In the streets and alleyways of Baltimore, Dawn Biehler and her colleagues have been hunting for mosquito larvae – with turkey basters.
“We go to a block and look for every single standing water container we can find,” she explains. “It could be as small as a bottle cap – we see a lot of those.” A small sample of water is sucked up into the tubes and brought back to the lab for analysis. This reveals how many larvae are present.
Biehler’s research at the University of Maryland, Baltimore County, is an effort to document the specific bits of man-made environments that encourage these insects to multiply. Besides feeding on the blood of humans and their pets, mosquitos are well-adapted to life in cities. Stagnant water is their breeding ground, so waste items that have collected rain are excellent places to find larvae.
Her team has found the greatest numbers of mosquitos in deprived areas, where fly-tipping is common and where sanitation is poor. “The neighbourhoods that have experienced disinvestment and lower economic status have about three times the level of mosquitos than the upper income neighbourhoods,” says Biehler.
These mosquitos are just one example of a species that, far from being driven to extinction by the dominance of humanity, are thriving. Some organisms just seem to have the knack of living alongside us – and those organisms will likely shape Earth’s ecosystem for millions of years to come.
The Anthropocene is the name given to the newest geological epoch, one in which human activity has been the dominant force for environmental change on Earth. The effects of our activity are visible everywhere: not just in cities but on farmlands and in national parks, forests and oceans. Species around the world are going extinct much faster than usual: perhaps 1,000 times the natural rate.
Our own species is clearly thriving. There are now more than seven billion men, women and children on Earth. But we are bringing a raft of other species with us that also appear to be doing well, either because, or in spite of, our impact on their habitats. Which, then, are the animals of the Anthropocene – and will they continue to thrive as the planet changes ever more rapidly?
It is not just pests that have proven adaptable to urban spaces
In her 2013 book Pests in the City: Flies, bedbugs, cockroaches and rats, Biehler explores how a handful of species have profited from human expansion – despite our attempts to purge them from our homes. She argues that it is often the very design of our buildings, or the untidiness of urban spaces, that make it so easy for these species to thrive.
For example, in North America cockroaches and other insects often make their homes within cavity walls in cheaper housing. From there, they can infest not just one apartment but several, by travelling via the gaps between dwellings.
Meanwhile, rodents find shelter, warmth and plentiful food in human households. Some have even evolved a resistance to common poisons, prompting worrisome headlines like, “How do we tackle poison-resistant super rats?”
But it is not just pests that have proven adaptable to urban spaces. A variety of fungi, pollinating insects and plants have also taken up city living. What’s more, anthropogenic changes reach far beyond the city limits.
As Bradley Cardinale of the University of Michigan points out, managed forests tend to have a high turnover of trees. This has benefitted some species more than others, since they feed on young plants.
“Deer love these forest practices,” says Cardinale. “It’s been absolutely a huge boon for them, especially when we eradicate the wolves as well. Certain species go hand-in-hand with human activities.”
Similarly, Cardinale points out that squirrels adore parkland. Not only are natural predators absent, there is a ready supply of food in the form of nuts, since trees are well looked-after throughout the seasons.
Cardinale says that the biodiversity in water bodies impacted by farming often goes down, even though a handful of species, like certain cyanobacteria, do well
Human impact on local ecologies can sometimes have unexpected effects, as Brian McGill at the University of Maine notes. He points to a study published in 2014 that found common European bird species, such as house sparrows, were declining in numbers. This coincided with an increase in less abundant birds like grey herons.
Cardinale also cites the example of cyanobacteria, a group of photosynthesising bacteria that sometimes look like algae. A 2015 study found that many nitrogen-fixing cyanobacteria – those that use nitrogen to make compounds – were generally dominant in lakes with higher levels of nearby anthropogenic land use, such as farming.
Nitrogen levels in rivers and lakes are often higher than natural, thanks to fertilisers applied to farmland being washed into the water. However, the authors of the study note that there could also be other factors at play, such as increased water temperatures – that might also be due to farming.
Cardinale says that the biodiversity in water bodies impacted by farming often goes down, even though a handful of species, like certain cyanobacteria, do well. As a result, an individual success story is not necessarily a sign of good news for local wildlife. Sometimes the individual in question is just a vigorous survivor.
However, there is one group of animals that does seem to be coping rather well in the Anthropocene: cephalopods.
These sea-dwelling creatures include all squid, octopus and cuttlefish. Bronwyn Gillanders at the University of Adelaide and colleagues recently investigated stories that the population of the Australian cuttlefish was declining.
But when they combed through reports of cephalopods in fishing catches over the last 61 years, the team found that estimated numbers of all groups were rising. Their results were published in 2016.
It is also difficult to know how humans have affected the numbers of some species, because we do not have data from before we started significantly impacting the environment
“We weren’t really expecting to see an increase,” says Gillanders. “What was surprising was that every single group and every single dataset came out very similar.”
There was even good news for the Australian cuttlefish. “Its numbers actually increased in the last few years as well.”
It is not clear why cephalopods are proliferating. One possibility is that we are fishing the animals that prey on them, says Gillanders. It could also be a result of the oceans warming thanks to climate change. But these are just suggestions and, importantly, Gillanders has found plenty of evidence for human activity having a negative impact on other marine species.
It is simply quite difficult to know how humans have affected the numbers of some species, because we do not have data from before we started significantly impacting the environment, says Chris Thomas at the University of York.
That said, one thing that is clear is that cephalopods have been around for a long time: over 480 million years. “I wonder whether perhaps they are a little bit more hardy,” says Gillanders. It is not clear why one species should be more hardy than another, but it may be that some are more adaptable in the face of upheaval.
To really understand this, we need a global picture of how biodiversity across the globe is changing over time.
In 2014, Maria Dornelas of the University of St Andrews and colleagues published a survey of 100 environments from all over the world. The team found that, even though many individual species had died out, overall the sites were not becoming less biodiverse. Instead, in many places the changeover of species was increasing.
“This doesn't prevent the fact that there are many individual sites with strong declines, just that they're balanced with strong gains,” says McGill, a co-author.
However, the paper’s conclusions have not been generally accepted. Cardinale and colleagues published a detailed critique in 2016. “They did make a very good point that, in the habitats that humans have yet to destroy, biodiversity is increasing,” says Cardinale. But his paper argues that the dataset is simply not good enough to assess what is happening to global biodiversity, because it does not cover a representative sample of Earth’s ecosystems.
Despite these complexities, scientists do agree that the current rate of extinction is higher than it has been for the majority of Earth’s history
The extreme complexity of biodiversity studies is illustrated by a study of global shark populations published in 2014. The authors show that there has been a steep decline in the populations of many shark species, but at the same time there has also been an increase in the discovery of new shark species, particularly in shallow waters. As a result, they write, “models cannot predict the global number of sharks”.
Despite these complexities, scientists do agree that the current rate of extinction is higher than it has been for the majority of Earth’s history. But that may not be the whole story.
Chris Thomas has long warned that key habitats could be lost if we do not do more to protect them. However, in 2013 he surprised some by suggesting that biodiversity could actually increase during the Anthropocene.
“Speciation by hybridization is likely to be a signature of the Anthropocene,” he wrote in the journal Nature. “A new hybrid species of Rhagoletis fruitfly has colonized invasive honeysuckle in North America. A primrose species, Primula kewensis, arose by hybridization and continues to be propagated in London’s Kew Gardens.”
Thomas’s point is that environmental change can act as an encouragement to evolution.
In other words, a shock to ecosystems does not necessarily have to result in biological apocalypse.
It’s commonplace that species will have part of their distribution beyond the original space where they evolved – Chris Thomas
This has been true of previous mass extinctions – at least in the long run. Most famously, the mass extinction of 66 million years ago wiped out all dinosaurs except for birds, clearing the way for the rise of mammals and ultimately of human beings.
Natural selection is primed to ensure that species will continue to adapt to changing surroundings – unless, of course, the change is too drastic for them. To return for a moment to mosquitos, one distinct subspecies has evolved in an unusual place: the London Underground rail network. It has adapted to breed in tighter spaces than its above-ground cousins, and does not need to feed on birds: it has turned instead to rodents and humans.
Because change and adaptation are normal, Thomas takes a relatively relaxed view of so-called invasive species, which are often targeted for eradication. “It’s commonplace that species will have part of their distribution beyond the original space where they evolved,” he says. “A vast majority are relatively rare and produce relatively few changes.” This is even true for species that are brought into new areas suddenly, as a result of human activity.
In seemingly stark contrast, some conservationists argue that we should engage in “rewilding”: returning altered habitats to their former state, often by reintroducing species that have been exterminated, such as wolves or elephants.
However, Thomas says there are limits to our ability to restore past ecosystems. “It won’t create the same thing that was there before, because too much water’s gone under the bridge,” he says. “The climate has changed, there’s more carbon dioxide in the atmosphere.”
It is also difficult to decide which species to reintroduce and which long-lost time period was the “golden era” for a particular patch of land. There are no scientific criteria, so Thomas says that “in the end, it is a social decision”.
Rapidly-evolving flora and fauna may outpace humanity’s imprint on the planet
Instead of trying to bring back lost ecosystems, we might do better to prioritise the conservation of species we stand to lose, and not worry too much about change in general. After all, the Earth really is changing. There is only so much we can do about that.
Still, it seems there is a kernel of hope in the Anthropocene. Many plants and animals are displaying incredible resilience and adaptability. That does not mean everything is fine – on the contrary, many species and ecosystems will surely need our help if they are to survive – but it does suggest that the situation is far from hopeless.
That leaves one lingering question: what about us? If the extinctions continue and global biodiversity falls, scientists like Cardinale question whether we will still have enough access to essential resources like food. There is gathering evidence that climate change, in particular, will reduce the global food supply.
In the end, rapidly-evolving flora and fauna may outpace humanity’s imprint on the planet. As Cardinale says, “Nature’s been doing it for 3.6 billion years. We don’t have that experience.”
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