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A 1960—71 study of populations of color—banded ant—following antbirds of three species on a tropical—forested lowland reserve, Barro Colorado Island, showed that the small species (Spotted Antbird, Hylophylax naevioides) remained stable at about 20 pairs/km2. A medium—sized species, the Bicolored Antbird (Gymnopithys bicolor), decreased from about 3 pairs to 1.5 pairs/km2. A large species, the Ocellated Antbird (Phaenostictus mcleannani), declined from 1.5 pairs/km2 to near extinction–only one female remained in early 1971. Two of three other species that regularly follow army ants showed relatively stable populations, but a third large species (Barred Woodcreeper, Dendrocolaptes certhia) declined from two pairs to local extinction. Prior to 1960 a very large ground—cuckoo that follows ants had already become extinct there. Thus, the three largest of the seven original species that regularly followed ants were gone or nearly gone by 1970. The decrease in numbers of regular ant—following birds was not made up by increases in occasional followers. Detailed studies of antbirds showed no clear reasons for declines, except that annual mortalities of adults were high in Ocellated Antbirds (about 30%) compared to Spotted Antbirds (15%—17%) and nest losses perhaps higher in the former (96% compared to 91%). Nest mortalities were slightly lower (88%) and adult mortalities intermediate (about 25%) in Bicolored Antbirds. Female Ocellated Antbirds had higher mortalities than males. The antbirds renest repeatedly during long nesting seasons, up to 14 times per year for Ocellated Antbirds. However, to replace females of this species under Barro Colorado conditions 19 nestings per year would be needed. Concurrent listing of all birds of the island showed that 45 species of breeding birds, 22% of the avifauna present when the island was made a reserve, had disappeared by 1970. No new species replaced them. Of the lost species 13 are forest birds, in danger if forests are cut elsewhere. The other species, second—growth and forest—edge birds, have been crowded out by growth of the forest. Loss of species from this tropical reserve, especially the part apparently caused by the small size and isolation of the reserve, poses problems for conservation and ecological studies of tropical biotas. It is suggested that large future reserves have corridor zones to each other, that is, that intensive human use not preempt too much area nor interrupt immigration of animals or plants from one refuge to another.

Flights (movements from place to place) were studied in 11 species of Lepidoptera which differed in size, mate—locating behavior, food plants of larvae and adults, oviposition behavior, population size, areal extent of populations, and habitat features possibly limiting flights. and population size were quantified. Mate—locating behavior influences the difference in flights of ♂ ♂ and ♀ ♀; males tend to remain in sites favorable for mating, while ♀ ♀ tend to disperse more than ♂ ♂. Multibrood polyphagous species feeding on early successional plants had the farthest flights. Conversely, single—brood species feeding as larvae on perennial trees or shrubs had short flights. There is apparently a genetic component to flight distances; taxonomically similar species had similar distances, flights of several species were similar in successive years, and flights of reared individuals were similar to those of native individual. Two species in different superfamilies occupying the same habitat convergently developed similar flight distances. A migratory species had more unidirectional flight than the nonmigrants. There is a strong positive correlation between the size of the area in which the majority of individuals of a population are concentrated and flight distances for the 11 species. No correlation exists with population size, but mean densities are inversely correlated with flight distances. The significance of these correlations is discussed. It ispostulated that heritable behavioral mechanisms and local extinction influence the relationships between flights, area, and density.

Counts of invertebrates by sweep-netting and beating the stratum of rain forest vegetation between 6 in (15 cm) and 6 ft (1·8 m) have been made during rainy season conditions in N. Brasil and on Barro Colorado Island, Panama Canal, the latter from measured volumes of habitat. This work has been repeated on BCI during the dry season. The average population densities of total catches were 0·97 m 3 for rainy season and 1·09 m 3 for dry season. Species diversity was very high, therefore numbers per species extremely low. It is suggested that such low population density brings hazards of local extinction, and that the maintenance of such a system depends on the existence of extensive habitat from which recolonisation can occur. If this is so, reserves in rain forest will have to be very large, to ensure long-term survival of many of the invertebrate species.

We examine the qualitative behavior of differential equations for the proportion of insular patches of each of two kinds of habitat occupied by each of two species with characteristics rates of migration between patches and of local extinction within a habitat. Certain migration and extinction rates result in stable coexistence, even of closely similar species; others lead to competitive exclusion, even when each species is competitively superior in one kind of habitat. For a community of many species in many habitats, we surmise qualitative limits to the subdivision of resources, and alternative stable communities. Our results extend to species that live in successional or ephemeral habitats. We therefore conjecture equilibrial theories for the number of patches of habitat occupied by insular species, fugitive plants and invertebrates, or infesting parasites.

A model is developed for the coexistence and exclusion of species over a region of similar habitable patches. Since the balance of local extinction and colonization would leave some patches unoccupied even without competitors, species may coexist even when all the patches are the same. Regional competition coefficients are found when species affect the local extinction or migration rates of each other. Rare species can regulate each other and even exclude other species completely.

It is now generally believed that progressive desiccation has been the principal climatic trend during the late Pleistocene of Israel. This gradual shift in the climate towards a drier regime has presumably been the cause of the local extinction of the more tropical components of the Eastern Mediterranean fauna. The impact of the European glacial sequence on the one hand and of the close proximity of a great desert on the other on the evolution of the climate of Israel, and on the rodent fauna of the land through the ages, is yet to be understood in depth. An attempt has been made in the following pages to collate all the available information on the environmental changes in relation to rodent faunas in the Pleistocene of Israel.

Attempts to understand evolutionary processes in cave organisms have been largely limited to speculation on the causes of loss of photoreceptor organs and pigmentation, increase in size and complexity of tactile sensory structures, and certain modifications in physiology (see review in Barr, 1968). Little is known of the population genetics of cave organisms or of the genetic changes accompanying the transformation of epigean (surface-dwelling) forms to troglobites (obligate cavernicoles). Yet genetic information is essential to the confident development of theories of troglobite evolution. For this reason, we have compared the genic character and degree of variability in troglobitic and epigean populations of the characid fish Astyanax mexicanus in Mexico. Students of cave biology have generally accepted the thesis that the ancestors of many troglobites entered caves as troglophiles (facultative cavernicoles) before the end of the Pleistocene, and became isolated with the local extinction of surface populations as a result of climatic changes associated with glaciation (Barr, 1968). If this is true, many troglobites have been living

to environmental extremes of drought and flash floods. 2. The natural life span for most Rhinichthys oscults in the Chiricahua Mountains is less than 3 years. None was found to reach 4 years. 3. High mortality rates during summer drought must be interpreted, in part, as an acceleration of death among older fish since most fish are expected to die before they are 3 years old. Among younger fish, drought directly elevates the mortality rate. 4. Mortalities during a drought are caused directly by the disappearance of water and indirectly by starvation of the fish which are crowded into reduced habitat with inadequate food. 5. Flash floods are an important cause of mortality among fish of the year if they occur while the fish are very small. The greatest potential loss of fish of the year occurs in late summer when the initial flash flood induces major reproduction and is then followed by another flash flood. 6. Flash floods are not a significant cause of mortality among older fish. 7. Temporary waters may persist in some sections through a period of wet years. The populations of fish inhabiting them are derived from upstream sections during a flash flood. All fish that were located in downstream temporary sections in the spring of 1960, were one-year-olds. 8. Temperatures do not rise to lethal levels in any of the flowing streams. In exposed shallow isolated pools, the temperatures may become lethal for oldcr fish, but not for fish of the year. 9. Predators play a minor role. Only one, the garter snake, Thamnophtis cyrtopsis, is common and apparently important. 10. Although fish populations are greatly reduced by a year of drought, there is no immediate threat of their local extinction.

The problem of the maintenance of marine littoral populations and especially that of the European oyster (O. edulis) in Great Britain as discussed by Gross and Smyth in Nature1 is one of great interest. In all species it is reasonable to assume that the properties of each particular organism give a measure of its attunement to the environment in its recent past, if not to the present. The supreme criterion and one hard fact of the sum of its relationships to life conditions is the number of young (larvae) produced during the life of the individual. This provision of young has ensured survival of the species in the past against predators, parasites, competitors and normal and abnormal deviations in the total of chemico-physical conditions over the range of the environment. In a given locality, however, it is reasonable to infer that extinction may occur or tend to occur if the full span of life is not attained by the normal adult population. If, therefore, the normal span of life is reduced in any locality, fewer young will be produced over that period of time which has ensured survival in the species as a whole, and a combination of local unfavourable conditions—or indeed any single one of a significant nature—will reduce the chance of survival and may result in local extinction.

The term interspecific competition has been used with various meanings (Birch 1957) and this necessitates a brief review to clarify the terminology employed in this paper. The principal source of confusion is failure to discriminate between observations on individual behaviour and the effects of one population upon another. Park (1954) and Brian (1956a) have used the terms 'exploitation' and 'interference' to describe the two types of interaction at the individual level and their use in place of 'competition', where appropriate, would result in much greater clarity. Exploitation occurs where two species have a resource in common and interference is simply damage (excluding predation and parasitism) to the individuals of one species by individuals of another. It is not possible at present to predict from observations on individual behaviour the effects produced on the population densities, but it is hoped that such predictions will be possible in the future when more examples of population interaction have been demonstrated and analysed in terms of individual behaviour. The definition of interspecific competition used here is: interspecific competition is occurring if lower densities of the competitors are found than would be the case if each were present alone. Odum (1953, p. 166) uses the term in this sense, and 'amensalism' for an interaction which lowers the density of only one of a pair of interacting species. It is possible that each of two species having a predator or parasite in common would show a sustained increase in density if the other were experimentally removed. Competition would not have its conventional meaning if applied to this case, but it is convenient to have one term to cover all cases of the same type of population interaction. It is also necessary to have a term for any factors which can be demonstrated to counteract changes in population density caused by irregular environmental changes, they are called stabilizing factors here. Andrewartha & Birch (1954) claim that such factors are not important, since animal populations are unstable and undergo repeated local extinction, but this is obviously not the case with many observed populations including those in the present study. Williamson (1957) has defined interspecific competition as the result of two or more species sharing a 'controlling' (stabilizing) factor, but this is a less practical definition since one does not usually know what is stabilizing populations, and one cannot conclude that two species were sharing a stabilizing factor if an increase in density of either is produced by removal of the other. Crombie (1947) and Williamson (1957) have considered the possibility of stable interspecific competition. It has been stated that one competitor always replaces the other (e.g. Brian 1956a, p. 341, and Park 1954, p. 223), but this is not the case, and only follows from simple mathematical models based on false premises or in some simple laboratory experiments. The terminology is straightforward. Processes are not given different names when they do not proceed to completion, for example chemical reactions and genetic polymorphism. Polymorphism is more than an analogous phenomenon, since at least one possible

THE Lamellibranch Spisula subtruncata (Da Costa) is reported in various old records as occurring abundantly in parts of the Clyde Sea Area. For example, in “The Mollusca of the Firth of Clyde”, 1878, p. 33, A. Brown writes: “Exceedingly abundant a little above low water in Ettrick and St. Ninian's Bays, Bute; and in Fintry Bay, Cumbrae. It is common also all along the Ayrshire coast, and in most sandy bays throughout the district. In Cumbrae they are known as ‘Aikens,’ and are used both for food and bait”. Further confirmation is found in the Medusa records and in the fauna and flora published for the British Association in 1901—records of almost thirty years age and older.

A series of tables lists land mammals common to North America and Eurasia, their probable times of migration by subepochs throughout the Cenozoic, and important groups that did not migrate in given subepochs. Major faunal interchanges occurred in early Eocene, late Eocene, early Oligocene, late Miocene, middle to late Pliocene, and Pleistocene. There was little or no interchange in the middle Eocene and middle to late Oligocene. Each interchange involved migration in both directions, but there was probably more movement from Eurasia to North America than in the opposite direction. All interchanges were selective, and they became increasingly limited from Eocene to Pleistocene. The most important selective influence was probably the relatively cold climate of the land connection. This connection was probably from Siberia to Alaska throughout the Cenozoic and was in almost continuous existence with important interruptions in parts of the Eocene and Oligocene and perhaps shorter interruptions at later times. The measurement of faunal resemblance in general is discussed, and mammalian faunal resemblance between Eurasia and North America charted for the Cenozoic. Changes in resemblance are correlated with five factors: faunal interchange, extinction of autochthones, divergent evolution, local extinction of migrants, and migration from other regions.