Levels of Organization, Population EcologyChapter 3

We study ecology at several levels

• 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

Levels of ecological studies

• 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

Each organism has habitat needs

• 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

The niche: a multidimensional concept

• 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

Population sizeAll populations show characteristics that affect their future dynamics

• Population size: the number of individuals present at a given time• Can increase, decrease, cycle, or

remain the same

Humans drove passenger pigeons, North America’s most abundant bird, to extinction

Population density

• 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

• 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

Sex ratios and age structure

Four factors of population growth or decline• 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

Population growth rate

• 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 population growth

• 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

Limiting factors restrain population growth• 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

• 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

Population density affects limiting factors• 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

Perfect logistic curves aren’t often found

Carrying capacities can change

• 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

Reproductive strategies vary among species• 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

Population changes affect communities• 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 cloud-

forest 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

Conserving biodiversity• 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’s protection is paying off• 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

Conclusion

• 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