Limiting Factors - Can There Be More Than One?

Limiting factor: a resource or component that constrains a population's size.

Examples: food, water, nesting sites, predators, parasites, reproduction rates, finite chemicals (iron, phosphorous, nitrogen, potassium, etc.)

Considering all those examples, doesn't it seem like there's more than one limiting factor at a time?  Don't they all affect the population at once?

Can there be more than one limiting factor at a time?

The answer, counter-intuitively, is no.  Liebig's Law of the Minimum states that there can only be one limiting factor and uses an analogy of a barrel:

The lowest slat prevents the water (which represents the population) from increasing any higher.  If that resource were to become much more abundant then the population could rise again to create a new limiting factor.  

Video illustrating this: 

I hear you objecting: Yes, but aren't all those factors acting at once on the organism's population?  Say, aren't predators and limited food both keeping the population low?  Both are acting on the organism's population, but the strongest force is the one keeping the population at the suppressed rate that it currently is.  If you were to change that one most limiting factor the population would rise.  Change anything else and it would stay stationary (in theory).

The below illustration is something I played with to also add time into the equation showing how that limiting factors can change and how there are secondary effects of either too much or too little of a factor.  For example, too little sunlight and a plant can't photosynthesize well, too much and the plant becomes scorched; too little water and the plant desiccates, too much and it drowns.  

To use a plant as the example again (because their needs are simple), an understory plant's population is limited by a lack of sunlight.  Say a tree dies, falls over and opens up the canopy.  The aforementioned plant's population will now rise to a new level.  That level will soon be constrained by a new factor, such as limited water.  If rain came, then the population would be constrained by limited space or a chemical necessary for photosynethesis such as nitrogen.  The assumption is that a species will always produce more offspring than survive and that it is in a constant battle of increasing its population and then having it constrained.

There are some scientists that would like to play around with introducing iron into strategic locations in the ocean to boost marine productivity (since it's a limiting factor for phytoplankton) and possibly offset global warming by absorbing an abundance of CO2. More here. 

This idea of the weakest link breaks the chain also has been used in many other systems scenarios, such as management, economics, health, government, et cetera.

Pix and vid:

http://en.wikipedia.org/wiki/Liebig%27s_Law

http://www.youtube.com/watch?v=hxPeGrl_0-4

Why Women are Chimeras and Sex Determination

Genetically speaking, what makes a male a male and a female a female?  There is no uniform answer in nature:

Mammals (and some insects), as you well know, use a XX and XY system.  Essentially, if you have a Y chromosome you become a male...right?...

Reptiles/amphibians/some insects, on the other hand, use a system where if you have a different chromosome than the 'normal' you become a female.  In the lingo, you're male if you have WW chromosome and you're female if you have ZW.  *Note, the letter has zip to do with the shape.  I was told growing up that the mammal Y chromosome gets its name from the fact that it was shaped like a Y.  Not true.  It's a tiny 'x' and behaves the same as every other chromsome does, by crossing over during meiosis.  The 'x' terminology began because in early genetics they needed to give it a name and 'x' sounds cool.  ha

Some other insects have no sex chromosome, but instead the lack of an 'X' determines the sex.  They're either X or XX.

Plants, fungi, protozoans and inverts don't even use sex chromosomes altogether.

Exceptions within humans:

47 XXX - female

48 XXXX - female
47 XYY -- so called supermales
48 XXXY - Extreme Klinefelters males (a male with partial female development, like breasts, etc.)
48 XXYY - Extreme Klinefelters males

Then there's even more exceptions!  You can have XX males and XY females!  How's that?  Well, I kind of lied earlier when I said that it was the Y chromosome that make a male.  It's actually a few genes on the Y and those genes can get mutated or moved over to the X chromosome.  The most important controls testes development.  As embryos we're all females.  Then, if you have the right genes, the sex gonads get told to turn into testes.  If you don't have  that gene, then you develop as a female.

It gets even more awesome!

So, how do mammals deal with the problem of females having too many X chromosomes? If they displayed both of the chromosomes there could be some pretty dangerous developmental differences between males and females.  Well, they turn one off!  In fact, they don't just turn it off, they glom it into a blob on the side of the nucleus wrapped up in RNA and proteins in tombed so that it doesn't express.  What makes this awesome is that which X is randomly chosen.  It could be the one from the father or it could be the one from the mother.  

Translation: female mammals are mosaics!  Some of their cells use one X chromosome and some cells use the other!  Mammal males are roughly all the same genetically across their cells, but females aren't!  Their cells use different DNA!  In fact, you can have whole patches that use one X and other patches using the other.  This is called mosaicism, for obvious reasons.  In kitty cats this produces some fun fur patterns that are -only- in the females.  Basically, the dad had one color X and the mother another X and the baby displays both at once, but in different patches!  (Why patches?  Why not a blend of different cells?...Dunno.)

Barr Body in nucleus: 

Mosaicism in kitties:

This one could have happened other ways, but most likely was due to mosaicism.  One hint could come from if this cat is female.

Humans still have this.  For us the patches don't usually manifest as a calico/tortoiseshell appearance, but instead can have effects like ectodermal displaysia, which is the patching of working and nonfunctional sweat glands: 

There are other kinds of mosaicism, like that of so called chimeras, which are the result of fraternal embryos fusing together.  What makes this particularly wild is that this has been an issue with welfare genetic testing and for organ transplants.  One mother was accused of being a surrogate mother for someone and committing welfare fraud when a DNA test showed her as not being the mother of her kids (another article).  Another instance showed that a kidney transplant couldn't go through since the relatives weren't close enough genetically.

Chimerism

Pix:

http://www.mooseyscountrygarden.com/garden-journal-03/garden-journal-2003-late-june.html

http://myitchyfingers.wordpress.com/2011/08/22/odd-encounter/

http://americantransman.com/2010/08/09/part-4-male-gender-identity-in-an-individual-with-complete-androgen-insensitivity-syndrome/

http://genetics.thetech.org/ask/ask443

What Makes Something Sticky?

Bare bones answer: electrons being attracted to protons.

Molecules are collections of atoms.  Within that molecule the distribution of protons (positively charged) and electrons (negatively charge) is not even.  Often, but not always, there are more negative charges on one side and more positive on another.  The negative side wants to stick to another positive sided molecule and vice versa.  Broadly speaking these are called van der Waals forces or intermolecular forces  if the attraction does not share electrons (well, for the most part...).


Chemists and physicists further break those broad categories into dipole-dipole forces (between molecules of differing charge distribution), hydrogen bonding (also a dipole molecule, but with a higher contrast between positive/negative), and ionic interactions (between ions), London dispersion forces (temporary redistribution of electrons resulting in charge asymmetry) and others shown below.


You can see these forces in action with tape.  Stick some tape on a substance and then pull it up and some of whatever it was stuck to will invisibly now be on the tape in the form of stolen electrons (this shows there was at least some electron sharing, but not to the level of covalent or ionic bonds).

These two pieces were stuck together so that they now have the same charge and therefore repel each other

One piece was stuck to something and then removed taking some electrons with it.  It's attracting to the tape that wasn't stuck to anything and because of that didn't pick up any extra electrons and therefore has more of a positive charge.

Cool animation of all this: http://web.mst.edu/~gbert/INTERACT/intermolecular.HTM
Pic from: http://www.science.uwaterloo.ca/~cchieh/cact/applychem/hydration.html
Pic from: http://en.wikipedia.org/wiki/Static_electricity
*http://www.dummies.com/how-to/content/sticky-chemistry-intermolecular-forces.html
http://www.ccmr.cornell.edu/education/ask/index.html?quid=1118
http://www.npr.org/templates/story/story.php?storyId=12507754