Diversity Across Taxa

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When investigating diversity across taxa, the following issues are worth considering:

• how might measures of diversity depend on which taxonomic level is being investigated?
• why are some higher taxonomic groups so speciose?
• how does the dispersion of species within taxa vary?

Taxonomic diversity can be categorised in other ways than species numbers, such as by size and trophic level, and the diversity of one taxon can be linked to that of another unrelated taxon, through co-evolution and competitive exclusion.

Diversity at Different Taxonomic Levels

The pie chart shows the number of described animal species by taxon, with data from [Brusca & Brusca 1990] .

Find outwhether diversity is higher amongst terrestrial or marine animals, at the phyla and species levels.

• In finding out whether most animals terrestrial or marine, does it depend on what taxonomic level you look at?

See [May 1994] to help answer this question.

Rotifers

Rotifers: 1,800 species.

Urochordata

Urochordates (tunicates): 3,000 species.

Ectoprocta

Ectoprocts (bryozoa): 4,500 species.

Echinodermata

Echinoderms: 6,000 species.

Cnidaria

Cnidarians: 9,000 species.

Porifera

Poriferans (sponges): 9,000 species.

Nematoda

Nematodes: 12,000 species.

Myriapoda

Myriapodes (centipedes and millipedes): 13,120 species.

Annelida

Annelids: 15,000 species.

Platyhelminthes

Platyhelminthes (flat worms): 20,000 species.

Crustacea

Crustaceans: 32,000 species.

Vertebrata

Vertebrates: 47,000 species.

Chelicerata

Chelicerates: 65,000 species.

Mollusca

Molluscs: 100,000 plus species.

Insecta

Insects: 827,175 plus species.

Speciose Groups

Three groups of organisms in particular, notably the beetles, nematodes, and fungi, are noted for being highly speciose.

• What are the characteristics of these groups that have resulted in so many species being formed in them?
  

Monotypic Taxa

• Does the taxonomic position of a species influence its contribution to biodiversity?

Most taxonomic hierarchies have many branches. For example, the conifers (order: Coniferales) contain seven familes, 65 genera and 607 species [Parker 1982] .

However, some taxa are highlymonotypic, and the species in these taxa may be considered of more value.

Ginkgo

The ginkgo, Ginkgo biloba, is extinct in the wild but widely cultivated.

Coelacanth

The coelacanth, Latimeria chalumnae, found in waters around the Comoros Islands.

Note that another possible species was discovered in 1998 in Indonesia.

Red Panda

The lesser or red panda, Ailueus fulgens, from Asia.

Tuatara

Tuataras, Sphenodon sp., are found on a few islands of New Zealand.

Size Range of Various Organisms

Taxonomic groups can be divided into different size classes. In this table, each step represents a 1,000 times increase in mass.

• How many orders of magnitude does this range cover?

Diversity of Trophic Levels

Taxonomic diversity may vary at different trophic levels.

An ecosystem dominated by primary producers would be less diverse than one with herbivores and carnivores as well.

• Can you think of any ecosystems which are less / more diverse in terms of trophic levels than others?

Plant - Insect Co-evolution

• What taxonomic group do a large number of flowering plants rely on for pollination?

• What taxonomic group is a major food source for insects?

• The flowering plants and insects are two highly diverse groups linked by co-evolution.

• Can you think of any other similar examples where co-evolution might lead to increased species diversity?

Competitive Exclusion

• Might the numbers of species in one taxon be inversely affected by the numbers in another?
• Is there any evidence for competitive exclusion (or replacement) of one taxon by another?