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Credit: Dr. Robyn Zerebecki, CC BY-SA 4.0.

Researchers measure cordgrass along the Atlantic coast.

Research from Dr. Robyn Zerebecki and collaborators demonstrates that intraspecific variation can have ecosystem-level consequences.

By Katie Sperry, PhD Candidate at Northeastern University

Take a look

Observing patterns in nature has led to many ecological discoveries. Famously, observed variation in Galapagos finches’ beaks sparked the theory of evolution. Notable differences in body size of mainland versus island populations, such as the tiny white-tailed deer that inhabit the Florida Keys, led to the “island rule”; on islands with constrained resource availability, animals can’t grow as big.

Salt marshes, coastal grasslands periodically inundated by tides, are dominated by smooth cordgrass. When walking through a salt marsh, you notice that from the back of the marsh toward the creek, the smooth cordgrass gets taller. At the back of the marsh, short form cordgrass just brushes your calf. At the front of the marsh, tall form cordgrass can tickle your chin. This pattern has long been observed but satisfying explanations for why this it occurs have been thin on the ground.

Big differences over small scales

Conditions at the front and back of the marsh vary. At the front, cordgrass is more frequently inundated and more fully submerged, while at the back tidal inundation is less frequent and deep. This creates an environmental gradient in both salinity and submersion. With sea level rise, this gradient will move up the marsh, and the ability of salt marsh plants to respond will determine the marsh’s ability to persist. An important piece of predicting this response is understanding whether plants will be able to grow taller as their microhabitat becomes more submerged.

Research led by Dr. Robyn Zerebecki, and published in The American Naturalist in 2021, investigated whether cordgrass height is a case of fine-scale local adaptation. In other words, are short and tall plants adapted to their respective zones even though they exist side by side? To do this, they used several experiments; a greenhouse experiment to examine whether height is a heritable trait, and a reciprocal transplant experiment to test for local adaptation to the two zones.

Tall does it all

They found that fine-scale local adaption and phenotypic plasticity both contribute to cordgrass height. However, the extent that local adaptation gives a “home field advantage” differs between the height forms. When plants from the short zone were planted in the tall zone, they were unable to grow taller. Plants from the tall zone, however, demonstrated a plastic response – growing tall in the tall zone and short in the short zone.

Researchers also have reason to believe that this will be the case not only in the region where they conducted the reciprocal transplants, but all along cordgrass’s Atlantic range. In addition to their greenhouse and field experiments, researchers conducted a population genetic survey of short and tall form cordgrass at sites along the Atlantic coast – including sites in the Plum Island Ecosystems LTER. At each site, they found genetic differentiation between short and tall form plants, suggesting the implications for sea level rise on short versus tall form cordgrass hold across its Atlantic range.

Differences within a species matter a lot

As tides creep higher and the underlying stress gradient pushes landward, these results suggest that the ability of tall form cordgrass to move higher up the marsh is key for persistence. Short form plants were unable to grow taller in response to increased submergence; therefore, short cordgrass is unlikely to persist at the new marsh edge. Persistence at this lower edge will depend instead on the ability of tall plants to move up the slope of the marsh.

Anticipating ecosystems responses to change is key to creating conservation plans. Often, though, responses are assumed to be uniform across a species – seldom is within-species variation accounted for. This work underscores that intraspecific variation can be an important factor in understanding how a system will respond to global change like sea level rise. To better understand how to preserve ecosystems facing big changes, we might need to get better at noticing differences within a species.