Tag Archives: population

Is Homo sapiens a boom-bust species?

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abandoned school
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.

graph of world population over last 12,000 years

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.

fertility rates across the world
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.

graph of world population into the future based on a fertility rate of 1.7
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.

Projected future workforce by nation
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.

dystopian art of a city
How do economies and societies function when it gets to the point that they are obviously shrinking?

The maddening truth: Feeding crows and jays harms other birds

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Backyard bird feeders bring nature close to people, increase appreciation for the natural world, and, in winter, help birds survive. But research also documents some adverse effects, such as the potential to spread avian diseases and to inflate the population of nest predators. There’s a lot of attention on the former, but the latter issue probably does a lot more harm.  

Close to half of all bird nests fail due to predation. Corvids (ravens, crows, jays, and magpies) play a big part in that; they are among the most voracious nest predators, taking other birds’ eggs and chicks. Squirrels and rats are also major nest predators. Open cup nesters are especially vulnerable.

jays and crows eating peanuts
Crows and jays are especially fond of peanuts. Feeding them may depress other bird populations.

Corvids are part of the natural ecosystem, but the problem goes beyond “natural” because many corvid populations are artificially higher due to anthropogenic food subsidies – human garbage, bird feeders, scraps, etc. In short, corvids are often what biologists call human-subsidized predators. Millions of restoration dollars have been spent trying to protect declining species from corvids (e.g., Marbled Murrelet, Least Tern, Snowy Plover, Piping Plover, and Desert Tortoise). Additionally, local populations of otherwise common species are known to be at risk from corvids (e.g. Common Murre, Red-capped Plover). And research shows they can depress bird populations in residential neighborhoods.

The Research

Crumb Clean sign
Sign from Big Basin State Park. Corvid-control measures may include removal (killing or transferring elsewhere), camper education, improved food storage, and food waste management and garbage control, among other measures. There is a vast literature on the topic.

By providing food, humans have a remarkable ability to inflate corvid populations. Brunk et al (2021) examined efforts to reduce Steller’s Jay densities at campgrounds in Big Basin State Park in California in order to protect endangered Marbled Murrelets. Jay densities within the campgrounds were nine times higher than in the surrounding forest. Previous research documented that the Steller’s Jay juvenile survival rate in the campgrounds was over 90%, possibly the highest figure ever recorded for a bird; 50% is more typical. After the installation of new garbage cans, food lockers, as well as extensive camper education, the jays disbursed and densities fell to natural levels. I’m proud to have been a part of that project.

There is a long list of papers that show the depressing impact that high numbers of jays and crows can have on the productivity and populations of other birds. Here are a few examples focusing on songbirds:

Jokimäki et al (2020) looked at nest predation rates in nine European cities, finding that cats and corvids had a significant impact on other birds’ nest success in urban and suburban areas.

Hanmer et al (2017) experimented with artificial thrush nests with quail eggs placed in a natural context but in the vicinity of bird feeders that were offering peanuts. Nests near active feeders were far more likely to be depredated than nests that were far away from peanut feeders. The predators were jays, magpies and squirrels.

Malpass et al (2017) conducted an experiment in seven neighborhoods in Ohio, adding bird feeders during the breeding season to some areas. The neighborhoods with the most feeders had triple the numbers of American Crows and Brown-headed Cowbirds compared to the neighborhoods with the least feeders.  American Robin nest success fell to just 1% in the high-feeder neighborhoods, compared to 34% in the areas with the fewest feeders. Northern Cardinals, perhaps because their nests are generally more hidden, managed to avoid these impacts.

eggs painted to look like Marbled Murrelet eggs are injected with a chemical to cause jays to vomit
In the Santa Cruz Mountains of California, eggs painted to look like Marbled Murrelet eggs are injected with a chemical that causes jays to vomit. This has trained some jays to avoid them.

Jokimäki and Huhta (2000) looked at bird assemblages across urban, suburban, and natural areas in Finland. They found a dramatically higher nest predation rate in the managed urban parks (53 to 92%) compared to more wild suburban parks (22 to 67%). Additionally, most ground nesting species simply avoided the urban areas. Crows and jays were listed among the primary nest predators.

Stoate and Szczur (1994) reported that corvid removal led to dramatic improvement in the hatching success of thrushes and Chaffinch. Some ground-nesting and thicket-nesting species (Dunnock, Yellowhammer, etc) were unaffected by corvids.

Slagsvold (1980) found that Fieldfare populations (the Scandinavian counterpart to the American Robin) doubled after the removal of Carrion Crows (the counterpart to the American Crow). Smaller songbirds also sought protection and nested near Fieldfares. Many songbirds based nest site decisions on the presence or absence of crows.

There are several meta-analyses, studies of studies, that try to summarize the big picture. Most of these include a wide array of avian and mammalian predators—and most focus on ground-nesting shorebirds and gamebirds, where conservation and hunting interests fuel research funding.

graph of nest survival
Hanmer et al (2017) examined the fate of artificial nests placed 5 and 10 meters away from feeders with peanuts, feeders with peanuts and a protective cage guard to exclude predators, and empty feeders. Approximately 90% of nests near empty feeders survived the first day, and 50% remained a week later. Near active peanut feeders, these numbers fell to 50% and just 10%. Guarded feeders only slightly reduced the impact.

Cote and Sutherland (1997) is typical. It reviews 29 studies, focusing on avian (crows, gulls, grackles) and mammalian (foxes, skunk, marten) nest predation on gamebirds (pheasants, quail, etc) and some songbirds. Removing predators had a large positive effect on the prey species’ productivity and post-breeding population.

It is important to add that not all studies have found a link between corvids and reduced densities of other birds. This seems to be truer in rural contexts. Furthermore, impacts seem to be species-specific. I can imagine, for example, that chickadees and nuthatches, which nest in small cavities, are relatively protected from jay and crow nest predation.

Finally, there is Madden et al (2015), a meta-analysis focusing exclusively on corvids, which caused some confusion. It focused mostly on impacts to gamebirds and did not include jays. It is curious because its summaries and conclusions do not match the data they present. Based on their data, Madden could have concluded with this:

“The presence of crows had a negative impact on other bird species’ productivity in 66% of cases. In 10% of cases, a decline in abundance was also detected. These results suggest that, in certain contexts and for many species, large crow populations may create population sinks or actual declines in the populations of other birds.”

That would have been consistent with other studies, but the paper didn’t end that way. Instead, it summarized the data in ways to bury important results, failed to include some well-known cases involving endangered species, and concluded with this head-scratching statement:

“Our review shows that although there is no consistent pattern with regard to corvid impacts on other bird species, the most commonly reported effect is that corvids have no negative impact on prey species abundance or productivity. When combining experimental and correlative studies (326 cases), most cases (81%, n=264) showed no negative influence of corvids on either abundance or productivity of birds…”

“Most commonly” meant more than 50%. Because they lumped magpies with crows, and lumped abundance with productivity, they were able to say this (barely). I am astonished this paper passed thru peer review with this sweeping, and deceptive, summary intact. Some backyard birders, in defense of laying out a smorgasbord of peanuts on their back patio, have cited this paper on social media.

There’s a lot more I can say about this paper, but suffice it to say their own numbers (see Table 4) imply that two-thirds of studies involving crows found significant impacts to other species’ productivity.

Conclusion: No peanuts, and use caution in the breeding season

Make no mistake—bird-feeders in winter are associated with increased bird survival. There are lots of papers about that. I feed birds in winter (but no peanuts!). In the spring and summer, however, when my juncos and sparrows have departed, those same feeders disproportionately attract jays, crows, and squirrels, as well as House Sparrows, Eurasian Starlings, and Brown-headed Cowbirds. It’s no wonder that open cup nesters have a tough time. In the summer, I maintain hummingbird feeders and have a little pond and fountain that attracts bathing warblers.

Bird feeding in spring and summer, and especially offering peanuts which disproportionately attract crows and jays, likely depresses the populations of other songbirds in the vicinity. This is probably a big reason why many suburban neighborhoods can be largely devoid of cup nesters (e.g. warblers, vireos, flycatchers) while still having cavity and thicket nesters (e.g. chickadees, nuthatches, wrens, towhees).

juvenile American Robin
A good sentinel species may be the American Robin. If your neighborhood is devoid of breeding robins, and especially fresh juvenile robins, it may be due to excessive numbers of jays or crows.

References

Benmazouz et al. 2021. Corvids in urban environments: A systematic global literature review. Animals.

Brunk et al. 2021. Reducing anthropogenic subsidies can curb density of overabundant predators in protected areas. Biological Conservation.

Cote and Sutherland. 1997. The effectiveness of removing predators to protect bird populations. Conservation Biology.

Hanmer et al. 2017. Provision of supplementary food for wild birds may increase the risk of local nest predation. Ibis.

Jokimäki et al. 2020. Land-sharing vs. land-sparing urban development modulate predator–prey interactions in Europe. Ecological Applications.

Jokimäki and Huhta. 2000. Artificial nest predation and abundance of birds along an urban gradient. The Condor.

Madden et al. 2015. A review of the impact of corvids on bird productivity and abundance. Ibis.

Malpass et al. 2017. Species-dependent effects of bird feeders on nest predators and nest survival of urban American Robins and Northern Cardinals. The Condor.

Shutt and Lees. 2021. Killing with kindness: Does widespread generalised provisioning of wildlife help or hinder biodiversity conservation efforts? Biological Conservation.

Slagsvold. 1980. Habitat selection in birds: on the presence of other birds species with special regard to Turdus pilaris. Journal of Animal Ecology.

Stoate and Szczur. 1994. Game management and songbirds. The Game Conservancy Review of 1993.

Wilson's Warbler
In 2021, Shutt and Lees reminded us that we (humans and jays and warblers) are all connected:  “Generalised provisioning is enthusiastically promoted by many conservation organisations as a means to foster connection with nature and help wildlife. However, such a vast input of additional resources into the environment must have diverse, ecosystem-wide consequences. Direct effects…  are generally positive in leading to increased survival, productivity and hence population growth. However, we argue that the wider implications for the recipients’ non-provisioned competitors, prey and predators are underappreciated and have the potential to generate pervasive negative impacts for biodiversity.”

Mojave Desert bird populations plummet due to climate change

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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.”