A definition of generation length was developed specifically for
seagrasses by regional workshop participants, as no definition was
apparent in the literature. Generation length is defined by the
IUCN Red List Guidelines (IUCN, 2010) as the average age of parents
of the current cohort (i.e., newborn individuals in the population).
For seagrasses, generation length was defined as the
recruitment rate via sexual reproduction. Recruitment rate for
each species was calculated from the time a seed or seedling is released
from the parent plant through the time of creating a reproductive,
mature plant – that is, the time needed for the seedling to
establish, grow, and produce seeds. For example, the recruitmentrate for Posidonia sinuosa was estimated to be approximately
20 years, based on its relatively low pollination viability and slow
growth rate (Smith and Walker, 2002). By contrast, the recruitment
rate of Halophila hawaiiana was estimated to be less than
2 years as it flowers relatively quickly, is fast growing, and has
a turnover rate of approximately 15 days (Herbert, 1986). Where
recruitment rate could not be determined for a given species,
information from similar known species’ recruitment rates was
used (Hemminga and Duarte, 2000). Although seagrasses reproduce
both asexually (clonally) and sexually, asexual reproduction
does not create a new, genetically distinct individual; rather the
same individual is colonizing a new area, increasing the size of
the clone. Asexual reproduction contributes to persistence, however
it does not provide greater evolutionary potential or increased
resilience to environmental change, i.e., does not
contribute recruitment of genetically new individuals into the
population.