Why populations can’t be saved by a single breeding pair

3 04 2018


© Reuters/Thomas Mukoya

I published this last week on The Conversation, and now reproducing it here for CB.com readers.


Two days ago, the last male northern white rhino (Ceratotherium simum cottoni) died. His passing leaves two surviving members of his subspecies: both females who are unable to bear calves.

Even though it might not be quite the end of the northern white rhino because of the possibility of implanting frozen embryos in their southern cousins (C. simum simum), in practical terms, it nevertheless represents the end of a long decline for the subspecies. It also raises the question: how many individuals does a species need to persist?

Fiction writers have enthusiastically embraced this question, most often in the post-apocalypse genre. It’s a notion with a long past; the Adam and Eve myth is of course based on a single breeding pair populating the entire world, as is the case described in the Ragnarok, the final battle of the gods in Norse mythology.

This idea dovetails neatly with the image of Noah’s animals marching “two by two” into the Ark. But the science of “minimum viable populations” tells us a different story.

No inbreeding, please

The global gold standard used to assess the extinction risk of any species is the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Read the rest of this entry »

Penguins cheated by ecosystem change

13 03 2018

Jorge Drexler sings “… I was committed not to see what I saw, but sometimes life is more complex than what it looks like …”*. This excerpt by the Oscar-winning Uruguayan singer seems to foretell the theme of this blog: how the ecological complexity of marine ecosystems can elicit false signals to their predators. Indeed, the fidelity of marine predators to certain feeding areas can turn demographically detrimental to themselves when the amount of available food shrinks. A study of jackass penguins illustrates the phenomenon in a context of overfishing and ocean warming.


Adult of jackass penguin (Spheniscus demersus) from Robben Island (South Africa) — in the inset, one of the first juveniles released with a satellite transmitter on its back. The species is ‘Endangered’ under IUCN’s criteria (28), following a recent halving of the total population currently estimated at ~ 80,000 adults. Jackass penguins are the only penguins living in Africa, and owe their common name to their vocalisations (you can hear their braying sounds here); adults are ~ 50 cm tall and weigh ~ 3 kg. Photos courtesy of Richard Sherley.

Surface temperature, dissolved oxygen, acidity and primary productivity are, by and large, the top four environmental factors driving the functionality of marine ecosystems (1). Growing scientific evidence supports the idea that anthropogenic warming of the atmosphere and the oceans correlates with this quartet (2). For instance, marine primary productivity is enhanced by increased temperatures (3), but a warmer sea surface intensifies stratification, i.e., stacked layers of seawater with contrasting physical and chemical properties.

In coastal areas experiencing ‘upwelling’ (where winds displace surface water, allowing deep water laden with nutrients to reach the euphotic zone where plankton communities feast), stratification weakens upwelling currents and, in turn, limits the growth of plankton (4) that fuels the entire trophic web, including our fisheries. The study of these complex trophic cascades is particularly cumbersome from the perspective of large marine predators because of their capacity to move long distances, from hundreds to thousands of kilometres (5), with strong implications for their conservation (6).

With those caveats in mind, Richard Sherley and colleagues satellite-tracked the movement of 54 post-fledged, juvenile jackass penguins (Spheniscus demersus) for 2-3 years (7). All individuals had been hatched in eight colonies (accounting for 80% of the global population), and were equipped with platform terminal transmitters. Jackass penguins currently nest in 28 island and mainland locations between South Africa and Namibia. Juveniles swim up to 2000 km in search of food and, when approaching adulthood, return to their native colonies where they reproduce and reside for the remainder of their lives (watch individuals swimming here).

The natural history of this species is linked to the Southern Hemisphere’s trade winds (‘alisios’ for Spanish speakers), which blow from the southeast to the tropics. In the South Atlantic, trade winds sustain the Benguela Current, the waters of which surface from some 300 m of depth and fertilise the marine ecosystems stretching from the Western coasts of South Africa to Angola (8). Read the rest of this entry »

Bring it back

13 02 2018

Protea compacta in fynbos, a form of shrubland at Soetanysberg, South Africa. Photo: Brian van Wilgen

Restoration of lost habitats and ecosystems hits all the right notes — conservation optimism, a can-do attitude, and the excitement of seeing biologically impoverished areas teem with life once more.

The Strategic Plan of the Convention on Biological Diversity includes a target to restore at least 15% of degraded ecosystems. This is being enthusiastically taken up in many places, including through initiatives such as the Bonn Challenge, a global aspiration to restore 350 million hectares of deforested and degraded land by 2030. This is in recognition of the importance of healthy ecosystems in not just conserving biodiversity, but also in combating climate change. Peatlands and forests lock away carbon, while grassland diversity stabilises ecosystem productivity during extreme weather events. So how can we make sure that these restoration efforts are as effective as possible? Read the rest of this entry »

Drivers of protected-area effectiveness in Africa

31 01 2018
Bowker_et_al-2017-Conservation_Biology. Fig. 1

Subtropical and
Tropical Moist Broadleaf Forest of
Africa with 224 parks surrounded
by a 10-km buffer area. ©
2016 Society for Conservation Biology.

I’ve just read an interesting paper published in late 2016 in Conservation Biology that had so far escaped my attention. But given my interest in African conservation recently (and some interesting research results on the determinants of environmental performance for that region should be coming soon out of our lab), the work caught my eye.

The paper by Bowker and colleagues asked a question that has been asked previously regarding the ‘effectiveness’ of protected areas — do they succeed in limiting forest loss? While forest loss itself is not necessarily indicative of biodiversity erosion in any given area (for that, you need measures of species trends, etc.), it is arguably one of the most important drivers of species loss today.

The first set out to differentiate ‘effective’ from ‘ineffective’ protected areas, which was a simple binary variable related to whether there was less deforestation inside the protected area relative to comparable points outside (effective), or greater than or equal to deforestation outside (ineffective). The authors then related this binary response to a series of biophysical and social indicators. Read the rest of this entry »

Influential conservation ecology papers of 2017

27 12 2017

Gannet Shallow Diving 03
As I have done for the last four years (20162015, 2014, 2013), here’s another retrospective list of the top 20 influential conservation papers of 2017 as assessed by experts in F1000 Prime.

Read the rest of this entry »

Human population growth, refugees & environmental degradation

7 07 2017

refugeesThe global human population is now over 7.5 billion, and increasing by about 90 million each year. This means that we are predicted to exceed 9 billion people by 2050, with no peak in site this century and a world population of up to 12 billion by 2100. These staggering numbers are the result of being within the exponential phase of population growth since last century, such that some 14% of all human beings that have ever lived on the planet are still alive today. That is taking into account about the past 200,000 years, or 10,000 generations.

Of course just like the Earth’s resources, human beings are not distributed equally around the globe, nor are the population trends consistent among regions or nations. In fact, developing nations are contributing to the bulk of the global annual increase (around 89 million per year), whereas developed nations are contributing a growth of only about 1 million each year. Another demonstration of the disparity in human population distributon is that about half of all human beings live in just seven countries (China, India, USA, Indonesia, Brazil, Pakistan, Nigeria, and Bangladesh), representing just one quarter of the world’s total land area. Read the rest of this entry »

Keeping lions from livestock — building fences can save lives

23 06 2017

Seeing majestic lions strolling along the Maasai Mara at sunset — a dream vision for many conservationists, but a nightmare for pastoralists trying to keep their cattle safe at night. Fortunately a conservation success story from Kenya, published today in the journal Conservation Evidence, shows that predation of cattle can be reduced by almost 75% by constructing chain-link livestock fences.

The Anne K. Taylor Fund (AKTF) subsidises over 70% of the cost of building a fully fortified chain-link livestock enclosure (‘boma’) to keep cattle safe from predators at night, in the hope that this will lessen the retaliatory killings of lions by frustrated farmers. While lions, leopards and cheetahs draw in crowds of tourists who marvel at their strength and beauty, living alongside big predators can be tough. Traditionally, local people keep their animals overnight in bomas made of acacia thorns — but depredation by lions and other large carnivores cause losses of on average more than nine head of cattle per year, or US$1870 that farmers see disappear down the throat of big, hairy animals. Building a solid fortification of chain-link fence costs just $890, of which the AKTF paid $638, helping to make this an affordable option for hard-pressed locals. Read the rest of this entry »