11:15 〜 11:30
[MIS03-05] 日本の海洋島において野生化ヤギと海鳥の活動が表層土壌の化学的特性におよぼす影響
キーワード：媒島, 世界自然遺産, 土壌中の植物有効態リン酸, 土壌の置換酸度, 外来種, 土壌流出
Ogasawara Islands, subtropical oceanic islands in north-western Pacific of Japan, have been listed as Natural World Heritage by UNESCO since 2011 because of their valuable ecosystems sustaining many indigenous spices including plants and snails. Nakoudojima Island and its peripheral reefs are the important components of the heritage, but the island has been exposed to the serious influence of soil erosion damaging the indigenous ecosystems. Many areas on the island have been covered with native forests before the introduction of goats (Capra hircus) of ca. 200 years ago. The introduced goats had been naturalized on the island since 1945 at the latest, and they had destroyed the native vegetation by grazing and trampling, resulting in a shift of the native forests into grasslands and loss of the surface soils. To fix the problem, all the feral goats on the island had been eradicated in 1999. Although the recovery of seabird nesting for brown booby (Sula leucogaster) and wedge-tailed shearwater (Puffinus pacificus) has been observed, the soil erosion has still been serious and plant biomass has been very low in some places on the island at least in 2014.
To conserve the indigenous natural ecosystem and help the recovery of the natural vegetation, we conducted field survey and investigated the chemical properties of surface soil (0‐5 cm) and aboveground plant biomass in relation to the topography of this island. Under grassland vegetation where influence of feral goat would have been severe, the chemical properties of surface soils, such as soil pH value, total C and N contents, exchangeable acidity, and plant-available phosphate (Bray II P), were highly variable even in a small area. By comparing soil profile characteristics under natural and disturbed vegetation and distribution patterns of these soils on the island, the changes in the soil chemical properties were reasonably assigned to the effect of soil erosion caused by feral goats for increased soil acidity (mainly found in inland valley area) and to the effect of seabird activities for increased Bray II P and soil acidity (mainly found in outer rim area with high altitude). It was also clarified that the high soil acidity was significantly related to the low productivity of plant biomass. Soil erosion would have removed surface soils having weak acidity and exposed subsoils having strong acidity to the ground surface, resulting in inhibition of plant growth and delay of vegetation recovery. Based on the findings obtained in the present study, several options were proposed to stop soil erosion and to recover the vegetation, although careful preliminary examination would be necessary for applying them.
To conserve the indigenous natural ecosystem and help the recovery of the natural vegetation, we conducted field survey and investigated the chemical properties of surface soil (0‐5 cm) and aboveground plant biomass in relation to the topography of this island. Under grassland vegetation where influence of feral goat would have been severe, the chemical properties of surface soils, such as soil pH value, total C and N contents, exchangeable acidity, and plant-available phosphate (Bray II P), were highly variable even in a small area. By comparing soil profile characteristics under natural and disturbed vegetation and distribution patterns of these soils on the island, the changes in the soil chemical properties were reasonably assigned to the effect of soil erosion caused by feral goats for increased soil acidity (mainly found in inland valley area) and to the effect of seabird activities for increased Bray II P and soil acidity (mainly found in outer rim area with high altitude). It was also clarified that the high soil acidity was significantly related to the low productivity of plant biomass. Soil erosion would have removed surface soils having weak acidity and exposed subsoils having strong acidity to the ground surface, resulting in inhibition of plant growth and delay of vegetation recovery. Based on the findings obtained in the present study, several options were proposed to stop soil erosion and to recover the vegetation, although careful preliminary examination would be necessary for applying them.