Warming Threatens to Propel the Expansion of the Exotic Seagrass Halophila stipulacea

Abstract

The spread of exotic species to new areas can be magnified when favored by future climatic conditions. Forecasting future ranges using species distribution models (SDMs) could be improved by considering physiological thresholds, because models solely based on occurrence data cannot account for plasticity due to acclimation of individuals to local conditions over their life-time or to adaptation due to selection within local populations. This is particularly relevant for the exotic seagrass Halophila stipulacea, which colonized the Mediterranean Sea a century ago and shifted its thermal niche, coping with a colder regime. Here, we used two hybrid models combining correlative SDMs with the thermal limits for growth of native and exotic H. stipulacea populations to predict the distribution of the species in its native (Indian Ocean and Red Sea) and exotic ranges (Mediterranean Sea and Caribbean Sea) under two scenarios forecasting limited (RCP 2.6) and severe (RCP 8.5) future climate changes by 2050 and 2100. Then, we assessed the differences between hybrid models based on native Red Sea thermal limits (niche conservatism: 17–36°C) and on exotic Mediterranean thermal limits (local adaptation: 14–36°C). At the Mediterranean exotic range, the local adaptation hybrid model accurately agreed with the present distribution of the species while the niche conservatism-based hybrid model failed to predict 87% of the current occurrences of the species. By contrast, both hybrid models predicted similar species distributions for the native range and exotic Caribbean range at present and projected that H. stipulacea will maintain its current worldwide under all future greenhouse gas emission scenarios. The hybrid model based on Mediterranean thermal limits projected the expansion of H. stipulacea through the western Mediterranean basin (except the gulf of Leon) under the most severe scenario (RCP 8.5) by 2100, increasing its distribution by 50% in the Mediterranean. The future expansion of H. stipulacea is related to its capacity to cope with warm waters and it may become a relevant species in the future, particularly under the projected decline of native Mediterranean seagrasses, resulting in important shifts in seagrass communities and overall ecosystem functions.

eDNA Reveals the Associated Metazoan Diversity of Mediterranean Seagrass Sediments

“…C. nodosa is of tropical affinity and has an intermediate growth rate and thermal thresholds [91]. In fact, P. oceanica is experiencing a well-documented regression due to habitat degradation and increasing summer heatwaves [32] and species distribution models project that the suitable habitat of the species could be completely lost by 2100 under the business as usual climate change scenario [92,93] while H. stipulacea will expand its distribution by 50% [43]. In fact, some meadows of H. stipulacea are currently growing on top of meadows that were P. oceanica or C. nodosa in the past [27].…”

“…Understanding the dynamics of seagrasses and how associated faunal communities within these habitats change through time is important to understand their response to environmental change and assist predictions about the future of these ecosystems, as well as the scope to rebuild biodiversity losses through active seagrass conservation and restoration. This is particularly important in the Mediterranean Sea, which is home to extensive seagrass meadows, including the endemic and most widespread seagrass Posidonia oceanica [36,37], the native Cymodocea nodosa which is increasing its distribution in some areas [38], as well as the exotic (Indo-Pacific origin) seagrass Halophila stipulacea which has been found mostly in the eastern Mediterranean since 1923 [39] but is expanding into the western Mediterranean [40][41][42] and is forecasted to continue doing so [43]. However, the Mediterranean is highly impacted by the urbanization of the littoral zone since the 1960s, eutrophication, the exploitation of living resources, high warming rates (0.65 • C and 0.25 • C decade −1 in the western and eastern Mediterranean), and the rapid spread of exotic species arriving from the Atlantic though the Gibraltar Strait or from the Indo-Pacific Ocean through the Suez Canal, resulting in an extensive loss of coastal habitats [32,44,45].…”

Tropicalization of seagrass macrophytodetritus accumulations and associated food webs

“…It generally colonizes habitats devoid of native seagrass, forms mixed meadows with C. nodosa (Winters et al, 2020), or colonizes dead matte of P. oceanica (Thibaut et al, 2022). Presently, H stipulacea is not replacing any native seagrass species, but this may occur when temperatures rise beyond the thermal tolerances of the native species (Wesselmann et al, 2021). In comparison, this invasive seagrass has already spread rapidly and displaced several native macrophytes in eastern Caribbean islands (Winters et al, 2020).…”

A mathematical model for inter-specific seagrass interactions: reproducing field observations for C. nodosa and C. prolifera

“…For instance, the introduction of invasive species that compete for resources with native ones can drastically affect the habitat conditions [Norton, 1976, de Villèle and Verlaque, 1995, Alós et al, 2016. The tropicalization of temperate latitudes due to global warming aggravates the spread of exotic species that are more resilient to higher water temperatures [Rius et al, 2014, Vergés et al, 2014, Wesselmann et al, 2021. Therefore, the addition of inter-specific interactions to the present numerical model is necessary to predict the dynamics of seagrasses under different global warming scenarios since each species have different responses to the thermal stress [Savva et al, 2018, Collier et al, 2011.…”