Additional Notes for Extinction Lecture (12/5/02)

 

(A) TYPES OF EXTINCTION

 

Along with Pseudoextinction and Terminal extinction, I introduced another type:

 

Local extinction (not on handout)- a species goes extinct somewhere within its geographic range, but not globally.   Global extinction is synonymous with ‘terminal extinction’.

For example...During an Ice Age, the climate changes rapidly in the north.  Northern critters need to adapt or get out of the way of advancing glaciers and severely cold climates.  Most species don’t move fast enough to migrate out of the way (like a tree for instance, trees don’t move very quickly...) and most species can’t adapt fast enough to the rapidly changing environment.  With no way out, the northern populations tend to go extinct.  If this is a widely distributed species (i.e. large geographic range), then it may only suffer a ‘local extinction’ because it may have southern populations that can survive and live on during and after the Ice Age.

 

 

(B) VULNERABLE OR NOT VULNERABLE TO EXTINCTION?....that is the question.

 

(1)   DETERMINISTIC TRAITS- Intrinsic traits that make species more or less vulnerable to extinction (they play a large role in background extinctions).  Please keep in mind that these traits are NOT mutually exclusive...

 

·        Geographic Range-  Species are better able to survive if they have a large geographic range

·        Rarity- If a species is rare (low population size) it is more likely to go extinct

·        Degree of Specialization- A species that is highly specialized is more likely to go extinct than one that is a generalist.  Take the Calivaria Tree for example, its reproductive success was entirely dependent on the Dodo.  When the dodo went extinct, the Calivaria Tree lost its ability to reproduce.  With no reproduction, it will perish.

·        Population variability- The more variation in a population, the more likely the species will be able to ride out perturbations like disease, introduced predators etc.  With increased variation, the species can ‘adapt’ more quickly to changes in their environment

·        Trophic Status-  The higher a species trophic status, the more vulnerable it is to extinction (i.e. carnivores are more likely to go extinct than herbivores).  Carnivores tend to have (a) small population sizes (b) long onset to sexual maturity and (c) produce few offspring in a lifetime.  This makes them more vulnerable to extinction...more so than rabbits, which tend to be numerous, reproduce like mad and have large litters of young.

·        Intrinsic rate of population increase-  This is a metric that captures how well a species is at recovering from some kind of perturbation.  If a species can reproduce quickly and efficiently (hence a high intrinsic rate of population increase), then it is more likely to ride out the perturbation and not go extinct

 

(2)   STOCHASTIC PROCESSES- unexpected events that are due to chance alone (i.e. natural disasters, meteor impacts etc)

·        If there were no deterministic factors, stochastic processes would still ensure that all species would go extinct sooner or later....why?  Think about the Gambler’s Ruin....

·        Gambler’s Ruin ---> HOUSE ALWAYS WINS.  Why is that?  When you gamble, your money fluctuates quite a lot (lose a little, win a little, lose a lot, win a lot)... but if you play long enough, at some point you WILL eventually loose all your money, guaranteed.  Once you hit $0.00, that’s the end of the game.  You can’t recover from it. The same thing happens with species and extinction.  They’re all playing the game of life...sometimes they’re winning (i.e. large healthy populations) but deterministic/stochastic factors cause them to lose sometimes (i.e. decrease in population size).  Some species can ride these waves of change better than others, but stochastic processes will lead even the heartiest species to extinction sooner or later...it’s just a matter of time.

 

GAMBLER’S RUIN (in terms of species going extinct):

 

 

THE STORY OF THE HEATH HEN...

 

This is a perfect example of how deterministic factors and stochastic processes can lead to the demise of a species.

 

The heath hen was a subspecies of the Greater Prairie chicken; therefore it was very tasty and easy to hunt.  Even though it had a large geographic range (Virginia to Maine) and was fairly abundant, humans hunted the heath hen to the point where its population declined rapidly and its geographic range decreased substantially.

·        By 1840, it was restricted to just a few places

·        1870 on, it was only found on Martha’s Vineyard

·        1908, people became concerned...they built a 1600 acre refuge to save the heath hen.  By this time, there were only 50 birds left

·        The population steadily increased until 1916 when a series of natural disasters struck Martha’s Vineyard:

-         huge fire which spread quickly due to strong winds

-         VERY hard winter

-         a predatory bird, the goshawk, invaded the area and ate many heath hens

-         if the previous disasters weren’t bad enough, the lingering heath hen population was stricken with a poultry disease

·        By 1927, there were only 11 males and 2 females left

·        In 1928, only 1 bird remained.....it was last seen on March 11^th, 1932.