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The species-area relationship (SAR), describing the increase in species richness with increasing area, and the latitudinal diversity gradient (LDG), describing the decrease in species richness with increasing latitude, are the oldest and most robust patterns in biogeography, yet connections between them remain poorly understood. Here, using 1,742 floras covering the entirety of North America north of Mexico (NAM) and including all of NAM's native species of vascular plants, we show that the slope of the SAR consistently decreases with increasing latitude. This trend is general and holds for subsets of the floras in eastern and western NAM. The southernmost latitudinal quarter of NAM exhibits SARs more than twice as steep as those of the northernmost quarter for both eastern and western regions. This decrease in SAR slope with increasing latitude is consistent with the environmental texture hypothesis and Rapoport's rule, and it suggests that more detailed studies of species endemism in relation to environmental and historical factors will yield significant insights into the underlying causes of SAR and LDG patterns.

The risk of liver cancer in relation to use of oral contraceptives was evaluated in a hospital-based case-control study conducted in five US cities from 1977 to 1985. Twelve new cases of liver cancer were identified in women aged 19-54 years; five controls selected from among patients hospitalized for acute conditions unrelated to oral contraceptive use were matched to each case on age (five-year categories), date of interview (three-year categories), and geographic location of the hospital. Among nine cases classified as having hepatocellular carcinoma, eight (89%) had used oral contraceptives; only 16 (36%) of 45 matched controls had used oral contraceptives. Among three other cases (two with cholangiocarcinomas and one with liver cancer of undetermined type), all had used oral contraceptives, compared with four of 15 matched controls. The results confirm the strong positive association between oral contraceptive use and hepatocellular carcinoma observed in earlier studies. Such an association is consistent with evidence that oral contraceptive use is associated with benign hepatic tumors in young women. However, the number of cases of liver cancer in the United States that are attributable to oral contraceptive use is probably small, because liver cancer is extremely rare in the United States.