Examples

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Example 1: Geographic range of marine prosobranch gastropods Roy et al. (1998) have assembled a database of the geographic ranges of 3916 species of marine prosobranch gastropods living in waters shallower than 200 m of the western Atlantic and eastern Pacific Oceans, from the tropics to the Artic Ocean. They have found that Western Atlantic and eastern Pacific diversities were similar, and that the diversity gradients were strikingly similar despite many important physical and historical differences between the oceans. Figure 8.8 shows the strong latitudinal diversity gradients that are present in both oceans.

The authors have found that one parameter that did correlate significantly with diversity in both oceans was solar energy input, as represented by average sea surface temperature. More, the authors continued saying that if that correlation was causal, sea surface temperature is probably linked to diversity through some aspect of productivity. They defend that if that is the case, diversity is an evolutionary outcome of trophodynamics processes inherent in ecosystems, and not just a by-product of physical geographies.

Example 2: Latitudinal trends in vertebrate diversity (http://www.meer.org/chap3.htm) Amphibians, absent from arctic regions, are well represented in the mid-latitudes (Figure 8.9A). Forty-seven species of amphibians are found in California (Laudenslayer and Grenfell, 1983). As might be expected given the warmth and humidity of much of the tropics and the inability of amphibians to thermoregulate, this group reaches its greatest diversity here. In fact, one of the three orders (groups of related families) of the class Amphibia, called caecilians (160 species of worm-like creatures), is restricted in its distribution to the tropics.

Latitude (degrees)

Figure 8.8 Latitudinal diversity gradient of eastern Pacific and western Atlantic marine proso-branch gastropods, binned per degree of latitude. The range of a species is assumed to be continuous between its range endpoints, so diversity for any given latitude is defined as the number of species whose latitudinal ranges cross that latitude.

Latitude (degrees)

Figure 8.8 Latitudinal diversity gradient of eastern Pacific and western Atlantic marine proso-branch gastropods, binned per degree of latitude. The range of a species is assumed to be continuous between its range endpoints, so diversity for any given latitude is defined as the number of species whose latitudinal ranges cross that latitude.

Figure 8.9 Diversity vs. latitude plots for three groups of terrestrial poikilotherms, showing what appear to be latitude-related anomalies in the region 15-30° that are probably a response to the less favorable conditions prevailing in the desert regions often found in those latitudes. Data show the highest number of genera in 5° latitude classes. Solid curve smoothed through points indicating highest diversity, dotted curve following the points suggestive of persistent anomaly. (A) Genera of amphibians. (B) Genera of lizards. (C) Genera of snakes.

Figure 8.9 Diversity vs. latitude plots for three groups of terrestrial poikilotherms, showing what appear to be latitude-related anomalies in the region 15-30° that are probably a response to the less favorable conditions prevailing in the desert regions often found in those latitudes. Data show the highest number of genera in 5° latitude classes. Solid curve smoothed through points indicating highest diversity, dotted curve following the points suggestive of persistent anomaly. (A) Genera of amphibians. (B) Genera of lizards. (C) Genera of snakes.

Reptiles, too, are represented by more species in the temperate latitudes. The diversity of lizards is shown in Figure 8.9B and for snakes is shown in Figure 8.9C. Both of these figures show slight decreases in diversity for these groups between 15 and 30° latitude. These are the latitudes at which most of the world's deserts are found. There are 77 species of reptiles in California (Laudenslayer and Grenfell, 1983). The two major groups of terrestrial reptiles, lizards and snakes, are represented by more species in the tropics than in higher latitudes. The pattern is even more pronounced for turtles.

Birds really increase in diversity in temperate latitudes. For example, at least 88-bird species breed on the Labrador Peninsula of northern Canada (55° N), 176 species breed in Maine (45° N), and more than 300 species can be found in Texas (31° N; Peterson, 1963). The total number of bird species found in California exceeds 540 (Laudenslayer and Grenfell, 1983); the total for all of North America is roughly 700 (Welty, 1976).

An indication of the latitudinal trend in mammalian diversity was provided by Simpson (1964) for continental North American mammals. Here again, species diversity is apparent with decreasing latitude. This analysis also shows that, superimposed on the latitudinal trend, is an effect due to elevation such that mountainous regions have more species of mammals than lowlands. There are 214 species of mammals in California (Laudenslayer and Grenfell, 1983).

A majority of all fish species are found in tropical waters. It is possible to get an indication of the diversity of fish in the tropics by considering two examples, one freshwater and one marine. The first example is provided by the dazzling array of coral reef fish. Something on the order of 30-40% of all marine fish species are in some way associated with tropical reefs and more than 2200 species can be found in a large reef complex (Moyle and Cech, 1996). Second, the Amazon River of South America, huge in comparison to most other river systems—3700 miles long, drains a quarter of the South American continent—has over 2400 species of fish. The Rio Negro, a tributary of the Amazon, contains more fish species than all the rivers of the United States combined.

Example 3: Trends within plant communities and across latitude Niklas et al. (2003) examined how species richness and species-specific plant density— number of species and number of individuals per species, respectively—vary within community size frequency distributions and across latitude. 226 forested plant communities from Asia, Africa, Europe, and North, Central, and South America were studied (60°4'N-41°4'S) using the Gentry database. An inverse latitudinal relationship was observed between species richness and species-specific plant density. Their analyses showed that the species richness increased toward the tropics and the reverse trend was observed for average species-specific plant density.

Example 4: Trends within marine epifaunal invertebrate communities Witman et al. (2004) tested the effects of latitude and the richness of the regional pool on the species richness of local epifauna invertebrate communities by sampling the diversity of local sites in 12-independent biogeographic regions from 62°S to 63°N. Both regional and local species richness displayed significant unimodal patterns with latitude, peaking at low latitudes and decreasing toward high latitudes (Figure 8.10).

Latitudinal gradients in biodiversity at the light of ecosystem principles Latitudinal gradients in biodiversity are easily interpretable at the light of the Ecological Law of Thermodynamics. Obviously, the higher solar radiation in the tropics increases productivity, which in turn is thought to increase biological diversity. In fact, Blackburn and Gaston (1996) found that one parameter that did correlate significantly with diversity in both oceans was solar energy input, as represented by average sea surface temperature. Moreover, these authors claim that if that correlation was causal, sea surface temperature is probably linked to diversity through some aspect of productivity. However, they could not establish the causal nexus, considering that productivity could only explain why there is more total biomass in the tropics, not why this biomass should be allocated into more individuals, and these individuals into more species.

This apparent inconsistency can nevertheless be explained within the frame of ecosystem principles. In fact, Jorgensen et al. (2000), proposed that ecosystems show three growth forms:

I. Growth of physical structure (biomass), which is able to capture more of the incoming energy in the form of solar radiation but also requires more energy for maintenance (respiration and evaporation).

II. Growth of network, which means more cycling of energy or matter.

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