I was watching Planet Earth, caves. This cave crab only lives in this one cave. There’s plenty of similar stories. Venus fly traps, transparent frogs, etc. that only live here and only here, or require extremely specific conditions.
All it would take is one thing to wipe it out, a fungus, a tidal wave, a drought, an invading species. Extinction happens all the time and always has. A new type takes a bazillion years to develop.
So why do we have so many different ones? Shouldn’t we be down to around a dozen by now? There are so many we can’t even count them all. That doesn’t make sense.
Species develop into ecological niches. There are bazillions of niches. Any place where food can be found is a niche. Bazillions of niches engender bazillions of species. That only takes a tiny amount of time in geologic terms. If a new niche opens up - an island appears from the sea or whatever - life will swarm over it in a comparative instant.
Wiping out a species is not a good survival tactic for a predator, even if that predator is a virus. Wipe out your food and you wipe yourself out.
But when that happens it just leaves a hole for a new set of predators and prey. So they move in. Fast. Your premise about taking time is wrong. To outsiders it’s as if they were always there.
The true surprise would be if there were a limited number of species. But that can’t possibly happen because the world is divided into niches. Only a perfectly homeostatic world could ever limit the number of species and that seems too extremely implausible to occur.
Wait, so evolution happens quickly? That would explain it, but that doesn’t sound quite right.
The same program showed cave formations that take 5,000+ years to form. But, I could mess it up in a weekend. If I keep it up, there won’t be many left anywhere.
Quickly is relative. You won’t see evolution happened around you in your lifetime. But we know that many evolutionary changes have occurred to humans over thousands or tens of thousands of years, including lactose tolerance, new lines of blood grouping, and possibly even language ability. Humans are very long-lived and a generation requires decades. With animals that have much shorter life spans, the number of generations in a thousand years is many times greater. That’s probably long enough for new forms to develop in an isolated niche.
Even 1,000 years is nothing over the span of history. Go back a million years and hardly any species on earth today would be found. If all the billions of species found on earth turn over every million years - a good first approximation for larger animals and orders of magnitude longer than needed for micro animals - then evolution takes place much more quickly than most people think.
It simply doesn’t matter if any niche is messes up. We know of a half dozen great extinctions in which 75-90% of all species of earth were wiped out. Each time, diversity came back to an even greater extent than before. It may have taken a few million years, but again that’s nothing on a geologic timescale.
As long as there is some life anywhere, there soon will be much life everywhere. Species will move to exploit empty niches. That’s why we can’t find any.
Why there are so many species on Earth is one of the major questions that biologists seek to answer.
Speciation doesn’t necessarily take a huge amount of time. As a rough estimate, it probably generally takes at least 10,000 years (but can be shorter) to maybe a couple of million years since lineages have separated. The polar bear is estimated to have split from the brown bear only about 150,000 years ago.
One of the key factors producing speciation is isolation. The reason each cave has its own unique fauna is because they are isolated from one another. If they weren’t, a species adapted to one cave would be able to spread to others, and the best competitor among the different cave species would drive the others to extinction. As it is, each one is confined to its own cave after it has become adapted to cave conditions. It was the evolution of similar but unique endemic species on each isolated island of the Galapagos that caused Darwin to postulate that each one had evolved on in place.
The OP recognizes that the current diversity of life on Earth is due to the balance between origination and spread of new species vs the extinction of old ones. It so happens that in general the origination of new species has kept pace with the extinction of old ones. If the extinction rate increases, as it has in the recent past, the total diversity of species will go down.
There aredead zones, now-lifeless niches. (There are others, but the oceanic ones are a large and easy example.)
Add99 is certainly correct that tiny local niches can, in some cases, be destroyed quite easily.
Surely we don’t want to cause one of those great extinctions in our own time if we can possibly help it–or we want to mitigate it as much as possible. In terms of the lifetimes of humans and human cultures, those “few million years” to recovery might as well be forever.
Hmm this post got me wondering. I wonder what the theoretical lower limit is for each species? I am sure we can do without a lot of insects and a lot of frogs and shit because they are not the ONLY food supply.
Because what you’re seeing in that programme are the survivors. That cave crab might have replaced another creature fitted that niche prior to one of the disasters you mention.
Most places don’t have terrible floods and plagues of fungus often enough to cause all members of all species in that area to become extinct. For those that do, there will always be some life form (often several) that can outlast those disasters.
For species that reproduce sexually, all you really need is two fertile survivors - the Adam and Eve of the cave crabs - perhaps lasting out the flood by climbing on top of the bodies of their fallen compatriots or living because they had a mutation which happened to make their skin repellent to transparent frogs.
True, for herd animals or others which live communally, small numbers would be a big deal, but even then, it takes two (baby) to give their species at least an outside chance.
There’s one large-animal econiche vacant right now – the bulky-body large-clawed bipedal leaf-eater niche. For over 100 million years, ever since broadleaf plants became common, there were animals in this niche – the segnosaurs/therazinsaurs in the Cretaceous, then the chalicotheres in Eurasia and the ground sloths in the Americas. The latter two groups developed early in, and survived during, the Tertiary, survived during the Ice Ages, and then – died out about 10-12,000 years ago. Nothing has evolved to fill that niche – yet.
I wouldn’t say that it just so happens. Everything in nature tends towards some sort of equilibrium. If the number of species on Earth were too small, then speciation would occur at a rate greater than extinction, and if it were too large, then extinction would occur at a rate greater than speciation. We can assume that the number of species we have now is the optimal number, simply by virtue of the fact that it is the number we have now.
But, humans upset all this by merit of adaptability, yes? Humans can adapt to almost any geography or food source, pushing other species out. We might not live in the ocean (although we probably could), but we dominate the food chain even there. We even live in space.
So specialization is trumped by adaptability (because we sure seem to be putting the smack down on everybody else)? It seems like this would have occurred long before, leaving just a few highly adaptable species running around everywhere.
Human adaptability is exceptional, due to our intelligence and technology. But generalists in the animal world are usually outcompeted by the specialists. They have the benefit of being able to utilize many niches, but in each of those niches they’ll lose most of the production to specialists. Extinction events with major habitat changes may give them a short term advantage, but odds are some populations of the surviving generalists will evolve into specialists, keeping the generalists in check.
That’s simply not true. Despite recent decreases since the end of the Pleistocene, diversity since the end of the Cretaceous has in general been going up. This graph shows the general upward trend in family diversity over time.
Species diversity in fact has a positive feedback, since every new species creates new niches for other species. Niches are not just determined by the physical environment, but by the biotic environment as well, and the latter may be the most important. The evolution of grasses created niches for grazers that previously had not existed. Every new plant species that evolves is a potential new food source for dozens of specialist microorganisms and insects, and they in turn can support new specialist predators. The enormous species diversity in the tropics is caused not only by the relatively benign physical environment, but by the extremely complex interactions between species.
While species diversity in a limited area over a restricted period of time may be determined by competitive interactions (and even that is disputed by the proponents of the Unified Neutral Theory of Biodiversity), in general it tends to trend upward, and is knocked back mainly during episodes of mass extinction caused by rapid environmental change.
Just to clarify with respect to my more recent post, it more-or-less keeps pace over shorter periods of time, but over very long periods of time the overall trend is an increasing number of species.
I think you have it backward. The question isn’t whether an animal has evolved to fill this niche, but whether the niche exists today in the first place.
The answer to that appears to be no. It’s true for environmental reasons: where are the enormous areas of broadleaf plants to be found that would support large numbers of herbivores? And it’s also true that a natural disaster has eliminated that niche: the coming of humanity.
I said earlier that it’s not a good idea for predators to completely eliminate their prey. Humans are currently an exception to that. There is abundant evidence that they played a part in the extinction of large mammals by overhunting. There is conclusive evidence that they have changed the landscape so drastically that former niches have completely disappeared.
Whether humans will continue to wipe out other species and do themselves in is one of the Great Debates I considered unanswerable and have no interest to take part in. But that macro-level niches exist for large mammals to evolve into seems to be a settled issue. There aren’t any. Niches only exist at the micro level today. I don’t know whether there exist islands or rainforest interiors that may be an exception. Even so, I’d still argue that those are micro niches and that any expansion out of them would be fatal.
Imagine a species of small crab that finds a nice cave somewhere. If there is plenty of food there, it is safe from predators, and thus there’s no reason to leave the cave. This particular group of crabs never leaves the cave and doesn’t interbreed with the rest of the population of crabs from the same species.
After a few hundred years, enough genetic drift might take place that crabs who live in this cave no longer will interbreed with others of what was once a similar species. If two types of animals don’t interbreed in the wild, they can be thought of as two species (although they may get sub-species identities first). Do this for each cave and underwater nook, and you can end up with dozens and even hundreds of species.
The greatest genetic diversity are the smallest creatures because their range can be extremely small. A single rock, a creek bed, or even a stand of trees. Larger creatures need more resources and tend to roam and mix which keeps genetic drift down. Thus, there’s only a single species of elephant in all of Africa. Yet, there are thousands of species of cockroaches on the same continent.
