Developmental Failure and Loss of Reproductive Capacity as a Factor in Extinction: A Nine-Year Study of Dedeckera Eurekensis (Polygonaceae)

Abstract

A BSTRACTMany long-lived perennial spe cies exhibit lowered reproductive capacity. Early studies of reproduct ive success in Dedeck era eurek en sis (Po lygonacea e) demonstr ated that the spe cie s e xhibited extremel y low reproducti ve su cce ss , low see dlo vule (S/O ) ratios (i.e., the percentage of ovules that produce filled seeds; 2.5 %), low germinability of filled seeds (3.5%), low seedling survivorship (11 .1% ), and lack of recruitment in natural populations. These results were attributed to genetic load , but thi s eli c ited co ntrove rsy , prompting long -term stud ies of the re lationshi p between the SIO ratio and en vironment. After nine year s o f monitoring, how ev er, the SIO ratio had not changed signifi cantly (2 .7%), and there was no significant correlation between precipitation and the SIO ratio. Controlled field ex perime nts dem on strated that neither resource ava ilability nor other eco logica l factors significantly influenced embryo abortion rates . Controlled se lf-po llinatio ns (N = 115) matured only one qu estionably filled seed , whereas intrapopul ation cro ss-pollination s (N = 192) produced signifi cantly mor e seed (S/O = 12.0 % ) . Previ ou s pollination studies demonstrated that the species has no primary poll inators and is onl y rarel y visited by a few gene ra list insects. However, the flower s typi call y se lfpoll inate in 2-3 da ys foll owing anthesis. Strong infe rence suggests that the loss of reproducti ve capacity in D. eurekensis may be the result of inbreeding depres sion due to the superimposition o f se lfpollination on a norm ally outcrossed species carr ying a high gen eticlsegregational load .

Reproductive success and genetic divergence among varieties of the rare and endangered Astragalus cremnophylax (Fabaceae) from Arizona, USA

“…Moreover, we found higher S/O ratios in taxa that had some degree of selfing. These data are consistent with other studies showing higher S/O ratios in taxa that are selfed rather than outcrossed (Wiens 1984;Wiens et al 1987Wiens et al , 2002Meney et al 1997;.…”

“…However, environmental fluctuation, as measured by precipitation, did not appear to be a factor contributing to low seed/ovule ratios and high embryo abortion in this taxon. These data are consistent with other studies that have demonstrated constancy of S/O ratios over multiple years and correlation of resource availability with greater flower and seed production but not embryo abortion (Maun and Cavers 1971;Stephensen 1984;Wiens 1984Wiens , 1989Harper and Wallace 1987;Roitberg 1989, 1994;Herrera 1991;Wiens et al 2002).…”

The mechanism of background extinction

“…Background extinctions are invariably the result of a series of interacting factors and their multiplicative effects (Gittleman & Gompper, 2001). Likewise, our studies of higher plants show that population decline is related to multiple environmental causes and genetic factors (Wiens et al ., 1989, 2002, 2011).…”

“…Unfortunately, with the exception of Harper (1977), their statements are brief and without elaboration: Jepsen, Mayr & Simpson (1949), Raup (1978, 1984, 1991), McKinney (1993), Rose (1998), Mayr (2001), and no doubt others. Mayr (2001) stated it clearly: ‘Whenever a population is no longer able to reproduce enough offspring to replace losses from natural causes, it will become extinct.’ Perhaps the lack of amplification by these authors was due to the paucity of fecundity data to support the hypothesis (Wiens et al ., 2002, 2011; Schoener et al ., 2003).…”

“…It is certainly counter‐intuitive. However, extraordinarily high rates of embryonic abortion (95–99%) in several species of flowering plants from the south‐western United States may give such an impression (Wiens et al ., 1989, 2002, 2011). Similarly high rates of embryonic abortion are also known in some flowering plants from Australia (Meney, Dixon & Pate, 1997; Cochrane et al ., 2001) and South Africa (Wiens et al ., 1983; Wiens, Davern & Calvin, 1988).…”