Agro-Ecosystems最新文献

The use of a “non-weed” concept by traditional agriculturalists in a tropical lowland environment is discussed. Interviews demonstrated that the non-crop components of cropping systems are classified according to use potential on one hand, and effects on soil and crops on the other. A detailed vocabulary for the management of these plants is presented. Such a classification indicates that local farmers understand the intricate role of non-crop plants in their agricultural activities, in direct contrast with modern day agriculture which considers all weeds as noxious, and thus to be eliminated. The ecological importance of this concept, and its potential contribution to a design for overall agroecosystem management is discussed.

Population data consisting of counts of larvae of three species of sugar cane moth-borer, Diatraea spp., and assessments of the extent of parasitism by Paratheresia claripalpis and secondary parasitism by Trichopria cubensis, have been analysed. Time-series analysis of the data revealed two regular cycles in population density, one with a period of about four months, the other with a period of about one year.

The four month cycle was apparent in all three species of Diatraea, although two species, D. saccharalis and D. impersonatella, were subject to parasitism and one, D. centrella, was not. Analysis of the annual cycle of the Diatraea-Paratheresia-Trichopria interaction indicated that the densities of these populations increased during the rainy season and decreased during the dry season.

From the time-series analysis, estimates of time-lags in host-parasitoid interaction responses and in the responses of parasitoid populations' growth rates to changes in their host populations' densities were made. Estimates of parasitoids' generation times could then be derived. These estimates have been compared with independent data collected from direct observations.

A population of Norway rats, Rattus norvegicus (Berkenhout), living on a small island was sampled on four occasions in 1974. Breeding began in the spring and ceased in late autumn. Sub-adult rats (< 150 g) were absent in March but they comprised 45.5% of the sample taken in June; they were even more numerous in September (65.9%) but their numbers were declining by December (36.9%). Pregnant and lactating females were trapped in June and September only.

The population was treated with a synthetic oestrogen reproduction inhibitor for 1 week in March 1975. The compound, the 3-cyclopentyl ether of 17α-hexa-1′3′-diynyloestra-1,3,5(10)-trien-17β-ol (BDH 10131), was included at 0.05% in oatmeal bait.

Further samples of rats were taken after the treatment, in April, July and September 1975 and in January 1976. Fertility was found to have been impaired; no sub-adult animals were caught and no actively breeding females were found. The capture of fewer individuals at post-treatment (29) than at pre-treatment (143) was a further indication that rat numbers remained low following the treatment.

In May 1976, 14 months after the BDH 10131 treatment, both sub-adult and pregnant animals were again present, providing evidence of renewed breeding and of population growth.

Arrhenatherum elatius (L.) Presl. (Oatgrass) and Bromus erectus Huds. (Bromegrass) were grown in pots in pure culture and with (a) root, (b) shoot and (c) root and shoot (total) competition with three levels of irrigation. There was one preliminary harvest of shoots and one final harvest of shoots and roots. Plant height, tiller number, and fresh and dry weight were recorded. The plant material was analyzed for protein and acid soluble nitrogen, for total ash, total lipids, carbon, DNA and RNA.

The experiment took place in an unusually dry summer. For this reason both grasses showed nearly the same competitive ability. There was a slight advantage to Arrhenatherum under root and total competition, especially with watering, and a slight advantage for Bromus under shoot competition, with respect to competitive coefficients based on dry matter production. The most distinct effect was the reduction of the root system of Bromus under total competition with watering. The data clearly indicate that water is one of the factors for which Arrhenatherum and Bromus compete.

Although there were only small changes in dry matter production, there were clear differences in the chemical composition of the plants. In the main part of the experiment, Arrhenatherum was able to increase its protein content under root and total competition, whereas Bromus showed a similar tendency only under shoot competition. These data confirm the earlier hypothesis that higher protein content of the green parts causes higher growth rate and thus indicates the stronger competitor. Both grasses exhibited an increase of carbon content under competition, whereas DNA and RNA, inversely related to each other, reacted specifically to root and shoot as well as total competition. These findings cannot be explained without further investigation.