Jugami Biology

Biology · Lesson 15

Conservation Biology

About 1 million species are threatened with extinction (see IPBES).

This lesson covers why the variety of life matters, what is driving its loss, why small populations are fragile, and the tools people use to bring species back.

Sketch of one large forest square with a big dark interior, then the same square cut by roads into four smaller squares, each with a smaller dark interior, joined by a vermilion arrow. Together the four interiors are about half the size of the original one.

1. Why biodiversity matters

Biodiversity is the variety of life, from genes to species to whole ecosystems. How energy and nutrients move through ecosystems is covered in Ecology and Ecosystems, and how species are named and grouped in Classification.

People depend on nature for food, clean water and protection from storms, often called ecosystem services. The 2019 IPBES global assessment found that up to US$577 billion of crops each year are at risk from the loss of pollinators, and 100 to 300 million people face more floods and hurricanes because coastal habitats are being lost (see IPBES).

The IUCN Red List, set up in 1964, tracks the extinction risk of species worldwide (see IUCN).

2. Five direct drivers

IPBES ranked the direct drivers of change in nature by their global impact so far, largest first (see IPBES):

  1. Changes in land and sea use
  2. Direct exploitation of organisms
  3. Climate change
  4. Pollution
  5. Invasive alien species

3. Habitat loss and fragmentation

When forest is cleared, what is left is often broken into fragments. A global study found that 70% of the world's remaining forest lies within 1 km of an edge. Across experiments on five continents, fragmentation cut biodiversity by 13 to 75%, with the worst effects in the smallest and most isolated fragments (see Haddad and others 2015).

Forest near an edge is “subject to the degrading effects of fragmentation” (see Haddad and others 2015), often called edge effects. A small patch has more edge for its area, so less of it is interior, as the model below computes.

In 1967 Robert MacArthur and Edward O. Wilson argued that an island's area and its distance from the mainland together set the balance between new species arriving and old ones dying out (see Princeton University Press). A habitat fragment surrounded by farms is a kind of island.

Explore: cut the forest into patches

The 1 km square cut into 9 patches of 300 m, each with a small dark interior 100 m across.InteriorEdge zone, 100 m deepRoad, 50 m wideSquare: 1 km

Turn on JavaScript to change the number of patches. The numbers for each cut:

  1. One patch, each 1000 m on a side. Habitat 100 ha, of which 64 ha is interior, 64%, and 36 ha is edge zone. Roads take 0 ha. Total edge length 4 km.
  2. 4 patches, each 475 m on a side. Habitat 90.3 ha, of which 30.3 ha is interior, 34%, and 60 ha is edge zone. Roads take 9.8 ha. Total edge length 7.6 km.
  3. 9 patches, each 300 m on a side. Habitat 81 ha, of which 9 ha is interior, 11%, and 72 ha is edge zone. Roads take 19 ha. Total edge length 10.8 km.
  4. 16 patches, each 212.5 m on a side. Habitat 72.3 ha, of which 0.3 ha is interior, under 1%, and 72 ha is edge zone. Roads take 27.8 ha. Total edge length 13.6 km.
  5. 25 patches, each 160 m on a side. Habitat 64 ha, of which 0 ha is interior, 0%, and 64 ha is edge zone. Roads take 36 ha. Total edge length 16 km.

A model, not real data. Every number is computed: 50 m roads, and an edge zone assumed to be 100 m deep; real edge effects reach different depths for different species.

4. Invaders, harvest, pollution and climate

Invasive species. The brown tree snake reached Guam by accident in the late 1940s or early 1950s. It has eliminated 10 of the 13 native birds on Guam, as well as many lizard species (see USDA APHIS). Zebra mussels were first found in North America in Lake St. Clair in 1988. Scientists generally agree they were carried from Europe in the ballast water of ocean-going ships. Up to 10,000 can cover a single native mussel (see USGS).

Overexploitation. The passenger pigeon was hunted to extinction; the last one, Martha, died in 1914 (see Smithsonian). In 1992 Canada closed the northern cod fishery off Newfoundland after the stock collapsed (see Canadian Encyclopedia).

Pollution. The insecticide DDT built up in bald eagles' food and made their eggshells thin. By 1963 only 417 nesting pairs were known (see US Fish and Wildlife Service). Rachel Carson's Silent Spring in 1962 stirred public concern, and in 1972 the EPA cancelled DDT's uses (see EPA).

Climate change is the third-ranked driver, and IPBES notes that greenhouse gas emissions have doubled since 1980 (see IPBES).

5. Small populations

In a small population, chance matters more. Each generation passes on only a sample of the parents' genes, and in a small sample allele frequencies wander by luck. This is genetic drift (see Coop, section 4.1).

Drift wears away heterozygosity, the chance that two gene copies differ. In an ideal population of N individuals, about 1/2N of it is lost each generation, so after t generations Ht = H0(1 − 1/2N)t (see Coop, section 4.1).

Small populations also suffer inbreeding, mating between relatives. It raises homozygosity and exposes harmful recessive alleles, which lowers fitness (see Coop, chapter 11).

How small is too small? Mark Shaffer defined a minimum viable population as the smallest isolated population with a 99% chance of surviving 1,000 years despite chance events (see Shaffer).

Explore: drift in a small and a large population

Four runs for N = 10 that drift to 0 or 1, and four runs for N = 250 that stay near 0.5.10N = 10Generation 04010N = 250Generation 040

Turn on JavaScript to run new simulations. Run 1: Generation 40. N = 10: theory keeps 13% of the starting heterozygosity, so 87% is lost; these 4 runs keep 0% on average. N = 250: theory keeps 92% of the starting heterozygosity, so 8% is lost; these 4 runs keep 90% on average.

PopulationAfter 10 generationsAfter 20After 40
N = 1060%36%13%
N = 25098%96%92%

The table shows the share of starting heterozygosity that theory says is left.

Each run samples 2N gene copies at random every generation, starting from an allele frequency of 0.5. The 4 runs scatter around the theory line rather than following it, because each run is a chance outcome. Theory line from Coop's notes.

6. Conservation tools

Protected areas. The IUCN defines a protected area as a clearly defined space, recognised, dedicated and managed to conserve nature in the long term (see IUCN). Yellowstone, set aside in 1872, was the world's first national park (see National Park Service).

Corridors. Strips of habitat can link fragments. At the Savannah River Site, corridors increased directed movement between patches for butterflies, small mammals and bird-dispersed plants (see Haddad and others 2003).

Captive breeding and reintroduction. By 1987 every wild California condor had been taken into a captive breeding program. Releases began in 1992, and the population has grown to 410 birds (see US Fish and Wildlife Service). In Yellowstone, 41 wolves from Canada and Montana were released from 1995 to 1997 (see National Park Service).

Genetic rescue. In 1995 eight female pumas from Texas were released among inbred Florida panthers. Panther numbers tripled and heterozygosity doubled (see Johnson and others).

Laws and treaties. The US Endangered Species Act was passed in 1973 (see US Fish and Wildlife Service). CITES, in force since 1975, controls international trade in listed wild species (see US Fish and Wildlife Service).

Explore: match each threat to a tool

Turn on JavaScript to match them yourself. Here they are with answers.

ThreatToolWhat was done
A forest is cut into small patches by farms.Protect or connect habitatCorridors of habitat joining the patches help animals and seeds move between them.
Northern cod off Newfoundland are fished close to collapse.Limit harvest or tradeCanada closed the northern cod fishery with a moratorium in 1992.
A pesticide is thinning bald eagle eggshells.Remove the causeThe EPA cancelled the uses of DDT in 1972, and eagles recovered.
Florida panthers show signs of inbreeding.Restore the populationIn 1995, eight female pumas from Texas were released to bring in new genes.
Rare animals and plants are sold across borders.Limit harvest or tradeCITES uses permits to control international trade in listed species.
Brown tree snakes could ride cargo from Guam to other islands.Remove the causeOn Guam, trained dogs search outbound cargo so snakes do not spread.
Wolves have been gone from Yellowstone for decades.Restore the populationWolves from Canada and Montana were released in the park from 1995 to 1997.
A wild geyser basin could be sold to private owners.Protect or connect habitatCongress made Yellowstone the first national park in 1872, setting the land aside.

Cases from the sources for this section and USDA APHIS for Guam's detector dogs.

7. Success stories

Bald eagles rose to at least 9,789 nesting pairs, and on 28 June 2007 the US Fish and Wildlife Service announced their removal from the list of threatened and endangered species (see US Fish and Wildlife Service).

In 2016 NOAA divided humpback whales into 14 distinct populations and found that nine had recovered enough to need no listing (see NOAA Fisheries).

Anyone can help. Citizen science projects let volunteers collect data. Audubon's Christmas Bird Count began on Christmas Day 1900 as an alternative to a holiday bird hunt (see Audubon).

8. How do we know?

Conservation grew from parks, extinctions, field studies and laws.

  1. 1872 Congress establishes Yellowstone, the world's first national park, on 1 March. (National Park Service)
  2. 1914 Martha, the last passenger pigeon, dies at the Cincinnati Zoo on 1 September. (Smithsonian)
  3. 1962 Rachel Carson publishes Silent Spring, raising public concern about pesticides. (EPA)
  4. 1964 The IUCN Red List of Threatened Species is established. (IUCN)
  5. 1967 Robert MacArthur and Edward O. Wilson publish The Theory of Island Biogeography. (Princeton University Press)
  6. 1973 Congress passes the Endangered Species Act. (US Fish and Wildlife Service)
  7. 1975 CITES comes into force on 1 July. (US Fish and Wildlife Service)
  8. 1995 Gray wolves are released in Yellowstone, and eight Texas pumas are released in Florida. (National Park Service; Johnson and others)
  9. 2019 IPBES publishes its global assessment: about 1 million species are threatened with extinction. (IPBES)

Quiz: Conservation biology check

Talk about it

  1. One big reserve or several small ones of the same total area: which would you choose, and why?
  2. Why might a population of 50 animals be in danger even if it is not shrinking?
  3. Bringing Texas pumas to Florida mixed two lineages. What are the risks and benefits?
  4. Which of the five drivers can you see where you live?

Sources

  1. IPBES (2019). Media release: Nature's dangerous decline “unprecedented”; species extinction rates “accelerating”. Gives the 1 million threatened species, the five drivers in order, crop and coastal risks, and the doubling of emissions since 1980. ipbes.net
  2. IUCN Red List. Background and history. States that the Red List was established in 1964. iucnredlist.org
  3. Haddad, N. M. and others (2015). Habitat fragmentation and its lasting impact on Earth's ecosystems. Science Advances 1: e1500052. doi:10.1126/sciadv.1500052
  4. Princeton University Press. MacArthur, R. H. and Wilson, E. O. The Theory of Island Biogeography, first published 1967. Book page. press.princeton.edu
  5. USDA APHIS Wildlife Services. Operational activities: brown treesnake. Covers its arrival on Guam, its effect on birds and lizards, and detector dogs in cargo. usda.gov
  6. US Geological Survey. Zebra mussel frequently asked questions. usgs.gov
  7. Smithsonian National Museum of Natural History. Martha, the last passenger pigeon. si.edu
  8. The Canadian Encyclopedia. Cod Moratorium of 1992. thecanadianencyclopedia.ca
  9. US Fish and Wildlife Service. Bald eagle species page. Covers DDT, the 417 pairs of 1963, the 9,789 pairs and delisting in 2007. fws.gov
  10. US Environmental Protection Agency. DDT: a brief history and status. epa.gov
  11. Coop, G. Population and Quantitative Genetics, open course notes, University of California, Davis, version 1.2. Section 4.1 derives Ht = H0(1 − 1/2N)t; chapter 11 covers inbreeding depression. github.com
  12. Shaffer, M. L. (1981). Minimum population sizes for species conservation. BioScience 31(2): 131–134. doi:10.2307/1308256
  13. IUCN. Effective protected areas. Gives the IUCN definition of a protected area. iucn.org
  14. National Park Service. Birth of a national park. Yellowstone National Park. nps.gov
  15. Haddad, N. M. and others (2003). Corridor use by diverse taxa. Ecology 84: 609–615. doi:10.1890/0012-9658(2003)084[0609:CUBDT]2.0.CO;2
  16. US Fish and Wildlife Service. California Condor Recovery Program. fws.gov
  17. National Park Service. Wolf restoration in Yellowstone: reintroduction to recovery. nps.gov
  18. Johnson, W. E. and others (2010). Genetic restoration of the Florida panther. Science 329: 1641–1645. doi:10.1126/science.1192891 Free text: ncbi.nlm.nih.gov
  19. US Fish and Wildlife Service. About the Endangered Species Act. fws.gov
  20. US Fish and Wildlife Service. CITES frequently asked questions. States that CITES went into effect on 1 July 1975. fws.gov
  21. NOAA Fisheries (2016). Media release: Successful conservation efforts pay off for humpback whales. noaa.gov
  22. National Audubon Society. About the Christmas Bird Count. audubon.org