2013 Akira Okubo Prize Announcement

The Society for Mathematical Biology and the Japanese Society for Mathematical Biology are pleased to announce that the 2013 Akira Okubo Prize will be awarded to Dr. Nanako Shigesasda, Professor Emeritus of Nara Women's University, Japan. In 2013, the Okubo Prize is awarded to a senior scientist whose lifetime achievements have been exemplary in developing innovative theory, in establishing superb conceptual ideas, in solving difficult theoretical problems, and/or in uniting theory and data to advance a biological subject. Professor Shigesada's outstanding accomplishments strongly fit the criteria and the spirit of the research of Professor Akira Okubo, in whose memory the Prize was established.

In her long and successful career, Dr. Shigesada has made uniquely significant contributions to the fields of spatial ecology, and the ecology of invasion. In the 1970's she was an active member of a group organized by the late Professor Ei Teramoto who was a pioneer of mathematical ecology in Japan. The group published papers on structure, stability and efficiency of ecosystems under the name Mumay Tansky, which was an acronym of the names of the six members. In 1979, Dr. Shigesada turned her attention to the observational study of the spatial distribution of ant lions by a Japanese ecologist, Masaaki Morisita. She generalized the phenomenological concept of environmental density, which represents the degree of how unfavorable a habitat patch can be. She introduced a novel model that included population pressure due to mutual interference between individuals combined with environmental potential, regarding how favorable a habitat is. After explaining Morisita's observations using a spatially discrete model, she extended it through a continuous, nonlinear diffusion-advection model and was able to explain how coexistence of competing species can arise through spatial segregation. This pioneering work on density-dependent diffusion has continued to have a significant impact on studies of animal dispersal and spatial distributions.

Since Skellam's seminal work in 1951, the speed of traveling waves has been one of the central questions investigated for reaction diffusion models. Although most models predicted constant wave speeds of animal range expansion, data for the speed of invasion of non-native insects, plants and birds show considerably different patterns. This was a puzzle pointed out clearly by Prof. Akira Okubo himself in the 1980's. For the initial establishment of an invasive population in a small area and for range expansion with or without acceleration, Dr. Shigesada identified three patterns. She formulated a stratified diffusion model by combining the generation of new colonies by long-distance migrants with the short range expansion by neighborhood diffusion and explained the three expansion patterns by assuming three forms of the colonization rate. This very important research solved a long standing puzzle using an innovative approach.

In the recent two decades, Dr. Shigesada began studying pine wilt disease which is caused by the pinewood nematode with a pine sawyer beetle as vector. Describing the population dynamics of pine sawyers and infected trees using a simple discrete-generation model, she estimated beetle densities and parameter values for the model and found that there is a threshold host density above which the disease can spread, and that the minimum density critically depends on the eradication rate. She also modeled the spatial spread of the disease by incorporating an empirically estimated distribution kernel and found that long-range dispersal is necessary to explain the rapid expansion of the disease. These were novel approaches in analysis of pest control strategies.

A major feature of Dr. Shigesada's research has been the explicit introduction of spatial heterogeneity. To assess the effects of spatial heterogeneity on the speed of traveling waves, she considered an environment in which the growth and diffusion rates vary periodically. She defined a traveling periodic wave as a frontal wave that shifts by a characteristic distance with a lapse of a characteristic period of time and calculated the minimum velocity of the waves by concentrating on the leading edge of the waves. She extended her model to combine population growth and diffusion in a two-dimensional fragmented environment and clearly explained the range expansion pattern by introducing a frontal envelope for the two-dimensional spread. These efforts are illustrative of her prominent ability to solve a complicated problem using fresh new approaches.

Dr. Shigesada has long been recognized as one of the leading researchers in mathematical biology, and the textbook (1997) written with her principal collaborator, Kohkichi Kawasaki, on biological invasions has had a significant impact. Prof. Shigesada has also greatly contributed to the education of young researchers at Kyoto University, Nara Women's University and Doshisha University. Recently, she has served as the Research Supervisor for the Basic Research Program PRESTO in the research area "Innovative Models of Biological Processes and its Development", supported by the Japan Science and Technology Agency for 2007-2013 and has had a great impact on young mathematical biologists in Japan. She has contributed for many years to the Japanese Society for Mathematical Biology for which she has served as Secretary General and President.

Based upon her contributions to developing original theory arising from observational studies and for advancing our understanding of spatial processes and patterns in natural systems, the Committee enthusiastically awarded the Akira Okubo Prize to Prof. Nanako Shigesada.

2013 Akira Okubo Prize Committee:

Toshiyuki Namba (Chair) Louis Gross Yoh Iwasa Denise Kirschner Toru Sasaki Jonathan Sherratt

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