Effects of local interactrions on the dynamics and diversity of populations

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The purpose of this study is to examine the effect of grazer-periphyton interactions on the spatial structure of periphyton. Three experimental tanks with no snail, 5 snails, and 10 snails were used. The dynamics of spatial distribution of periphyton was estimated by image processing analysis. Then, the results were analyzed using semivariogram. In addition, individual-based models were conducted to examine the factors affecting the change in spatial distribution of pheriphyton.The results of the experiments show that the growth of periphyton biomass in a focal area was negatively affected by the biomass in the area, but positively affected by the biomass in the neighboring areas. For the case of no grazing, little or no spatial patches were formed. With increase of snail grazing, the more conspicuous patches were formed. The results of individual-based simulation suggested that when the effects of the periphyton biomass in neighboring areas on the periphyton biomass were positive, the patches could be formed. The intermediate degree of grazing pressure form more clear patches. In addition, when snails grazed periphyton in area where the snails once grazed, the size of patches tended to become larger. This suggests that snails in the experiments tended to graze the same area. The present study indicats that the interactions between snails and periphyton and among neighboring periphyton would cause the structured spatial distribution of periphyton.