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. 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.
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.
Some, perhaps, but not the large browser niche.
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.
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.
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 chartsurprised 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.
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.
Just to nitpick this minor point, neither chalicotheresnor 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.
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.
