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Environmental Science Unit 2: Biodiversity and populations

Biodiversity keeps ecosystems resilient. Population growth depends on births, deaths and the limits of the environment.

Covers: Species, genetic and ecosystem diversity, ecosystem services, island biogeography, r- vs K-selected species, population growth, human population

Study guide

Biodiversity exists at three levels: genetic diversity within a species, species diversity (richness, the number of species, and evenness, how equally they're represented), and ecosystem diversity across habitats. Higher biodiversity makes ecosystems more productive and resilient to disturbance. Ecosystems provide services worth a great deal to people: provisioning (food, timber, medicine), regulating (pollination, water purification, climate regulation), supporting (nutrient cycling, soil formation) and cultural (recreation).

Island biogeography explains species richness on islands and habitat fragments. Larger islands support more species (more habitat and resources), and islands closer to the mainland receive more colonists. Fragmented habitats act like small islands, losing species.

Species have different reproductive strategies. r-selected species, like insects and weeds, have many offspring, little parental care, short lives and fast maturity; they thrive in unstable environments. K-selected species, like elephants and whales, have few offspring, lots of care and long lives, and their populations stay near carrying capacity. Survivorship curves (type I, II and III) show when in life most individuals die.

Populations grow exponentially when resources are unlimited and logistically when limited, leveling off at the carrying capacity. Density-dependent factors such as food, disease and competition regulate growth. The rule of 70 estimates doubling time: 70 ÷ percent growth rate.

Human population grew rapidly after the industrial and agricultural revolutions. Age structure diagrams predict growth: a wide base means rapid growth. The demographic transition model describes how countries move from high birth and death rates to low ones as they develop, and total fertility rate, replacement level (about 2.1) and access to education affect future growth.

Type I curves show low death rates until old age, as in humans and elephants (typical of K-selected species). Type II curves show a constant death rate throughout life, as in many birds. Type III curves show very high death rates among the young and then few deaths among survivors, as in insects, fish and plants (typical of r-selected species). Every species has a range of tolerance for conditions like temperature, pH and dissolved oxygen. Near the edges of that range organisms are physiologically stressed and reproduce less, and beyond it they die.

Generalists, which use many foods and habitats, cope well with change and often become invasive. Specialists, which have narrow niches, are more vulnerable to habitat loss and environmental change.

Key ideas

• Biodiversity includes genetic, species and ecosystem diversity.
• Ecosystem services: pollination, clean water, climate regulation.
• r-selected species have many offspring with little care; K-selected species have few with lots of care.
• Growth rate = (births − deaths)/population.
• Rule of 70: doubling time ≈ 70 ÷ percent growth rate.

Formulas and rules

Common mistakes

• Mixing up r- and K-selected traits.
• Using the growth rate as a decimal in the rule of 70 (use the percent).

Words to know

Rule of 70

Doubling time ≈ 70 ÷ growth rate in %.

r-selected species

Many offspring, little parental care, short lives.

K-selected species

Few offspring, lots of care, long lives.

Ecosystem service

A benefit nature provides people, like pollination.

Island biogeography

Bigger, closer islands have more species.

Genetic diversity

Variation in genes within a species.

Species richness

The number of species in an area.

Species evenness

How equally individuals are spread among species.

Carrying capacity

The largest population an environment can support.

Survivorship curve

Type I (most die old), II (steady), III (most die young).

Age structure diagram

Shows a population by age; a wide base means rapid growth.

Demographic transition

Countries move from high to low birth and death rates as they develop.

Total fertility rate

Average children per woman; replacement level ≈ 2.1.

Habitat fragmentation

Breaking habitat into small, isolated patches.

Sample practice questions

19 more questions like these, from level 1 to 5, are in the practice set.