Demography of the endangered fragrant prickly apple cactus, Harrisia fragrans

The population dynamics of the endangered fragrant prickly apple cactus of southeastern coastal Florida were studied in order to predict future population growth and structure. Plants in two populations were tagged, mapped, and measured each year for 5 yr. Recruitment and mortality were recorded. These small populations declined dramatically with high mortality and low recruitment rates. Transition matrices based on stem length were constructed and analyzed. The dominant eigenvalues indicated declining populations. Projection analysis also indicated population decreases, but it underestimated the actual declines. This result is understandable since the predicted stable length distributions (analogous to stable age distributions) were different from the actual length distributions at the end of the fifth year. In 8 yr, the two populations declined by 55.3% and 59.8%. Elasticity matrices indicated the overwhelming importance of the largest individuals to the survival of the populations, yet changes in reproductive rate had negligible demographic effects. Actual length distributions at the end of the fifth year were projected to the eighth year and compared with actual length distributions in the eighth year. For population 1, there was a significant difference, in which there were actually more small individuals and fewer large individuals than projected, but results were not significantly different for population 2. A population viability analysis indicated that modal time to extinction for both populations was <20 yr.