bio 1b lec 22, 23, 24

Human population explosion

Rapid acceleration in human population growth associated with improved food production, sanitation, medicine, industrialization, and fossil fuel use.

Exponential vs. logistic population growth

Exponential growth assumes a constant per-capita growth rate; logistic growth slows as a population approaches carrying capacity.

Population size vs. growth rate

Population size can continue increasing even while the percentage growth rate declines; the global growth rate peaked around 1962–1963 at 2.2% annually.

Demographic transition Stage 1

Birth rates and death rates are both high, resulting in relatively low population growth.

Demographic transition Stage 2

Death rates decline while birth rates remain high, producing rapid population growth.

Demographic transition Stages 3 and 4

Stage 3: birth rates fall while death rates remain low, slowing growth. Stage 4: birth and death rates are both low, producing low or stable growth.

Age structure pyramids and population momentum

A wide pyramid base indicates many young individuals and potential future growth; a narrow base suggests slower growth or possible decline. Youthful populations may keep growing after fertility declines.

Anthropocene and anthropogenic impacts

The Anthropocene describes extensive human influence on Earth systems; anthropogenic means caused by human activities. Environmental impacts also depend on consumption, technology, energy, and food production.

Importance of nitrogen

Nitrogen is essential for proteins, DNA, and chlorophyll; most atmospheric N₂ is unavailable directly to organisms, and nitrogen availability can limit biological production.

Nitrogen fixation

Conversion of atmospheric nitrogen gas (N₂) into biologically available nitrogen, including through microbes and lightning.

Nitrogen assimilation and ammonification

Assimilation: plants absorb ammonium or nitrate and incorporate nitrogen into organic molecules. Ammonification: decomposers convert organic nitrogen into ammonium.

Nitrification vs. denitrification

Nitrification converts ammonium to nitrite and nitrate; denitrification converts nitrate into gaseous nitrogen returned to the atmosphere.

Human impacts on nitrogen cycling

Fossil fuel burning, Haber–Bosch synthetic fertilizers, nitrogen-fixing crops, livestock manure, and wastewater increase nitrogen inputs to ecosystems.

Eutrophication and hypoxia

Excess nutrients stimulate algal and microbial growth; decomposition consumes dissolved oxygen, causing hypoxia and potentially aquatic dead zones.

Gulf of Mexico dead zone

Nitrogen from fertilizers and other sources enters the Mississippi River basin and reaches the Gulf, promoting biological production and decomposition that deplete oxygen and reduce aquatic habitat.

Carbon source vs. carbon sink

A carbon source releases carbon into the atmosphere, such as fossil fuel burning or deforestation. A carbon sink removes atmospheric carbon, such as growing forests.

Net atmospheric carbon flux

Atmospheric CO₂ increases when total carbon inputs exceed carbon removal, even when carbon sinks continue absorbing CO₂.

Seasonal atmospheric CO₂ fluctuations

Atmospheric CO₂ decreases during periods of high photosynthesis and rises when photosynthesis declines; these seasonal fluctuations occur alongside a long-term increase.

Greenhouse effect

Greenhouse gases such as CO₂, CH₄, and water vapor absorb and re-emit infrared radiation, warming Earth's climate system.

Climate models and hindcasting

General circulation models simulate interactions among atmosphere, oceans, and land. Hindcasting tests models against past observations; models including human influences better reproduce modern warming.

Representative Concentration Pathways (RCPs)

Scenarios describing different greenhouse gas concentration trajectories based on factors such as population, economic development, and carbon efficiency.

Positive vs. negative climate feedback

Positive feedback amplifies an initial change; negative feedback counteracts it. These terms describe the direction of the response, not whether it is beneficial.

Ice-albedo feedback

Warming melts reflective ice, exposing darker surfaces that absorb more solar radiation, causing additional warming. This is positive feedback.

Plant-mediated climate feedbacks

Increased photosynthesis and NPP can remove more atmospheric CO₂ (negative feedback); increased tree mortality can raise atmospheric CO₂ (positive feedback).

Humidity and wet-bulb temperature

Warmer air can hold about 7% more moisture per 1°C. High humidity limits sweat evaporation, increasing wet-bulb temperature and dangerous heat stress.

Unequal human impacts of climate change

Vulnerability differs with age, health, poverty, location, infrastructure, and existing climate. Already hot and dry agricultural regions often face greater crop-yield losses.

Three biological responses to climate change

Organisms may decline or go extinct, shift their distribution or seasonal timing, or adapt in place.

Extirpation vs. extinction

Extirpation is disappearance of a species from one local area while it survives elsewhere; extinction is disappearance of the entire species globally.

Phenology and phenological mismatch

Phenology is the timing of life-history events. A mismatch occurs when interacting species shift their seasonal timing at different rates, such as birds breeding after peak caterpillar abundance.

Traits affecting climate-change vulnerability

Large populations, broad distributions, strong dispersal, high genetic variation, short generation times, and generalist niches generally increase opportunities to persist or expand under climate change.

Conservation biology

A multidisciplinary science that uses ecology, genetics, evolution, and other fields to understand and prevent biodiversity loss.

HIPPO framework

The five major anthropogenic threats to biodiversity: Habitat loss, Introduced species, Pollution, human Population, and Overexploitation. Climate change interacts with these threats.

Habitat loss and fragmentation

Habitat destruction, degradation, or fragmentation reduces resources and carrying capacity, isolates populations, decreases gene flow, and increases extinction risk.

Introduced vs. invasive species

Introduced species are transported by humans outside their native ranges. Some become invasive and harm native species through competition, predation, disease, or food-web disruption.

Pollution threats to biodiversity

Pollution can cause toxicity, bioaccumulation, biomagnification, eutrophication, and behavioral or physiological disruption.

Overexploitation

Harvesting or killing organisms faster than populations can replace themselves through reproduction; particularly dangerous for slowly reproducing species.

Human appropriation of NPP

Humans use approximately 25% of global terrestrial net primary productivity, placing substantial pressure on ecosystems and biodiversity.

Ecosystem service value

The value of natural processes that benefit people, such as water filtration, pollination, erosion control, and flood protection; these services can be compared with the cost of replacing them.

Existence value vs. intrinsic value

Existence value is the benefit people gain from knowing a species exists. Intrinsic value means a species has value or a right to exist independent of human usefulness.

Equilibrium Theory of Island Biogeography and reserve design

Larger reserves generally have lower extinction rates, while reserves closer to source populations have higher immigration rates; large, less isolated reserves tend to support more species.

Wildlife corridors

Connections between habitat patches that facilitate dispersal and gene flow, reduce isolation, and help maintain larger effective population sizes.

Genetic drift and inbreeding in small populations

Genetic drift randomly changes allele frequencies and can eliminate alleles in small populations. Inbreeding increases homozygosity and can expose harmful recessive alleles, reducing fitness.

Extinction vortex

A self-reinforcing cycle in which small population size increases genetic drift and inbreeding, reduces genetic diversity and fitness, lowers reproduction, and causes further population decline.

California condor recovery

Condors declined because of habitat loss, pesticides, and lead poisoning. All 27 remaining birds were captured for captive breeding, with managed genetic diversity, releases, and efforts to reduce lead exposure.

Effective conservation strategies

Identify and address the causes of decline through habitat protection, reducing pollution and overexploitation, restoring connectivity, genetic management, and balancing ecological and social considerations.