Effects of marginality on plant population performance

Aim - Populations at the edge of a species' distribution range may differ substantially from central populations. Peripheral populations may have either a high evolutionary potential or be prone to extinction, but the processes driving these outcomes are still unclear. Peripheral plant populations have been the subject of numerous studies and reviews, with many focusing on their genetic characteristics. In this review, we consider the effect of marginality on demographic species-specific traits. Location - World-wide. Methods - We reviewed the literature based on direct comparisons between central and peripheral plant populations. Strict inclusion criteria were applied to avoid biased analysis that may arise as a result of inaccurate boundary considerations or inappropriate comparisons. We inferred from the published data whether a certain trait had a better performance in central or peripheral populations (reliability of the abundant centre hypothesis, ACH). Results - There have not been enough studies on plant performance to allow for generalizations on the effects of marginality on plants. ACH expectations were not met in most cases and specific responses to marginality were observed at the species and population levels. Population and plant size more often met the ACH assumptions, suggesting that most geographically peripheral populations are also ecologically marginal. The availability of resources, the reproductive strategy, the level of ploidy and the ability to cope with interspecific competitors seem to drive the numerous exceptions to the ACH expectations. Main conclusions - The large numbers of exceptions to the ACH expectations suggest that a new comprehensive theory is needed to explain the effects of marginality in plants and to identify any general patterns. From the theoretical point of view, we propose that population history and dynamics should be considered when attempting to explain the processes that occur in peripheral plant populations.