Ecology Lab Quiz

26 November 2002

Name_____________________________

A population of duck-billed platypuses has been determined to have the following stage transition matrix:

1. At what age does reproduction start in this species?

 AGE 1

2. Why are the 1-1, 1-2, 1-3+, 0-1 etc. cells 0?

BECAUSE  1-YR OLDS CAN’T BE 1 YR-OLD IN THE NEXT YEAR, ETC. THE ONLY CELLS IN THE MATRIX BELOW THE AGE-CLASS O ROW THAT HAVE VALUES ARE FOR 1-YR OLDS TURNING INTO 2-YR OLDS, 2-YR OLDS TURNING INTO 3-YR OLDS, AND 3+-YR OLDS SURVIVING. 

3. If the population includes 10 individuals in age class 0, 30 in age class 1, 40 in age class 2, and 30 in age class 3+, how many newborns will there be in the next year/generation?

THE 30 1-YR OLDS HAVE 9 OFFSPRING (30 * 0.3), THE 2YR OLDS HAVE 20 (40 * 0.5) AND THE 3+ GROUP HAVE 9 (30 * 0.3), SO THERE ARE 9+20+9=38 OFFSPRING. 

4. Given the same age structure as in question 3, how many age-class 2 individuals will there be in the next year/generation?

15, OR 0.5 * THE 30 INDIVIDUALS IN AGE-CLASS 1. 

5. What is a “sensitivity analysis” in ecological modeling and why do we use them?

IN A SENSITIVITY ANALYSIS, WE CHANGE ONE PARAMETER IN OUR ECOLOGICAL MODEL (SURVIVORSHIP AT A GIVEN AGE, FOR EXAMPLE) AND SEE HOW THAT CHANGES OVERALL POPULATION SIZE. WE THEN CHANGE ANOTHER PARAMETER AND SEE IF WE GET A LARGER OR SMALLER CHANGE IN POPULATION SIZE. IN THIS MANNER WE CAN DETERMINE WHICH OF THE MODEL PARAMETERS IS MOST SENSITIVE TO CHANGE, AND HENCE WOULD BE THE MOST EFFECTIVE WAY TO MANAGE THE POPULATION FOR WHATEVER END WE DESIRE (YEILD OF TUNA, SURVIVORSHIP OF RED-COCKADED WOODPECKERS, ETC.) 

6. Why is the outer edge of a tree ring darker than the wood laid down earlier in that year?

THE OUTER CELLS, LAID DOWN IN LATE SUMMER AND EARLY AUTUMN, ARE SMALLER AND HAVE THICKER CELL WALLS. 

7. How would a forester trying to maximize profit from a timber stand use tree ring data?

SAID FORESTER WOULD LOOK FOR THE PERIOD IN WHICH THE TREE RINGS WERE WIDEST. THIS INDICATES THAT THE TREE STAND IS GROWING FAST. HARVESTING WOULD BE CARRIED OUT AT THE AGE WHEN THE TREE RINGS START NARROWING, INDICATING THAT THE TREES AREN’T GROWING MUCH, SO THEY SHOUD BE HAVESTED AND SOLD TO MAKE ROOM FOR YOUNGER, FASTER GROWING INDIVIDUALS. 

8. Dendrochonologists have used tree rings to chronicle climate changes (especially periods of drought) 9000 yrs before present, yet the oldest standing trees are only 4000 yrs old. How is this possible?

BY MATCHING UP RING PATTERNS FROM LIVING TREES WITH DEAD OR FOSSILIZED TREES. IF YOU CAN FIND A DEAD TREE WHOSE OUTERMOST RINGS MATCH A PATTERN FOUND IN THE INNERMOST RINGS OF A LIVING TREE, YOU CAN EXTEND YOUR RECORD OF CLIMATE BACK TO THE EARLY RING PERIOD OF THE OLDER DEAD TREE. 

9. Why is it desirable to replicate an experiment?

BY REPEATING EXPERIMENTS WE CAN CALCULATE MEANS AND STANDARD DEVIATIONS FOR OUR RESPONSE VARIABLES. WITHOUT KNOWING ABOUT THE DEGREE OF VARIABILITY IN WHATEVER WE’RE MEASURING, WE CAN’T BE CONFIDENT THAT OUR RESULTS ARE RELIABLE. FROM A PRACTICAL POINT OF VIEW, WE CAN’T USE STATISTICS UNLESS WE HAVE A NUMBER OF OBSERVATIONS OF OUR VARIABLES. 

10. Why could our experiment on the effect of a controlled burn on short-leaf pine growth be considered “pseudoreplication”?

BECAUSE WE REALLY ONLY HAVE 1 EXPERIMENTAL TREATMENT. WE CAN’T TELL IF THE DIFFERENCES WE SAW (OR DIDN’T SEE) IN THE CONTROL AND EXPERIMENTAL SITES WERE THE EFFECT OF THE EXPERIMENTAL TREATMENT OR CAUSED BY SOME EXTRANEOUS VARIABLE—PERHAPS THE SOILS WERE DIFFERENT ON THE TWO SIDES OF THE ROAD AND THIS HAD A BIGGER INFLUENCE ON TREE GROWTH THAN THE EXPERIMENTAL BURN. TO AVOID THIS PROBLEM, WE WOULD NEED TO REPEAT THE CONTROLLED BURN IN SEVERAL DIFFERENT AREAS. WITH ENOUGH REPLICATIONS, THE EFFECTS OF OUTSIDE INFLUENCES SHOULD CANCEL THEMSELVES OUT—WE WOULDN’T EXPECT TO HAVE THE SAME DIFFERENCES IN THE SOIL EVERY TIME WE SET UP CONTROL AND BURN PLOTS.