Biodiversity is the totality of all inherited variation in the life forms of Earth, of which we are one species. We study and save it to our great benefit. We ignore and degrade it to our great peril.
Imagine you’re about to set off on a bike ride. Your bike has the usual parts – wheels, handlebars, pedals, frame, and all the little screws and bolts holding it together (Figure 1). If you were asked to give up one piece of the bike, which one would you choose? Maybe the basket at the front? It would still be rideable. What if you had to remove 10 parts? In deciding which pieces to remove and whether it’s safe to ride it afterward, you’ll weigh the importance of each part to the bike’s overall function.
Scientists are grappling with similar questions about ecosystems. In 1981, American scientist duo Paul and Anne Ehrlich equated extinctions with losing rivets from an airplane wing and having to evaluate whether it could still fly, much like the bike example above. (Ehrlich and Ehrlich, 1981). The Ehrlichs’ “rivet-popper” hypothesis suggests that it’s not wise to lose species because each one may play an ecosystem role. Through the many species they contain, ecosystems provide essential services to human societies, such as food provision, nutrient cycling, and water purification (See our Environmental Services and Economics module). Are certain species more crucial than others?
“But the Anthropocene isn’t a novel phenomenon of the last few centuries. Already tens of thousands of years ago, when our Stone Age ancestors spread from East Africa to the four corners of the earth, they changed the flora and fauna of every continent and island on which they settled - all before they planted the first wheat field, shaped the first metal tool, wrote the first text or struck the first coin.”
- Historian Yuval Noah Harari, Hebrew University of Jerusalem
People have been modifying the habitats they inhabit for thousands of years. Archaeological and paleoecological evidence shows that by 12,000 years ago, humans lived on almost three quarters of land on Earth, and by 10,000 years ago they were using land-altering practices such as burning, hunting, farming, and domestication of animals (Ellis at al. 2021). Today, we associate human use of natural areas with degradation and extinction of species. But that was not always the case.
Hunter-gatherers and early farmers, through lower intensity subsistence practices, in some cases had neutral or positive impacts on biodiversity. Forest gardens, multiple crops, nomadic populations, and field rotations from fallow to cultivated made for diverse landscapes with high biodiversity. A study (Armstrong 2021) of forest gardens in British Columbia, which were cleared and cultivated by Indigenous communities until two centuries ago, revealed that they still have more diverse plants and animals than the conifer forests around them. Cultural stewardship practices of native inhabitants, including planting edible species like hazelnuts, cranberry, and wild ginger, made for more ecologically complex and diverse habitats (See our Biodiversity I: Patterns module).
Today, higher human densities on Earth and more intensive practices such as industrial agriculture and global supply chains, have tipped the scales towards negative impacts of humans on biodiversity.
In the photos above (Figure 2), what evidence can you find of how humans have changed their environments? Your list may get pretty long. What other changes do humans make to natural landscapes as we live, work, and play in them?
All animals modify their habitats to some degree as they nest, find food, or otherwise use resources. Humans are exceptional at altering habitats to meet our needs for shelter and food, plus distinctly human needs like entertainment. As a result, nearly every habitat in the world has been altered by people. A recent global assessment estimated that 75% of terrestrial and 66% of marine environments have been significantly altered by humans (IPBES 2019). Why do these changes matter?
In subsistence economies, such as those of Indigenous Peoples, humans and biodiversity were largely compatible. In global market economies supplying dense human populations, they’re not (Otero et al. 2020). For example, more than 85% of global wetlands have now been converted to other, lower-biodiversity uses. Just as a bicycle with missing parts may not function as well, ecosystems with lower biodiversity mean worse function. As habitat is lost, you’ll find fewer large animals, disrupted interactions between species, lower breeding success, and myriad other changes. As human populations continue to grow and consume resources, other species are increasingly deprived of resources and nudged towards extinction.
What features break up the forest in this landscape? (Figure 3)
Conversion of natural habitats to human uses breaks up ecosystems into patches of habitat, often separated by man-made barriers. Small land conversions of land may promote biodiversity by creating more diverse habitat conditions. For example, in northernmost Patagonia, the monkey-puzzle tree (Araucaria araucana) was planted and maintained through localized burns to clear areas around it by Native Peoples. With the elimination of these controlled fires and the advent of large farming from Euro-American settlement, monkey puzzle trees are now endangered (Nanavati at al. 2022).
Large-scale land conversions, leaving small patches, support less biodiversity since some organisms lack sufficient habitat and others cannot freely move as needed because of roads, parking lots, or other man-made barriers. Animals with big home ranges such as lions and other top predators, don’t do well in small patches without enough prey to sustain a large enough population (Lawrence and Fraser 2020). Norwegian ecologist John D. Linnell calculated home range sizes for Eurasian lynxes and found that protected areas in Scandinavia are mostly too small to support them. The outcome is that lynxes are preying on sheep in semi-natural forest areas, thereby affecting people’s livelihoods (Linnell at al. 2001). In contrast, organisms with broader habitat tolerances, such as pigeons, raccoons, or dingoes, may actually thrive in patches, therefore persisting in urban areas (Andrén et al. 1985).
Consider how you’d define your home range and what sorts of resources you depend on locally. What do you do when the resources you need are not available?
Species that are not native to a particular ecosystem colonize habitats all over the world. Pet Burmese pythons escaped into the Florida Everglades; American gray squirrels were deliberately released in Britain, and the emerald ash borer beetle reached the U.S. in cargo containers from Asia. These introduced species, no longer contained by their predators or parasites, often outcompete native ones for resources. Introduced species may become invasive, thriving in their new habitat free of restricting factors like specific predators or limited food supplies. Rabbits, for example, after deliberate introduction to Australia in the 1800s, became prolific and continue to damage livestock and natural habitats, despite various control efforts.
Introduced species can have particularly stark impacts on the native species of islands. For example, the biodiversity of the Hawaiian Islands changed dramatically after repeated arrivals by humans. First, Polynesians came, bringing pigs and rats to the islands. Then, cats were introduced by European explorers and colonists. The two waves of new predators fed on the eggs and hatchlings of ground-nesting birds such as geese. Before the introductions, Hawaii had at least seven species of native geese, of which only the nēnē, or Hawaiian goose (Branta sandvicensis), survives today. With no way to escape, the other six “moa-nalos” (vanished fowl) were driven to extinction by the introduced predators they were not adapted to avoid (National Park Service 2021).
The problems with introduced species are not limited to terrestrial habitats. In this diagram of invasions of nonnative marine species into other waters, what trends do you notice?
Invasive marine species are often introduced via shipping routes (Figure 4). The largest concentration of introduced species is between Africa and Asia, along the major shipping corridor called the European-Asian sea route. As humans began to travel around the world, we transported other species around, often unintentionally. Commercial shipping is implicated in an estimated 44-78% of invasions by non-native species into North American waters that either cling to ships’ hulls or ride in ballast water (stored in the hull; Elçiçek et al. 2013).
A study by Canadian marine biologist Jesica Goldsmit and colleagues assessed ecological risk based on ships discharging their ballast water at ports in the Canadian Arctic. They tallied up total ballast water discharged per year per port. They focused on three invasive species: the periwinkle snail Littorina littorea, soft shell clam Mya arenaria, and red king crab Paralithodes camtschaticus. Given shipping routes and ballast water discharges, they found that the risk of introduction of these invasives was higher for domestic ships operating within Canadian waters because they weren’t subject to ballast water inspections and reporting (Goldsmit et al. 2019).
While not all introduced species become invasive, the overall result of introduced species tends to be lower biodiversity. However, some introduced species become valuable to humans, such as earthworms in cropland soils that are mostly non-native species from Europe; the honeybees brought to the New World by English settlers; or the cattle introduced by Spaniards. The diversity of species in every part of Earth has changed dramatically over time and will continue to do so.
Global changes and biodiversity
“A species is there, and it's abundant for quite a long period of time, and then at some point it's no longer there - and so, when you look at that bigger picture, yes, you realize that either you change and adapt, or, as a species, you go extinct.”
- Kenyan paleontologist Louise Leakey, National Geographic Explorer in Residence (National Public Radio 2014).
Global change has shaped biodiversity since the beginning of life on Earth. Before humans, there were five mass extinctions, periods when biodiversity plummeted. Each mass extinction was caused by a combination of global changes, including shifts in climate, huge volcanic events, ocean current flows, and/or changes in atmospheric gases (see our Factors that Control Regional Climate module). These suites of related changes led to drastic shifts in climate and habitats all over the world. Of course, life on Earth marched on after these mass extinctions, but many species were lost forever, and new species emerged to take their place. For example, the mass extinction that included the loss of nearly all the dinosaurs was what paved the way for the diversification and dominance of the mammals.
What are some global changes occurring across Earth today?
Scientists concur that we’re in the midst of the sixth mass extinction on Earth, the first one caused by humans. The drastic changes on our planet stem from the human tendency and ability to alter our surroundings in almost every conceivable way, including water flow, temperature, nutrient cycles, forest cover, variety of plants and animals, and even the global climate. Some alterations benefit other species, but at the scale and intensity of today’s land use practices, most do not.
“Many indigenous communities rely on nature for everything – from food and water to their livelihoods and culture. Because of this intimate relationship with nature, we are the first ones to feel the impact of the climate crisis.”
- Indigenous Kichwa biodiversity researcher Johnson Cerda, 2020, Senior Director at Conservation International.
Climate change affects ecosystem conditions at all scales - from local rainfall patterns to global ocean currents. Changing conditions make habitats more or less hospitable to humans and the other species that rely on them. Indigenous Peoples, given their physical and spiritual connections to their landscapes coupled with lower capacity to relocate, are disproportionately impacted by climate change. For example, as precipitation decreases, the Western Apache Peoples encounter less robust deer and elk populations, low river levels for fishing, and scarcer water for subsistence farming (Gauer et al. 2021).
Scientists such as Italian biologist Michela Pacifici have come up with ways to assess the resilience of other species to climate changes - what range of temperatures they can tolerate, what they feed on, how fast they reproduce, and how common they are. All animals have upper thermal limits – maximum temperatures that they can tolerate. Pandas get heat-stressed in temperatures above 25˚C (77˚F; Yuxiang Fei et al. 2016), whereas some Andean iguanas can tolerate temperatures up to 40˚C (104˚F). (Guerra-Correa, 2020). Based on nearly 100 studies of plant and animal tolerances to environmental extremes, Pacifici mapped out the species most vulnerable to climate change (Pacifici et al. 2015).
In Pacifici’s map (Figure 5), where do you see concentrations of vulnerable species? Why there?
Note that vulnerable species are concentrated in the Poles, where ice is melting, and in areas near the equator, such as the Amazon, where fires are becoming more frequent. As conditions shift outside of livable ranges, organisms either move, adapt, or die, depending on their resilience to change and their ability to migrate. Thinking about how humans handle environmental changes, to what extent do the biological outcomes - move, adapt, or die - apply?
Moving to better habitat
“Expected anthropogenic climate change will redistribute the locations where specific climatic conditions favorable to the survival of a species will occur.”
- American ecologist Osvaldo E. Sala, Arizona State University
Scientists that map biodiversity by tracking the ranges of various species see evidence that many are migrating in response to climate change. Tasmanian ecologist Gretta Pecl estimates that at least a quarter of life on Earth, and possibly much more, is in the process of relocating. For example, her work shows how ocean animals like snappers, rays, and sea urchins are moving towards the South Pole as oceans warm along Tasmania’s coast. The shifts disrupt thousands of years of cultural practice by indigenous ice-fishing peoples of the region. Climate change affects not only ocean wildlife, but also the people who depend on it.
Animals migrating in response to climate change also face novel situations and threats. For example, North Atlantic Right whales have shifted their feeding routes northward in response to warming temperatures in the Gulf of Maine. In their new Gulf of St. Lawrence habitat, these whale populations suffer increased ship strikes and fishing gear entanglements. As new management plans are drafted to protect the whales, Canadian fishermen will suffer restrictions such as seasonal closures of St. Lawrence fishing areas (Meyer-Gutbrod et al. 2021).
Adapting to changing habits
When conditions change, animals may or may not have the capacity to adapt. What choices does this Arctic fox have (shown here in its winter fur) as warming weather with less snow cover increasingly changes its winter habitat to shades of brown and green?
Arctic foxes respond to seasonal changes by shedding their white winter fur and replacing it with brown fur in the spring. The change is mediated by seasonal changes in sunlight from short winter days to longer summer days. Warmer temperatures and less snow, therefore, do not provide the cues to molt to a brown coat any sooner, despite the need for camouflage (Denali Education Center 2022). Along with many other Arctic animals, adapting to climate change will require longer-term natural selection for a modified schedule of fur shedding. With climate change occuring at such a rapid pace, it is unclear if the foxes will have enough time to adapt.
When climate conditions change, some organisms can adapt. American Pikas, with naturally high body temperatures, prefer cooler habitats. Originally from Asia, pikas spread into North America five million years ago when the climate was cooler. Over geologic time, pikas have retreated to high mountains in the western U.S. and Canada. During hot weather, they stay cool by taking refuge in the shade of rock piles. There may come a tipping point when temperatures in the rocks rise beyond what pikas can tolerate, forcing them to migrate or go extinct, but for now they appear to be adapting (Smith 2021).
Cold-blooded animals (ectotherms), such as insects or lizards, may have an advantage in adapting because of their ability to tolerate more extreme temperatures. Ectotherms rely on outside temperatures to regulate their body temperature, hence their name (ecto = outside; therm = heat). Many have mechanisms to avoid freezing, like natural antifreeze chemicals in their blood. As the climate warms, some insects benefit from higher metabolisms and increased reproduction, which may lead to unpredictable shifts in populations of pollinators and crop pests (Gérard 2020; Deutsch 2018).
Still, the immediate advantages of high temperatures do not ensure long-term gains. Portuguese marine biologist Carolina Madeira used sea snails (Stramonita haemastoma) to examine short versus long-term impacts of temperature in a laboratory setting. She found that the snails could acclimate to higher water temperature over short periods, but grew more slowly from the thermal stress. Insects and other ectotherms can usually adapt to natural cyclical variations in global temperatures, but the current temperature increase is occurring on a much faster time scale (Madeira et al 2018).
Generally, any species will have a threshold beyond which temperatures are intolerable, forcing individuals to migrate or die. Not all species have the ability to migrate. A study by Colombian biologist Cristian Román Palacios modeled whether animal and plant species could survive climate change by migrating. The model, which included over 500 animal and plant species, indicated that if migration is the only option, more than 50% of them face extinction. But, taking into account adaptations like the pikas finding cooler refuges, the percentage facing extinction is closer to 30% (Román-Palacios and Wiens 2018). Whether a particular species adapts, migrates, or goes extinct in response to climate shifts will depend on the amount of change in relation to its capacity to adjust its habits or range.
Failing to move or adapt
For those species that do not succeed in adapting or migrating, climate changes and other sustained global changes can be fatal. As climate continues to warm on Earth, biodiversity is expected to plummet (see our Factors that Control Earth's Temperature module). For example, American biologist Barry Sinervo estimated that climate change could wipe out 80% of the world’s lizard species by 2080 (Sinervo et al. 2010).
As habitats continue to change globally, we face big questions about how biodiversity will change. Which species can adapt by adapting or moving? Which species will go extinct? As climate warms, we can expect to see increasing disruptions in how ecosystems function across the globe. The Fourth National Climate Assessment predicts more frequent and severe storms, droughts, erosion, and flooding. Each of these disruptions may cause significant changes to biodiversity (see our Environmental Services and Economics module).
Biodiversity in the anthropocene
The Anthropocene, or “Age of Man”, is what scientists call the current period of dramatic Earth changes caused by human activities. When the Anthropocene began is debatable, but its long-term impacts are clear. Habitats have been altered, ecosystems are functioning differently, and biodiversity is lower. Earlier humans, with lower densities and less intensive resource exploitation, altered the landscape in ways that allowed other species to persist. Modern human practices leave little ecological room for other species (Figure 7).
Thinking about the bicycle again, some other species (parts) are missing, leading us to suspect that its essential systems of brakes or steering might not work. You are riding the bicycle anyway because it's the only one you've got, as we are living on Earth despite the lost species. You may find it more difficult to ride with so many of the parts missing, and the bike may not last as long as it would have with all of its parts intact.
The upkeep and repair of the bicycle that is Earth is in our hands. Recognizing that the sustainability of Earth for living organisms, including humans, is at stake, people around the world are working to maintain biodiversity.
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