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Testing limiting similarity in Quaternary terrestrial gastropods

John Warren Huntley¹, Yurena Yanes², Micha Kowalewski¹, Carolina Castillo³, Antonio Delgado-Huertas⁴, Miguel Ibáñez³, María R. Alonso³, José E. Ortiz⁵ and Trinidad de Torres⁵

¹ John Warren Huntley and Micha Kowalewski. Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061. jhuntley{at}vt.edu
² Yurena Yanes. Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802
³ Carolina Castillo, Miguel Ibáñez and María R. Alonso. Departamento de Biología Animal, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez s/n 38206, La Laguna, Tenerife, Canary Islands, Spain
⁴ Antonio Delgado-Huertas. Laboratorio de Biogeoquímica de Isótopos Estables, Estación Experimental del Zaidín (CSIC), Prof. Albareda 1, 18008, Granada, Spain
⁵ José E. Ortiz and Trinidad de Torres. Laboratorio de Estratigrafía Biomolecular, Escuela Técnica Superior de Ingenieros de Minas de Madrid, C/Ríos Rosas 21, 28003, Madrid, Spain

The hypothesis of limiting similarity, which postulates that morphologically and/or ecologically similar species will differ enough in shape, size, or other variables to minimize competition, has been controversial among ecologists and paleoecologists. Many studies have reported the occurrence of limiting similarity in modern environments or in time-averaged fossil deposits; however, empirical high-resolution time series demonstrating limiting similarity over longer time scales are lacking. We have integrated radiocarbon-calibrated amino acid dating techniques, stable isotope estimates, and morphometric data to test the hypothesis of limiting similarity in late Quaternary land snails from the Canary Islands over a period of 42,500 years. We tested for both ecological character displacement (two closely related species will differ in size in order to minimize competition in sympatry and these differences will be minimized in allopatry) and community-wide character displacement (overdispersion of body size among competitors in a guild). Multiple proxies of body size consistently show that two endemic congeneric pulmonate gastropod species (Theba geminata and T. arinagae) maintained a difference in size from 42,500 B.P. through the last occurrence of T. arinagae 14,900 B.P., with a concomitant trend of a decreasing body size. Theba geminata body size did not converge on that of T. arinagae and variation in T. geminata body size did not increase significantly following the extinction of T. arinagae; therefore, ecological character displacement and release did not occur. Community-wide character displacement was found in only one time bin over the last 42,500 years. These results suggest that limiting similarity is a transient ecological phenomenon rather than a long-term evolutionary process. This study not only demonstrates the problems inherent in biological "snapshot" studies and geological studies of time-averaged deposits to test limiting similarity adequately, but it also presents a more adequate research protocol to test the importance of interspecific competition in the history of life.