Is Homo sapiens a boom-bust species?

Projections suggest the human population will fall 50% by the year 2300.

The boom: Life in the fast lane

The basic facts are clear. Homo sapiens cruised along for most of our 200,000 years with a small population, probably less than a hundred thousand. Then something happened. We expanded, emigrating out of Africa and across the globe. About 10,000 years ago, when our population was around four million, we developed agriculture. Cities and large organized societies – and money and writing – came along later. The human population really took off. This growth became exponential. Homo sapiens passed 1 billion around 1800, 2 billion in 1928, and 5 billion in 1987. Our impact on the earth has been so dramatic that geologists have coined a new geological epoch: the Anthropocene. When I was a child, I was told there were more humans alive on the earth than had ever existed in the history of the species. If that’s not the definition of a population boom, I don’t know what is.

Today we’re just over 8 billion, but we’ve passed an inflection point. The rate of increase is decreasing, from 2% per year in the 1960s to less than 1% now. The S-curve is forming. The top of the curve is bending, ultimately to be replaced by a decline. Demographers predict that human population will peak in 2064 at about 9.7 billion. After that, the decrease may be equally precipitous.  

In most “developed” nations, the fertility rate (the average number of children that a woman has in her lifetime) has fallen below 1.6. The fertility rate is 1.23 in Spain, 1.24 in Italy, 1.34 in Japan, 1.44 in Austria, 1.53 in Germany, 1.56 in the UK, and 1.64 in the United States. It takes a fertility rate of at least 2.0 to maintain a steady population (not counting immigration). The countries with the highest fertility rates, over 4.0, are in sub-Saharan Africa and Afghanistan, though even those are declining sharply. Every nation in the world may have a shrinking population by 2100.

Without immigration, high-income nations with low fertility rates will see much more rapid population declines. Regardless, all nations may have shrinking populations by 2100. Source.

The bust: It’s the end of the world as we know it

The worldwide average fertility rate is predicted to be 1.7 by 2100. That implies a population decline of about 9% each generation (about 25 years). This, in turn, implies a 50% decline in total population within 200 years, and a 90% decline, back below one billion, in 600 years. That’s basic math. That’s a freefall, a massive shrinking of human society.

The implications of a worldwide average fertility rate of 1.7.

This kind of population collapse is not unheard of in the natural world. Many species, from lemmings to locusts to crabs to passenger pigeons, are known as boom-bust species, with wild swings in their populations. Some research suggests that wild swings in animal populations may be the norm.

The bust – the fall, the decline, the collapse – is often driven by hitting the carrying capacity of the resources they depend on. Simply put, the species runs out of habitat or runs out of food. Life, literally, becomes too difficult. Animals may die or simply fail to reproduce.

When the world is running down, you make the best of what’s still around

What is it like to be an individual living during a bust cycle? Apart from a few perturbations in history (see Collapse by Jared Diamond), it’s a world that humans are unaccustomed to.

The economy will be shrinking, both supply and demand. The stock market will fall daily, for most businesses will be in decline. Stores will be closing, never to re-open.

The number of working age adults (assuming some migration between nations).

By definition, deaths will outnumber births, which means the demographic age mix will be decidedly older. By definition, many won’t have children to look after them. They will need to rely on paid caregivers, which will be in short supply.

We have a hint of that economic upheaval now, with supply shortages as we emerge from the Covid pandemic. Recent research suggests that half the lost workers in the US today are actually a result of Trump’s crackdowns on legal immigration. It is immigrants, after all, who make up the difference in the US’s low fertility rate. Those labor shortages, and the resulting supply chain problems and inflation, are caused by more people aging out of the labor force than aging in. This can only be mitigated by immigration.

When immigrants are no longer available, how does a nation manage a declining economy, a permanent recession? Public services – police, fire, water, electricity – will be difficult to provide everywhere. Service will be terminated in rural areas. Outlying areas and small towns will be abandoned as people seek services (primarily medical at first) in larger cities. Schools, healthcare clinics, stores, and homes will crumble as the population concentrates into remaining urban centers. All production, especially agriculture, will need to efficient, relying less and less on labor. With a lack of private investment, will government need to play a stronger role to guarantee the provision of goods and services?

What does this mean for culture? So many of the values, customs, and social rules of the past ordered growing societies, supporting the family, state, and nation. In a declining society, the center, the things we hold dear, does not hold. Things fall apart. Sure, one can strive for personal peace and meaning, but family lines will end. There are no “greatest” generations building infrastructure for the future.

Will there be a collective future to aspire to? Perhaps divisions of the past – gender, race, nationality – will lose meaning in the face of a shared human condition. On the other hand, we already see those tensions exacerbated as people glimpse the future, desperate to cling to the past.

Perhaps a shrinking economy can be managed. Perhaps there is a soft landing to societal collapse, to shrinking our footprint to a new steady-state in a sustainable harmony with nature.

We don’t know yet. We’re still on the crest of the wave, the top of the curve, enjoying the best of times. It’s our kids and grandkids and great grandkids who will come up against that carrying capacity. And that’s exactly why many are choosing not to have kids. Others don’t have kids because they have other economic or personal opportunities, thanks to our societal successes. But even in this decision, they feel no imperative to have kids. Indeed, the imperative is to reduce our impact on the earth. It looks as if that will happen. For the earth, a short-lived Anthropocene will no doubt be a good thing. For the people living during the collapse, perhaps not.

How do economies and societies function when it gets to the point that they are obviously shrinking?

Mojave Desert bird populations plummet due to climate change

Two recent papers concluded that many breeding bird species in southern California and Nevada deserts have declined dramatically due to climate change.

In their abstract, Iknayan and Beissinger (2018) summarized, “We evaluated how desert birds have responded to climate and habitat change by resurveying historic sites throughout the Mojave Desert that were originally surveyed for avian diversity during the early 20th century by Joseph Grinnell and colleagues. We found strong evidence of an avian community in collapse.”

They re-surveyed 61 sites originally surveyed by Grinnell teams in the early 20th century (primarily between 1917 and 1947).

Of 135 species assessed (which included some wintering and migrating species, as well as breeding species), 39 had significantly declined; only one (Common Raven) had increased. This was in stark contrast to similar assessments they conducted of Sierra and Central Valley sites, where more species had increased than decreased and there were no overall declines (not to say there weren’t winners, losers, and range shifts within those regions).

Figure 1B from Iknayan and Beissinger (2018). Every study site had fewer species than previously– on average each site had lost 43% of their species.

Detailed analyses suggested less rainfall and less access to water was the primary driver. Habitat change only affected 15% of the study sites and was of secondary importance. They found no evidence of expansion of species from the hotter, drier Sonoran Desert (e.g. Phainopepla, Verdin, Black-throated Sparrow) into the Mojave Desert.

Consistent with a community collapse, declines were greatest among species at the top of food chain — carnivores such as Prairie Falcon, American Kestrel, and Turkey Vulture. Insectivores were the next most impacted, and herbivores the least. But the declines affected both common and rare species, both generalists and specialists.

Figure 1B from Iknayan and Beissinger (2018), which I’ve augmented with species labels from the database available in the supplementary materials. Other significant losers (red dots), in order of degree of decline, included Western Kingbird, Western Meadowlark, Black-chinned Sparrow, Lawrence’s Goldfinch, Bushtit, Ladder-backed Woodpecker, and Canyon Wren. The yellow dots are newly invasive species: Chukar, Eurasian Collared-Dove, Eurasian Starling, and Great-tailed Grackle.

A follow-up study by Riddell et al (2020), also involving Iknayan and Beissinger, focused on the thermoregulatory costs — the water requirements to keep cool — for the declining species. They found that “species’ declines were positively associated with climate-driven increases in water requirements for evaporative cooling and exacerbated by large body size, especially for species with animal-based diets.” Larger species get much of their water from the insects they eat. They estimated larger species would have to double or triple their insect intake to meet their water needs, though insect abundance is lowest July thru September.

American Kestrels were among the biggest losers in the study, struggling to meet their cooling needs.

Intriguingly, they found that 22 species had actually declined in body size over the last century, consistent with Bergmann’s Rule, and had reduced their cooling costs up to 14%. These species fared better. Current climate change, however, is at least ten times more rapid than any previous warming event, during which many species evolved. They estimated cooling costs have already increased 19% and will reach 50% to 78% under most scenarios, far outstripping any species’ ability to evolve through the current rapid warming.

These results stand in stark contrast to the Pacific Northwest, where many of the same bird species (e.g. Anna’s Hummingbird, Turkey Vulture, Northern Mockingbird) are increasing. This is consistent with projections which generally show individual declines along species’ southern edge and expansions at the north edge of their range (see Audubon climate projection maps for individual species).

Iknayan and Beissinger conclude, “Our results provide evidence that bird communities in the Mojave Desert have collapsed to a new, lower baseline. Declines could accelerate with future climate change, as this region is predicted to become drier and hotter by the end of the century.”