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- Title
AN ECOLOGICAL STUDY OF THREE CLOSELY RELATED PSOCID SPECIES.
- Authors
Broadhead, Edward; Thornton, Ian W. B.
- Abstract
1. An ecological study has been carried out at Malham in Yorkshire over a period of two years, of three closely related psocid species, Elipsocus mclachlani Kimmins, E. westwoodi McLachlan and E. hyalinus (Stephens). 2. These species occur together on the same trees in the wooded areas around Malham Tarn at an altitude of 381-458 m (1250-1500 feet). Mclachlani and west wood! are both very abundant. Hyalinus, although less abundant, is nevertheless quite common. Both adults and nymphs of all three species live on the bark of the branches, where the eggs are deposited also. The adults, although fully winged, fly little and then for only short distances, but males fly much more frequently than females. An ichneumonoid parasitises the nymphs, killing them at the last instar. 3. The distribution of Elipsocus nymphs along the edge of a pure larch stand was found to be fairly uniform. On single larch branches, a non-random distribution is produced by the formation of clusters of resting nymphs at the base of the short lateral shoots on the underside of the twigs. Negative phototaxis and low thigmokinesis are responsible for this aggregation. 4. The densities of each species differ on different tree species. Six tree species were sampled—hawthorn, beech, sallow, spruce, yew and dead branches of larch. Mclachlani shows an aggregation on the dead larch branches (i. e. significantly greater numbers than expected assuming that distribution was uniform over all tree species); westwoodi shows an aggregation on hawthorn, and hyalinus on sallow and spruce. Westwoodi and hyalinus are more uniformly distributed over these tree species than is mclachlani. Some differentiation of habitat is, therefore, apparent, although not to the extent of complete spatial separation of their populations, and mclachlani has a more restricted ecological niche than the others. 5. Both mclachlani and wesiwoodi are abundant on larch. Mclachlani predominates greatly over westwood on the dead branches, which arc thickly encrusted with lichen, whereas wesiwoodi is the predominant species on those portions of the living branches sampled, which carry Pleurococcus and only small amounts of lichen. The proportions of these two species, both on the living and on the dead branches, are similar throughout nymphal and adult life. 6. Laboratory experiments, in which nymphs of these two species in turn were presented with the choice of a living twig with Pleurococcus and a dead one with lichen, indicate that mclachlani nymphs actively seek the dead twig and westwoodi nymphs the living twig. Some olfactory response appears to be concerned and the antennal flagellum plays no significant role in this selection of the habitat. 7. Laboratory experiments with adult females indicate that, in the presence of both abundant lichen and abundant Pleurococcus, westwoodi feeds almost entirely on Pleurococcus., whereas mclachlani takes Pleurococcus but also a considerable amount of lichen as well. This food preference appears to account in large measure for the observed distribution on larch in the field. The adult females oviposit where they are feeding. 8. All three species have one generation per year at Malham and overwinter in the egg stage. Mclachlani and westwoodi are very similar in the times of appearance and of maximum abundance of each nymphal instar and of the adults, whereas hyalinus is a little later at all stages. The rapid build up of adult numbers to the peak level, followed by a rapid decline, is another feature common to mclachlani and westwoodi, whereas the hyalinus adults emerge over a longer period, so that adult density only gradually rises to its maximum and then gradually declines. Heavy mortality occurs in the late instars in the one species investigated (mclachlani). In both weslwoodi and mclachlani females live twice as long as the males in the field. Reproductive capacity of these two species is not very great—the average number of eggs laid per female in the field being not more than 65 and 48 in westwoodi and mclachlani respectively. 9. The species differ markedly in the sex ratio of adults emerging in the field. Females outnumber males. The ratios (female/male) are 6.6 for westwoodi, 1.8 for mclachlani., and no males in hyalinus. 10. The microclimate at the surface of larch branches has been investigated. These data and continuous temperature and humidity records, when compared with the tolerance ranges of the three species, indicate that the adults at Malham never experienced temperatures or humidities outside their tolerance range during the period of the investigation. The species are sensitive to prolonged exposure to humidities below about 70% R.H., suggesting that the humidity factor may set a limit to population size in drier areas.
- Subjects
YORKSHIRE (England); ENGLAND; PSOCIDAE; LARCHES; HAWTHORNS; FOOD habits; PSOCOPTERA
- Publication
Oikos, 1955, Vol 6, Issue 1, p1
- ISSN
0030-1299
- Publication type
Article
- DOI
10.2307/3564823