Effect of Population Size on the Prospect of Species Survival

• Published in: Folia geobotanica 2006-01, Vol.41 (2), p.137-150
• Main Author: Münzbergová, Zuzana
• Format: Article
• Language: English
• Subjects: Demography; Flowers; Habitat conservation; Plants; Population characteristics; Population estimates; Population growth rate; Population size; Seed production; Seeds
• ISSN: 1211-9520; 1874-9348
• DOI: 10.1007/BF02806475

Many recent studies have demonstrated a negative effect of small population size on single plant traits. However, not much is known about the actual consequences of reduced plant performance on the long-term prospect of species survival. I studied the effect of population size on population growth rate and survival probability in the rare perennial herb Scorzonera hispanica occurring in fragmented grasslands. Its performance was measured using several traits related to reproduction in 21 populations ranging in size from 3 to 2475 plants. These data were then connected with data on full demongraphy of the species from three of the studied populations. Two different matrix models differing in the number of transitions based on measurements in the populations differing in size were used to explore the relationship between population size and population growth rate. Both matrix models showed that despite the decline in seed production in small populations, population growth rate is never significantly different from one, and the populations could thus be expected to survive in the long run. Calculations of extinction probabilities that take into account demographic and environmental stochasticity, however, showed that populations below 100 flowering individuals have a high probability to become extinct. This demonstrates that demographic and environmental stochasticity is an important driver of the fate of small populations in this system. This study demonstrates that estimation of population growth rate can provide new insights into the effect of population size on population growth and survival. It also shows how matrix models enable the combination various pieces of information about the single populations into one overall measure, and may provide a useful tool for the standardization of studies on the effects of population size on population performance.