(i) Species diversity or richness

A basic "statistic" of zoogeography is species diversity or "richness" which describe the numbers of species within a perscribed area, without regard to their numerical abundance or ecological importance in a given community.

There are three basic kinds of richness indices:

1. Alpha diversity or the numbers of species within a generally small area (e.g., a hectare, sq. km, or a defined habitat such as a single lake).

2. Beta diversity or the change in the species richness between communities across an environmental gradient over a relatively small distance (e.g., the change across a salinity gradient or across a mountain slope). Beta diversity is often estimated by calculating a species turnover rate:

Turnover (measure of Beta diversity) = [ (a + b) / c ] x 100%, where:

a = number of species unique to area "A"

b = number of species unique to area "B"

c = total species pool (sum of ALL species in area A and area B). Say area A has 15 unique species (of 20 species total), area B has 10 unique species (of 15 species total). The two areas, therefore, must share 5 species and the total species pool is (20+10) = 30 species.

The the turnover is: [(15 + 10)/30] x 100% = 83%.

From the BC fish faunal similarity data in the last series of lectures, the least faunal turnover was between Vancouver Island the the Queen Charlotte Islands (about 14%), whereas the greatest turnover was between these two areas and the Yukon River drainage (87%).

3. Gamma diversity or the numbers of species across a very large area such as a biome or a continent. Gamma diversity tends to be a function of both alpha and beta diversity, i.e., it will trend to be high in areas of high alpha diversity and particularly across a diversity of habitats. Gamma diversity is simply the sum of all species across each area (without double counting shared species). In the table below, an "X" means that a species (1-9) is present in one of three areas (Forest, grassland, and scrub ecosystems).

Species	Forest	Grassland	Scrub
1	X
2	X	X
3	X
4	X	X
5		X
6		X	X
7			X
8			X
9			X

The gamma diversity across all areas is 4 + 2 + 3 = 9.

(ii) The latitudinal gradient in species diversity

Described by some as the only true "law" of ecology is the observation made even by early zoogeographers that there tend to be more species (higher alpha, beta, and gamma diversities) in the tropics than at higher or lower latitudes.

Recall the pattern for North American birds and mammals.

Reduced richness of North American freshwater fish towards the poles.

In addition, marine invertebrates show a similar latitudinal trend

As do foraminiferan zooplankers in the open ocean.

Below is shown general trrends for combinations of taxa from North America (from Ricketts et al. 1999).

The latitudinal gradient has also been observed in ancient faunas, e.g., fossil remains of brachiopods in deposits 270 million years old (ranging from, 7 to 80 degrees N latitude, Willig et al. 2003)!

This tendency towards richer diversity in tropical habitats is, therefore, widespread amongst many animal (and plant) taxa and begs for a universal and singular causal mechanism.

Before investigating potential mechanisms, however, note that there are some exceptions.

Sandpipers (Scolopacidae), for instance, show greater species diversity in Arctic and temperate habitats then farther south in North America.

Similar "reversed" gradients are known in seals (Phocidae) and Penguins (Spheniscidae).