Community Ecology and Life History

Let's talk about what makes up the life history of a population. How and when do species reproduce? What factors influence their lifespans? How does this impact community succession? SO MANY QUESTIONS!

And now some answers...

Life history characteristics influence population demography through interconnected traits resulting from natural selection and environmental conditions (Smith & Smith, 2003, p. 234). The theory behind life history investigates patterns among populations such as their mode of sexual reproduction (sexual or asexual), mating system, age structure, sex ratio and survivorship/mortality curve (Smith & Smith, 2003, p. 219). It explains why and how organisms have evolved to maximize their reproduction rate while ensuring their survival (Smith & Smith, 2003, p. 264).

Life history explores interactions between extrinsic and intrinsic factors. Extrinsic factors are influenced by environmental conditions affecting age-specific rates of mortality and reproduction. Intrinsic factors amount to tradeoffs among traits within an organism such as energy available energy available for reproduction, maintenance or growth (Smith & Smith, 2003, p. 264).

A key concept of life history variation is a species' energy allocation for reproduction. An organism can only uptake limited amounts of energy, and thus, there will be tradeoffs between the number and size of offspring. Life History patterns reveal a population's allotment of available energy and resources and explain a population's relationship between fecundity, survival, relative offspring size, and age at reproductive maturity. For example, when an organism has larger offspring, they produce fewer numbers because they have allocated more energy to bigger offspring (Molles, 2012, p. 264).

When adults have low chances of surviving, the population will reproduce at a younger age in order to ensure their genes are passed on. Organisms with high mortality rates will allocate more of their energy to reproduction. Whereas low mortality rates lead to a later age of reproductive maturity. Organisms that have a higher change of living as adults will reproduce later in life and use less of their energy and resource for reproduction (Molles, 2012, p. 271).

Scientists attempt to organize life history characteristics using different classification systems. Grime's characterized plants depending on three different environmental conditions: R, C and S (Smith & Smith, 2003, p. 228). Charnov's classification model delineates reproductive effort, offspring size and benefit-cost rations without factors of time and size (Molles, 2012, p. 280). Perhaps the most known model was developed by MacArthur and Wilson: the r and K Selection concept (fig 2.1).

Figure 2.1: The binary view of life history traits characterized a r or K (Johnson, 2012, p. 729)

No species is exclusively "r" selected or exclusively "K" selected. Instead species fall somewhere along a continuum (Johnson, 2012, p. 729). Humans are "K" selected in that we are iteroparous (reproduction multiple times during a lifespan) and relatively large mammals with slow development, yet humans are currently "r" selected in our growth rate due to technological advancements that have raised our carrying capacity allowing for continued exponential growth. Generally, populations in rapidly changing habitats express r-selected adaptations, while populations of related organisms in more ecologically stable and competitive habitats have more K-selected attributes (Molles, 2012, p. 275).

Life history characteristics among species may change during primary succession towards community equilibrium because the species within the community will modify the habitat, impacting the resources available. Primary succession represents an ecological succession beginning with bare substrate or rock, void of soil, often occurring after a natural disaster such as volcanic eruptions or glacial retreat. Climax community (equilibrium) refers to the final ecological succession.

However, climax community can be elusive and take a very long-time due to continuous flux in local climate (or intrusive human activities) (Johnson, 2012, p. 741). The 1971 study of Glacier Bay, Alaska revealed trends in community changes during succession (Molles, 2012, p. 443). First, pioneer species exhibiting r- selected traits such as mosses and lichens colonized exposed land. Next low shrubs and herbs moved in, increasing diversity. As the ecosystem matured, the more r-selected species were replaced by more K-selected species, often time trees (Molles, 2012, p. 444). There are some commonly observed features of successional changes including greater amounts of biomass, primary production, respiration and nutrient retention; all guiding succession towards a climax or stable state (Molles, 2012, p. 447).

References and Good Reads:
Johnson, George B. (2012). The Living Word: Seventh Edition. McGraw Hill Education, University of
Washington, St. Louis, Missouri.
Molles, Manuel C. (2012). Ecology Concepts and Applications: Fifth Edition. McGraw Hill
Education, University of New Mexico.
Smith, Robert, L. and Smith, Thomas, M. (2003). Elements of Ecology: Fifth Edition. Pearson
Education, Inc. San Francisco, California.