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Bird communities in transition: the Lago Guri islands.

INTRODUCTION

Notwithstanding a substantial literature dedicated to investigating species loss on land-bridge islands, to date there has been no concerted study of such islands in the period just subsequent to isolation, when it is presumed that changes in faunal composition occur most rapidly. In the analogous situation of habitat patches isolated by other habitat types, rapid species loss subsequent to isolation has been documented in several studies (Lovejoy et al. 1986, Robinson et al. 1992, Kattan et al. 1994, Stouffer and Bierregaard 1995). In "relaxing" from the initial supersaturated condition to an eventual dynamic equilibrium, habitat fragments and land-bridge islands must inevitably pass through a series of transitional states, characterized by faunal assemblages and species-abundance relationships that would not otherwise occur in nature (Brown 1971, Diamond 1972, Wilcox 1978, 1980, Terborgh and Winter 1980, Heaney 1984, 1986).

Here we report on the bird faunas of set of 7-yr-old land-bridge islands in Lago Guri, Venezuela, a 4300 [km.sup.2] hydroelectric impoundment on the Rio Caroni in the State of Bolivar. We ask whether the avian species composition of these islands is better understood as representing residual populations in a system driven primarily by extinctions, or as representing a dynamic balance maintained by high colonization pressure and turnover (Haila et al. 1993).

STUDY AREA AND METHODS

The Raul Leoni dam that contains the Guri impoundment was constructed in two stages. The first stage, completed in 1968, raised the water to 217 m above sea level from a base of 120 m. Construction of the second stage followed several years later and began, in 1983, to raise the water again. The final level of 270 m above sea level was attained in October 1986 (Morales and Gorzula 1986).

The formation of Lago Guri has created scores of forested islands that range in size over nearly four orders of magnitude from [less than]0.1 ha to [approximately equal to]650 ha (Alvarez et al. 1986). As of mid-1993, when the first censuses reported here were conducted, the islands had been isolated for 7 yr.

Twelve study islands were selected in April 1993 from a much larger number of candidate islands: four small ([approximately equal to]1 ha) "near" islands, four small "far" islands, three medium islands ([approximately equal to]10 ha; one "near," one "far," and one at an intermediate distance) and one large island, 350-ha Danto Machado. For our purposes, "near" refers to islands that are [less than or equal to]500 m from a larger landmass, and "far" refers to those located [greater than or equal to]1 km from a larger landmass. A nearby mainland "control" site was located on a peninsula that juts into the lake. We selected this situation to mimic as closely as possible the physical conditions characteristic of islands (exposure to edge and prevailing easterly winds).

The habitat of all study islands and the mainland is semideciduous tropical dry forest. The height of the canopy is mostly between 15 and 20 m, and only scattered trees rise above 25 m. Forest composition is somewhat patchy due to a history of fires in the region. Most of our sites had gentle topography and well-drained soils, with the exception of two islands, Miedo and Lomo, that were steep and somewhat rocky. The forests of the mainland were high graded for commercially valuable timber species before and during the construction of the Raul Leoni dam, but no logging took place on any of the islands. Further information on the structure and composition of the forest of Lago Guri can be found in Huber (1986).

To document the bird faunas of the study islands and mainland, we first employed the spot map method because it offered the potential of yielding comprehensive censuses of the small- and medium-sized islands. However, the spot map technique, in providing a unique record for each site, suffers from a severe sampling limitation in the lack of replicates. We therefore conducted a second round of censuses of all sites in 1995 using the point count method (Ralph et al. 1993). In 1993, described as a normal year by EDELCA (Electrificacion del Caroni) scientific staff, the rains began in early May and the canopy had leafed out by the middle of the month. However, 1995 was described as a particularly dry year, and some elements of the canopy (e.g., Bursera simaruba, Bursuraceae) had not yet leafed out by mid-June. In both years, however, fledglings of many species appeared in June.

Spot map censuses

We conducted spot mapping from 19 May to 27 June 1993, taking advantage of the peak breeding season. By the end of the period, each of the 12 islands and the control plot had been censused four times. Nesting activity at Lago Guri coincides with the period of leafing out of the semideciduous canopy, which, in turn, begins just before or in synchrony with the onset of the annual rainy season.

To facilitate spot mapping, we cut trail grids on all 12 islands and at the mainland control site. On small islands we simply cut two perpendicular trails that crossed in the center. On medium islands, the trails were circumferential and located 25-50 m inland from the shore, with a connecting trail across the center. On Danto Machado and the mainland we cut rectangular grids, the parallel legs of which were located 100-200 m apart, in keeping with the observation that the territorial song of most forest-dwelling bird species can be heard to distances of at least 100 m (Emlen and DeJong 1981). All of us had prior experience censusing birds in tropical South America, and the vocalizations of many Lago Guri species were known to us at the outset (Terborgh et al. 1990). Vocalizations that were new to us were quickly learned.

All small- and medium-sized islands were censused in their entireties. The bird communities of Danto Machado (the "large" island) and the mainland were sampled in single plots encompassing 23 and 26 ha, respectively. All censuses were conducted in the early morning hours, beginning at first light and ending around 0900, when vocal activity tended to subside. An individual observer normally covered two small islands, one medium island, or half of the Danto Machado or mainland plot in a morning.

In tabulating the results, we distinguished five categories of birds: (1) resident, forest-dwelling, and presumably breeding species; (2) visitors of presumably nonbreeding species; (3) species with large spatial requirements that often visited islands but were not resident on them; (4) edge species; and (5) aquatic species.

A male/pair was considered to be a resident only if it was recorded in the same area on two or more censuses. In most cases, resident individuals were recorded on three or all four censuses. Individuals or species that were recorded only once were regarded as visitors, with the exception of some night birds and hummingbirds that were not readily detected in the regular censuses and were counted as residents. Species with large spatial requirements, such as raptors, vultures, parrots, pigeons, swifts, and swallows occurred daily on or over nearly all islands, even the most isolated ones, but in most cases were probably not resident breeders. Such species are not included in the tallies unless there was evidence of nesting within an island's forest.

We distinguished edge species as a separate category because the margins of all landmasses in and around Lago Guri are occupied by "ghost forest," the boles and crowns of trees killed by inundation. Several groups of birds are strongly attracted to the dead trees - woodpeckers, woodcreepers, flycatchers, swallows, House Wrens, and a few others - and use them both for foraging and nesting. Edge species were not tallied with resident breeders because we felt that their presence was attributable to the ghost forest, and not to the living forest of the islands. Aquatic species, such as cormorants, darters, herons, rails, terns, and kingfishers, were not included within the reported census results.

Point counts

In the second round of censuses, we conducted point counts between 11 June and 28 June 1995. Each small island was represented by two 10-min counts from a position in the center where the trails crossed. One count was done early in the morning (before 0800), and the other later (after 0800). Because the small islands are widely scattered and accommodate only one point each, visits to successive islands often entailed 20-30 min of intervening travel. Medium islands were surveyed twice with four or five points each located 200-250 m apart along the trails (Ralph et al. 1993). The direction taken by the observer was reversed in successive surveys. Finally, Danto Machado and the mainland were represented by eight points each located [greater than or equal to]200 m apart, repeated once. In sum, the islands and mainland were collectively represented by a total of 38 points, each of which was censused twice.

Birds registered at each point were estimated, when first detected, to be less than or more than 50 m from the observer. The mean number of registrations within 50 m at each point was then converted to density per hectare by dividing by the area of a circle of radius 50 m = 0.79 ha. Bird densities on islands [less than]0.79 ha in area were also normalized to 1 ha.

Analysis of point counts

Point count results were analyzed by a nested ANOVA using the mean number of registrations [less than or equal to]50 m at each point as the basic response variable. Islands were grouped into four categories, which were then treated as fixed effects in the ANOVA model: (1) small, far islands, (2) small, near islands, (3) medium islands, and (4) large landmasses (Danto Machado and the mainland). Individual islands were then nested within island categories with "island" considered as a random effect. The various points censused on each landmass provided replicates; however, only one point was censused on each of the small islands, resulting in an unbalanced design. Analysis was performed on square-root (n+1) transformed means of the two replicate censuses at each point using only registrations at distances of [less than or equal to]50 m. The residuals were approximately normally distributed. Data were analyzed for all forest-dwelling species detected in the counts, and for truncated counts from which all pigeons and hole-nesting species had been removed.

Island areas

We surveyed and mapped the perimeters of all the small and medium islands with a hand-held compass to determine the areas occupied by forest. Distances between islands and between islands and the mainland were taken from a bathymetric chart of the lake prepared by EDELCA.
TABLE 1. Area and isolation of Lago Guri landmasses.

                                           Distance
                                              to
                                            nearest        Distance
                                            larger            to
                                           landmass        mainland
Locality                    Area (ha)        (km)            (km)

Small, far islands
Colon                        0.6(*)           1.8             4.2
Iguana                       1.4(*)           1.9             4.5
Miedo                        0.7(*)           2.6             3.2
Triangulo                    2.3(*)           1.0             4.9

Small, near islands
Bumeran                      0.9(*)           0.1             1.5
Palizada                     1.8(*)           0.4             0.4
Perimetro                    1.7(*)           0.5             3.4
Rocas                        0.6(*)           0.1             0.1

Medium islands
Corral (near)               12.3(*)           0.2             0.2
Lomo (intermediate)         12.0(*)           0.5             2.2
Panorama (far)              11.1(*)           1.5             1.9

Large islands
Danto Machado               23.1(**)          2.2(***)        2.2

Mainland
Terra Firme                 26.0(****)        0.0             0.0

* Area of island; entire island censused.
** Area censused; area of Danto Machado is [approximately equal
to]350 ha.
*** A similar-sized island lies 0.4 km to the west.
**** Area censused.




RESULTS

Geographical properties of the islands

Areas of the eight small islands ranged from 0.6 to 2.3 ha, and of the three medium islands from 11.1 to 12.3 ha (Table 1). Distances to the nearest larger landmass varied from 1.0 to 2.6 km for small, far islands, and from 0.1 to 0.5 km for small, near islands. Of the three medium islands, Corral was closest to the mainland (0.2 km), while the other two, Lomo and Panorama, were 2.2 and 1.9 km, respectively, from the mainland, but closer to an intervening large island with an area of [approximately equal to]150 ha. Danto Machado, the 350-ha "large" island, was [greater than]2 km from the mainland, but [less than]0.5 km from another, somewhat smaller island.

The water level of Lago Guri fluctuates on an annual cycle. The lake is drawn down for power generation during the dry season by as much as 17 m, and reaches its lowest level in May or June. The impoundment refills during the rainy season, which extends from May through September. At low water level, some of the islands we are studying are briefly connected to other islands, or to the mainland, by emergent necks of bare, unvegetated lake bottom (Bumeran, Corral, Perimetro, Rocas, Triangulo).

[TABULAR DATA FOR TABLE 2 OMITTED]

Spot map censuses: species numbers

During the spot map censuses of the 12 islands and mainland control plot, we recorded a total of 142 bird species, of which 128 occupy terrestrial habitats. The mean number of resident forest-dwelling species documented on small and medium islands, respectively, were 8.4 and 11.7 species, but the totals were highly variable within each class of islands. These numbers represent [approximately equal to]7 and 9% of the available pool of terrestrial species. The bird community of Danto Machado proved to be much richer, with 58 resident species tallied in the 23-ha census plot. The 26-ha mainland plot contained 43 resident species (Table 2).

Appreciable numbers of forest-dwelling visitors were registered in all censuses, and on several islands the number of species recorded as visitors exceeded the number of resident species (e.g., Corral). Visitors were especially prevalent on small and medium near islands [a cumulative total of 56 records of visitors vs. 43.5 pairs of resident forest birds (129%)]. Visitors were less frequent on small and medium far islands [31 visitors vs. 91.5 resident pairs (34%)]. The relative number of visitors recorded on Danto Machado and the mainland was markedly less than on the small and medium islands collectively [34 vs. 318.5 pairs of resident birds (11%)].

In contrast to resident and visitor species, resident edge species were evenly distributed, with a mean of 3.4 species over all small and medium islands. The Danto Machado and mainland plots, with longer censused perimeters, supported a mean of six resident edge species. Among the islands, only Lomo, which is steep-sided and lacks a fringe of ghost forest, did not support resident edge birds. Visitor edge species were relatively infrequent, with a mean of 2.1 species over all sites.

Spot map censuses: avian densities

The densities of resident, forest-dwelling species were highly variable among islands. In general, small islands supported higher densities of resident pairs per hectare (mean = 8.9 pairs; N = 8) than did the medium islands (mean = 2.1 pairs; N = 3), Danto Machado (8.3 pairs), or the mainland (4.9 pairs). In particular, two medium islands, Lomo and Corral, were nearly devoid of resident forest birds, harboring a mean of only [approximately equal to]1 pair/ha. In contrast, the resident populations of some of the small islands (when normalized to 1.0 ha) exceeded 10 pairs/ha.

The numbers of pairs of resident edge birds were quite consistent across the small islands (mean = 4.1 pairs; N = 8). Medium islands supported more variable numbers of edge pairs, in part due to the absence of edge species on Lomo. Danto Machado and the mainland, consistent with the longer perimeters of the census plots, supported more edge pairs than did most islands.
TABLE 3. Avian densities on Lago Guri islands and mainland via spot
maps and point counts, normalized to 1.0 ha.

                                                          Point
                                                         counts:
                                                          hole
                                        Point            nesters
                       Spot maps:      counts:             and
                         total          total            pigeons
                         census        census            removed
                       (mean no.      (mean no.         (mean no.
Landmass        N       prs./ha)       prs./ha)          prs./ha)

Small, far      4         10.0            7.7               4.0
Small, near     4          7.7            9.3               5.4
All small       8          8.9            8.5               4.7
Medium          3          2.1            4.1               2.9
Large           2          6.6            3.0               2.3




Point counts: avian densities

Analysis of square-root transformed data demonstrated significant differences in density among islands (F = 3.47, df = 3, 24, P = 0.0071), but not among island categories (F = 2.63, df = 3, 9, P = 0.11). The model accounted for 71% of the variation in the system. Deleting pigeons and hole nesters, the full model accounted for 70% of the variation, and significant differences were still found among islands (F = 4.96, df = 3, 24, P = 0.0008), but not among island categories (F = 0.73, df = 3, 9, P = 0.56).

Bird densities on the medium islands proved to be unusually variable, for special reasons to be explained below. If data from the medium islands are removed from the analysis, there are no longer significant differences between islands and a significant island category effect appears (F = 7.27, df = 2, 7, P = 0.02). However, when pigeons and hole nesters are removed, the island category effect disappears (F = 2.43, df = 2, 7, P = 0.16).

Results of the point counts agreed with those of the spot map censuses conducted 2 yr earlier in showing higher resident bird populations on small islands relative to the larger landmasses (Table 3). However, the two census methods did not agree in all details. Point counts generated higher densities for the medium islands and lower densities for both Danto Machado and the mainland than did the spot map censuses.

Species composition

No species was found on all islands. The five most widespread were Myiarchus venezuelensis (9/11 small and medium islands), Vireo olivaceus and Myiodynastes maculatus (8/11 islands), and Lepidocolaptes souleyettii and Leptotila verreauxi (7/11 islands). Apart from these five common species, the faunas of the small and medium islands were surprisingly variable. In all, 34 species, including 14 nonpasserines (41%) were resident on one or more of these islands (exclusive of edge species). The mean number of islands per species was 3.0 for the 11 islands. With only scattered exceptions, species were present on most islands as single pairs, resulting in a pronounced evenness of abundances.

The birds that inhabited the ghost forest around most of the islands, in contrast to those that occupied the living forest, comprised a relatively consistent set of species that prominently included Columba cayennensis, Dryocopus lineatus, Melanerpes rubricapillus, Pitangus sulphuratus, Troglodytes aedon, and Tyrannus melancholicus.

Many of the 34 species that were recorded as forest residents on one or more of the 11 small and medium islands, were recorded as visitors on other islands (detected on only one census). Moreover, we recorded an additional 24 species of visitors to the forests of the 11 islands. The majority of these were detected only once on just one or two islands. If one were to include visitors as well as resident species in comparing these islands, the differences between their communities would be far more striking than the similarities. With the exception of an owl (Pulsatrix perspicillata) twice flushed from Palizada, all species of birds that were recorded on small and medium islands, but not on Danto Machado or the mainland, were nonforest species (Camsiempis flaveola, Florisuga mellivora, Myiozetetes cayennensis, Myiozetetes similis, and Sporophila lineola).

DISCUSSION

The results contain four noteworthy features. (1) Small, 1-ha islands were found to support approximately twice the density of resident birds as the mainland control plot, determined either by spot mapping or point counts. Avian density on some small islands reached the equivalent of 12-16 pairs/ha. (2) Medium-sized ([approximately equal to]10 ha) islands, on average, supported only slightly greater numbers of resident species (mean = 11.7 pairs; N = 3) than did small islands (mean = 8.4 pairs; N = 8), negating the expectation of a steep species-area curve for this recently formed set of land-bridge islands (Wright et al. 1985).(3) Again, contrary to the findings of some other studies, the resident bird communities of the small and medium-sized islands did not form highly ordered nested sets. No species was resident on more than 9 out of the 11 islands in these two size classes, and only five species occupied more than 6 of the 11 islands. (4) Surprising numbers of transient, nonresident individuals of many species appeared and disappeared on nearly all islands, especially on ones located within 500 m of a larger landmass.

Density of resident birds

Small islands. - Bird densities (number of resident pairs of forest-dwelling species per hectare) averaged twice as high on the eight small islands as on the mainland control plot, although high interisland variance precluded a significant effect of "island category" in the ANOVA conducted on the point count data. On the smallest islands, resident species occupied areas of [less than] 1 ha, while on the mainland no species consistently occupied such small territories. These results contrast with those of Stouffer and Bierregaard (1995) who reported sharp declines of avian abundance and species richness in 1- and 10-ha forest fragments in Brazil.

High densities of resident birds on small Lago Guri islands, relative to the mainland, could have several possible explanations. One is that the density recorded at the mainland control site is in some way anomalously low, leaving the impression of high densities on the islands. Although we cannot exclude this possibility altogether, we did conduct two types of checks. First, in 1993 we conducted a cursory spot map census at a second mainland site, located [approximately equal to]5 km southwest of the first, and found densities similar to those at the thoroughly censused control site. Second, in 1995 we conducted two sets of point counts on the mainland, one at the site used for the 1993 spot map census, and the other at a locality [approximately equal to]2 km to the northeast. None of these four efforts to estimate mainland bird densities resulted in a value [greater than]4.9 pairs/ha, which is less than half the density recorded on most of the small islands.

A second possible explanation for high avian densities on small islands is that these islands were especially attractive to two categories of birds that were present in seemingly high numbers: cavity nesters and pigeons. A number of cavity nesters commonly nest in the ghost forest, among them, Dryocopus lineatus, Lepidocolaptes souleyettii, and Myiodynastes maculatus. These species conduct much of their foraging in the living forest, and were therefore counted as forest residents. Pigeons are strong fliers that regularly commute to distant food sources. They may have been attracted to small islands as potentially predator-free nesting havens. All significant effects of island category disappeared after the two groups of species were removed from the point count data, suggesting that there was no significant effect of island category independent of the presence of cavity nesters and pigeons on small islands.

Some previous studies of small-island bird communities have documented higher densities than those recorded on an adjacent mainland, and attributed the observations to "ecological release" (Crowell 1962, MacArthur et al. 1972, Yeaton and Cody 1974, Cox and Ricklefs 1977, Arcese et al. 1992). The term is usually employed in the context of exploitation competition, in which it is imagined that in the absence of mainland competitors, the species present may profit from unexploited resources and expand in numbers (George 1987). However, other possibilities are no less plausible in the case of the Lago Guri islands. One is that arthropod food resources might be absolutely more abundant on small islands due to the absence of army ants, or perhaps host-specific parasites of invertebrate folivores (Terborgh et al., in press). Alternatively, birds (e.g., pigeons) might be attracted by a perceived paucity of nest predators, thereby trading off greater commuting distance for improved nesting security.

Medium islands. - In a striking anomaly, two of the three medium islands, Corral and Lomo, held fewer resident pairs and species of birds than did some of the small islands, despite offering 10 times the area of habitat. Distinguishing these two islands from the other medium island (Panorama) and all the small islands is the presence of capuchin monkeys (Cebus olivaceus). Capuchins are also found on Danto Machado and the mainland. To test the possibility that capuchins were imposing high levels of nest predator pressure on Lomo and Corral, we deployed artificial nests stocked with Coturnix quail eggs on all the islands and the mainland. Nowhere were [greater than]30% of the nests raided, except on Lomo, where 100% were (Terborgh et al., in press). The capuchins of Corral may simply not have discovered the artificial nests. The situation is paralleled by the case of Barro Colorado Island, Panama, where high rates of depredation of artificial nests (Loiselle and Hoppes 1983) have been associated with high densities of a mesopredator (coati mundi) and the extinction of a number of bird species (Willis 1974, Terborgh and Winter 1980, Karr 1982).

These results offer circumstantial evidence that capuchins have decimated the bird communities of Lomo and Corral by imposing unsupportable levels of nest predation. If true, the high impact of capuchins on these islands can be understood as a consequence of ecological amplification. The capuchins of Lomo and Corral are confined to 11-12 ha, whereas troops of the same species on the Venezuelan mainland routinely occupy 150-200 ha (Robinson 1986).

Nested sets

Other studies of island groups have documented the occurrence of nested sets of species (Nilsson 1986, Bolger et al. 1991). Nested sets could arise in an archipelago in which successful colonization events were rare. In such a situation, ability to persist would largely determine which species occupied any island. Species with the greatest ability to persist would be the only occupants of the smallest class of islands, while those showing somewhat less persistence (or requiring a larger area to support a given population) would occupy the next larger class of islands, and so on (Pimm et al. 1988).

Instead of an orderly size-related pattern of species composition, the bird communities of Lago Guri islands show highly variable composition. One possible interpretation is that the variability is a transient condition in communities undergoing relaxation. Randomness in the order of extinctions on different islands might result in the observed lack of consistency in their species complements. However, a different interpretation seems more plausible.

Persistence may not be a compelling factor in regulating the species composition of small and medium Guri islands. Instead, variable composition may be generated by high rates of extinction and invasion from a large pool of potential colonists, as has been suggested for small islands in the Gulf of Finland under the term, "sampling colonization" (Haila et al. 1993). Spot map censuses recorded the presence of 58 species on the small and medium Guri islands (including visitors), while the mean species number for the two classes of islands, respectively, was only 9 and 12 species. High turnover of species and individuals is probable in light of the fact that a large majority of the populations of small Lago Guri islands consist of only a single pair.

High incidence of visitors

Visitors were so frequent that nearly every census turned up individuals of species not recorded at the site either previously or subsequently. A high flux of visitors, even through the "far" islands, may account for the lack of any apparent distance/isolation effects (no significant "island category" effect after removal of pigeons and hole nesters). Which species breed on a given island in a given year may have more to do with fluctuations in the size of mainland populations than with the properties of the island in question, since it is clear that a much larger number of species can potentially breed on small Guri islands than actually do on any island at a given time.

Most studies of land-bridge islands have assumed negligible rates of recolonization subsequent to the onset of isolation (Brown 1971, Diamond 1972, Terborgh 1974). It now appears that this assumption should be reexamined, although the colonization rates of islands distant enough to be over the horizon are unlikely to be as high as those documented here. Given the large number of avian immigrants observed on Lago Guri islands, and on small islands in Lake Gatun, Panama (Wright 1985), one must wonder why re-establishment of extinct birds on Barro Colorado Island, Panama has not been documented (Karr 1982, 1990).

A comment on methods

Finally, we wish to point out the benefits of employing both the spot mapping and point count methods of censusing birds. Unlike point counts, spot mapping does not lend itself to replication or statistical tests of differences between sites. Because of these deficiencies, the method has lately fallen out of favor. Nevertheless, spot mapping proved invaluable in this case by revealing the high flux of visitors through most of the censused sites, possibly the most interesting result of the study. Point counts are unable to distinguish between residents and visitors, a fact that leads to over-estimation of densities at sites where visitors are frequent. Otherwise, point counts will tend to underestimate densities, because not all territory holders at a point will be detected during a 5- or 10-min count.

CONCLUSIONS

Our findings suggest that a dynamic and variable community composition on small and medium Guri islands is the result of high extinction rates coupled with intense colonization pressure from a rich pool of potentially breeding species. In this scenario, the founding communities present when the waters of Lago Guri reached their final level have already collapsed and been reconstituted. Species loss on some islands may have been accelerated by relict populations of "mesopredators," such as the capuchins of Lomo and Corral (Soule et al. 1988). Changes in composition thus appear to have been driven by a combination of biological (nest predation) and stochastic (recurrent colonization and extinction) processes. Bird species numbers on the smallest islands may by now have settled down to an unusually dynamic equilibrium; further changes are no doubt in store for the medium and larger islands.

ACKNOWLEDGMENTS

We are grateful to Blgo. Luis Balbas of EDELCA for generous support with myriad logistical and administrative details. We thank EDELCA, CONICIT, and PROFAUNA for permits necessary to conduct the research in the Guri impoundment. Clemencia Rodner, Ana Rita Bruni, and Viviana Salas of Audubon de Venezuela are warmly thanked for generous hospitality and vital assistance of many kinds. We are grateful to Lisa Davenport for aiding with the point counts and to Marty Jarrell for help with data processing. Miles Silman and Susan Paulsen provided invaluable statistical expertise. The research was supported through a MacArthur Fellowship to J. Terborgh.

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Author:Terborgh, John; Lopez, Lawrence; S, Jose Tello
Publication:Ecology
Date:Jul 1, 1997
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