The current overarching paradigm in seagrass conservation is that meadows which are home to large, luscious late succession seagrass species are healthy, pristine ecosystems, while meadows displaying patches of disturbance and consisting of short, fast-growing colonizing species are often classified as degraded.
While logically this makes sense, as we often associate areas of disturbance with anthropogenically driven habitat destruction, it neglects to take into account the role that megaherbivore grazing has on the local environment and on ecosystem functioning. In seagrass systems, primary megaherbivores became ecologically extinct long before we noticed the meadows themselves were disappearing too. This shifts what we believe to be the baseline state of the meadows, as we have only witnessed them in the absence of their major herbivores and not in their original state.
A recent commentary published in Nature Ecology & Evolution, suggests that we need to re-evaluate what we consider a “pristine” environment and what we consider “degraded” in terms of seagrass meadows. In fact, we have become so concerned with saving an idealized concept of a “pristine”, undisturbed seagrass meadow that we have forgotten that organic systems do often exist in a state of flux and cycle through different successional phases following natural disturbances.
The commentary comes after increased green turtle densities, resulting from successful conservation efforts at several nesting beaches, have dramatically impacted local seagrass foraging sites. After centuries of low population numbers, researchers were surprised at how the hungry sea turtles were able to shift the species composition of meadows and adapt their feeding schemes to the shift in seagrass structure and species composition.
Green Turtles (Chelonia mydas)
Green turtles are the largest hard-shelled sea turtles and the only extant member of the genus Chelonia. On average, they measure between three to four feet in length and weigh between 300 and 350 pounds. They have a global distribution and inhabit tropical and subtropical oceans across the world. There are two distinctive populations, one inhabiting the Atlantic Ocean and the other inhabiting the Pacific Ocean, although they are also sometimes found in the Indian Ocean. Like most turtle species, green turtles undergo extensive migrations between feeding and breeding grounds. Nesting sites are found in around 80 countries, where females undertake nocturnal excursions to lay clutches of eggs on the same beach every year.
Historically, green turtles were exploited for their meat, eggs and shells and experienced severe population declines over the past couple of centuries. However, in many countries, it is now illegal to hunt sea turtles or collect their eggs, although they still face threats in the form of climate change, pollution and fishing industries. Many nest sites are now protected, and marine protected areas have been established in many places to help protect their feeding grounds. As a result of these actions, green turtle populations have been experiencing localized increases.
Green turtles are the only species of turtle which is primarily herbivorous. Their diet consists of a variety of seagrass species and algae, the consumption of which turns their fat green and ultimately gives them their name. Adults are often found grazing in shallow lagoons where there is plenty of seagrass.
Seagrass: Successional Development of A Pioneer Species
Similar to terrestrial environments, marine seagrass meadows experience successional development with pioneer species moving in first to unvegetated areas before other species are able to establish in the area.
Pioneer species are hardy species and although they are known primarily as the first species to colonize a new area, they are also found in disturbed ecosystems that were previously biodiverse. They are often fast-growing and may facilitate the re-establishment of late successional species by enriching the substrate with the necessary nutrients and stabilizing the sediment. They are good colonizers and thrive in disturbance but are often outcompeted once late successional species become established in the area.
Climax species typically are more suited to a predictable and stable ecosystem. They are good competitors within stable environments but poor colonizers. In seagrasses, climax species are often larger and slow growing. Due to the loss of major herbivores, such as sea turtles and sirenians, these climax species were previously believed to be indicative of a healthy seagrass meadow but recovering densities of turtles in some areas are causing researchers to reconsider this view.
Effects of Green Turtles On Seagrass Meadows
The impact of increased green turtle population numbers has been observed at five green turtle feeding grounds in the Pacific Ocean, Atlantic Ocean, Indian Ocean and the Caribbean and is indicative of historical ecosystem dynamics.
The researchers observed that when green turtle densities are low, they graze on competitively dominant, long-lived seagrass species, targeting the younger, more nutritious leaf tissue which occurs at the base of the shoot. As grazing intensifies, turtles maintain specific grazing patches within the meadow that they revisit and graze repeatedly, as this increases plant nutrient content by encouraging new growth within the seagrass. While productivity may initially increase, it will start to decline if it is grazed too often or if it experiences prolonged grazing. Once it reaches a point where there is too much disturbance, pioneer species are able to outcompete and replace the climax species. Turtles often shift towards random grazing at this point. If this occurs patchily throughout the meadow, it can positively impact the meadow as minor disturbances have been shown to increase genetic and species diversity by opening up space in the sediment for new individuals to establish.
However, if the meadow becomes overgrazed, the pioneer shoot densities will also decline and the turtles will resort to digging up rhizomes, targeting nutritious belowground tissues. Without their rhizomes, the seagrasses are unable to grow back, even in the absence of disturbance, and this eventually leads to meadow collapse, triggering turtle migrations to new foraging grounds.
Based on these observations, the researchers suggest that we should shift away from the idea of conserving unchanging, pristine seagrass ecosystems, that appears to be incompatible with the presence of high densities of megaherbivores, and rather embrace the inherent dynamic nature of ecosystems. While conservation targets and management plans should do their best to mitigate anthropogenic disturbances to natural environments, they should also strive to account for the impact of disturbance dynamics and acknowledge that natural ecosystems also occur in a state of flux.