Understanding the spatial and temporal exposure of biodiversity to specific threats is crucial for targeted management and their conservation. This is particularly true for species threatened with extinction and that are exposed to seasonal disturbances. Consequently, several efforts have been undertaken to determine the specific exposure of species of conservation concern, such as marine turtles, to disturbances [1
]. Marine turtles use both terrestrial and marine environments throughout different life stages during which they are exposed to an array of anthropogenic and environmental disturbances at various scales [3
]. Most of the studies examining the exposure of marine turtles to disturbances have focused on exposure to anthropogenic threats such as fisheries bycatch [6
], coastal development [8
], and light and plastic pollution [11
]. Only a few studies have been focused on determining the exposure and potential disturbance of marine turtles to environmental disturbances such as those from tropical storms and hurricanes [13
], hereafter “named storms” for simplicity.
Named storms are amongst the world’s most destructive natural hazards [13
] and are accompanied by strong winds, intense rainfall, and storm surges that have the potential to flood and/or substantially modify the morphology of coastal areas [15
]. Several of the world’s marine turtle nesting grounds are exposed to named storms, either during or outside their nesting season [13
]. This is problematic because named storms can affect marine turtles in the long term, over several generations, by shaping their nesting habitat (through beach erosion) and in the short-term, during the incubation of their clutches (6–8 weeks), by increasing localized (temporal and spatial) mortality of their eggs and decreasing hatchling production [17
]. Further, decreases in temperature from increased rainfall and cloud cover during named storms can play a role in influencing phenotype and sex ratios of hatchlings from eggs deposited on beaches [22
], as both incubation duration and sex of marine turtle hatchlings is affected by the sand temperature during incubation [25
Given the potential impacts of named storms on the reproductive output of marine turtles and their habitat, it is speculated that these events can have substantial selective pressures on marine turtles and can play a major role in structuring the current distributions of individuals and populations [13
]. However, there is limited information on the exposure of marine turtle nesting grounds to named storms and the spatial and temporal variability of impacts across species and nesting grounds (see [13
]). Furthermore, these studies are outdated; Dewald and Pike [14
] investigated the exposure of marine turtle nesting grounds in the north–western Atlantic and north–eastern Pacific Oceans to named storms from 1970–2007 and Fuentes et al. [13
] investigated the exposure of marine turtle nesting grounds in the south Pacific Ocean to named storms from 1969–2007. Considering that named storm frequency and intensity are changing in response to warmer environments [27
], with named storm activity in the North Atlantic showing an apparent increasing trend in frequency and intensification [28
], there is a need to revisit earlier studies and reassess the exposure of marine turtle nesting grounds to recent storm activities.
To address temporal gaps on the assessments of the exposure of marine turtle nesting grounds to named storms and to determine the potential impact of storms on different species of marine turtles, we investigated the extent to which marine turtle nesting grounds along the coast of the continental United States of America (USA) have been exposed to named storms between 2008 and 2018. Further, we assessed the temporal and spatial overlap between named storms and nesting grounds for loggerhead turtle (Caretta caretta), green turtle (Chelonia mydas), leatherback turtle (Dermochelys coriacea) and Kemp’s ridley turtle (Lepidochelys kempii) in the region. To illustrate some of the potential impacts of named storms on the reproductive output of marine turtles, we used hurricane Florence, which hit North Carolina, USA in 2017, as a case study and explored how this hurricane affected the hatching success of nests laid by four species of marine turtles nesting in the impacted region.
All species of marine turtles nesting in the continental USA are exposed to named storms and associated impacts. Variation in exposure to named storms was driven by the spatial distribution of each population’s nesting grounds, with populations nesting in the northern extent of the eastern USA from Georgia to North Carolina being the most exposed. Consequently, loggerhead turtles were found to be the most exposed species, with the northern management unit having significantly higher exposure levels than all other loggerhead management units. Kemp’s ridley turtles, in contrast, were found to be the least exposed species to named storms given their restrained geographic extent, with nesting only on the western coast of the Gulf of Mexico.
The impact that each nesting ground species, and ultimately populations will experience from named storms is a function of their exposure to the storms, the temporal overlap between the storms and nesting and hatching, the importance of each of the nesting grounds in terms of numbers of incubating eggs during the storm, and nest placement on the beach [13
]. In our study, a significantly higher number of named storms occurred during the summer months which overlaps with marine turtle nesting and hatching season in the region. However, leatherback turtles and Kemp’s ridley turtles have been less exposed to storms as they nest earlier in the season (the first half of their nesting season occurs outside the hurricane season) and thus their eggs are exposed to storms for a shorter period (only the beginning of the storm season, which historically has had fewer storms). Nevertheless, our work indicates that storm frequency is becoming more evenly distributed throughout the storm season, due to the increased duration of storm effects, which could have a larger effect on all marine turtle nesting and hatching in the southeast USA due to the prolonged duration of storm effects.
This increase in exposure can have negative impacts on the reproductive output of marine turtles at each nesting ground. As indicated by our case study and other studies, named storms can dramatically decrease hatching success and overall productivity [18
]. Impacts on the overall productivity of nesting populations will be driven by the importance of the nesting ground (in terms of the percentage of turtles from a population that nests on that site) that the named storm hits. Indeed, Fuentes et al. [13
] found that the impacts of named storms to the northern Great Barrier Reef green turtle population can be 100 times higher if it hits the main nesting sites for this population rather than a trivial nesting ground. In our study, high levels of exposure to named storms were recorded in very high-density nesting grounds used by the northern management unit (Cape Island and Lighthouse Island) and high-density nesting grounds (Cape St. George and St. Joseph Peninsula) used by the north-western management unit. Aperiodic impacts from named storms might be an issue particularly for the north-western management unit as it is one of the smallest loggerhead management units within the Northwest Atlantic Ocean Distinct Population Segment [60
]. Thus, any impact to this management unit can have severe implications for population stability.
Nest placement on a beach will also affect the impact that named storms will have on overall reproductive output, with nests being placed closer to the high tide line being more vulnerable [61
]. In our case study, distance (i.e., km) from where the hurricane made landfall seems to have made little impact on the overall hatching success of loggerhead nests deposited post-Hurricane Florence. In contrast, green turtle eggs on beaches further away from the hurricane had significantly higher hatching success. This might be due to species-specific differences in nest-site selection relative to the dune line: on the same nesting grounds, green turtles tend to lay their eggs closer to the dune line than loggerhead turtles, which tend to nest closer to the high tide line [63
]. This difference may protect green sea turtle nests from excessive washover.
Even though named storms can directly impact nests that are incubating at the time of landfall, some of the reproductive strategies used by marine turtles can help mitigate the impacts of these discrete events and allow faster than expected recovery from disturbances [14
]. Traits include large clutches (approximately 100 eggs/clutch), laying multiple clutches within a nesting season (2–7 clutches depending on species) which are spaced throughout the season (2–3 weeks apart), and extended nesting seasons (several months) [64
]. Nevertheless, despite marine turtles having these traits and the ability to withstand the loss of a single cohort of eggs [65
], frequent disturbances to incubating eggs (and thus low hatching success over time) from named storms can potentially have a profound effect on the longer-term reproductive output of a population, potentially exerting substantial selective pressures [13
]. This might have been the case for marine turtles nesting in the eastern Queensland coast of Australia, as they currently nest in areas with historically low incidence of tropical cyclones [13
]. It was hypothesized by the authors that areas with a high incidence of tropical cyclones might have substantially affected nesting and/or hatching success over time, reducing the number of turtles that return to nest in these areas over several generations [13
], which would mean that cyclone activity was an important factor in shaping the distribution of marine turtle nesting grounds along the eastern Queensland coast. Such effects still need to be assessed in the USA.
As climate change progresses, storms may become more frequent and intense [68
] and sea level may continue to rise, potentially causing significant alteration of the physical structure of nesting grounds. This coupled with contemporary anthropogenic threats (e.g., coastal development, pollution), may hinder the ability of marine turtle traits to buffer impacts from named storms. This highlights the need to understand how predicted changes in named storm frequency and distribution may affect marine turtle populations; studies such as the present one will provide baseline information for comparison. Future studies should also explore how different pressures (e.g., storm surge, temperature, rainfall), associated with named storms affect hatching success to provide further insights into how named storms with different characteristics may affect marine turtle reproductive output at a site. Importantly, cumulative and synergetic effects from other climatic processes and anthropogenic stressors also need to be considered. Climate processes will not act in isolation and they may produce unexpected changes in combination with local conditions and other pressures [69
]. For example, seawalls and other structures to protect beaches in Florida from erosion associated with named storms can impede reproductive female marine turtles from accessing nesting habitat [71
]. Additionally, the number of structures placed on some beaches increased following named storms [72
], suggesting that in the future nesting females may face impediments to nesting grounds more frequently, as the frequency of named storms increases.