Patterns of variation in species richness throughout the region
The TEP fauna includes 1,089 known species of shorefishes.
As measured by overlap in ranges of different species the sections of the coastline with the greatest numbers of species (700-740) occur along Costa Rica and Panama.
Species richness per section of coastline declines to the north and south of these two countries but slowly - the southern coast of Ecuador and the southern edge of the mouth of the Gulf of California still have about 600-630 species, and the tip of Baja California and the south-western Gulf of California have about 585.
Even sites near the limits of the region (northern Peru, the central Gulf of California and southern Baja) are included in the ranges of ~450 species.
Looking at the pattern of variation in species richness in greater detail we find that the overall pattern described above, with the greatest richness in the Costa Rica/Panama area also occurs in many species-rich families (e.g the Muraenids, Engraulids, Clupeids, Ariids, Batrachoidids, Atherinids, Sciaenids, Blenniids, Gobiids, and Tetraodontids).
Other families follow this same general pattern but with the richest zone spread more widely to the north (to central Mexico) and/or south (to Ecuador) of the Costa Rica/Panama section; these include the Urolophiids, Dasyatids, Ophichthids, Scorpaenids, Lutjanids, Haemulids, Dactyloscopids, and Cynoglossids.
In a few families (Carcharinids, Serranids, Carangids, and Scombrids) richness is spread nearly uniformly throughout the three major provinces, without any obvious concentration.
Other families have more idiosyncratic richness distributions: the Pomacentrids have slightly greater richness around the mouth of the Gulf of California and at the offshore islands (except Clipperton); in the Tripterygiids and Labrisomids richness is greatest in the northern part of the region, around the Gulf of California.
Chaenopsids and Gobiesocids have two centers of richness - the Gulf of California and Nicaragua to Panama.
Care should be taken in interpreting this numeric displays because some apparent centers of richness at the tip of Baja reflect the occurrence of vagrant Californian Province species in the northern edge of the TEP.
Regional patterns of richness among species with different biological attributes often follow the general pattern (greatest richness in Costa Rica/Panama) described above.
This occurs for species resident in the TEP (not surprisingly vagrants are concentrated at the edges of the region), those living on sand and mud bottoms, or in the water column; demersal or bottom living species in general; species living inshore and those living offshore; l carnivorous and planktivorous species; and non-marine species.
Other groups show somewhat different patterns: reef living species and marine species show a broad peak in richness between central Mexico and Colombia; omnivorous species have two peaks (Costa Rica/Panama and the Galapagos).
The most atypical pattern is for transpacific fishes: while there is a peak of richness in Costa Rica and western Panama (about 146 species) the same number occur at the Galapagos and slightly lower numbers at the other offshore islands (123 at the Revillagigedos, 109 at Clipperton, 125 at Cocos and 110 at Malpelo).
In terms of the percentage of the fauna represented by those transpacifics they are much more important components of the island faunas than the continental fauna: while they constitute 18.6% of the Costa Rica/Panama fauna their percentage representation in the oceanic-island faunas is 68% for Clipperton, 49% for the Revillagigedos, 41.7% for Cocos, 38.2% for Malpelo and 33.9% for the Galapagos.
The general pattern of latitudinal change in species richness described here for the TEP fish fauna (declining richness with increasing latitude) is an example of a general phenomenon for animals and plants in marine and terrestrial environments.
For coastal marine organisms this has been ascribed to latitudinal variation in energy supply, as reflected in sea surface temperatures, and nutrient levels due to river inputs and upwellings (see. Macpherson, 2002, for a recent discussion).
In the present case we need to consider the extent to which the regional pattern is influenced by differences in sampling effort at various parts of the TEP.
While all countries have been sampled there has been a concentration of effort in two areas - the Gulf of California, and Costa Rica to Panamá.
Despite the coincidence of these areas of enhanced sampling and species richness it seems likely that the sampling is not the primary factor involved.
Those two areas also have features likely to support richer faunas, namely a diversity of environmental conditions, including: large groups of nearshore islands that offer a range of turbidity conditions; large, semi-enclosed bays; large highly active estuarine systems; substantial areas of both rocky shores and soft-bottoms; and areas with varying or different temperature and productivity regimes (e.g. Panama and Costa Rica both have areas with and without large seasonal upwelling systems).
The Costa Rica - Panamá area, with the highest species richness in the TEP, also is in the region with highest sea surface temperatures and high levels of input of nutrients from rivers and upwellings.