Allee Effect

22 01 2013
A "crackle" of black abalone on San Nicolas Island (B. Blaud).  Crackle: a group of black abalone.

A “crackle” of black abalone on San Nicolas Island (B. Blaud). Crackle: a group of black abalone.

The Allee effect was first described by its namesake, Warder Clyde Allee in the 1930s in an article on goldfish.  His article (which I cannot access but have a summary through other sources) investigates the effect aggregation has on overall fitness on the individual goldfish, in this case.  He found that goldfish grow more rapidly in a tank that holds more individuals.  Although he never comes right out and says, “the Allee effect is defined as…”, the generally accepted definition is that higher densities within a population lead to improved individual fitness.  This basically means that larger group sizes provide individuals with more opportunities to mate, defend themselves, feed themselves, and/or can work together to alter their environment in a beneficial manner to the whole group.

 

The term, Allee effect, was often misused and erroneously applied, however, so Stephens et al. (1999) decided to take it upon themselves and define the principle in clear terms laden with scientific jargon and information.  I have a secret to admit: when I see scientific papers, my eyes gloss over.  If the paper has equations, game over.  I check out completely.  If you talk to researchers, many admit to only reading the abstract (a roughly 200 word summary of the paper at the very beginning of the paper), looking at the figures, and skimming the conclusion.  With the overwhelming amount of publications out there, that really is the only way to see if the paper is relevant to what we’re studying and saves a considerable amount of time.  Between the jargons, big words scientists use to justify the tens to hundreds of thousands of dollars spent on earning advanced degrees; you can tease out the basic information of what they’re trying to get across.

 

Black abalone on San Nicolas Island (B. Blaud)

Black abalone on San Nicolas Island (B. Blaud)

Stephens et al. (1999) paper, titled “What is the Allee effect” has large amounts of jargon as well as confusing equations.  I was able to bear down and wade through it.  I need to justify the exorbitant amount of money being spent on my education as well, and prove that I understand demographic stochasticity as much as the next overly educated person.  Side note: I think that if my spelling autocorrect in Word doesn’t know that it’s a word, I should either.  But then again, the autocorrect doesn’t know my last name, so this opinion doesn’t have strong support.  But as usual, I digress; back to the point.  After exploring the numerous examples Allee lays out to support his principle, Stephens et al. (1999) is able lay out a clear definition:

 

…we thus define the Allee effect as: a positive relationship between any component of individual fitness and either numbers or density of conspecifics.

 

Basically, the more of you there are, the better you all will be in terms of certain basic needs (reproduction, food, protection, etc).  There are limitations with this.  Too many individuals and you become competitors for resources, such as food, so oftentimes there is a threshold.  A population size below 50, for example, is bad; a population between 50 and 100 is good; and, a population over 100 is bad.

 

Stephens et al. (1999) continue with their definition to explore the distinction between the component Allee effect and the demographic Allee effect.  The distinction lies basically in the scale, where the component Allee effect is the positive relationship between any measurable component of individual fitness (breeding, feeding, or defense) and population density; the demographic Allee effect is the positive relationship between the overall individual fitness and population density.  So adding component Allee effects together produces an overall demographic Allee effect.  This is where a lot of the fun equations come into existence, but I’m going to avoid all that because I don’t want to get away from the whole point of this post, which is: how does the Allee effect apply to my project.

 

Black abalone on San Nicolas Island (B. Blaud)

Black abalone on San Nicolas Island (B. Blaud)

This leads back to the basic core question I’m trying to answer dealing with recovering black abalone populations on San Nicolas Island.  How close together do two black abalone need to be in order to successfully reproduce?  My question delves into a component Allee effect, just one aspect of individual fitness that’s investigated as a result of population density.  The more dense abalone are, the closer they are located, the more their gametes will mix and more babies will be made.  When 99% of the population was eliminated, numbers drastically decreased and density was also affected.  As they are recovering, numbers are slowly rebounding, but more interesting is the rate the population density is increasing.  Black abalone are clustering closer together as they recover, which leads us to another interesting story about the Nearest Neighbor Data.  But that is a story for another time…

 

Stephens, P.A., W.J. Sutherland, and R.P. Freckleton.  1999.  What is the Allee effect?  Oikos, Vol. 87(1): 185-190.