Richness and endemism of the freshwater fishes of Mexico
Topiltzin Contreras-MacBeath1, Marlem Brito Rodríguez 2, Valentino Sorani 3, Chris
Goldspink 4 & Gordon McGregor Reid 5
1 Centro de Investigaciones
Biológicas, Universidad Autónoma del Estado de Morelos, México
2 Facultad de Ciencias
Biológicas, Universidad Autónoma del Estado de Morelos, México
3 Centro de Investigación en
Biodiversidad y Conservación, Laboratorio Interdisciplinario de Sistemas de
Información Geográfica, Universidad Autónoma del Estado de Morelos, C.P. 62260,
Morelos, México
4 Manchester Metropolitan
University, All Saints Building, All Saints,
Manchester,
M15 6BH, UK
5 Chester Zoo,
Upton-by-Chester, Chester CH2 1LH, UK
1 topis@uaem.mx (corresponding
author), 2 merlam_14@hotmail.com, 3 vsorani@yahoo.com.mx,4 CGoldspink@aol.com,5 gordonr@chesterzoo.org
Abstract:A study of
richness and endemism of the freshwater fishes of Mexico, was carried out in
order to identify hotspots and inform conservation efforts. This
was done by mapping and overlaying individual species distributions by means of
geographical information systems based on museum data. The study was
able to confirm several previously proposed centres of freshwater fish richness
(Southeastern Mexico, the Mesa Central, the Bravo-Conchos river system and the
Panuco and Tuxpan-Nautla rivers). Seven
areas with high ‘Corrected Weighted Endemism’ Index values were identified,
with the valley of Cuatrociénegas recognized as a true centre. An alarming
result was the identification of a “Ghost” centre of endemism (Llanos El
Salado) in southwestern Nuevo León, where six endemic cyprinodont species are
all ‘extinct’ or ‘extinct in the wild’. Forty-nine single site endemics that are distributed all over Mexico
were identified. The Chichancanab
lagoon in the border between Yucatan and Quintana Roo, where a flock composed
of six endemic cyprinodonts is present needs special mention. Three hotspots of
richness plus endemism were found in Mexico, the most important of which is the
Mesa Central where impacts by human activities have had a detrimental effect on
fish populations.
Keywords: Conservation, hotspots,
ichthyodiversity, Meso-America, species.
Spanish
Abstract Resumen:Se realizó
un estudio de la riqueza y endemismo de los peces dulceacuícolas de México, con
la finalidad de identificar “hotspots” y dirigir esfuerzos de conservación. Lo
anterior se llevó a cabo mediante el mapeo y la superposición de las
distribuciones de especies con el uso de sistemas de
información geográfica y bases de datos de colecciones científicas. Los resultados confirman varios centros de endemismos propuestos
previamente (Sudeste mexicano, la Mesa central, el sistema Bravo-Conchos, así
como el de los ríos Panuco y Tuxpan-Nautla). Se identificaron además
siete áreas con altos valores en el Índice de Endemismo Ponderado, siendo el
Valle de Cuatro Ciénegas un verdadero Centro. Un
resultado alarmante fue el haber encontrado un centro de endemismo “fantasma”
(Llanos El Salado) al Sudoeste de Nuevo León, donde seis especies de
Cyprinodóntidos endémicos están ahora extintos o extintos en la naturaleza. Se encontraron además cuarenta y nueve especies endémicas a una
sola localidad. Destaca el caso de la laguna de Chichancanab ubicada en
la frontera de los estados de Yucatán y Quintara Roo, donde existe un grupo compuesto de seis especies de Cyprinodóntidos. Se
encontraron tres “hotspots” de riqueza y endemismo, dentro de los que destaca
la Mesa central, donde los impactos de las actividades humanas han tenido un
fuerte impacto sobre las poblaciones de peces.
doi: http://dx.doi.org/10.11609/JoTT.o3633.5421-33 | ZooBank: urn:lsid:zoobank.org:pub:6D0D9D42-7628-4BFB-9273-48A8A32D4700
Editor: Rajeev Raghavan, Conservation
Research Group (CRG), St. Albert’s College, Kochi,
India. Date
of publication: 26 February 2014 (online & print)
Manuscript details: Ms #
o3633 | Received 22 May 2013 | Final received 06 February 2014 | Finally
accepted 08 February 2014
Citation: Contreras-MacBeath,
T., M.B. Rodríguez, V. Sorani, C. Goldspink & G.M. Reid (2014). Richness and endemism of the freshwater fishes of Mexico. Journal
of Threatened Taxa 6(2): 5421–5433; http://dx.doi.org/10.11609/JoTT.o3633.5421-33
Copyright: © Contreras-MacBeath et al. 2014. Creative Commons
Attribution 3.0 Unported License. JoTT allows unrestricted use of this
article in any medium, reproduction and distribution by providing adequate
credit to the authors and the source of publication.
Funding: Mexican Education Ministry
through PROMEP and the Noth England Zoological Society.
Competing Interest: The
authors declare no competing interests.
Author Contribution: Topiltzin
Contreras-MacBeath coordinated the study and participated in each step. C.R.
Goldspink, and Gordon McGregor Reid participated in the design, management and
implementation of the study, its research objectives and the provision of
academic rigour. Marlem Brito
Rodríguez participated in spatial analysis, integration of the freshwater fish data base of Mexico, richness map, endemism map and final
cartography. Valentino Sorani
participated in spatial analysis methodology development and manuscript
revision.
Author Details: Topiltzin Contreras is head of the
Conservation Biology Research Group at the “Universidad Autónoma del Estado de
Morelos” in Mexico, he is also Chair of the IUCN/SSC Freshwater Conservation
Sub-Committee, he is also currently Minister for Sustainable Development of the
Government of Morelos, Mexico. C.R.Goldspink, previously worked for the
International Biological Programme (Netherlands) Freshwater Section but
currently involved with local and worldwide conservation issues. Particular
research interests include the management of isolated lakes in North West
England and the potential threats from invasive species and climatic change. Senior Lecturer at the Manchester Metropolitan University. Gordon McGregor Reid is the former
Director General Director of Chester Zoo, and Former Chair of the IUCN/SSC
Freshwater Fish Specialist Group. Valentino
Sorani is head of the Interdisciplinary GIS Laboratory of the Research
Center in Biodiversity and Conservation of the “Universidad Autónoma del Estado
de Morelos”. Professor of GIS. Expert
in land planning, remote sensing and spatial analysis. Marlem Brito Rodríguez is responsible
for the GIS area of the Interdisciplinary GIS Laboratory of the Research Center
in Biodiversity and Conservation of the “Universidad Autónoma del Estado de
Morelos”. She is an expert in remote sensing, cartography and spatial data bases.
Acknowledgements: We would like to thank CONABIO for
providing access to their databases, as well as the GBIF for their records.
For figures, images, tables -- click here
Mexico is the southern most country
of North America, extending into the Isthmus of Tehuantepec. It has a total area of 1,964,375km2and is bounded by a coastline of 7,828km to the west (Pacific) and 3,294km to
the east, which includes parts of the Gulf of Mexico and the Caribbean (CNA
2008). The country has a wide range of
terrestrial habitats and formations consisting of volcanic mountain ranges and
extensive arid deserts, and is estimated to contain at least 10% of the known
world terrestrial biodiversity including plants, reptiles, amphibians and
mammals (Espinosa et al. 2008). This taxonomic diversity reflects the wide range of habitats represented
in Mexico, regional differences in climate and its position between the
Nearctic and Neotropical biogeographical regions (Morrone 2005).
Similarly,
a wide range of aquatic systems occurs across the country with over 50
large rivers and 70 large lakes represented. The rivers Yaqui, Fuerte, Mezquital, Lerma-Santiago,
and Balsas drain into the Pacific, whilst the Bravo, Pánuco, Papaloapan, Grijalva
and Usumacinta rivers drain into the Gulf of Mexico (Lara-Lara et al. 2008). There are over 70 lakes ranging in size
from 10–100 km2. Lake Chapala in Jalisco is the largest lake, followed by Cuitzeo and
Pátzcuaro in Michoacán, Catazajá in Chiapas, del Corte in Campeche, Bavicora
and Bustillos in Chihuahua, and Catemaco in Veracruz. Though, there have been various studies
on the distribution of plants and animals across the country (Ramamoorthy et
al. 1993, Flores-Villela & Gerez 1994, Koleff & Soberón 2008) there
have been relatively few studies on aquatic biota (Domínguez-Domínguez et al.
2006b; Huidobro et al. 2006; Aguilar-Aguilar et al. 2008). The freshwater fish fauna is
particularly diverse with more than 616 species described of which 264 are
endemic (examples of some Mexican freswater fish species are shown in images
1–7). A further 115
species are known to be exotics (Miller et al. 2009). In spite of this diversity there have been very few attempts to formally
describe regional differences in fish species assemblages to better inform
conservation action.
In this study, we attempt to
identify and define regional differences in biodiversity in terms of the
‘hotspot’ approach which has proved successful in
identifying areas of conservation importance elsewhere in the world (Myers 1988, 1990; Myers et al.
2000). Both species number
(richness) and endemism are used in defining the ‘hotspot’ approach by
combining two independent measures of biodiversity. Myers (2003) had stressed that “there
is an urgent need to document freshwater ecosystems which could prove to be one
of the most species-rich hotspots, certainly in terms of fish and one of the
most severely threatened of all hotspots”. Freshwater ecosystems therefore provide
ideal candidates for applying the biodiversity hotspot approach (Mittermeier et
al. 2010).
Thus,
the main aim of the current study was to re-evaluate the status of freshwater
fishes in Mexico in terms of their species richness and degree of endemism, and
identify and map local ‘hotspots’ to further direct conservation planning and
debate in the country. Large-scale studies of this kind on freshwater fish have
largely been confined to Brazil and Africa (Nogueira et al. 2010; Darwall et
al. 2011).
The analysis used during this study
was based on the geographical procedures employed by Aguilar-Aguilar et al. (2008). This
consisted of developing geo-referenced distribution records for the freshwater
fishes found in Mexico. Empirical
data were derived from the National Mexican Biodiversity Authority (CONABIO)
and records held by the Global Biodiversity Information Facility (GBIF). These sources yielded 36,174 countrywide
records and maps for 563 of the 650 known species listed for the country. These data, which account for ca. 86% of
the total fish fauna, also included many of the introduced or exotic species (Contreras-MacBeath 2014).
For each species, spatial records
were converted to a vector format using ArcGis 9.3 and superimposed onto a
1:4,000,000 scale map of Mexico obtained from the geo-information module of
CONABIO. Each map was then compared
to previously published species accounts (Miller
et al. 2009) and those mentioned in Fishbase (Froese & Pauly 2010) to
eliminate inconsistencies and false positives (Fielding & Bell 1997). Of the total records examined, 358 were
rejected representing 1% of the total. A grid map of Mexico was then produced consisting of 249 grid cells
assigned 1–31 from west to east and from
A to M North to South. Each 1oX1o grid
cell equivalent to 12,345km2 was subsequently converted to raster format. This
marker allowed a comparison of the geographical layers. Once the grid was obtained, each
grid-cell was assigned consecutive numbers from 1 to 249.
For
evaluation, data of each species was transformed to raster format, obtaining a
matrix in binary format, with grid-cells with a value of “1” where the species
was present and “0” where it was not. This grid was then combined with the distributional ranges of each
species using the “combine” command of Arc Info 7.0, which from a series of rasters, creates a new one that shows in its database all
the possible combinations among all the rasters used in the operation.
The
end result was a coverture in matricial format with an associated table that
contains 534 columns, each representing a species, and 249 lines that represent
grid-cells. In this way, presence
and absence information was available for every species in each grid-cell. The total for each line represented
species richness in each grid-cell, while the total for each column indicated
the distributional range for each species. With this information, a map of Mexico was constructed showing species
richness in three categories: Low (1–30 species), medium (31–60
species) and high (61–90 species).
The
endemism analysis was based on the ‘Corrected Weighted Endemism’ Index CWEI
(Crisp et al. 2001; Linder 2001; Aguilar-Aguilar et al. 2008) using 216 of the
264 known Mexican endemic species, because of the availability of point
data. To calculate the Weighted
Endemism Index (WEI), each species was weighted for the inverse of its range
such that each endemic species in a grid-cell had a maximum ‘weight’ of 1; if a
species was present in three grid-cells, it has a weight of 0.33 and one in 30
grid-cells has a weight of 0.03. To
calculate the value for each grid-cell, the weight values for all the species
in that grid-cell was added so that cells with a high number of restricted
range species had a higher score than grid-cells with fewer restricted range
species (Linder 2001). To correct for the correlation with species richness, and to
generate the CWEI, the ‘weighted’ endemism was divided by the total number of
species in a grid-cell (Crisp et al. 2001). A map that differentiates between areas
of low, moderate and high endemism was subsequently produced with the results
of this analysis.
Due
to the fact that the former is a coarse analysis that considers relatively
large areas, and that it is known that some Mexican freshwater fish species
have highly restricted distribution ranges that would not be identified by this
means, we further analyzed single site endemics (extant species) separately, so
as to have a finer view of endemism.
To
identify possible conservation ‘hotspots’ of richness plus endemism, the two
previous maps were combined by creating two raster maps both with the same grid
size, a grid position, and the same geo-reference, the first for species
richness and the second for species endemism. A cross operation was performed by
overlaying the two raster maps by comparing cells at the same positions in both
maps and keeping track of all the combinations that occur between the values or
classes in both maps. A cross-table
and the output cross-map were obtained. The results were stored in an output cross-table and an output
cross-map. In the table associated
with the output raster, a new item was created containing a unique value for
all the possible combinations of the two input classes, as well as the class
number and description of the first input map (richness) and those of the
second input map (endemism) which makes it possible to identify every single
combination. The number of pixels
occurring for each combination was counted, as well as the areas of the
combinations calculated. A
biodiversity quality indicator was then defined by assigning to every
combination a value, following the decision rule:
IF
Vr + Ve < 4 then BQI = poor else if 4≤ Vr +
Ve ≤5 then BQI = medium else BQI = high
This
provided the basis for the production of a map, which correlates richness and
endemism.
Distribution
and range
Fish species records were obtained from 235 of the 249 grid-cells
covering Mexico which represented 94.3% of the country area. Those areas without confirmed records
are shown in Figure 2 as white squares and are generally associated with arid
regions of country, namely the provinces of El Salado and Mapimi.
In terms of distribution
patterns, species varied from having very restricted distributions occurring in
only one grid cell (23.3% of the total) up to those, which were widely
represented across the country (Fig. 1 ). Ten species are represented in 45 to 66
of the grid cells, namely Astyanax aeneus (66 cells), Astyanax
mexicanus (65), Micropterus salmoides (62), Poecilia mexicana(60), Poecilia sphenops (55), Gambusia affinis (54), Lepomis
macrochirus (51), Oreochromis mossambicus (47), Heterandria
bimaculata (46) and Sphoeroides annulatus (45). Astyanax aeneus was the most
widely represented species, occurring in 66 cells equivalent to ca.
491,093km2 or 26.5% of the country area.
Many of the widely dispersed
species correspond to the distribution maps previously described by Miller et
al. (2009) and Froese & Pauly (2010), reflecting natural geographical
boundaries. These species include Astyanax
aeneus, Astyanax mexicanus, Poecilia mexicana, Sphoeroides
annulatus and possibly Poecilia sphenops, if sites with populations
of un-determined taxonomic status are included (Miller et al. 2009). Other widely distributed
species are those, which are known to be associated with human activities,
especially aquaculture. The CichlidOreochromis mossambicus (commonly referred to as Mozambique Tilapia) is
now the seventh most widely distributed freshwater fish in Mexico. In addition, four native species, which
have had relatively small original distributions in Mexico, have now been
translocated to many other parts of the country. Species such as Micropterus salmoidesand Lepomis macrochirus have both been widely stocked for sport
fishing (Contreras-MacBeath et al. 1998) and Gambusia affinis is often
used for the control of mosquito larvae (Miller et al. 2009). Similarly, Heterandria
bimaculata has spread through aquaculture activities (Contreras-MacBeath et
al. 1998). It is alarming that half
of the ten most widely distributed freshwater fishes of Mexico are exotics
(Gozlan et al. 2010).
In contrast, 124 species
(23.3% of the total) have ranges restricted to only one cell. Most of these restricted species are
from five families comprising 67% of the total namely, Poeciliidae (19 spp),
Atherinopsidae (16 spp), Cyprinodontidae (12 spp), Cyprinidae (11 spp) and
Goodeidae (9 spp).
When examined by their range
areas, it is apparent that the overall pattern for all species is skewed to the
right (Fig. 1), consistent with the generalized model described by Gaston
(1998). That is, most species have
a relatively small range size and a few have relatively large range size. This pattern has been observed for many
other taxonomic species assemblages (Bell 2001; Jetz & Rahbek 2002).
For Mexico, Ceballos (2001)
found that that 50% of mammal species and 8% of birds could be considered as
‘rare’ based on the criterion (50,000km2) employed by IUCN. Similar
results were found for reptiles and amphibians (Ceballos 2001), which
correspond to the value, observed here for fish. Thus based on the 50,000km2criterion, 57% of Mexican freshwater fish should be considered as rare.
Moreover, if each 1ogrid cell is equivalent to 12,345km2, then 100 species found within
this category (19%) roughly fit the criterion of ‘restricted range’ species
proposed by Nogueira et al. (2010) who found that of the 2587 freshwater fish
species known for Brazilian freshwaters, 819 (32%) had distribution ranges not
exceeding 10,000km2. Similar to Brazil, these values could be used in the identification of
site-scale conservation priorities in Mexico.
Richness
The number of fish species found across the 249 grid cells varied from
one up to 90 species per cell with an overall mean of 19 species per cell (Fig.
2). In terms of richness, the
grid-cell with the highest richness (S=90) was located in south-easternMexico within the Papaloapan River basin. This includes Lake Catemaco, los Tuxtlas (Biosphere Reserve) and the
coastal lagoon of Sontecomapan in the State of Veracruz. Of the 90 species recorded, six were
exotics, especially tilapias introduced for aquaculture. The 86 remaining species were
distributed amongst 27 families, the most speciose of which were Poeciliidae
with 18 and Cichlidae with 10. Both
these groups represent 32% of the total fish diversity in the grid-cell.
If families are grouped
according to Myer’s (1949) classification of freshwater fish based on their
tolerance to salt water (primary, secondary and ‘peripheral’) (Berra 2007),
‘peripheral’ families dominate, with 22 families and 50 species, while there
are only two primary families with five species and three secondary families
with 29 species. This clearly
reflects the influence of the paleogeographical history of the region, as well
as marine radiations and incursions into the freshwater fish fauna (Myers 1966;
Miller et al. 2009). This is
consistent with what has also been described for the whole of North America
(Lévêque et al. 2008).
From Fig. 2, it is evident
that the highest values of species richness are found in four distinct centers
formed by grid cells which correspond well with the
ichthyofaunal provinces described earlier by Miller et al. (2009).
(1) The first center is
localized in southeastern Mexico, with the largest number of species per
grid-cell (62–90) concentrated in the Grijalva-Usumacinta, Coatzacoalcos
and Papaloapan rivers (darkest color in the map). This region corresponds with the area of
highest availability of water in Mexico (Bunge 2010). If the Yucatan Peninsula is included in
this centre, 39 grid-cells are represented and this corresponds to the
Usumacinta ichthyofaunal province (Miller et al. 2009). These findings are in accordance with a
similar analysis carried out for helminth parasites of freshwater fish
(Aguilar-Aguilar et al. 2008). This
general pattern is also similar to that found for terrestrial vertebrates with
the highest species richness in southeastern Mexico (Koleff & Soberón
2008).
This centre has 44 families
and 265 species, 16 of which are exotics. Within this group, the two most diverse families are Cichlidae and
Poeciliidae with 42 and 40 species respectively. These are
followed by the Gobiidae with 12, Eleotridae 11, and Ariidae, Cyprinodontidae
and Gerreidae with 10 species each. Perhaps due to the large river systems, availability of water and large
floddplains (Aguilar 2003; Lara-Lara et al. 2008; Bunge 2010), there are
species with large distributions, such as Sciades guatemalensis (34 grid
cells), Astyanax aeneus (33) (Image 1), Cichlasoma urophthalmus(31), Poecilia mexicana (31), Gambusia yucatana (29), Belonesox
belizanus (28), Rocio ocotal (28), Parachromis friedrichsthalii(27), Heterandria bimaculata (24) and Cichlasoma salvini(23). By contrast, there are 71
species with distributions represented by only one grid-cell equivalent to
12,345km2.
(2) The second centre occurs
in central Mexico mainly within the Lerma-Santiago river system, which
corresponds to the central part of the Mesa Central ichthyofaunal province
(Miller et al. 2009). It has eight
grid-cells that range from 33–52 species per cell, with the highest
values concentrated in the region of Michoacan lakes (Patzcuaro & Cuitzeo)
as well as Lake Chapala, which has been well documented previously
(Domínguez-Domínguez et al. 2006b, Huidobro et al. 2006, Mercado-Silva et al.
2006). This province also
corresponds with the findings of Aguilar-Aguilar et al. (2008) for fish
helminths. It has 23 families and
122 species of freshwater fish, including 19 exotics. The most diverse family
is Goodeidae with 25 species, but in the whole of the Mesa Central there are
about 41 species (Domínguez-Domínguez et al. 2006a). Other diverse families are
Atherinopsidae, Poeciliidae and Cyprinidae, with 22, 13 and 12 species
respectively. The two species with the widest distribution (eight grid-cells)
are Goodea atripinnis, and the exotic Oreochromis mossambicus. These are followed by a group of species
with relatively wide distributions (seven grid-cells) such as Algansea
tincella, Notropis sallaei, Chirostoma arge, Ilyodon
whitei, Poeciliopsis infans, and the exotics Oreochromis aureusand Xiphophorus hellerii. The last species is native to Mexico but not to to the Mesa Central
(Miller et al. 2009), following the definition proposed by Copp et al. (2005). In this region, there were 55 species
having distributions within one grid-cell.
(3) The third center is
localized in northern Mexico, along the border with the United States, and
corresponds to the Bravo-Conchos river system. It comprises eight grid-cells that range
from 32–57 species, the most diverse one of which includes the Rio Bravo
delta and part of Laguna Madre. This suggests that its high diversity is a reflection of the influence
of the presence of many peripheral fish species. It has 30 families and 122 species, with
only three exotics. The three most
diverse families are Cyprinidae, Poeciliidae and Cyprinodontidae with 24, 14
and nine species respectively. As
in the previous region, species distributions range from 1–8 grid
cells. There are five species with
a wide distribution, Astyanax mexicanus, Gambusia affinis, Ictalurus
punctatus, Lepomis macrochirus and Micropterus salmoides, all
native to the region. However, 57
species have distributions which include only one
grid-cell.
(iv) The fourth center is found in eastern Mexico
along the central portion of the Gulf, and comprises the Pánuco and
Tuxpan-Nautla rivers, the latter of which, corresponds to the Tamesi-Pánuco
complex ichthyofaunal province. It
consists of seven grid-cells with a range of 33 –41 species. It has 32 families and 125 species, 15
of which are exotics. The most diverse families are Poeciliidae and Cichlidae
with 24 and 11 species respectively. Three species are distributed throughout the whole centre (seven
grid-cells), Astyanax mexicanus, PoeciliaMexicana and Herichthys cyanoguttatus. In the case of this last
species, Miller et al. (2009) suggested that for this region it could be a
different species. There are also
53 species with distributions of only grid-cell.
There are three other
grid-cells with relatively high values of species richness, two are along the
Pacific coast, one in Guerrero and the other in Sinaloa, but this reflects the
influence of ‘peripheral’ fish species. One of the highlighted grid-cells has
33 species and is located in central Mexico between the States of Morelos and México
reflecting the influence of the higher Lerma and Balsas basins. However, this high number of species is
mainly due to the presence of 14 exotic species, which represent 42% of the
total. Based on native species
alone, this cell would fall in the low richness category with only 19 species.
Exotics species account for 64% of all species within the State of Morelos
(Contreras-MacBeath et al. 1998).
Endemism
Based
on the definitions applied here, Mexican endemics appear to be widely
distributed across the country. Of the endemic species identified, 216 occurred
within 174 of the 249 grid-cells (colored grid-cells Fig. 3) representing 70%
of the country area. There were
also areas of moderate endemism throughout the country, but with no distinctive
pattern apparent.
Based
on the calculated indices (CWEI) of endemism, seven individual grid-cells
showed high values. These are
highlighted in the map (dark color Fig. 3). Most occur in northern Mexico and are
associated with arid ecosystems, areas of relatively low species richness and
with species of restricted distributions. These grid-cells can be grouped in one of two categories. Grid-cells 1 and 2 can be considered as
true centers of endemism.
The
first center of endemism (Fig. 2(1)) compromises the valley
of Cuatrociénegas in central Coahuila, which is a 1,000km2 desert valley that has the greatest number of endemic species of any
place in North America (Stein et al. 2000). Much of its biotic diversity is
associated with a complex array of thousands of geothermal springs, marshes,
lakes and streams (Souza
et al. 2006). It has the highest richness of all the
areas of endemism identified in this paper with 10 endemic fish species, but
its high index of endemism is dependent on the presence of Cyprinella
xanticara, Cyprinodon atrorus, Cyprinodon bifasciatus, Etheostoma
lugoi, Gambusia longispinis, Herichthys minckleyi, Lucania
interioris and Xiphophorus gordoni. This is one of the eight globally
outstanding freshwater ecoregions identified for Mexico by Revenga et al.
(2000).
The
second center of endemism (Fig. 2(2)) consists of two springs included in two
endorheic basins in Southwestern Nuevo León; El Potosí (Ejido Catarino
Rodríguez, Municipio de Galeana), which is the type locality for Cyprinodon
alvarezi and Megupsilon aprorus (both now extinct in the wild) and
the Ojo de Agua la Presa in Bolsón de Sandia which was inhabited byCyprinodon veronicae, Cyprinodon longidorsalis (both now extinct in
the wild), Cyprinodon inmemoriam and Cyprinodon ceciliae (both
extinct). Due to the fact that all six species identified for this center are
extinct or extinct in the wild, it is best regarded as a “ghost” center of
endemism. Eleven years ago, the
freshwater ecosystems in this area, which roughly corresponds to “Llanos El
Salado” ecoregion were regarded as vulnerable by Abell
et al. (2000) but have now all disappeared due to water abstraction. Clearly,
further studies are required to identify the stressors whichhave contributed to the extinction of these species, to avoid similar problems
occurring elsewhere. The remaining
areas of endemism (compared to the valley of Cuatrociénegas and Llanos
El Salado), are highlighted by values of CWEI due to
the fact that they are in areas with low species richness of endemics, and
those present have restricted distributions.
The
third center (Fig. 3) occurs in the northern part of Coahuila, on the border
with the United States. This center
is defined by two species: Gambusia krumholzi from Río de Nava andPrietella phreatophila from a series of caves in the same region. Although both of these sites are found
within part of the río Bravo basin, they are restricted to México.
Center
(4) is triggered by Gambusia alvarezi, which is endemic to the Ojo de
San Gregorio spring near Ciudad Parral Chihuahua and center (5) triggered by Cyprinodon
fontinalis and Cyprinella bocagrande, both from Bolsón de los
Muertos in Chihuahua. Center (6) is
localized in the Baja California Peninsula and is triggered by Gobiesox juniperoserrari, thatis known only from a series of pools in Arroyo “Las Pocitas” in Baja California
Sur. Center (7) is localized to the
south of the country in Río Tehuantepec, Atoyac basin in the State of Oaxaca,
and is triggered by two species, Notropis imelda andPoeciliopsis lutzi.
Single
site endemics
In
order to understand in detail the distribution of endemic species, we set out
to identify species with discrete distributions known only from one site. From our analysis, we found that 43
species in 11 families can be regarded as single site endemics. The most diverse family is
Cyprinodontidae with 11 species, followed by Goodeidae (9), Cyprinidae (7),
Cichlidae (5), Poeciliidae (3), Atherinopsidae and Heptapteridae with two
species each, and then there are three families with one species each
(Fundulidae, Profundulidae and Characidae) (Fig. 4).
Single
site endemics are distributed all over the country (Fig. 5). Some species have a very restricted
distribution; for example Zoogoneticus tequila (Image 2) is now
only found in a 4m diameter spring in Tehuchitlán Jalisco (De la Vega-Salazar
et al. 2003) and Cyprinodon julimes from the Chihuahuan desert that
occurs in a 742m2 thermal spring (De
la Maza et al. 2010). Others
localized endemics are found in larger water bodies such as Bramocharax
caballeroi restricted to Catemaco Lake in Veracruz with a surface area of
72.54km2 (Torres-Orozco & Sanatta 1998). There are also other local endemics such
as Poeciliopsis catemaco and Xiphophorus milleri which occur in Lake Catemaco, but
these were not considered because they can also be found in streams and
tributaries of the lake (Miller et al. 2009).
It
is important to mention a species flock composed of six endemic species of
Cyprinodonts (Cyprinodon beltrani, C. simus, C. maya, C.
labiousus, C. esconditus & C. verecundus) occurs in
Laguna Chichancanab in the border between Yucatan and Quintana Roo (García-Moreno
et al. 2008), but their occurrence could not be identified by the methods used
here CWEI method (Crisp et al. 2001). There is also a small region in central-western Mexico, where there are
several endemic species of the upper Río Ameca, (Yuriria amatlana, Allotoca
goslinei, Algansea amecae, Ameca splendens and Zoogoneticus
tequila) which could be treated as a center of endemism. The last species mentioned above is now
confined to a single small spring in Tehuchitlan Jalisco (De la Vega-Salazar et
al. 2003; Magurran 2009).
Richness
and endemism hotspots
In
order to identify biodiversity hotspots (Myers 1988; Crisp et al. 2001;
Aguilar-Aguilar et al. 2008; Myers et al. 2000), richness and endemism were
correlated and a map constructed (Fig. 6). Three main hotspots with high values; (1) the Mesa Central, (2) the Central-southeastern and (3)
the Lower Rio Bravo River were identified. These are aso priority areas for
conservation in terms of their parasite biodiversity and threats by Aguilar et
al. (2010).
Among
these, the Mesa Central stands out and includes the headwaters of the Río Ameca
and the Lerma-Chapala-Santiago system extending into the Río Pánuco basin. This whole region includes four of the
eight globally outstanding freshwater ecoregions described by Revenga et al.
(2000) that are distributed along central Mexico. This region has been described as
transitional between the Nearctic and Neotropical provinces (Morrone 2005;
Huidobro et al. 2006; Corona et al. 2007) and contributes to its high species
richness and endemism for fish (Miller 2005). Within this region, two hotspots
can be identified which have the highest value for the combination of richness
and endemism.
The
first hotspot, consists of an area known as the “Bajo
Lerma” (Díaz-Pardo et al. 1993) and includes headwaters of the Río Ameca and
Lake Chapala. It has 48 species of
which 38 are Mexican endemics (79%). The second hotspot is formed by two adjacent grid-cells that cover the
“Medio Lerma”, including lakes Patzcuaro, Cuitzeo and Yuridia. It has 63 species, of which 49 are
Mexican endemics (77%). Unfortunately, this region of Mexico has also been identified as one of
the most seriously downgraded/impacted areas (rivers and lakes) by human
activities (Bernal-Brooks 1998; Fisher et al. 2003; von Bertrab, 2003; Garrido
et al. 2010). This has had a
negative impact on the native fish fauna of the region (De la Vega-Salazar
2006; Domínguez-Domínguez et al. 2006a; Domínguez-Domínguez et al. 2008;
Mercado-Silva et al. 2009; Magurran 2009) and has resulted in the extinction of
six species, namely Chirostoma bartoni, Chirostoma charari, Evarra
bustamantei, Evarra eigenmanni, Evarra tlahuacensis and Skiffia
francesae (Contreras-MacBeath 2005).
The
Central-southeastern hotspot includes the area with most water resources in
Mexico for the rivers Coatzacoalcos, Papaloapan and Grijalva-Usumacinta
(CONAGUA 2008; Bunge 2010). This
hotspot is more relevant because of its richness of 208 species, rather than
its endemics comprising of only 30 species (14.4%). Maybe due to the abundance of water in
this hotspot, as well as the relatively low human population, it is one of
Mexico’s regions with lowest pollution as indicated by measures of Biochemical
Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) (CONAGUA 2010).
Nevertheless, Rio Grijalva is highlighted as an area that has witnessed large
habitat modifications due to the construction of four large dams (Garrido et
al. 2000). There are als two known extinct species in this hotspot namely Atherinella
callida, Priapella bonita (Harrison & Stiassny 1999).
The
last of the hotspots identified is a small area in northern Mexico that
corresponds to the lower Rio Bravo. As in the former case, this hotspot is
highlighted mainly by its richness of 90 species, rather than the occurrence of
seven endemics (7.7%). As with many
Mexican freshwater ecosystems associated with arid regions, it has been
severely impacted by human activities (Contreras-Balderas & Lozano-Vilano
1993). Surveys have demonstrated
that the original freshwater fish fauna has been retreating from the lower
reaches and is being replaced by brackishwater and marine invaders
(Contreras-Balderas et al. 2002). This river which had a runoff of over 12,000 million cubic meters/year
in 1962 had less than 2% of that figure in 2002 and was dry for months in the
delta region, both in 2002 and 2004 (Contreras-Balderas et al. 2008).
Data
on species richness and endemism are both crucial for identifying regional and
national ‘hotspots’ and for directing conservation effort. Identifying and mapping these centres by
means of geographical information systems based on museum data have confirmed
several previously identified centres of freshwater fish richness including
those of southeastern Mexico, the Mesa Central, the Bravo-Conchos river system
and the Panuco and Tuxpan-Nautla rivers. A further seven areas with high CWEI endemism values have been
identified including the valley of Cuatrociénegas recognized as a true
centre. Unfortunately, an important
area previously identified as a center of endemism, has now been identified as
a “Ghost” centre of endemism (Llanos El Salado) in southwestern Nuevo León, due
to the loss of six endemic cyprinodont species that were previously present in
this area. In addition, 49 single
location endemics were found to be widely dispersedacross Mexico. One site, the
Chichancanab lagoon on the border between Yucatan and Quintana Roo, contains a
flock of six endemic cyprinodonts. Many of the other single site endemics which are threatened species should be considered
as areas triggering ‘Alliance for Zero Extinction’. However, three hotspots of richness and
endemism in Mexico including the Mesa Central have been seriously compromised
by human activities that are very detrimental to fish species populations.
This
study has provided a countrywide analysis of the distributional patterns of most
of the known species of freshwater fish in Mexico, using the most up-to-date
information available. However,
further work is required on a smaller scale to identify and resolve local
conservation issues. However, our results, provide
important baseline reference material which can stimulate further conservation
initiatives and debate at both local and national level. The next step is to complete the IUCN
threat assessments for each species. When completed, it will be possible to
cross-reference our findings with the distributional patterns of threatened
species and define explicit conservation goals. Our study has also shown that methods,
which have previously been applied to terrestrial taxa, can also be applied
successfully to aquatic taxonomic groups.
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