Abstract
The island rule suggests that, when mainland animals are isolated on islands, large animals tend to become smaller, while small animals tend to become larger. A small frog in eastern Brazil, Phyllodytes luteolus (Wied-Neuwied, 1824), is widely distributed in association with bromeliads. At the end of the last glaciation, parts of the mainland became islands due to rising sea levels, thereby isolating frog populations on these islands. If the island rule holds, we predicted that frogs on islands would tend to be larger than frogs on the mainland. We compared sizes (weight and length) of 30 randomly selected male frogs from the mainland with 30 from an island in the state of Espirito Santo, Brazil. We also sampled population density on the island and mainland because concurrent with changing sizes, depending on the causal relationship, density may also change. As predicted, island frogs tended to be larger (both in snout-vent length and weight) and were much more abundant. While not specifically addressed in this study, the absence of predators and interspecific competitors may explain both of these trends.
Highlights
The island rule hypothesis suggests that after becoming isolated on islands, mainland animals tend to change in size, with smaller animals increasing and larger animals decreasing in size (FOSTER 1964, VAN VALEN 1973)
While density may change on islands relative to the mainland, complex dynamics and coexisting species must be understood to predict the direction of those changes (DIAMOND 1970a,b, MCARTHUR et al 1972, CASE 1975, MCGRADY-STEED & MORIN 2000, WHITTAKER & FERNANDEZ-PALACIOS 2007)
Frogs were found in four species of bromeliad: Aechmea blanchetiana (Baker) LB Sm., A. nudicaulis (L.) Griseb., Quesnelia quesneliana and Vriesea neoglutinosa Mez
Summary
The island rule hypothesis suggests that after becoming isolated on islands, mainland animals tend to change in size, with smaller animals increasing and larger animals decreasing in size (FOSTER 1964, VAN VALEN 1973). Smaller population sizes are expected when competition is stronger (MCARTHUR et al 1972), and the absence of competitors on islands may lead to increased population size (CASE 1975) This will depend upon resource availability and whether resources were the limiting factor on the mainland. A frog that obligatorily uses bromeliads, Phyllodytes luteolus (Wied-Neuwied, 1824) is small (SVL: 24-25 mm, HADDAD et al 2013), widely distributed in eastern Brazil (FROST 2013, SALLES & SILVA-SOARES 2010) and is found, along with several species of tank bromeliads, on the mainland and on nearby islands (PEIXOTO 1995, OLIVEIRA et al 2015). We predict that density will change on the islands relative to the mainland because at least some of the potential predators and competitors are absent from the island
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