Lecture 2: Threats to biodiversity Flashcards
Lecture outline + focus
Outline
* Geological time and anthropogenic extinctions in context
– Australia’s Pleistocene extinctions
– North America’s Pleistocene extinctions
* Recent extinctions
* Diamond’s “Evil Quartet”
– introductions
– habitat loss
– overkill
– chains of extinction
Consider:
* The major drivers of historical extinctions
* The potential impact of humans migrating across the globe
* Evidence for competing hypotheses
Anthropogenic extinctions
repeated glaciation events in the pleistocene allowed human migration
Much of human history:
– human population low
– biodiversity impacts often assumed to be low
however: 150 genera of megafauna (> 44kg) extant 50,000 years ago and 97 gone by 10,000 years ago (Barnosky et al. 2004)
Possible to quantify impacts most accurately:
– where human arrival can be dated with relative precision
– where continental isolation shows clear impact on naïve fauna – e.g. Australia and North America
Australia’s late-Pleistocene extinctions
Fossil evidence:
– diverse and striking fauna in Australia
– within the last 100,000 years
Many spp. Were large-bodied – e.g.:
*Varanus priscus – c. 2 tons? – a large monitor lizard
* Dromornis – c. 600kg? – largest bird ever?
* Diprotodon optatum – c. 2.5 tons? - a wombat larger than a white rhino
* many large bodied kangaroos, including the short-faced kangaroos (now all extinct)
* Thylacoleo carnifex – close to the size of a lion
Fossil record difficult to work with:
– record is patchy – depending on sediment and tissue types
– timing of extinction very uncertain - I.e. finding the absence in the record does it show extinction date or period of poor preservation? Finding a fossil cannot confirm time when the species was abundant etc.
– body size difficult to ascertain
However, evidence suggests:
Australia and New Guinea lost 55 spp. of mammals, as well as large birds and reptiles, c. 45,000 years ago
variety of competing hypotheses for Australia’s “megafauna” extinctions:
e.g. climate change (glacial intervals), habitat destruction (clearing for farming or habitation) and overkill
Best evidence:
not climate change: climate was formerly very variable hence, change unlikely to explain abrupt and widespread extinctions, as well as size-selectivity (e.g. Prideaux et al.2007)
see: timing and potential impact consistent with hunting by humans (Johnson 2006)
loss of megafauna according to the appearance of humans in Australia
North America’s late-Pleistocene extinctions
- Humans arrived later than in Australia– some dispute but generally believed to be about 14000 - 12000 bp – coincidentially at the same time as the ice retreat at the start of the Holocene
– “Clovis peoples” characterised by tools adapted to the hunting of large mammals
– previously, N. American fauna would have rivalled that of Africa
arrival of people coincides with the extinction of at least 15 genera of large mammals
As with Australia, causes of extinction debated:
e.g: climate change? Overkill? habitat destruction? Or a combination of factors?
Prevailing view:
- “native” (as opposed to “Western”) humans were low in numbers and active “conservationists”, in touch with the landscape and fauna so had little or no impact on wildlife
Emerging viewpoint (remains controversial):
– north American natives were far more abundant than usually acknowledged
* population of pre-Columbian N. America may have been ~ 1 million (Stannard 1992)
* increasing evidence suggests that native peoples routinely overexploit wildlife resources
(e.g. see Broughton 1997 and citations in Kay 2007)
* evidence suggests that N. America was already human-dominated and depauperate in wildlife before European arrival (e.g. Kay 1998, 2007)
‘There was no “wilderness”. In fact, the idea that North America was a “wilderness” untouched by the hand of man before 1492 is a myth… As resource use intensified over the last several
thousand years, a parallel increase in the violence within and between prehistoric societies occurred (i.e. people were fighting over scarce resources)’
- Kay (1998), p490
Implications:
– pre-Industrial humans unlikely to have been natural “conservationists”
– overkill can account for many of the late-Pleistocene extinctions
– N. American extinctions are highly coincident with arrival of humans
– overall, humans and overkill, rather than climate change, are implicated in most mass extinctions between the late-Pleistocene and recent centuries
Human arrival and extinctions
Tend to correlate see graph from Johnson (2009) in lecture notes
Remember correlation is not causation – humans often reached new lands during periods of glaciation.
However arrival tends to be associated with extinctions, climate shifts are common throughout suggesting extinctions are caused by human arrival and not climate change.
Emerging consensus implicates overkill – reproductive rate rather than animal size predicts extinction.
Discovery of ‘naïve’ species not adapted to avoid or escape hunting humans – associated with arrival rather than habitat destruction as extinctions occur in correlation with arrival dates.
Growing view: Climate interacted with human actions
South American extinctions
– some megafauna persisted for thousands of years after human arrival and after the climate warmed
– seen as evidence that both climate and human arrival interacted to lead to extinctions
– not inconsistent with gradual attrition I.e. unsustainable harvest over time
see: Barnosky, A.D. and Lindsey, E.L. (Year) ‘Timing of Quaternary megafaunal extinction in South America in relation to human arrival and climate change’
Extinction on 5 landmasses
Extinctions on 5 land masses:
– fitted models containing human arrival times + 4 climate variables
(^ typically, speed of change in climate) – dependent variable was observed rate of megafaunal extinction per 10,000 years
see: Prescott, G.W., Williams, D.R., Balmford, A., Green, R.E. and Manica, A. (2012) ‘Quantitative global analysis of the role of climate and people in explaining late Quaternary megafaunal extinctions’, Proceedings of the National Academy of Sciences of the United States of America (PNAS), [online] 109(42), pp. 16005–16010. Available at: https://doi.org/10.1073/pnas.1118466109
Growing view: climate interacted with human impact
Prescott et al. (2012) concluded:
– predictions are better if human arrival AND climate are included in
models
– but, modelling is coarse
– authors acknowledge that estimates of the role of human impacts are
conservative
– as with Barnosky & Lindsay (2010), doesn’t really acknowledge potential
for attrition (rather than blitzkrieg)
Recent extinctions
Human-caused extinctions are not novel.
However:
* recent centuries have brought a dramatic increase in human population
* extinctions now threaten a much wider variety of species
* species face new threats
Summary
- Major “megafauna” extinctions in many parts of the world in the late- Pleistocene
- Main theories: climate change and overkill
- Overkill, owing to gradual but unsustainable harvest, overwhelmingly supported
- No evidence that early humans lived in harmony with nature; accumulating evidence to contrary
- Interaction of climate and human arrival now receiving increasing support
- Recent centuries have seen an explosion in human population growth – new threats to biodiversity
