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This is something to consider regarding the current 'extinction crisis'.
Some people don't understand what the 'big deal' is, with a species going extinct here, and another one going extinct there. They may think there's plenty species left; they may not see what the value is of having many species in the world; they may think that the species going extinct, are not 'fit' or 'competitive' enough; they may think that species going extinct, will be replaced by evolution; or they may not think at all.
So: let's look at the first belief, that there are 'many species left' despite extinctions.
What many people don't realise is that we live in times of great biodiversity impoverishment.
Considering the large, 'dominant' or most visible species – our own group, land vertebrates: mammals, birds, reptiles, amphibians and so on.
We currently have about 5,000 species of mammal on the planet. The diversity that still exists, is quite bewildering for someone starting to study them. And yet the mammals, especially the large mammals, we have now are only a shadow of the diversity that used to exist only a short (geologically speaking) while ago.
Perhaps as many as half of all the large mammals that used to exist about a million years ago, are extinct today. A million years may sound like a lot, but the Earth has existed 4.5 billion years already.
Large mammals that used to exist only a million years or so ago, include representatives of groups entirely extinct now: mastodons, chalicotheres, sivatheres, glyptodons, giant ground sloths, sabre-toothed cats, diprotodons. Suffice it to say, these are large mammals unlike any that exist today (although mastodons are somewhat like elephants).
Then there were species related to ones that still exist, but different kinds and a much greater overall diversity.
We only have two (or maybe three, if the African Forest Elephants are recognised as a different species) elephant species left, in parts of Africa and Asia. Elephants and Mammoths used to occur as many other different species, also in Europe and other parts of Asia, and in North America. Several different species of mammoths. There were also different elephant species including pygmy ones on islands.
There were also several different species of rhinos, in Europe and America as well, including forest rhinos, steppe rhinos and woolly rhinos of the ice ages.
There were several camel species in the Americas. There were short-necked giraffe species in Europe. There were several different species of hippo in Europe and Asia. Even in Africa, where there are now only two hippo species, there were at times six species or perhaps more. A group that has only one species today, the Pronghorn of America, used to be represented by a diversity of species almost as great as exists among the antelopes of Africa. There were several different species of bison in America and Eurasia, including ones with incredibly long horns, similar to certain long-horned buffalo that lived in Africa. There were 'giant' deer and antelope, and giant pigs also.
Some species that still exist today, had much larger distributions in the past. The African Spotted Hyena, for example, once ranged over large parts of Europe and Asia as well.
I can give even more examples, but the bottom line is this: the large mammal diversity we have today is a small fraction of the diversity that used to exist on the planet only a short while ago. A heck of a lot of species have already become extinct. We therefore have an 'impoverished' global ecology in terms of mammal species. If we lose any more, we are just so much the poorer.
The extinctions probably started about five million years ago, when humans were still very much monkey-like, and were related to climate changes; this intensified during the ice ages, but still: a great many species made it through one glaciation after another, but then suddenly became extinct. There is plenty of evidence to show that humans may have contributed to the extinctions.
This is still controversial but I'll write something about it. The further back in time we go, the more difficult it becomes to pinpoint the reason for an extinction. It's like trying to solve a crime that happened ten thousand years ago. There's not much evidence left. But the more recent the extinctions, the more it becomes possible to say what the cause was – and for the most recent mammalian extinctions known, the finger of blame points unequivocally at us humans. The Steller's Sea Cow (a huge marine mammal related to Dugongs and Manatees); the South African Blue Buck; the Yangtze River Dolphin; the Thylacine; all these and many more would still exist if it wasn't for us humans. In a place like Madagascar for instance, in about a thousand years, about half of the species of lemurs disappeared. Many of the extinct ones were larger and weirder than any that are left. Is it a coincidence that humans arrived on the island at the same time as the extinctions started?
Then let us consider birds. Currently there are about 10,000 species of bird left in the world – and that is a lot, compared to mammals. The diversity of living birds is incredible, and there is incredible beauty in them as well. But only a short while ago, there might have been as many as 12,000 to 13,000 species! An incredible cataclysm of extinctions happened especially in the islands of the Pacific ocean: from New Zealand to Hawaii, incorporating Indonesia, the Philippines, the Solomon Islands, and almost all of the other thousands of islands ... there was a holocaust of bird species, within the last few thousands of years. NOT at all incidentally this coincided with the spread of humans to all these islands.
Just to explain how it happened. Many of the Pacific islands are of volcanic origin, having risen up out of the ocean at some (geologically fairly recent) time. They were barren at first and then became progressively colonised by seeds, by insects, and then by birds and bats. But non-flying mammals can't easily arrive on them. So the birds that arrive there find a place without mammalian predators (or reptilian predators like snakes). They can nest on the ground; they needn't fly as much. Many species become poor fliers or even flightless. Staying on a single island, they evolve and become different from the same species on other islands. In fact the thousands of islands in the region have become a kind of 'factory' for bird diversity in this way! Many small islands consequently have bird species unique to them.
So imagine now these thousands of small islands, with birds on them that have changed and adapted, in areas with no mammal predators; they nest on the ground, they fly poorly or not at all, and they don't even realise they should flee from a potential predator. Humans arrive, and with them, animals like dogs, cats, pigs, rats and sometimes even snakes. The humans will easily catch birds for food - and so, too, the dogs, cats and snakes. Rats and pigs will devour eggs or small nestlings on the ground, or even adult birds who don't know they should flee. Small islands had only small populations of birds; such small populations could be rapidly wiped out entirely.
This thing happened on island after island. It is estimated that from 750 to 1,800 species of bird became extinct on the Pacific islands alone. The human settlers probably did not understand just how badly they had disturbed the island ecology. The concepts of 'extinction', evolution and biodiversity, are ones that humans have only recently come to understand fairly well.
Similar extinctions happened on Indian Ocean islands: Madagascar, Mauritius, Reunion and others. Though less well documented, it is very probable that vast extinctions happened in the Caribbean islands as well. We will never know just how many species became extinct; birds are fragile beings and their remains, even when preserved well, don't give us many clues of what they were like in life.
So: the world's bird fauna is also hugely impoverished right now. Incredibly interesting groups like the Moas of New Zealand, the Elephant Birds of Madagascar, and many more, are now gone – lost within only a couple of thousand years, a geological blink of an eye.
Similar extinctions happened with reptiles, though this is also hard to document. Most reptiles are small species, mostly living in forests, not leaving many remains when they disappear; who knows how many species of small lizards and snakes were lost? With these, also, there must have been considerable diversity on islands. Small reptiles can drift to islands on pieces of driftwood, and today there are still some very interesting island species left. They, too, would have been impacted by mammalian predators like cats or rats.
Of larger reptiles, we know about many species of giant ground tortoise that occurred on islands that were wiped out by humans. These huge tortoises had no defence and were taken for their meat and fat. Several species were wiped out on Indian ocean islands.
There were also land crocodiles on Indonesian islands that were wiped out entirely. There are no similar land-living crocodiles left in the world.But reptiles were not hit quite as hard as birds or mammals; nevertheless, there has been a diversity reduction among them, over the last thousands of years.
Moving to amphibians, we only have a few thousand species left, and there seems to be a process in operation that is wiping them out right now. There have been 'crashes' in amphibian diversity all over the world, only in the past few decades! And we don't know exactly why; maybe climate change; maybe pollution; maybe introduced diseases; maybe habitat destruction; maybe new predators introduced by humans; maybe a mix of all of those. Whatever the case, many unique and extremely interesting amphibian species have been lost, and this quite recently!
Amphibian diversity might be or become the most impoverished of all.
Ponder all of this! There has been a very significant drop in diversity among the land-living vertebrates over the past few thousand years; what we have left is only a fraction of what we used to have! We cannot really afford losing even more! Conservation must now be an ultra-high priority!
Let us get to this point: what is the value of having many species in the world? Two words: ecological resilience. The ecology of the planet has been based on a great diversity of creatures for most of its recent existence – say, the past five hundred million years. Over this period, animal and plant diversity has been steadily rising. A 'primitive' ecosystem would have had only a few species. Say, an ancient coal forest, with about a dozen species of tree, perhaps only five or so species of reptile and amphibian, ten or twenty species of insects, etc. Such an ecosystem is stable while environmental conditions stay the same, but a small disturbance can wipe out most species. But over time, species have evolved and diversified so that today, in the most diverse ecosystems like tropical rain forests, there are thousands of species of plants and insects, and hundreds of species of birds, mammals, reptiles and amphibians co-existing in the same habitat. Such an ecology is more resilient than one that is less diverse; a small change might wipe out many species but many more are guaranteed to remain.
The last may seem like I'm contradicting myself, saying that because diversity is high, the ecology can withstand the extinction of many species. So, seeing as diversity recently has been high, doesn't it hold true that the ecology could recover from the extinction crisis?
It does hold true. The global ecology has been incredibly diverse before the current extinction crisis, and so can probably recover from the extinctions and regain or even surpass its previous levels of diversity.
But there are some serious problems. The way humans have 'taken over' the natural world has 'fragmented' ecosystems everywhere. The natural world now exists, not as a continuous integrated, interlocked system of local ecosystems, but as a huge number of small 'patches' of natural land, separated from each other by farms and areas of human habitation. Species can no longer easily move from one ecosystem to another.
The species that remain have been seriously reduced in number. Their natural ranges have been seriously reduced.
Meanwhile, humans are still increasing in number and taking over more and more of the planet's surface. 'Natural' ecosystems just shrink and shrink.
This has never happened before in the history of the Earth. Even during the worst extinction crises, like at the end of the Permian or at the end of the Cretaceous, a 'wild' world was left over for the surviving species to colonise freely; and subsequently, evolution could go on and produce new species freely, in time restoring ecological networks and biodiversity.
Another factor is the time factor. It takes a lot of time for evolution to replace extinct species. In some way, extinct species can never be replaced; there would perhaps never again be something exactly like a dinosaur or a mammoth or a sabre-toothed cat. But evolution can make new species to replace lost ones – though not exactly similar, they may fulfil similar ecological functions. But this takes time! It may take tens to hundreds of thousands of years before a new species evolves out of an old one. It may take millions of years before global biodiversity is restored to previous levels, after a massive extinction event. It may take millions of years for food webs to be restored to their previous intricacy. Disturbed ecosystems lose productivity and resilience, which takes a lot of time to recover. It took the world about twenty million years to recover, ecologically speaking, from the extinction of the dinosaurs for example.
What is the deal if ecosystems become impoverished? The worse they are impoverished, the more 'unstable' they become. Unstable means they are vulnerable to disturbances, and, when disturbed, may 'collapse'. They cease to function as they did before. If we consider forests: forests are very important for making oxygen, for improving soil fertility, for protecting soil against being washed away by rain, and probably many other 'functions' of which we currently have no idea. If a forest ecosystem 'collapses' – say, all the trees die – then all these functions are no longer carried out; oxygen levels drop, carbon dioxide levels rise; the soil is eroded and washed away by the rain, and a 'desert' landscape remains, one that can no longer support much life. It may take hundreds or thousands of years to rehabilitate such a wasteland back into a forest.
Globally we have many such ecosystems on land and in the ocean, and all of them carry out functions that are valuable or even essential to living things ... plants, animals, and ourselves.
The extinction crisis is threatening all of this. Imagine: if globally ecosystems collapse, turning the whole world into a 'wasteland', that needs millions of years to be rehabilitated back into something resembling its previous level of functioning.