Determinants of extinction in fragmented plant populations: Crepis sancta (asteraceae) in urban environments

Abstract

Local populations are subject to recurrent extinctions, and small populations are particularly prone to extinction. Both demographic (stochasticity and the Allee effect) and genetic factors (drift load and inbreeding depression) potentially affect extinction. In fragmented populations, regular dispersal may boost population sizes (demographic rescue effect) or/and reduce the local inbreeding level and genetic drift (genetic rescue effect), which can affect extinction risks. We studied extinction processes in highly fragmented populations of the common species Crepis sancta (Asteraceae) in urban habitats exhibiting a rapid turnover of patches. A four-year demographic monitoring survey and microsatellite genotyping of individuals allowed us to study the determinants of extinction. We documented a low genetic structure and an absence of inbreeding (estimated by multilocus heterozygosity), which suggest that genetic factors were not a major cause of patch extinction. On the contrary, local population size was the main factor in extinction, whereas connectivity was shown to decrease patch extinction, which we interpreted as a demographic rescue effect that was likely due to better pollination services for reproduction. This coupling of demographic and genetic tools highlighted the importance of dispersal in local patch extinctions of small fragmented populations connected by gene flow.

Inferring contemporary dispersal processes in plant metapopulations: comparison of direct and indirect estimates of dispersal for the annual species Crepis sancta

“…Finally, the low selfing rates found in the two networks are congruent with those obtained in the same populations based on multilocus heterozygosity (Dornier and Cheptou, 2012).…”

“…In our study, we controlled for the presence of null alleles using the XN data set for the JC network and our results suggested that immigration rate was indeed inflated by the presence of null alleles. Although such results were not obtained for the HM network, it is likely that the presence of null alleles biased estimates in the same way for the two networks, given the absence of significant genetic structure among the networks (Dornier and Cheptou, 2012). Although changing the genetic data set for JC was found to affect immigration, the immigration rate remained relatively high and was unlikely to be due to human error or marker quality.…”

Effects of fragmentation on plant adaptation to urban environments

“…We used eight neutral microsatellite markers [36,41] to assess levels of neutral genetic diversity within each population and between the fragmented and unfragmented (and urban and rural) sets of populations. We extracted DNA using maternal leaf tissue from 28-30 individuals per population using QIAGEN DNeasy Plant Mini kits.…”

Experimental separation of genetic and demographic factors on extinction risk in wild populations

“…Despite the importance of understanding the relative roles of demography and genetic processes for developing sound conservation decisions, experimental tests of these processes in nature are still sparse. Correlations between population size, heterozygosity, life history features, and local extinction have been observed (Pimm et al 1988, Saccheri et al 1998, Saether et al 2005, Dournier and Cheptou 2012, and experiments provide evidence that some of the mechanisms associated with small population size affect short-term components of population fitness (Jime´nez et al 1994, Newman and Pilson 1997, Madsen et al 1999, Nieminen et al 2001). However, we lack experimental information on the relative importance of different processes on long-term population dynamics and persistence.…”