Recent extinctions
Duncan Golicher
The current period has been termed the “anthropocene”
Many, if not all, ecosystems have been altered by human activities
Terrestrial ecosystems frequently lack populations of large mammals that may play a role as keystone species
However, although populations have been reduced in size or lost locally, how many keystone species have actually gone extinct in recent times?
Most of the recently documented extinctions have been of endemic species on islands
Small islands have witnessed most extinctions
However, some large islands such as Madagascar have also lost many species
The populations of continental species have been reduced, but some remnant populations typically persist
As biodiversity is concentrated in the tropics more tropical species have been lost than temperate species
Many extinctions not recorded. Dark extinctions (Boehm and Cronk 2021)
Islands species tend to be endemic, but islands have fewer species overall
The loss of any species is more likely to disrupt trophic interactions
Introducing species to islands tends to have a major impact on ecosystem structure and function
Many island species went extinct as a result of European exploration and settlement beginning in the 15th century
Polynesian colonisation and the introduction of rats, pigs and chickens may have caused even more.
Latitude and topography are important
Flat temperate areas -> very low rates of extinction
Montane tropical areas -> higher rates of extinction
Many extinctions (often dark extinctions) may be occurring as tropical montane forests are lost.
Habitat loss, fragmentation and hunting leads to local loss of large predators
May affect top down control
The UK has lost wolves, bears, lynx, large birds of prey (eagles) etc.
However, these species are still extant (not extinct) and could be reintroduced from continental remnants
As continents are large the complete eradication of an entire species is less likely
Extensive extinctions of large mammals occurred in North and South America, Eurasia, Africa and Australia in the late Pleistocene between 50,000 and 10,000 years before present
Mammoths, Mastodons, Giant ground sloths, saber tooth cats, giant beaver
Coincided with the expansion of Homo sapiens populations
Modern humans reached Australia around 60 thousand years ago
“Great leap forward” in hunting techniques around the same time
Three conflicting theories
Climate change
Hunting
Disease
Probably involved some elements of all three, although disease less likely
(Lyons, Smith, and Brown 2004)
Atmospheric C02 at the last glacial maximum may have been 75 ppm below the level at the beginning of the Holocene (“Atmospheric CO 2 Concentrations over the Last Glacial Termination” 2001)
Could have been a low point in net primary productivity
Large herbivores can digest low quality food in bulk
However, if net primary productivity was low they could have a negative energy balance
During the Pleistocene, megaherbivores were more diverse and widespread than today on all continents.
By the early Holocene (c.11 700 kyr BP), hundreds of species of large herbivores and their predators had disappeared from the continents
The late quaternary extinctions were more selective than any event in the preceding 65 million yr of mammalian evolutionary history, disproportionately targeting large mammals (>10 kg)
Could be due to disease (unlikely), climate change alone (why not previously?), hunting alone (why all the large mammals including equines .. hard to hunt) or reduced primary productivity (climate and low C02) + hunting (seems most likely)
Extinctions of large herbivores led to extinctions of their predators
Were large herbivores “landscape architects?”
(Gill 2013)
Reduced herbivory led to increase in trees
Increase in trees led to more fuel build up
More fires, thus more charcoal
(Hitchmough and Vera 2002)