Andrew L. J. Ford

Pseudo-absences, pseudo-models and pseudo-niches: pitfalls of model selection based on the area under the curve

The area under the curve (AUC) of the receiver operator characteristic (ROC) graph is regarded as an objective measure of the discrimination accuracy of predictive models. AUC scores calculated from background values, or pseudo-absences, have been proposed as a method of model selection for species distribution models (SDMs) fitted to presence-only data. However, the utility of AUC as a measure of model performance when data on confirmed absence are unavailable has not been fully investigated. We fitted SDMs using informative climatic variables for 2000 species of Mesoamerican trees. As a reference, we also built ‘pseudo-models’ using Gaussian random fields with no biological meaning. AUC correctly selected SDMs fitted to single environmental variables over ‘pseudo-models’ fitted to single random fields in almost all cases. However, when all seven variables were included in the models, AUC erroneously selected complex pseudo-models over complex climate models in 17% of the cases. The spatial distribution patterns predicted by the pseudo-models differed from the results derived from climate-based models, even when overall AUC scores were similar. Both model and pseudo-model AUC values increased when presence points were few and spatially aggregated. The results show that AUC calculated from presence-only data can be an unreliable guide for model selection. Pseudo-absences have ill-defined properties that challenge the interpretation of AUC values. Inference on multidimensional niche spaces should not be supported by AUC values calculated using pseudo-absences.

Ice surface velocity patterns on Seward Glacier, Alaska/Yukon, and their implications for regional tectonics in the Saint Elias Mountains

Abstract Seward Glacier, on the Alaskan/Yukon border along the Gulf of Alaska, sits atop an important structural and morphological junction in the Saint Elias orogen. It is situated at the intersection between the Fairweather and Bagley strike–slip faults, and in the hanging wall of the Malaspina and Chugach–Saint Elias thrust faults. An ice surface velocity map of Seward Glacier derived from interferometric synthetic aperture (InSAR) reveals a complex flow pattern, which implies there is a previously unmapped northwest-trending supra-/subsurface ridge crossing the Seward. Analysis of additional remote-sensing images, ASTER, ERS SAR and the InSAR coherence map, confirms this observation. The presence of this ridge leads to a set of tectonic models describing the possible interaction of the underlying faults.

Airborne spectral imagery for archaeological prospection in grassland environments—an evaluation of performance

The new generation of aerial photographers is using different wavelengths to sense archaeological features. This is effective but can be expensive. Here the authors use data already collected for environmental management purposes, and evaluate it for archaeological prospection on pasture. They explore the visibility of features in different seasons and their sensitivity to different wavelengths, using principal components analysis to seek out the best combinations. It turns out that this grassland gave up its secrets most readily in January, when nothing much was growing, and overall the method increased the number of known sites by a good margin. This study is of the greatest importance for developing the effective survey of the worlds landscape, a quarter of which is under grass.