DryadLab Package: Extinction Risk

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Text and excel version of the dataset in Dryad - cleaned up with log10 values already added.

Teacher Background

Background extinction and mass extinction

Extinctions of species have occurred gradually and continuously throughout the history of life, creating a turnover of species through time. This is background extinction. However, there are particular points in time when a very large number of species go extinct, this is a mass extinction. Historically we recognize 5 big events including the most recent event, which occurred approximately 65 million years ago at the K-T boundary when the non-avian dinosaurs went extinct. The causes of these past events are varied and still debated but include climate-change, volcanic activity and asteroid impacts.

Insert image about background vs mass extinctions from Understanding Evolution website -> evolution.berkeley.edu/evolibrary/search/imagedetail.php

Although estimates vary on the precise numbers of current extinctions, most scientists agree that we are very close to entering a sixth mass extinction event due to human actions such as habitat destruction, pollution and overexploitation. For example, the IUCN (an international union of scientists and conservation organizations) estimates that 20% of the world’s 5,494 livings mammals are currently threatened with extinction. Given their rates of decline, it is estimated that all threatened mammals may go extinct within 1000 years, which exceeds the rates of background extinction estimated from the fossil record.

Resources/Further reading
Barnosky et al. 2011 Has the Earth's sixth mass extinction already arrived? Nature 471, 51-57.

Non-random extinctions

It has long been recognized that extinctions appear to be non-random with respect to which species survive and which go extinct. For example, it has been shown that large carnivores that specialize on large vertebrate prey (known as hypercarnivores) evolve repeatedly but have far shorter durations within the fossil record than other mammalian species. One of the most extreme examples of repeated hypercarnivore evolution is the saber-tooth cat-like form, which has evolved multiple times within the history of mammals.

Insert image from Valkenburgh et al. 2007 Integrative and Comparative Biology 47(1), 147-163 showing skulls of two different sabertooth skulls from very different time periods

The hypothesized explanation for the short duration of these hypercarnivorous clades is that their large size and highly specialized diets, which required lots of morphological adaptations, means they are unable to switch to other prey types during environmental perturbations.

Many other traits that have been proposed to predispose species to extinction as well (see table below).

Hypothesized modes of action
Related traits in student dataset
Small populations
Experience increased demographic stochasticity and inbreeding, and so may be wiped out by a local catastrophe.
Geographic range area (km2)
Population density per km2
Slow reproductive rate
More likely to experience population decline when mortality is increased because they are less able to compensate with increased fecundity. This trait may be particularly important for hunted species because hunting directly increases mortality.
Gestation length
Weaning age
Age at sexual maturity
Age at first birth
Inter-birth interval
Poor dispersal abilities
Are less able to move away from local threatening processes or to live in fragmented habitats. Poor dispersal may also be linked to small geographic range.

Ecological specialization
If availability of the specialist resource changes, may be unable to switch to other more common resources. Also, specialization is often linked to other traits such as low population density because specialized resources often have a patchy distribution.
Dietary specialization
Habitat breadth
Higher trophic levels
Are more vulnerable to disturbance of species lower down the food chain. Also, carnivory is often linked to other traits that may increase threat, such as low population densities and small population sizes.

Large group size
Means that the species may be preferred by hunters because they are more visible and easier to track.
Average group size
Large home range
Makes it harder for species to live in fragmented habitats. Large home ranges also mean that the species will have a low population density.
Home range (km2)
High human impacts
Humans are the cause of most, if not all, current threatening processes. Therefore species may experience more threatening processes if they live close to lots of humans or in countries that are less economically developed because this can force the citizens to exploit their natural resources unsustainably.
Mean human population density per km2 across the species range
Minimum human population density per km2 across the species range
Gross National Income (US $) across the species range
Large body size
Is often a composite trait linked to other traits that may increase threat level through other traits, such as slow reproductive rates and low population density. Also, hunters often prefer to hunt large species

Body mass (g)

Links to include

IUCN Red List of Threatened Species

Other articles and resources on extinction:

  • including jablonski and stuff on UE
  • Paper on body size and extinction risk:

Johnson, C. N. 2002 Determinants of loss of mammal species during the Late Quaternary ‘megafauna’ extinctions: life history and ecology, but not body size. Proc. R. Soc. B 269, 2221–2227. http://dx.doi.org/10.1098/rspb.2002.2130

Images to include

  • This figure would require permission from the publisher:


Fig. 7 from: Van Valkenburgh, B. et al. (2007) Déjà vu: the evolution of feeding morphologies in the Carnivora. Integrative and Comparative Biology 47(1), 147-163 doi: 10.1093/icb/icm016


  • These images appear on the web without any copyright statement; they may be freely available: