Land use changes and sea level rise may induce a “coastal squeeze” on the coasts of Veracruz, Mexico

Global Environmental Change 29 (2014) 180–188

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Land use changes and sea level rise may induce a ‘‘coastal squeeze’’ on the coasts of Veracruz, Mexico Ma. Luisa Martı´nez 1,*, Gabriela Mendoza-Gonza´lez 1, Rodolfo Silva-Casarı´n, Edgar Mendoza-Baldwins Instituto de Ingenierı´a, Universidad Nacional Auto´noma de Me´xico, Cd. Universitaria, 04510 DF, Mexico

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 April 2014 Received in revised form 17 September 2014 Accepted 23 September 2014 Available online

‘‘Coastal squeeze’’ refers to the process in which coastal ecosystems are threatened by the combination of sea level rise (SLR) and the presence of a physical barrier, such as human infrastructure. This situation prevents the landward migration of ecosystems and species, as the coastline moves inland, and they are thus exposed to local extinction. Our objective was to explore coastal squeeze in the state of Veracruz, Mexico, through the study of urban expansion on the coast, an analysis of coastline geodynamics, and a projection of the potential effect of SLR on the distribution of two focal plant species which are endemic to the coastal dunes of Mexico. Urbanization of the coast, parallel to the shoreline, has been taking place increasingly rapidly, displacing ecosystems, both natural (mangroves, beaches and coastal dunes) and transformed (cultivated fields and pastures). Taking into consideration the geodynamic trends of the coastline and an analysis of its historical evolution, it can be seen that the coastal strip is eroding at rates that vary from slow to very rapid. Finally, the results of ecological niche modeling indicate that, under scenarios of SLR, the potential distribution of the two focal species would diminish: Chamaecrista chamaecristoides by 6–28%, and Palafoxia lindenii by 2–15%. These results indicate that ‘‘coastal squeeze’’ is likely in the study area, and that measures to limit or mitigate this process are required. Such measures could include urbanization programs which limit development to appropriate zones, the restoration and rehabilitation of deteriorated ecosystems and the conservation of those ecosystems which are still healthy. ß 2014 Elsevier Ltd. All rights reserved.

Keywords: Coastal dunes Mexico Coastal squeeze Sea level rise Ecological niche modeling

1. Introduction It has been estimated that around 41% of the world’s population live within 100 km of the coast and that 10% are concentrated in a very narrow strip, which is only 10 m.a.s.l. (McGranahan et al., 2007). Human encroachment on coastlines throughout the world has become increasingly intense and extensive (Nordstrom, 2008; Martı´nez et al., 2013). In many cases, the degradation and loss of coastal ecosystems is a consequence of both local and distant factors. Local activities, such as land use change, waste disposal, agriculture, mining and even military activities have an immediate effect on the

* Corresponding author. Tel.: +52 228 8421800x4215. E-mail addresses: [email protected], [email protected], [email protected] (M. Luisa Martı´nez), [email protected] (G. Mendoza-Gonza´lez), [email protected] (R. Silva-Casarı´n), [email protected] (E. Mendoza-Baldwins). 1 Current address: Facultad de Estudios Superiores Iztacala, UNAM, Unidad de Biotecnologı´a y Prototipos. Laboratorio de Recursos Naturales, Av. de Los Barrios 1; Los Reyes; 54090, Tlalnepantla de Baz, Estado de Me´xico, Me´xico. http://dx.doi.org/10.1016/j.gloenvcha.2014.09.009 0959-3780/ß 2014 Elsevier Ltd. All rights reserved.

coast. Modifications to fluvial regimes, by the construction of inland dams or irrigation schemes and forest clearance, can alter sediment budgets, which eventually has consequences on the coast. Similarly, pollutants released in the water basin can cause serious environmental degradation on the coast, over time. These actions, which may take place far from the coast, can modify soil properties, alter natural processes, reduce topographical variability, cause fragmentation, degrade or eliminate habitats, reduce biodiversity and threaten endemic species in the littoral zone (Nordstrom, 2008; Martı´nez et al., 2014). In addition to increasing human encroachment, the coasts are also vulnerable to another element of pressure: mean sea level rise (SLR), associated with climatic variability and climate change (Li et al., 2009; Mendoza-Gonza´lez et al., 2013), to which the effect of subsidence can be added in some cases. For almost 25 years, various authors have been predicting that SLR will increase coastal erosion (Carter, 1991; Feagin et al., 2005, 2010; Nicholls and Cazenave, 2010; Ranasinghe et al., 2012) and that, in combination with changing temperature and precipitation, the spatial distribution of the biota may be altered (Metzing and Gerlach, 2001;

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Greaver and Sternberg, 2007; Peterson et al., 2010; Ciccarelli et al., 2012; Mendoza-Gonza´lez et al., 2013). The expected response to coastal erosion is the landward migration of certain species in line with the new coastline, due to the shifting of wet and dry zones and the increase in saline intrusion (Greaver and Sternberg, 2007; Provoost et al., 2011). However, local extinction of the species that are less tolerant of flooding and salinity may occur, and be followed by the occupation of newly flooded zones by species that are characteristic of these environments (Greaver and Sternberg, 2007). One factor that limits the possible landward migration of species and coastal ecosystems is the existence of rigid barriers in the continental coastal zone, which block migration. This produces ‘‘coastal squeeze’’, which is defined as ‘‘the loss of coastal habitats, where the high water mark is fixed by a hard structure (sea wall, or a city) while the low water mark migrates landwards in response to SLR’’ (Doody, 2004; Schleupner, 2008; Comin et al., 2010; Pontee, 2013). The term ‘‘coastal squeeze’’ was originally employed to describe conditions in salt marshes and estuaries (Nicholls, 2004; Schleupner, 2008; Doody, 2013; Torio and Chmura, 2013), but it also occurs on rocky coastlines (Jackson and Mcvenny, 2011) and on beaches and coastal dunes (Schleupner, 2008; MendozaGonza´lez et al., 2013). The problem can be further exacerbated when poorly designed coastal protection structures create a static or erosive coastline (Comin et al., 2010). Mexico is no exception to these worldwide trends: nearly 30% of the population lives within 100 km of the coast (Martı´nez et al., 2007). Here, coastal squeeze is a growing reality on large parts of the coastline, where the impact of human activities has increased, causing a transformation of the landscape and possible scenarios of SLR. Coastal squeeze has important ecological, economic and social impacts. Furthermore, damage to coastal ecosystems reduces their ability to protect the coast against extraordinary events such as storms and hurricanes (Costanza et al., 2008). One of the states of Mexico with an increasing risk of coastal squeeze is Veracruz, located in the central region of the Gulf of Mexico. The coast of Veracruz (745 km long) is of great ecological, social and economic importance at local, regional and national level. Veracruz state has one of the largest areas of sand dunes in Mexico (106,600 ha), with a wide variety of different types of dune (with parabolic dunes being the most abundant types in the country) (Martı´nez et al., 2014). The dune systems in Veracruz are unique worldwide, since they also feature rare forms such as the ‘‘star dunes’’ and gegenwall dunes found in the central part of the coast of Veracruz (Hesp et al., 2011). Furthermore, the vegetation that grows on the dunes of Veracruz is one of the most diverse in the country, because of the occurrence of tropical forests on dunes (Moreno-Casasola et al., 1998; Martı´nez et al., 2014). One fifth of the cities of Veracruz are coastal and 27% of the population, (about 2 million people), live less than 20 km from the coast (Mendoza-Gonza´lez et al., 2012). The Veracruz coastline has commercial ports of great importance at national level: Tuxpan, Veracruz and Coatzacoalcos process 24% of all the cargo that passes through Mexican ports (SCT, 2012). Moreover, 11% of national

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electricity generation takes place on the coasts of Veracruz, in the thermo electrical center at Tuxpan and the nuclear power station at Laguna Verde (SENER, 2014). Finally, the coast of Veracruz is a popular tourist destination, both nationally and internationally (Propin-Frejomil and Sa´nchez Crispı´n, 2007; Martı´nez et al., 2014). Over the last 30 years, these human activities have brought about rapid changes in land use, with a 36% reduction in the original forest cover and intense soil erosion (SEFIPLAN, 2005). Added to this, the National Institute of Ecology and Climate Change has predicted that most of the coastal zone of Veracruz is at risk from sea level rise. http://www2.inecc.gob.mx/cclimatico/edo_sector/ estados/veracruz.html. Considering the importance of the Veracruz coast, the growing human pressure and the predictions of SLR, it is important to determine the risks and probability of coastal squeeze to which this coastline is exposed. This will enable the proposal of preventative and remedial measures. The objective of this study is, therefore, to explore coastal squeeze on the coastline of Veracruz. Based on pre-existing information, patterns of land use change and urban expansion on the coast were studied, coastline geodynamics were analyzed and the potential effect of SLR on the distribution of two focal plant species that are endemic to the coastal dunes of Mexico was projected. From the results obtained, we identified areas with a high risk of erosion and species extinction as well as zones that could function as refuges for biodiversity and where conservation should be a priority. From these exercises, various proposals are outlined for the protection and restoration of the beach and dune ecosystems at risk, which protect human settlements and infrastructure. 2. Methods 2.1. Land use change Land use changes were examined in three areas with contrasting tourist activities and different patterns of urban growth; Boca del Rı´o, Chachalacas and Costa Esmeralda. The area studied in the three sites is similar and the three share a warm humid climate, with annual precipitation that ranges from 1018 mm in Chachalacas to 1694 mm in Boca del Rı´o. The urban, demographic and economic characteristics of the three sites are different however (Table 1): Boca del Rı´o is the most urbanized site, while Chachalacas has the least area covered by urban infrastructure. High-resolution aerial images from 1995 and 2006 were used to elaborate land use polygons in order to analyze changes that have taken place. ArcView 3.2 was used to digitalize the land use and this was then verified in the field. The area analyzed in each case extended from the coastline to 2.5 km inland. Finally, transition matrices of the land use changes were calculated. 2.2. Sea level rise It has been predicted that SLR will potentially take place 2–4 times more rapidly than the increment observed in the past

Table 1 Social and economic characteristics of Boca del Rio, Chachalacas and Costa Esmeralda in Veracruz, Mexico. Site of study

Municipality

Surface (ha)

Urbanization

Predominant economic activities

Population (inhabitants)

Boca del Rı´o

Boca del Rı´o

2290

Tourism and commerce

141,906

Chachalacas

´ rsulo Galva´n U

2630

Intense: mostly urban infrastructure Low: Suburban infrastructure

Costa Esmeralda

Tecolutla, San Rafael, Nautla

3060

Medium: Sparse urbanization

Data from INEGI (2007).

Agriculture, Livestock production, fishery, tourism, commerce Agriculture, Livestock production, fishery, commerce, tourism

26,909 25,680

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(Meehl et al., 2007) and at levels that vary from 20 cm in 2025 with a temperature increase of 1.5 8C (Maul, 1993) to 1.4 m in 2100 with an increase of 4.1 8C (IPCC, 2001, 2007). However, these estimates do not include the accelerated changes in the ice caps that have been observed more recently (Rignot et al., 2014). This is important because, for example, it has been calculated that the polar cap of Greenland contains a volume of water that is equivalent to a SLR of 6 m (Li et al., 2009). For this reason, this study projected two extremes of SLR: one of 1 m and the other of 6 m, with a spatial resolution of 1 km and a vertical resolution of 1 m (Li et al., 2009). These scenarios consider the connectivity of continental water at the coast with the ocean, as well as zones of low elevation that already contain bodies of water (Li et al., 2009; Peterson et al., 2010). The models used to explore SLR (Li et al., 1998) were combined with the SLR vulnerability assessment performed by Ortiz Pe´rez and Me´ndez Linares (2004) for low lying coasts in the Gulf of Mexico and Caribbean. This study uses the classification of the Mexican coastline according to the analysis of geodynamic behavior conducted by Silva et al. (2011) to define the instability tendency of the Veracruz coastline as a function of tectonic characteristics and morphological expression (forms of erosion and accumulation), as well as the hydrological and hydrographic characteristics of the fluvial and estuarine network. This classification is an extension of the work conducted by Ortı´z Pe´rez and Me´ndez Linares (2004), who used

multitemporal information from satellite images, aerial photographs and field verification. 2.3. Ecological niche modeling Ecological niche modeling is based on the analysis of the environmental conditions in sites where a species is known to be present. The ecological niche is an n-dimensional hyperspace where a set of biotic attributes (biological interactions, vegetation type), abiotic conditions (pluvial rate, topography, solar radiation, temperature) and mobility (lack of geographic barriers) allow a population to remain with a positive relative growth rate (r > 1), without the need for immigration (Grinell, 1917; Hutchinson, 1957). In the modeling exercises of this study, efforts were focused on two recognized dunebuilding species (Gallego-Ferna´ndez and Martı´nez, 2011) that are endemic to the coastal dunes of Mexico: Chamaecrista chamaecristoides and Palafoxia lindenii. C. chamaecristoides (Fabaceae) is a low shrub that colonizes mobile sands and facilitates the successional process (Martı´nez and Moreno-Casasola, 1998). It has a wide distribution along the Gulf of Mexico, and two separate populations in the states of Jalisco and Michoaca´n (Martı´nez and MorenoCasasola, 1998). P. lindenii (Asteraceae) is also a low shrub and colonizer of mobile sands and is endemic to the coasts of Veracruz and Tabasco (A´lvarez-Molina et al., 2013). Both species are very tolerant of burial in sand (Martı´nez and Moreno-Casasola, 1996).

Fig. 1. Atlantic coastal provinces of Mexico (Moreno-Casasola and Castillo, 1992; Ortı´z Pe´rez and Me´ndez-Linares, 2000). The location of the three zones of study where land use changes were analyzed (Boca del Rı´o, Chachalacas and Costa Esmeralda) is highlighted.

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In the databases of the CONABIO (Comisio´n Nacional para el conocimiento y uso de la Biodiversidad-National Commission for the knowledge and use of Biodiversity), the herbarium of INECOL (Instituto de Ecologia, A.C, Institute of Ecology) (XAL) and previously published literature (Moreno-Casasola and Castillo, 1992; Moreno-Casasola et al., 1998) we looked for georeferenced locations where each species has been collected. With this information we built our own database of locations for each species. All the records that we found were used, except for those outside the known distribution for each species (for example, at more than 5 km inland), or records that were not on coastal dunes (Lo´pez-Portillo et al., 2011). In this way, uniquely occurring data were used in order to minimize bias when inferring the geographic distribution models (Martı´nez-Meyer, 2005). In order to characterize the environmental variables necessary for the niche modeling exercise, 19 bioclimatic variables obtained from the WorldClim project (www.worldclim.com; Hijmans et al., 2005) were used, with a resolution of 0.00838/px (per 1 km2). These variables are derived from monthly temperature (maximum and minimum) and precipitation data that represent annual and seasonal trends as well as extreme conditions (see Mendoza-Gonza´lez et al., 2013 for more details). From this information, ecological niche modeling exercises were conducted with the potential distribution of each one of the focal species, using the GARP (Genetic Algorithm for Rule-set Production) algorithm (Stockwell, 1999). This method relates climatic conditions and records of locations where the species are distributed. These conditions represent, in part, the ecological niche of our focal species. For each species, 100 models were run with different percentages of training and testing records, depending on

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the total number of records for each species. We had 75 records for C. chamaecristoides (60% were used for training and 40% for testing) and 60 records for P. lindenii (70% were used for training and 30% for testing). The 10 best subsets were then chosen as a function of their errors of omission and commission (Rojas-Soto et al., 2012). Finally, the results closest to the median of commission errors were chosen and the final consensus map was constructed (Anderson et al., 2003). Two limitation of this type of exercise is that it does not consider the biotic dimensions of the niche (such as interactions between species) or the potential for dispersal (Sobero´n, 2010), which leads to the generation of commission errors (over-prediction of the potential distribution of the species). To minimize this type of error, the maps obtained were edited considering historical and biotic attributes (Sobero´n and Nakamura, 2009). A buffer of 5 km from the coastline was used for the two study species. This is the maximum distance from the coast at which the dunes that support the focal species of this study may be found (Lo´pez-Portillo et al., 2011). Atlantic coastal provinces were also used (Moreno-Casasola and Castillo, 1992; Ortı´z Pe´rez and Me´ndez-Linares, 2000), and were defined by the temperature, precipitation, topography and substrate, in order to validate and regionalize the predicted models with the conditions of the niche utilized by the species (Fig. 1). 3. Results 3.1. Land use change In the geographic information system we established eight different categories of land use for the study sites: beach, dune,

Table 2 Area covered by different land uses in the study sites, for 1995 and 2006. Values are shown in hectares and as the percentage represented by each land use within the analyzed landscape. Land use

Beach Dune Mangrove Scrub Tropical forest Cultivated fields Pasture Urban zone Total

1995

2006

% Change

Area (ha)

% of the landscape

Area (ha)

% of the landscape

Boca del Rı´o 37 – 254 – –

1.6 – 11.1 – –

30 – 213 – –

1.3 – 9.3 – –

9.5 77.8

140 1901 2284

6.1 83.2

3.4 5.5

217 1776 2284

Land use Beach Dune Mangrove Scrub Tropical forest Cultivated fields Pasture Urban zone Total

Chachalacas 1995 Area (ha) 24 682 – 507 15 485 803 80 2596

Land use Beach Dune Mangrove Scrub Tropical forest Cultivated fields Pasture Urban zone Total

Costa Esmeralda 1995 Area (ha) 54 – 352 53 – 1717 806 132 3114

0.3 – 1.8 – –

% of the landscape 0.9 26.3 – 19.5 0.6 18.7 30.9 3.1

2006 Area (ha) 27 581 – 632 26 492 696 142 2596

% of the landscape 1.0 22.4 – 24.3 1.0 19.0 26.8 5.5

% change 0.1 3.9 – 4.8 0.4 0.3 4.1 2.4

% of the landscape 1.7 – 11.3 1.7 – 55.1 25.9 4.2

2006 Area (ha) 51 – 278 53 – 1642 870 220 3114

% of the landscape 1.6 – 8.9 1.7 – 52.7 27.9 7.1

% change 0.1 – 2.4 0.0 – 2.4 2.1 2.8

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mangrove, scrub, tropical forest, cultivated fields, pasture and urban zone. Boca del Rı´o was already intensely urbanized in 1995, hence there were only four land uses found at that site: beach, mangrove, pasture and urban zone (Table 2). In the period analyzed, the urban area of Boca del Rio expanded even more, while the area with other land uses declined. In contrast, we observed seven land uses in Chachalacas: beach, dune, scrub, tropical forest, cultivated fields, pastures and urban zone (Table 2). The site has a larger area of natural ecosystems, and many of the changes were related to natural processes. For example, the beach increased in area due to sediment accumulation; the coastal scrub expanded on the previously mobile dunes, as well as on the pastures; the tropical forest also increased in coverage. The urban area almost doubled, predominantly along the length of the coast, on the beach and coastal dunes. Finally, at Costa Esmeralda, six land uses were found: beach, mangrove, scrub, cultivated fields, pasture and urban zone. Over the period analyzed, the area covered by beach and mangrove decreased; no changes were recorded in the coastal scrub, while the pasture and urban zone expanded. The urban footprint increased on the beach and dunes, but also on the pastures, mangroves and cultivated fields. Natural ecosystems were transformed into cultivated fields.

Fig. 2. Mexican coastal geodynamics. Modified from Silva et al. (2011).

3.2. Sea level rise The results of the SLR vulnerability assessment showed that the three areas most affected by this phenomenon were the Laguna Madre system and the delta plain of the Rı´o Bravo in the north of the Gulf of Mexico; the mouth of the Rio Papaloapan and the Laguna del Alvarado in the central-east; and the delta complex of the Grijalva-Mexcalapa-Usumacinta and the Laguna de Te´rminos in the central-south of the Gulf of Mexico (Fig. 2). It thus seems that the impact of SLR in our study sites will be less intense, because they are relatively far away from the most affected areas, although more detailed studies are necessary for such a local scale. According to the classification of the coastal geodynamic trend presented by Ortı´z Pe´rez and Me´ndez Linares (2004), with the modifications made by Silva et al. (2011) (Table 3) based on an analysis of the historical position of the coast (particularly of the most vulnerable sandy and mixed sand and gravel low zones), a classification of the Mexican coast was produced showing: (a) regressive, (b) transgressive and (c) stable zones (Fig. 2). Marine regression occurs when areas of submerged seafloor are exposed above sea level (progradation). Marine transgression occurs when flooding from the sea covers previously exposed land (erosion) (Monroe et al., 2006). Stable zones can respond to long term

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Table 3 Criteria for determination of sea level change on the Mexican coastline (Ortı´z Pe´rez and Me´ndez-Linares, 2000). Characteristics

Instability of the coastline

Tectonic regime, direction of movement Morphological expression

Hydrological characteristics and patterns of the hydrographic network

Sedimentary environment

Some ecological implications

Types of coast Coastal zones with transgressive behavior (erosion)

Coastal zones with regressive behavior

 Landward displacement of the coastline  Erosion of beaches, destruction of low lands and of infrastructure  Subsidence, sinking

 Seaward advance of the coastline  Accretion of beaches, growth of delta plains through sedimentary accumulation. Domination of cumulative forms  Ascent, rising with rapid sedimentation

 Barrier islands with retrenchments and parapets of beach loss on the leeward flank, slope ruptures ‘‘berms’’, outcrops of ‘‘beach rock’’  Presence of intertidal plains (salt marshes), flooding plains surrounding the infralittoral zone  Interior tidal delta  Landward migration of active dunes  Rising groundwater level with an increase in marshy conditions or waterlogging  Reduction of the slopes of fluvial courses  Increase of estuarine conditions upstream with widening of the river mouths  Increase in water level, with a greater connection between the lotic and lentic environments (increased integration of the hydrographic network). Landward migration of the shoreline  Domination of cumulative and sedimentary forms in the supralittoral zone  The basal sediments are coarse overlying alternate muds and sands to terminate in a series of very fine deposits on the surface  Reduced substrate thickness  Accumulation of very fine sediments in the inland mixed aquatic areas by the flocculation of particles  Increased salinization of the soils and water  Destruction of the vegetation  Modification of the habitat  Landward migration of the mangrove

 Rocky and cliff coastlines, abrasion terraces, plains of coastal ridges, formation of bars, growth of barrier islands, spits and dune fields

external agents of disturbance without losing the current or potential functions of the coast. In the particular case of Veracruz, it can be seen that generally the coastal strip is being eroded at rates that vary from slow to very rapid (Fig. 2), and that this is associated with the effects of geology and of SLR. Table 3 shows that the coastal zones exhibiting transgressive behavior are those where the coastline is moving landwards and processes of erosion and subsidence are ongoing. In these areas, an increase in groundwater level is predicted, along with increased connection between the lotic and lentic environments. Some of the ecological implications of these scenarios include the salinization of the soils and water, destruction of the vegetation, modification of habitats and landward migration of coastal ecosystems. 3.3. Ecological niche modeling The results of the niche modeling indicate that the two focal species are limited by different sets of bioclimatic variables with different threshold values. They are distributed in areas with annual mean temperatures of between 23 and 26 8C and within a wider range of precipitation from 1000 to 3400 mm per year. The potential distribution of C. chamaecristoides reveals a wide distribution all along the coastline of the Gulf of Mexico, while that of P. lindenii is restricted to the central region of the Gulf coast, comprising the state of Veracruz and western Tabasco. It was observed that the current distribution of C. chamaecristoides could decline by 6.19% and 28.23% under SLR scenarios of 1 m and 6 m, respectively, while the distribution of P. lindenii would decrease by 2.28% and 15.68% under these scenarios. (Fig. 3).

 Decrease in the sub-surface groundwater level with superficial settlement of the land  Increase in the slope gradient and area of the longitudinal profile of the fluvial courses  Incision of river channels in the high part of the coastal plain with formation of low terraces  Antecedent and hanging valleys  Elevated carsticity levels above the current groundwater level  Disintegration of the network of lotic and lentic environments (formation of isolated water bodies). Seaward migration of the shoreline  Very fine materials in the base, with particle size increasing gradually and becoming coarser as it approaches the beach deposits  Increased substrate thickness

 Decreased salinization of the soils and water  Modification to the conditions of the original habitat  Modification in the pattern of vegetal succession

4. Discussion The results of this study indicate that there is increasing coastal squeeze on the Veracruz coast, due to accelerated urbanization along the coastline and processes of erosion, causing a potential loss of habitats and thus local extinction of species endemic to the Mexican coastal dunes is possible. We discuss the phenomenon of coastal squeeze, considering the dynamics of the beaches and coastal dunes, and present an analysis of the three study sites. 4.1. Coastal squeeze Beaches and coastal dunes are dynamic ecosystems, capable of responding to different factors of natural perturbation that form part of their own dynamics (Trenhaile, 1997; Ruessink and Jeuken, 2002). The possibilities for resisting and responding to external agents of perturbation, without losing the current or potential functions of the coast, have morphological, ecological and socioeconomic components (Klein et al., 1998). The presence of infrastructure on the coast, for example, modifies its morphology and functioning and thereby limits its capacity for response (Pontee, 2011). The coast of Boca del Rı´o is classed as an intensely urbanized zone. Erosion (slow to rapid) occurs there, due to the combination of SLR and the sinking, or subduction, caused by the displacement of the Cocos tectonic plate by that of North America (Silva et al., 2011). The extent of the changes to the focal species, predicted by the ecological niche modeling exercises, ranges from low to medium. This clearly exemplifies a situation of coastal squeeze and so definition of ecological protection policies and risk mitigation programs for the coast of Boca del Rı´o is

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Fig. 3. Current potential distribution of Chamaecrista chamaecristoides and Palafoxia lindenii. Percentages indicate changes in distribution area (in terms of the percentage of pixels where the focal species is potentially distributed), under SLR scenarios of 1and 6 m. Purple: current distribution, yellow: absence. Blue: Zones vulnerable to flooding under the modeled SLR.

considered important. This type of action would include the rehabilitation of beaches and dunes (Ley Vega de Seoane et al., 2007; Nordstrom, 2008), conservation of coastal fragments that are still intact and functional and the adoption of new policies concerning the use of these fragments (Pranzini and Williams, 2013). In contrast, Chachalacas is a zone where erosion of the coastline is slow, as confirmed by the land use changes observed from 1995 to 2006. Overall, the results show a slight growth in the area of the beach near the coastal dune system, but there is erosion in the urbanized area. The urban coverage almost doubled, displacing areas of pasture and dunes (3.1% and 5.5%, respectively). The likelihood of land use change in Chachalacas is greater than in the other study sites because the area covered by natural ecosystems is larger, especially in contrast with Boca del Rı´o. We found that the focal species were moderately threatened in this zone and therefore Chachalacas is not considered to be experiencing ‘‘oppression’’. Nevertheless, if the dunes are not protected and urban encroachment continues along the coastline, the situation could change, as has already occurred in salt marshes (Doody, 2004, 2013) and coastal dunes (Metzing and Gerlach, 2001) in England and Germany, respectively.

Finally, the Costa Esmeralda site shows coastline erosion at a very slow rate (Ortiz-Pe´rez and Me´ndez-Linares 2000; Silva et al., 2011). Between 1995 and 2006, the areas of beach and mangroves decreased while the area of urban coverage almost doubled (4.2% and 7.1%). At this site, under extreme SLR scenarios, the loss of both focal species is predicted, bringing about their local extinction. This area clearly exhibits a process of ‘‘oppression’’ and, therefore, failure to implement coastal management and protection policies will undoubtedly lead to serious damage to coastal ecosystems and infrastructure. 4.2. Capacity of response (ecosystem; society) The presence, or potential risk, of coastal squeeze will lead to changes in both natural and anthropized ecosystems, as well as for the human populations that inhabit these zones (Nicholls et al., 1999; Doody, 2004; Feagin et al., 2005; Schleupner, 2008). Where ecosystems are affected by erosion and their response of landward migration is limited by physical barriers some means of assuring the integrated response of the system are required in order to maintain both the functioning of the ecosystem and the opportunities for economic growth (Klein et al., 1998). It is therefore important to

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explore the capacity of response presented by each species and each ecosystem, with the aim of formulating and implementing appropriate policies for the management of the coastal zone. 4.2.1. Response of ecosystems and species The species that live on dunes differ between systems, regions and continents, due to climatic variations and the fact that they present different biogeographic and phylogenetic histories (Hesp et al., 2011; Gallego-Ferna´ndez and Martı´nez, 2011). Despite these differences, however, many plant species typical of coastal dunes share physiological and morphological responses to the characteristic restrictions of these environments, such as extreme temperatures, drought, low nutrient availability, disturbances, salinity and substrate mobility (Hesp, 1991; Hesp and Martı´nez, 2008; Maun, 2009; Gallego-Ferna´ndez and Martı´nez, 2011). Under the scenarios of SLR, the tolerance of the plants to substrate salinity and seawater flooding gain importance. In this sense, Greaver and Sternberg (2007) found a clear oceanic influence in coastal dune plants, reflected in the plastic response of plant tolerance that can include the capacity to utilize seawater. In the case of the focal species of this study, C. chamaecristoides and P. lindenii present moderate tolerance to salinity (Martı´nez et al., 2002); while it is known that C. chamaecristoides cannot utilize seawater (Mendoza-Gonza´lez et al. unpublished data), there is no information on this for P. lindenii. This suggests that the two focal species of this study could be affected adversely by coastal squeeze, with a high risk of extinction in the case of urbanized coastlines that impede landward migration. Similar results have been observed with other tropical coastal dune species. In an earlier study in the Gulf of Mexico and the Caribbean, Mendoza-Gonza´lez et al. (2013) performed an ecological niche modeling exercise in order to explore the responses of six coastal dune plant species to climate change and a global sea level increment of 1 m. They found that the distribution of local endemics would probably be more affected than the more widely distributed pantropical species, and identified the central part of the Gulf of Mexico, and the north of the Yucatan Peninsula as potential refuges in the future. Interestingly, these regions have recently been recognized as areas of plant species richness concentration (Mendoza-Gonza´lez et al. data not published). 4.2.2. Response of society With reference to human development on the coast, it is important to consider the following: (a) to limit and regulate urban expansion in coastal zones in such a way that permits and favors the natural dynamics of these environments (Turner et al., 2007). This has the additional benefit of providing natural protection against the impact of storms and hurricanes by coastal dunes (Martı´nez et al., 2012 and references therein); (b) to restore and rehabilitate beaches and coastal dunes in urbanized coastal zones (Nordstrom, 2008); and (c) to consider that the beaches and dunes support an often unappreciated biodiversity, as well as providing unique ecosystem services, hence the conservation of systems that are currently intact and functional should be a priority. 5. Conclusions The data analyzed in this study suggest that the coastal zones of the state of Veracruz are exposed to a process of coastal squeeze that can not only eliminate and destroy coastal ecosystems and species, but also endanger infrastructure and human lives due to the growing risk of erosion and flooding events. For this reason, it is important to establish measures to counter and to mitigate coastal squeeze, such as: (a) local and long-term studies to evaluate changes in coastal habitats and on the coastline, and to identify possible causes; (b) the implementation of strategies which balance the natural dynamics of the coastal zones with human

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interests; (c) the fomentation of appropriate urbanization programs within coastal management schemes which establish areas that are suitable for urban development; (d) considering the option of eliminating hard structures from zones with an imminent risk of collapse and marine intrusion, in order to permit flooding and landward migration of the coastal ecosystems; (e) the restoration and rehabilitation of deteriorated ecosystems and conservation of those that are intact and functional. Given the gravity of the loss of coastal ecosystems, these actions would have immediate benefits for society and would help us to face uncertain future changes, while maintaining development opportunities on the coast. Acknowledgements This study received partial funding form grant CONABIOJM027. MLMV and GMG are very grateful for the hospitality of UNAM. We are thankful to Keith MacMillan and Jill Taylor for translating and editingan earlier version of this text. The authors greatly appreciate the comments and revisions made by two anonymous reviewers to earlier versions of this text, which helped us make it clearer.

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