Does disturbance enhance genotypic diversity in clonal organisms? A field test in the marine angiosperm Zostera marina

Abstract

Physical disturbance has often been invoked to control genotypic diversity in sessile clonal organisms, yet experimental evidence is lacking. I studied the effects of physical disturbance on genet dynamics and genotypic diversity in a clonal marine angiosperm, Zostera marina (eelgrass). In replicated plots of 1 m2, the vegetation canopy was removed in gaps of zero (control), 25%, 50% and 75% of the area (n = 6 replicates). Before removal and during two consecutive years, the genotypic composition was determined using genetic markers (DNA microsatellites) in a 5 x 5 pixel grid per plot. An aggregate index of genet dynamics summarizing recruitment, increase, loss and decrease of clones was maximal at intermediate disturbance levels (quadratic polynomial P = 0.02). Physical disturbance also increased the occurrence of new genotypes, possibly reflecting recruitment (linear model, P < 0.05). Contrary to expectations, there was no competitive advantage of more heterozygous genotypes over less heterozygous ones. In the absence of disturbance, in particular, clones with lower individual heterozygosity were more likely to increase in area over a 1-year time period than more heterozygous ones, while there was no such correlation in plots with disturbance (logistic model, P(disturbance x heterozygosity) = 0.036). Undisturbed plots revealed background recruitment independent of canopy gaps, suggesting that Z. marina exhibits a strategy of continual recruitment. Effects of experimental disturbance (linear or quadratic) on clonal diversity were not detectable. Instead, initial (pretreatment) clonal diversity accounted for between 68% and 91% of the variance in diversity, indicating remarkable resilience of genotypic diversity in the face of physical disturbance.

Genetic diversity and connectivity remain high in eelgrass Zostera marina populations in the Wadden Sea, despite major impacts

“…In a recent experimental study in the Baltic Sea, Reusch (2006) showed, in a series of 1 m 2 plots with varying percentages of vegetation canopy removed, that genet turnover, recruitment and clonal stability were maximum at intermediate disturbance levels. Natural disturbance by waterfowl and invertebrate grazing have been noted by many (Nacken & Reise 2000, Schanz et al 2002, and higher genetic diversity of Zostera marina populations in the Baltic Sea have been documented after extensive grazing by swans (Hammerli & Reusch 2003).…”

“…This translates to the dual importance of habitat quality (water clarity, nutrients, hydro-dynamics and geomorphology) and the genetic potential of the populations (genetic variation, population structure and connectivity in relation to mating systems and demography). Integration of these 2 factors is a major goal in a management context (Ouborg et al 2006), and the power of this integration in eelgrass conservation and biodiversity is exemplified in studies that have modeled the importance of positive feedbacks between eelgrass and turbidity in the Wadden Sea (van der Heide et al 2007) and manipulative experiments documenting the importance of genetic diversity to enhanced ecosystem function in response to disturbance (Reusch 2006) and to global warming (Reusch et al 2005).…”

Evidence for persistent seed banks in dwarf eelgrass Zostera noltii in the German Wadden Sea

“…Consequently, meadow or patch composition may range from nearly every shoot arising from a separate seed (Coyer et al 2004a, Olsen et al 2004) to a single, long-lived clone with little, if any, recruitment (Reusch et al 1999). Factors affecting this variation include patch isolation (Reusch 2003), poor dispersal (Häm-merli & Reusch 2003a), intraspecific density-dependent competitive interactions (Hämmerli & Reusch 2003b, Reusch 2006, low seed production (Loqués et al 1990, Alexandre et al 2006, local seed predation (Fishman & Orth 1996) and the absence of a seed bank (Hootsmans et al 1987, Harrison 1993.…”

“…As seed banks provide a genetic reservoir for changing environmental conditions, subsequent recruitment can strongly influence population structure and genotypic diversity, especially for clonal plants (Eriksson 1989, Morris et al 2002, Koch et al 2003, Barrett et al 2005, Reusch et al 2005, Reusch 2006, Reusch & Hughes 2006, After the near total loss of subtidal eelgrass Zostera marina meadows to a wasting disease in the early 1930s, dwarf eelgrass Z. noltii has emerged as the primary seagrass species inhabiting the Wadden Sea, particularly in the northern regions (Reise & Kohlus 2008). Dwarf eelgrass is usually confined to the upper intertidal zone of sheltered sandy and/or muddy European coastlines, but sometimes it is found in the shallow subtidal zone (den Hartog 1970).…”

Numerous mitigation transplants of the eelgrass Zostera marina in southern California shuffle genetic diversity and may promote hybridization with Zostera pacifica

“…The higher frequency of smaller clones (4-6 linear m) is likely the consequence of most Z. marina meadows being located in sheltered bays and harbors characterized by more active dynamics, intra-specific competition, repeated seedling recruitment (Eriksson, 1993;Reusch, 2006) and, of course, human impact and mitigation. For Z. pacifica, large clonal meadows were initially predicted but found in only two of 11 meadows.…”