A Brief Background of Modern Biodiversity

Biodiversity is all aspects of variety in the living world.  Most of the time when people use the term, they are actually referring to Species Richness, the number of species in an area.  Although species richness is part of biodiversity, biodiversity also encompasses genetic and habitat diversity. 

Genetic Diversity (variation) is the differences that exist among individual in the alleles they carry.  Genetic diversity is responsible for the differences we see in phenotypic traits or characteristics between organisms in a population.  (Review section 2 of the course if you don’t recall these details).

Habitat Diversity is the number of different habitats in an area.  Habitat is where animals and plants get their food, water, shelter, and find mates, basically where they live.  Different types of habitats, harbor different types of species because, as we saw in the biogeography lesson, species are specialized to different kinds of environmental conditions (niches).  

Global species richness is estimated to be 8.7 million species, but we have only named 1.95 million species.  We know there is a lot more species out there than what we have named because we continue to find new species at a high rate. If we were close to having named all the species on earth, it would be extremely rare to find a new species.  There are many different techniques we can use to estimate the total number of species on the planet.  They all indicate there is a huge number of species still to be discovered.  Scientists estimate that describing all remaining species would take more than 300,000 taxonomists up to 1,200 years of work and cost $US 364 billion.

Unfortunately, populations of plants and animals are in decline and some species have already gone extinct.  The size of vertebrate populations has declined by 60% on average since 1970 and a large number of species are now considered to be at risk of extinction.  Extinction is when the last individual of a species dies without reproducing, eliminating the unique genetic code of that species forever. 

The International Union for the Conservation of Nature (IUCN) is tasked with assessing extinction risk.  It is a very time and data intensive endeavor.  For most of the world’s species we do not have the data to make adequate assessments.  The IUCN has assessed almost 116,000 species and determined that 31,000 species are at risk of extinction.  This is 27% of assessed species!  Other assessments indicate:

• We could lose one million species from climate change alone this century

• 500,000 species have insufficient habitat for long term survival

• 25-50% of all species could go extinct this century

The IUCN estimates for specific groups are:

 

When we examine the causes of population declines and species extinctions, they are the result of the human activities we outlined last week:  a growing human population, an increase in average resource use per person, and rapidly advancing technologies (I=PxAxT) that have resulted in:

• Habitat Destruction

• Overexploitation

• Non-native Species

• Global Climate Change

• Ocean Acidification

• Toxic and Nutrient Pollution

See the Summary Below of Some Statistics on the World's Resources

 

 

Case Studies of Human Impacts on the Natural World

Habitat Destruction:  The Lemurs of Madagascar

 

 

Lemurs are endemic to the island of Madagascar.  They live there and no where else in the world.  There are 111 species and subspecies of lemurs, 105 of which are threatened with extinction.  They are hunted as food, but the primary factor in their decline has been habitat destruction.  Almost 90% of the original forest cover of the island has been removed for farmland and firewood. I took the picture below from the window of a plane while flying from central Madagascar to Western Madagascar.

 

Some people might say who cares? What’s so special about lemurs? Some interesting things about lemurs are:

• Madame Bertha’s Mouse Lemur is the smallest primate in the world (1.2 oz)

• A recently extinct lemur was 350 pounds, about the size of a male gorilla

• Black lemurs are the only primate species other than humans known to have blue eyes

• The black and white ruffed lemur is the primary pollinator and seed disperser for an endemic palm species, the Traveler’s Palm

• Male ring-tailed lemurs use jump fights and stink fights to compete for breeding. Ring-tail lemur scent contain at least 203 different chemical compounds.

Watch the Lemur Stink Fight Below

 

Does it matter if lemurs go extinct?  That’s something you have to decide.

Overexploitation: Savannah Elephants in Africa

 

African elephants are primarily killed for their tusks, a type of modified tooth.  The ivory is used to make jewelry and home decorations. Their populations were estimated to be over 12 Million in 1800.  Today, there are only about 300,000.  Between 2007 and 2014, 30% of Africa’s Savannah Elephants were killed.  On average 55 African elephants are killed each day!

 

Some people might say who cares? What so special about elephants? Some interesting things about elephants are:

• They have 3 sets of molars that erupt sequentially

• Males are usually solitary and generally don’t breed until age 30

• Females live in herds led by a matriarch

• They are the largest living land animal

• Their closest living relatives are Hyraxes and Dugongs

 

Does it matter if elephants go extinct?  That’s something you have to decide.

Marine animals are also at risk from over exploitation. 100 Million sharks a year are killed by humans, primarily as by-catch (accidental catches while fishing for some other species) or for their fins.

Non-native Species: The Brown Tree Snake on Guam

The brown tree snake was introduced to the island of Guam accidentally in the 1950’s.  The song birds of Guam did not evolve with a native snake predator on the island, so did not have the adaptions to avoid predation by this introduced snake.  As the brown tree snake reproduced and spread across the island, the native song birds declined as indicated in the graphs below developed from survey data:

 

 

 

Does it matter if a few bird species on some remote island go extinct?  That’s something you have to decide.

We will discuss global climate change and ocean acidification in detail in next week’s lessons.

Why Should We Care About Extinctions?

Scientists are concerned about species going extinct for a variety of reasons. 

• When we lose a species, we lose the unique genetic code that allows its development and functioning, and we lose any species that may have evolved form that species in the future through speciation events.

• Species in ecosystems have evolved dependencies on each other: predator and prey, mutualists, parasites and hosts, herbivores and the plants they feed on. When a species goes extinct, other species may go extinct as a result of the loss of the species they depended on.  When a species goes extinct, not because humans did something directly to it, but because humans drove to extinction a species that they depended on, we call these extinction Secondary Extinctions.  This is concerning because it creates a domino effect.  The more species humans drive to extinction the worse secondary extinctions get, so eventually we could see ecosystems collapsing as a result of these chain reactions of extinctions.

• Humans depend on natural systems for a whole host of resources and services: food, pollination of crops, oxygen production, medicines, recreation, etc. The higher the rate of extinction, the faster the rate of loss of these benefits that nature provides.  We do not know how many species can be lost before humans are significantly affected, but the prospect of the loss of a quarter of the world’s species by the end of the century is concerning.

Why Should We Care About Population Declines?

We focus a lot on the extinction of species because extinction is irreversible, and it is one metric we can use to measure human impacts on the planet, however, this does not mean that we should not be concerned about population declines. Think about elephants.  300,000 elephants may seem like a lot to us, but if we consider that there were over 12 Million elephants 200 years ago, that means the species population size has decreased by 97.5% in 200 years.  This would be equivalent to the human population dropping from 7.8 Billion to 195 Million.  We should be concerned about population declines even if species' population numbers seem high to us or the species does not seem to be in imminent danger of extinction becasuse:

• As population sizes decline, we lose genetic diversity.

  • Genetic diversity affects a species ability to respond to environmental change through adaptation (review section two if this is unclear). As humans cause significant change on the planet through all the means we have previously discussed, it becomes harder for species to respond to these changes because of the loss of genetic diversity.

  • The greater the genetic diversity within a population, the more different species that could descend from a species through speciation events in the future over time because there are more different traits for evolutions to work with.

  • The lower the genetic diversity, the more similar individuals in a population are. Therefore, it is more likely that genetic problems caused by homozygous recessive genotypes will express and cause further declines in the population.

  • Loss of genetic diversity results in a loss of traits in a population. The fewer different traits in a population the more reduced the human experience is as we interact with these species.  With fewer natural compounds being produced by the species, there are fewer potential medicines that could be discovered.  With less differences between individuals, there are fewer differences in the photographs of animals people take, or the behaviors they can witness, etc.

• A species may become functionally extinct. Species fill roles with in their environment.  As a population declines, its numbers may get so low they can’t fill their role anymore even if they are not extinct.  The American Chestnut was one of the most common tree species in the temperate deciduous forests of North America. In 1904, humans accidentally introduced a non-native fungus from China that killed off most of the American Chestnuts.  They are not extinct.  Their numbers are just drastically reduced.  There were seven moth species that depended on the American Chestnut for their survival.  All seven of these moth species went extinct when the chestnut declined.  The functional extinction of the American Chestnut from the introduction of a non-native disease resulted in a series of secondary extinctions.

• If a population is declining, there is something causing that decline. If the cause of the decline is not recognized and addressed, the population will continue to decline and the ultimate outcome will be extinction.

• Small populations are more susceptible to extinction than large populations. There are a variety of reasons why this is true, but two interesting ones are inbreeding and the Allee effect.

  • Inbreeding is when close relatives breed with each other at a higher rate than would be expected in a large randomly breeding population. As population size declines, there are a limited number of breeding partners so pretty soon every individual is related to every other individual through some recent ancestor (great grandparent, grandparent, etc.).  With so few individuals to carry different alleles, there is a limited number of alleles that keep getting exchanged through breeding in the population.  If there is a negative version of a gene that is recessive, it expresses more often because homozygous recessive genotypes occur at a higher rate since all individuals are closely related to each other and genetic diversity is low.  These individuals will be less successful at growing, surviving, and reproducing, so the population size will decline further.

  • The Allee Effect is when reproductive success declines as population size declines. This is a problem because when population size is low, you want high reproductive success so that the population can quickly recover and not go extinct.   For species suffering from the Allee Effect, their low reproductive rates make them more likely to go extinct before population numbers can increase. 

    • Broadcast spawners, like the black abalone are good examples of this.

 

• • • In broadcast spawners, the males release sperm into the water around them and the females release eggs into the water around them.  The water mixes up the sperm and the eggs resulting in fertilization of the eggs.  This works well if there are many males and females in close proximity to each other as in the picture above.  However, as the population declines, the distance between individuals increases, which means the likelihood of a sperm contacting an egg in the water column decrease and reproductive success declines.

Because of all these negative consequences for small populations, we want to protect species before their numbers get too low.  We want to make sure that there are enough individuals to ensure the long term survival of a species. 

• • • The minimum number of individuals necessary to ensure the long term survival of a species is termed the Minimal Viable Population.  We determine the minimal viable population by using stochastic computer models. We measure what resources a species needs, the variability of these resources through time, and the variability of birthrate and death rates through time for this species, then program these variables into our model allowing them to vary at random.  We run the models a very large number of times and get out predicted probabilities of survival for a certain period of time based on certain population sizes.  We might find that 1000 individuals of our species of interest would ensure an 80% chance of survival for 50 years.  If we feel like this risk is too high, then we could look at other numbers, maybe 5000 individuals will result in a 95% probability of survival for 100 years.  If we’re comfortable with this, we can find the average area an individual needs to secure its resources (its home range) and multiply this area times 5000. We now know how much land we need to protect in a nature preserve (national park) to ensure the long term survival of this species.  We call this the minimum dynamic area.  However, we might look at the amount of land required and the resources that we would have to forgo to protect these 5000 individuals and decide as a society that we are not willing to commit that amount of resource and will accept the risks of only protecting a thousand.

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A Brief Overview of the Fossil Record

We ended our last lesson discussing human impacts on natural systems, focusing on the endangerment and extinction of species.  We saw that as a result of the growing human population, increases in average resource use per person, and advancements in technology, species are in decline from overexploitation, habitat destruction, non-native species, climate change, ocean acidification, and pollution.

We may lose 25-50% of all species by the end of this century.  Is this normal, or is something unusual happening as a result of the way humans are interacting with the planet?  To understand the significance of these extinctions, we have to place what is happening today into the context of life on the planet before humans evolved.  That is our goal today.

Life on the planet is more than 3.5 billion years old.  For most of the existence of life on the planet, life was single-celled organisms.   Multi-cellular life, where organisms are made of a large number of specialized cells interacting with and dependent on each other to make one living organism, evolved about 650 million years ago and was well developed by 550 million year ago.  This is the beginning of a world that would be more recognizable to us as a multi-cellular organism surrounded by a planet filled with multi-cellular organisms (although if you always wore microscopes in front of your eyes, you would recognize that the world today is still well populated by single-celled species also).

Watch This Video For a Brief Overview of Life on the Planet

 

 

Mass Extinction and Adaptive Radiation

 

The graph above depicts the history of life on the planet at the family level (remember there are often many species, in many genera, in each family).  This is a conservative way of looking at the fossil record.  If it is confusing to you, you can think of this graph as depicting the increase and decrease of species on the planet over time, but if it was actually species, rather than families, the peaks of the graph would be higher and the valleys lower. 

When multicellular life originated (about 650 Million years ago), it was a new way of living in the world that gave species opportunities that were not available to their single-celled ancestors.  As multi-cellular species evolved, mutation, natural selection, sexual selection, and genetic drift resulted in more new traits and strategies for living that allowed multicellular life to develop into more and more species over time. This continued for about 200 million years with biodiversity of multicellular species generally increasing until about 444 million years ago when something strange happened.  444 million years ago a Mass Extinction occurred (the End-Ordovician Extinction).  A mass extinction is a significant decline in biodiversity (usually around 70% or more of all species) that occurs in a relatively short period of time (relative to the 650 million year history for multicellular life on the planet) is global in extent (affects species around the entire planet) and cuts across taxonomic lines (affects species in different Kingdoms, Phyla, etc).  This was the first of five mass extinction events that occurred in the history of multicellular life on the planet before humans evolved.

Following the End-Ordovician Extinction, the number of species on the planet recovered (re-diversified) over millions of years.  This happened because all the species that went extinct left open the roles they filled in the environment (niches).  The species that did not go extinct speciated over time, filling the roles (niches) left open by the species that went extinct.  Mutation, natural selection, sexual selection, and genetic drift occurred within the surviving lineages driving this diversification.  This is Adaptive Radiation writ large.  Instead of a single species encountering a few open opportunities that it can speciate into on an island (like the Galapagos Finches or Caribbean Anoles), a large number of ancestral species (whatever did not go extinct) encountered large numbers of open opportunities across the entire planet.  What life looked like before the mass extinction and what it looked like after was different because all species could trace their ancestry only to those that survived the End-Ordovician Extinction. 

This pattern of extinction and radiation was repeated four more times before humans evolved on the planet.  Each time huge numbers of species were wiped out across the entire globe, and large numbers of new species arose over time as the surviving species radiated, giving rise to a diversity of ancestral species.  Each time this happened, what life looked like after the event was different than what it looked like before, because all species could trace their ancestry back to only those species that survived the extinction event, and the genes that they carried.  The species that went extinct left no ancestors.

 

Think about dinosaurs.  They dominated the roles of large organisms in ecosystems for over 130 million years.  Brontosaurs eating plants; Tyranosaurs eating land herbivores; Plesiosaurs (actually a marine reptile) eating fish; etc; but after the end-cretaceous event (see below), only birds (a dinosaur lineage) survived to produce ancestral species.  Previous to the end-cretaceous event, mammals existed, but they were not the large dominant species in ecosystems. Those rolls were filled by dinosaurs.  After the end-cretaceous extinction, the mammal species that survived, radiated to fill the roles left open by the dinosaur extinctions. Today the species filling the roles of major large organisms in ecosystems are mammals (and a few flightless bird species); Zebras eating plants; Lions feeding on land herbivores; Sea lions eating fish; etc. (By the way, sharks evolved about 450 million years ago and survived all of these events.)  If an asteroid had not smashed into the planet 66 million years ago, in all likelihood, dinosaurs would still dominate the world’s ecosystems and zebra, lions, sea lions, and humans would never have existed. 

This pattern of radiation, mass extinction, radiation, mass extinction, was repeated many times over the last 650 million years.  Each time the course of evolution was altered as the species that went extinct left no descendants.  All future life evolved only from those individuals (and the genes they carried) of those species that survived the mass extinction event.

During periods between mass extinctions, species still went extinct, sometimes at slightly higher rates, sometimes at slightly lower rates, but the rate of speciation was usually enough to compensate for these background extinctions.  Scientists have estimated from the fossil record that the background (or historical) extinction rate is one species per million species on the planet per year.  If there were 8 million species on the planet, we would expect 8 species a year to go extinct under normal circumstances.

We can also look at the history of life through extinction rates (how many families went extinct in each million year period -- see the chart that follows) rather than the number of extant (existing) families (see above).  Explore This Interactive Chart. (Links to an external site.)  You’ll see the extinction rate spike during each of the five mass extinction events.

The End-Permian and End-Cretaceous Events

We are going to focus on two of these mass extinction events because these are the ones that scientists know the most about.  However, when I say scientists know the most about these, recognize the difficulties of putting a cause to an event that took place 66 million years ago in the case of the End-Cretaceous event or 252 million years ago in the case of the End-Permian event  (Note: I use End-Permian and End-Cretaceous to name these events, but there is a diversity of names that you might see for the same event, for instance K-T event was previously in vogue for the End-Cretaceous event, and different terms seem to come in and out of fashion regularly). Geological forces over time obscure the evidence of mass extinction events.  For instance, a continental plate may subduct under another plate, carrying evidence into the mantle of the earth where it is melted and destroyed.  Also, the data we have on mass extinction events isn’t consistent.  Many times, we have a better record of what marine species/families went extinct than we do of what land species/families went extinct. 

In the sciences, we are always collecting more data and having to re-evaluate what we know in light of the new data.  This is one of the really powerful things about the sciences that allows the advancement of human knowledge, but when the data is sparse or the interpretation less clear, we will see different opinions amongst scientists on the explanation of a particular phenomenon.  This is true of scientists who study mass extinctions.

With all of this in mind, let’s look into the causes of historical mass extinction events. 

The End-Permian extinction occurred about 252 million years ago. This is the mass extinction event we learned about in the “Your Inner Reptile” video we watched in a previous lesson.  The End-Permian extinction was the most significant of the five historical mass extinction events.  Estimates are that over 95% of all the species on the planet at that time disappeared.  Although there is some difference of opinion on this, the End-Permian Extinction seems to have been a more protracted event than the End-Cretaceous event.  The current leading hypothesis for the cause is prolonged volcanic activity that burned enormous amounts of coal, releasing greenhouse gasses and sulfur dioxide into the atmosphere.  This set off a series of changes on the planet that included climate change and ocean acidification.  This is disconcerting because these are similar consequences to what we see today from burning fossil fuels.

The End-Cretaceous extinction appears to have been a more rapid extinction event with some estimates indicating that the bulk of the dying occurred in six months or less.  It is well established that an asteroid impacted the planet at this time and was a contributing factor, if not the major driver of this mass extinction.  An asteroid about 6.2 miles across smashed into what is now the Gulf of Mexico off the Yucatan Peninsula.  This would have ejected rock though the atmosphere that would have burned up as it re-entered (like shooting stars).  So much debris burned up in the atmosphere, that it super-heated the planet for a period of time.  Molten rock resulted in wildfires globally, and between the smoke from the fires and particulate matter ejected by the impact, sunlight would have been blocked out of the atmosphere, resulting in a crash in energy capture through photosynthesis.  As plants declined, so did the herbivores that fed on them, and the carnivores that fed on those.  It has been estimated that nothing over the size of a large house cat survived on land.  What did survive may have been able to burrow and hibernate to make it through these difficult times.

Click Here to Watch a Video on the End-Cretaceous Event (Links to an external site.)

 

Context from the Fossil Record

From examining the fossil record, we see that extinctions are not unusual. From origin to extinction, the average life span of a species is between 1 million and 4 million year.  We expect one species per million to go extinct every year.  However, mass extinction events are unusual.  We have only seen 5 in the 650 million years of multi-cellular life on the planet.  They are caused by major physical disruptions on the planet that are so rare, and so severe, they are beyond the ability of any species to adapt to.  Although we have focused on a narrow set of causes for each event we examined, a growing number of scientists think that mass extinctions are the result of a convergence of multiple severe and unusual events.

In the fossil record, we see that at least a small percentage of species have always survived through mass extinction events, and these have become the progenitors of all species to come.  Remember that when a species goes extinct, we lose the unique genetic code that coded for it, and it leaves no descendant species.  Also, for those species that do survive, they will have experienced significant population declines resulting in declines in genetic diversity.  It is from this small percentage of species with their reduced genetic diversity that all species in the future descend.  Adaptive radiation allows re-speciation of the planet, but it can take tens of millions of years for biodiversity to bounce back to what it was before the mass extinction and what comes back is different than what existed before the event.  Mass extinctions change the course of evolution.

Watch This Video on Post Extinction Adaptive Radiations and the Formation of Modern Fauna

 

The Sixth Mass Extinction

When we look at the rate of extinction today, it is estimated to be 1000 times higher than the background (historical) extinction rate. We have 8.7 million species on the planet.  We would expect one species per million to go extinct each year under normal circumstances.  This equates to an expected extinction rate of 8.7 species per year if humans were not on the planet.  Scientists estimate that we are actually losing greater than 5000 species a year.  In addition, the rate of extinction is accelerating.

 

When we look at the causes of these extinctions, they are the result of human activities: overexploitation, habitat destruction, non-native species, climate change, ocean acidification, and pollution that result from the growing human population, the increase in average use per person, and the advancement of technology.  Scientists predict that we will lose 25 to 50% of all the species on the planet by 2100 and possibly 70% over the next several centuries.  This level of extinction is equivalent to what we have seen in previous mass extinction events. We are in the early stages of the Sixth Mass Extinction.

One could say, what does it matter, mass extinctions have occurred before and life has always rebounded.  This is true.  It is extremely unlikely that humans would cause all life to go extinct and, given enough time, if the planet were to recover from the damage we have done, life would re-radiate.

Even so, humans should be concerned about this because we are dependent on nature for a diversity of products and services.  The biodiversity we lose will not rebound in any meaningful time frame.  We have existed as a species for only about 195,000 years and biodiversity could take 10’s of millions of years to recover to its pre-sixth extinction level.  Can humans survive this mass extinction event?  We have no idea, but scientists are concerned!  Humans have never lived in the kind of world we are creating, 

This mass extinction is similar in many ways to what we have seen in the past.  A significant decline in biodiversity that is global in extent, occurs in a short period of time (several 100 years for the bulk of it), and cuts across taxonomic lines.  It is the product of major disruptions in the life support system of the planet. 

However, it is different than other mass extinctions, as well. For the first time, a single species is driving the mass extinction: humans.  We are a species that knows what we are doing and what many of the consequences will be.  We have the ability to choose to stop it or let it proceed.  The dinosaurs could not have foreseen the coming of the asteroid and the effects that it would have.  They could not have stopped it.  We can see the causes of the Sixth Extinction.  We can predict their outcomes.  We can choose to take a different path.  Will we?

Watch This Video Summary of Extinctions

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