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Herbivores without predators

Discussion of the distribution and abundance of living organisms and how these properties are affected by interactions between the organisms and their environment

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Re: Genetic Instability

Postby cyanodave » Tue Mar 27, 2012 4:04 am

JorgeLobo wrote: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.
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Postby JorgeLobo » Wed Mar 28, 2012 11:53 am

reference please
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Re: Herbivores without predators

Postby marquitosgm » Thu Mar 29, 2012 9:11 pm

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
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Re:

Postby marquitosgm » Thu Mar 29, 2012 9:18 pm

JorgeLobo wrote: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.
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Re:

Postby cyanodave » Thu Mar 29, 2012 11:34 pm

JorgeLobo wrote: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.
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Re: Herbivores without predators

Postby marquitosgm » Fri Mar 30, 2012 8:05 am

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.
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Postby JackBean » Fri Mar 30, 2012 9:31 am

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.
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Postby JorgeLobo » Fri Mar 30, 2012 2:10 pm

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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