animal diversity essay

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• A definition of animals
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Parazoa: a cellular level of organization

Radiata: a tissue level of organization, acoelomates.

• Pseudocoelomates, or aschelminths
• Social levels of organization
• Types of skeletons and their distribution
• Translating movement into locomotion and feeding
• The nervous system
• Water/vascular systems
• Reproduction and life cycles
• Competition and animal diversity
• Evolution of ecological roles
• Humans and the environment
• Appearance of animals
• Rise of vertebrates
• Diagnostic features
• Annotated classification
• Critical appraisal

• What are the basic functional systems of animals?
• How are mammals distinct from other animals?
• Why is the platypus a mammal?
• How do fish sleep?
• How do fish hear?

Animal diversity

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• Biology LibreTexts - Animal
• New Hampshire PBS - Wildlife Journal Junior - Animalia
• Animal Diversity Web - Animal
• East Tennessee State University - Basic Principles of Animal Form and Function
• animal - Children's Encyclopedia (Ages 8-11)
• animal - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

The diverse appearance of animals is mostly superficial; the bewildering variety of known forms, some truly bizarre, can be assorted among a mere half-dozen basic body plans. These plans are established during the embryonic stages of development and limit the size and complexity of the animals. Symmetry, number and relative development of tissue layers, presence and nature of body cavities, and several aspects of early development define these fundamental modes of organization.

Although the two phyla in this subkingdom, Porifera (sponges) and Placozoa, lack clearly defined tissues and organs, their cells specialize and integrate their activities. Their simplicity has been adaptive, and sponges have remained important in benthic marine habitats since their origin. The sessile, filter-feeding way of life shown by sponges has favoured a body plan of radial symmetry , although some members have become asymmetrical. The shape of the creeping, flattened placozoans is irregular and changeable.

The two coelenterate phyla ( Cnidaria and Ctenophora ) advanced in complexity beyond the parazoans by developing incipient tissues—groups of cells that are integrally coordinated in the performance of a certain function. For example, coelenterates have well-defined nerve nets, and their contractile fibres, although only specialized parts of more generalized cells, are organized into discrete muscle units. Because discrete cells of different types do not carry out the internal functions of the animals, coelenterates are considered to be organized at only a tissue level.

The integration of cells into tissues, particularly those of nerve and muscle, permits a significantly larger individual body size than is possible with other modes of body movement. Flagella and cilia become ineffective at rather small size, and amoeboid movement is limited to the size a single cell can attain. Muscles contract by a cellular mechanism basically like that used in amoeboid locomotion—interaction of actin and myosin filaments. Through coordinated contraction of many cells, movement of large individuals becomes possible.

Coelenterates, like parazoans, have only two body layers, an inner endoderm primarily for feeding and an outer ectoderm for protection. Between the endoderm and the ectoderm of coelenterates is the mesoglea , a gelatinous mass that contains connective fibres of collagen and usually some cells. Both layers contain muscle fibres and a two-dimensional web of nerve cells at the base; the endoderm surrounds a central cavity, which ranges from simple to complex in shape and serves as a gut, circulatory system , and sometimes even a skeleton. The cavity is also used for gamete dispersal and waste elimination.

Cleavage of a fertilized egg produces a hollow sphere of flagellated cells (the blastula ). Invagination of cells at one or both poles creates a mouthless, solid gastrula; the gastrula is called the planula larva in species in which this stage of development is free-living. The inner, endoderm cells subsequently differentiate to form the lining of the central cavity. The mouth forms once the planula larva has settled. Although the details of early development are different for parazoans and coelenterates, most share a stage in which external flagellated cells invaginate to form the inner layer, which lines the cavity, of these diploblastic (two-layered) animals. This is characteristic of invagination during the development of all animals.

All coelenterates are more or less radially symmetrical. A radial form is equally advantageous for filtering, predatory, or photosynthetic modes of feeding. Tentacles around the circumference can intercept food in all directions.

Bilateria: an organ level of organization

All animals except those in the four phyla mentioned above have bilaterally symmetrical ancestors and contain three body layers (triploblastic) with coalition of tissues into organs. The body plans that are generally recognized are acoelomate, pseudocoelomate, and coelomate.

Acoelomates have no internal fluid-filled body cavity (coelom). Pseudocoelomates have a cavity between the inner (endoderm) and the middle (mesoderm) body layers. Coelomates have a cavity within the mesoderm , which can show one of two types of development: schizocoelous or enterocoelic. Most protostomes show schizocoelous development, in which the mesoderm proliferates from a single cell and divides to form a mass on each side of the body; the coelom arises from a split within each mass. Deuterostomes show enterocoelic pouching, in which the endoderm evaginates and pinches off discrete pouches, the cavities of which become the coelom and the wall the mesoderm. The animals in these major divisions of the Bilateria differ in other fundamental ways, which are detailed below.

Unlike sessile sponges or floating jellyfish , the Bilateria typically move actively in pursuit of food, although many members have further evolved into sessile or radial forms. Directed movement is most efficient if sensory organs are located at the head or forward-moving end of the animal. Organs of locomotion are most efficiently arranged along both sides, a fact that defines the bilateral symmetry; many internal organs are not in fact paired, whereas muscle layers, limbs, and sensory organs almost invariably are. The diffuse nerve net of coelenterates coalesces into definite tracts or bundles, which run posteriorly from the anterior brain to innervate the structures of locomotion.

Flatworms (phyla Platyhelminthes , Nemertea , and Mesozoa) lack a coelom, although nemerteans have a fluid-filled cavity at their anterior, or head, end, which is used to eject the proboscis rapidly. The lack of a fluid-filled cavity adjacent to the muscles reduces the extent to which the muscles can contract and the force they exert ( see below Support and movement ). Because most also lack a circulatory system, supplying muscle tissues with fuel and oxygen can be no faster than the rate at which these substances diffuse through solid tissue. Flatworms are thus constrained to be relatively flat and comparatively small; parasitic worms, which do not locomote, can achieve immense lengths (e.g., tapeworms), but they remain very thin. The larger of the free-living flatworms have extensively divided guts, which reach to within a few cells of the muscles, thus compensating for the lack of a circulatory system. Most flatworms have but one opening to the gut. Nemerteans, in addition to a coelom-like housing for their proboscis, have attained a one-way gut and a closed circulatory system. Both increase their ability to move food and oxygen to all parts of the body. Flatworms are considered to be the ancestors of all other Bilateria.

Pseudocoelomates , or aschelminths

The pseudocoelomates include the nematodes, rotifers, gastrotrichs, and introverts. Some members of some other phyla are also, strictly speaking, pseudocoelomate. These four phyla of tiny body size (many species no larger than the bigger protozoans) are placed together in part because they lack mesoderm on the inner side of the body cavity. Consequently, no tissue, muscular or connective, supports the gut within the coelomic fluid. For tiny organisms, this is advantageous for conservation of tissue: there is no reason to evolve or to maintain a tissue that is not functionally important. The inconspicuousness of most of these phyla has led to a slow advancement in understanding their phylogenetic position in the animal kingdom.

Animal Form and Function

Introduction to animal diversity.

Figure 1. The leaf chameleon (Brookesia micra) was discovered in northern Madagascar in 2012. At just over one inch long, it is the smallest known chameleon. (credit: modification of work by Frank Glaw, et al., PLOS)

Animal evolution began in the ocean over 600 million years ago with tiny creatures that probably do not resemble any living organism today. Since then, animals have evolved into a highly diverse kingdom. Although over one million extant (currently living) species of animals have been identified, scientists are continually discovering more species as they explore ecosystems around the world. The number of extant species is estimated to be between 3 and 30 million.

But what is an animal? While we can easily identify dogs, birds, fish, spiders, and worms as animals, other organisms, such as corals and sponges, are not as easy to classify. Animals vary in complexity—from sea sponges to crickets to chimpanzees—and scientists are faced with the difficult task of classifying them within a unified system. They must identify traits that are common to all animals as well as traits that can be used to distinguish among related groups of animals. The animal classification system characterizes animals based on their anatomy, morphology, evolutionary history, features of embryological development, and genetic makeup. This classification scheme is constantly developing as new information about species arises. Understanding and classifying the great variety of living species help us better understand how to conserve the diversity of life on earth.

• Biology. Authored by : OpenStax. Provided by : OpenStax College. Located at : http://cnx.org/contents/[email protected]:1/Biology . License : CC BY: Attribution

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INTRODUCTION to Diversity of Animals

This bee and  Echinacea  flower could not look more different, yet they are related, as are all living organisms on Earth. By following pathways of similarities and differences—both visible and genetic—scientists seek to map the history of evolution from single-celled organisms to the tremendous diversity of creatures that have crawled, germinated, floated, swam, flown, and walked on this planet.

Section Outline

While we can easily identify dogs, lizards, fish, spiders, and worms as animals, other animals, such as corals and sponges, might be easily mistaken as plants or some other form of life. Yet scientists have recognized a set of common characteristics shared by all animals, including sponges, jellyfish, sea urchins, and humans.

The kingdom Animalia is a group of multicellular Eukarya. Animal evolution began in the ocean over 600 million years ago, with tiny creatures that probably do not resemble any living organism today. Since then, animals have evolved into a highly diverse kingdom. Although over one million currently living species of animals have been identified, scientists are continually discovering more species. The number of described living animal species is estimated to be about 1.9 million, 1  and there may be as many as 6.8 million.

Understanding and classifying the variety of living species helps us to better understand how to conserve and benefit from this diversity. The animal classification system characterizes animals based on their anatomy, features of embryological development, and genetic makeup. Scientists are faced with the task of classifying animals within a system of taxonomy that reflects their evolutionary history. Additionally, they must identify traits that are common to all animals as well as traits that can be used to distinguish among related groups of animals. However, animals vary in the complexity of their organization and exhibit a huge diversity of body forms, so the classification scheme is constantly changing as new information about species is learned.

• 1 “Number of Living Species in Australia and the World,” A.D. Chapman, Australia Biodiversity Information Services, last modified September 28, 2022, https://www.dcceew.gov.au/science-research/abrs/publications/other/numbers-living-species/executive-summary

© OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License. https://openstax.org/books/concepts-biology/pages/1-introduction

Introduction to Living Systems Copyright © by Dr. Becki Brunelli. All Rights Reserved.

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Chapter 32 - an introduction to animal diversity.

Chapter 32 An Introduction to Animal Diversity Lecture Outline

Overview: Welcome to Your Kingdom

• Biologists have identified 1.3 million living species of animals.
• Estimates of the total number of animal species run far higher, from 10 to 20 million to as many as 100 to 200 million.

Concept 32.1 Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers

• There are exceptions to nearly every criterion for distinguishing an animal from other life forms.
• Animals take in preformed organic molecules through ingestion, eating other organisms or organic material that is decomposing.
• The multicellular bodies of animals are held together by extracellular structural proteins, especially collagen.
• Animals have other unique types of intercellular junctions, including tight junctions, desmosomes, and gap junctions, which hold tissues together.
• These junctions are also composed of structural proteins.
• Animals have two unique types of cells: nerve cells for impulse conduction and muscle cells for movement.
• In most species, a small flagellated sperm fertilizes a larger, nonmotile egg.
• The zygote undergoes cleavage, a succession of mitotic cell divisions, leading to the formation of a multicellular, hollow ball of cells called the blastula.
• The resulting development stage is called a gastrula.
• A larva is a sexually immature stage that is morphologically distinct from the adult, usually eats different foods, and may live in a different habitat from the adult.
• Animal larvae eventually undergo metamorphosis, transforming the animal into an adult.
• Animals share a unique homeobox-containing family of genes known as Hox genes.
• Many of these regulatory genes contain common modules of DNA sequences called homeoboxes.
• All animals share the unique family of Hox genes, suggesting that this gene family arose in the eukaryotic lineage that gave rise to animals.
• Hox genes control cell division and differentiation, producing different morphological features of animals.
• Hox genes in sponges regulate the formation of channels, the primary feature of sponge morphology.
• In bilaterians, Hox genes regulate patterning of the anterior-posterior axis.
• The same conserved genetic network governs the development of a large range of animals.

Concept 32.2 The history of animals may span more than a billion years

• Various studies suggest that animals began to diversify more than a billion years ago.
• Some calculations based on molecular clocks estimate that the ancestors of animals diverged from the ancestors of fungi as much as 1.5 billion years ago.
• Similar studies suggest that the common ancestor of living animals lived 1.2 billion to 800 million years ago.

Neoproterozoic Era (1 billion–542 million years ago)

• These fossils are known as the Ediacara fauna, named for the Ediacara Hills of Australia.
• Ediacara fauna consist primarily of cnidarians, but soft-bodied mollusks were also present, and numerous fossilized burrows and tracks indicate the presence of worms.

Paleozoic Era (542–251 million years ago)

• During this period, known as the Cambrian explosion, about half of extant animal phyla arose.
• Fossils of Cambrian animals include the first animals with hard, mineralized skeletons.
• Predators acquired adaptations that helped them catch prey.
• Prey acquired adaptations that helped them resist predation.
• More oxygen may have provided opportunities for animals with higher metabolic rates and larger body sizes.
• The evolution of the Hox complex provided the developmental flexibility that resulted in variations in morphology.
• These hypotheses are not mutually exclusive; all may have played a role.
• Vertebrates (fishes) became the top predators of marine food webs.
• By 460 million years ago, arthropods began to adapt to terrestrial habitats.
• Two of these survive today: amphibians and amniotes.

Mesozoic Era (251–65.5 million years ago)

• Few new animal body plans emerged among animals during the Mesozoic era.
• Animal phyla began to spread into new ecological niches.
• In the oceans, the first coral reefs formed.

Cenozoic Era (65.5 million years ago to the present)

• Insects and flowering plants both underwent a dramatic diversification during the Cenozoic era.
• This era began with mass extinctions of terrestrial and marine animals.
• Among the groups of species that disappeared were large, nonflying dinosaurs and the marine reptiles.
• Large mammalian herbivores and carnivores diversified as mammals exploited vacated ecological niches.
• Our ancestors were among these grassland apes.

Concept 32.3 Animals can be characterized by “body plans”

• Zoologists may categorize the diversity of animals by general features of morphology and development.
• Certain body-plan features shared by a group of animals define a grade.
• Sponges lack symmetry.
• Some animals, such as sea anemones, have radial symmetry.
• A bilateral animal has a dorsal (top) side and a ventral (bottom side), a left and right side, and an anterior (head) end and a posterior (tail) end.
• Cephalization also includes the development of a central nervous system concentrated in the head and extending toward the tail as a longitudinal nerve cord.
• Many radial animals are sessile or planktonic and need to meet the environment equally well from all sides.
• Animals that move actively are generally bilateral.
• Their central nervous system allows them to coordinate complex movements involved in crawling, burrowing, flying, and swimming.
• Sponges lack true tissues.
• In all other animals, the embryo becomes layered through the process of gastrulation.
• Ectoderm, covering the surface of the embryo, gives rise to the outer covering and, in some phyla, to the central nervous system.
• Endoderm, the innermost layer, lines the developing digestive tube, or archenteron, and gives rise to the lining of the digestive tract and the organs derived from it, such as the liver and lungs of vertebrates.
• Animals with only two germ layers, such as cnidarians, are diploblastic.
• In these animals, a third germ layer, the mesoderm, lies between the endoderm and ectoderm.
• The mesoderm develops into the muscles and most other organs between the digestive tube and the outer covering of the animal.
• The inner and outer layers of tissue that surround the coelom connect dorsally and ventrally and form mesenteries that suspend the internal organs.
• Animals that possess a true coelom are known as coelomates.
• Some triploblastic animals have a cavity formed from blastocoel, rather than mesoderm. Such a cavity is a “pseudocoel” and animals that have one are called pseudocoelomates.
• Some animals lack a coelom. These animals are known as acoelomates, and have a solid body without a body cavity.
• Its fluid cushions the internal organs, helping to prevent internal injury.
• The noncompressible fluid of the body cavity can function as a hydrostatic skeleton against which muscles can work.
• The presence of a cavity enables the internal organs to grow and move independently of the outer body wall.
• Current research suggests that true coeloms and pseudocoels have evolved many times in the course of animal evolution.
• Thus, the terms coelomate and pseudocoelomate refer to grades, not clades.
• The differences between these modes of development center on cleavage pattern, coelom formation, and blastopore fate.
• Some protostomes also show determinate cleavage, where the fate of each embryonic cell is determined early in development.
• Most deuterostomes show indeterminate cleavage, whereby each cell in the early embryo retains the capacity to develop into a complete embryo.
• As the archenteron forms in a protostome, solid masses of mesoderm split to form the coelomic cavities, in a pattern called schizocoelous development.
• In deuterostomes, mesoderm buds off from the wall of the archenteron and hollows to become the coelomic cavities, in a pattern called enterocoelous development.
• In many protostomes, the blastopore develops into the mouth, and a second opening at the opposite end of the gastrula develops into the anus.
• In deuterostomes, the blastopore usually develops into the anus, and the mouth is derived from the secondary opening.

Concept 32.4 Leading hypotheses agree on major features of the animal phylogenetic tree

• The relationships between these phyla continue to be debated.
• Traditionally, zoologists have tested hypotheses about animal phylogeny through morphological studies.
• Currently, zoologists also study the molecular systematics of animals.
• New studies of lesser-known phyla and fossil analyses help distinguish between ancestral and derived traits in various animal groups.
• This creates a phylogenetic tree that is a hierarchy of clades nested within larger clades.
• Whether the data are “traditional” morphological characters, “new” molecular sequences, or some combination of the two, the assumptions and inferences inherent in the tree are the same.
• Two current phylogenetic hypotheses can be compared: one based on systematic analyses of morphological characters and the other based on recent molecular studies.
• Both trees indicate that the animal kingdom is monophyletic, representing a clade called Metazoa.
• Sponges branch from the base of both animal trees.
• They exhibit a parazoan grade of organization, without tissues.
• Recent molecular analyses suggest that sponges are paraphyletic.
• All animals except sponges belong to a clade of eumetazoans.
• The common ancestor of living eumetazoans acquired true tissues.
• Bilateral symmetry is a shared derived character that helps to define a clade called the bilaterians.
• The name deuterostome refers to an animal development grade and also to a clade that includes vertebrates.
• The hypotheses also disagree on some significant points, including the relationships among the bilaterians.
• This assumes that these two modes of development reflect a phylogenetic pattern.
• The molecular evidence assigns two sister taxa to the protostomes: the ecdysozoans and the lophotrochozoans.
• As the animal grows, it molts the old exoskeleton and secretes a new, larger one, a process called ecdysis.
• While named for this process, the clade is actually defined mainly by molecular evidence.
• Some animals, such as ectoprocts, develop a lophophore, a horseshoe-shaped crown of ciliated tentacles used for feeding.
• Other phyla, including annelids and mollusks, have a distinctive larval stage called a trochophore larva.
• Animal systematics continues to evolve.
• Systematists are now conducting large-scale analyses of multiple genes across a wide range of animal phyla, in an effort to gain a clearer picture of how the diversity of animal body plans arose.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 32-1

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Conservation

In defense of biodiversity: why protecting species from extinction matters.

By Carl Safina • February 12, 2018

A number of biologists have recently made the argument that extinction is part of evolution and that saving species need not be a conservation priority. But this revisionist thinking shows a lack of understanding of evolution and an ignorance of the natural world. 

A few years ago, I helped lead a ship-based expedition along south Alaska during which several scientists and noted artists documented and made art from the voluminous plastic trash that washes ashore even there. At Katmai National Park, we packed off several tons of trash from as distant as South Asia. But what made Katmai most memorable was: huge brown bears. Mothers and cubs were out on the flats digging clams. Others were snoozing on dunes. Others were patrolling.

During a rest, several of us were sitting on an enormous drift-log, watching one mother who’d been clamming with three cubs. As the tide flooded the flat, we watched in disbelief as she brought her cubs up to where we were sitting — and stepped up on the log we were on. There was no aggression, no tension; she was relaxed. We gave her some room as she paused on the log, and then she took her cubs past us into a sedge meadow. Because she was so calm, I felt no fear. I felt the gift.

In this protected refuge, bears could afford a generous view of humans. Whoever protected this land certainly had my gratitude.

In the early 20th century, a botanist named Robert F. Griggs discovered Katmai’s volcanic “Valley of Ten Thousand Smokes.” In love with the area, he spearheaded efforts to preserve the region’s wonders and wildlife. In 1918, President Woodrow Wilson established Katmai National Monument (now Katmai National Park and Preserve ), protecting 1,700 square miles, thus ensuring a home for bear cubs born a century later, and making possible my indelible experience that day. As a legacy for Griggs’ proclivity to share his love of living things, George Washington University later established the Robert F. Griggs Chair in Biology.

That chair is now occupied by a young professor whose recent writing probably has Griggs spinning in his grave. He is R. Alexander Pyron . A few months ago,  The Washington Post published a “ Perspective” piece by Pyron that is an extreme example of a growing minority opinion in the conservation community, one that might be summarized as, “Humans are profoundly altering the planet, so let’s just make peace with the degradation of the natural world.” 

No biologist is entitled to butcher the scientific fundamentals on which they hang their opinions.

Pyron’s essay – with lines such as, “The only reason we should conserve biodiversity is for ourselves, to create a stable future for human beings” and “[T]he impulse to conserve for conservation’s sake has taken on an unthinking, unsupported, unnecessary urgency” – left the impression that it was written in a conservative think tank, perhaps by one of the anti-regulatory zealots now filling posts throughout the Trump administration. Pyron’s sentiments weren’t merely oddly out of keeping with the legacy of the man whose name graces his job title. Much of what Pyron wrote is scientifically inaccurate. And where he stepped out of his field into ethics, what he wrote was conceptually confused.

Pyron has since posted, on his website and Facebook page, 1,100 words of frantic backpedaling that land somewhere between apology and retraction, including mea culpas that he “sensationalized” parts of his own argument and “cavalierly glossed over several complex issues.” But Pyron’s original essay and his muddled apology do not change the fact that the beliefs he expressed reflect a disturbing trend that has taken hold among segments of the conservation community. And his article comes at a time when conservation is being assailed from other quarters, with a half-century of federal protections of land being rolled back, the Endangered Species Act now more endangered than ever, and the relationship between extinction and evolution being subjected to confused, book-length mistreatment.

Pyron’s original opinion piece, so clear and unequivocal in its assertions, is a good place to unpack and disentangle accelerating misconceptions about the “desirability” of extinction that are starting to pop up like hallucinogenic mushrooms.

In recent years, some biologists and writers have been distancing themselves from conservation’s bedrock idea that in an increasingly human-dominated world we must find ways to protect and perpetuate natural beauty, wild places, and the living endowment of the planet. In their stead, we are offered visions of human-dominated landscapes in which the stresses of destruction and fragmentation spur evolution. 

White rhinoceros ( Ceratotherium simum ). Source: Herman Pijpers/ Flickr

Conservation International ditched its exuberant tropical forest graphic for  a new corporate logo  whose circle and line were designed to suggest a human head and outstretched arms. A few years ago, Peter Kareiva, then chief scientist for The Nature Conservancy,  said , “conservationists will have to jettison their idealized notions of nature, parks, and wilderness,” for  “a more optimistic, human-friendly vision.” Human annihilation of the passenger pigeon, he wrote, caused “no catastrophic or even measurable effects,” characterizing the total extinction of the hemisphere’s most abundant bird — whose population went from billions to zero inside a century (certainly a “measurable effect” in itself) — as an example of nature’s “resilience.”

British ecologist Chris Thomas’s recent book, Inheritors of the Earth: How Nature is Thriving in an Age of Extinction, argues that the destruction of nature creates opportunities for evolution of new lifeforms that counterbalance any losses we create, an idea that is certainly optimistic considering the burgeoning lists of endangered species. Are we really ready to consider that disappearing rhinos are somehow counterbalanced by a new subspecies of daisy in a railroad track? Maybe it would be simpler if Thomas and his comrades just said, “We don’t care about nature.’’

Enter Pyron, who — at least in his initial essay — basically said he doesn’t. He’s entitled to his apathy, but no biologist is entitled to butcher the scientific fundamentals on which they hang their opinions.

Pyron began with a resonant story about his nocturnal rediscovery of a South American frog that had been thought recently extinct. He and colleagues collected several that, he reassured us, “are now breeding safely in captivity.” As we breathed a sigh of relief, Pyron added, “But they will go extinct one day, and the world will be none the poorer for it.” 

The conviction that today’s slides toward mass extinction are not inevitable spurred the founding of the conservation movement.

I happen to be writing this in the Peruvian Amazon, having just returned from a night walk to a light-trap where I helped a biologist collect moths. No one yet knows how many species live here. Moths are important pollinators. Knowing them helps detangle a little bit of how this rainforest works. So it’s a good night to mention that the number of species in an area carries the technical term “species richness.” More is richer, and fewer is, indeed, poorer. Pyron’s view lies outside scientific consensus and societal values. 

Pyron wasn’t concerned about his frogs going extinct, because, “Eventually, they will be replaced by a dozen or a hundred new species that evolve later.” But the timescale would be millennia at best — meaningless in human terms — and perhaps never; hundreds of amphibians worldwide are suffering declines and extinctions, raising the possibility that major lineages and whole groups of species will vanish. Pyron seemed to have no concerns about that possibility, writing, “Mass extinctions periodically wipe out up to 95 percent of all species in one fell swoop; these come every 50 million to 100 million years.”

But that’s misleading. “Periodically” implies regularity. There’s no regularity to mass extinctions. Not in their timing, nor in their causes. The mass extinctions are not related. Three causes of mass extinctions — prolonged worldwide atmosphere-altering volcanic eruptions; a dinosaur-snuffing asteroid hit; and the spreading agriculture, settlement, and sheer human appetite driving extinctions today — are unrelated.

Rio Pescado stubfoot toad ( Atelopus balios ). Source: De Investigación y Conservación de Anfibios/ Flickr

The conviction that today’s slides toward mass extinction are not inevitable, and could be lessened or avoided, spurred the founding of the conservation movement and created the discipline of conservation biology.

But Pyron seems unmoved. “Extinction is the engine of evolution, the mechanism by which natural selection prunes the poorly adapted and allows the hardiest to flourish,” he declared. “Species constantly go extinct, and every species that is alive today will one day follow suit. There is no such thing as an ‘endangered species,’ except for all species.”

Let us unpack. Extinction is not evolution’s driver; survival is. The engine of evolution is survival amidst competition. It’s a little like what drives innovation in business. To see this, let’s simply compare the species diversity of the Northern Hemisphere, where periodic ice sheets largely wiped the slate clean, with those of the tropics, where the evolutionary time clock continued running throughout. A couple of acres in eastern temperate North America might have a dozen tree species or fewer. In the Amazon a similar area can have 300 tree species. All of North American has 1,400 species of trees; Brazil has 8,800. All of North America has just over 900 birds; Colombia has 1,900 species. All of North America has 722 butterfly species. Where I am right now, along the Tambopata River in Peru, biologists have tallied around 1,200 butterfly species.

Competition among living species drives proliferation into diversified specialties. Specialists increasingly exploit narrowing niches. We can think of this as a marketplace of life, where little competition necessitates little specialization, thus little proliferation. An area with many types of trees, for instance, directly causes the evolution of many types of highly specialized pollinating insects, hummingbirds, and pollinating bats, who visit only the “right” trees. Many flowering plants are pollinated by just one specialized species.

Pyron muddles several kinds of extinctions, then serves up further misunderstanding of how evolution works. So let’s clarify. Mass extinctions are global; they involve the whole planet. There have been five mass extinctions and we’ve created a sixth . Past mass extinctions happened when the entire planet became more hostile. Regional wipeouts, as occurred during the ice ages, are not considered mass extinctions, even though many species can go extinct. Even without these major upheavals there are always a few species blinking out due to environmental changes or new competitors. And there are pseudo-extinctions where old forms no longer exist, but only because their descendants have changed through time. 

New species do not suddenly “arise,” nor are they really new. They evolve from existing species, as population gene pools change.

Crucially for understanding the relationship between extinction and evolution is this: New species do not suddenly “arise,” nor are they really new. New species evolve from existing species, as population gene pools change. Many “extinct” species never really died out; they just changed into what lives now. Not all the dinosaurs went extinct; theropod dinosaurs survived. They no longer exist because they evolved into what we call birds. Australopithecines no longer exist, but they did not all go extinct. Their children morphed into the genus Homo, and the tool- and fire-making Homo erectus may well have survived to become us. If they indeed are our direct ancestor — as some species was — they are gone now, but no more “extinct” than our own childhood. All species come from ancestors, in lineages that have survived.

Pyron’s contention that the “hardiest” flourish is a common misconception. A sloth needs to be slow; a faster sloth is going to wind up as dinner in a harpy eagle nest. A white bear is not “hardier” than a brown one; the same white fur that provides camouflage in a snowy place will scare away prey in green meadow. Bears with genes for white fur flourished in the Arctic, while brown bears did well amidst tundra and forests. Polar bears evolved from brown bears of the tundra; they got so specialized that they separated, then specialized further. Becoming a species is a process, not an event. “New” species are simply specialized descendants of old species.

True extinctions beget nothing. Humans have recently sped the extinction rate by about a thousand times compared to the fossil record. The fact that the extinction of dinosaurs was followed, over tens of millions of years, by a proliferation of mammals, is irrelevant to present-day decisions about rhinos, elephant populations, or monarch butterflies. Pyron’s statement, “There is no such thing as an ‘endangered species,’ except for all species,” is like saying there are no endangered children except for all children. It’s like answering “Black lives matter” with “All lives matter.” It’s a way of intentionally missing the point. 

Chestnut-sided warbler ( Setophaga pensylvanica ). Source: Francesco Veronesi/ Wikimedia

Here’s the point: All life today represents non-extinctions; each species, every living individual, is part of a lineage that has not gone extinct in a billion years.

Pyron also expressed the opinion that “the only reason we should conserve biodiversity is for ourselves …” I don’t know of another biologist who shares this opinion. Pyron’s statement makes little practical sense, because reducing the diversity and abundance of the living world will rob human generations of choices, as values change. Save the passenger pigeon? Too late for that. Whales? A few people acted in time to keep most of them. Elephants? Our descendants will either revile or revere us for what we do while we have the planet’s reins in our hands for a few minutes. We are each newly arrived and temporary tourists on this planet, yet we find ourselves custodians of the world for all people yet unborn. A little humility, and forbearance, might comport.

Thus Pyron’s most jarring assertion: “Extinction does not carry moral significance, even when we have caused it.” That statement is a stranger to thousands of years of philosophy on moral agency and reveals an ignorance of human moral thinking. Moral agency issues from an ability to consider consequences. Humans are the species most capable of such consideration. Thus many philosophers consider humans the only creatures capable of acting as moral agents. An asteroid strike, despite its consequences, has no moral significance. Protecting bears by declaring Katmai National Monument, or un-protecting Bears Ears National Monument, are acts of moral agency. Ending genetic lineages millions of years old, either actively or by the willful neglect that Pyron advocates, certainly qualifies as morally significant.

Do we really wish a world with only what we “rely on for food and shelter?” Do animals have no value if we don’t eat them?

How can we even decide which species we “directly depend’’ upon? We don’t directly depend on peacocks or housecats, leopards or leopard frogs, humpback whales or hummingbirds or chestnut-sided warblers or millions of others. Do we really wish a world with only what we “rely on for food and shelter,” as Pyron seemed to advocate? Do animals have no value if we don’t eat them? I happen not to view my dogs as food, for instance. Things we “rely on” make life possible, sure, but the things we don’t need make life worthwhile.

When Pyron wrote, “Conservation is needed for ourselves and only ourselves… If this means fewer dazzling species, fewer unspoiled forests, less untamed wilderness, so be it,” he expressed a dereliction of the love, fascination, and perspective that motivates the practice of biology.

Here is a real biologist, Alfred Russell Wallace, co-discoverer of evolution by natural selection:

I thought of the long ages of the past during which the successive generations of these things of beauty had run their course … with no intelligent eye to gaze upon their loveliness, to all appearances such a wanton waste of beauty… . This consideration must surely tell us that all living things were not made for man… . Their happiness and enjoyments, their loves and hates, their struggles for existence, their vigorous life and early death, would seem to be immediately related to their own well-being and perpetuation alone. —The Malay Archipelago, 1869

At the opposite pole of Wallace’s human insight and wonder, Pyron asked us to become complicit in extinction. “The goals of species conservation have to be aligned with the acceptance that large numbers of animals will go extinct,” he asserted. “Thirty to 40 percent of species may be  threatened  with extinction in the near future, and their loss may be inevitable. But both the planet and humanity can probably survive or even thrive in a world with fewer species … The species that we rely on for food and shelter are a tiny proportion of total biodiversity, and most humans live in — and rely on — areas of only moderate biodiversity, not the Amazon or the Congo Basin.”

African elephant ( Loxodonta africana ). Source: Flowcomm/ Flickr

Right now, in the Amazon as I type, listening to nocturnal birds and bugs and frogs in this towering emerald cathedral of life, thinking such as Pyron’s strikes me as failing to grasp both the living world and the human spirit. 

The massive destruction that Pyron seems to so cavalierly accept isn’t necessary. When I was a kid, there were no ospreys, no bald eagles, no peregrine falcons left around New York City and Long Island where I lived. DDT and other hard pesticides were erasing them from the world. A small handful of passionate people sued to get those pesticides banned, others began breeding captive falcons for later release, and one biologist brought osprey eggs to nests of toxically infertile parents to keep faltering populations on life support. These projects succeeded. All three of these species have recovered spectacularly and now again nest near my Long Island home. Extinction wasn’t a cost of progress; it was an unnecessary cost of carelessness. Humans could work around the needs of these birds, and these creatures could exist around development. But it took some thinking, some hard work, and some tinkering.

It’s not that anyone thinks humans have not greatly changed the world, or will stop changing it. Rather, as the great wildlife ecologist Aldo Leopold wrote in his 1949 classic A Sand County Almanac , “To keep every cog and wheel is the first precaution of intelligent tinkering.”

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Animal Diversity

A section of Diversity (ISSN 1424-2818).

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This section deals with all aspects of animal diversity, from evolution, to phylogeny, adaptation, and conservation, from the species up to community interactions with ecosystems. As Diversity is a generalist journal, specialized or local studies will be evaluated only if discussion leads to/illustrates general questions/conclusions in relation with diversity. Experimental and field studies are favored, but theoretical studies will be considered if dealing with an original approach to evaluate animal diversity.

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