Module #60

Causes of Declining Biodiversity

Module Introduction:

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• We have seen that declines in biodiversity are happening around the globe.
• In "Science Applied 2: How Should We Prioritize the Protection of Species Diversity?” on page 184, we discussed biodiversity hotspots, which are areas around the world rich in biodiversity.
• Many organisms in these biodiversity hotspots face threats of extinctions due to human activities.
• However, threats to biodiversity exist throughout the world.
• In this module, we build on the basics of population and community ecology from Chapter 5 to understand how a number of factors can all affect biodiversity.

Module 60: Causes of Declining Biodiversity

Essential Knowledge

5.8 Impacts of Overfishing (Modules 32, 60)

• Overfishing has led to the extreme scarcity of some fish species, which can lessen biodiversity in aquatic systems and harm people who depend on fishing for food and commerce.

Essential Knowledge

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8.2 Human Impacts on Ecosystems (Modules 25, 41-44, 51-54, 57, 60)

• Organisms have a range of tolerance for various pollutants. Organisms have an optimum range for each factor where they can maintain homeostasis. Outside of this range, organisms may experience physiological stress, limited growth, reduced reproduction, and in extreme cases, death.
• Coral reefs have been suffering damage due to a variety of factors, including increasing ocean temperature, sediment runoff, and destructive fishing practices.
• Oil spills in marine waters cause organisms to die from the hydrocarbons in oil. Oil that floats on the surface of water can coat the feathers of birds and fur of marine mammals. Some components of oil sink to the ocean floor, killing some bottom-dwelling organisms.
• Oil that washes up on the beach can have economic consequences on the fishing and tourism industries.
• Oceanic dead zones are areas of low oxygen in the world’s oceans caused by increased nutrient pollution.

Essential Knowledge

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8.2 Human Impacts on Ecosystems (Continued)

• An oxygen sag curve is a plot of dissolved oxygen levels versus the distance from a source of pollution, usually excess nutrients and biological refuse.
• Heavy metals used for industry, especially mining and burning of fossil fuels, can reach the groundwater, impacting the drinking water supply.
• Litter that reaches aquatic ecosystems, besides being unsightly, can create intestinal blockage and choking hazards for wildlife and introduce toxic substances to the food chain.
• Increased sediment in waterways can reduce light infiltration, which can affect primary producers and visual predators. Sediment can also settle, disrupting habitats.
• When elemental sources of mercury enter aquatic environments, bacteria in the water convert it to highly toxic methylmercury.

Essential Knowledge

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8.4 Human Impacts on Wetlands and Mangroves (Modules 13, 60)

• Wetlands are areas where water covers the soil, either part or all of the time.
• Wetlands provide a variety of ecological services, including water purification, flood protection, water filtration, and habitat.
• Threats to wetlands and mangroves include commercial development, dam construction, overfishing, and pollutants from agriculture and industrial waste.

Essential Knowledge

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9.8 Invasive Species (Modules 59, 60)

• Invasive species are species that can live, and sometimes thrive, outside of their normal habitat. Invasive species can sometimes be beneficial, but they are considered invasive when they threaten native species.
• Invasive species are often generalist, r-selected species and therefore may outcompete native species for resources.
• Invasive species can be controlled through a variety of human interventions.

Essential Knowledge

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9.10 Human Impacts on Biodiversity (Modules 59, 60, 61)

• HIPPCO (habitat destruction, invasive species, population growth, pollution, climate change, and over exploitation) describes the main factors leading to a decrease in biodiversity.
• Habitat fragmentation occurs when large habitats are broken into smaller, isolated areas. Causes of habitat fragmentation include the construction of roads and pipelines, clearing for agriculture or development, and logging.
• The scale of habitat fragmentation that has an adverse effect on the inhabitants of a given ecosystem will vary from species to species within that ecosystem.
• Global climate change can cause habitat loss via changes in temperature, precipitation, and sea level rise.
• Some organisms have been somewhat or completely domesticated and are now managed for economic returns, such as honeybee colonies and domestic livestock. This domestication can have a negative impact on the biodiversity of that organism.
• Some ways humans can mitigate the impact of loss of biodiversity include creating protected areas, use of habitat corridors, promoting sustainable land use practices, and restoring lost habitats.

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Causes of Biodiversity Loss (HIPPCO)

The 6 major causes of biodiversity loss can be summarized with the acronym HIPPCO.

Habitat Loss

• For most species the greatest cause of decline and extinction is habitat loss and most habitat loss is due to human development.
• Specialist species requiring specialized habitats are particularly prone to population declines. An example would be the Northern Spotted Owl of the Pacific Northwest, which requires old growth forests that are attractive for logging.
• A species may decline in abundance or become extinct even without complete habitat destruction as a result of habitat fragmentation.

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Some regions of the world experienced large declines in the amount of forested land from 1980 to 2000 while other regions have shown little change or have seen increases in forest cover.

Habitat destruction also promotes zoonotic diseases (e.g. Hendra virus and COVID-19) by exposing people to new animal populations and viral reservoirs.

Marine Habitat Loss

• In marine systems, there has been a sharp decline in the amount of available coral habitat which has generated a cascade of impacts and species losses similar to cutting down rainforests.
• The percentage of coral that remains alive in coral reefs has declined sharply in the Caribbean from 1977 to 2012.
• Coral bleaching results from ocean acidification, pollution and global temperature rise which ultimately result in marine habitat loss.

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Marine habitat loss is the result of a combination of a number environmental impacts including ocean acidification, pollution and temperature rise resulting in coral bleaching and loss of marine habitat.

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Alien and Invasive Species

• During the past several centuries, humans have frequently moved animals, plants, and pathogens around the world.
• Globally, invasive alien species pose a serious threat to biodiversity by acting as predators, pathogens, or superior competitors to native species.
• Examples: quagga and zebra mussels, cane toad, Kudzu vine, etc.
• Native species: Species that live in their historical range, typically where they have lived for thousands or millions of years.
• Alien species: A species living outside its historical range. Also known as exotic species or nonnative species.
• Invasive species: An alien species that spreads rapidly across large areas. Invasive species are usually generalist species rather than specialists.

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Questions to Consider:

1. Are invasive species inherently “bad” or are they just exploiting open niche space generated by anthropogenic activity?
2. Do invasive species restore balance to ecosystems in the long-term? (most ecologists say “no”).
3. Should humans be considered invasive?
4. How can we control the spread of invasive species through biological, chemical and physical means?
5. What traits help make an invasive species successful?

Characteristics of Invasive Species

• Invasive species are typically generalists rather than specialists and usually have a high reproductive rate (r-selected rather than K-selected).
• Invasive species tend to dominate because they have been freed from top-down controls such as their natural predators, parasites and pathogens.
• Invasive species, tend exploit vulnerable ecosystems that have been heavily disrupted, usually due to human activity. This means that invasive species are generally resilient.

Over the decades, there has been a steady increase in the number of alien species in Europe. This example shows the number of non-native species recorded in terrestrial ecosystems.

Controlling Invasives

Controlling invasives centers around 3 strategies:

• Physical removal from the landscape by mowing, trapping, prescribed burns, isolation/quarantine measures, etc.
• Chemical controls such as pesticides, poisons and chemical ecology methods such as pheromones to disrupt and reduce populations of invasive species.
• Biological controls such as the introduction of a natural predator of the invasive species to prey on the invasive and control its growth.

Consequences of Controlling Invasive Species

Physical and Chemical Controls

• The physical removal process may damage the landscape or harm nontarget species and chemical controls (e.g. pesticides) can impact nontarget species.

Biological Controls

• Can have unintended consequences such as predation of nontarget species.
• The newly introduced predator will still compete with native species, can even prove toxic or invasive itself.
• Cane toads were introduced to combat cane beetles that were feeding on cane sugar plants in Australia → Disaster.

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Invasive Species: Zebra Mussels

• Zebra mussels are invasive species from freshwater systems in Europe and Asia introduced to the Eastern U.S. and Great Lakes.
• Introduction:
• Mussels attach to boats and boat trailers
• Ballast water discharge from cargo ships
• Prevention:
• Inspection/cleaning of boats
• Educational campaigns

You don’t need to know EVERYTHING about EVERY invasive species listed in these notes, but you should be an expert on at least one or two and recognize multiple names.

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Consider the common traits shared across all examples.

Extra Resources for Invasive Species

• Nat Geo: Invasive Species 101
• SciShow: Invasive Species

Ecological Impact of Zebra Mussels

• High reproductive rate allows zebra mussels to displace native species.
• Enhanced competition for resources with native species.
• Zebra mussels are filter feeders* → increased water clarity, light penetration and primary productivity.
• Introduction of zebra mussels can support other species such as fish and waterfowl that feed on mussels directly or the enhanced algal populations.

*Quagga and Zebra mussels are filter feeders but selectively avoid blue-green algae, which generate harmful/toxic algal blooms, increasing the damage of eutrophication.

Invasive Species: Cane Toads

• Farmers in Australia were growing cane sugar, a non-native plant.
• The cane beetle was accidentally introduced along with cane sugar plants and began eating cane sugar, limiting crop yields.
• Cane toads were intentionally introduced to combat the cane beetle. The toads ate everything, quickly proliferating and spreading across the continent.
• Native species were outcompeted and natural predators fell victim to the cane toad’s toxins, further harming wildlife.

Cane Toads: An Unnatural History (1988). One of the greatest, most overly dramatic (but accurate!) documentaries of all time.

Cane Toad Success

The success of cane toads stems from 5 key traits:

• Toughness: the large size of the toads buffers them from ecological stresses like water availability, salinity, etc.
• Flexibility: cane toads are generalists and opportunists.
• Mobility: cane toads can travel over a mile in a single night.
• Poison: cane toads are protected from predators by their potent toxins.
• Reproductive success: cane toads are r-selected species can have have 2 clutches of eggs per year with each clutch containing 40,000 eggs.

These ‘Toad Tactics’ are described in Rick Shine’s Cane Toad Wars and offer a blueprint for the success of other invasive species.

Invasive Species: Rats (and Mice)

• Rats have reached global distribution by stowing away in ships and travelling with people.
• Rats are highly opportunistic generalists with high reproductive growth rates, making them excellent invasive species.
• Rats wreak havoc on local ecosystems that are not familiar with them, decimating native species especially ground nesting birds on remote ocean islands.
• Rats cause economic damage by feeding on crops and damaging infrastructure.

National Geographic: Saving a Remote

Island’s Birds-

by Removing Its Mice

Invasive Species: Kudzu Vine

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• Kudzu (“the vine that ate the south”) is native to Japan and southeast China, but introduced to the U.S. in 1876. Farmers were encouraged to plant kudzu to combat erosion in their fields until the 1950s.
• The excessive growth rate of kudzu combined with the favorable climate of the southeastern US helped make kudzu a successful invasive species.
• Kudzu grows over native plants (e.g. wildflowers and trees), smothering/shading them to death.

Invasive Species: Asian Carp

• Asian carp is a catch all terms and include Bighead, Grass, Black and Silver carp.
• Silver carp were intentionally introduced to combat excessive algal growth found in aquaculture operations as well as in the holding ponds of sewage treatment plants.
• Some silver carp escaped containment and entered the Mississippi River, working their way toward the Great Lakes.
• It is feared that invasive Asian carp will outcompete the native fish species of the Great Lakes already weakened from introductions of other invasives such as lampreys.

Historically, the Great Lakes were remote and experienced little introduction of non-native aquatic species, but that all changed when the canals (such as the Erie canal) were built.

Invasive Species:

Spotted Lanternfly

• Spotted Lanternfly (SLF) is an invasive plant-hopper native to China, India, and Vietnam; it is also established in South Korea, Japan and the U.S. SLF was discovered in NJ in 2018.
• This insect has the potential to greatly impact agricultural crops and hardwood trees. SLF feeds on the plant sap of many different plants including grapevines, maples, black walnut, and other important plants in NJ.

• SLF usually does not kill native plants directly. Instead, as it feeds, the insect excretes honeydew (a sugary substance) which can attract bees, wasps, and other insects. The honeydew also builds up and promotes the growth for sooty mold (fungi), which can cover the plant, forest understories, patio furniture, cars, and anything else found below SLF feeding.

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Other Invasive Species

• Purple loosestrife
• Snakehead fish
• Emerald ash borer
• Sea lamprey
• Burmese pythons

Pollution

• Threats to biodiversity come from toxic contaminants such as pesticides, heavy metals, acids, and oil spills.
• Other contaminants, such as endocrine disruptors, can have sublethal effects that prevent or inhibit reproduction.
• Pollution sources that cause declines in biodiversity also include the release of nutrients that cause algal blooms and dead zones as well as thermal pollution that can make water bodies too warm for species to survive.
• Recall the case studies on atrazine, neonicotinoids and microplastics.

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Population Growth

• IPAT Equation: As the human population expands, so too does its demand for resources and overall environmental impact.
• Conservation strategies like the 3 R’s can help to limit the impacts of population growth and reduce our environmental footprint.

Climate Change

• While the impacts of climate change will vary from species to species, overall it is expected that biodiversity will be negatively impacted by climate change.
• For example, in some regions, a species may be able to respond to warming temperatures and changes in precipitation by migrating to a place where the climate is well suited to the species niche. In other cases, this is not possible as species are pushed to their ecological limits.
• Consider the compounding impacts of ocean acidification, ocean warming, sea level rise and coral bleaching.

Overexploitation

• Overexploitation causes declines in populations and species.
• Hunting, fishing, and other forms of harvesting are the most direct human influences on wild populations of plants and animals.
• Most species can be harvested to some degree, but a species is overexploitation when individuals are removed at a rate faster than the population can replace them. 
• In the extreme, overexploitation of a species can cause extinction.
• Consider The Ocean of Life and The Sixth Extinction.

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Tragedy of the Commons: Bison

• When Europeans arrived in North America, they saw wildlife as a commodity to be exploited and implemented an unregulated market for the trade of wildlife. Bison were used for food, belts and furs, skulls were ground up as fertilizer, but most left to rot.
• We overexploited bison to the point of near extinction; 30-60 million in 1820 → 325 in 1884 (genetic bottleneck).
• Along with bison, pronghorn antelope, bighorn sheep, mule deer, elk, beavers, wolves and grizzlies were nearly wiped out.

What issues of environmental externalities and equity does this example of overexploitation pose?

Efforts to save the remaining bison by William Hornaday and even Teddy Roosevelt were rooted in racism as efforts to preserve “the survival of white masculinity”.

Module Review:

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• In this module, we learned that the biodiversity of our planet is declining for a number of reasons.
• The primary causes of this decline are the loss of habitats and the fragmentation of habitats that species need to survive and reproduce.
• Exotic species are being moved around the world with the increased global movement of people and materials.
• Many populations of these exotic species remain small and cause no discernible harm, but some become invasive species that spread quickly and have harmful effects on native species.

Module Review (Continued):

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• Overexploitation of species can cause larger declines in population sizes and, in some cases, extinction.
• Current regulations within and among countries are designed to limit harvesting to sustainable levels, although these regulations are not always successfully enforced.
• Toxic compounds including pesticides, heavy metals, and spilled oil can also be detrimental to species either by direct lethal effects or by altering communities and ecosystems.
• Finally, climate change has the potential to alter populations and the long-term persistence of species, but more time is needed to determine if these predictions will come true.