Evolution question: Why DON'T niches get filled

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Evolution question: Why DON'T niches get filled

Postby sociotard » Thu May 09, 2013 3:37 pm UTC

Evolution was explained to me as a drive to fill a niche. An asteroid wipes out all the big animals, but those big animals filled niches, so the remaining little animals diversify, evolve to fill the niches, and now we have big animals again.

Except, sometimes the niches don't get filled. My question is why. Why do niches sometimes go unexploited?

The case that prompted the question is New Zealand Kakapo. It's a flightless parrot that is well adapted to living on an island with no predators, so it got screwed over when humans came to the island with rats and cats and so forth.

But wait, why would an island with no predators exist?

The model I was shown was that birds got blown onto the island, and some of the parrots turned into flightless couch potatoes to save calories. But why didn't any of the other birds evolve to eat the flightless parrots? For that matter the "nothing but birds" line isn't quite true either. New Zealand has a bunch of seals and related mammals. Why didn't one of them turn their flippers back into legs and go fill the new niche of ground based predator? If there's enough time for a Tinamous to completely lose its wings, grow 12 feet tall and be a Moa, it seems like there's time for a seal to try something new.

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Re: Evolution question: Why DON'T niches get filled

Postby idobox » Thu May 09, 2013 4:16 pm UTC

Prior to introduction by humans, the only mammals in NZ were bats and seals, which are not in good position to become land predators.
NZ fauna is dominated by birds, and they occupy all niches. Falcons are a common predator, and the Haast's Eagle was a freaking giant Eagle of doom that ate birds bigger than ostriches. Maori legens also talk of animals big enough to kill humans.

I think there are no native big predators on NZ because humans were better hunters, and starved (or hunted) them to extinction.
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Re: Evolution question: Why DON'T niches get filled

Postby HungryHobo » Thu May 09, 2013 4:27 pm UTC

particularly for small environments it can be a case of there not being enough food to support a breeding population.

many of them are tiny islands, lets say something did go predatory, it takes something like 8 square miles to support one tiger and you need more than 1, you need more than 2, you need a couple of hundred for things to be reasonably stable.

And there can be a kind of inertia, if you've got a population of seals and 100 of them near some islands are getting an advantage from having better legs but 10,000 of them are getting an advantage from having better flippers that doesn't mean they'll automatically split into 2 species if the 2 groups still mix together a lot and keep swapping genes.
They might have an advantage on land but then are more likely to get eaten by sharks while traveling between islands.

sometimes a niche can get filled but if it's too small all the individuals who've evolved to fill it can be killed when some change comes along because their population was too small to include much variation to allow them to cope with change.
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Re: Evolution question: Why DON'T niches get filled

Postby sociotard » Thu May 09, 2013 4:50 pm UTC

I knew about Haasts eagle. And yet, nothing adapted to eat the Kakapo, as far as I know. They had almost no defences, which tells me they had almost no predation. I guess the falcons had trouble with the forest cover?

Again, if a parrot can become a land-based herbivore, why can't a bat become a land-based predator? Bats can kind of run. I watch them do it at the zoo. A few generations chasing stuff on the ground and it seems like they could get awesome at it.

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Re: Evolution question: Why DON'T niches get filled

Postby Scyrus » Thu May 09, 2013 4:59 pm UTC

sociotard wrote:Evolution was explained to me as a drive to fill a niche. An asteroid wipes out all the big animals, but those big animals filled niches, so the remaining little animals diversify, evolve to fill the niches, and now we have big animals again.


I understood it as there existing variety in a population, then the population is exposed to an "obstacle", and those who are more fit (eg. have more phenotypical advantages), are able to reproduce and pass along their genes, while those who are unfit eventually die out. If more than an "obstacle" arises, and they are sufficiently different for there to exist two or more groups of "fit" individuals, then over time the successive generations of the two groups will specialize in their particular traits, and will eventually become different enough to be classed as different species (speciation).

Not many predators appeared to prey on the Kakapo because either nothing happened to the other animal's food supplies, which would force some of them to search alternative sources, or when something did happen, the entire population of that/those species was/were unfit for whatever reason and they all starved to death. It's not like *(insert predator)* can sprout *(insert advantageous limb or appendage)* to go hunt *(insert victim)*, and if some of them had the capacity, there would be no reason to do so if there were enough food in their domain. I don't think animals evolve to fill niches, but rather survive while others fail to do so when tested. I could be wrong, though.

HungryHobo wrote:sometimes a niche can get filled but if it's too small all the individuals who've evolved to fill it can be killed when some change comes along because their population was too small to include much variation to allow them to cope with change.

I agree.

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Re: Evolution question: Why DON'T niches get filled

Postby Dr. Willpower » Thu May 09, 2013 6:10 pm UTC

Scyrus wrote:Not many predators appeared to prey on the Kakapo because either nothing happened to the other animal's food supplies, which would force some of them to search alternative sources, or when something did happen, the entire population of that/those species was/were unfit for whatever reason and they all starved to death. It's not like *(insert predator)* can sprout *(insert advantageous limb or appendage)* to go hunt *(insert victim)*, and if some of them had the capacity, there would be no reason to do so if there were enough food in their domain. I don't think animals evolve to fill niches, but rather survive while others fail to do so when tested. I could be wrong, though.


You have the right idea. If we assume that other factors aren't involved (particularly extinction), then if an advantageous niche exists, its a matter of time before a creature will grow to fill it. There's two ways this can happen, either natural selection forces them into the niche (which can happen in one generation), or all non-fatal mutations thrive and the process becomes essentially random (this can take many generations, or possibly never happen). Although, with each new generation, the likelihood that a population of X generations would not have filled an advantageous niche gets smaller. But at each generation the likelihood of filling the niche is essentially the same.

Of course if a niche is not advantageous, genetically, then a creature should never be expected to fill it without natural selection (although it is still possible for this to happen).
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Re: Evolution question: Why DON'T niches get filled

Postby Heisenberg » Thu May 09, 2013 6:22 pm UTC

sociotard wrote:But wait, why would an island with no predators exist?

It didn't. There were avian predators, but those primarily hunted during the day, so they were avoided by the Kakapo going nocturnal. The only one who hunted by night was the Laughing Owl, which totally killed and ate Kakapos until mammals showed up and ate all of the Laughing Owl's food, including Kakapos.

Source: Your wikipedia article on Kakapos.

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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Thu May 09, 2013 9:37 pm UTC

Evolution is not a "drive" to do anything. Evolution is the change of populations of living organisms over time. One (big, important, awesome, interesting) mechanism of evolution is natural selection, the enrichment of traits that increase the survival of the organism (in that particularly environment). Evolution is not a "drive" to fill a niche. An empty niche is a problematic concept to begin with. "Niches" appear and disappear as other organisms evolve (the niche of clothing louse didn't exist until humans invented clothing), and niches are only "filled" when an organism exists that evolves into it. And even when they are filled, competing species can elbow the species that has "filled" a niche out of it. Darwin did think that increasing competition over an increasingly crowded niche space drove an increase in diversity and complexity (and, as Darwin saw it, progress) over the history of life ... but that doesn't mean that evolution is a drive to fill niches.

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Re: Evolution question: Why DON'T niches get filled

Postby qetzal » Thu May 09, 2013 9:41 pm UTC

Evolution takes time and is contingent. What an organism can evolve into depends heavily on what it already is. When it comes to evolving to fit some new niche, sometimes it's a situation of "you can't get there from here." Or if you can, it might take so long and so many steps that "there" wouldn't exist any longer.

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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Thu May 09, 2013 10:17 pm UTC

What qetzal said.

Also, as evolution is not directed by any external power or intelligence or plan, there is nothing to "see" an "empty niche." In order to fill an empty niche, an organism essentially has to wonder into it, due to proximity and random mutation.

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Re: Evolution question: Why DON'T niches get filled

Postby Xanthir » Fri May 10, 2013 12:22 am UTC

sociotard wrote:Evolution was explained to me as a drive to fill a niche. An asteroid wipes out all the big animals, but those big animals filled niches, so the remaining little animals diversify, evolve to fill the niches, and now we have big animals again.

Except, sometimes the niches don't get filled. My question is why. Why do niches sometimes go unexploited?

Enuja and qetzal got it.

While it's often useful, as a thinking shortcut, to consider evolution as "trying to fill a niche", that's anthropomorphization, and not actually true.

In reality, niches get exploited randomly, as creatures discover new foods to eat, or new ways to obtain food, or new behaviors that help them out (or evolve things that let them do one of the preceding things). Animals and evolution don't *try* to fill niches, but they often randomly stumble onto them, and then become successful because nobody else is occupying that niche yet. It's that "randomly stumble onto them" part that doesn't always happen, because it's random.
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Re: Evolution question: Why DON'T niches get filled

Postby dudiobugtron » Fri May 10, 2013 2:22 am UTC

sociotard wrote:Again, if a parrot can become a land-based herbivore, why can't a bat become a land-based predator? Bats can kind of run. I watch them do it at the zoo. A few generations chasing stuff on the ground and it seems like they could get awesome at it.

I'm not sure what this is in response to, and I'm not sure if this goes against what you're saying, but: they can. And did. In New Zealand. Those were probably the bats you saw at the zoo even.
http://en.wikipedia.org/wiki/New_Zealan ... #Behaviour
(Bolding mine):
Short-tailed bats spend only around 30% of their foraging time catching insects in the air, typically flying less than 2 m (6 ft 7 in) above the ground, and a further 40% feeding from plants. The remaining 30% is spent hunting on the forest floor, a higher proportion than any other species of bat. To assist with this unusual style of hunting, short-tailed bats are able to fold their wings inside a protective sheath formed from their membranes, and the wings have only a very limited propatagium, making them more flexible and mobile. Movement along the ground is also assisted by strong hind limbs and a robust pelvic girdle, and by the additional talons on their claws.
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Re: Evolution question: Why DON'T niches get filled

Postby ImagingGeek » Fri May 10, 2013 1:23 pm UTC

sociotard wrote:Evolution was explained to me as a drive to fill a niche. An asteroid wipes out all the big animals, but those big animals filled niches, so the remaining little animals diversify, evolve to fill the niches, and now we have big animals again.

Others have touched on this, but its worth stating this explicitly.

The above is not part of evolutionary theory - evolution is descent with modification. Filling a niche is but one possible outcome of evolution. It is not the 'purpose' of evolution, nor does niche filling have to occur for evolution to occur. Most adaptive evolution is organisms evolving within their existing niche - i.e. becoming better at exploiting its resources, or adapting to stay in a changing niche. Most evolution is neutral (non-adaptive), so the bulk of evolution isn't even 'directed' at niches.

sociotard wrote:Except, sometimes the niches don't get filled. My question is why. Why do niches sometimes go unexploited?

The case that prompted the question is New Zealand Kakapo. It's a flightless parrot that is well adapted to living on an island with no predators, so it got screwed over when humans came to the island with rats and cats and so forth.

But wait, why would an island with no predators exist?

There are a number of flaws with this argument:
  1. Small predators do exist. A more accurate question would be why didn't any predators big enough to prey on the kakapo evolve?
  2. Even the revised question is wrong - there were/are a number of bird species capable of preying on the kakapo. So we need to revise the question again - why didn't any land predators large enough to prey on the kakapo evolve?
  3. Others have given a number of good answers to the revised-revised question - insufficient time to evolve a predator, not enough area to support a species of large predators, evolutionary/morphological 'blockades' between the existent animals and a conceivable predator, etc.

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Re: Evolution question: Why DON'T niches get filled

Postby idobox » Fri May 10, 2013 4:44 pm UTC

ImagingGeek wrote:There are a number of flaws with this argument:
  1. Small predators do exist. A more accurate question would be why didn't any predators big enough to prey on the kakapo evolve?
  2. Even the revised question is wrong - there were/are a number of bird species capable of preying on the kakapo. So we need to revise the question again - why didn't any land predators large enough to prey on the kakapo evolve?
  3. Others have given a number of good answers to the revised-revised question - insufficient time to evolve a predator, not enough area to support a species of large predators, evolutionary/morphological 'blockades' between the existent animals and a conceivable predator, etc.

Bryan

And also, competition with existing predators.
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Re: Evolution question: Why DON'T niches get filled

Postby Newt » Fri May 10, 2013 5:20 pm UTC

Just because a mutation would be advantageous doesn't mean that it will ever occur.

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Re: Evolution question: Why DON'T niches get filled

Postby DaBigCheez » Fri May 10, 2013 5:52 pm UTC

Really, it's in part a question of the fitness landscape - if a bird having slightly smaller wings means they're slightly more agile and slightly more able to catch insects, or having slightly larger wings means they're slightly better at sustained flight and slightly better at hopping from island to island to find seeds, that's one thing. However, it will introduce a much larger delay or prevent the transition from occurring at all if the area between here and there involves going through a transitional state with much lower fitness; maybe fish have become plentiful but the bird's beak is adapted to eat seeds, and while a bird that had more water-repellent wings, talons with stronger claws, and a beak and digestive system better suited to eating fish could thrive with that food source, it may be unlikely for that to come about because a fish-eater beak and digestive system would suck at eating seeds, and they can't catch the fish without wing/talon modifications that take both substantial random chance to come about and potentially more energy to grow. So, even though they're kinda running out of seeds and there's all these delicious fish right there (higher fitness peak), you can't easily et there from here (bigger trough in the fitness landscape between the two peaks).
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Re: Evolution question: Why DON'T niches get filled

Postby Copper Bezel » Fri May 10, 2013 7:57 pm UTC

Newt wrote:Just because a mutation would be advantageous doesn't mean that it will ever occur.


Individual mutations inevitably will, given the time scale. But particular adaptations generally take more than a single mutation, and they may have to appear more than once to spread into the population, etc., etc.
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Re: Evolution question: Why DON'T niches get filled

Postby Technical Ben » Sat May 11, 2013 8:09 am UTC

Seems to be a problem here. We are assuming in this particular case there is a niche/drive/selection for predators. Can we confirm that is the right assumption? If we make it erroneously, then of cause such a theory of "predators will top the web of life" will appear. It may be that it's simple more energy efficient to be a couch potato in the ecosystem of NZ. Thus there is no selective pressure for predator populations, and possibly selective pressure against them.

As DaBigCheeze says, evolution cannot plan ahead. Nor can chance mutations. So if we do not observe the required mutations or the evolution in this particular fitness landscape, what should we conclude about this particular example? (The environment may not even provide a route of evolutionary progress to the fittest required animal, let alone the mutations, as here?)
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Re: Evolution question: Why DON'T niches get filled

Postby Copper Bezel » Sat May 11, 2013 8:51 am UTC

Couch potatoes are big bags of concentrated nutrients that don't really try very hard not to be eaten. In most cases, yeah, there is a benefit to being something that can and will eat them. Of course, large enough and sparse enough herbivores can be in a position of having no natural predators, but the kakapo isn't in that class.
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Re: Evolution question: Why DON'T niches get filled

Postby Technical Ben » Sat May 11, 2013 12:10 pm UTC

Again, that's assuming it's easier to eat a couch potato over eating what the couch potato eats. That's a poor assumption IMO. It may be true in some environments, but I see no requirement for it to be true in all environments (or all webs of life if including those in our consideration).

We would first need to check and prove that a predator is always the selected for and easiest route for the animal. As far as I know, the easiest route/action is the one that will have most success, and be selected for the most. So can we assume a predator to the Kakapo will always be more successful than a Kakapo? If for an instance imaging just Kakapos on an island, what would cause selection so one group of Kakapos suddenly start becoming predators to the others? If we cannot assume every environment would cause such a split between Kakapo descendants, then neither can we assume it would apply to other island creature who are not yet Kakapo predators.

If there is no "niche" in the first place, we would be wasting time looking for the prevention of a niche being filled. Perhaps we should look at where the niches are first? Although I agree there are things that would interfere and prevent a selection for the fittest of the current environment (such as random natural disasters), I'm not sure if the example of a Kakapo is an example of this.
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Re: Evolution question: Why DON'T niches get filled

Postby gmalivuk » Sat May 11, 2013 2:07 pm UTC

Technical Ben wrote:Again, that's assuming it's easier to eat a couch potato over eating what the couch potato eats. That's a poor assumption IMO. It may be true in some environments, but I see no requirement for it to be true in all environments
It's an advantage to eat the couch potato since it has already done the hard work of concentrating the nutrients from the food it eats.

What this means is that there is a niche for eating couch potatoes. What it doesn't mean is that there's any probable evolutionary path for some subpopulation of couch potatoes themselves to become the predators.
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Re: Evolution question: Why DON'T niches get filled

Postby tomandlu » Sun May 12, 2013 2:07 pm UTC

The more I think about it, the more it seems a bloody interesting question - why were there no small land-based predators in NZ? Surely it's big enough? No stoats, foxes, rats, or their local equivalent? There may be no imperative to fill a niche, but if it's there one might reasonably expect life to occupy it unless there was some other disincentive. I wonder if the marsupial nature of NZ mammals played a part - certainly, the invasion of placental mammals from N. America to S.America didn't seem to go well for the SA marsupials...
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Sun May 12, 2013 2:34 pm UTC

Here is an online book chapter (I found it by searching for "predators oceanic islands evolution," for which it was the second result, but I actually know the author: he's an evolutionary biology professor at UCI, and knows both teaching and evolution quite well) about islands, that provides some useful context. It doesn't explicitly answer your question, but it does mention that the species number at equilibrium (equal number of species additions by speciation and immigration to number of species extinctions) is lower for smaller islands. This is known as the "species area curve." In general, if you want to know more about this stuff, look into island biogeography, the species area curve, extinction, and adaptive radiation. A lot of what we know about evolution in general comes from islands.

An implication of the fact the smaller the area, the fewer the species is that this lower species number means there aren't as many predators. An interesting hypothesis that pops into my mind is that, if we were to stop introducing them, in evolutionary time maybe a lot of those introduced predators would go extinct, and the herbivores would again evolve a kind of predator naivete.

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Sun May 12, 2013 4:18 pm UTC

It's an interesting question; I wager it has to do with when [island] became geographically isolated. It follows that weirdo ground or burrowing birds could develop, because birds can be blown onto islands much easier than mammals or reptiles can be. I wager there's something about the difficulty or distance from the main land that determines how likely an island is to be seeded with land predators.

So, the ecological timescales involved means that if you have a geographically isolated landmass, the only thing that can get seeded there is birds and insects, and there's just enough time involved for birds to adopt some ground niches. The lack of ground predators is why you see such insane behavior on the part of birds (Kiwis, seriously, what the fuck guys?), and such enormous insects like the Weta.
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Sun May 12, 2013 4:37 pm UTC

Izawwwlgood, the species area curve suggests that it doesn't matter how much time you've got: because species become extinct, immigration is limited, and speciation is slow, the equilibrium total number of species is small. You're right that birds and insects colonize islands much better than mammals do, but that doesn't mean you'd expect to have a large compliment of land predators given enough time. You're less likely to develop or retain land predators at all, because species become extinct. The low number of species on islands is not just about ecological time scales, it's also about evolutionary time scales.

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Sun May 12, 2013 4:47 pm UTC

Enuja wrote:You're less likely to develop or retain land predators at all, because species become extinct.
Is there evidence of ground predators being introduced to island communities previously, insofar as island wide extinctions being observed?
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Sun May 12, 2013 4:56 pm UTC

I'm sorry, I don't understand your question. The chapter I linked talks about extinctions on islands, associated with humans and introductions of other non-native predators by humans, but do you mean predator-caused extinctions previous to humans arriving on islands? Humans have arrived on islands throughout our evolutionary history, and extinctions have followed. I don't personally know of any good evidence of non-human related immigration of predators to islands, but I'm honestly not sure how you'd get that evidence at all, if you're only interested in things that happened millions of years ago. I also don't know why you'd only be interested in things that happened before humans could record them, and in the absence of humans.

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Sun May 12, 2013 5:08 pm UTC

I quoted the statement I was uncertain about, and asked a question about it. Let me try again to be more clear;
Enuja wrote:You're less likely to develop or retain land predators at all, because species become extinct.
To me, this sounds like you're saying that a given ecology is unlikely to develop land predators, because they and their prey will become extinct. This makes sense if you imagine fairly small ecologies, like islands; introduction of a land predator will simply wipe out the native populations, leading to he extinction of both. What I'm asking is if there is evidence of this happening sans human intervention.

Enuja wrote:I also don't know why you'd only be interested in things that happened before humans could record them, and in the absence of humans.
I don't know why you'd only be interested in ecology that occurs in the presence of humans, nor why you'd imagine that humans can't record the evolutionary history of islands?
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Sun May 12, 2013 5:52 pm UTC

I think we're talking about extinctions on islands caused by the introduction of predators.

I've never been particularly interested in paleontology, and I'm also quite cautious about the sampling issues with paleontology. I'm more interested in the natural experiments for which we have lots of data. And I've heard a little bit about earlier extinctions, in the context of human immigration and animals that humans bring with them. But I don't, personally, know of any evidence about extinctions that don't involve humans. Plenty with humans dumping species on islands and then leaving, of course. But, for some reason, as far as I understand, you're disinterested in, or think there is something wrong with, evidence from human-caused introductions? If that's what you think, I really don't understand that.

More evidence is better than less evidence. We've got lots of high-evidence natural experiments with human introductions to islands. I think it makes sense to figure out ecological and evolutionary processes using current and historic evidence, and then extrapolating to paleontological examples. Paleontological evidence is really good for some things (speciation, phylogentic realtionships, ect.), but for figuring out what happened on ecological time scales (like the introduction of species and extinction specifically caused by that predator), not so much.

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Sun May 12, 2013 6:04 pm UTC

Enuja wrote:I think we're talking about extinctions on islands caused by the introduction of predators.
Yes, that's what I'm discussing as well.

Enuja wrote:I've never been particularly interested in paleontology, and I'm also quite cautious about the sampling issues with paleontology. I'm more interested in the natural experiments for which we have lots of data. And I've heard a little bit about earlier extinctions, in the context of human immigration and animals that humans bring with them. But I don't, personally, know of any evidence about extinctions that don't involve humans. Plenty with humans dumping species on islands and then leaving, of course. But, for some reason, as far as I understand, you're disinterested in, or think there is something wrong with, evidence from human-caused introductions? If that's what you think, I really don't understand that.
I think you grossly misunderstood my question then; I'm not at all suggesting that I'm disinterested in human caused extinction events via introduction of invasive species. I'm asking if there are examples of extinction events caused by non-human mediated introductions of ground predators to island populations that previously didn't have them, because your statement was;
Enuja wrote:You're less likely to develop or retain land predators at all, because species become extinct.

Now, obviously human interactions via introductions of new species cause a glut of data, because it's a massive change in the ecosystem. I'm wondering if your statement has any previous supporting evidence for being true sans human interaction.

Enuja wrote:Paleontological evidence is really good for some things (speciation, phylogentic realtionships, ect.), but for figuring out what happened on ecological time scales (like the introduction of species and extinction specifically caused by that predator), not so much.
Yes; you could tell a lot about genetic analysis of island oddities (Kiwis, Weta's, etc) in terms of likelihood of previous mass extinction events however. That's all I'm asking about; is there evidence of mass extinction events of oddity ground lifeforms on island populations.
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Re: Evolution question: Why DON'T niches get filled

Postby tomandlu » Sun May 12, 2013 6:12 pm UTC

Izawwlgood wrote:
Enuja wrote:You're less likely to develop or retain land predators at all, because species become extinct.
Is there evidence of ground predators being introduced to island communities previously, insofar as island wide extinctions being observed?


I read something about NZ's largest bird predators becoming extinct as the area wasn't enough to support them. I haven't read Enuja's link yet, but I will. Bottom line, there does seem to be a lot of evidence that you need a substantial land-mass to support predator populations - I'm just surprised that NZ isn't large enough. I suspect that part of the answer is that the path from NZ's past to now is convoluted beyond imagination or discovery. The bit that makes me wonder about the role marsupials is that, when rats, etc. were introduced by humans, they were/are very successful. That said, predators are presumably always the last to arrive and will always be the first to die out if the environment cannot support them - still, interesting. Must remember that link sitting there...
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Re: Evolution question: Why DON'T niches get filled

Postby Technical Ben » Sun May 12, 2013 10:15 pm UTC

Izawwlgood, what is the difference between a predator riding a log to an island, and a predator riding a humans boat to an island? Say a rat for example?

There is at least one difference, I'll see if you notice it I think you hinted at it with "glut". What would we be controlling for by testing the difference (between human introduced predators and "naturally" introduced ones)? It's a good question that you brought up. :)

PS, was the marsupial "wolf" a predator? It would seem there were some about perhaps? Was it human intervention that removed them?

PPS, aaaaahhh... a fact finding hunt on Moas, which I erroneously thought could be a predator to small creatures like the cassowary, I found this: https://en.wikipedia.org/wiki/Haast%27s_Eagle
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Sun May 12, 2013 11:22 pm UTC

Izawwlgood wrote:I think you grossly misunderstood my question then;
Very likely: as I keep saying, I'm having a very hard time understanding the meaning and intent of your questions.
Izawwlgood wrote:I'm not at all suggesting that I'm disinterested in human caused extinction events via introduction of invasive species. I'm asking if there are examples of extinction events caused by non-human mediated introductions of ground predators to island populations that previously didn't have them,
I don't personally know of any examples of this. But this may be because I'm not particularly familiar with the literature that would contain these examples. Feel free to look in the literature for evidence of this yourself. I'm not going to. Technical Ben just linked to a Wikipedia article which states that it was the human-caused extinction of Moa that led to the extinction of the Haast's Eagle (a Moa predator) on New Zealand. Humans were present at a lot of places, at a lot times, so it's hard to find relatively recent evidence that doesn't involve us.

(repeating a bit of your quote to provide context)
Izawwlgood wrote:I'm asking if there are examples of extinction events caused by non-human mediated introductions of ground predators to island populations that previously didn't have them, because your statement was;
Enuja wrote:You're less likely to develop or retain land predators at all, because species become extinct.

Now, obviously human interactions via introductions of new species cause a glut of data, because it's a massive change in the ecosystem. I'm wondering if your statement has any previous supporting evidence for being true sans human interaction.
I thought we were now talking about predator-caused extinctions, but when I said that you're less likely to develop or retain land predators on islands, I was not talking about introductions as the direct cause of extinction. I was talking about the observation that the smaller the island, the fewer the species. That's it. Introductions of predators really do appear to cause extinctions on islands, but this mechanism is not necessary to make predators less common on islands. On islands, all extinctions are more common than on larger land masses, because you don't have regular immigration of nearby populations to the island, to make up for extirpation (local extinction). When all extinctions are more common, it seems likely to me that you're much, much less likely to retain predators. To have predators, you need both the plants and the herbivores. To have herbivores, you need only the plants. I know of lots of evidence that there are fewer species on smaller islands. I do not know (and have not looked for) any research onto whether predators are disproportionally less likely to be present on islands, but this seems somewhat superfluous. Land predators are a relatively small percentage of land animal species, and given that there are fewer species, it seems obvious that there are fewer predators. But I am not an expert on islands, extinction, or predators, so I don't really know.

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Mon May 13, 2013 12:06 am UTC

Your distinction of land predators and predators is what I'm wondering about though. NZ obviously has a bunch of predatory animals, just not many land predators (or rather, the introduction of land predators was/is fairly devastating to local ecosystems). This point doesn't seem remotely superfluous, given that we're talking about a specific niche and it's probability of being filled. And my point was that because we are less likely to see ground based animals washing their way over to islands, it seems like geography is the prime factor in reducing the likelihood of ground predators getting to an island.

We are talking about predator induced extinction, because we are talking about land predators filling the more or less empty niche on island ecosystems. I'm not sure what was confusing about that?

In any case though, you say you aren't an expert on the matter (neither am I!) so it's possible my question will require research to answer.
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Mon May 13, 2013 12:59 am UTC

I was just trying to stick to our example, since New Zealand does have /has had non-land predators. I wasn't trying to make a theoretical distinction between land and non-land predators. So you asking me why I was making a distinction I wasn't trying to make (and wasn't aware anyone was trying to make) is why I was very, very confused. You're probably correct about the difficulty of immigration of ground predators to oceanic islands, but I don't know how useful it will be for you to make a distinction between "ground" predators and other types of predators. Different islands are going to have different species and different ecologies, and I think it's the details and historic contingency that matter, not some theoretical distinction between "types" of predators. But I could be wrong, and you should feel free to look for research specifically on "ground" or "land" predators on islands. But I can't even give you good keywords (like I did above for island evolution), because I don't know the theoretical categories biologists might use to ask and answer your questions.

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Mon May 13, 2013 1:21 am UTC

Because we've been talking about land/non-land predators this whole time?

Waitwait, lets back the hell up;
Is this question not
sociotard wrote:The case that prompted the question is New Zealand Kakapo. It's a flightless parrot that is well adapted to living on an island with no predators, so it got screwed over when humans came to the island with rats and cats and so forth.

But wait, why would an island with no predators exist?

We're asking about why there are/were no land based predators on an island community like NZ. We seem to have gotten turned around with you assuming something about my interests in the utility of some types of observations or that I was implying that eventually you'd get land predators.
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Re: Evolution question: Why DON'T niches get filled

Postby Enuja » Mon May 13, 2013 1:41 am UTC

Some of sociotard's premise is incorrect, as lots of people in this thread have shown. I think a more accurate/interesting/useful way to phrase the question would be "Why would a species exist that is vulnerable to new predators?" in large part because New Zealand did have predators. It was not an "island without predators," just an island without predators that were good at targeting Kakapo. The niche "predator" was not empty: just the niche "Kakapo predator" (and other "niches," some predatory, some not, and some, like "feline parasite" not just empty, but not yet possible). And the general category of island endemics vulnerable to new predators is not limited to New Zealand, nor to "ground" predators. The most famous predator to cause problems in islands is probably the brown tree snake, an arboreal predator.

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Re: Evolution question: Why DON'T niches get filled

Postby sociotard » Mon May 13, 2013 1:57 am UTC

My original question was as to why there weren't any ground based predators. I think the comment of sight-based vs. scent/hearing based predation was what really answered my question; mammals dominated the Kakapos not because they were land-based but because they were using different sense organs. (except humans, which are land based and sight based predators, but humans just sort of dominate all so we don't count)

I suppose my question could change to "why didn't any birds develop good senses of smell/hearing. After all, humans are sight based animals that evolved from scent/hearing based animals, so I suspect the opposite can happen. And I know some owls have really good hearing. (and the kakapo's predator was an owl, so maybe that is what made the difference)

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Re: Evolution question: Why DON'T niches get filled

Postby Izawwlgood » Mon May 13, 2013 3:26 am UTC

I misread the question than to be 'why would an island have no ground predators', which I answered in my first post. Enuja, the Brown Tree Snake you linked is an invasive species; if anything, that underlines the point that some islands don't have any ground predators.

Since we're talking about NZ, where a burrowing parrot, flightless bird, and grasshoppers the size of small birds exist, talking about a lack of ground based predators is rather apropos.
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Re: Evolution question: Why DON'T niches get filled

Postby tomandlu » Mon May 13, 2013 7:38 am UTC

You learn something every day - I hadn't taken in that, prior to human interference, NZ didn't even have any marsupials. A few species of bat and some sea mammals, plus the odd sea-snake, but apart from that it was just birds and invertebrates. One article I read suggested that the lack of certain trace elements disfavoured mammals and favoured birds, but I've no idea where on the crackpot scale that falls.
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