- 名古屋大学森林科学研究 (ISSN:13442457)
- vol.22, pp.27-47, 2003-12
特定のアリ種に巣場所と栄養体を提供する代わりに植食者からの防衛をその共生アリに委ねている。東南アジア熱帯に分布するオオバギ属のアリ植物の4種、M.winkleri, M.trachyphylla, M.beccariana, M.bancana を対象に、どのような生態学的要因がアリ植物-共生アリ間の種の組み合わせの特異性を高めているのかということについて実証的に明らかにするために本研究は行われた。これら4種が分布している、マレーシア国サラワク州ランビルヒルズ国立公園とその近隣の二次林において、すべての観察・実験が行われた。In the tropics, many species of plants have mutualistic symbiosis with ants. The myrmecophytism is a typical mutualism between plants(myrmecophytes) and ants. Macaranga (Euphorbiaceae) is a tree genus of approximately 280 species, and includes many obligate myrmecophytic species. Macaranga myrmecophytes provide nest sites and food (food bodies) for their symbiont ants; in turn, the plant-ants protect their host plants from herbivores and clinging vines. One species of Macaranga myrmecophyte has a symbiotic relationship exclusively with only one or two ant species that are specialized to colonize the myrmecophyte or a few Macaranga species, including it. In some localities, multiple Macaranga myrmecophytic species coexist in the same microhabitat ; the spatial distributions of mature trees of such sympatric Macaranga species are highly overlapped at a microhabitat scale. However, the species-specificity in the partnership of the Macaranga-Crematogaster myrmecophytism has been highly maintained there. In this study, first, we examined the mechanisms that maintained such high species-specificity in Macaranga-Crematogaster myrmecophytism. As a key process that is involved in the mechanisms, we focused on settling-plant selection of the partner Macaranga seedlings by single foundress Crematogaster. The field observation showed that foundress queens are able to select correctly their partner plant species, and thus that species-specificity is consequently through the settling-plant selection. However, a number of Macaranga myrmecophytes were observed to be settled by foundress queens of non-partner ants, which settled into stem internodes of Macaranga seedings of sympatric Macaranga myrmecophytes species in the field. This means that settling-plant selection by foundress queen is insufficient to maintain the high species-specificity. Therefore, we hypothesized that the higher mortality in ant colonies and plants that are colonized by the ant species that are not specific to the plants might complementarily maintain the species-specificity. Second, to test the hypothe sis, we experimantally swapped the partner species of symbiont ants between the three Macaranga myrmecophyte species and monitored the survival rates of the plants and ant colonies. The results support the hypothesis. When queens of a ant species that is non-specific to a plant species were forced to colonize the seedlings of the plant, the mortality rates were significantly higher than those when they colonized seedlings of the other plant species to which the ant species is specialized. Third, to examine the intraspecific variation in the status of the ant-plant symbiosis among microhabitats of different light conditions, we investigated the species composition of nesting ants and the herbivory damage on M.bancana saplings by field observation and sampling in the primary and secondary forests in Sarawak. In addition, the effectiveness of non-ant (physical and chemical) defenses were estimated by feeding the larvae of a polyphagous lepidopteran with M.bancana leaves from saplings in the two types of forests. The results suggest that the symbiosis between ants and M.bancana is looser and the non-ant-defenses are stronger in secondary forests, where light is more intense, than in primary forests.