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Questions about population dynamics  RSS feed

 
pollinator
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Xisca Nicolas posted a few hours ago in another thread (about making choices to have biological children over adoption or deliberate childlessness--you can find that thread here ... https://permies.com/t/80/53917/population-family-planning-adoption). But, reading through that massive (3 Pages!!!) thread I got totally side-tracked by one of her "biological facts"... specifically this one:

3) Most carnivores have more offsprings than herbivores. This is striking that we see big packs of antilopes, zebras etc, and few predators, when you see antilopes with 1 young, and lions with maybe 4! Do they die more?



This fact, (or is it?) taken completely out of context, got me wondering about population dynamics and just WHY it is possible for most predators to individually have more offspring yet be less numerous overall than most prey species. I started thinking about this and couldn't stop.

So ... here I am, asking all of you to help me try to figure out why this is so and what the implications may be for our (human) over-populated planet. What are the triggers that regulate population? Famine vs. plenty? Size? (Bears tend to be larger than other predators and usually have less than 3 offspring at a time, while smaller prey species like rabbits and mice have oodles of babies and have them multiple times each year.) Culture/society--including pack behavior? (Wolves only allow alpha males and females to breed, for example; and a new baboon alpha male will routinely kill all the offspring of his predecessor, for another example).

Are we more similar to predators than prey or vice versa? If the former, then why can't we seem to get our act together and control our own population explosion?

I realize this is a bit more, I don't know, philosophical(?) than the usual permies post, but it has captured my imagination and I would like some feedback, if possible, so I can get it out of my head. It's becoming like a song I can't quit humming over and over.

 
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One thing that might be a factor is that many larger predators (including humans) spend longer raising their children and thus don't breed every single year whereas many prey species breed every year or even multiple times a year. Plus predators food is harder to come by than herbivore food for the most part I think.
 
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Deb Stephens wrote:Are we more similar to predators than prey or vice versa?



If I remember correctly from "Sapiens" by Yuval Noah Harari, humans were pretty harmless creatures until we learned to control fire and tools. Before that time, we are pretty innocuous and were preyed upon by many animals. Probably not the full answer you wanted, but it's a start. If you haven't read the book, I highly recommend it.



Deb Stephens wrote:...why can't we seem to get our act together and control our own population explosion?



In 2012, the global fertility rate was 2.5 children per woman, and that number is trending downwards. Most "developed" countries are already below 2 children per woman. The continuing "explosion" is due to people just living longer. I recommend this video about population statistics:

 
pollinator
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I have always said that the way to save humanity is to educate all the women of the world. In developed nations where women can go to school into their twenties, and pursue careers afterwards, childbearing is delayed, and the number of children born on average drops, versus conditions where having children is a survival strategy.

Never mind the significant sea change that will occur when this happens, this fact alone will curtail population growth to the point where countries with negative population growth will either increase immigration or offer incentives for people to have children.

This is a far cry from the alarmists clamouring about not having children to save the planet, but I agree with some of the above regarding the need to look at the actual growth numbers.

In a world this calamitous, cutting down our population is a reckless move. If the Thwaites and Pine Island glaciers in Antarctica were to collapse, the resultant surge in ocean levels could kill hundreds of millions rather quickly, and leave hundreds of millions more homeless climate refugees. We are heading toward many potential population bottlenecks, and I don't see that discussed anywhere.

I think the permacultural solution is to widen the scope of our system to, you guessed it, the solar system. The waste populace (I am being facetious, I don't actually view anyone, or many things at all, as waste) becomes the feedstock for another permacultural enterprise: the colonisation of the Sol system.

There are some strong opinions that growth should be unfettered, that we should be allowed to seek prosperity for ourselves. If that is the case, and I haven't seen any indication that humanity at large will be dissuaded from it, we need a larger system, more resources, larger inputs.

As to the original question, I think it's just a question of different ways of doing. As mentioned before, carnivores tend to have one litter a season, with larger numbers of offspring, and prey tend to have multiple litters per season. Some, though, like certain ungulates, have singletons or sometimes twins, whereas others, like rabbits, have larger litters and breed like, well, rabbits.

While incomprehensible in absence of history, the evolutionary development of different predator/prey dynamics is probably responsible for the specific differences, and the details, of individual reproductive patterns.

-CK
 
pollinator
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here we have a lot of coyotes, and as a sheep farmer I do not like that. I have defenses in place for my sheep like a Livestock Guard Dog and good fences so I have only lost one sheep to predators (a crow), but I admit I have semi-boasted that hunting coyote is done a lot here. Behind my house is 7000 acres of forest and field with no houses whatsoever, and the area hunters will often remove 60-90 coyotes a year.


The sad part is, for some reason the coyotes sense how many of them can be in a given area, so while the coyote hunters are taking out large number of coyotes every year, the coyotes themselves breed back fo fill the gap left behind. In the years the coyote hunters do not get as many coyotes (crust on the snow, deep snow, etc), the coyotes have fewer babies.


As a sheep farmer this should not come as a surprise to me, because in the fall I "flush" my sheep with pastures teeming with great feed because they see that great feed, instinctually think they can raise more lambs because of it, and their reproductive systems drops more eggs giving me twins and triplets. In the years I have over-grazed in the breeding season (fall), they have produced more singles.


...

Interestingly enough, my state and town are stagnant on population. I have just as many people in my town now as when I was a kid 30 years ago. Because I have been here so long, I have noted the trend has been, people stay about 8 years then pack up and move.
 
pollinator
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Several years ago, a very interesting book was published: "What to Expect When No One is Expecting", which looked at the perception that the global population was exponentially increasing, and the reality that demographic trends have actually stabilized and will soon cause the population of many nations to decrease.  So much has been written about over-population, but few scholars have ever explored the topic of population collapse.  It's already happening in many countries.

Simplistically, in order for a population to remain stable, each woman has to have 2.1 children.  In most developed nations, that "replacement" number isn't being met.  Most nations in Europe are at 1.7 or lower.  Russia has a population that is in freefall due to a number of variables, one of which is the low birth rate per woman (1.4 children per).  Japan is experiencing the same, as women are waiting far longer before having a child, or foregoing children and marriage all-together.  China, for all the foreboding about 1.3 billion people, is going to experience a population free-fall in the decades to come due to the 1-child policy it enforced for almost 30 years.  Their social engineering has led to the selective abortions of millions of baby girls (since parents want a son to carry on the family name) and a gross imbalance now of women to men (men being much more populous).  Basically, every child in China now has 6 adults to care for them, and ultimately, whom they will need to care for (2 parents and 4 grand-parents).  Every year, the average age of the Chinese population is raising at a significant rate.  Down the line, it will lead to major problems, as there will not be enough people to care for and support the demographic time-bomb of aging Chinese.

In city after city in Russia, Japan, Germany, Italy . . . schools are closing as kids are being bussed farther and farther so that they'll have classmates.  Its the same thing that's been happening in rural America for 30 years, as schools are consolidated and average classroom sizes reflect the fact that the average family size continues to drop, even as the population of rural counties collapses. 

Yes, the number of people on the planet continues to grow, but the rate of growth has decelerated significantly.  If it were not for African and Islamic nations which still have a high birth rate per woman, the global population would be static and not growing.  If America didn't have such generous immigration quotas (and porous borders), our population would be in decline as well.



 
gardener
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The video is very encouraging, but there is one factor not accounted for: resource availability. The current population is using non-renewable resources at a rate that will make them relatively more expensive to extract in the future - not just dollar-wise, but energy-wise. When it takes 1/100th of the energy value of a resource to extract and use it, it is very productive. When it takes 1/10th of the value to produce it, it becomes less valuable. When it takes 1/3 of the value to produce it, it is hardly worth the effort, and vastly more needs to be produced just to end up with the same amount of end product. Many resources are headed in that direction.

Another point is pollution or waste. As more is produced, more waste is generated, and that waste sticks around and builds up, potentially faster than it degrades. Even without increasing production, the amount of waste or pollution is going to keep increasing for a long time, degrading some kinds of productivity the more it builds up.

This is a small summary of some of the information developed in the studies underlying the 1972 book "Limits to Growth", which modeled simplified trends for the next century, and has shown to be accurate for the ensuing 40-some years so far. Even the most optimistic modified-behavior projections starting from 1972 lead to a crash or downturn by around the middle of this century, and the business-as-usual projections (which we have followed very closely so far) lead to a serious crash in population, food, health, and quality of life well before the end of the century.

It would be wonderful if the optimistic predictions of the video were to come true, and it is worthwhile to strive for them, but I think that if the one big point near the end, seriously reducing first-world consumption, is not achieved, the world as a whole is likely to be doomed to a bleak future for the next century or more as population is forcibly reduced to a sustainable carrying capacity - which will be lower than what it would be if we were on a sustainable track now.
 
Marco Banks
pollinator
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One other variable that isn't being discussed, but which has played a role in human population for centuries is the potential for disease to cause a mass epidemic that decimates the human population.  There is a never-ending battle between humanity and the viruses that desire to kill us.  At some point, there is a high likelihood that a virus will get loose and will take out a significant percentage of humanity.  There have been sci-fi movies on this for decades, but the reality only grows more likely with globalization.

30 years ago, we first heard about AIDS.  It seemingly came out of nowhere.  About 70 million people globally have been diagnosed with AIDS, 30 million have died of it.  But AIDS is a tremendously difficult disease to catch if you use even a modicum of reason and prevention.  Comparatively, viruses like West Nile, Zika, Hantavirus, or Denge are much easier to contract.  All are deadly.

If a virus like ebola or marburg were ever to escape to a populous nation like India or one of the teeming cities of Asia where overpopulation has already made the existing healthcare facilities inadequate, it would have the potential to kill hundreds of millions of people. 

We are naive if we think that science will always stay one step ahead of viruses.  For all of the billions of dollars that have been thrown at AIDS research, we still do not have a cure and it continues to kill.  As unscientific and draconian as it sounds, if there were to be a mass outbreak epidemic, the only way they'd be able to control its spread would be to cease transportation and, basically, quarrentine entire nations.  Close the airports and train stations, stop all ship traffic, etc.
 
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Maybe evolution itself demands higher numbers of predator offspring to offset how evolution works:

My recent study (unpublished Masters) of > 200 large mammal species showed that evolution is driven by (factors pertaining to) energy. Not only that, but there was an area affect, that might also be seen as a population effect, where animals with more real estate (thus resources) evolve faster than those with less room and resources.

I tested animals with larger ranges or higher energy latitudinal positioning vs their closest relatives sharing the same continent. This was no island theory this was hardcore putting obstacles in my way each step till my supervisor was practically hairless. Energy drives evolution.

A population effect seems relatively straight forward. More mating events = more opportunities for mutations -> a percentage of these mutations carried forward.

Now, in the arms race between predator and prey (look at the predator prey cycle) the larger populations of prey would eventually have an advantage due to this population effect on microevolution. But with predators bearing larger litters thus more genetic diversity this 'gap' might be closed somewhat. Another factor to consider is predators often prey on multiple species. Add in the level of difficulty in securing a meal for young predators vs prey...

The reason there are so many prey compared to predators again is to do with energy. In a model world 100% of captured solar energy is available to herbivores, this is rapidly diminished to 10% of energy available to carnivores in the same system. The rest was lost via the herbivores as heat, gas, and soil builders.

I realise this answer is not an explanation of the phenomenon but a possible explanation for 'some of the why'.

In a permie context - bigger populations are good for evolving organisms to cope in a rapidly changing world. Wherever there is too much or too little, water also becomes a critical factor in evolution. Trading seeds and livestock in breeding programs might overcome limitations in localised genetic diversity through crossing strains and species with closely related counterparts from higher or lower latitudes and different base substrates.

 
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There's a lot of oversimplification going on. Sorry if I repeat stuff that has already been stated.

1) Large herbivores, the ones which birth offspring less frequently, are the minority of herbivores. They have looooong gestation periods and birth well-developed offspring which are able to run at birth or within a few hours after. Their offspring have high survival rates because they are well developed at birth. They can run, jump and travel with mother and/or herd, though they may be left unattended at times. (Think baby deer hiding in the thicket.)

2) Marsupials may have large litters, but only enough teats for a few totally undeveloped at birth, tiny larval-type babies to latch onto and survive. To my understanding, most marsupials are mostly herbivorous and opportunistically insectivorous or scavengey. The opposum is a weirdo 'cause it has like 15 teats. It may sometimes be a predator, but it's certainly not a very good one. Only a few offspring are able to receive the milk and care of the mother, after all is said and done.
3) Primates, which are mostly herbivorous and frequently also insectivorous, tend to have only 1 or 2 undeveloped, helpless babies. Babies can cling to mother's back, and are not left unattended.

^These groups invest a lot of energy into a smallish number of babies. They don't generally don't leave babies unattended.

4) Small herbivores and mostly-herbivorous-omnivores like rabbits, squirrels and other rodents have short gestation periods and large litters of immature, helpless young. I'm guessing these make up a larger portion of the herbivorous mammalian biomass than do the large and hoofed varieties, simply because they are so much more numerous. Babies are left unattended while the mother searches for food.
5) Carnivores in the relevantly named order tend to have litters of undeveloped, helpless offspring. They need to leave these offspring frequently to go kill stuff. The survival rates are relatively low.

^These groups invest a smaller amount of energy into a lot of babies. They generally leave babies unattended to search for food. They have higher rates of dominant-male-killing-rival-babies, infanticide (nervous hamster mommy = cannibal hamster mommy) and abandonment.

The pattern I see has less to do with the diet of the animals, and more to do with their behavior towards their offspring. Animals that build a nest, fill it with helpless babies and leave them for extended periods see fewer offspring survive to maturity. Animals that invest a lot of time in one or two offspring they keep with them at all times, see a large portion of those offspring survive to maturity. Of course there are exceptions to the rule in all groups. Like bears. I'm just addressing the general schemes that seem apparent to me.

I think the complication with people is that we like to have sex. We generally need to have sex in order to maintain pair bonding. Orangutans don't have to deal with this nonsense because orangutan moms are all single moms, they only come into estrus when their baby's 8 years old and ready to be more independent. (Or if it dies before that, but that's sad times.)
Low fertility and frequent estrus may be an adaption to allow women to have frequent sex without being injured by intercourse or just not being in the mood. I really can't think of any other large mammals that cycle on a monthly basis.
I think we need to be careful not to imply that it would be better for the planet if a bunch of people died off. People like me take this really personally and start wondering if we shouldn't just off ourselves. It's defeatest thinking that assumes there's not a more efficient and sustainable way of sustaining ourselves. I don't think it's appropriate to suggest that infant mortality is a good thing, or that giving birth to a human being is a bad thing. To me, every human life is precious.
 
Dc Brown
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While I have no issue with anything you've stated it is worth noting the patterns of evolution I discovered in large mammals aligned with results of previous studies of small mammals, marsupials, endotherms, exotherms, insects, bacteria, fungi and plants...

Energy. Temperature. Calories. Primary Production. We've yet to test this on hydrothermal vent communities which I recommend, as they are without solar but have very distinct energy gradients radiating out from the vents.

Evolution is bad for an individual but good for a species. Caloric restriction will extend your life. Lots of energy will shorten it.

 
After burning through the drip stuff and the french press stuff, Paul has the last, ever, coffee maker. Better living through buying less crap.
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