Anthropogenic threats to biodiversity and ecosystems

Human activities are the leading threat to Earth’s biodiversity. Subsumed for agriculture, tropical dry forests and grasslands have almost completely disappeared. Many species are barely surviving on a fraction of their former ranges, in increasingly fragmented landscapes. Dams disrupt freshwater ecosystems, while overfishing, pollution, and habitat destruction threaten coastal regions and the oceans. As we have seen, plants and animals transported around the globe by humans can change entire ecosystems. No corner of the globe is untouched: pollutants are carried tens of thousands of kilometers through the atmosphere and contaminate both poles. We are changing Earth’s atmosphere through the industrial release of carbon dioxide, which is dramatically altering the climate.

Habitat loss and fragmentation

Habitat loss and fragmentation are related processes, typically occurring simultaneously, and together are the primary threat to biodiversity globally. Habitat loss occurs when an environment is modified to the extent that a particular organism can no longer survive there, while ecosystem loss refers to the disappearance of an entire ecosystem. Conservation measures often focus on protecting threatened ecosystems, such as the acacia forests in the Senegal River basin, or threatened species such as orangutans. Fragmentation is usually a product of habitat loss and is best thought of as the subdivision of a formerly contiguous landscape into smaller units. Many forests, for example, have been cleared piecemeal for human activities, as satellite imagery clearly shows (see the following essay and Figure 2). In Sumatra and Borneo (home to the last populations of the critically endangered orangutan), forests are converted primarily for agriculture (palm oil) but also for mining, road development, and logging. When habitat is fragmented, the species that depend on it not only lose part (or all) of their range, but also face new risks, such as higher exposure to predators, invasive species, and being hunted or collected by humans (e.g., young orangutans are targeted for the illegal pet trade). Fragmentation also isolates populations, disrupting the dispersal and migration of individual plants and animals (and their genetic material) across a landscape (see Figure 1 for a management approach to mitigate the effects of fragmentation).

Ecosystems that are especially vulnerable to loss or fragmentation are typically those in greatest demand for human use, for example for agriculture, resource extraction, or other development (e.g., roads, housing). Conservation goals often compete with these demands. For instance, when reviewing Rwanda’s national park system, the government must weigh a growing human population’s need for additional agricultural land against the need of endangered mountain gorillas for habitat. In Mozambique, Gorongosa’s boundaries have both expanded and retracted in response to similar pressures over its history.

Figure 1. Landscape fragmentation by roads. In addition to fragmenting the environment, roads make remote areas more accessible to hunters, markets, and invasive species. Roads—especially heavily traveled ones—also threaten wildlife because of collisions or an inability to cross the physical barrier. One solution is to establish corridors of natural habitat so animals can connect with other populations. This photograph shows a newly constricted wildlife overpass in Singapore that connects two sections of a rain forest nature reserve separated by an eight-lane highway. Its success as a wildlife corridor will be carefully monitored with camera traps and ultrasonic recordings. Other conservation initiatives work to keep areas “roadless.” Wikimedia/Benjamin P. Y-H. Lee (University of Kent)

Figure 2. Modern extinction rates (vertebrates). Approximately 1.3 million animal species have been described since 1758, of which 66,178 are vertebrates; of those, 39,223 vertebrate species have been formally evaluated (determined as extinct, threatened with extinction, species of least concern, etc.) by the International Union for the Conservation of Nature (IUCN). This graph is a conservative estimate of the cumulative percentage of evaluated species that have gone extinct or are extinct in the wild since the 1500s: mammals in blue, birds in purple, other vertebrates (reptiles, amphibians, and fishes) in yellow, and all vertebrates combined in green. Note that lower extinctions in the category “other vertebrates” may reflect a shortage of data for these groups (for which most species are not yet evaluated by the IUCN). The dashed black line represents the number of extinctions expected to occur under a constant standard background extinction rate of 2 E/MSY (2 extinctions per 100 years per 10,000 species); based on this background rate, the total number of vertebrate species that went extinct in the last century (or 100 years) should have taken about 800 to 10,000 years to disappear. © Ceballos, G., Ehrlich, P. R., Barnosky, A. D., García, A., Pringle, R. M., & Palmer, T. M.(2015, June 19). Science Advances 1(5) e1400253 (CC BY-NC 4.0)

Invasive species

Invasive species are arguably the second most important threat to biodiversity globally, threatening individual species and even entire ecosystems. The frequency and scope of this threat has increased along with the evolution of transportation and commerce. While not all introduced species have a detrimental effect, others can cause local or global extinctions and significantly disrupt ecosystems, as we saw with the zebra mussels’ impact on the Hudson River. The brown tree snake is directly credited with the local extinction of over half of Guam’s native bird and lizard species and two (of only three) native bat species. Chestnut blight, a fungus introduced from Japan, is estimated to have killed some 4 billion American chestnut trees in North America in the first half of the 20th century. Rabbits have created major environmental disruption in Australia, where they were introduced as a game animal, also costing millions per year in lost agricultural production.

Overexploitation

Humans use wildlife and other natural resources in order to survive, but overexploitation is a critical problem and plays a powerful role in biodiversity loss. Overexploitation was almost certainly responsible for numerous historical extinctions, such as the elephant birds and giant lemurs of Madagascar, the giant kangaroos of Australia, and the moas of New Zealand. More recently, passenger pigeons, once North America’s most abundant bird, were hunted to extinction by the end of the 19th century.

Overharvesting, unsustainable use, and illegal trade threaten not only the continued survival of some plants and animals, but also that of ecosystems, communities, and local economies that depend upon those species. For instance, the collapse of the North American cod fishery in the 1990s led to strict management measures, including the complete closure of the Canadian fishery, which affected thousands of Canadian fishermen and communities (see also Box 1). In theory, some level of exploitation should be sustainable. The difficulty is determining what level is manageable in the long term—and enforcing it. Moreover, harvest theory (such as maximum sustained yield; see Modeling Population Growth) is often too simplistic to address the complexities of natural systems, and regulations must balance conservation with human needs.

Figure 3. Status of the Atlantic cod fishery off Newfoundland, in fish landings (1850–2000). Atlantic cod stocks off the east coast of Newfoundland collapsed in 1992 due to overharvesting. This resulted in the complete closure of the Canadian fishery, affecting thousands of Canadian fishermen and their communities. Adapted from Inman (2011) by Nadav Gazit/AMNH

As one species became overharvested, fishing pressure shifted to other species further down the food chain. Between 1950 and 1994, species fished shifted gradually from long-lived bottom-dwelling fish that eat other fish (e.g. cod, halibut) to lower-trophic-level invertebrates and open-water species that eat plankton. Non-target species may also be exploited inadvertently. For instance, many commercial fishing methods catch other species along with the intended target, known as “bycatch.” The impact can be substantial. For example, commercial shrimping, which has one of the highest bycatches of any fishery, discards five pounds of bycatch for every pound of shrimp harvested.

It is still possible to restore fish stocks. A 2016 study that compared current and alternative fisheries management scenarios found that if reforms (e.g., minimum size criteria, restrictions on certain fishing practices) were implemented on a global scale, the median fishery could reach recovery targets in under 10 years. Nearly every country in the world would see increases in food provisioning and fishery profits.

Pollution

Every day, thousands of pollutants are discharged into the environment, fundamentally altering ecosystems and threatening biodiversity on multiple scales. Pollutants tend to be classified by the environment they affect (e.g., air pollution, water pollution). Many pollutants are not regulated, and many tend to persist in the environment so that the effects are felt even after regulations are put in place. Some are directly toxic to living organisms, such as insecticides like DDT (dichloro-diphenyl-trichloroethane), which accumulate in animal tissues or interfere with vital processes such as reproduction or immunity (see Figure 4). Sometimes the effects are indirect, as with fertilizer runoff, which is nontoxic but in large quantities harms aquatic systems by causing excessive algal growth. Noise and light pollution can threaten species by disrupting their behavior. For example, artificial lights on beaches can discourage female sea turtles from nesting or can distract sea turtle hatchlings from heading to the ocean.

Figure 4. Bioaccumulation in marine organisms. The accumulation of substances (such as pesticides or other chemicals) in an organism is known as “bioaccumulation.” As illustrated here, the substance becomes more concentrated at each trophic level; the animals at the top are affected most severely. This applies to humans as well. For example, human industrial activity has increased the amount of mercury ingested by marine species, which concentrates in shellfish and fish, accumulating faster than it can be broken down. The largest, oldest fish absorb the most over time. Anything that eats these fish—including humans—also ingests the higher level of mercury. Because high mercury levels are linked to developmental issues (by affecting the central nervous system), the U.S. Environmental Protection Agency and the Food and Drug Administration set consumption guidelines for fish with the highest mercury levels, like tuna, swordfish, and shark. ©Nadav Gazit/AMNH

Climate change

Climate is central to the geographic distribution of plant and animal life on Earth and the stability and productivity of ecosystems. The effect of anthropogenic climate change on plants and animals worldwide depends largely on how much the climate is warming, and how fast. As the climate warms, we’re seeing shifts in vegetation patterns and species distributions, and mismatches in seasonal cycles (i.e., phenology). One example is that some plants are blooming earlier in the season, but their pollinators and/or the species that use those plants as a critical food resource, are still arriving later in the season. These shifts may fundamentally alter ecosystem composition and function. For example, warming temperatures allow the mosquitoes that transmit malaria, Zika, and dengue fever to widen their range. Conversely, as alpine species are pushed to the (vertical) limits of their habitat, they may disappear entirely (Figure 5). Species that are able to adapt to climate change may encounter other barriers, such as habitat fragmentation. Species that are confined to a small fragmented habitat, such as coastal wetlands, are particularly at risk if they are unable to migrate because of coastal development, for instance. Climate change additionally stresses already endangered systems, such as coral reefs. Because of ocean acidification (the process by which the ocean absorbs carbon dioxide from the atmosphere, reducing its pH levels), coral reefs are becoming less able to absorb the calcium carbonate they need to maintain their skeletons.

Figure 5. American pika (Ochotona princeps) geographic range estimates for the past, present, and future in the western US. Climate change is leading to local pika population extirpations and geographic range contractions as the lower elevation boundary of their range is shifting upslope. Pikas are directly stressed by increasing temperatures—both increased heat stress and also increased cold stress, which may arise from warmer temperatures that reduce snowpack. Normally, snowpack acts as an insulator during winter; this loss may lead to pikas freezing or depleting their food supplies while attempting to thermoregulate. While some areas have the option for populations to disperse to higher (and cooler) elevations, many populations have and will become extirpated as temperatures continue to rise. According to the IUCN, the effects of climate change on the American pika should be considered an early warning, and they should be continuously monitored throughout their range to demonstrate how similar species could adapt and be resilient in the face of climate change. Range map ©HHMI; pika, ©GlacierNPS

Figure 6. American people and communities at risk from global sea level rise. In recent years, all coastal states have been subject to severe flooding and damage from coastal storms as a result of global climate change. Roughly a third of the U.S. population—more than 100 million people—lives in coastal counties, but Florida, Louisiana, New York, and California residents are the most vulnerable, as these states have the most residents living on low-lying coastlines (land less than 3.3 feet above high tide). According to the Union of Concerned Scientists, even if global warming emissions were to drop to zero, the changes we have already made to the atmosphere would lead to 1.2 to 2.6 feet of global sea level rise by 2100. More likely, based on emission trends and the extent of ocean warming and land ice loss, the average sea level is projected to rise to the 3.3-foot mark before 2100. ©Union of Concerned Scientists (2015); www.ucsusa.org/sealevelrisescience

As with temperature change, sea level rise—the result of melting land ice (e.g., glaciers) and warming ocean temperatures (warm water expands)—is expected to vary regionally and have dramatic regional consequences. Small island states and low-lying coastal areas are particularly vulnerable because of the increased risk of coastal flooding and the impact of storm surges. Other consequences of sea level rise are the loss of beach, wetland, and mangrove habitats (see Figure 6).

“Wicked” problems

If all these threats to biodiversity and ecosystems seem complex and overwhelming, that’s because they are! Moreover, the exact nature of a conservation problem can be difficult to assess. Many threats to biodiversity acting together can have effects that are highly synergistic and difficult to tease apart. In Gorongosa, for instance, direct threats like illegal resource use are exacerbated by complex underlying threats that stem from poverty and politics. Given the inherent complexity and dynamism of social and ecological systems, it is impossible for conservation planners and managers to predict all the potential consequences of their actions. Like the Gorongosa Restoration Project, many conservation initiatives have multiple—and competing—objectives that encompass both biodiversity protection and poverty reduction. Success is ultimately subjective. This kind of problem is often referred to as “wicked.”

Wicked problems are not easily defined and lack clear solutions. The problem itself can be subject to multiple interpretations, eludes resolution, and defies generalization. Each problem—habitat loss, for example—is potentially linked to other problems such as overexploitation, population growth, or political instability. Its components are interconnected, and changes to any part of the system may have unintended consequences elsewhere. In contrast, problems such as building an engineered structure are simple, because they can be solved by technical solutions that apply equally in different places.

Wicked problems can be overwhelming, but a systems-thinking approach can help. Recall from the beginning of the course that when we look through a systems-thinking lens, connections become more visible. We see the world—or in this case, the problem—as elements and processes that connect and interact in dynamic ways to form a whole. A systems thinker asks how changing one or a set of parts (e.g., losing or reintroducing a species) or changing their connections (for example, a change in behavior, like Gorongosa lions targeting warthog instead of buffalo), can affect other parts of the system (Figure 7). A systems thinker also understands that connections among parts may change over time, and that they can affect each other through feedback loops in ways that are indirect, or delayed in time. This helps us better understand the complex and wicked nature of conservation problems as well as to plan incremental, iterative interventions that address a component of the problem—while simultaneously learning more about how the whole system functions. Considered “adaptive,” this type of management promotes experimentation and learning. As we will discuss in the next essay, adaptive management is important for many of today’s conservation strategies.

Figure 7. Male lion in Gorongosa National Park. In the absence of large populations of buffalo and zebra, lions have changed their feeding behavior and now target smaller prey, such as warthogs, bushbucks, and reedbucks. A systems thinker might ask: Is pride size being limited by a preference for smaller prey? How will this shift change the behavior of the prey species? Will there be cascading effects or feedback loops that will affect lion recovery or the larger Gorongosa ecosystem? ©Jeff Reed

Conclusion

In August 2016, 12 years after Crutzen had introduced the term, the Anthropocene Working Group for the International Commission on Stratigraphy formally recommended that geologists adopt the proposed new epoch—the “Anthropocene,” a geological age marked by human impact. It may take decades before this recommendation is officially adopted (if at all), but many scientists and environmentalists hope the debate will bring attention to anthropogenic threats to Earth’s biodiversity. In the next two weeks, we will discuss how humans are seeking to conserve and recover ecosystems, and what values and visions are in play.

Adapted from M. Laverty and E. Sterling, Threats to Biodiversity: An Overview, Network of Conservation Educators and Practitioners module.

By Kimberley Landrigan, Ana Luz Porzecanski, and Suzanne Macey

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