# Why so many species? **URL:** \ **Category:** Factual Questions\ **Created:** [August 12, 2010, 12:36am UTC](https://boards.straightdope.com/t/why-so-many-species/550000 "2010-08-12T00:36:39Z")\ **Posts on this page:** 13\ **Page:** 2
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [August 12, 2010, 4:01pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/21 "2010-08-12T16:01:10Z")
> [@Add99](#): > > 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. Specialists are more efficient. If you have an enzyme that allows you to detoxify milkweed, and no other organism does, you have a food source that no one else can compete for. So if you CAN digest milkweed, there will be strong selective pressure to specialize on milkweed, and pretty soon we find that’s your sole food source. Now, it might seem like a big mistake to specialize in eating only one plant species, because if something happens to your food species you’ll be in trouble. But evolution doesn’t have foresight. If specialist organisms tend to have more offspring than generalists, then we’ll see more specialist organisms. It’s true that the specialists are more vulnerable to environmental change, but the generalist can’t compete with the specialist in the specialist’s particular niche, and so specialists tend to evolve into every niche. Of course, it’s a bit misleading to classify a species as a generalist or a specialist, because every species is both, and how you’d classify a species depends on what characteristic you’re looking at. So a monarch butterfly is a dietary specialist, but in other respects it could be considered a generalist–it has a very wide geographic range, for instance. So it turns out not to be true that generalists tend to push out specialists, until we’re left with a planet with a very few generalist species. Of course there have been times where particular species for various reasons take over–after the Permian mass extinction, one of the few survivors was a mammal-like reptile called [Lystrosaurus](http://en.wikipedia.org/wiki/Lystrosaurus). And it became the single most common species in the world after the extinction, in some places 95% of the animals were Lystrosaurs. But did Lystrosaurus survive because it was a generalist, or a specialist? It must have had some sort of special characteristic that allowed it to survive the Permian extinction when almost every other species went extinct, but it also must have been a generalist in that it was able to exploit the new post-extinction environment. So defining Lystrosaurus as either a specialist or a generalist is a mistake. It was specialized in some ways and generalized in other ways. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 12, 2010, 4:35pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/22 "2010-08-12T16:35:53Z")
> [@Exapno\_Mapcase](#): > > 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. The niche still exists. You underestimate the area of tropical forests still remaining. There are large tracts of forest remaining in Amazonia, and some areas of Africa and Asia. (I’ve been in such areas in Amazonia and Africa.) There are very large browsing herbivores in these forests in Africa and Asia today, namely elephants. Up until a few thousand years ago very large browsers also existed in South America, including both mastodonts and ground sloths. The forests themselves haven’t changed that much. The niche for these species is still there, it is just currently vacant. > [@](#): > > There is conclusive evidence that they have changed the landscape so drastically that former niches have completely disappeared. Some, perhaps, but not the large browser niche. > [@](#): > > 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. Untrue. The extinction of most megafauna at the end of the Pleistocene was probably due to a combination of overhunting by humans and environmental change (both natural and human-caused). However, enough suitable habitat probably exists today that at least the herbivores could survive in the absence of direct human persecution. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [August 12, 2010, 6:50pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/23 "2010-08-12T18:50:55Z")
> [@Colibri](#): > > Untrue. The extinction of most megafauna at the end of the Pleistocene was probably due to a combination of overhunting by humans and environmental change (both natural and human-caused). However, enough suitable habitat probably exists today that at least the herbivores could survive in the absence of direct human persecution. Perhaps I’m being unclear. I meant that no niches exist which would allow for the evolution of new species to split off from current ones to inhabit them. Certainly niches exist that allow large animals to persist. What I don’t see is how animals would get to these areas or split off from others in these areas, given the human interference that is likely. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 12, 2010, 7:11pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/24 "2010-08-12T19:11:31Z")
> [@Exapno\_Mapcase](#): > > Perhaps I’m being unclear. I meant that no niches exist which would allow for the evolution of new species to split off from current ones to inhabit them. Certainly niches exist that allow large animals to persist. What I don’t see is how animals would get to these areas or split off from others in these areas, given the human interference that is likely. That’s somewhat of a different issue. The potential niche is still there for a large browser. Ecologists differentiate between the _fundamental_ niche, the full range of environmental and biological conditions under which a species can exist, and the _realized_ niche, the part of its fundamental niche it is restricted to by interactions with competitors or predators. The presence of humans has greatly restricted the realized niche of many species, but that doesn’t mean the fundamental niche isn’t still there. The niche of very large browser is currently vacant in the Americas, due to the extinction of mastodonts and ground sloths. If humans were to cease hunting, and allow large areas of forest to persist, over the course of several million years this vacant niche would most likely be filled by the increase in size of existing species such as tapirs and capybaras. In the long run, you might have rodents as big as rhinos, as there were in the past in South America. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [August 13, 2010, 4:48pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/25 "2010-08-13T16:48:43Z")
Species are small variations from other related forms. Considering the size of the earth relative to its inhabitants, and the diversity of ecological niches, the number of species is not very surprising. This [chart](http://en.wikipedia.org/wiki/File:AnimalsRelativeNumbers.png)surprised me though. Anybody have any suggestions why Arthropods are so much more diverse than other phyla (assuming the chart is accurate)? ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 13, 2010, 6:55pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/26 "2010-08-13T18:55:21Z")
> [@TriPolar](#): > > Species are small variations from other related forms. Considering the size of the earth relative to its inhabitants, and the diversity of ecological niches, the number of species is not very surprising. > > This [chart](http://en.wikipedia.org/wiki/File:AnimalsRelativeNumbers.png)surprised me though. Anybody have any suggestions why Arthropods are so much more diverse than other phyla (assuming the chart is accurate)? First, the basic arthropod body form is nearly infintely adaptable. Lobster claws, lobster swimmerets, millipede legs, preying mantis arms, tick claspers, grasshoppr legs, spider legs, barnacle filter-feeding mechanisms – all adaptations of the same basic body part. Same thing goes for mosquito and butterfly proboscises, dragonfly larva jaws, etc. By comparison, the number of things a priapulid worm can adapt into is very limited. Second, most arthropods are small, meaning they can have adequate breading populations in very small niches. Imagine a beetle specialized to live off the juices of tender shoots of the jubjub tree. Well, even if the jubjub tree flourishes only in a 100-square-mile area of the Amazon rain forest, and only a few thousand jubjub trees survive, each has several hundred shoots where new growth is happening – meaning there are nearly a million locations where that beetle can live. Add several thousand similar niches on other plants, each with their own specialist beetle occupying it… Very few vertebrates can do that kind of specialization and maintain a breeding population in it – among land animals, only the koala comes to mind. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [August 13, 2010, 6:58pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/27 "2010-08-13T18:58:01Z")
> [@Polycarp](#): > > Very few vertebrates can do that kind of specialization and maintain a breeding population in it – among land animals, only the koala comes to mind. Panda. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [August 13, 2010, 7:08pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/28 "2010-08-13T19:08:34Z")
> [@Polycarp](#): > > First, the basic arthropod body form is nearly infintely adaptable. Lobster claws, lobster swimmerets, millipede legs, preying mantis arms, tick claspers, grasshoppr legs, spider legs, barnacle filter-feeding mechanisms – all adaptations of the same basic body part. Same thing goes for mosquito and butterfly proboscises, dragonfly larva jaws, etc. By comparison, the number of things a priapulid worm can adapt into is very limited. > > Second, most arthropods are small, meaning they can have adequate breading populations in very small niches. Imagine a beetle specialized to live off the juices of tender shoots of the jubjub tree. Well, even if the jubjub tree flourishes only in a 100-square-mile area of the Amazon rain forest, and only a few thousand jubjub trees survive, each has several hundred shoots where new growth is happening – meaning there are nearly a million locations where that beetle can live. Add several thousand similar niches on other plants, each with their own specialist beetle occupying it… Very few vertebrates can do that kind of specialization and maintain a breeding population in it – among land animals, only the koala comes to mind. I suspected that would be the case. Small size is probably the primary factor, giving them the niches. Its the proportional scale that surprised me. But I guess it shouldn’t have. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [August 13, 2010, 7:26pm UTC](https://boards.straightdope.com/t/why-so-many-species/550000/29 "2010-08-13T19:26:59Z")
> [@Colibri](#): > > If humans were to cease hunting, and allow large areas of forest to persist, over the course of several million years this vacant niche would most likely be filled by the increase in size of existing species such as tapirs and capybaras. In the long run, you might have rodents as big as rhinos, as there were in the past in South America. R.O.U.S.s? I don’t believe they exist. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 14, 2010, 5:12am UTC](https://boards.straightdope.com/t/why-so-many-species/550000/30 "2010-08-14T05:12:02Z")
> [@Sailboat](#): > > R.O.U.S.s? I don’t believe they exist. They did a few million years ago: [Josephoartigasia monesi](http://discovermagazine.com/2009/jan/079) ---
**Author:** ![Grendel\_s\_Father](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grendel\_s\_Father](https://boards.straightdope.com/u/Grendel_s_Father)\ **Post date:** [August 16, 2010, 2:23am UTC](https://boards.straightdope.com/t/why-so-many-species/550000/31 "2010-08-16T02:23:11Z")
> [@Colibri](#): > > > [@Quartz](#): > > > > There’s the giant panda in China and the koala in Australia. > > Neither is bipedal, and neither is nearly as large-bodied as the animals that **Polycarp** is talking about. Just to nitpick this minor point, neither [chalicotheres](http://en.wikipedia.org/wiki/Chalicothere)nor ground sloths were bipeds. Chalicotheres had long forelimbs and were quadrupedal knuckle-walkers, like gorillas. Ground sloths probably could rear up onto their hind legs to feed, but they didn’t walk bipedally. Pandas and gorillas seem to inhabit similar niches, maybe given a few million years (and barring their being driven to extinction by humans) they could evolve to become more chalicothere-like. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 16, 2010, 4:35am UTC](https://boards.straightdope.com/t/why-so-many-species/550000/32 "2010-08-16T04:35:15Z")
> [@Grendel\_s\_Father](#): > > Ground sloths probably could rear up onto their hind legs to feed, but they didn’t walk bipedally. At least some ground sloths, such as _Megatherium_, were apparently able to walk bipedally at least some of the time: [Bipedalism and quadrupedalism in Megatheriurn: an attempt at biomechanical reconstruction](http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1996.tb01842.x/abstract) I believe some trackways also indicate bipedal locomotion, though I haven’t been able to find a link. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [August 16, 2010, 5:20am UTC](https://boards.straightdope.com/t/why-so-many-species/550000/33 "2010-08-16T05:20:45Z")
Your question about caves doesn’t take into account one thing about isolated environments. They are only relatively isolated. Wipe out a species of cave dwelling insects that are uniquely well adapted to one cave, and the food which they so efficiently used will become abundant. The odd invader won’t have to evolve to get there, he can get blown in on an unusually large windstorm. But this time, the invader lays her eggs, and a very large number of individual insects (with an unusually small genetic diversity) will survive and procreate. Lots of niches have that sort of populations. Not unique species, yet. Just a very limited gene pool, with no communication to other gene pools. Speciation can proceed at it’s normal pace for insects, with a thousand generations in a thousand years, and then it might well have a unique species, conserving characteristics beneficial in this environment, and losing some no longer of benefit. Evolution: It’s not a plan, it’s just results. Tris [Previous page](https://boards.straightdope.com/t/why-so-many-species/550000.md?page=1)