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SkepticJ
2012-Nov-06, 01:03 AM
I was lounging in the tub last night and the question arose in my mind, "Why do predator animals exist?"

Herbivores are predators of autotrophs, but why do they have predators, and sometimes those predators have predators?

The total available energy doesn't go up with increasing complexity of the food web, so why aren't there just more herbivores and no predators?

Noclevername
2012-Nov-06, 01:15 AM
I was lounging in the tub last night and the question arose in my mind, "Why do predator animals exist?"

Herbivores are predators of autotrophs, but why do they have predators, and sometimes those predators have predators?

The total available energy doesn't go up with increasing complexity of the food web, so why aren't there just more herbivores and no predators?

The total energy does not go up, but the energy available is more concentrated and in a more easily digested form. Why spend most of the day grazing and converting grass into animal protein when a cow has already done it for you?

Strange
2012-Nov-06, 01:16 AM
Because things will evolve to exploit any resource, I assume.

If there is an animal that nothing is preying on then it is a potential resource for which there is no competition (yet). So there is an advantage for the first species to start chowing down.

Jens
2012-Nov-06, 01:31 AM
Because things will evolve to exploit any resource, I assume.


I think basically that's true, and somebody might ask, "what about the apex predators," but in fact even as humans we have predators, things like TB germs and malaria, for example.

ravens_cry
2012-Nov-06, 01:44 AM
Those are more parasites than predators. In fact, it is in the best interests of a pathogen to not kill its host, as that means the end of the gravy train for them.

Solfe
2012-Nov-06, 01:48 AM
I think creatures evolve to consumer an particular energy source, then evolve to use many energy sources. Once that occurs, there exists the potential to consume something similar in nature to ones itself. Whichever creature is able to consume the most capable prey or the widest variety of prey is the apex predator.

We happen to be able to eat lions and tigers and bears, but in reality, I suspect they eat more humans than we do them. We are a glitchy apex predator. We soft kill creatures while dominating the environment. That isn't exactly predation. Bears and such consider us snack food if we don't have our normal barriers or weapons.

ShinAce
2012-Nov-06, 02:10 AM
They also provide a feedback mechanism. If herbivores thrive too much, they eat all the plants and then starve. Predators help keep populations in check.

Swift
2012-Nov-06, 02:55 AM
Because things will evolve to exploit any resource, I assume.
That would be my answer. Life seems to have a tendency to evolve to exploit all available resources, both for energy and for "building materials". The first thing that comes along and can obtain these from a previously unused resource gains a huge advantage - they get that resource all to themselves.

Predators date back at least to early multicellular creatures and I suspect there were one-celled predators too.


They also provide a feedback mechanism. If herbivores thrive too much, they eat all the plants and then starve. Predators help keep populations in check.
Completely true, but that would not be a driver for the development of predators. A predator doesn't evolve to save plants, or to save herbivores from overpopulating. You can have a "stable" ecosystem of just plants and herbivores, though I suspect it tends to be rather cyclic.

ravens_cry
2012-Nov-06, 03:15 AM
From a plants perspective, a herbivore is a kind of predator.
Could you have a stable ecosystem with only plants and decomposers? What about literally just plants?

Solfe
2012-Nov-06, 05:32 AM
From a plants perspective, a herbivore is a kind of predator.
Could you have a stable ecosystem with only plants and decomposers? What about literally just plants?

I wish I could provide a credible answer, but I want to reference every Sim Earth game I played in the 1990's. :) The Gaia Hypothesis Scenario was pretty compelling.

novaderrik
2012-Nov-06, 11:06 AM
because meat is awesome.. that's why there are predators.

starcanuck64
2012-Nov-06, 05:54 PM
Nature tends to want to fill vacuums and I think that also applies to evolution.

The line between predator and herbivore is often blurred, deer for instance will hunt young birds for the calcium in their bones, it's not hard to see them becoming more of an omnivore or even predators given the right conditions and enough time.

http://news.nationalgeographic.com/news/2003/08/0825_030825_carnivorousdeer.html


Red deer on the Scottish island of Rum may be eating the heads and legs of live seabird chicks as a way to get minerals they need to grow their antlers.

Scientists believe this surprising addition to the red deer's diet stems from mineral deficiencies in the vegetation they eat. By munching on Manx shearwater chicks, the deer are able to get the extra calcium they need.

This is a video of a buck eating a bird.

http://www.youtube.com/watch?v=sQOQdBLHrLk

Swift
2012-Nov-06, 06:55 PM
Could you have a stable ecosystem with only plants and decomposers? What about literally just plants?
Yes. A terrarium is an example of this - you can have a sealed container with just soil, plants, moisture and it will survive as long as you supply light. I suspect there may be some isolated natural ecosystems like this - a rock in the middle of the ocean with just a few plants growing on it, for example. But I don't know that such ecosystems are stable over long periods of time (decades, centuries, etc.).

Though even there you might argue that there are almost certainly things like bacteria, fungii, and other microbes in such an ecosystem.

Swift
2012-Nov-06, 09:04 PM
A little googling found a book chapter from Yale University entitled "Origins and early evolution of predation" (http://www.yale.edu/ypmip/predation/Chapter_11.pdf) that people might find interesting.

Abstract:

Predation, in the broad sense of an organism killing another organism for nutritional purposes, is probably as old as life itself and has originated many times during the history of life. Although little of the beginnings is caught in the fossil record, observations in the rock record and theoretical considerations suggest that predation played a crucial role in some of the major transitions in evolution. The origin of eukaryotic cells, poorly constrained to about 2.7 Ga by geochemical evidence, was most likely the ultimate result of predation among prokaryotes. Multicellularity (or syncytiality), as a means of acquiring larger size, is visible in the fossil record soon after 2 Ga and is likely to have been mainly a response to selective pressure from predation among protists. The appearance of mobile predators on bacteria and protists may date back as far as 2 Ga or it may be not much older than the Cambrian explosion, or about 600 Ma. The combined indications from the decline of stromatolites and the diversification of acritarchs, however, suggest that such predation may have begun around 1 Ga. The Cambrian explosion, culminating around 550 Ma, represents the transition from simple, mostly microbial, ecosystems to ones with complex food webs and second- and higher-order consumers. Macrophagous predators were involved from the beginning, but it is not clear whether they originated in the plankton or in the benthos. Although predation was a decisive selective force in the Cambrian explosion, it was a shaper rather than a trigger of this evolutionary event.

starcanuck64
2012-Nov-06, 10:47 PM
Although predation was a decisive selective force in the Cambrian explosion, it was a shaper rather than a trigger of this evolutionary event.

The appearance of complex vision probably had a big part to play in this.

http://www.ncbi.nlm.nih.gov/pubmed/22158247


The existence of large, macrophagous nektonic predators possessing sharp vision--such as Anomalocaris--within the early Cambrian ecosystem probably helped to accelerate the escalatory 'arms race' that began over half a billion years ago.

Delvo
2012-Nov-07, 03:02 AM
I was lounging in the tub last night and the question arose in my mind, "Why do predator animals exist?"For one thing, they probably got here first, because their food did. Animals appear to predate plants, so for a while they could not possibly eat plants but could possibly eat each other. Of course, you could point out that photosynthesizing microbes were here before animals and equate to plants in a practical sense, but those came along after non-photosynthesizing, more animal-like microbes. (Photosynthesis was not the original starting point; life originally thrived on chemical nutrients in the water, which had a significantly higher concentration back then.)


The total available energy doesn't go up with increasing complexity of the food webIt does. Adding more layers on top doesn't diminish the the layers below; it just adds more layers to the total.

Animals eating plants don't destroy the plants' energy. They transfer it to themselves while creating openings for new plants to fill. Then the ecosystem contains about the same amount of plant growth, plus the energy that went into the animals. Add another layer of animals eating those animals, and you get the same thing again. They don't destroy the herbivores' energy. They transfer it to themselves while creating openings for other herbivores to fill. So instead of only the energy of about the same supply of plants and herbivores, the ecosystem contains the energy of the plants, herbivores, and carnivores.

Eventually, of course, 100% of the energy that ever entered the system (from sunlight absorbed by plants) leaves it (mainly as infra-red, because that's the frequency range radiated by things in our general range of temperatures). Life is just where it pauses for a while on the way (sort of like the old saying that soil is a mountain on its way to the sea). But in a thickly layered food chain/web, more of it stays there longer than in a simpler ecosystem, for a greater total amount present at any given time.


why aren't there just more herbivores and no predators?What would the additional herbivores eat? Once an environment has as many herbivores as it can, the plants are all taken and can't feed any more, so there's nothing else left available for any additional animals to eat except animals.


Predators date back at least to early multicellular creatures and I suspect there were one-celled predators too.Here's a video of a cell chasing another cell :D: http://www.youtube.com/watch?v=BNizygcJA9A

Also, consider the results of the latest experiments trying to approximate the oceans' early chemical conditions. Not only do amino acids and nucleic acids join together and start spontaneously forming small random protein and DNA/RNA chains, but fatty acids also come together and start spontaneously forming microscopic round enclosed bubble-like structures called "vacuoles". Just due to laws of chemistry and thermodynamics, they tend to grow by incorporating more free-floating fatty acid molecules from their surroundings into their surfaces and letting in more of those other organic acids than they let out. Once they get too big to remain intact, they split into smaller ones which continue the trend. And when they're in contact with each other, the larger one grows by incorporating fatty acid molecules into its surface straight from the smaller one. So these vacuoles not only are structured like a simper version of cells (lipid outer membrane, water inside with higher concentration of organic compounds than outside) and grow and divide like cells, but also consume others like some cells and multicellular organisms do... all before even developing most of the traits we normally think of as making a cell a cell.

SkepticJ
2012-Nov-08, 03:36 AM
It does. Adding more layers on top doesn't diminish the the layers below; it just adds more layers to the total.

No, it doesn't. There's no such thing as a free lunch--see below.


Animals eating plants don't destroy the plants' energy.

They don't destroy energy--which can't be done--but energy is lost to entropy at each level. That's why there's not that many levels to food webs. If your assertion was true, it would be physically possible for predators to have predators that have predators, that have predators, that have predators, tens, hundreds of layers deep. If they could, why don't we see this in nature?


Eventually, of course, 100% of the energy that ever entered the system (from sunlight absorbed by plants) leaves it (mainly as infra-red, because that's the frequency range radiated by things in our general range of temperatures). Life is just where it pauses for a while on the way (sort of like the old saying that soil is a mountain on its way to the sea). But in a thickly layered food chain/web, more of it stays there longer than in a simpler ecosystem, for a greater total amount present at any given time.

I don't think this is correct. Energy can be stored basically forever without losing it. Look at oil, coal, natural gas . . . energy from long-dead organisms, which originally came from the Sun. Energy is lost when it's used. The more critters you have running around eating one another, fighting, the more complexity, the more it is used up.

I think there'd be more active biomass if there was only: producers, consumers, and decomposers.

If nothing ate or decomposed dead things, they'd just pile up and stay there like styrofoam until there wasn't any more simple raw materials to make new living things out of, and then no more life.

Predators are just "bad guys" that have been around a long, long time.

BioSci
2012-Nov-08, 05:32 PM
"Why do predator animals exist?"


A simple, (evolution-based) answer: Because they can. :)

(species evolution is not design or goal based - but more of a "random walk" through what is possible and what "works".)

PlutonianEmpire
2012-Nov-09, 04:52 AM
I wish I could provide a credible answer, but I want to reference every Sim Earth game I played in the 1990's. :) The Gaia Hypothesis Scenario was pretty compelling.
Fun times. Great memories. Love that game. :)

Shame the PC version had glitchy plate tectonics. But still awesome in that I could actually name my custom planets. :)

One of the scenarios on the SNES version has one of my two most favorite video game songs. :)

Ara Pacis
2012-Nov-09, 08:26 AM
Why do predators exist?

Because prey exists.

It's not about choice. It's about chemistry.

Robert Tulip
2012-Nov-09, 10:11 AM
A simple, (evolution-based) answer: Because they can. :) (species evolution is not design or goal based - but more of a "random walk" through what is possible and what "works".)

Yes, evolution of predation illustrates that mutation is random but selection is determined by the possibilities of the niche. As Murphy might have said, whatever can happen will happen.

When a predator has offspring that are more successful killers, those offspring have relatively more young, driving cumulative adaptation towards successful predation. And meanwhile the prey continually adapt as well with the most successful at avoiding being eaten the ones having most young, producing the arms race of the Red Queen, both sides running as fast as possible just to stay in one place.

Evolution is a bit like guessing a hidden card, with the eventual successful random guesser the one who gets to reproduce, and hand on the genes for guessing the card to all its descendents, building on the precedent so the card hider has to evolve a different strategy to get back in the game.

Predation evolves towards an evolutionarily stable strategy, a long equilibrium where each side is co-adapted to the other, with the stability of the geozoom punctuated by occasional catastrophes such as the current anthropocene. If predators are too successful, like virulent plagues, they wipe out their food source and die themselves. Humans seem to be the most successful predators in the history of our planet...

tnjrp
2012-Nov-09, 10:18 AM
"Predation" is simply a term describing a specific form of gathering of and competing for resources. It is not really different at heart from herbivorism (the term can in fact be used for herbivorism too) or indeed, very deep down, ingestion of inorganic matter for nourishment. With the notable difference that the inorganic matter doesn't put up the sort of resistance that organic things often do, obviously.

On the evolution of predation:
http://www.yale.edu/ypmip/predation/Chapter_11.pdf

Swift
2012-Nov-09, 01:39 PM
On the evolution of predation:
http://www.yale.edu/ypmip/predation/Chapter_11.pdf
ToSeeked! :D

A little googling found a book chapter from Yale University entitled "Origins and early evolution of predation" (http://www.yale.edu/ypmip/predation/Chapter_11.pdf) that people might find interesting.

tnjrp
2012-Nov-09, 01:46 PM
Oops. I was sure I was going to post something else but... nevermind. Shall have to look it up later.

Delvo
2012-Nov-11, 06:05 AM
No, it doesn't. There's no such thing as a free lunchI didn't assert one.


They don't destroy energy--which can't be done--but energy is lost to entropy at each level. That's why there's not that many levels to food webs. If your assertion was true, it would be physically possible for predators to have predators that have predators, that have predators, that have predators, tens, hundreds of layers deep. If they could, why don't we see this in nature?You're talking about the fact that each new trophic layer has less energy in it than the one below, which I haven't said they don't, so your sentence starting with "if your assertion was true" is false, responding to an assertion that I haven't made.

What I did say was that an environment with more trophic layers has more energy in living things than it would if it had fewer trophic layers, because each layer does contain an amount greater than zero, so adding a layer does add to the total (even if it adds less than the one below it did).


Energy is lost when it's used. The more critters you have running around eating one another, fighting, the more complexity, the more it is used up.Used, but not "used up", because of the nature of the supply. Energy's entry point into biological ecosystems is photosynthesis, and the amount of photosynthesis happening in an ecosystem is not reduced by animal activity. Destruction of one plant just creates a place for another plant or group of plants to grow.

Rhaedas
2012-Nov-11, 06:46 PM
The total available energy doesn't go up with increasing complexity of the food web, so why aren't there just more herbivores and no predators?

Two points that I didn't see in the comments so far (but could have missed).

The total energy put into the system does increase. The Sun continues to shine.

Evolution doesn't try to maximize energy levels or efficiency. If a variation of something works well enough to allow reproduction within a population, then it may continue into the next generation, and change how energy is gathered. Or not. So far predators seem to work out okay, even if it's not the best system. It's good enough, and that's all that evolution requires.

Gondolla
2012-Nov-14, 03:31 PM
Whether you eat plants or people it all boils down to tiny particles and what you need from them. When life only existed on a cellular level microorganisms ate other microorganisms. When they evolved I'm guessing the same organisms continued to feed as they always had but some grew up to be plants and others legged, toothy things.

Noclevername
2012-Nov-14, 06:49 PM
The total energy put into the system does increase. The Sun continues to shine.


The total energy as you go up the food web. Animals don't use photosynthesis (well, there's one sea slug that does, but it's a one-off). Adding more sunlight doesn't affect plants already eaten.

TooMany
2012-Nov-22, 01:48 AM
They don't destroy energy--which can't be done--but energy is lost to entropy at each level. That's why there's not that many levels to food webs. If your assertion was true, it would be physically possible for predators to have predators that have predators, that have predators, that have predators, tens, hundreds of layers deep. If they could, why don't we see this in nature?


I don't think that increasing entropy is the limitation on levels of predation. Rather I would propose that it is in part size. In the most common case, the predator is larger than the prey by some finite amount. The biggest modern predators are whales, whereas it may have tyrannosaurs in the past. There are practical limits to size on both ends. In addition to the size issue, each predator seems to claim a certain "space" within an ecological system. There may not be enough such spaces to make the hierarchy very deep. In other words (I'm conjecturing) too deep a hierarchy would collapse into a shallower one.

SkepticJ
2012-Nov-22, 07:57 AM
I don't think that increasing entropy is the limitation on levels of predation. Rather I would propose that it is in part size. In the most common case, the predator is larger than the prey by some finite amount. The biggest modern predators are whales, whereas it may have tyrannosaurs in the past. There are practical limits to size on both ends. In addition to the size issue, each predator seems to claim a certain "space" within an ecological system. There may not be enough such spaces to make the hierarchy very deep. In other words (I'm conjecturing) too deep a hierarchy would collapse into a shallower one.

I don't see a reason why predators couldn't be much smaller than their prey.

Imagine an animal that is like a hornet, but it stings with a venom that has a potency comparable to ricin. These "hornets" are basically aerial driver-ants, they rove in colonies. If need be, one of them can risk taking one for the team and fly up a hippo's nose and sting it.

Shaula
2012-Nov-22, 09:23 AM
I don't see a reason why predators couldn't be much smaller than their prey.

Imagine an animal that is like a hornet, but it stings with a venom that has a potency comparable to ricin. These "hornets" are basically aerial driver-ants, they rove in colonies. If need be, one of them can risk taking one for the team and fly up a hippo's nose and sting it.
It is not so much being able to kill them as being able to eat them. It is generally riskier and harder to go after prey much larger than you, and after a certain point the pay-off stops increasing. It doesn't matter to a mouse sized creature if it kill something the size of a cat or the size of an elephant - it gets the same meal. Also the smaller you are the longer you have to stay in one place eating these huge beasties. Which makes you a target. That ricin-tolerant anteater is going to love a huge swarm of snacks stuck eating their kill for an hour thanks to their tiny jaws!

Ara Pacis
2012-Nov-23, 08:46 AM
Aren't Hyenas smaller than Wildebeests?

tnjrp
2012-Nov-23, 09:46 AM
A significant number of preadors are actually smaller than their prey animals, massively so if you include parasitism under predatory behaviour as at least some biologists do. Most mammalian predators do tend to prefer prey that is not several magnitudes larger however, but aren't averse of attacking much larger animals when the need arises. Canines in particular are known for attacking much larger prey such as adult ungulates in packs if they can't find suitable smaller prey.

TooMany
2012-Nov-25, 12:32 AM
A significant number of preadors are actually smaller than their prey animals, massively so if you include parasitism under predatory behaviour as at least some biologists do. Most mammalian predators do tend to prefer prey that is not several magnitudes larger however, but aren't averse of attacking much larger animals when the need arises. Canines in particular are known for attacking much larger prey such as adult ungulates in packs if they can't find suitable smaller prey.

At least one study of Arctic wolves suggests that they mainly subsist on mice. Their packing allows them to take down much larger prey. I doubt that a lone wolf can be successfully hunt an elk.

There are certainly many exceptions to the rule that predators are larger than prey, but it is commonly the case. Perhaps because there is danger in attacking prey larger than yourself (as Shaula pointed out). Even loins, though quite large, tend to hunt in packs. Humans have long hunted in packs as well (as do chimps when they hunt).

TooMany
2012-Nov-25, 01:07 AM
Originally Posted by SkepticJ:


They don't destroy energy--which can't be done--but energy is lost to entropy at each level. That's why there's not that many levels to food webs.

I don't think that increasing entropy is the limitation on levels of predation.

On second thought, your argument about entropy is similar to mine about size. If you look just at size and think of upper case "A" as bigger than lower "a" and a two letters ("aa") as indicating "many" and "<" means "eaten by", then you could in theory have:


aa < B < cc < D < ee < F < gg < H ...

for as many levels as you please. But in nature, I think aa < B is more common than B < cc. I don't think such a hypothetical food chain is limited by entropy either.