Dominance of Certain Species and Groups.
In [Fig. 21] a numerical analysis is given of the different orders of insects represented in Rothamsted soil. The ascendency of the Hymenoptera and Collembola is almost entirely due to the occurrence of three species in large numbers, viz., the ant Myrmica lævinodis and the Collembola, Onychiurus ambulans and O. fimetarius. In the unmanured plot these two Collembola constituted 13 per cent. of the insects and the species of ant accounted for nearly 28 per cent. In the manured plot they amounted respectively to 27 per cent. and 36 per cent. of the insects present. Next in order of numerical ascendency are larval Diptera, mainly belonging to the families Cecidomyidæ, Chironomidæ, and Mycetophilidæ. The Diptera are followed by the Coleoptera, whose most abundant representatives are larval Elateridæ (wireworms).
1, Collembola; 2, Thysanura; 3, Orthoptera; 4, Thysanoptera; 5, Hemiptera; 6, Lepidoptera; 7, Coleoptera; 8, Diptera; 9, Hymenoptera; 10, Diplopoda; 11, Chilopoda; 12, Areinida; 13, Acarina.
Fig. 22.—Number of species of different orders of invertebrates present in the manured and unmanured (or control) plots at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)
In point of view of number of species present ([Fig. 22]), Coleoptera take the front rank; in the unmanured plot they are very closely approached by Collembola and Diptera.
Passing from the insects, the next assemblage of organisms in point of number of individuals are the smaller worms. The difficulties attending the specific identification of these organisms are great, and, in the present survey, the Nematodes and all the smaller Oligochætes have not been separated.
The abundance of the Myriapoda is mainly due to the prevalence of Diplopoda, which are represented by four species. The Chilopoda almost entirely consist of a single species Geophilus longicornis.
The dominant group of the Arachnida is the Acarine family Gamascidæ, which are represented by about a dozen species.