Honors Biology Unit 20 Review: Conservation Biology & Environment

The greenhouse effect is natural and necessary. Sunlight warms Earth's surface, the surface gives off infrared radiation (heat), and greenhouse gases like CO2, methane (CH4), nitrous oxide (N2O) and water vapor absorb and re-emit that heat. Without them, Earth would average about -18°C instead of about 15°C.

The problem is the enhanced greenhouse effect. Burning fossil fuels and clearing forests raised atmospheric CO2 from about 280 ppm before the Industrial Revolution to over 420 ppm today, and average surface temperature has risen more than 1°C. Effects include sea level rise, species shifting toward the poles or uphill, earlier flowering and migration, and coral bleaching.

The ocean absorbs some of the extra CO2, which forms carbonic acid. This ocean acidification makes it harder for corals, clams and some plankton to build calcium carbonate shells.

Ozone depletion is a separate problem. CFCs depleted stratospheric ozone, which absorbs much of the Sun's harmful UV radiation. The 1987 Montreal Protocol phased out CFCs, and the ozone layer is slowly recovering.

Worked example. Average ocean surface pH has dropped from about 8.2 to about 8.1. How much did hydrogen ion concentration change? pH is a base-10 log scale, so a drop of 0.1 means $[H^+]$ was multiplied by $10^{0.1}$. Since $10^{0.1} \approx 1.26$, the ocean has about 26% more hydrogen ions than before. A small pH change is a big chemical change.

Sustainability means meeting today's needs without compromising the ability of future generations to meet theirs. For renewable resources, use must stay within the rate of replenishment over time.

Renewable resources include sunlight, wind, forests and fish. Forests and fish can replenish on a human timescale if harvest does not exceed regrowth over time. Coal, oil, natural gas and metal ores are non-renewable on that timescale, though metals can often be recycled.

An ecological footprint estimates the productive land and water needed to support a person or country. One ecological footprint estimate puts humanity's annual demand at about 1.7 Earths' worth of biocapacity. This estimate depends on accounting methods and includes the area needed to absorb CO2 emissions.

For waste, the order matters. Reduce first, then reuse, then recycle. Recycling aluminum saves about 95% of the energy needed to make new aluminum from ore, but never making the can saves all of it.

Worked example. A fish population grows logistically, $\frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right)$, with $r = 0.4$ per year and $K = 10{,}000$. At $N = 9{,}000$, growth is $0.4 \times 9{,}000 \times 0.1 = 360$ fish per year. At $N = 2{,}000$, it is $0.4 \times 2{,}000 \times 0.8 = 640$. At $N = 5{,}000$, it is $0.4 \times 5{,}000 \times 0.5 = 1{,}000$. In this model, growth peaks at $K/2$, so the maximum sustainable yield is 1,000 fish per year if the population is held at 5,000. Real populations vary, and their growth and carrying capacity are uncertain. Atlantic cod off Newfoundland declined after heavy fishing, and a fishing moratorium began in 1992.

Biodiversity exists at three levels. Genetic diversity is variation within a species, species diversity is the number and relative abundance of species in an area, and ecosystem diversity is the variety of habitats across a region. Genetic diversity matters because it is the raw material for natural selection. A population with more variation is more likely to survive a new disease or a warmer climate.

Current extinction rates are estimated at 100 to 1,000 times the natural background rate. Habitat loss is a major driver, along with overexploitation, invasive species, pollution and climate change. Their relative effects vary among ecosystems and species. Fragmentation splits habitat into isolated patches, which shrinks populations and adds edge effects. Pollutants can biomagnify up food chains. DDT and its breakdown products built up in bald eagles and thinned their eggshells. The United States banned most uses of DDT in 1972.

Some species have effects far larger than their numbers suggest. After wolves returned to Yellowstone in 1995, elk numbers and grazing patterns changed, and willows recovered along some streams. Wolves may have contributed to this trophic cascade, though other factors also affected elk and willow recovery.

Conservation works in situ, protecting species where they live, and ex situ, protecting them elsewhere. By 1987 the last wild California condors had been taken into captivity, bringing the captive population to 27. Captive breeding and releases have since grown the population to over 500. Laws like the US Endangered Species Act (1973) and the CITES treaty on wildlife trade support these efforts.