The variations between scenarios 1 and 2 are primarily owing to variation in respective initial abundances, and the rate of decrease in population is almost identical. However, the scenario 3 estimates vary significantly from the other scenarios both in terms of rate of decrease in population as well as overall final abundance.
The final abundance estimates are useful when it is used with respect to the initial abundance, as this gives an indication about the growth rate/ decline rate, and the sustainability of the metapopulation.
With the growth rate at 1.08, the final abundance is higher in all the cases. the final abundance grows by 16-20% in scenarios 1 and 2, while the final abundance is about equal to the initial abundance in scenario 3. However, the probability of extinction is scenario 3 is slightly lower than the other scenarios.
Similar pattern is visible when the growth rate is at 1.02. the final abundance reduces by 58-59% in scenarios 1 and 2, while it reduces by 64% in scenario 3. Probability of extinction is about the same in all cases.
4. How do the results differ if the average dispersal distance is reduced to 150 map units (15 km or 9.3 miles) or increased to 200 map units (20 km or 12.4 miles) from the original distance of 18.5 km (11.5 miles)?
The overall pattern remains same. however, the effect of dispersion distance on final abundance is less pronounced that of change in growth rate. On increasing the dispersion distance to 20 kms, the final abundance reduces by 14-19% in scenarios 1 and 2, and to 33% in scenario 3.
Changing the carrying capacity of Philip Flat changes the overall pattern. When the carrying capacity of the Philip Flat is increased to 130, the reduction in final abundance is lower in scenario 1 at 15% as compared to 20-25% in scenario 2, and 34-38% in scenario 3.
The PVA is highly sensitive to errors in the growth rate.