Extinction of many species is accelerating at an alarming rate, leading some scientists to dub this period as the sixth mass extinction in Earth’s history. While past mass extinctions, like the asteroid impact that wiped out many dinosaurs, were natural events, the current extinction crisis is primarily caused by human activities. A study published in Nature Communications to calculate all bird extinctions dating back to the Late Pleistocene, has found a correlation that suggest that humans have been enabling species extinction for a long time, perhaps going back to over 100,000 years.
Looking at the history of bird extinction
Birds do not have a clear fossil record due to the fragility of their bones. This makes understanding when and why some birds have become extinct difficult. Many researchers have focused on studying bird extinctions caused by humans over the last 500 years. From around 1500 CE, records and observations about bird extinctions are more accurate and complete.
Correlation between bird extinction and human presence
Over this span of 500 years, numerous bird species sensitive to habitat destruction, particularly those with limited flight capabilities—most notably the Dodo—have been severely impacted. Additionally, the introduction of invasive or non-native species has contributed significantly to the decline and extinction of many of these birds.
Research published in Nature Communications has analyzed known bird extinction records, along with using models to fill in the gaps left by the incomplete fossil record, to calculate how many birds have gone extinct since the Late Pleistocene.
As humans move into new areas on Earth, bird extinction rates rise
As part of this study, researchers observed that the disappearance of numerous bird species from the fossil record frequently aligns with periods of human expansion or heightened activity in specific regions over the last 100,000 years, particularly since the Late Pleistocene epoch.
Combining data on approximate dates of given extinctions for birds and when humans arrive to a given area shows a remarkable corelation. It is clear most extinctions happen during the last 7,000 years as humans populated every corner of the Earth. Species such as Āmaui (Myadestes woahensis) from Hawaii, Lyall’s Wren (Traversia lyalli) from Aotearoa New Zealand, and the Colombian Grebe (Podiceps andinus) from South America are examples of previously common birds that disappeared as humans expanded into the regions where these birds were from.
Higher rates of bird extinction on Pacific islands
The rate and intensity of extinction is particularly evident in islands in the Pacific, where many birds had uniquely adapted to isolated areas. Many of these birds were flightless or very sensitive to changes in the environment, particularly when invasive species began to appear. Research also suggest that the pace of extinction has been accelerating and will accelerate further, with an additional 226—738 bird extinctions predicted over the next few hundred years.[1]
Arrival of humans is the strongest predictor of bird extinction
In this new research, scientists used linear models applying spatial properties of land mass, temperature, precipitation, elevation, plant richness, and other inputs to estimate the likelihood other bird species, potentially unknown, to have gone extinct at given time intervals based on rates of extinction known from other periods where it is evident birds disappeared. Over the last 100,000 years, isolation distance, elevation, temperature, and the presence of rodents have been identified as some of the contributing factors to species extinction.
The research found that human arrival (R2 = 0.13) was the strongest predictor of extinction compared to other factors such as isolation distance, elevation, temperature and native rodents, and this rate can be used to estimate unknown bird extinctions. A general linear model (GLM) was used to provide estimates for species richness in given areas that could then be estimated to have been lost over time and space. From these, researchers were able to extrapolate the timing and likelihood of extinction events based on human arrival.
New Zealand has the most complete bird extinction records
New Zealand, where 640 bird species are documented to have gone extinct, serves as a reference point for estimating the number of bird species that have gone extinct globally. This is because New Zealand is one of the rare locations where the extent of bird extinctions is believed to be fully or almost fully documented. The rate and pace of extinction from this known dataset could be used to estimate other regions and act as a baseline to control estimates.
About 12% of all bird species have gone extinct in the last 100,000 years
In total, 1,000 possible timelines of extinction were identified for various regions, including different archipelagos. From this analysis, it is estimated that between 1,300 and 1,500 bird species have gone extinct since the last ice age about 100,000 years ago. This represents approximately 12% of the overall bird population that has existed during this period.
By applying estimated rates of extinction and projections from linear models, it is possible to deduce that approximately 55% of extinctions remain undocumented or involve species that were never known to us. It is estimated that about 61% of these undocumented extinctions likely took place in the Pacific region, which is known for its many specialized bird species that were particularly vulnerable to extinction following human contact. These extinctions may have resulted from direct actions, such as hunting and deliberate killing by humans, or indirectly through changes in habitat or environmental alterations that accompanied human arrival.[2]
Humans have been shaping extinction rates for thousands of years
What is clear from this new research is that humans have been shaping extinction rates for thousands of years, even during our early evolution. Certain bird species have shown particular sensitivity to human presence, influenced by the ways in which we modify and impact their habitats and environments. This includes activities such as hunting, farming, the introduction of other species, and various other human activies.
Birds significantly contribute to environmental health through pollination, seed dispersal, and scavenging animal remains. By causing the extinction of certain bird species, humans have indirectly influenced and altered the evolution of ecosystems. This impact is now believed to extend back to periods when humans were migrating and evolving across the globe. [3]
References
[1] For more on extinctions as affected by human activity and the rate of extinction, see: Pimm, S., Raven, P., Peterson, A., Şekercioğlu, Ç.H., Ehrlich, P.R., 2006. Human impacts on the rates of recent, present, and future bird extinctions. Proc. Natl. Acad. Sci. U.S.A. 103, 10941–10946. https://doi.org/10.1073/pnas.0604181103.
[2] For more on the new method on estimating bird extinctions, including during the last 100,000 years and for species never known to science, see: Cooke, R., Sayol, F., Andermann, T., Blackburn, T.M., Steinbauer, M.J., Antonelli, A., Faurby, S., 2023. Undiscovered bird extinctions obscure the true magnitude of human-driven extinction waves. Nat Commun 14, 8116. https://doi.org/10.1038/s41467-023-43445-2.
[3] A newspaper article on the scientific article explaining the new extinction calculation can be found here: https://www.theguardian.com/environment/2023/dec/19/human-driven-extincition-of-bird-species-twice-as-high-as-thought-study-says-aoe
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Fonte : National Geographic
