Dept of Biology, Lewis and Clark College		Dr Kenneth Clifton
	Biology 141 Lecture Outline

Population size, extinction, and an introduction to ecological communities

What happens when populations decrease in size: the concept of  "threatened" and "endangered" species.

Why are species being threatened with extinction? Main reasons:

habitat loss or disruption.

overexploitation.

environmental degradation, e.g. pollution, global warming.

introduced species: competitors, predators, diseases, etc.

Why are populations that are small more likely to go extinct than larger populations are?

stochastic (unpredictable) events are more likely to bring a small population's numbers to zero.

environmental variability -- no year is exactly like the next.

demographic variability -- the law of averages doesn't apply very well to small populations, birth and death rates and sex ratios may fluctuate randomly.

How can we determine a species' minimum viable population size (MVP)?

we need to define "viable".

mathematical models are essential.

How does population size relate to the concept of "resiliency"

A case study: using a mathematical model to promote recovery of the loggerhead sea turtle, Caretta caretta, a threatened species. (reference: Crouse, D. et al. 1987. A stage-based population model for loggerhead sea turtles and implications for conservation. Ecology 68:1412-1423.)

Threats to the turtle.

Intuition steers us to certain measures; are they the most effective ones?

Mathematical models take us beyond intuition: Crouse's study.

used a population model using age-specific survival and fecundity.

analyzed whether increasing juvenile survival would cause the population's r to change from negative to positive. Answer: no.

then analyzed whether there was some other life stage where help would be more effective. answer: yes. Increasing survival of pre-reproductive adults could reverse the declining trend.

Is this possible? yes, by using turtle excluder devices (TEDs) on shrimp trawlers

Policy implications

good science is not enough, political will is required.

TEDs became a requirement on U.S. shrimp trawlers in 1991.

Crouse's study was based on an exponential model, appropriate for a species whose numbers are so far from K

Now let's start to think about groups of species:  Communities

Communities are populations of different species that co-occur in time and space.

 

Typically this includes organisms of diverse biological characteristics (e.g., sedentary / mobile, plants / animals, short-lived / long-lived, etc.)

 

Defining the boundaries of a community?

 

Is the notion of "communities" a biologically useful and reasonable approach?

 

Cohesive units vs. overlapping, independently operating populations

 

So... what ties communities together?

 

To answer, examine the "role" that species play within communities

 

Trophic roles: a revisit to trophic relationships and the concept of food web

 

By definition, species interact within a community

Types of interspecific interactions we will study include::

Predation and parasitism: what's the difference?

 

Competition (interspecific vs intraspecific).

 

Mutualism

 

Commensalism

 

Might these relationships vary through time?

 

Thinking about succession within a community

X

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