Deep in the meadow, under the … seagrass, a bed of temporally stable diversity?

Genetically diverse populations are often more stable and productive. For habitat-forming organisms, such as seagrasses, this results in increased habitat complexity and more abundant associated communities (e.g., Hughes and Stachowicz 2004, Reusch et al. 2005). 
Spatial patterns of genetic diversity have then be used to infer spatial variation in functioning. This relies on the assumption that genetic diversity is stable over time.  But are estimates of population history, connectivity and the ecological significance of these patterns reliable? Are they temporally stable?

Few studies actually look at temporal stability in genetic variation, despite it’s utility (see another post on temporal dynamics here).
Reynolds et al. (2017) assessed the stability of spatial patterns of allelic richness, clonal richness and relatedness and genetic structure in the eelgrass Zostera marina.
The structure of genetic diversity in seagrass meadows can vary with the age (i.e., well-established vs. restored), the life history (i.e., perennial vs. annual) and the intensity of human influences (i.e., urban vs. rural).
Reynolds et al. used published estimates of genetic diversity and stability sampled in meadows from California and Virginia over a period of 5-12 years.
Neither diversity nor differentiation (as measured by Fst) changed over time.
In Bodega Bay, differences among tidal heights were also remarkably consistent, but unlike some other intertidal organisms (such as Chondrus crispus), the high intertidal was more genetically diverse than the subtidal!
But, seagrasses are clonal, so they might have just re-sampled the same giant clones? Yet, this wasn’t the case in their seagrass meadows as large, persistent clones were not represented.
In an analogous study, Becheler et al. (2014) found sign significant temporal patterns of genetic differentiation within plots at the same site and same tidal elevation. So, what’s the difference between within site and between site variation?
Becheler et al. attributed their patterns of temporal differentiation to the increasing dominance of some clones within plots and within sites, rather than changes in population genetic diversity over time.
When Reynolds et al. re-analyzed the data set from Becheler et al. at larger spatial scales, they found consistency of differentiation between sites over time. Little variation from both studies at larger spatial scales is due to time relative to space.
They argue that

at least in the absence of catastrophic disturbances, even decadal patterns of genetic structure and diversity can provide surprisingly accurate depictions of genetic patterns among eelgrass assemblages.

Genetic data can, therefore, be used for management actions. For example, population genetic surveys can be used to pinpoint priority conservation sites where there is high diversity and high connectivity.
Now we just need to do these types of studies in other ecosystem engineers, like intertidal and subtidal seaweeds!
R Becheler, E Benkara, Y Moalic, C Hily and S Arnaud-Haond (2014) Scaling of processes shaping the clonal dynamics and genetic mosiacs of seagrasses through temporal genetic monitoring. Heredity 112: 114-121.
A R Hughes and J J Stachowicz Genetic diversity enhances the resistance of a seagrass ecosystem to disturbance. PNAS 101: 8998-9002.
T B H Reusch, A Ehlers, A Hämmerli and B Worm (2005) Ecosystem recovery after climatic extremes enhanced by genotypic diversity. PNAS 102: 2826–2831.
L K Reynolds, J J Stachowicz, A R Hughes, S J Kamel, B S Ort and R K Grosberg (2017) Temporal stability in patterns of genetic diversity and structure of a marine foundation species (Zostera marina). Heredity 118, 404-412

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