The birds and the seas: body size reconciles differences in the abundance–occupancy relationship across marine and terrestrial vertebrates

Despite some fundamental differences in production processes and the ecology of consumer species on land and in the sea, further understanding of pattern and process in both biomes might be gained by applying common methods of macroecological analysis. We develop methods that reconcile apparent differences in abundance and occupancy for marine and terrestrial vertebrates, as exemplified by fish and birds. These recognize and take account of those aspects of the life history and ecology of marine and terrestrial animals that influence their abundance, distribution and trophic role. When abundance and occupancy are averaged within species over time we show that variation within a region is less for birds than fish, but when abundance and occupancy are averaged over space, the difference between birds and fish disappears. Further, we develop size rather than species‐structured abundance–occupancy relationships for fish assemblages and demonstrate that patterns of intra‐size class variation that are very similar to intraspecific variation in bird species, over both time and space. We argue that this result reflects the relative importance of body size and species identity respectively in determining trophic roles in marine and terrestrial environments. Selection of the appropriate analytical unit on land (species) and in the sea (size) helps to reconcile apparently divergent macroecological patterns, especially when these are driven by contrasting patterns of energy acquisition and use.