Back to Table of Contents

Back to  Ecology Home

Dr. D.'s Home Page

Department of Biological  and Physical Sciences

School of Science  and Mathematics

B. How do populations change; "the motion picture" (population dynamics)

  2. Age-specific population dynamics and why do we care

b. Why do organism reproduce and die as they do (life history patterns)

Aldo Leopold stated that to understand ecological systems "we must think at right angles to evolution and examine the collective behavior of biotic materials. This calls for a reversal of specialization; instead of learning more and more about less and less, we must learn more and more about the whole biotic landscape".
 
• Evolution cannot be understood without the ecological knowledge of the interaction between environment and survival and reproduction.
• Ecological systems cannot be understood without knowledge of how organisms adapt to each other and the environment.

 

So first a quick review of evolution (click here)
 
 


Life History Patterns - optimization of traits that maximize the passing of genes on to future generations.

If success in passing on genes determines the evolution of a population, why shouldn't all organisms grow rapidly, reproduce shortly after birth, produce many large offspring, reproduce frequently, and take extensive care of young?


 
 
 
 

The key to understanding 'life history patterns' is to understand the following typical life history diagram:


(each point represents one of 14 lizard population; roughly based on a study by Tinkle, 1969)



 

To express this mathematically (as stated in the previous lecture):

For a population that has existed over many generation,

mean R₀ = sum of l_am_a = 1

if l_a high, m_a is low

if m_a high, l_a is low
 








 


Therefore, some basic questions in examining life history patterns:
• What environmental factors explain the allocation of energy  between and within survival and reproduction for a given species?
• Do life history traits for various populations tend to fall into definable combinations ('packages') of 'strategies'?

 

But first,in what ways can a limited amount of energy be allocated by an individual?
Analogous to a financial budget
 

Survival

• growth
• maintenance
• acquisition of more energy
• escaping predators
• etc.

 

For example:

• Generalist vs specialist
For example

What are the advantages and disadvantages of specializing on a particular resource versus being able to use a wide variety of resources?


 

In  what kind of environment (in terms of predictability and period) would a generalist be favored?

 

Woodstork	HIV	Red-cockaded Woodpecker

 
 

• Territory - defense of an area based on nesting, foraging, mating (suggests a resource in short supply resulting in intraspecific competition)

 
 

What are the advantages and disadvantages of  large versus small territories?


 
 
 
 


Reproduction (7-48% of physiological energy devoted to reproductive effort).

Some examples on different 'options' for the allocation of  energy and effort toward reproduction:

Timing of Reproduction
 

If a dog can produce between ~2-10 pups/liter, why hasn't greater liter size been selected for?
 

Does reproduction at the present time have future costs in terms of survival and future reproduction?
 
 

Profits of immediate reproductive investment are at the cost of prospects of future reproduction (which depends on survival)

Organism display a continuum of strategies, defined between to extremes:
 

semelparity - one massive reproductive effort



Many organisms are semelparous, e.g. mayfly larvae, salmon, and the annual plant Cenchrus biflorus

iteroparity - few offspring in a series of reproductive events

What kind of environmental conditions might select for semelparity, variable or constant?
 
 

Roughly a trade off between m_a (immediate reproductive investment) and V_a (prospects of future reproduction which depends on survival).

Given two similar species (above) in two environments that differ in age-specific survivorship, which species is more likely to evolve toward semelparity?
 


 

Parental investment


 
 

Can a plant allocate energy to parental investment?


http://www.botany.hawaii.edu/faculty/webb/BOT410/Angiosperm/Seeds/Seed-5Ricinis.htm 
 

Other examples:

 
 
 

For size of offspring, the trade off may be number per brood:


 

But the benefits of parental investment to offspring may include both greater growth and lower mortality:
 


http://www.umsl.edu/~biology/Bio220/LifehistLecture/lifehistory.html
 

Is a trait necessarily selected for if it improves the efficiency at which the genes for that trait are passed on to the next generation?
 
 
 
 

Some examples parental investment in relationship to environmental conditions:
 

David Lack (1948) examined how many eggs (Clutch size) should  starlings lay based on future success of offspring.

 
   

Clutch size	No. of nests w/  that clutch size	% surviving  after 3 mo.
1	65	none
2	328	1.8
3	1278	2.0
4	3956	2.1
5	6175	2.1
6	3156	1.7
7	651	1.5
8	120	0.8
9, 10	28	none


Because it physically possible for Swiss Starlings to produce clutchs of 10, why don't all female do this, thereby passing on as many genes as possible?

Why is the percent surviving low for the lowest clutch sizes?

Which clutch size has the most successful number of offspring? Which clutch size is most commonly laid?  Does this support the concept of life history patterns as 'the optimization of traits that maximize the passing of genes on to future generations'?
 
 
 



Mola mola, the ocean sunfish drift about in the open ocean and produce 200 million eggs/brood.

Would parental care be difficult in such an environment?

 
 

In highly unpredictable environments, what parental allocations of energy might increases the success of their offspring?

"Fugitive species"

  


    Other allocation of energy to reproduction include time to maturity, parental size, and fecundity
      

Delay of first reproduction in Albatroses attributed to time needed to learn sufficient foraging skills, feeding 10-100 miles offshore for up to 1 week.



A long delay in maturation must be associated with high survivorship:


 
 
 
 
 

  Do life history traits for various populations tend to fall into definable combinations ('packages') of 'strategies' ?  

The concept of r-K Selection:
Two extremes for reproductive strategies dealing with environmental variability/predictabilty in which species lie along a continuum.
 

r-selection	K-selection
<-----------------	------------------->


 
 

Comparison of environments:

	r-selection	K-selection
variability	variable, unpredictable	stable, predictable
populations	uncrowded, variable	crowded
mortality	density-independent	density-dependent
resources	abundant	scarce
competition	lax or variable	keen and constant

Comparison of life history traits selected for in the above environments

	r-selection	K-selection
body size	small	large
competitive ability	low	keen
maturation	early	delayed
offspring	many small	few larger
parental care	minimal	greater
number of  reproductions	semelparity	iteroparity
length of life	short	long

Are the traits logically connected to one another (e.g. would you expect delayed maturation be associated with keen competitive ablility)?


Within a given environment, are r and K species mutually exclusive?  How can the same environment be favorable for both r-selected species and K-selected species?



Unfortunately, this r-K concept is not an example of a fundamental law of ecology:  

   


Are native freshwater mussels r or K?
 

	freshwater mussel (Unionacea)	Asiatic Clam (Corbicula)	Zebra mussel (Dreissena)
life span	6->100 yr	1-5 yr	4-7 yr
age at maturity	6-12 yr	0.25-.75 yr	1-2 yr
fecundity (young/adult/breeding season)	200,000-17,000,000	35, 0000	30,000-40,000


Typical life cycle of a freshwater mussel:

 

Responses to environmental variability may not be a simple dichotomy. 
For example:


Winemiller and Rose (1992) recognize, for example, that delayed maturation is not always associated with low fecundity it species adapted to taking advantage of infrequent (periodic) favorable conditions.






Stearns (1976) identifies 6 life history patterns as response to combinations to 3 types of environmental variability:

• regular fluctuations (cycles)
• unpredictability
• generation time relative to period of fluctuations
  

e.g. predictable environmental in which period of cycles is short compared with generation time.  Such an environment might select for sychronizing breeding to optimal times or to releasing young to swamp predators

Whereas an environment in which period of cycles is long compared with generation time, might select for more 'generalist' strategies with continuous breeding or with 'resting stages'.

 
 
 
Cicadas : insects whose nymphs live underground living off root juices emerge every 17 yrs (3 spp.) or 13 yrs (southern sp.) into adults to breed and lay eggs. Long life span is greater than life span of their predators, but Why 13 and 17?

 
Bottom line: selection will favor different life history "packages".  Reproductive-survival traits are not just a hodge-podge and packages are related to environmental predictability.  

Back to Table of Contents

Back to  Ecology Home

Dr. D.'s Home Page

Department of Biological  and Physical Sciences

School of Science  and Mathematics