Megafaunal extinctions, refugia and phylogeography Flashcards
(41 cards)
Explain the terms: Extirpation, Extinction and Mass extinction.
- Extirpation: Loss of a regional/local population of a species but not in other place(s), leaves potential for re-introduction.
- Extinction: Loss of all populations of a species, species is lost.
- Mass extinction: Loss of all populations of multiple diverse species, species are lost and major ecosystem shifts occur.
How many mass extinctions have occurred since the rise of multicellular organisms? Which?
There has been five mass extinctions since the precambrian ~ 600 M ya.
- 1st: occurred in the end of Ordovician (444 M ya) – about 85% of all species went extinct.
Proposed cause: Tectonic plate shifts and intense glacial/interglacial periods moving the shoreline a lot - changes in climate and ocean chemistry. - 2nd: Late Devonian (360 M ya) – 75% of all species went extinct.
Proposed cause: Rapid growth and diversification of land plants generated rapid and severe global cooling. - End Permian (250 mya) – The largest, with 95% of species going extinct.
Proposed cause: Intense volcanic activity in Siberia. This caused global warming. Elevated CO2 and sulfur (H2S) levels from volcanoes caused ocean acidification, acid rain, and other changes in ocean and land chemistry. - End Triassic (200 mya) – 80% of all species went extinct, paved the way for the age of the big reptiles.
Proposed cause: Underwater volcanic activity in the Central Atlantic Magmatic Province (CAMP) caused global warming and a dramatic change in the chemical composition of the oceans. - End Cretaceous (65 mya) – 76% of species went extinct, the event that killed off the non-dinosaurs, started the age of mammals that we live in today.
Proposed cause: Asteroid impact in Yucatán, Mexico. This caused a global cataclysm and rapid cooling. Some changes may have already pre-dated this asteroid, with intense volcanic activity and tectonic uplift. - It is debated if we are in a sixth mass extinction today, but these previous ones are often called “the big five”.
All of the “Big Five” were caused by some combination of rapid and dramatic changes in climate, combined with significant changes in the composition of environments on land or the ocean (such as ocean acidification or acid rain from intense volcanic activity).
What is the definition of “megafauna”?
Megafauna = Large vertebrate taxa weighing over 45 kg.
What characterizes megafauna?
The characteristics of megafauna are:
- Long lived (usually)
- Few offspring
- Slow population growth rate
- Few or no natural predators (adults) - Thermoregulatory advantage in cold climates
- Often well adapted to seasonality in food supply
A lot of megafauna still alive today, but many also extinct. Give three examples of extinct Pleistocene megafauna.
- Ground sloth
- Woolly mammoth
- Cave bear
- Aurochs
- Moa
What time period is referred to as the Late Quaternary? How much of megafauna has been lost during this time?
Late Quartenary = 126,000 years ago to present (late Pleistocene =126,000 to 11,700 years ago + Holocene = 11,700 years ago to present).
During the late Quaternary 68% of megafauna has gone extinct, most in northern Eurasia and south America, but sub-Saharan Africa not as affected - at least acc to the fossil record, but as preservation is not as good at lower latitudes and more studies have been done in the northern hemisphere, this might not be accurate. During this time after the last glaciation the temperature fluctuated a lot, could be an explanation.
What could be an alternative explanation for the massive loss of megafauna during the late quaternary?
It is possible that taxonomic over-splitting of fossil species is artificially inflating the number of species lost during the late quaternary. For example, there are 50 named species of Pleistocene horse from North America, whereas there only thought to be 1-2 ‘true’ species… Many regarded their finds as specific species, but upon looking over similar finds systematically, it is likely that many specimens represent intraspecies variation. BUT many highly distinct groups were lost, so there is strong support for this indeed being a mass extinction, just not of the magnitude it’s currently viewed at.
Describe an approach that can be used to determine the time of extinction for a given species.
One way to determine the time of extinction for a species is to find the most recent sample, which represents the LAD or last appearance date for the species (or a population, such as mainland woolly mammoths). This does however require extensive field work at many locations and always opens for the possibility of a later LAD if a more recent specimen is found. This also provides information about the last habitat/region that the species inhabited, which can be useful in conservation efforts, ecology, or population history.
How can you increase the resolution and robustness of establishing the LAD for a species?
sedaDNA can be very useful in establishing LAD, as even though skeletal/macrofossils are absent, one can look at when the LAD was. If sedaDNA confirms presence of the species up until a given point (dated layer) but not after, this provides evidence for the LAD, although not nearly as strong evidence as fossils.
Megafaunal extinctions happened on all continents, but which were hit hardest? How does this relate to human range at the time?
North and South America, together with Australia, were hit hardest - neither of which were inhabited by humans. Areas of human home range (Africa) or with long exposure to hominins (Asia) were less affected by megafaunal losses (Love)
but this paper (Bergman et al 2023, Nature communications) argues that humans likely had a bigger impact that previously thought; https://www-nature-com.ezp.sub.su.se/articles/s41467-023-43426-5 They showed through DNA analysis of modern animals (population genetics) that the effective population sizes did not cycle with glacial periods, but instead decreased specifically in the last 50 kya - consistent with expansion of homo sapiens. Although their data show decreasing populations in America that predates human arrival, which could reflect either increasing cooling towards the glacial maximum (most of these species are warmth-adapted species) combined with later anthropogenic suppression of populations, or earlier human arrival dates
When did the woolly mammoth go extinct? Where?
The wooly mammoth was extirpated on the mainland between 12-10 000 years ago around the Bering sea. But survived on two islands (previously highlands when the sea level was low): St. paul island (extirpated ~5 600 years ago, and Wrangel island where they finally went extinct ~4 000 years ago.
Some evidence have been brought up for later survival of megafauna on the mainland well into Holocene, what evidence is this?
eDNA from woolly mammoth, woolly rhino and other megafauna suggest that they survived longer on the mainland than what the fossil record shows, aka “ghost range” well into Holocene, but this provides weak evidence. sedaDNA or eDNA is not strong evidence as it can migrate/get reworked and redeposit at a different layer than it’s origin - so without bones it’s hard to be sure. And yes, there are bones of extant taxa (reindeer) throughout these Holocene time intervals, so the problem is likely not preservation issues.
Name five candidate explanations for the extinction of the Pleistocene megafauna and explain each briefly.
- Hyper-disease: Humans might have been a vector for hypervirulent pathogen, that might have more impacted megafauna more due to long generation times and slow recovery. However, there is no hard evidence for this hypothesis as there have been very little signs of pathogens in megafaunal remains (or aDNA in general but the field is growing so we might see more of this in the future).
- Asteroid impact - ”The Younger Dryas Impact Hypothesis”: There is some weak evidence of an asteroid impact ~12,900 ya, in north America which could explain both extinctions and the YD cooling. Mainly a carbon rich layer that extends over large parts of NA at the time, but this study has been widely refuted but debates over the robustness of the hypothesis are still ongoing.
- Climate changes: This is one of the main candidates today. The main extinction window (supported by radiocarbon dating) falls within the transition from the late glacial to B-A (14,7ka) with rapid heating followed by rapid cooling that continued into the YD (12,9ka) until the beginning of Holocene when the temperatures rapidly rose again and continued during early Holocene (11,7 until ~10ka). A period with drastic and rapid changes in temperature.
- Human overkill: Extinctions approximately coincided with human expansion into new territories, since timings for extinctions are asynchronous across the continents, this likely had a bigger impact than climate, which on one hand does impact different locations differently, but the match to human expansion is better.
- Fire: Warm temp –> dryer –> more fire –> extreme environment that is not easy to handle –> big changes in environment which can impact many different taxa. The rapid heating during the beginning of the B-A could have caused this pattern of events to unfold.
The most likely explanation is a combination of different candidates, such as Climate changes, fire and human overkill. It is likely that the climate fluctuations and environmental changes that followed left much of megafauna vulnerable/reducing population sizes, and coupled with human hunting that might have been the last straw.
Does the potential candidates for extinction differ between mainland and islands? Motivate.
On continents/mainland: a combination of climate and associated environmental changes reducing population sizes, coupled with human hunting is the most likely explanation.
On islands on the other hand, hunting was likely not as big of an issue, instead island populations are usually small and therefore more vulnerable to fluctuations, so it is more likely one of the alternatives, e.g. climate change, env change or hunting if it was accessible to humans.
There are four major things to consider when it comes to the megafaunal extinctions, which and what conclusion do you draw when taking those into consideration?
- Southern Asia and Sub-Saharan Africa were minimally impacted.
- Small mammals and birds, insects, and plants did not undergo an extinction pulse.
- The terminal Pleistocene was probably not the time of most severe change in climate during the Pleistocene. Dozens of other severe changes occurred.
- Extinctions were asynchronous and approximately coincided with several millennia after human arrival
Taking all these points into consideration, the megafaunal extinctions remain a mystery. If the climate change was too extreme, other smaller taxa would likely have been more impacted and it would have impacted lower latitudes as well. The evidence is most consistent with extinction by attrition/a thousand cuts, many things that went badly for the megafauna with little time to recover in between.
Explain the concept of ”Evolutionary anachronism” (Ghosts of evolution) and provide an example.
Evolutionary anachronism: “Attributes of living species that are best explained as a result of having been favorably selected in the past due to coevolution with other biological species that have since become extinct”.
It could also be explained as “Legacies of extinction”, and a good example is the steppe tundra: a highly diverse and productive ecosystem that supported large biomasses of herbivorous megafauna during the Pleistocene and generally disappeared when most megafauna went extinct, but remnants of this habitat survives today in the Altai region (where Kazakhstan, Russia and Mongolia meet) but will likely continue to disappear. This ecosystem is thought to have protected carbon-rich permafrost and in light of recent global warming, there are ideas popping up about attempting to restore it (e.g colossal).
Provide two other examples of legacies of extinction.
- Avocados: They have too big seeds to be naturally dispersed today, so likely coevolved with giant ground sloths and are since dependent on humans for dispersal.
- Wildfires: Megafauna are important for nutrient cycling and preventing the build-up of combustible materials in forested and grassland landscapes. Extinctions of megafauna results in increased wildfire risk, unless managed by humans through clearing or controlled burns.
- Pronghorn: The pronghorn Antilope of North America is a fast runner but has no living predators that can match its speed… The extinct American cheetah (no relation to living cheetahs) was the Pronghorn’s predator during the Pleistocene.
- Mosquitos: Swarms of mosquitos in northern Siberia tundra during the summer months. Why? Mosquitos typically need a blood meal to reproduce but there are only sparsely distributed reindeer in these environments today. –> These mosquitos might have evolved to reproduce without a bloodmeal – an adaptation to a megafaunal depauperate landscape.
Give three examples of species that have gone extinct recently.
- Stellar’s seacow (Hydrodamalis gigas) Extinct ~250 years ago
- Haast’s Eagle (Hieraaetus moorei) Extinct ~750 years ago
- Giant moa (Diornis maximus) Extinct ~750 years ago
Should the Late Quaternary megafaunal extinctions (current day) be considered a ’mass extinction’?
Not really when comparing to previous ones, which have altered both ecosystems and climates - which is not happening to a big extent today. But the sheer speed of which climate change is occurring today has never really happened before, so we are entering unknown territory of what will happen. We should be cautious and try to help almost extinct species through conservation efforts. Once gone, it will be hard to get them back.
Define the term “Quaternary refugium”.
A region that a species inhabits during the period of a glacial/interglacial cycle that represents the species’/populations’ maximum contraction in a geographical range.
Briefly explain the expansion/contraction model.
Species have specific adaptations, and therefor will respond differently during diff stages in a glacial/interglacial cycle. Species expand and contract their ranges in concert with the changing climate.
What is the range of temperate taxa in Europe during glacials in general?
The range of temperate taxa in Europe during glacials: Short answer: in the south (Iberia, Italy, Greece, Turkey). Overall the Ice sheet covered northern Europe, central Europe was tundra so the only place that was habitable was southern Europe = southern refugia.
What is the range of cold-adapted taxa during interglacials in general?
The range of cold-adapted taxa during interglacials (when Europe overall is temperate or warmer) is in the very northern regions (polar refugia) and at high altitudes in the south (cryptic southern refugia).
Oaks are a temperate taxa and has been found in Scotland during glacial periods, how?
Oaks surviving in Scotland is strange, could have been spread by animals but unlikely. More likely is that the oak trees survived in a microclimate much further north than they should survive = a cryptic northern refugia.
