8.5 Threats to species and ecosystems

The major threats to species and ecosystems cross lines of terrestrial, freshwater and marine. This section will be relevant to Chapter 9, on oceans, as well as to this chapter. Additional threats to marine species and ecosystems will be added in Chapter 9.

A history of threats to species and ecosystems

The threats to terrestrial biodiversity have changed dramatically over human history. In prehistory, overhunting of the enormous land birds, the moas, of New Zealand and of the giant mammals of the Ice Ages – wooly mammoths, dire wolves, giant sloths, etc. was the primary means by which humans reduced biodiversity.

During the so-called Age of Exploration, during which primarily European explorers traveled to places that were not Europe, we continue to see overhunting, partly to feed the explorers and partly to fuel demand for fur and other commodities at home. During this period, North America lost the great auk, Steller’s sea cow, the Labrador duck, and the Atlantic sea otter. Almost 100 bird species of Hawai’i are extinct, eliminated by avian malaria brought in by Captain Cook in mosquitoes in his water casks. Elsewhere, species losses are less well documented but include the dodo and various giant tortoise species.

As colonization occurred and unfamiliar forms of agriculture and livestock production were imposed on continents, hunting, poisons, and trapping were used to control predators and to secure commercially valuable wildlife and plant products. Predators were extirpated from many places, but outright extinctions were much fewer. The thylacine, or Tasmanian tiger was one loss during this period (Fig 1).

More recently, we are beginning to see losses due to massive changes in river systems brought on by damming. The baiji, a freshwater dolphin of the Yangtze River in China was last seen in 2002 and is considered extinct. Several salmon species have declined precipitously, although their cultural, recreational, and food value have led to recovery efforts that have staved off extinction, so far.

Ninety species of amphibians are believed to be extinct as a result of a fungal disease spread around the world by travelers, including, perhaps, researchers. Three species of North America bats have decline precipitously due to a fungal disease unintentionally brought over from Europe. Climate change is believed to have claimed the golden toad of Costa Rica. Three species of migratory bats in North America have declined precipitously since wind farms became common, although the industry has contributed to efforts to reduce losses.

Twenty years ago, it was relatively easy to outline the major threats to terrestrial biodiversity – habitat loss and degradation, invasive species (including diseases), illegal harvest and overharvest, secondary effects. It is becoming more difficult to rank the causes of biodiversity loss.

Certainly, habitat loss and degradation remains the most obvious source of blunt-force trauma. Species cannot persist without space in which to live. Massive losses to agriculture occurred over a century ago in North America and millennia ago in many other places, but continue in the present in places like Brazil and Indonesia, as we saw in Chapter 7. Urban sprawl also continues to eat land. These are forms of true habitat loss – the land is no longer available for habitat. In other cases, habitat may be degraded by pollution, overgrazing and other overuse, by fragmentation by roads, development, agriculture, and other land use.

Habitat fragmentation is a slightly less direct form of loss – theoretically, it can occur without any actual loss in acreage, although some loss is usually involved. Edges of fragments are different from core areas that still resemble original continuous habitat. Edges are open to humans, dogs, cats, trash, noise, sunlight, wind, invasive species and other disturbances, depending on the nature of the habitat. So, although the land may still be there, its value in terms of the original habitat is degraded. As a result, overall area of useable habitat is reduced for species that need characteristics of the original habitat. Some species need areas of a certain minimum size; fragmentation reduces the number of habitat patches that they can use. New species may benefit from the edge conditions, but they are often generalist species that were already common.

Invasive species are often listed as the number 2 or number 3 threat to biodiversity. Most people don’t recognize non-native species, and they are so ubiquitous in many places that they go unnoticed. But their impacts can be devastating. Buffel grass was brought to the US Southwest in the 1930s as cattle forage for the desert. It was deliberately planted from the late 1930s until 1980, and it was noticeably invasive by the 1990s. It has spread throughout the Sonoran desert, where continuous vegetation was historically absent and wildfires were rare as a result. As buffelgrass has taken hold, it has supported the spread of wildfires over much larger areas than could previously support fire. Native cactus, notably including saguaro, are impressively vulnerable to fire and die when burned (Fig 2). By changing the fire regime of an entire biome, buffelgrass is threatening endemic native species and converting patchy Sonoran desert vegetation into a nonnative grassland.

Illegal harvest affects both plants and animals, from elephant ivory and rhino horn to rosewood and mahogany and a host of animal and plant parts used for medicinal and other purposes around the world. Some of this harvest is for personal use – some illegal pangolin harvest (see previous section) is for food, for example. But much of the illegal harvest is for trade, and natural resource trade is the one of the largest illegal markets in the world. Smuggling of illegal natural resource products can occur simultaneously with drug smuggling, leveraging profits from both kinds of trade.^[1] Overharvest is a related harm that can occur when harvest is unregulated or poorly regulated. Harvest of plants, fish, and invertebrates is often unregulated, leaving these species unprotected.

Secondary effects are unintended consequences of actions or substances intended for other purposes. In Chapter 7 we saw that use of neonicotinoid pesticides as seed coatings and genetic modification of crop plants with Bt genes have significantly increased the levels of pesticides in agricultural environments, leading to declines in pollinators including bees and butterflies. Bees and butterflies were not specific targets of either form of pesticide, but because both are broad-spectrum pesticides, they also affect “good bugs” and reduce biodiversity. The coal-fired powerplants that heated homes and powered industries in the last century and, in some places, in this century, were never intended to wipe out forests or render lakes devoid of most living things, but their emissions caused acid rain, which killed some organisms outright and degraded habitats to the extent they were unusable by many species. Such unintended consequences are sufficiently common and severe in their impacts that they rank among the leading causes of biodiversity loss.

This was the list of leading threats to biodiversity before climate change became a significant concern. These threats still exist, and all remain hugely important concerns. But climate change both exacerbates some of these threats – for example, heat waves and warming, generally, can make some otherwise suitable habitats unsuitable, contributing to habitat loss, and invasive species are spreading poleward and higher in elevation under warmer and sometimes wetter conditions – and adds to them.

Climate-change impacts on species and ecosystems

As climate change accelerates, its effects are increasingly clear. The news is not good, and reading about it is not uplifting. Losses have already occurred and more are inevitable. Most people find it difficult to face the details of these impacts, and it is important for readers to remember why they became interested in sustainability in the first place. We cannot solve all problems, but by seeking to solve those we can, and being ambitious about our abilities, we can limit overall harm. Understanding the drivers of loss is part of reducing loss.

Impacts to species

Direct climate-change impacts to species include impacts related to changes in average and extreme temperature and precipitation. All species have climatic conditions they can tolerate and conditions for which they are so unsuited that they will not settle in those areas. Researchers refer to the climate envelope of tolerable conditions for species, and model climate envelopes by studying the temperature and moisture regimes in the range of the species – the total area inhabited by all individuals of the species. As climate changes, the climatically suitable area for species also change. Species can move to newly climatically acceptable areas, adapt in place to changes in climate, or they can be extirpated from the unsuitable area. [Note: extirpation describes loss from a particular area. Extinction describes the complete loss of the species from the planet.]

A review of information on range shifts in a wide variety of plant and animal groups shows that some species and some groups are clearly shifting poleward or upward in elevation, apparently to better match their temperature preference or towards areas of newly appropriate moisture availability (which may sometimes be downslope, against the temperature change). However, the majority of species have not (perhaps not yet) shown such shifts. Highly mobile groups – birds, insects, and fish – were among the species groups to show significant poleward shifts, overall (Fig 3).^[2] This group includes a number of commercially important marine species.

Note that species that currently inhabit polar and high elevation locations have no colder places to move to and are limited to adapting in place or disappearing. Adding to the stress, polar and high-elevation locations are also warming faster than the planetary average – 4 times faster, for polar regions – narrowing the window for adaptation. Species adapted to using sea ice – penguins, polar bears, seals, and walruses, among others – are declining as this habitat disappears. Species adapted to living on or under snow, or foraging in snow fare much less well when warm spells bring rain that then freezes in thick layers, entombing ground-level vegetation in ice that hooves cannot break up and small mammals cannot burrow through, and collapsing polar bear dens on polar bear cubs that cannot stay warm when drenched and then frozen. The developing possibility that shipping lanes will develop in the so-called Northwest Passage north of the Northern Hemisphere will expose marine and coastal species through the region to the noise and pollution of international shipping along with the possibility of oil spills from damaged and grounded ships.

Less mobile species are limited in their ability to move in order to remain within their climate envelopes. Plant species with small seeds that are easily carried by wind or rain have a clear advantage over heavy-seeded species such as baobab, Brazil nut trees, and oaks. Amphibians, reptiles, smaller mammals, flightless insects, and many other less-mobile animal groups also have limited options.

Mobile species are not guaranteed success in finding climatically suitable areas simply because they are mobile. New diseases, parasites, predators, or competitors may make climatically suitable areas unavailable. For species near the edge of a biome, a move may take them into a different, and less suitable habitat type. Food plants or prey species may be absent in the potential new home, requiring a change in diet that may or may not work for the moving species.

Invasive species are among the species shifting in response to climate change, including pest species and diseases. The US Center for Disease Control notes that several diseases are already expanding in the US as a result of changes in temperature and precipitation (including flooding).^[3] Warmer, wetter conditions are hospitable to a wider range of species than colder, drier conditions, so more of the planet is becoming available to more species, and parasites and diseases spread with them. Climate refugeeism also contributes to the spread of disease.^[4]

Climate change combined with high human mobility is not only spreading wildlife diseases but also supporting the development of novel diseases as a result of close contact between humans and animals. Avian influenza was first detected in domestic geese in China in 1996, and spread through contact with wild birds.^[5] It was first observed in humans in 2020 and has not become dangerous or widely problematical. However, impacts on waterfowl, birds of prey, and wildlife species have been more severe. More than 20 endangered California condors died when the disease first reached their populations in 2023. In Canada, more than 25,000 gannets, a seabird, have died.^[6] And, of course, tallies of all mortality in all species are unavailable, because monitoring at that level does not occur. OneHealth is an initiative of the World Health Organization that works to provide an integrated framework for health information for humans, wildlife, and ecosystems.^[7]

Results of changes in climatic extremes can be less subtle than range shifts in response to changes in climate averages. A “blob” of unusually hot, nutrient-poor, unproductive marine water developed off the northwestern coast of North America during 2014-2016, in usually highly productive waters. In response to the lack of nutrients, the marine food web of the area collapsed, and an estimated 4 million seabirds – common murres (Fig 4) – were estimated to have died of starvation, the largest single reported wildlife mortality event in the modern record.^[8] Researchers are concerned that changes to the food web and to the murre population may persist.^[9][10]

Impacts to ecosystems

Ecosystems are collections of plant and animal species, along with abiotic elements including soil and water, plus the processes that bind them all together. As climate change leads to shuffling of species around lands and waters, species move, or fail to move, as single units, each responding to its own tolerances and needs, depending on its own mobility.

As a result of differential responses of species, the ecosystems that managers have become accustomed to managing are changing. In some cases, the changes are as simple as redrawing biome lines, as is occurring in some northern areas where woody vegetation is spreading into the tundra, converting it first to a more shrub-dominated system and then to boreal forest.

In other ecosystems, the biome type is maintained, at least for now. Familiar species may become less common or more common, and new species move in. New invasive species appear. Often, the ecosystem remains recognizable, with a slightly different cast of species.

As species assemblages are shifting, so, too, are disturbance regimes. Increased disturbance may increase mortality, generally, creating more holes in forest canopies or more bald spots in grasslands. Ecosystem services may be diminished and the higher disturbance level may favor invasive species, further diminishing ecosystem services. In some cases, novel ecosystems may form, unlike anything managers in the area have dealt with before.

In ecosystems where fires occur naturally with some regularity, a century of fire suppression has often already predisposed them to burn more intensely as trees and brush have accumulated in unnatural density. More frequent and more severe droughts, including flash droughts, can render vegetation so dry that even if fires have burned through fairly recently, remaining vegetation may still easily burn if an ignition source – lightning, a stray campfire, the catalytic converter of a vehicle pulled off on the side of the road – is available. In some once-forested areas, forest fires have become so frequent that forests can no longer persist – young trees are repeatedly killed back before they can establish a forest – and the areas are converting to shrublands: more shrub species can handle being burned back to their stumps than tree species can. These shrublands may represent new assemblages of species because of the new role of fire in shaping the ecosystem – these may be novel ecosystems.

If an extremely wet season in a usually drier environment leads to a rich, diverse forest or woodland understory in spring, then by summer it may become a layer of dry, fine fuel, easily ignited and easily blown around by fire updrafts. In this way, increased variability in weather, with unusual wetness followed by later dryness, can create fires of greater intensity and frequency.

Persistent or frequent drought, alone, can also convert areas from forested to unforested, because trees are also less able to manage extreme dryness than shrubs. Outbreaks of bark beetles occur naturally in the northern hemisphere, but are occurring more frequently and with greater severity, assisted by droughts that weaken tree defenses. The dead needles and branches of dying trees become fuel for wildfire, creating a triple threat to boreal forests. Pest species are often favored by warmer conditions, and invasive species pressure may also shape ecosystems affected by climate change.

All disturbance regimes have the potential to modify the ecosystems in which they occur, if changes are big enough. Rivers that were once lined with riparian wetlands may lose those wetlands if large floods regularly wash away the accumulated soil and rotting vegetation and scour the riverbed down to rock. Droughts severe enough to dry rivers back to isolated pools may extirpate fish species that cannot persist in small, warm pockets of water.

Managers seeking to conserve biodiversity must plan not only for movement of native species but also movement of invasives, implications of new combinations of species, and results of changes in the disturbance regimes of their areas. If harvested species are being managed, impacts of climate change on their population sizes and mortality rates must also be considered.

Ecosystem changes under climate change are most observed in the Arctic, where climate change is proceeding at a greater pace, with woody species advancing into the tundra, tundra melting into thermokarst lakes, and boreal forest both expanding north and collapsing under more intensive fire regimes. However, the most at-risk major ecosystem under climate change is clearly warm-water (shallow-water) coral reefs. We will study these more carefully in the next chapter.

One class of species has an out-sized impact on ecosystems and deserves attention under climate change. Ecosystem engineers are species that have unusually large impacts on other species and on ecosystem processes. Beavers cut down trees and dam rivers, creating ponds and wetlands where forests and rivers once occurred. Prairie dogs create extensive burrow systems that support an entire suite of other species and rework soils over large areas. Tropical termites support their own suite of species and affect soils and nutrient levels throughout their biomes. Reef-building coral organisms create complex structures that provide homes for a immense diversity of species. Whereas, in most ecosystems, the loss or arrival of individual species does not affect the ecosystem in major ways, the loss or arrival of ecosystem engineers may change ecosystems significantly.

Ecosystem and species impacts on climate change 

Carbon sequestration

Climate regulation is an increasingly important ecosystem service, provided especially by wetlands, peatland forests, tropical forests, and tundra, which sequester significant carbon (remove it from atmospheric circulation, into biomass or soil, for meaningful amounts of time) . So-called nature-based solutions to climate change include the protection of these ecosystems and efforts to increase the amount of carbon they hold and the length of time for which they hold it. However, humans continue to drain wetlands, and clear and burn peatland forests and tropical forests. Climate change, itself, is melting tundra, releasing methane and carbon dioxide, and increasing wildfires that darken the land surface and further hasten warming. These anthropogenic factors can turn ecosystems into major sources of GHG emissions, as we have seen. If we can control these factors, ecosystems can continue their natural roles in regulating climate change through carbon uptake and sequestration. However, as ecosystem services are reduced in climate-stressed ecosystems, the amount of climate regulation that can be performed by those ecosystems may decrease.

At present, researchers calculate that natural systems and agricultural soils may be able to absorb approximately one third of greenhouse-gas emissions. However, most of the science behind our understanding of the details of carbon cycling in ecosystems in very young, and researchers stress that we need a much more solid understanding of all phases of carbon cycling in the natural world in order to understand and benefit from the potential for natural systems to mitigate climate change.^[11]

Unlike livestock, single species in natural systems mostly do not have a major impact on carbon cycling and climate change. However, termites, by virtue of their role in breaking down large volumes of dead woody material in the tropics, and their production of methane, are often mentioned. Climate change may expand their range and their impacts. Wild ruminants burp methane, just as cattle, sheep and goats do, however their populations are very small relative to livestock populations, and they usually receive at most a passing note.

Other impacts on climate

In addition to sequestering substantial carbon, tropical rainforests also affect climate change through teleconnections. Because of the very large amount of water vapor these forests “breathe out” through evapotranspiration, they affect precipitation patterns around the world. Degradation of these forests reduces rainfall at great distances.^[13] And all forest types are now seen as important for supporting local and regional rainfall.^[14]

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.