Effect Of Topographic Complexity On Species Richness In The Galapagos Islands

Aim The accumulation of species through time has been proposed to have a hump-shaped relationship on volcanic islands (highest species richness during intermediate stages of an island's lifespan). Change in topographic complexity (TC) of islands over time is assumed to follow the same relationship. However, TC can be measured in different ways and may not have the same impact across taxonomic groups. Here, we quantify TC across the Galapagos Islands and test the assumption that TC follows a predictable trajectory with island age. Subsequently, we ask whether including TC improves statistical models seeking to explain variation in species richness across islands. Location Galapagos Archipelago, Ecuador. Taxon Native and endemic terrestrial animals and plants. Methods For each island, we generated eight TC indices from a 30-m resolution digital elevation model. We tested for a relationship between each index and island age, and whether it significantly contributes to observed variation in species richness, using 11 different models for 12 taxonomic groups across the Galapagos Islands. Results Four TC indices were significantly negatively correlated with either island age or ontogenetic age and only one index followed the hump-shaped relationship with age. No index consistently contributed to the variation in species richness for all taxonomic groups. However, for all 12 taxonomic groups, incorporating at least one TC index in modelling species richness improved one or more models. The most common TC index improving models was standard deviation of slope, although each index improved at least five models across all taxa. Different factors predicted taxon-specific richness, and habitat diversity was significant for all taxa. Main conclusions Topographic complexity is an important component influencing species richness, but its impact likely differs among taxonomic groups and different scales. Therefore, future studies should incorporate broad, multi-dimensional measures of TC to understand the biological importance of TC.