Dear Howie, There are a couple of different issues that relate to your question.
1) Other species don’t DELIBERATELY regulate their population size - if they don’t have consciousness (as we understand it), they can’t make that decision even if it would be advantageous in terms of not over-exploiting their resources and experiencing a population crash later as a result. Being consciously aware of the consequences of our actions, we have the option of making that choice as a society. 2) Populations can stabilize when lambda equals 1 - that is, on average each individual leaves one surviving offspring in the next generation. This can result from changes in the birth rate, changes in the death rate, or both. Usually both change, but one may change more than the other. Limited resources can certainly lead to reduced reproduction - for example, lower flowering and seed production in perennial plants, fewer eggs laid in birds, etc. This allows the organism to devote more resources to functions related to survival. But if conditions are bad enough that such reallocation is necessary, there is likely to be at least a modest increase in deaths as well. 3) The effects of environmental conditions on survival and reproduction depend on the life history of the organism. While an elephant or a tree can afford to skip a year of reproduction in the interest of boosting the chances of survival (because if they survive they will have many more chances to reproduce), an annual plant does not have this luxury. Because it will die at the end of the season no matter what, in dry or otherwise unfavorable years annual plants put MORE energy into making seeds and less into producing leaves etc. 4) Humans are closer to the elephant side of the life history continuum. A woman can produce at most one offspring a year, if we she is sedentary and resources are plentiful; hunter-gatherers tend to have a child only every 3-5 years, because they need to move around more to obtain resources and it is hard to do that if you have multiple kids who can’t walk well at the same time. Regardless of the spacing, we invest a lot in those offspring - it takes at least 14 years before they would have a reasonably high chance of surviving on their own, and in that time parents and other guardians must provide food and protection and teach them all the skills they need. 5) The basic life history outlined in #4 seems to have resulted in a pattern that would otherwise seem odd from an evolutionary point of view: as societies become wealthier and better educated, the number of children born per woman always drops (once child mortality drops sufficiently), sometimes below replacement rate (www.bit.ly/1W3sOag<http://www.bit.ly/1W3sOag>). Essentially, it is a continuation of our tendency to invest more resources in a small number of offspring rather than a small amount of resources in each of a large number of offspring. And as a society becomes wealthier, the amount of resources it takes to raise a child successfully goes up. In the US, the cost of raising a child in a middle-income family to age 18 is over $240,000 - and that doesn’t count college tuition or other later costs that a parent might want to cover in order to ensure that their child has the best chance of success. To make enough money to give even 1-3 kids all the basics, it is necessary in many families for both parents to work, which cuts into the time they can invest in parenting those kids. Essentially, once the question “how many kids do I need to have to ensure at least one survives?” can be laid to rest by medical technology, the decision for many people comes down to “how many kids can I have while still giving them the best possible start in life - and not going insane myself?”…and for developed countries, those individual decisions add up to reduced population growth. 6) In non-human species, if species A can maintain a positive population growth rate while species B has a stable or declining population under a particular set of conditions, species B will be outcompeted and will go extinct (at least in that area). Within a species, genes or behaviors that lead to having more surviving offspring under a given set of conditions will tend to spread - this is how natural selection and cultural evolution work. However, there are tradeoffs - there will always be a point where having too many births reduces fitness because the parent has had to shift too many resources away from their own survival (leading to earlier death and fewer total offspring) or from resources provided to offspring (reducing offspring survival). On a population level, a high intrinsic rate of increase (lots of births under good conditions) will lead to rapid population growth, but may lead to “boom and bust” cycles under certain conditions. That is unpleasant for the individuals experiencing a “bust” (humans would certainly like to avoid such situations!), but not necessarily bad in terms of species-level survival. 7) In the modern human world, the effects of differences in birth rate on competition between nations is complex. Many nations that have experienced reduced birth rates for some time still have growing populations because they take in immigrants (many of whom are young working adults), which reduces the potential negative effects on the availability of workers or the integrity of social security systems built on the assumption that there will always be many more workers than retirees. Conversely, many of the nations that still have high birth rates are hampered in their development by the need to provide for their rapidly growing population. It is true that the relative numbers of people with different ancestry will shift if some nations have higher population growth rates than others, but it would be racist to worry about that. It may be somewhat more legitimate to worry about cultural shifts that may result from high birth rates in country A vs country B coupled with immigration from A to B…but I suspect these worries are overblown. Especially if they are welcomed and treated well, immigrants usually integrate quite well into into their new nations - second-generation immigrants in the US, for example, generally have English as their first language and many habits that are distinctively American. It is likely true, however, that as the world population as a whole begins to stabilize we will need to revise the assumptions behind things like our retirement age, how social security works, etc. For example, if health of older adults also improves, it may be possible to raise the retirement age, or for more people to work part-time beyond age 65 - which would at least partly correct for the imbalance between workers and retirees. Emily Moran Assistant professor, UC Merced On Jan 20, 2016, at 6:38 AM, Howard S. Neufeld <[email protected]<mailto:[email protected]>> wrote: Hi all - I am currently working on an abstract about global climate change for a regional biology meeting in the southeast, and I wanted to say something about the control of natural populations of organisms, but I am not sure if the statement I want to make is true, so I’m asking for some advice and counsel on this. Here’s the question: Has any population of organisms (humans excluded) regulated and reduced their population size by lowering their birth rate instead of increasing their death rate? And have any slowed their rate of increase by raising the age at first birth? Most of the examples I know of natural population control do so by increasing the death rate. Some further comments: If resources get scarce as populations increase in density then behavioral changes could lead to reductions in the birth rate, but under resource scarcity I would assume that the death rate would go up also. I know about density-dependent and density-independent controls on population growth, but here, I’m looking for explicit examples where populations decrease birth rate without increasing the death rate. You may wonder why I’m asking this. It's because I’m wondering if humans can, in the long-term, reduce their population by lowering the birth rate without increasing the death rate. Yes, some countries are already on that path (Japan, for example), but economists and social and political scientists seem to have a problem with such demographic changes, particularly in a free-market situation where an aging population, even if sustainable, is viewed as less competitive and therefore at risk of losing out (whatever that means) to younger, more dynamic populations. It suggests to me that ecology and society are fundamentally at odds here, and that future societies may require paradigm shifts in the way they operate if humans are to actually create a sustainable society. But that’s another story. For now, I’d be really interested to hear explicit examples if anyone has any. Thanks. Howie Neufeld -- Dr. Howard S. Neufeld, Professor Director, Southern Appalachian Environmental Research and Education Center (SAEREC) Chair, Appalachian Interdisciplinary Atmospheric Research Group (AppalAIR) Mailing Address: Department of Biology 572 Rivers St. Appalachian State University Boone, NC 28608 Tel: 828-262-2683; Fax 828-262-2127 Websites: Academic: http://biology.appstate.edu/faculty-staff/104 Personal: http://www.appstate.edu/~neufeldhs/index.html SAEREC: http://saerec.appstate.edu<http://saerec.appstate.edu/> AppalAIR: http://appalair.appstate.edu<http://appalair.appstate.edu/> Fall Colors: Academic: http://biology.appstate.edu/fall-colors Facebook: https://www.facebook.com/FallColorGuy
