure are changing populations and communities
• Factors threatening biodiversity have complex social, economic, and political roots
• We must understand these factors to solve problems
• Millions of people are working to protect biodiversity and to safeguard ecological and evolutionary processes

• Costa Rica was losing forests at the world ’ s fastest rate
• Now, 25% of its area is under protection
• Ecotourism: tourists visit protected areas
Ecotourism provides thousands of jobs and billions of dollars to
Costa Rica
’ s economy

• The fundamentals of evolution and population ecology are integral to environmental science
• Natural selection, speciation, and extinction help determine Earth ’ s biodiversity
• Understanding how ecological processes function at the population level is crucial to protecting biodiversity