Many species exhibit a strongly female-biased phenotypically expressed sex ratio that likewise is in some cases genetic and in others possibly due to varying responses to environmental conditions. The "shy male" hypothesis lacks support in explaining most of this female bias. Examples of distinct male bias in expressed sex ratios also exist. Sex ratios based on genetic information on non-expressing plants is known for a very limited number of species.  Some species, perhaps more than we realize, have sexual plasticity. That is, they have different gender expressions in different years, possibly dependent on age or available energy resources. This can be due to hormonal expressions of the same or neighboring plants. When sexual reproduction fails, asexual reproduction by specialized propagules can compensate, and this is especially true for dioicous species. Because of the energy cost of producing sporophytes, males might have the energy needed for producing asexual structures. In addition, clonal growth and fragmentation can help the species spread. A modeling study suggests that disturbance level (weather, pollution, fire, etc) affects genders differentially, hence maintaining both sexes in the long term. Epiphytes are frequently isolated on a tree with only one sex present. They can benefit from asexual reproduction and have a higher than average percent of propguliferous taxa.