The checkered history of checkerboard distributions: reply

Diamond et al. (2015) raise three criticisms of Connor et al. (2013). The first is that by analyzing each archipelago separately rather than analyzing species pairs using their entire or global geographic ranges, Connor et al. (2013) have misinterpreted the factors that affect the geographic ranges of congeneric species pairs. The second is that Connor et al. (2013) did not plot the geographic ranges of species pairs. Finally, Connor et al. (2013) did not include information on vagrancy. The checkered history of checkerboard distributions is characterized by its pioneer (Diamond 1975) and subsequent followers (Diamond and Gilpin 1982, Gilpin and Diamond 1982, 1984, Sanderson et al. 2009) examining the pairwise geographical distributions of species pairs within archipelagos. Connor et al. (2013), as in previous work (Connor and Simberloff 1979, 1983, 1984, Simberloff and Collins 2010, Collins et al. 2011), followed this convention since it appeared to be part of the definition of and the tradition for inferring competitively determined checkerboard distributions. It is conceivable that one could attempt to analyze rigorously the global pairwise distributions of species, but Diamond et al. (2015) have not done so. Furthermore, such an analysis would raise new issues. For example, how should patchy distributions within larger islands like New Guinea be treated when one scores checkerboard distributions? How should the barriers to dispersal among island groups within archipelagos, as proposed by Mayr and Diamond (2001), inform the analysis? Diamond et al. (2015) marshal only a single example to support their contention that, by analyzing the entire or global distributions of species, one would detect many pairs of species that display checkerboard distributions because of competition. Furthermore, their critique is based on the simple inspection of a map, which is tantamount to Diamond’s (1975) original basis for inferring that competition had affected the geographical distribution of species: that a checkerboard distribution is prima facie evidence for competitive interactions shaping geographical distributions; in essence, checkerboards arise only because of competition. They claim that merely by visually examining the ranges of Macropygia mackinlayi andM. nigrirostris they can tell that the distribution of these two species requires an explanation involving interspecific competition—a clear case of déjà vu all over again. However, Mayr and Diamond (2001) provided compelling evidence for the existence of barriers to dispersal within archipelagoes, and barriers likely exist between archipelagoes as well. Any analysis would need to account for potential dispersal limitation both within and between archipelagoes. Connor et al. (2013) motivated the three attributes that they claim define a ‘‘true checkerboard,’’ a species pair that would have geographical distributions consistent with competitive interactions: (1) the pair would have exclusive island-by-island distributions, (2) their geographic ranges would overlap more than expected were they independently determined, and (3) the pair would share one or more of the island groups defined by Mayr and Diamond (2001) and mapped by Simberloff and Collins (2010) and Collins et al. (2011) for the Solomon Islands and the Bismarck Archipelago, respectively. These three criteria were intended to provide an operational definition of a ‘‘checkerboard’’ distribution sensu Diamond (1975) and Mayr and Diamond (2001). Diamond et al. (2015) do not object to this definition, yet as mentioned above they feel confident that their visual inspection of the ranges satisfies the second criterion. In the analysis conducted by Connor et al. (2013), the pair of Cuckoo Doves in question met Manuscript received 10 June 2015; revised 6 July 2015; accepted 7 July 2015. Corresponding Editor: P. de Valpine. 1Department of Biology, San Francisco State University, 1600 Holloway Avenue, San Francisco, California 94132 USA 2Department of Biology, Rhodes College, Memphis, Tennessee 38112 USA 3Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996 USA 4 E-mail: efc@sfsu.edu