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为什么数十亿信鸽在一个世纪内消亡?

Why Billions of Passenger Pigeons Died in Under a Century

#音乐#
[♪INTRO]
在北美 候鸽曾经种群庞大
The passenger pigeon was once the most abundant bird
或许甚至在全球
in North America—maybe even the world.
在19世纪上半叶
In the first half of the 19th century,
估计数量在30到50亿只
estimates put the number between 3 and 5 billion, which
今天的北美 鸽子数量本应达到整个鸟类的
would have been at least one-third of the total bird population
三分之一
of North America today.
在1833年还能看到鸽群遮天蔽日的景象
But while flocks could block out the sky in 1833,
截止1900年再也 见不到
by 1900 there were no wild birds to
野鸽子
be found.
1914年 甚至连动物园里也见不到鸽子
And by 1914 there were none alive even inzoos.
到底是什么令鸽群的数量
So what happened to make the population go
在不到一个世纪的时间里 从数十亿降到零
from billions to zero in less than a century?
要想理解鸽子如何消亡 我们要理解它们怎样生活
To understand how passenger pigeons died, we need to understand how they lived.
因为……都是我们的错
Because… it was kind of our fault.
候鸽生长在北美的东部
Passenger pigeons were native to eastern
在北美大湖偏北与墨西哥湾偏南
North America and migrated between the regions around
之间迁徙
the Great Lakes in the north and the Gulf of Mexico in the south.
它们成群结队地迁徙
They traveled in huge flocks with hundredsof thousands of pals.
当它们栖息或者筑巢时
When roosting or nesting,
这么多的鸟儿落在树上 这很不寻常
it wasn ’ t uncommon for so many birds to perch on a tree that
因为它们加起来的重量可以压折树枝
the branches would snap under their combinedweight.
有时鸽子会落在不同的树梢
Sometimes birds would even perch on top of each other just
以便睡觉
for a place to sleep.
很安逸
Real cozy.
庞大的种群让它们
This strength in numbers was their main defense
足以抵御天敌 狼 狐狸还有鼬鼠
against predators like wolves, foxes, weasels,
还有老鹰
and hawks.
本地的食肉动物可以吃掉它们
The local carnivores could eat themselves sick and
疾病也没能缩减种群数量
still not make a dent in the flock.
生态学家称之为捕食者饱满效应
Ecologists call this predator satiation,
是一种生物界很普通的
and it ’ s a pretty common defense tactic in both
防御策略
plants and animals.
也就是为什么成群的橡树两年内不结果
It ’ s why groups of oak trees produce almost no acorns for two years,
到了第三年
then on the third
结出很多果
year it’s an acorn party.
也是蝉虫阶段性生育的原因
It ’ s also why cicadas reproduce periodically,
为什么大马哈鱼在它们的生育季节阻塞河道聚集繁殖
why salmon clog streams during their breeding season,
也是为什么现在北美驯鹿成群迁徙
and why caribou travel in massiveherds. Now,
候鸽聚集喂养幼鸽
passenger pigeons worked together to raise
觅食坚果 种子
their young and to find food like nuts, seeds,
浆果
and berries.
当人类开垦森林来耕种庄稼
So when humans started cutting down forests to plant crops,
鸽子就追随着谷物
pigeons went after those grains
同时
too. And,
你可以想见 农夫对它们肯定不待见
as you might guess, that made them pretty unpopular with farmers. But,
不是吗
really,
当我们开始捕杀鸽子作为商品 作为食物来源的时候 一切都变了
everything changed when we started hunting the pigeons as a commercial food source…
因为它们吃上去味道不错
because they were apparently super tasty.
加上运输网的蔓延
The expansion of train networks made it possible
使得猎杀 运输鸟儿成为可能
for hunters to kill and ship the birds nationwide.
且不费劲
And it wasn’t hard.
因为它们成群飞行 飞得又低
Because they traveled in dense, low-flying flocks,
猎人只要抬手
hunters could basically just swing
就能打下来几只
a stick and hit a couple birds.
来满足需求
To satisfy the demand,
捕猎者去诱捕 以及用火去烧
hunters started using baited traps and controlled fires to kill
将它们一网打尽
more birds in one go.
他们甚至在鸽巢下方燃烧硫磺来熏死鸽子
They even burned sulfur below pigeon nests to suffocate them,
等它们从树上掉下来
collecting the bodies as
然后收集起来
they fell from the trees.
鸽肉如此美味
The pigeons were apparently so tasty
没有人会追究猎杀鸽子
that no one questioned whether eating birds killed
用火烧 用烟熏是否是好主意
by fire and smoke was a good idea or not.
甚至在1870晚期
Even as late as the 1870s,
人们还认为鸽子这么多
people thought there were so many pigeons that it wasn ’ t
猎杀几只也没关系
possible to hunt them too much.
可一旦种群数量锐减 情势就不容乐观
But once their flock numbers dipped too far, the situation got dire.
截止1900 纵贯中西部 只有动物园里有鸟儿生活
By 1900, the only surviving birds lived in zoos across the midwest.
1914年9月1日
And on September 1st, 1914,
最后一只捕捉的候鸽-玛莎,继玛莎华盛顿后死去
the last captive passenger pigeon, named Martha after Martha Washington,
时年29岁
died at the age of 29.
回望以前 科学家试图指出
Looking back, scientists have tried to figure
为什么的短羽候鸽不能
out why the passenger pigeon couldn ’ t survive
活下来
in smaller flocks — like,
或许它们不能保卫自己 扩张种群
maybe they couldn ’ t defend themselves or breed successfully.
2014年
In 2014, a group
一群科学家发表论文指出 鸽群数量的波动
of scientists published a paper suggesting that passenger pigeon populations
甚至在人类到达之前
likely fluctuated even before modern humansarrived.
事实上 它们已经在衰竭
In fact, they may have been in a natural decline
但是又恢复了过来
that they had recovered from in past cycles.
接着人类来了 就乱套了
But then humans came in, messed around,
给了鸽群致命的最后一击
and ended up being the last nail in the coffin.
科学家用生态学模型
These scientists used ecological models that generally say
巨大的种群意味着
that bigger populations mean
巨大的有效数量规模
larger effective population size — which
可以推测有多少个体在繁衍后代
is a measure of how many individuals are making
为稳定基因库做出贡献
babies and contributing to the total genepool.
中性理论分子进化论提出
And the neutral theory of molecular evolution suggests
有效的种群规模
that the effective population size
必须由基因多样性来保证 这个叫中性突变
should be linked to DNA variations called neutral mutations,
实际上从根本上并不影响
which don ’ t really affect
生存
survival. Basically,
更多的成鸟 从理论上讲 意味着更多的中性突变
more breeding adults means moreneutral mutations randomly cropping up.. So,
随时发生
theoretically,
庞大的种群的基因应该有高度的多样性
large populations should result in high genetic diversity from bird
鸟类如此
to bird.
但是 根据对基因多样性 DNA的分析
But — according to their DNA analyses — genetic variation
候鸽种群的基因多样性并不高
between passenger pigeons was low,
不管数量有多少
despite how many there were.
基于这个事实
Based on that fact,
研究者估计鸽群数量的锐减
the researchers estimated that the pigeon population dipped down in cycles,
可以解释为为什么
which could explain why the billions
18世纪那么庞大的鸟群竟都是
of birds seen in the 1800s seemed like not-so-distant
近亲
relatives.
这个观点被花粉记录证实
This idea was seemingly supported by pollen records
在过去的两万年间
from the last 20,000-odd years suggesting
橡子演化出丰富的后代巨大的鸟群却不能存续
that acorn production varied enough that huge bird populations couldn ’ t always survive. However,
2017年的一个研究表明
a 2017 study suggested that the lack
基因多样性的匮乏由于快速的进化
of genetic diversity came from rapid evolutionary
适应所代替
adaptation instead.
这些科学家们指出
These scientists pointed out that there are plenty
庞大的种群 但它的规模
of populations whose size doesn ’ t
没有配套基因多样性
correlate with genetic diversity.
当然这也与突变基因的传播有关
And this might have to do with how new mutations can spread.
当生物体产生精子和卵子
When organisms make sperm and eggs,
会存在一个混合与适配 组合成新DNA的过程
there ’ s a mix-and-match process where DNA gets swapped
这一过程叫重组
around called recombination.
这就使得每一个后代有一套独特的基因
This makes it so every offspring has a unique set of genes.
重组牵扯大块DNA的变化
But recombination involves swapping big chunksof DNA.
优良基因传播很快 整个的基因得以传播
So as an advantageous gene spreads like wildfire, whole stretches of DNA spread.
这就能解释了为什么
That could explain why there doesn ’ t
在一些大种群里基因的多样性似乎不多
seem to be a lot of genetic diversity in some big
如候鸽 尽管有中性理论支持
populations — like passenger pigeons — despite what the neutral theory suggests.
所以研究人员认为:鸽群数量会持续壮大
So these researchers think that the pigeon population was always big,
而它们
and that their
数量的锐减不太正常
huge drop-off in their numbers probably wasn ’ t normal…
其实是我们错了
so it was largely our fault.
这是一个沮丧的消息 是我们导致了鸽群的消亡
It ’ s a gloomy thought that we caused the passenger pigeon extinction,
但它却导致了
but it did inspire
第一部旨在保证物种可持续的野生动物法的出台
the first wide-scale laws aimed at speciesconservation.
1890 几乎没有针对
In the 1890s, there was almost no regulation
猎杀野生动物的规范
on hunting or killing any kind of wildlife.
但在1900
But in 1900,
物种的消失 相关法律开始禁止州际间运送
this looming extinction inspired a federal law prohibiting interstate shipping
非法猎杀的野生动物
of illegally hunted game.
13年后 数周的Mclean行动催生了
13 years later, the Weeks-McLean Act added a ban
当万物复苏时 禁止猎杀
on hunting migratory birds in the spring,
春季候鸟
when many species are breeding.
今天 禁止出口用于时装业的野生鸟类羽毛
It also banned imports of wild bird feathersused for fashion. Today,
我们有了许多类似的保护法律
we have many such laws protecting
我们关注着濒危物种
wildlife and we keep a much closer eye on endangered
所以我们就有希望防止像候鸽类似现象的发生
species — so we can hopefully prevent more stories like the passenger pigeon.
谢谢观看这一集的《科学秀》
Thanks for watching this episode of SciShow,
我们集体的劳动成果
which is produced by Complexly — a group
我们相信:对世界认识越深
of people who believe the more we understand about the world,
我们会生活地
the better we are at being
越好
humans.
尽管人类不总是对的
Even if humans aren’t always so great.
如果你想要多了解
If you want to learn more about all kinds
与我们同在的各种动物
of animals that we still share the planet with,
浏览Animal Wonders@youtube.com/animalwondersmontana!
check out Animal Wonders at youtube.com/animalwondersmontana!
#音乐#
[♪OUTRO]

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视频概述

或许不是人类介入的原因

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收集自网络

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Tiger_woo

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审核员YX

视频来源

https://www.youtube.com/watch?v=twr53QVGh0E

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