THE OCCURRENCE AND DISTRIBUTION OF ACRASIEAE IN FORESTS OF SUBTROPICAL AND TROPICAL AMERICA

Forests of the subtropical and tropical regions of North America harbor cellular slime molds not found in the soils of temperate deciduous forests investigated previously. However, most species found in the temperate forest are common in the tropics. Although the diversity of forms is greater in the soils of tropical forests the numbers of Acrasieae per unit of soil are comparable. Characteristic of tropical and subtropical forest soils are Acrasieae bearing crampon bases, of which four new species of Dictyostelium are presently known. Also present, but less frequently isolated, are two other new species of the genus Dictyostelium and two still undescribed species of the Guttulinaceae. Occasional isolates of D. purpureum and D. discoideum were found that produce macrocysts, which seem, also, to be confined to tropical and subtropical areas. Macrocysts were previously knowrn only in D. mucoroides and D. minutum isolated from temperate forest soils. The occurrence and distribution of Acrasieae in warm temperate desert and mesquitescrub, in subtropical hammock, and in tropical thorn, deciduous, seasonal evergreen, rain, and cloud forests were investigated. Acrasieae were well represented in all of these forests except desert. The number of species and the total populations were largest in seasonal evergreen forests. The composition of the cellular slime mold populations and the dominant species within these populations could be related to the soil environment as expressed by the dominant vegetation. THIS PAPER represents the fourth in a series on the Acrasieae in nature. Earlier papers dealt with isolation techniques and the relation of cellular slime mold distribution to tree associations within temperate deciduous forests (Cavender and Raper, 1965a, b, c). The present study is concerned with the occurrence of these organisms in a wide variety of forest formations present in or near the tropics of North America. METHODS-Soils for analysis were collected from the following forest areas: (1) Giant cactus forest in southwestern Arizona and in Sonora, Mexico. (2) Mesquite-scrub forest in southern Texas and in Tamaulipas, Mexico. (3) Thorn forest in Sonora, Sinaloa, and Oaxaca, Mexico. (4) Tropical deciduous forest in eastern and western Mexico and in Guanacaste, Costa Rica. (5) Subtropical hammock forest in southern Florida. (6) Seasonal evergreen forest in Veracruz, M\lexico. (7) Rain forest in several areas of Costa Rica. (8) Cloud forest in Puebla, Mexico, and in Cartago, Costa Rica. 1 Received for publication 23 August 1967. This study was supported in part by research grants from the National Science Foundation (G-24953) and the National Institutes of Health (AI-04915), United States Public Health Service. 2 Present address: Department of Biology, Wabash College, Crawfordsville, Indiana. Five collecting trips of from 2 to 5 weeks' duration were made to the above areas between 1961 and 1964. The help of Dr. Ted Cavender, University of Michigan, and Professor Raul Sequeira, University of Costa Rica, is gratefully acknowledged in carrying out the field trips in A'lexico and in Costa Rica, respectively. Usually 10 soil samples of about 40 g each were collected from individual forest sites. These samples were kept moist and were processed for cellular slime-mold populations as soon as possible after being brought to the laboratory. Techniques of sampling for and isolating the Acrasieae were described in an earlier paper (Cavender and Raper, 1965a). Frequencies and relative densities of Acrasieae in forest soils were calculated as previously described (ibid); and populations from one or more collection sites representative of each of the above forest types are reported here in tabular form (Table 1), as was done earlier for temperate deciduous forests (Cavender and Raper, 1965b, c). Acrasian populations in soils from five areas, each representing a different forest type, are illustrated diagrammatically in the manner utilized for those reports. Special attention will be accorded new species and varieties that were first isolated during the course of this investigation. For characterizations of cellular slime molds isolated but previously known, the reader is referred to the summary descriptions and comments in Bonner (1967). R ESUTLTS-Giant cactus forest-The saguaro