Levels of Organization, Population Ecology

1. Levels of Organization, Population Ecology Chapter 3

2. 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

3. 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

4. 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

5. 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

6. Population size All 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

7. 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

8. 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

9. Sex ratios and age structure • 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

10. 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

11. 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

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

13. 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.

14. 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

15. 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

16. Perfect logistic curves aren’t often found

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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