John Terborgh

Trophic Downgrading of Planet Earth

Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind’s most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

Drought sensitivity of the Amazon rainforest

Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 × 1015 to 1.6 × 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.

Five New World Primates : A Study in Comparative Ecology

Launching a new series, Monographs in Behavior and Ecology, this work is an intensive study of five species of New World monkeys--all omnivores with a diet of fruit and small prey. Notwithstanding their common diet, they differ widely in group size, social system, ranging patterns, and degree of territorialityOriginally published in 1984.The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

Structure and Organization of an Amazonian Forest Bird Community

To help fill the gap in detailed knowledge of avian community structure in tropical forests, we undertook a census of a 97—ha plot of floodplain forest in Amazonian Peru. The plot was censused over a 3—mo period spanning the 1982 breeding season. The cooperative venture entailed ≈12 person—months of effort. Conventional spot—mapping was the principal method used, but several additional methods were required to estimate the numbers of non—territorial and group—living species: direct counts of the members of mixed flocks, saturation mist—netting of the entire plot, opportunistic visual registrations at fruiting trees, determination of the average size of parrot flocks, color banding of colonial icterids, etc. Two hundred forty—five resident species were found to hold territories on the plot, or to occupy all or part of it. Seventy—four additional species were detected as occasional—to—frequent visitors, wanderers from other habitats, or as migrants from both hemispheres. By superimposing territory maps or t...

Oddity and the ‘confusion effect’ in predation

Abstract We report on two sets of experiments designed to clarify the roles of sensory ‘confusion’ and prey ‘oddity’ as they interact to influence the hunting success of a pursuit predator, the largemouth bass ( Micropterus salmoides ), on silvery minnows ( Hybognathus nuchalis ). Bass quickly captured solitary minnows, but performed many unsuccessful attacks and took much longer to make a capture as prey school size was increased. At school sizes of eight and above, bass were effectively stymied, demonstrating the ‘confusion effect’. The inclusion of one or two ‘odd’ (blue-dyed) minnows in a school of eight greatly increased the ability of bass to capture both normal and odd prey, but this effect of oddity disappeared at a school size of 15. The implications of these results for understanding the adaptive basis of mixed species flocks, herds and schools is discussed.

DOMINANCE AND DISTRIBUTION OF TREE SPECIES IN UPPER AMAZONIAN TERRA FIRME FORESTS

Amazonian forests are the largest and most diverse in the tropics, and much of the mystery surrounding their ecology can be traced to attempts to understand them through tiny local inventories. In this paper we bring together a large number of such inventories scattered across immense areas of western Amazonia in order to address simple questions about the distribution and abundance of tropical tree species in lowland terra firme forests there. The goal is to describe patterns of commonness and rarity at local (1 ha), landscape (∼104 km2), and regional (>106 km2) scales, and to fuse the results into a more complete picture of how tropical tree communities are structured. We present estimates of landscape-scale densities for ∼1400 taxa, based on data from tree plots scattered over large tracts of terra firme forest in eastern Ecuador and southeastern Peru. A database of morphological, ecological, and other traits of >1000 of these species compiled from the taxonomic literature is then used to explore how species that are common in the inventories differ from species that are rare. Although most species show landscape-scale densities of <1 individual/ha, most trees in both forests belong to a small set of ubiquitous common species. These common species combine high frequency with high local abundance, forming predictable oligarchies that dominate several thousand square kilometers of forest at each site. The common species comprising these oligarchies are a nonrandom subset of the two floras. At both sites a disproportionate number of common species are concentrated in the families Arecaceae, Moraceae, Myristicaceae, and Violaceae, and large-statured tree species are more likely to be common than small ones. Nearly a third of the 150 most common tree species in the Ecuadorean forest are also found among the 150 most common tree species in the Peruvian forest. For the 254 tree species shared by the two data sets, abundance in Ecuador is positively and significantly correlated with abundance ∼1400 km away in Peru. These findings challenge popular depictions of Amazonian vegetation as a small-scale mosaic of unpredictable composition and structure. Instead, they provide additional evidence that tropical tree communities are not qualitatively different from their temperate counterparts, where a few common species concentrated in a few higher taxa can dominate immense areas of forest. We hypothesize that most Amazonian forests are dominated at large scales by oligarchies similar in nature to the ones observed in Ecuador and Peru, and we argue that the patterns are more indicative of regulation of relative abundances by ecological factors than of nonequilibrium chance-based dynamics. The paper concludes with a discussion of the practical applications of predictable oligarchies over large areas of unexplored forest.

Preservation of Natural Diversity: The Problem of Extinction Prone Species

with which we share this planet. Some day, of necessity, man must reestablish a steady state with nature. Where on the scale of population this steady state will lie and how we shall arrive there are difficult questions that are being forced upon us with an ever greater urgency by an accelerated pace of world events. With certainty the stressful transition period which we are entering now will impose extraordinary pressures on virtually all existing species, not the least on our own.

Distribution on Environmental Gradients: Theory and a Preliminary Interpretation of Distributional Patterns in the Avifauna of the Cordillera Vilcabamba, Peru

A new theoretical approach to the study of distribution is presented in this paper. The central concerns the types of interactions between organisms and their surroundings which may function to impose limits on the occurrence of species on a smooth unifactorial environmental gradient. The theory is constructed of a set of three complementary and mutually exclusive models which arbitrarily are given the power of accounting for all possible distributions. Each of the models predicts a different patter of distribution within a group of organisms and each contains two or more unique features which serve to distinguish it from the others. In their simplest form the models state that the occurrence of species is limited respectively by: (i) physical or biological conditions that vary in parallel with the measured gradient, (ii) competitive exclusion and (iii) environmental discontinuities (ecotones). Predictions of each model include (a) the shape of population density curves, (b) the shape of congruity (faunal attenuation) curves, (c) distributional patterns at the termini of gradients, and (d) the form oft he frequency distribution of ecological amplitudes. Application of the theory is demonstrated with data obtained in a study of the distribution of bird species on a uniform elevational gradient in the Eastern Andes of Peru. A series of four expeditions to the Northern Massif of the Cordillera Vilcabamba, a vast undisturbed mountain wilderness, provided information on the upper and lower limits of occurrence of over 410 species of forest birds. Faunal composition and the relative abundances of many species wee estimated at each of 15 stations through large netted samples of birds (170—604 individuals). Following a protocol described in the text, the upper and lower limits of 261 species were assigned to one or another of the three models. Certain limitations of method result in a small number of unavoidable errors in these assignments; hence the outcome of the partitioning procedure is only a first approximation. As evaluated by this preliminary analysis, the three mechanisms of distributional limitation differ appreciably in their importance in the Vilcabamba avifauna. Ecotones account for less than 20% of the distributional limits, competitive exclusion for about one—third of the limits and gradually changing conditions along the gradient for about one—half of the limits.

Bird Species Diversity on an Andean Elevational Gradient

This paper analyzes patterns of bird species diversity on an elevational transect of the Cordillera Vilcabamba, Peru. Major changes in climate and vegetation are encompassed by the tran- sect which extended from the Apurimac Valley floor at 500 m to the summit ridge of the range at > 3,500 m. Four vegetation zones are easily discerned-lowland rain forest, montane rain forest, cloud forest, and elfin forest. In progressing upwards there is a monotonic trend toward decreasing canopy stature and reduced number of plant strata. The vegetation gradient provided the opportunity to examine the relation between bird species diversity and habitat complexity in an entirely natural setting. The decrease in forest stature with elevation was closely paralleled by decreasing avian syntopy (the total number of bird species cohabit- ing the forest at a given elevation). Bird species diversity was shown to be highly correlated with foliage height diversity, using either four or five layers in the foliage height diversity calculation (r = .97), and less well correlated using three layers, as defined previously by MacArthur (r = .84). At this superficial level the trend in bird species diversity seemed to be adequately explained as a response to the vegetation gradient. This preliminary conclusion was found to be illusory when the elevational trend in syntopy was reexamined separately for three major trophic subdivisions of the fauna. The number of insectivores decreased 5.2-fold from the bottom to the top of the gradient, frugivores decreased by a factor of 2.3, and nectarivores showed no change. It was now clear that the diversity in each of these trophic categories was responsive to environmental influences other than, or in addition to, the gradient in habitat structure. Additional factors implicated by the available evidence are competitive interactions with other taxa at the same trophic level, changing composition of the resource base as a function of elevation, and declining productivity at high elevations. Analysis of netted bird samples revealed an unexpected diversity maximum in the lower cloud forest zone. The immediate cause of this was a relaxation of the vertical stratification of foraging zones, such that an anomalously large fraction of the species present entered the nets. The excess diversity was found to consist almost entirely of insectivores. Several factors appear to contribute to the ultimate causes of the diversity maximum: greater patchiness of the montane forest due to the rugged topogra- phy, a higher density of foliage near the ground, and possibly increased resource productivity. A correlation between diversity and density in the netting results suggested a causal connection mediated via resource levels. The conclusion is that diversity is a complex community property that is responsive to many types of influences beyond simply the structure of the habitat.

TREE SPECIES DISTRIBUTIONS IN AN UPPER AMAZONIAN FOREST

Not a single tree species distribution in the Amazon basin has been reliably mapped, though speculation regarding such distributions has been extensive. We present data from a network of 21 forest plots in Manu National Park, Peru, totaling >36 ha and sited over an area of -400 km2, to explore how tree species are distributed across upper Amazonia at a variety of spatial scales. For each of 825 tree species occurring in the plots we asked three questions: (1) Does the species have a large or small geographic range? (2) Is the species restricted to a single forest type, or is it found in several? (3) Is the species locally abundant anywhere or is it scarce everywhere? The answers served to classify a subset of species under Rabinowitzs classification scheme for rare species. Three main conclusions emerged. First, the great majority of tree species at Manu are geographically widespread. Every species identified to date occurs elsewhere in South America, outside the department of Madre de Dios; more than two-thirds of them have been collected 1500 km away in Amazonian Ecuador. Second, 15-26% of species appear to be restricted to a single forest type, when forest types are defined by historical river dynamics (i.e., terra firme forest, mature floodplain forest, swamp forest, and primary successional floodplain forest). The proportion of restricted species declined with increasing sampling effort, mak- ing 15% a more reliable figure. Third, while 88% of species occurred at densities of 1.5 individuals/ha. Extrapolating these results provides a first guess at how tree species are distributed across the western portion of the Amazon basin. We conclude with the suggestion that most tree species in the region are habitat generalists occurring over large areas of the Amazonian lowlands at low densities but large absolute population sizes.