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Journal of Coastal Conservation 2: 13-22, 1996 EUCC; Opulus Press Uppsala. Printed in Sweden - Coastal sand dune vegetation: an extreme case of species invasion - Coastal sand dune vegetation: an extreme case of species invasion
Castillo, S.A.1 & Moreno-Casasola, P.2*
1Laboratorio de Ecología, Facultad de Ciencias, UNAM, México 04510 D.F., México; 2Instituto de Ecología, A.C. Apdo. Postal 63, Xalapa 91000, Veracruz, México; *Fax + 52 28 186910 ext 1202; E-mail: Abstract. The coastal sand-dune flora of the Gulf and Carib-
terized by a sandy, nutrient-poor substrate of siliceous bean region of Mexico was analyzed to understand differ- particles with variable amounts of calcareous sand. There ences in floristic composition and richness found along the are systems with narrow beaches and one or more paral- coast. Each of the 655 species reported was classified accord- lel dune ridges 3-8 m high. There are also topographically ing to its ecology and distribution range by checking herbaria complex systems with 25-m high dunes with different specimens, literature and specialists. Three groups were degrees of stabilization. Caribbean dunes in Mexico are formed: (a) species with predominantly coastal distribution;(b) ruderal or secondary species frequently found inland, formed by ridges of calcareous sand, derived from coral common of disturbed areas such as roadsides, abandoned reefs and shells as well as from the limestone Peninsula fields or forming part of secondary growths; (c) inland spe- of Yucatan. Both regions have many floristic and cies frequently found in other vegetation types such as tropi- physiognomic differences (Sauer 1967; García 1987; cal dry or seasonal forest and grassland. A total of 71 coastal species, 237 ruderal/secondary and 336 species from other In patch dynamics, natural systems are seen as mo- community types were found. The distribution of these groups saics of local, suitable patches colonized by various was analyzed along 44 sites of the Gulf and Caribbean, in the species at different times. Species ranges show that a different dune habitats and for the dominant growth forms.
species’ internal geographical abundance is usually dy- Coastal species are more widely distributed; they predomi-nate in habitats with sand movement and the herbaceous namic, even when the range limits are stable. Thus, component prevails. Ruderal/secondary species and especially invasions can be conceived as temporary disturbances those belonging to other vegetation types frequently appear in leading to spatial dynamism and viewed more broadly, only one or two sites occupying more protected or stabilized species ranges can be regarded as being in a constant habitats. The two latter groups considerably increase species state of flux, both internally and externally (Hengeveld richness of sand dune flora, but also pose interesting prob- 1989). Plant community boundaries are not static; ecotones show a constant exchange of species. Seeddispersion can take place between communities that are Keywords: Coastal dune; Species distribution; Richness.
Invading species have been identified as those being indigenous to some region other than the area being Introduction
invaded. In many cases, this is not always clear-cut: it isdifficult to define community boundaries, modification The floristic composition of a plant community is of scale in the analysis makes results differ, and popula- the result of its disturbance history, its present dynam- tion density as well as species presence should be taken ics, the life history (attributes) of the species colonizing into account to define the native range of a species. For and reproducing successfully in that particular environ- the purpose of this paper a difference is made between ment and species availability (Guevara 1982). Sand exotics and invaders. Exotics can be defined as alien dune vegetation is a clear example of this and we believe species introduced intentionally or non intentionally to that invasion of species with different traits are impor- the region. Often, ecosystem invasion by exotic plants tant contributors to floristic richness and biodiversity.
can have detrimental effects. Exotics displace native Once their propagules have dispersed into the dune species by competing for resources, interfere with suc- systems, species attributes (such as germination, estab- cessional processes, alter disturbance regimes and dis- lishment and growth requirements) will be important in rupt food chains (Van Wilgen & Richardson 1985; defining in which part of the dune mosaic invading MacDonald & Frame 1988; Elliott & White 1989). We define invaders as species with an occasional presence Coastal sand dunes in the Gulf of Mexico are charac- for that type of community (arbitrary frequency values Castillo, S.A. & Moreno-Casasola, P. of 10-20% for the sites) which can have low or high nity invasibility varies according to the characteristics population numbers on a site and are frequently charac- of the community and the life history traits of the po- teristic of other community types. Huston (1994) con- tential invaders (Ewel 1986). Disturbed communities siders that invasions potentially lead to an increase in are more often subject to invasions than intact commu- species richness, as the invading species are added to the nities and disturbance regime (type, frequency, dura- existing species pool, although numerous examples dem- tion and magnitude of disturbance) is recognized as the onstrate that they can also lead to extinctions.
most important overall factor allowing invasion of ex- In biological invasions there are two components, otic species (Elton 1958; Myers 1984; Ewel 1986; Fox modulated by timing and chance: the invasive species & Fox 1986; Crawley 1987; Rejmánek 1989; DeFerrari and the invaded community. Consequently, commu- & Naiman 1994; Huston 1994). However, there are Fig. 1. Location of the 44 beach and dune study sites sampled (*). Sites include the Gulf (Northern Gulf- sites 1 to 6 and Central Gulf-
sites 7 to 29) and the Caribbean (sites 30 to 44). Other beaches and dunes studies (reported in the literature) are indicated with a dot.
See text for references.
- Coastal sand dune vegetation: an extreme case of species invasion - several examples in which invasion occurs in the ab- several species mainly because of absence of herbaria sence of disturbance (Mack 1985; Burdon & Chilvers collections and these were left out of the calculations 1977; Kruger 1977). Huston (1994) considers that there (Amaranthus arenicola, Eragrostis excelsa, E. yucatana, has been little progress in generalizations about which Eriochloa boxiana, Mandevilla subsagittata, Matelea ecosystems are most likely to be invaded.
yucatanensis, Panicum rigidulum, Paspalum clavu- Coastal dunes are very dynamic systems comprising liferum, Pithecellobium grisebachi, Solanum yucata- a wide variety of habitats with different physical and biotic conditions, allowing the existence of species with Species distribution patterns were divided into three very diverse life history traits. They can be visualized as a permanently changing environment with distinct de- (1) species with a predominantly coastal distribution grees of stabilization closely correlated with the topog- (sand dunes, coastal marshes or mangroves) henceforth raphy and the disturbance produced by sand movement (Barbour et al. 1985; Moreno-Casasola 1986; Maun & (2) inland ruderal or secondary species frequently found Lapierre 1984) and slack inundation (van der Laan inland and common inhabitants of disturbed areas such 1979; Sykes & Wilson 1987; Grootjans et al. 1991). It as roadsides, abandoned fields or secondary growths; results in a vegetation mosaic which has been described (3) inland species, frequently found in other vegetation The aim of this work is to understand the floristic types (OT) which can be considered mature, such as composition, richness and variability of the regional tropical seasonal forest, oak woodland or grassland.
sand-dune systems along the Gulf and Caribbean through For most of the analyses the complete floristic list a detailed analysis of species composition, trends in was used (655 species). Where geographical or local species distribution patterns, species richness under dif- habitat distributions were analyzed only data for the 44 ferent habitats and disturbance regimes and growth forms sites sampled with the same methodology and intensity that compose the present dune flora. Strategies for con- were utilized (492 species), so as to avoid errors caused servation and management of the dune flora will be by intensity of sampling during floristic collections.
influenced by the resulting species analysis.
Species numbers varied considerably among sites We accumulated a list of 655 species for the coastal and distribution patterns. A total of 71 (10.83%) coastal beach and dune vegetation along the Gulf and Carib- species (C) were found. The sand dune flora also com- bean Sea of Mexico. A data base was elaborated to prised 237 ruderal/ secondary species (R/S- 36.2 %), as allow for consultations. Data from the vegetation analy- well as 336 species characteristic of other communities sis of 44 beach and dune sites (Fig. 1) totalling 492 (OT- 51.3 %). Species richness per site (Fig. 2) is very species was used (Moreno-Casasola et al. 1982; Espejel variable (18 - 157 species). The number of species in the 1984; Moreno-Casasola & Espejel 1986; García 1987; three categories (out of a total of 492) in each of the 44 Castillo et al. 1991). This list was augmented with sites also varies considerably. The mean number of C- information from publications (Flores 1984; Gonzalez- species found was 15.5 (± 4.6), of R/S-species was 18.5 Medrano 1972; Poggie 1962; Puig 1976; Sauer 1967) (± 9.7) and OT-species was 21.4 (± 14.7).
and herbaria specimens in the National Herbarium at the Although the coastal flora between the Gulf and the National Autonomous University of Mexico (MEXU) Caribbean is quite distinct in species numbers and com- and in the Herbarium of the Institute of Ecology in position (Sauer 1967; Moreno-Casasola & Espejel 1986; Xalapa (XAL). Finally, a partial list of some of the Espejel 1984; Moreno-Casasola 1988) the same trends species belonging to each species type is given in Table in species number per pattern of distribution is main- 4. The complete list is available from the authors.
tained (Table 1). To further understand this distribution, Each of the 655 species was assigned to one of a more detailed analysis of the flora of the Gulf coast three categories of distribution patterns. This was done was made by subdividing this region’s flora into two by examining all specimens for each of the listed spe- distinct floristic groups based on distribution of pre- cies in the collections of both herbaria. A category was dominant growth forms, presence of endemics, biogeo- assigned when 60% or more of the specimens indi- graphical relations and general floristic composition cated a certain type of distribution. This was further (Moreno-Casasola 1988, 1991). The Northern Gulf group checked against the literature and by specialists (see (State of Tamaulipas and northern part of Veracruz – Acknowledgements). We were not able to classify sites 1 to 6 in Fig. 2 – has many similarities with the Castillo, S.A. & Moreno-Casasola, P. Fig. 2. Species richness along the 44 Gulf and Caribbean sites. Black low parts of columns represent coastal species (C), open middle
parts species from other vegetation types (OT), and shaded parts ruderal/ secondary species (R/S).
Texas coastal flora; the Central Gulf group (States of higher frequency values. This means that more species Veracruz and Tabasco – sites 7-29) has many different are found along a high number of beach and dune sites.
elements. Comparisons between both Gulf groups show The species most frequently occurring on the beach and that the Central group has the lowest proportion of C- embryo dunes along the Gulf coast are: Ipomoea species, sharing 32 species with the Northern group.
stolonifera, Oenothera drummondii, Chamaecrista Species numbers vary between floristic groups but pro- chamaecristoides, Schizachyrium scoparium var.
portions of the three categories are roughly maintained, littoralis, Palafoxia lindenii, Amaranthus greggii and especially when comparing both Gulf groups with the Fimbristylis spadicea. They are present in more than half of the sampled sites. Several of them are important Species presence along sites varied greatly. A high sand stabilizers in active dunes, which are found along number of species were very infrequent, appearing only this coast. Hibiscus pernambucensis is less frequent but in one or two sites. Species frequency distribution was also a characteristic tree species along these coasts.
analyzed for the three categories. Fig. 3a, b shows that Another group of beach species is mainly found along C-species, both along the Gulf and the Caribbean, have the Caribbean (Scaevola plumierii, Coccoloba uvifera, Table 1. Number of species (out of a total of 655) and percentage of coastal (C), ruderal/secondary (R/S) and species from other
vegetation types (OT) found in the Gulf and Caribbean regions. The Northern Gulf floristic group is found in Tamaulipas and North
Veracruz, the Central Gulf group inhabits central and southern Veracruz and Tabasco and the Caribbean group includes Campeche
and the Yucatan Peninsula (Moreno-Casasola 1991).
- Coastal sand dune vegetation: an extreme case of species invasion - Fig. 3. Frequency distribution of coastal species (C) along 44 sites on the Gulf (a) and Caribbean (b). Frequency distribution of
ruderal/secondary species (R/S) -shaded bars- and for species characteristic of other vegetation types (OT) -darkened columns-
along the Gulf (c) and Caribbean (d). See text for explanation.
Ambrosia hispida, Tournefortia gnaphalodes, Ernodea of R/S and 62 % of OT-species were recorded in one or littoralis, Suriana maritima, Tribulus cistoides) and still two sites (in contrast with 52 % of C-species in seven other species (Ipomoea pes-caprae, Sesuvium portula- sites). Values on the Y-axis are much higher in Fig. 3b castrum, Sporobolus virginicus, Canavalia rosea, than in Fig. 3a. Along the Gulf, 43.2 % of R/S-species Okenia hypogea, Croton punctatus) are widely distrib- appeared in only 1-2 sites and only seven species (Bidens uted in both areas. Coastal species in stabilized dunes pilosa, Panicum maximum, Commelina erecta, Iresine also vary between the Gulf and Caribbean. Schiza- diffusa, Lippia nodiflora, Porophyllum punctatum and chyrium scoparium var. littoralis, Schrankia quadri- Crotalaria incana) occur in more than a third of the valvis and Cenchrus tribuloides are frequent along the sampled sites (which is equivalent to ten sites). For the Gulf. In the Caribbean the following species are com- Caribbean 68.5 % of the species appeared in 1-2 sites mon: Tribulus cistoides, Ernodea littoralis, Scaevola and seven species (Lantana involucrata, Jacquinia plumierii, Coccoloba uvifera and Pithecellobium aurantiaca, Gossypium hirsutum, Gymnopodium flori- keyense. The latter form dense thickets.
bundum, Cenchrus echinatus, C. incertus and Por- The frequency distribution of R/S and OT-species tulaca pilosa) are also found in more than one third follows a much more pronounced J-curve (Fig. 3c, d).
of the sites (seven sites). Passiflora foetida, Por- Many of them occur in only one or two sites, and very tulaca oleracea, Commelina erecta, Macroptilium few appear in more than one third of the sites. Data for atropurpureum, Malvaviscus arboreus and Rivina laevis Gulf and Caribbean species together, showed that 55 % are widely distributed in the Gulf and Caribbean.
Castillo, S.A. & Moreno-Casasola, P. Table 2. Number and percentage of coastal species (C), ruderal/secondary (R/S) and species from other vegetation types (OT) in
habitats of the dune system. Percentages in bold letters indicate values along horizontal lines. Three main habitats based on the
subdivisions elaborated by Doing (1981) and Moreno-Casasola & Espejel (1986) are used. Dune habitats are characterized mainly
by sand movement (and low salinity). Water-dependent habitats are the slacks where the elevation of the water table produces
inundation during the rainy season and a higher concentration of nutrients. Stabilized areas — such as grasslands, thickets or tropical
forest — have a dense plant cover, no sand movement, roots are out of reach of the phreatic table and physical factors lose importance
as agents of disturbance.
OT-species show the same trend as R/S-species (Fig.
rises. OT-species also represent more than half of the 3c, d). Many of the Gulf species (60.9%) appear exclu- species present (53%), R/S represent more than a third sively in one or two sites and only a few appear in more (34.5%) and C-species 12.3%. In stabilized habitats than a third of them (Chamaesyce dioica, Cyperus articu- where there is no sand movement and conditions are less latus, Hydrocotyle bonariensis, Erigeron myrionactis, stressful such as grassland, thickets and tropical forest Chamaesyce ammanioides, Citharexyllum ellipticum, — including the protected area behind the foredune —, Cissus sicyoides and Psychotria erythrocarpa). Shared OT-species represent more than half (54%), R/S-spe- by the Gulf and Caribbean beaches and dunes are cies 38.5% and C-species only 7.4%. If we analyse Chiococca alba, Metastelma pringlei, Randia laete- distribution of C-species among habitats (Table 2, per- virens, Waltheria indica and Bursera simaruba. In the centages in bold letters), we find that 41% of them occur Caribbean 64.2% species occur in 1-2 sites and 11spe- in dune habitats, 37.8% in stabilized communities and cies appear in more than one third (Agave angustifolia, 21% in slacks or water-dependent habitats. R/S and OT- Coccothrinax readii, Cordia sebestena, Capparis species have low values in dune habitats (11 and 7.2% flexuosa, C. incana, Commicarpus scandens, Metopium respectively), intermediate values in slacks (20.5 and brownei, Thrinax readii, Bumelia retusa, Caesalpinia 22.9% respectively) and high percentages in stabilized vesicaria and Hymenocallis littoralis).
Coastal dune vegetation is characterized by a mosaic Growth forms also vary among the species groups of habitats in which environmental factors change. C, R/ (Table 3). Herbaceous species can be divided into (1) S and OT-species differ in their presence among habi- grasses and sedges — in which OT-species account for tats (Table 2). Dune habitats are formed by the sandy 48.3% and R/S-species for 33.3% — and (2) forbs beach, embryo dunes and foredunes in which sand move- (where R/S-species account for 45.1%). OT-species are ment is the dominating environmental factor. C-species the dominant group among shrubs (51.2%), and espe- predominate in the dune habitat (40.6%), R/S and OT- cially trees, where they represent 77.6%. In both shrubs species represent 31.21% and 28.1% respectively. Hu- and trees coastal species have few representatives. If we mid and wet slacks are those habitats which become analyse the growth forms of C-species (Table 3, per- inundated during the rainy season when the water table centages in bold letters), we find that 71% of them are Table 3. Number and percentage of coastal (C), ruderal/secondary (R/S) and species from other vegetation types (OT) for the various
growth forms. Other growth forms include cacti, agaves and epiphytes. Percentages in bold letters indicate values along horizontal
- Coastal sand dune vegetation: an extreme case of species invasion - Table 4. Partial list of coastal (C), ruderal/secondary (R/S) and species from other vegetation types (OT).
Ruderal / secondary
Other vegetation types
herbaceous (grasses and sedges: Schizachyrium scopa- cia farnesiana, Celtis pallida, Caesalpinia cacalaco, rium, Panicum amarum, Sporobolus virginicus, Trachy- Guazuma ulmifolia, Senna occidentalis, Thevetia pogon gouini, Uniola paniculata, Fimbristylis spadicea, gaumeri, Verbesina persicifolia). OT-species show a and forbs: Ipomoea pes-caprae, Canavalia rosea. See different trend. Herbaceous elements represent 43.9% Table 4). 27.5% of the C-species are woody (shrubs or (Eleocharis spp., Fimbristylis castanea, Nymphaea trees: Chrysobalanus icaco, Ernodea littoralis, Cocco- ampla, Typha domingensis, Rhynchosia americana) loba uvifera, Chamaecrista chamaecristoides, Hibiscus and woody species account for more than half (51.2% pernambucensis, Suriana maritima) and 0.7% have par- — Acacia cornigera, Bursera simaruba, Bumelia spp., ticular growth forms such as cacti (Opuntia stricta).
Cedrela odorata, Enterolobium cyclocarpum). Palms R/S-species are also mainly herbaceous (66.8% - were considered under shrubs and trees, because of Asclepias curassavica, Cenchrus echinatus, Commelina their physiognomy (Acrocomia mexicana, Thrinax erecta, Desmodium spp., Macroptilium atropurpureum, radiata, Coccothrinax readii, Scheelea liebmanii). No Rhynchelitrum repens, Sida rhombifolia, Solanum other data are available on various ecological or physi- nigrum) and the woody species represent 33.1% (Aca- Castillo, S.A. & Moreno-Casasola, P. Discussion
Mexico. 46% of the taxa were perennial, non-succulentforbs, 29% were perennial grasses and allies and 26 % Frequently, exotics are considered synonymous to woody perennials, mostly sclerophyllous. Percentages invaders. There are few data on dune colonization by of herbaceous and woody C-species in dune habitats are exotics from abroad (Rhynchelytrum repens, Cocos very similar to those found by Barbour et al. (1987).
nucifera, Casuarina equisetifolia). Exotics are known Most R/S and OT-species were found in only one or to compete with native species, even to the point of two sites. They invade the dunes and some propagules replacing them. It is difficult to imagine the implications are able to germinate and establish. Stabilized habitats that such invasions have had on dune systems, as studies and slacks have adequate conditions for their successful on dune communities are recent, and human activities establishment and reproduction. Their presence prob- both on the coastal plain and on dunes are quite old.
ably depends on nearby sources, time, chance, species Johnson & Barbour (1990) mentioned three exotics that attributes and adequate microhabitats in the dune sys- tend to take over habitats they invade in the coasts of tem. These are found further away from the beach: Florida: Agave sisalana, Schinus terebinthifolius and humid slacks and stabilized areas. OT-species are able Casuarina equisetifolia. In this study invaders (OT and to invade as well as S/R-species but with lower num- R/S) are those species which have not evolved in coastal bers. Slacks are also subjected to disturbances produced environments, but their life history traits allow them to by inundation. Wetland flora is adapted to these condi- colonize and reproduce successfully. Their presence is tions and easily invades new sites in which humid highly variable and they are not so frequent as to be conditions predominate. This would explain the high considered characteristic elements of the communities.
number of OT-species found in water dependent habi- 59% of invaders are found in just one or two sites. The tats. R/S-species are able to colonize newly opened analysis performed on the 655 species that constitute the spaces in which vegetation has died after a prolonged coastal dune flora of the Gulf and Caribbean of Mexico helps to understand why dune communities behind the Species adapted to stable mature communities (OT) foredune are so different both floristically and structur- are constantly invading and colonizing. Succession tends ally from one place to another. The dune system seems towards grasslands and tropical forests — dry and to tolerate and enrich its flora with invaders.
semideciduous forests — (Novelo 1978; Moreno- The results of this study show that species richness Casasola & Espejel 1986) where OT-species will domi- for any one site depends on the presence of several nate. They show the highest percentage of invaders groups of species, each of which predominates in par- (Table 2), having more OT-species than S/R species.
ticular habitats within the dune system and in different The standard deviation for OT-species number along periods of the stabilization process. There is essentially the 44 sites is the highest (21 ±14.7), the minimum being no biological difference between the process of invasion 6 and the maximum 184. In these habitats there is and the process of colonization or recolonization by usually an almost closed vegetation cover and probably native plants. Colonization after a disturbance is a fun- more competition among plants both for nutrients and damental process of plant succession (Huston 1994). C- water. Disturbances are more sporadic and less intense.
species, which include those solely adapted to sand Thickets and tropical forests on coastal dunes are mainly dune environments, mangroves and coastal marshes, formed by invaders (mostly OT-species) and with time have a more uniform distribution throughout the littoral will resemble these types of communities established areas (Fig. 2 and Table 1) although, numerically, they inland, although always with some coastal elements (C- often constitute a minor component on each site (Fig. 2).
Populations may disappear from any one area, but The results obtained for the three habitat types do they will soon recolonize and reestablish. They occupy not show any direct relationship between disturbance environments with stressful physical conditions (sand and invasibility. R/S-species account for approximately movement, airborne salinity, sea-water inundation) in one third of the richness in the three of them, regardless which other species are not successful such as beaches, of their disturbance regime. OT-species account for embryo dunes and active dunes. In these habitats distur- 54% of the flora of stabilized sites, where sand move- bance by sand movement is determinant and is con- ment and inundation are not important.
stantly opening new spaces for colonization. C-species During the last decades, many tropical sand dune are the most abundant here and they have probably areas have increasingly been bordered by agricultural evolved under these drastic conditions (Table 2). This land. The coastal plain has rich soils with crops and explains their higher frequency distribution along the managed grasslands for cattle. They are connected by sites (Fig. 2). Barbour et al. (1987) analyzed growth dirt roads and other types of corridors that favour the forms of beach vegetation along the northern Gulf of spread and persistence of weeds, thus facilitating - Coastal sand dune vegetation: an extreme case of species invasion - movement of plants through the landscape (DeFerrari and are associated with the first stages of dune & Naiman 1994; Romano 1990). Human activities fa- stabilization. Dune conservation should reproduce spa- vour the presence of weeds and ruderal species, many tially the temporal sequence of dune stabilization. By of which produce small, easily dispersed seeds. It is favouring this mosaic of habitats, species whose exist- probable that the disappearance of forests and the in- ence depend on the physical factors determining each crease in croplands, pastures and secondary vegetation habitat are guaranteed a place where they can reproduce will increase the R/S component and modify the com- successfully. Later stages favour the presence of invad- position of the vegetation covering stabilized dunes.
ers (OT and R/S) which drastically increase local This will increase the presence of herbaceous growth biodiversity. Their presence probably depends on nearby Many other physiological parameters will also be The data presented in this paper seem to imply a modified. As Barbour et al. (1985) have shown, beach contradiction between favouring habitats for C-species species show trends in life forms, leaf traits and physi- and facilitating stabilized conditions more adequate for ological behaviour (for example germination, growth OT-species (and quite a number of R/S-species) and and photosynthesis have broad temperature optima).
thus richer communities. We believe dune conservation We do not have information on these differences for C, strategies should take into account the simultaneous maintenance of several degrees of stabilization in nearby Currently, the result is a highly variable flora be- systems as well as preserved patches of communities tween different dune systems in a geographic region, the inland (forests, thickets, grasslands) which can act as coastal element (C) being the common link because of seed sources. Efforts should be encouraged to preserve its higher frequency distribution. At the regional level the dynamics and ecosystem functioning of dunes, which other vegetation types, i.e. mediterranean-type shrub- include both the habitats in which coastal species sur- lands, also show a high level of floristic variation al- vive successfully and those in which thickets and trees though structure remains fairly uniform (Griffin et al.
Biodiversity and its conservation is now becoming one of the main concerns of humankind. Coastal dune Acknowledgements. We are grateful to Jerzy Rzedowski,
systems occupy a very low percentage of the land Gonzalo Castillo, Patricia Dávila, Fernando Chiang and Javier environments. In subtropical and tropical regions it is Valdés for checking the assigned species categories, and to rapidly diminishing with the impressive growth of Sergio Guevara, Carlos Vazquez-Yanes and Guadalupe touristic developments. Species richness in dune sys- Williams-Linera for their helpful comments on an earlier tems cannot be approached in a simple way. It varies geographically (Gulf and Caribbean) as well as locally(Figs. 1 and 2), depending on the degree of stabilization,mosaic of environmental conditions, floristic compo- References
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