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Freshwater fish species range throughout the worlds lakes and rivers. The variety of species within geographic regions varies greatly from region to region. Locally, fish populations are often strongly influenced by non-temporal variables that separate fish into areas where they are particularly well adapted. Factors such as flora, flow, depth, temperature, substrate, and pH can all have an influence on species demographics for microhabitats. In more broad settings, such as a whole river or watershed, these variances do not influence the overall diversity of that region (Mora, 2005). There are areas, however, where environmental extremes exert strong influences and negatively affect species diversity. Extremes of temperature, pH, altitude, resource availability, and others can all depress or eliminate the number of species of fish that one would otherwise expect to find.

There are patterns of diversity, however, that ecologists and researchers can use to predict what assemblages of fish species will look like numerically for any given region despite the great morphological variance between said regions. In general, scientists use models for predicting patterns of latitudinal changes in species richness such as the mid-domain model. The mid-domain model states that there are two null regions of species diversity with an ever increasing richness towards the midpoint between these regions. The null points are at each of the poles and the midpoint is of course the equator (Hillibrand, 2004). This model is based primarily upon energy availability, typically a key motive force for diversity.

In this study, pH is the factor that is examined as a possible limiting factor for species diversity within tropical rivers in Costa Rica. An extreme pH would be expected to lower the number of species found within rivers in comparison to other areas throughout the worlds lacking such an environmental stressor. pH would be expected to be low due to soil pH of tropical soils, which are known to have pH’s in the 4-6 range and lack sedimentary rocks in mountainous areas (such as the study area), containing components such as calcium carbonate that raise water pH’s into being basic.