Chapter 18

Human Threats to Biodiversity

Human Threats to Biodiversity: Introduction

• the number of species is unknown: estimates range from 10 million to 100 million

• ~1.4 million species have been catalogued, many of them are beetles!

• rates of speciation and extinction are also unknown, although estimates exist

• it is widely believed that significant species loss is occurring, but it is easier to catalogue habitat loss than species loss

• estimates of habitat loss can lead to estimates of species loss, but that is complicated because certain species are more important than other species to their ecosystems: keystone species

Pleistocene and Holocene Extinctions:

• estimated 70% of large mammal genera of the late Pleistocene are extinct. Birds too.

• know significant examples include Madagascar, New Zealand, Polynesia, Hawaii

• North American extinctions include large mammals such as bison, antelope, mammoth, other mammals and large birds

• N. American and Australia also had climate change, hard to isolate the two effects

• overhunting was the impact prior to modern days

Modern Extinctions:

• habitat destruction and alteration from land use and land cover change is the main mechanism for modern extinctions

• for example, Europe has almost no land left of its original forest cover

• Today, greatest rate of species loss is found in the tropical forests: ~6% of land surface area, but >50% of the earth’s plant and animal species

• Of the ~9000 known bird species, almost half live in Amazon or Indonesia

Tropical Deforestation:

• for every 10x increase in the area of tropical forest, it is believed that the number of species doubles

• E.O. Wilson concludes that ~27000 species are lost each year from tropical deforestation. This is believed to be perhaps thousands of times greater than the average natural rate of extinction

• Tropical forests hold much of the nutrients in the biomass, not in the soil, and so when trees are removed few nutrients remain

• soils are acidic, and tree seeds are less tolerant to the stressful environmental conditions, so forests do not grow back easily, or with the same species

• many species live in relatively small geographic areas, and can get wiped out

Tropical Deforestation: Other environmental impacts

• temperature, hydrologic cycle, and nutrient cycling

• DECREASE IN: ET, precip, soil moisture, runoff

• INCREASE IN:temperature

Tropical Deforestation: how?

• logging for agriculture, ranching, hydroelectric projects, mining, human settlement

• 80,000 km2 per year cleared for agriculture

• slash and burn leaves nutrients in the soil. Buth, they are depleted within a few years. If given enough regeneration time, this works. Otherwise, problems with soil fertility, crop yields, and degradation results

• 50,000 km2 per year cleared for wood products (particularly Asia and West Africa)

• 20,000 km2 per year in latin america cleared for cattle ranching. Usually results in soil that can not be regenerated due to compaction

Tropical Deforestation: why?

• population growth

• uneven property ownership – many landless, poor people forced to move into previously uninhabited areas

• government sponsored resettlement programs: Brazil

• cash crops or mineral extraction for export

Hotspots of Habitat Loss: Norman Myers, Oxford

• Land only, no ocean

•areas with many species that live nowhere else, and that are in greatest danger of extinction as a result of human activities

• “biogeographical units” with distinct and identifiable assemblage of plant and animal species

• 25 “hotspots” units with at least 0.5% of the worlds 300,000 know vascular plants, lost at least 70% of its primary vegetation

Hotspots of Habitat Loss: Norman Myers, Oxford

• currently contain only 12% of their original cover

• 1.4% of earth’s land surface, but almost 50% of vascular plant species, and 35% of all vertebrate species (excluding fish)

Why should we care about biodiversity? Environmental Ethics

• instrumental value how does one species benefit another (usually humans)

• intrinsic value something has value for its own sake, regardless of how it benefits us

• rights do animals, plants, rocks have rights?

• biodiversity and food supply, ecosystem stability

Coral Reefs

• Coral is an animal found in shallow tropical seas

• most productive and diverse ocean ecosystem

• dynamic, fragile, vulnerable, yet often recovers quickly

• structures made of calcium carbonate, built from calcium and carbonate ions in the ocean water

• symbiotic relationship to algae necessary to survive

www.chbr.noaa.gov

Coral Reefs

• tolerate temperatures ~21C – 29C (~70F – 85F)

• require sunlight, so grows in the euphotic zone (several meters to 200 m)

• believed to contain at least 25% (33%?) of all marine species, including 700 coral species and >4000 fish species

Coral Reef Bleaching

•corals expel the zooxanthellae, single celled organisms

• symbiotic relationship – corals can not live without their nutrients

• corals loose their colors and die

• changes in temperature cause bleaching

http://earthobservatory.nasa.gov/Study/Coral/coral2.html

Science, May 4 2007, Vol 316, p. 678-681

Science, May 4 2007, Vol 316, p. 678-681

Threats to Coral Reefs

“besieged by pathogens, predators, and people, the ‘rainforests of the sea’ may soon face their ultimate foe: rising ocean acidity driven by CO2 emissions

Attempting to regrow reefs that were devastated by the tsunami of Dec 2004such attemps have been somewhat successful

Threats to Coral Reefs are mostly human

• divers destroying the reefs

• shipping and dredging destroying reefs

• pollution and sewage

• over fishing

• rising ocean temperatures

• increased acidification

• ~20% of the earth’s coral reefs have been destroyed in the last few decades

• another 50% on the verge of collapse

• vulnerability on “many fronts”: for example, over fishing and/or pollution and higher temperatures, could in conjunction, make them more vulnerable and less resilient to predators (certain starfish) and pathogens (certain algae)

• the coral species less resistant to bleaching will die first

Science, May 4 2007, Vol 316, p. 678-681

Coral Reef / CO2

• CO2 concentrations affect the availability of carbonate ions in the ocean water

• increase in CO2 results in increased acidity, and reduction of carbonate

• it this gets bad enough it will make it harder for the corals to grow

http://earthobservatory.nasa.gov/Study/Coral/coral2.html

Protecting Coral Reefs

• MPAs – marine protected areas

• range of restrictions, can include recreation, can bar fishing

• <3% of worlds reefs are within MPAs

• many unprotected reefs are being “fished out” for human food

• many MPAs are not working due to poor enforcement, and lack of local “buy in”

• reef management after bleaching to prevent over fishing can improve recovery

• no management scheme is considered viable in the long run unless carbon emissions are curtailed

• IPCC scenarios indicate potential ocean pH lower than in the last 20M yrs

• under all climate scenarios, reefs will be drastically different; under most scenarios reefs might not survive

Science, May 4 2007, Vol 316, p. 678-681