Lecture 3: the evil quartet Flashcards
(17 cards)
Origin of the term ‘evil quartet’
The biologist Jared Diamond coined the phrase ‘evil quartet’ to explain the four main human-induced causes of extinction which were:
1. habitat degradation
2. habitat fragmentation
3. the introduction of exotic species
4. over-harvesting
Introductions of species: examples
Lake Victoria: Nile Perch (lates niloticus)
A large edible fish species widespread across North and West Africa, introduced into Lake Victoria in East Africa for commercial exploitation. Population slowly grew initially but boomed in 1980’s. The species is highly predatory and is believed to have led to the extinction of 200 cichlid fish species within 10 years (Witte et al 1992)
Guam: brown tree snake
– Pacific island housing US military bases
– brown tree snake (Boiga irregularis) accidentally transported there by US military in 1950s
– of c. 30 species of native vertebrates, only 3 small lizard species remain in most forest
fragments
– all forest birds and mammals and most reptiles have been extirpated or are on the brink of extinction (Fritts & Rodda 1998)
Species are introduced for a variety of reasons
accidental transport
e.g. brown tree snake, rats, insects and marine species introduced by ballast water released by ships
e.g. ships off Oregon contained 367 marine species originating in Japanese waters (Carlton & Geller 1993)
European colonists took many spp. To newly conquered lands
e.g. sparrows introduced to Australia, Africa, the Americas to make the countryside more familiar
e.g. trout, carp, pigs, fallow deer, widely-naturalised to provide hunting / food
biological control
e.g. foxes released in Australia to control rabbits and weasels and cats introduced to NZ for same reason
horticulture and agriculture
e.g. nonindigenous grasses introduced to the N American prairies for grazing outcompete native grasses and are not as useful to native species
e.g. Rhododendron introduced as a decorative now dominates British vegetation
e.g. GMOs – esp. crops – threaten to “escape” and eclipse native vegetation as “superweeds”
Issues with introduction of species
predation on naïve prey
– results in greatest vulnerability on islands, particularly when introduced as bio-control
Bird extinctions since 1600
See: Groombridge (1992)
^ notably highest levels of extinctions on islands
Competition
- particularly pronounced for many invasive plants e.g. rhododendron
Genetic introgression
- hybrids (often resulting from male dominance) threaten genetic integrity of native species e.g. Ethiopian Wolves threatened by domestic dogs as the domestic males outcompete native males changing the gene pool
Introduced diseases
Introduced diseases
Arrive with other introductions, or are spread by humans / domestic stock
e.g. squirrel pox – introduced with grey squirrels which are immune and caused huge decline in red squirrels
Chytrid fungus introduced to the Amazon largest cause of decline in amphibians
Dutch elm disease and ash die back – individual saplings with disease brought over and planted in the UK transferring diseases
Gorillas are susceptible to human diseases e.g. measles
Introductions remain a major problem
With increasing travel & ‘globalisation’
The pet trade is also now amongst commonest routes of introduction e.g. > 15% of Florida’s reptiles & amphibians are exotics, mainly from the pet trade (Hambler 2004, p85)
Invasives in UK –mink released from fur farms, american craw-fish, himalayan balsam, boa
And American signal crayfish have outcompeted native white claw
Habitat loss includes:
destruction
degradation e.g. by pollution, conversion
fragmentation – edge effects (e.g. Robbins et al., 1989) - edge of habitat lower quality than central area
And climate change may alter, degrade or even destroy habitats altogether
> 80% of birds, mammals & plants on the IUCN red list are threatened (at least partly) by habitat loss / degradation
“For terrestrial and freshwater ecosystems, land-use change has had the largest relative negative impact on nature since 1970” (IPBES, 2019)
Habitat loss: Perhaps most overt as deforestation
Formerly, much of the world’s land surfaces were forested
– the clearing of the world’s forests is one of the most striking human impacts
– much of Europe (since medieval times), Canada and the USA (in recent centuries) have been cleared
– half the world’s rainforests were cleared between 1830 and 1984
No doubt that deforestation leads to catastrophic loss of biodiversity (e.g. Brook et al. 2003)
– Singapore has lost 95% of primary forests since 1819
– highly conservative estimates are of 50% loss of spp. – since records began in 1870 (i.e. after much habitat already lost)
Habitat loss: Perhaps most overt as deforestation: tropical forests
Deforestation in tropical forest – the most diverse and complex of terrestrial habitats is caused by:
- Slash-and-burn agriculture 61%
- Logging 18%
- Dams 9%
- Plantations 6%
- Cattle ranching 6%
(figures based on c. 165,000km2 per year (Myers 1994))
^ current figure for global deforestation is about the same but proportions have changed
Logging and cattle-ranching are now the leading issues
see figure on habitat loss predictions from Laurance (2007) in notes:
left figure rural pop density increase results in less forest
Right figure red=no. Of people in rural areas In blue = no. Of people in urban area
^ shows human preference for urban conditions which requires more development/destruction of forest
Habitat loss vs degradation/conversion
is loss compensated for by conversion (e.g. to plantations)?
* emerging evidence suggests not (e.g. Barlow et al. 2007)
* Agricultural intensification led to population declines in species reliant on the primary habitat (Gregory at al. 2019)
is it better to have smaller areas of intensive utilisation, rather than investing in mitigation?
Land sharing vs land sparing?
* important implications for UK agriculture
* remains an open question but see Phalan et al. (2011)
Note:
Habitat loss is not limited to terrestrial habitats
– e.g trawling for Britain’s seabed –> “marine desert” (Times online, June 2009)
Habitat loss: there are different drivers in different habitats
water extraction
* water for agriculture / human consumption
* threatens many habitats with drying-out
mining / extraction
* pollution, infrastructure
grazing
* competition with wild ungulates
energy
* infrastructure, dams, pollution
^ sustainable energy can impact migration and behaviour and cause decline in species
Pollution: a ubiquitous problem impacting land and water and air:
Land: litter, waste disposal, contamination of land, pesticide accumulation (e.g. in the 1980’s EDT built up in insects, accumulated in small insectivorous birds which then has egg shell thinning effects on raptors that consumed them), mercury leakage from gold mines in Amazon
Water:
* litter (even on uninhabited islands)
* pesticides, herbicides (toxicity)
* fertilisers and sewage (eutrophication)
* oil spills (direct effects, toxic algal blooms)
* PCBs, heavy metals (bioaccumulation)
Air:
* power station waste gases containing S and N → acid rain
(^ transferred by UK wind currents leading to our polluted rain forming and falling in Scandinavia)
– 50% of Europe’s forests have suffered damage
– increasing problem in developing countries
* ↑ ozone (O3) in lower atmosphere, damaging to plants and animals
* ↓ ozone in upper atmosphere (breakdown promoted by CFC pollution from aerosols, solvents and coolants) results in increased incidence of UV radiation, health risks and negative consequences for marine organisms
* CO2 build up → greenhouse effect
Climate change
see IPCC 2021 figure in notes
Changes to climate across the globe
– Temperatures risen by 1-1.5oC since 1950
– Further rises of at least 1.5oC by the end of the century – current estimates upto 3!
Polar regions most likely to be affected
Changes in patterns of precipitation associated with the impacts of warming
– Less uniform – harder to predict patterns and affects?
– Most likely to threaten tropics, where water determines productivity?
Climate change: Widespread evidence for impacts on biodiversity
(e.g. Walther et al. 2002)
changes in phenology
-flowering and leaf unfolding in plants – changes in timing of spring abundance
-migration and hatching in birds and butterflies (e.g. Lawrence et al., 2021)
-breeding or hibernation times in many other spp.
^ this results in phenological mismatches that threaten ecological links
range shifts and invasions (e.g. Howard et al., 2018)
direct impacts on populations (e.g. Stephens et al., 2016)
changes span a huge range of taxa and are already pronounced
If changes are this obvious following a change in global mean temp of 0.6C over the past 100
years … how much more change is to come with global mean temp predicted to rise by 1.5-5.7oC by
2100 (IPCC, 2019)?
Overkill: Not just a problem of the past
bushmeat hunting is resurgent in many areas
- bushmeat hugely problematic in West Africa
- almost 5 million tons of wild mammal meat consumed annually & many spp. exploited unsustainably (Fa et al. 2002)
fisheries continue to exploit unsustainably in many areas
- large marine predatory fish have declined by ~ 90% over the last 50 years (Myers & Worm 2003)
- few exploited fish currently reach their pre-Industrial body size
Maximum yield quotas often not adhered to or enforced
all spp. of sea turtle are currently endangered, most by overkill
- caught deliberately for meat, eggs and shells
- e.g. ~35,000 turtles killed every year in Mexico
- also fisheries bycatch (e.g. suffocation in nets)
see: rarity induced by hunting Courchamp et al. 2006
Overkill: Further abundant evidence from:
– collectors (e.g. birds’ eggs in UK)
– trophy hunters (e.g. big-horn sheep permits ~$400,000)
– pet trade (see graph above)
– luxury foods / (e.g. a single bluefin tuna ~$100,000 in Japan)
– traditional medicines (organised crime threatens tigers & rhino)
Chains of extinction aka “Coextinction”
See Sumption and Flowerdew 1986
– relatively unstudied / difficult to document
– mutualists (like the large blue), plant-pollinator systems, and parasites most at risk (Dunn et al. 2009)
A recent example (Nichols et al. 2009) suggests: dung beetles may face extinction in areas where large mammals are heavily hunted – with consequences for:
nutrient recycling
soil aeration
parasite suppression
seed dispersal
Summary
Understanding the causes of extinction is, in most cases, straightforward
Introduced species and diseases have, historically, had the greatest effect on biodiversity
Habitat loss and degradation have become bigger issues – especially through climate change
Climate change will disrupt ecological linkages - coextinctions are likely to become more of a problem … but remain understudied
