Posted by: Adam Kay | November 13, 2011

Zombie apocalypse

Population ecology and the coming of the zombie apocalypse

In 300 A.D there were 50-60 million humans, by 1804 there were 1 billion, and last week, the UN reported that there are now 7 billion of us. And more are coming. Given that the planet has a limited resource base, the human population cannot grow forever. Demographers describe constraints on population growth with a metaphor, the carrying capacity, which is the maximum population size that a habitat can support indefinitely. Population growth rate decreases to 0 as the population size approaches the carrying capacity. The processes that bring about this decrease in population growth rate are called negative density-dependent factors. These factors include the crappy aspects of civilization – food and shelter scarcity, violence among individuals, higher risk of disease, waste accumulation – as well as behaviors that reduce birth rates. As of now, resource use per person is still increasing as the global economy continues to grow (albeit unevenly). But it’s impossible for the human population to increase in size indefinitely – even if technology increases resource use efficiency. Population growth rates will decrease, and this decrease will be due to negative density-dependent factors. Hopefully, we’ll figure out a way to reduce birth rates and resource use rates without mayhem. I’m not optimistic.

And I don’t think I’m alone in this view. My feeling is that people are getting a stronger sense that human impacts on ecosystems are becoming increasingly severe and show no signs of abating. These impacts will cause greater and greater challenges for us, but how exactly they will affect population growth and resource use is unclear. Will there be gradual modifications in lifestyles until we emerge as a globally sustainable society? Or will there be catastrophic transitions that massively restructure society? I imagine that most people have a general sense that the world is crowded, it’s getting more crowded, and this trend can’t continue forever. They feel that change is coming, but they are uncertain about what that change will look like.

Zombies_NightoftheLivingDeadSo what’s going to happen? One option that is getting a lot of attention is a zombie apocalypse. The idea of zombies originates from the depiction in Haitian culture of bringing corpses back to life using witchcraft. In modern culture, the zombie concept was popularized by the classic 1968 film “Night of the Living Dead”, in which the dead become re-animated and then seek out the living as food. Although the zombie concept has remained a part of American culture since that time, its prevalence has exploded over the last few years. A quick search for “zombies” on amazon.com brings up over 30 titles (!) published since 2006, including “The Zombie Survival Guide: How to Live Like a King After the Outbreak” by Etienne DeForest, “Pride and Prejudice and Zombies: The Classic Regency Romance – Now with Ultraviolent Zombie Mayhem!” by Jane Austen and Seth Grahame-Smith, and one that I highly recommend, “World War Z: An Oral History of the Zombie War” by Max Brooks. I’m currently reading “Zone One” by Colson Whitehead, about a military unit assigned to “eliminate” any remaining zombies in New York City about 2 years after a zombie apocalypse. I started reading this book because of all of the critical acclaim it’s received; the New York Times described it as “a cool, thoughtful and, for all its ludic violence, strangely tender novel, a celebration of modernity and a pre-emptive wake for its demise.” But I’m still trying to figure out why the zombie genre has become so popular, and I wonder if it has to do with global ecology.

Zombieism is a strong negative density-dependent factor. It’s interesting that the cause of zombieism in “Night of the Living Dead” was the release of radiation from a satellite. That was 1968 – the height of the space age and the threat from the great Communist menace. In today’s literature, zombieism is a combination of cannibalism (zombies eat humans) and an emerging infectious disease (getting bit by a zombie gives you the zombie plague, which kills you (within hours or days) and then re-animates you (within minutes) hungry for human flesh). In the real world, cannibalism occurs in a variety of species and often becomes more prevalent at higher population densities. My favorite example of cannibalism is from the spadefoot toad genus Spea, a group of desert-dwellers that inhabit rain-filled pools. Spea tadpoles often have to mature rapidly to reach adulthood before ephemeral ponds dry upConditions indicating high tadpole densities trigger the development of a cannibalistic Infectious disease transmission is more likely when population density and contact among members of the same species are high. Cannibalism and infectious disease can be interconnected. For example, tiger salamanders are more likely to develop a serious hemorrhagic disease when they eat infected members of their own species. In humans, observations of a Papua New Guinea tribe have shown that eating human brains increases the risk of catching the deadly disease kuru


Zombieism is an even tighter connection between cannibalism and infection disease. For the zombie disease, the plague actually leads to cannibalistic behavior. In fact, it seems to create a singular and insatiable drive for the flesh of the living. Every successful attack spreads the disease, and the positive feedback can quickly lead to an apocalypse. (The dynamics of this disease have actually been modeled in a book on host-disease interactions). The only hope for the living is to create a well-protected refuge and hone your anti-zombie defensive skills (you can kill a zombie by destroying its brain).morphology, and the ensuing carnage leads to a reduction in population size. Infectious disease transmission is more likely when population density and contact among members of the same species are high. Cannibalism and infectious disease can be interconnected. For example, tiger salamanders are more likely to develop a serious hemorrhagic disease when they eat infected members of their own species. In humans, observations of a Papua New Guinea tribe have shown that eating human brains increases the risk of catching the deadly disease kuru.

So how is the recent explosion (epidemic?) of zombie literature related to today’s population ecology? Maybe zombie literature describes a type of worst-case scenario as the world gets more crowded and our natural support systems become more degraded. Imagining worst-case scenarios may prepare us psychologically when (if?) real chaos actually emerges. And even though the destruction brought about by a zombie apocalypse is horrific (even parts of the campy “Night of the Living Dead” are hard to watch), the end result is a massively reduced population size and a lot of civilization’s infrastructure left intact. That scenario will have broad appeal. Survivors of the zombie apocalypse won’t need to think about the ecological consequences of their actions – they’ll be able to use a leaf blower while sitting in their Hummer without having to think about what some crazy environmentalist has to say about it.Life will be good. So it might be time to hone your skills as a writer of zombie literature; that fantasy may become even more appealing for us in the future as the impact of climate change and habitat destruction become even more acute. Or, better yet, learn how to slay zombies.   

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