Aerial Plankton and Its Conditions of Life

Summary 1. Among animals which can be transported by air currents three main types have been distinguished: (a) large forms, from a length of roughly one decimetre down to a few millimetres; (b) small forms varying between some millimetres and about 1/10 mm.; (c) animals of a microsize, smaller than 1/10; mm. 2. Large organisms are usually distributed by wind in typhoon and hurricane areas alone. During the Glacial Period, however, cyclonic disturbances proceeded along other routes, in this way creating distributional paths which are now no longer in use. 3. Small organisms have considerable possibilities of distribution by convection air currents and winds at moderate altitudes. Examples are given of such distribution over great distances. But as the animals are often strictly specialized ecologically (herbivores, parasites, etc.) they have particular difficulties to overcome in their new surroundings. 4. Numbers of micro‐organisms are constantly being driven up into the air to return again to earth in rain showers or downward air currents. 5. Micro‐organisms are very resistant to unfavourable factors met with in the air‐sea. Some may be distributed through the air in an anabiotic stage. Being often hermaphrodite or parthenogenetic, many of them can give rise to progeny from a single individual which happens to arrive in suitable surroundings. Their resistance to low temperature, low barometric pressure and drought is superior to that of all other organisms. Nevertheless, in comparison to larger forms, they are very sensitive to radiations, especially ultra‐violet, which seem to check their distribution more than that of larger forms. 6. The explanation of the fact that the microforms do not flourish everywhere is found in their very strict biotopical specialization. But wherever a biotope suitable for a certain micro‐organism exists, that organism will appear there as soon as sufficient time has elapsed to allow it to be transported through the air and to settle in the locality. The numbers which arrive are a factor in the survival and establishment of the invading species. 7. No geographical borders or barriers exist for microforms. They are often cosmopolitan, or else regionally distributed around the whole globe in certain climatic belts. 8. Under favourable conditions, especially in humid air, the harmful influence of radiation is diminished, and microforms may be transported alive by winds over greater distances than in clear and dry weather.