Introduction - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 5

However, consider the extensive range between these extremes. To
cite just one example, a guide to hardwood restoration of bottomland sites
recommends spacing of seed or seedlings 0.9-1.8 m apart in rows separated
by 2.7 m (Allen et al. 2001: 46-47), well above the natural densities for most
hardwoods. These recommendations are partly based on the recognition
that there is likely to be high mortality among the introduced individuals
(often exceeding 90% after just 10 years). Plants with different life history
characteristics (e.g., annuals or short-lived herbs) will require other planting
strategies. In many restoration projects involving endangered species,
project managers will want to manage planting density for optimal survival,
maintenance considerations, and genetic diversity preservation because of
the costs of obtaining suitable seeds and seedlings, and/or the low number
of available propagules.
How should a restoration project manager design a plan for
reintroduction? Are there differing population growth and genetic diversity
conservation outcomes that arise solely as a consequence of different
spationumeric arrangements of founders? Are such differences minor or
major? Do differences in life history characteristics affect these issues?
Should the seeds or seedlings be planted in a solid or hollow square? In
three lines? In subpopulations? How far apart should founding individuals
be? Is it best to plant founders at approximately the average density for
the species, or at greater or lesser densities? Perhaps the normal average
density would be optimal since this is the density that facilitates normal
establishment and ensures “normal” plant-environment interactions and
maintenance. Perhaps planting at greater than normal density better
ensures that needed pollinators and or seed vectors will be effective in
initially small populations. Might planting at higher densities promote
greater genetic mixing among founding and offspring individuals? When
does planting at higher densities become detrimental? If planting at
densities lower than average is superior for a given species, how much
lower provides “optimal” population growth, genetic diversity retention,
and avoidance of inbreeding? When do densities become so low that
Allee effects become noticeable? Are there spatially induced effects that
differ across different stages of population development? Suppose only
20 seeds of an extremely rare plant can be harvested. Does it matter how
those are spatially introduced? Or with such low numbers of founders, are
introduction processes geometrically neutral?
As noted above, there are economic repercussions to such considerations.
Obviously, planting 500 seedlings 0.5 km within the border of a preserve is
less costly than planting 20,000 seeds 10 km into a preserve. But are there
increased biological costs such as greater loss of genetic diversity in the
former case? How does distance of introduction from a preserve border
affect population growth and genetic diversity retention? If corridors are