Herbivores without predators

[Fnorbl]

Hi there.
I’m not that much into biology, so sorry if my question seems stupid.

As an exercise I wanted to program a small application simulating a very simple biotope.
Step one: some plants. They simply age, spread their seeds, and slowly cover all the area they can live on.
Step two: herbivores. They eat the plants, maybe only young plants. Now here is the question: if those herbivores have no predators at all, what is the most likely outcome?

• since the herbivores have no predators, their number increases and they eat up all the plants. Then those plants are gone and the herbivores die out as well.
• as before, but some seeds of the plants survive and they recover in a biotope without those herbivores
• there will be a balance concering the number of herbivores. Even without predators, their number will stabilize so they will be in a balance with the plants (what is the cause for this then?)
• depends on the herbivore. Some will munch up everything and die out, some others have ony so few young anyway that they will never be enough to wipe out the plants.
• some other thing will happen

Please enlighten me. 🙂

[JackBean]

Isn’t aim of any simulation to get the results?

combination of first three 😛
Their population will increase so much, that they eat all the plants, but some (both plants and herbivores) will survive and the population will start over. It won’t reach exactly balance, but will oscilate around some number of plants and herbivores (plants will be peak earlier)

[Fnorbl]

Thanks for the reply.

Well, as said it’s not supposed to be a scientific model or give interesting results, but more a programming exercise combined with my amateur interest in nature sciences.

So the reason why some of the plants would survive, despite a huge number of hungry herbivores, is that the herbivores senses are limited and they wouldn’t be able to locate all of those edible plants?

[JackBean]

look for example on the grasshopers in Africa. They come in huge amounts, eat everything and go elsewhere. But does that mean, that it will be dead land for next five years? No, it will be affected and the population of plants will be reduced, but some of the plants survive, some may give rise to the seeds (although that depends, when does the herbivore peak).

[animartco]

Herbivores can’t exist without predators. They have no way of regulating their own numbers so that they don’t consume all the plants. In the scenario that they consume most of the plants and start dying off from starvation, giving the plants time to recover, there would be so much damage to their genomes that they could never become a successful and viable species. It is only with predators that this can happen.

[JorgeLobo]

Damage to their genomes? Mind explaining that?

[animartco]

quote JorgeLobo:

Damage to their genomes? Mind explaining that?

Hi Jorgelobo. Sorry, I’m not good at exact terminology. But a population that goes through a cycle of near starvation every fifty years or so would be genetically unstable. A starving population tends to reduce in size. A well fed population increases. This regular unchanging yo yo effect would be very bad for any species.
There is also of course the fact that without predators the beautiful herbivores we have today would never have evolved. They would all look more like tortoises without shells.

[JorgeLobo]

Help me on this – how does starvation lead de facto to genetic instability and what do you mean be "genetic instability"?

[animartco]

quote JorgeLobo:

Help me on this – how does starvation lead de facto to genetic instability and what do you mean be “genetic instability”?

Hi Jorgelobo. I studied ecology, not genetics, but I think there are rules for the survival of any species, and they are set up something like this:-
There need to be Food chains, and the longer they are, the better. It’s rather like all the bones in your arm giving a big range of movement. If you only had two joints there would be lots of things you couldn’t do. A food chain gives a greater range of conditions for evolution, and leads to a greater diversity of forms. If the food chain is reduced evolution is slowed down for the species within it. Stagnation is something nature abhors. If a species can’t evolve fast enough, it can’t beat the disease organisms which keep ganging up on it. (Ok so crocodiles are successful, but they are a bit of a one off. The exception that proves the rule.)
So to answer your question, What I mean by genetic instabiliy is a slowed genetic response to threats in the environment.

[JorgeLobo]

I think you overinterpret – evolution is not a near term event and dynamics of disease are much more complicated than you suggest – both for the host and parasite. In any case, evolution is not going to addres the short term starvation issue we were discussing.

[sachin]

In such simulation

Considering that there are Edible plants and non edible plants both, the alternation in animal population will cause as per food availability. Again that will depend on life span and size of animal. The animals with larger life span and large body size will definitely vanish. Animals with small body size will show the alternating population pattern.

Just because predators and parasites exist in world large and highly reproducing animal races including humans could survive longer in an ecosystem.

[animartco]

I think Sachin makes a good point here. The smaller the animal the greater the number that will survive. Plus if there were no predators there would be no need for the animal to grow bigger, Perhaps the size wouldn’t yo yo a much as I thought .
On the problem of a change of plant type -eaten grass repaced by broadleaves, either the animal eats both or there are two species involved in the yo yo effect, a grazer and a browser. This wouldn’t actually make a difference to the yo yo though. It would simply mean one species ate one area and the other the other. Both are going to run out of their food source as they increase in number, and they would be lucky to avoid extinction in the resulting desert.
In fact, come to think of it, given a desert due to overgrazing coinciding wih a couple of years of poor rains it is very doubtful that any animal life would have developed on Earth without predation.

[cyanodave]

quote JorgeLobo:

Help me on this – how does starvation lead de facto to genetic instability and what do you mean be “genetic instability”?

When the numbers of a populations are diminished severely the gene pool is narrowed, this is like pruning a tree to the point where it has a single branch, as the species bounces back from its severe decline, individuals will be forced through what we call a ‘genetic bottleneck’, in which the whole species comes from a limited number of individuals, meaning serious inbreeding, genetic defects, susceptibility to disease, and overall species collapse. Thus the detrimental affects of the ‘yo-yo’ action are obvious- the smaller the population, the more prone to disease.

[JorgeLobo]

reference please

[marquitosgm]

Forget about genetics, ecology alone gives big enough an issue.

You need to consider:

1) Is your system closed or open? If closed -say an island or a fenced property- herbivores can’t migrate into or out of the system. They will eat all edible plants and start dying of starvation as food becomes scarce. Eventually they will become extinct. But if you allow migration to occur, a long term equilibrium is possible as indivuals depart and come back when plant populations have recovered. The grasshopers example. Immigration of plants (seeds can be transported by wind or by animals) can help too, but little.

2) Is there competition among different herbivore species? Basic rules of ecoly say that when two species rely solely on the same limited resource (food), the one that needs more is displaced by the one that needs less. In other words, animals that need more food will die first.

3lizards that feed on plants) Herbivore extinction may be a likely outcome and has ecological sense. But this will not happen if you set plant regeneration rate to be higher enough than herbivore increase. This is not unconceivable in islands inhabited by herbivorous lizards because they are not active in winter and plants can regrow.

4) As pointed by others in this thread, you should need more elements (predators; climate fluctuations, etc) in your system to achieve a stable situation. Expect it to oscillate up and down. However be warned: experience shows that even the most simple of such systems are not only hard to modelize, but unrealistic as well.

Hope this helps

[marquitosgm]

quote JorgeLobo:

reference please

Yes, please. We all have heard a lot about endogamic depression theory. But over the years I have only come across one single study showing it happening in wild populations. I remember it was a paper on european vipers published by late nineties, that’s all I can remember.

[cyanodave]

quote JorgeLobo:

reference please

apologies this is actually known as the bottleneck effect, according to the 6th edition Campbell Reece biology textbook. – "disasters such as… droughts" (or rather the resulting famine caused by a drought or similar occurrence such as overconsumption of a food source) "may reduce the size of a population drastically"- the fear with this model. When this occurs "by chance, certain alleles will be overrepresented among the survivors". this is simply explained, look at genetics as picking a skittle at random from a pack of assorted, when there are more skittles, there is a higher chance you will select a different skittle each time, however eliminate the majority and leave only a couple skittles to represent their ‘species’ you will only be capable of selecting 1 or 2 different kinds of skittles, and while this is upsetting due to the overwhelming lack of a rainbow for you to taste, it has more drastic implications when you think of it as a population, cutting off the majority of genes from a gene pool and leaving only a few individuals to replicate for the rest will end in poor resistance to disease and many genetic problems based on inbreeding- if you allow your two leftover skittles to reproduce, the next generations will be of the same colors, and should one color be the target of a disease or a person who fancies it above the other colors, half your population is eliminated.

[marquitosgm]

Yes, true. Bottleneck effect, associated genetic drift and inbreeding depression are all well theorised to reduce genetic variation, thus harming populations’ ability to whithstand environmental changes. Increased riskof extinction results.

But back in my years as a conservation biologist I remember this theory was not that much supported by data. Most well documented extinctios had more to do with ecological detrimental factors (either deterministic or stochastic) than with genetic depletion. In fact some widespread species have been shown to overcome bottlenecks in the past. In turn, very small, isolated island populations seem to be able to survive as far as their habitat is preserved and kept free of exotic predators and competitors.

If someone else can provide reference of genetics iduced declines or extinctions, I’d be grateful.

Back to the beggining, for our computer friend who started this discussion, ecology alone should suffice.

[JackBean]

First, if you consider, that the bottleneck effect will work and you will get only one or two versions of any gene, if you repeat the bottleneck, it can hardly get worst, so technically, there would be no second (or other) bottleneck.
Second and more importantly, the genes and genomes are mutating all the time, so probably new alleles will arise before new bottleneck comes. Of course that depends on how often such bottleneck should survive.

Anyway, in case of bottleneck like insufficient food, I doubt that many bad alleles would survive. Rather, you should get some strong individuals who are able to survive tough times.

[JorgeLobo]

Thanks Jack. I’ve been disappointed that the confident assertions made above offer no data or refercnes other than late generalities from biology texts. This discussion has been pretty sophomoric.

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