Life Histories

***The following notes are taken directly from –Life Histories Chapter 12 of  Ecology:  Concepts and Applications, 5th edition, by Manuel C. Molles Jr.***


*Diferent species, often living side-by-side, reproduce at vastly different rates over lifetimes that may differ by several orders of magnitude.

*Because all organisms have saccess to limited energy and other resources, there is a trad-off between the number and size of offspring:  those that produce larger offspring are constrained to produce fewer, while those that produce smaller offspring may produce larger numbers.

*Life history consists of the adaptations of an organism

that influence aspects of its biology such as the number of offspring it produces, its survival, and its size and age at reproductive maturity.

*Fecundity is the number of seeds or eggs produced by an or


*A polymorphic locus is one that occurs as more than one allele. (  /

*Plants vary widely in the number of offspring they produce, ranging from those that produce many small seeds to those that produce a few large seeds.

*There are four plant growth forms:

1. Graminoids:  grasses and grasslike plants.

2. Forbs:  herbaceous plants other than graminoids.

3. Woody plants:  species with woody thickening of their tissues.

4. Climbers:  vines and climbing plants.

*As recognized by the author, there are six seed dispersal strategies among plants, and they are classified as

1. Adhesion-adapted:  These seeds use hooks, spines, or barbs to aid in their dispersal.

2. Wind-dispersed:  These seeds have wings, hairs, or other structures that provide air resistance and so aid in their dispersal.

3. Ant-dispersed:  These seeds have an elaisome–a structure o

n the surface which contains oils attractive to ants.

4.vertibrate-dispersed:  These seeds have an aril–a fleshy covering that attracts birds and other vertebrates.

5. Scatterhoarded:  These seeds are gathered by mammals and stored in scattered hatches and hoards.

6. Unassisted dispersers:  These seeds have no specialized structu

res for dispersal.

*Germination is the process by which seeds begin to grow or develop, producing the small plant called a seedling in the process.

*Where adult survival is lower, organisms begin reproducing at an earlier age and invest a greater proportion of their energy budget into

reproduction; where adult survival is higher, organisms defer reproduction to a later age and allocate a smaller proportion of their resources to reproduction.

*Reproductive effort is the allocation of energy, time, and other resources to the procustion and are of offspring.

*The great diversity of life histories may be classified on the basis of

a few population characteristics.  Examples include: fecundity (number of offspring), survival relative to offspring size, and age at reproductive maturity.

R and K Selection

*R and K selection was one of the earliest attempts to organize information on the great variety of life histories that occur among species.

*R selection and K selection are the endpoints on a continuous distribution and most organisms are subject to forms of selection somewhere in between these two extremes.

*The most fundamental contrasts are between intrinsic 

rate of increase–which should be highest in R selected species, and competitive ability, whichshould be highest among K selected species.

*Development should be rapid under R selection and relatively slow under K selection.

*Early reproduction and smaller body size will be favored by R selection, while K selection favors later reproduction and larger body size.

*Life history information is playing a key role in the conservation of
they also increase our understanding of basic population ecology.


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