Aim Various techniques model a species' niche and potential distribution by comparing the environmental conditions of occurrence localities with those of the overall study region (via a background or pseudoabsence sample). Here, we examine how changes in the extent of the study region (ignored or underappreciated in most studies) affect models of two rodents, Nephelomys caracolus and Nephelomys meridensis.Location North-central South America. MethodsWe used Maxent to model the species' potential distributions via two methods of defining the study region. In Method 1 (typical of most studies to date), we calibrated the model in a large study region that included the ranges of both species. In Method 2, we calibrated the model using a smaller study region surrounding the localities of the focal species, and then applied it to the larger region. Because the study region of Method 1 is likely to include areas of suitable conditions that are unoccupied because of dispersal limitations and/or biotic interactions, this approach is prone to overfitting to conditions found near the occupied localities. In contrast, Method 2 should avoid such problems but may require further assumptions ('clamping' in Maxent) to make predictions for areas with environmental conditions beyond those found in the smaller study region. For each method, we calculated several measures of geographic interpredictivity between predictions for the species (cross-species AUC, crossspecies omission rate, and proportional geographic overlap).Results Compared with Method 1, Method 2 revealed a larger predicted area for each species, less concentrated around known localities (especially for N. caracolus). It also led to higher cross-species AUC values, lower cross-species omission rates and higher proportions of geographic overlap. Clamping was minimal and occurred primarily in regions unlikely to be suitable.Main conclusions Method 2 led to more realistic predictions and higher estimates of niche conservatism. Conclusions reached by many studies depend on the selection of an appropriate study region. Although detailed information regarding dispersal limitations and/or biotic interactions will typically be difficult to obtain, consideration of coarse distributional patterns, topography and vegetational zones often should permit delimitation of a much more reasonable study region than the extremely large ones currently in common use.

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Soil salinization limit crop yield and deteriorate product quality in arid and semi-arid agroecological regions. Under such conditions regulation of mineral nutrients may help to sustain crop productivity. Therefore, a pot experiment was carried out to identify optimal sulfur level and source for enhancing salt adaptability of sunflower (Helianthus annuus L.). A uniform salinity level of 100 mM NaCl was developed in each pot and then six S treatments viz. S0, control; S1, 20 mg sulfur kg-1 soil; S2, 40 mg sulfur kg-1 soil; S3, 60 mg sulfur kg-1 soil; S4, 80 mg sulfur kg-1 soil and S5, 100 mg sulfur kg-1 soil were applied by using elemental S and K2SO4 as sulfur source. Supplementation of sulfur caused reduction in tissue Na + accumulation and electrolyte leakage while increase in tissue K + and Ca 2+ with a subsequent increase in relative water content and photosynthetic rate of sunflower. Increasing sulfur levels in the form of K2SO4 depicted linear increase in plant growth and yield attributes up to 80 mg sulfur kg-1 soil beyond which there was decline in these growth characteristics suggesting that sulfur as K2SO4 at 80 mg sulfur kg-1 soil might be optimum in mitigating NaCl toxicity in sunflower.