A PCA-based modelling technique for predicting environmental suitability for organisms from presence records
- Robertson, Mark P, Caithness, N, Villet, Martin H
- Authors: Robertson, Mark P , Caithness, N , Villet, Martin H
- Date: 2001
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/442609 , vital:74014 , https://doi.org/10.1046/j.1472-4642.2001.00094.x
- Description: We present a correlative modelling technique that uses locality records (associated with species presence) and a set of predictor variables to produce a statistically justifiable probability response surface for a target species. The probability response surface indicates the suitability of each grid cell in a map for the target species in terms of the suite of predictor variables. The technique constructs a hyperspace for the target species using principal component axes derived from a principal components analysis performed on a training dataset. The training dataset comprises the values of the predictor variables associated with the localities where the species has been recorded as present. The origin of this hyperspace is taken to characterize the centre of the niche of the organism. All the localities (grid‐cells) in the map region are then fitted into this hyperspace using the values of the predictor variables at these localities (the prediction dataset). The Euclidean distance from any locality to the origin of the hyperspace gives a measure of the ‘centrality’ of that locality in the hyperspace. These distances are used to derive probability values for each grid cell in the map region. The modelling technique was applied to bioclimatic data to predict bioclimatic suitability for three alien invasive plant species (Lantana camara L., Ricinus communis L. and Solanum mauritianum Scop.) in South Africa, Lesotho and Swaziland. The models were tested against independent test records by calculating area under the curve (AUC) values of receiver operator characteristic (ROC) curves and kappa statistics. There was good agreement between the models and the independent test records. The pre‐processing of climatic variable data to reduce the deleterious effects of multicollinearity, and the use of stopping rules to prevent overfitting of the models are important aspects of the modelling process.
- Full Text:
- Date Issued: 2001
- Authors: Robertson, Mark P , Caithness, N , Villet, Martin H
- Date: 2001
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/442609 , vital:74014 , https://doi.org/10.1046/j.1472-4642.2001.00094.x
- Description: We present a correlative modelling technique that uses locality records (associated with species presence) and a set of predictor variables to produce a statistically justifiable probability response surface for a target species. The probability response surface indicates the suitability of each grid cell in a map for the target species in terms of the suite of predictor variables. The technique constructs a hyperspace for the target species using principal component axes derived from a principal components analysis performed on a training dataset. The training dataset comprises the values of the predictor variables associated with the localities where the species has been recorded as present. The origin of this hyperspace is taken to characterize the centre of the niche of the organism. All the localities (grid‐cells) in the map region are then fitted into this hyperspace using the values of the predictor variables at these localities (the prediction dataset). The Euclidean distance from any locality to the origin of the hyperspace gives a measure of the ‘centrality’ of that locality in the hyperspace. These distances are used to derive probability values for each grid cell in the map region. The modelling technique was applied to bioclimatic data to predict bioclimatic suitability for three alien invasive plant species (Lantana camara L., Ricinus communis L. and Solanum mauritianum Scop.) in South Africa, Lesotho and Swaziland. The models were tested against independent test records by calculating area under the curve (AUC) values of receiver operator characteristic (ROC) curves and kappa statistics. There was good agreement between the models and the independent test records. The pre‐processing of climatic variable data to reduce the deleterious effects of multicollinearity, and the use of stopping rules to prevent overfitting of the models are important aspects of the modelling process.
- Full Text:
- Date Issued: 2001
Parasitism of Plutella xylostella (Lepidoptera, Plutellidae): field and laboratory observations
- Waladde, S M, Leutle, M F, Villet, Martin H
- Authors: Waladde, S M , Leutle, M F , Villet, Martin H
- Date: 2001
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/442239 , vital:73969 , https://doi.org/10.1080/02571862.2001.10634397
- Description: Investigations carried out at Alice and its rural environs in the Eastern Cape province of South Africa, showed that parasitoids affecting the diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae) were Cotesia plutellae, Apanteles eriophyes, Diadegma sp., Diadromus collaris, Oomyzus sokolowskii and Hokeria sp. Cotesia plutellae was active all year round and accounted for 30–50% parasitism of P. xylostella. Field experiments using three cabbage cultivars, grown under pesticide-free conditions, indicated that C.plutellae achieved rates of parasitism as high as 90–95% for at least eight months of the year. In winter and early spring parasitism by C. plutellae was less than 30% in the field. This level of parasitism coincided with rising pest density resulting in a pest increase in early spring when C. plutellae appeared unable to control the high pest numbers. Towards the latter part of spring and early summer, C. plutellae parasitism and that of the other parasitoids increased. The possibility of using indigenous parasitoids especially C. plutellae in the management of P. xylostella in this area requires further investigation.
- Full Text:
- Date Issued: 2001
- Authors: Waladde, S M , Leutle, M F , Villet, Martin H
- Date: 2001
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/442239 , vital:73969 , https://doi.org/10.1080/02571862.2001.10634397
- Description: Investigations carried out at Alice and its rural environs in the Eastern Cape province of South Africa, showed that parasitoids affecting the diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae) were Cotesia plutellae, Apanteles eriophyes, Diadegma sp., Diadromus collaris, Oomyzus sokolowskii and Hokeria sp. Cotesia plutellae was active all year round and accounted for 30–50% parasitism of P. xylostella. Field experiments using three cabbage cultivars, grown under pesticide-free conditions, indicated that C.plutellae achieved rates of parasitism as high as 90–95% for at least eight months of the year. In winter and early spring parasitism by C. plutellae was less than 30% in the field. This level of parasitism coincided with rising pest density resulting in a pest increase in early spring when C. plutellae appeared unable to control the high pest numbers. Towards the latter part of spring and early summer, C. plutellae parasitism and that of the other parasitoids increased. The possibility of using indigenous parasitoids especially C. plutellae in the management of P. xylostella in this area requires further investigation.
- Full Text:
- Date Issued: 2001
Preliminary observations on the effects of hydrocortisone and sodium methohexital on development of Sarcophaga (Curranea) tibialis Macquart (Diptera: Sarcophagidae), and implications for estimating post mortem interval
- Musvasva, E, Williams, Kirstin A, Muller, Nikite W J, Villet, Martin H
- Authors: Musvasva, E , Williams, Kirstin A , Muller, Nikite W J , Villet, Martin H
- Date: 2001
- Language: English
- Type: text , Article
- Identifier: vital:7075 , http://hdl.handle.net/10962/d1009532
- Description: Larvae of Sarcophaga (Curranea) tibialis (S. tibialis) were reared at constant temperature on chicken liver treated with a steroid or a barbiturate at concentrations that would be lethal, half-lethal and twice-lethal doses for humans. Trends to greater mortality at higher drug concentrations were not statistically significant. Larvae exposed to either drug took significantly longer to reach pupation compared to those in the control, while larvae exposed to sodium methohexital passed through pupation significantly faster than those in the control. No systematic relationship was found between drug concentration and development time of larvae or pupae. The total developmental period from hatching to eclosion did not differ between treatments, implying that estimates of post mortem intervals- (PMI) based on the emergence of adult flies will not be affected by the involvement of these drugs in a case. On the other hand, anomalous pupation spans may indicate the presence of barbiturates. These findings are compared with patterns found in another fly fed other contaminants.
- Full Text:
- Date Issued: 2001
- Authors: Musvasva, E , Williams, Kirstin A , Muller, Nikite W J , Villet, Martin H
- Date: 2001
- Language: English
- Type: text , Article
- Identifier: vital:7075 , http://hdl.handle.net/10962/d1009532
- Description: Larvae of Sarcophaga (Curranea) tibialis (S. tibialis) were reared at constant temperature on chicken liver treated with a steroid or a barbiturate at concentrations that would be lethal, half-lethal and twice-lethal doses for humans. Trends to greater mortality at higher drug concentrations were not statistically significant. Larvae exposed to either drug took significantly longer to reach pupation compared to those in the control, while larvae exposed to sodium methohexital passed through pupation significantly faster than those in the control. No systematic relationship was found between drug concentration and development time of larvae or pupae. The total developmental period from hatching to eclosion did not differ between treatments, implying that estimates of post mortem intervals- (PMI) based on the emergence of adult flies will not be affected by the involvement of these drugs in a case. On the other hand, anomalous pupation spans may indicate the presence of barbiturates. These findings are compared with patterns found in another fly fed other contaminants.
- Full Text:
- Date Issued: 2001
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