Abstract
This paper focuses on the geographical distribution and the habitats of Bulinus tropicus, the snail intermediate host of the conical fluke, Calicophoron microbothrium. Bulinus tropicus is the freshwater snail species with the most extensive geographical distribution, as reflected by the collection sites of the 7 992 samples currently on record in the database of the National Freshwater Snail Collection (NFSC) of South Africa. The presence of this species was noted by collectors in a total of 14 different types of waterbodies, however, the largest number of samples was collected in dams and brooks and in habitats of which the water was described as standing, clear and fresh. The effect size was calculated for each variable separately to determine its importance in governing the distribution of this species. The data in the database were further processed and adapted to construct an integrated decision tree. This is a statistical model allowing the selection of those variables which can maximally discriminate between the frequency of occurrence of a given species and all other species in the database under specific conditions. The success of B. tropicus, as reflected by its extensive geographical distribution, can most probably be ascribed to the circumstance that population dynamic studies by various authors have shown, without any doubt, that it is highly r-selected and that it additionally has the ability to survive conditions of drought for elatively long periods. It does however, appear as if its limited tolerance to water with a relatively low conductivity could possibly be a limiting factor for its colonisation of specific areas in South Africa. As intermediate host of C. microbothrium which causes paramphistomosis in domestic stock and sometimes can lead to great losses, the wide distribution of B. tropicus has economic implications, especially for residents of informal, rural settlements whose livestock traditionally forms an integral part of their daily existence.
Highlights
Oor die afgelope dekade is verskeie alternatiewe struktuursisteme in Suid-Afrika voorgestel om die koste verbonde aan ligter staalstrukture te verminder
During the last decade various alternative structural systems were proposed to reduce the cost associated with lightweight steel structures
A typical small industrial building was completed in South Africa recently for less than half the cost of a similar conventional structure
Summary
Many alternative structural systems have recently been proposed in South Africa in an attempt to reduce costs of lightweight steel structures. Most of these comprise premanufactured components which are bolted together on site. Indien na groter strukture soos fabriekgeboue en vergadersale gekyk word, is die praktyk in Suid-Afrika oor die algemeen om die gebruik van dun koudgevormde profiele te beperk tot die daklatte wat die bekleding vasheg aan die hoofstruktuurdele. Die aanwending van ligte dunwandige koudgevormde profiele word ondersoek en ’n effektiewe kombinasie van hierdie elemente word voorgestel. Verskeie alternatiewe konfigurasies van strukture word geanaliseer en ekonomiese moontlikhede, volgens wat die kliënt benodig en verkies, word voorgestel. Die referaat toon ten slotte aan dat die gebruik van die voorgestelde metode lei tot ekonomiese strukture en bespreek die toepassings van die beginsels op ’n aantal werklike strukture
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