Introduction

It is expected that climate change will directly affect individual organisms, populations, species distribution and ecosystem function (^{IPCC, 2002}). The rise in sea level is probably the most important anthropogenic effect of climate change in the present century (^{Grinsted et al ., 2009}). It threatens to inundate and erode beaches, i.e. the principal nesting areas of sea turtles, where infrastructure or natural obstacles hinder the landward retreat of the beaches (^{Drews and Fonseca, 2009}).

The life cycle and biology of sea turtles are closely linked to their terrestrial environment. Changing conditions in nesting and foraging beach areas, which can limit their potential for growth and reproduction, are severe threats for populations that are already under pressure at the global level owing to high mortality as a result of fisheries, over-exploitation and habitat modification (^{Lutcavage et al ., 1997}).

Sea turtles are an ancient species group whose adaptive capacity is determined by the speed and geographic scale at which their environment changes. True, they survived the four great glaciations of the Pleistocene and many other, albeit lesser, events that caused the extinction of the great mammals, and they have retained physiological mechanisms that have allowed them to mitigate the effect of events whose cycles are of short or medium term. However, the likely effects of processes associated with the present accelerated increase in temperature are unknown.

In all sea turtle species, reproductive success depends mainly on the availability of terrestrial habitat. Sea turtles can emerge onto the beaches to deposit several batches of eggs during the course of a nesting season (^{Miller, 1997}). The loss of beach nesting habitat up to 2080 through a sea level rise of 4.2 mm per year suggests potentially adverse effects on their reproductive success and hence on their survival (^{Church et al ., 2001}; ^{IPCC, 2007}).

Sea turtle species are of interest for conservation and they are increasingly seen to be affected by a series of natural and anthropogenic hazards. However, despite the many efforts in the past two decades to understand and mitigate the threats to these turtles (^{Lutcavage et al ., 1997}; ^{Watson et al ., 2005}), until recently little attention has been paid to the threat posed to this group by climate change (^{Mrosovsky et al ., 1984}; ^{Davenport, 1989}).

The place where the turtles make their nests can affect their reproductive success: the fitness for survival and the success of their progeny (Bjordal and Bolten, 1992). Although it is not entirely clear why some beaches are favored by turtles for egg-laying, Mortimer (1982) identifies the following general requirements for a nesting beach.

The slope of the beach is a function of various factors that include the geomorphology of the zone, wave action, storms and the grain size of the sand (^{Bird, 1996}). The rise in sea level and an increase in the frequency and intensity of hurricanes will increasingly affect beaches.

Studies of changes in sea level through global warming predict a range of scenarios. Rises of 0.18 to 0.59 m are expected by the end of the 21st century (^{IPCC, 2007}). A rise in temperature of between 1.5° and 5 °C will bring a possible rise of 0.20 to1.65 m in sea level (^{Lara, 2008}). Predictions have included a rise of 0.75 to1.90 m for the period of 1990 to 2100, (^{Vermeer and Rahmstorf, 2009}), of 0.26 to 0.39 m by 2025, and of 0.91 to 1.36 m by 2075 (^{Hoffman et al. , 1983}).

The World Wildlife Fund (^{Fish and Drews, 2009}) estimates that climate change will adversely affect marine turtle populations as follows:

Studies of sea-level rises on beaches used for nesting by marine turtles (^{Delgado and Alvarado, 2005}) have shown a loss of ∼70% of the reproductive population of the black sea-turtle (Chelonia agassizi =Chelonia mydas agassizi ), also referred to as Eastern Pacific green turtle, a species classified as endangered (^{World Conservation Union, 2001}). The Official Mexican Norm NOM-59-SEMARNAT (Official memorandum 6 March 2002) lists it in the category "in danger of extinction". Sea turtles are of great tourist economic value and are highly vulnerable to human activity, at least during nesting. Their inclusion in most lists of threatened species or those in danger of extinction reflects their over-exploitation by humans (^{Pritchard, 1997}).

In Mexico, the loss of nesting habitat through rises in mean sea level has not been evaluated. Models of local beach inundation by sea-level rise are important in determining the risk; the present study is one of the first attempts to test it in Mexico. This study considers a range of scenarios involving sea-level rise caused by global warming, and evaluates the likely effects of the loss of beach nesting habitat in Colola Beach, Michoacán, Mexico.

Study area

Colola Beach (18°17¿47" N, 103°24¿3" W) is the principal nesting area of the black turtle (Chelonia mydas agassizii ). It is located along the Pacific coast of Mexico, a tectonically active coast. The Pacific coast in Michoacán is bordered inland by rough topography of the folded and fault-blocked mountains of the Sierra Madre del Sur and by narrow beaches (Figure 1). Colola beach is 4.8 km long and an average 150 m wide, reaching more than 200 m width in some sectors. It extends in NW-SE direction, with two minor rivers (arroyos) flowing to the east and west of the beach, and a rocky granite outcrop defines its western end, while the village of Colola is at its eastern end. The beach sand here is relatively coarse grained and the most abundant minerals include biotite, magnetite and zircon (^{Alvarado and Figueroa, 1989}). The Colola beach morphology is characterized by a steep slope (Figure 1). Although the beach morphology is highly dynamic and its characteristics change seasonally, we summarize here the general characteristics of this beach. The lower and seaward sector of the beach, the beach face, is steep and shows variable slope, ranging from 10° to 30°; it remains wet because of the influence of waves depending on the month of the year, when wave conditions, wind direction and intensity change. Landwards and up-profile, the backshore, shows a berm, characterized by a gentle slope, and marks the boundary between beach face and the beach mid-platform. The beach mid platform is flat and is the most extensive area barren of vegetation. Above the mid platform, there is an area with gentle slope and patches of vegetation (Distichlis spicata and Ipomoea pes-caprae ) (^{Perez, 2008}). The higher area of the beach is the upper beach limiting with the vegetation. The predominant vegetation is low deciduous forest, medium semi-evergreen forest, mangrove, bushy scrub of coastal dunes, and palms (^{Alvarado and Huacuz, 1996}). Climate is warm humid with storms and strong winds and rainfall during July-October, and a prolonged dry period from November to July (^{Alvarado and Huacuz, 1996}). Major hurricane and tropical storm activity prevails during June to November in the Eastern North Pacific (^{NHC, 2013}). The black sea-turtle breeding season coincides from August to March, with the hurricane and tropical storm activity, which suggests that morphologic changes along-beach and on the beach profile occur seasonally. Two zones are favored for turtle nesting: the middle zone of the beach profile, around100 m from the beach tide front, and alongshore on the western and central sectors of Colola beach (^{Fuentes et al ., 2010}).

Figure 1 Location of Colola Beach, Michoacán, Mexico. 

Methods

Results

The DEM, topographic profiles, and plot of nesting sites in the Colola Beach show that most of the nesting sites are located in the mid-and high-beach. The beach slope although variable alongshore, tends to be steep (10° to 30°) in the beach front facing the sea (Figure 2). The effects of different sea-level rise scenarios and inundation of the beach are illustrated on six crossed beach profiles traced from west to east on Colola beach (Figure 2). It is evident that the effect of a 0.5 m sea level rise is apparently smaller along shore. While a sea level rise of 1.4 m would affect mostly the foreshore along the Colola beach. Instead, a sea level rise scenario of 5 m would have a significant flooding effect in the beachfront and mid-platform of most of Colola beach, with more dramatic effect on the westernmost and easternmost sectors of this beach (Figure 2).

Figure 2 Topographic profiles of Colola Beach. Symbols: mamsl-meters above mean sea level; V.E. - vertical exageration. 

The simulations of beach inundation in Colola suggest that a rise of 0.50 m in sea level, predicted to occur within 50 years (^{Vermeer and Rahmstorf, 2009}), would affected the beachfront and produce 2% of area lost (Table 1). Although variable, the steep beach profile (10° to 30°) in the lower and seaward sector of the beach, the beach face, prevents from a larger damage and loss of the sea-turtle nesting area (Figure 3).

Table 1 Predicted loss of nesting area with different rise in sea level scenarios. 

Figure 3 Model shows a sea-level rise scenario of 0.50 m at Colola Beach, Michoacán, Mexico, as predicted within 50 years. Inundated areas are shown in red color. 

Modeling of a rise in sea level of 1.40 m, predicted for the next 100 years, suggests a significant loss of beach area (9%). Since the black sea-turtle tends to nest in the mid-and high-beach, the steep slope of the beachfront aids in preventing a major loss of nesting area (Figure 4, Table 1).

Figure 4 Model shows a sea-level rise scenario of 1.40 m at Colola Beach, Michoacán, Mexico, as predicted within 100 years. Inundated areas are shown in red color. 

The model of beach inundation by sea-level rise of 5 m shows a considerable loss of beach, sparing only isolated patches. Two rivers reach the sea at Colola Beach, and the rising seawater would encroach on their channels and thereby leave the turtles with practically half of the current nesting area (Figure 2 and ⁵ ). The projected loss of beach area is about 54% (Table 1). Although in this scenario some nesting individual sites would not be affected, the main nesting area, in the mid platform of the beach, would be completely lost (Figure 6). Since Colola is a wide beach (150 m average width), only those areas landwards and near the vegetation would be the least affected, but high tides and the proximity of the coastline would result in deleterious effects in these areas also (Figure 5).

Figure 5 Model shows a sea-level rise scenario of 5 m at Colola Beach, Michoacán, Mexico. Inundated areas are shown in red color. 

Figure 6 Model of Colola Beach showing a 5 m rise in sea level (inundated areas shown in red) and the positions of turtle nests (black dots). Insert shows 3D perspective. 

Discussion and Conclusions