Hybrids may succeed where invasive species fail as sea levels rise

Spartina densiflora has been moving into habitats in the San Francisco estuary, displacing the native Spartina foliosa, leading to a campaign to eradicate it. A study by Blanca Gallego-Tévar and colleagues has found that rising sea levels could cause more harm to the invasive species than the native plant. But they also found that a hybrid of the two species may combine genes to survive in the changing environment. “The hybrid S. densiflora × S. foliosa produced more biomass, tall tillers and rhizome reserves than the parental species. The hybrid also achieved high vegetative and reproductive fitness, even under the most stressful conditions imposed in our experiment,” wrote the authors in their paper.

Understanding how invasive species behave in habitats is critical, as we learn more about the ecosystem services they provide, said one of the co-authors of the study, Jesus Manuel Castillo Segura. “Invasive species outcompete native biodiversity, alter natural landscapes, change nutrient fluxes, etc. We don’t need invasive species because we normally have native plants that are suitable bio-tools for ecological restoration projects and other ecological applications.”

The study gathered plant material from the Benicia and Corte Madera salt marshes in the San Francisco Estuary. Here, Spartina foliosa was seeing its home invaded by Spartina densiflora. The team of botanists from Spain and the USA wanted to see how habitability would change in the future, and so they collected samples for experiments. The experiments combined two issues, invasive species and climate change, with the latter expected to drive sea level rise. The team combined the effects of the two problems by taking plants and subjecting them to increasing salinity and inundation in the lab, then measuring their biomass and many other plant responses. The ecologists found that the three plants each had different responses to the stresses. “Overall, the native S. foliosa expressed the traits of a slow-growing and stress-tolerant species. Spartina foliosa was less sensitive to increasing salinity than S. densiflora or the hybrid,” wrote the authors in their paper.

“In contrast with the native species, the invasive S. densiflora may be typified as a fast-growing species able to take advantage of low-stress conditions while tolerating moderate stress levels. Under non-stressful conditions, this species maintained high sexual reproduction and biomass accumulation with tall tillers. However, extreme conditions (hypersalinity and/or deep inundation) induced high leaf-rolling, increased proline concentrations and marked decreases in net photosynthesis rate and floret production.”

“The hybrid S. densiflora × S. foliosa produced more biomass, tall tillers and rhizome reserves than the parental species. The hybrid also achieved high vegetative and reproductive fitness, even under the most stressful conditions imposed in our experiment. High tiller production would drive rapid lateral expansion rates to colonize surrounding sediments and high floret production would increase the colonization capacity of the hybrid to medium and long distances if this taxon were not sterile.” 

“This study shows that climate change is even a more important environmental problem than biological invasions. Climate change may even promote the spread of alien species at the same time that impacts negatively on local biodiversity. We need to mitigate climate change now before it is too late,” said Castillo.

“What we are seeing is that the combination of different environmental stresses creates synergic effects on plants response. Sometimes, plants can deal adequately with just one abiotic stress but the plant responses get like blocked when two stresses are combined.”

“Climate change is exposing biodiversity to many different stresses at the same time. This is why we need to understand how biodiversity deals with stress mixes. We need to be able to anticipate the effects of climate change so we can adapt our artificial systems to global warming at the same time that we try to minimize its effects on natural ecosystems.”

While Gallego-Tévar and Castillo are based in Seville, they have found it extremely helpful to work with colleagues in the USA. “Our research group collaborates with the group of Dr. Brenda J. Grewell from the United States Department of Agriculture (USDA) in the University of California, Davis. We learn from each other.” Castillo added that while San Francisco may be thousands of kilometres from Seville, some of the problems in California are very close to home.

“Plants living in estuaries are similar to each other in many salt marshes around the world. Some of these halophytes are invasive species colonizing very distant marshes. For example, the cordgrass Spartina densiflora from South America is invading marshes in Southwest Iberian Peninsula and in California!”

The problems should interest anyone working on the consequences of climate change on the biodiversity, especially in estuaries and salt marshes, Castillo said. The problems the team tackle have the potential to be a long term problem. They conclude: “The sterility of the hybrid S. densiflora × S. foliosa currently limits its invasiveness, but, once established, it has a high ability to maintain growth and biomass production with increasing levels of salinity and inundation.”