Thomas D. Lambert

AMAZONIAN SMALL MAMMAL ABUNDANCES IN RELATION TO HABITAT STRUCTURE AND RESOURCE ABUNDANCE

Abstract Previous studies in tropical rain forests suggest that most small mammal species reach their highest densities in disturbed habitats; however, only a few sites have been examined. Consequently, habitat and resource use for many species is poorly understood. This is especially true in the Amazon Basin, where no studies of microhabitat associations of small mammals have been undertaken. We studied relationships with habitat variables and resource abundances for 5 species of marsupials and 9 species of rodents at a site in southeastern Amazonia. Small mammals were sampled with traps placed both on the ground and in the understory. Eight habitat variables were measured to quantify habitat structure. Measures of insect biomass were collected by the use of sticky traps, and fruit abundance was quantified. Patterns of habitat use were examined using logistic regression, multiple regression, and ordinations. Many species showed increased abundances with habitat features indicative of edge-affected or disturbed habitats, showing negative relationships with understory openness, understory woody-stem density, tree density, and tree size; and positive relationships with number of vines per tree, mean log size, number of logs, and volume of downed wood. We obtained support for the hypothesis that the cause of this pattern is increased resource abundances in these areas, because both insect biomass and number of fruiting trees showed similar relationships. However, for many species, measures of resource abundance were not important once habitat features were entered into the models, indicating that the relationship to resources is an indirect one.

Rodents on tropical land‐bridge islands

The results are reported of a survey of rodents on 10 forested land-bridge islands ranging in size from 0.2 to 350 ha in the state of Bolivar, Venezuela. The islands were contained within a lake formed c. 12 years before the study by the damming of the Caroni River for hydroelectric power. Rodents were sampled on each island by live-trapping along transects that sampled all available habitat types on each island, and microhabitat structure was measured at each trap station. A total of 674 captures of 359 individuals of six species of rodents was recorded. Species composition changed from the largest to the smallest islands, and small and medium islands (0.2–11 ha) displayed the typical effects of insularity, with fewer species and increased abundances and biomass. The largest island (350 ha) seemed to function more like a mainland. Most species were associated with a suite of microhabitat variables. It is suggested that release from top-down control by predators was responsible for higher abundances and biomass on the smaller islands and that predators moving between large islands and other nearby landmasses help maintain a mainland community structure on large islands. However, changes in species composition on smaller islands may be the result of patchy occurrences of some species before isolation, changes in microhabitat structure following isolation, and species-specific microhabitat requirements.

VARIATION IN SMALL MAMMAL SPECIES RICHNESS BY TRAP HEIGHT AND TRAP TYPE IN SOUTHEASTERN AMAZONIA

Abstract Despite its biological richness, the rodent and marsupial fauna of many parts of the Amazon Basin remains poorly known and the efficacy of different methods in assessing its diversity are poorly understood. We present results of small mammal trapping at a previously unsurveyed site in the Xingú Basin of the southeastern Amazon, Pará, Brazil; provide details on a new method for arboreal trapping; and compare species richness among traps at different heights and between different trap types. Mammals were livetrapped at 3 trap heights: ground, understory (1–3 m above ground), and canopy (mean height = 11.5 m ± 3.2 SD, range = 4.8–16.8 m, n = 76 trap stations). We recorded 1,769 captures of 1,178 individuals of 8 marsupial and 17 rodent species in 38,090 trap-nights (or 19,320 station-nights, where a station-night consisted of 1 Sherman and 1 Tomahawk live trap at 1 height for 1 night). Overall trap success was 6.1 individuals per 100 station-nights; success by trap position was 9.0% for ground (7,850 station-nights), 4.7% for understory (7,850 station-nights), and 2.9% for canopy (3,490 station-nights). Success by trap type was 4.7% for Shermans and 5.9% for Tomahawks, with Tomahawk traps showing a more rapid accumulation of species. Rarefied species accumulation curves showed little improvement with the inclusion of canopy trapping, which we attribute to high variability in the development of vertical structure at the site. We suggest that in areas with low and often-broken canopies, only ground and understory traps need be employed for long-term studies; however, some form of canopy trapping should be used during initial surveys so that the utility of arboreal trapping can be evaluated.

MICROHABITAT USE BY A TROPICAL FOREST RODENT, PROECHIMYS SEMISPINOSUS, IN CENTRAL PANAMA

Abstract We studied microhabitat use by Proechimys semispinosus (Central American spiny rat) by livetrapping along transects in 5 forested sites in central Panama. Microhabitat was quantified by measuring 14 variables at each sampling station. Relative abundance of P. semispinosus was 1.86 individuals/100 station-nights, which was typical of abundances of this species and other members of the genus. Spiny rats were distributed throughout sampled microhabitat space defined by factor analysis of 14 variables. We used logistic regression analysis to relate presence–absence at trap stations to microhabitat structure. Variables describing disturbed and younger forest were strong predictors of presence at trap stations. Associations of P. semispinosus with younger forest (smaller trees and lianas and lower canopies) and treefall gaps within older forest may have implications for regeneration of local forest patches through the activities of these rodents as seed predators and dispersers of seeds and arbuscular mycorrhizal fungi spores.

Does watching a monkey change its behaviour? Quantifying observer effects in habituated wild primates using automated radiotelemetry

In studies of animal behaviour, researchers have long been concerned that their presence may change the conduct of their study subjects. To minimize observer effects, researchers often habituate their study animals. The premise of this method is that, with sufficient neutral exposure, animals will stop reacting to humans. While numerous studies demonstrate that negative responses to humans decrease over time, the fact that an animal does not flee from or behave aggressively towards observers cannot be taken as evidence that it is not altering its behaviour in other, more subtle ways. Because remotely monitoring the behaviour of wild animals is difficult, it has not been possible to answer the critical question: do habituated animals change their behaviour when researchers are present? Here, we use data from an automated radiotelemetry system that remotely monitored the movement and activity of radiocollared animals to test whether observers affected the behaviour of seven habituated white-faced capuchins, Cebus capucinus. We found no evidence that observers influenced the ranging behaviour or activity patterns of their study subjects. Capuchins did not move faster, stop to rest less frequently, or display higher levels of activity when they were being followed compared to when they were alone. It has been suggested that researchers may embolden habituated study subjects, artificially increasing their relative dominance, but we found no relationship between observer presence and proximity to neighbouring social groups. Although it remains possible that observer effects existed but were too subtle to be detected with the remote sensing technology we used, the results of this study nevertheless provide reassuring evidence that humans can observe habituated wild animals without overly influencing the animals’ activity and movement patterns.

ECOLOGICAL CORRELATES OF TRAP RESPONSE OF A NEOTROPICAL FOREST RODENT, PROECHIMYS SEMISPINOSUS

Patterns of trap response in the Central Anmerican spiny rat (Proechirnys seniispitioslis), a frugivorous forest rodent, were examined in relation to age, sex, density and resource abundance. Populations on four small islands (designated nunmbers 4, 8, 14, anid 52) in Gatuni Lake (central Panama) were sampled by live-trapping for four nights every month for four years. Trappability was calculated as the proportion of all individuals known to be alive that were actually captured. Mean body mass at first capture was within the range of subadult body masses and differed among islands but not between sexes. Trappabilities summed over the entire study were generally low and differed among islands and age classes (juveniles, subadults and adults) and between sexes. In general, adults were more trappable than juveniles and subadults, and fenmales (particularly adults) were more trappable than males. Trappability and the nunmber of captures per individual varied seasonally and were generally greatest during December and January. Monthly estimates of these two variables were examined for autocorrelation, and the order of the autoregressive error model was determined separately for each island population. The arcsine of trappability anid the number of captures per individual were regressed on log,0 densities of spiny rats and of fruiting trees and lianias after controlling for autocorrelation. Both trappability indices were negatively related to the density of fruiting trees and lianas on islands 8 and 14, indicating that spiny rats may be seasonally food-stressed. The number of captures per individual was related positively to the density of spiny rats on island 52 and negatively on island 8. Results indicate that spiny rat populations must be sampled with greater intensity and duration than temperate forest rodenits.

Seed dispersal by rodents in a lowland forest in central Panama

We studied the removal of seeds of three species of large-seeded tree ( Astrocaryum standleyanum, Attalea butyracea and Dipteryx oleifera ) from three different heights within six study plots in a lowland forest in central Panama. Fresh fruits with intact seeds fitted with industrial sewing bobbins were placed within semi-permeable exclosures. Removed seeds were tracked to deposition sites, and seed fate was determined. Removals were likely perpetrated by two small rodents, the strictly terrestrial Proechimys semispinosus and the scansorial Sciurus granatensis , because they were the most abundant small rodents in the study site during the study period and were of sufficient size to remove large seeds. Rodent abundance and fruit availability were estimated by conducting censuses. Nine microhabitat variables were measured at each deposition site to determine if these two rodents were preferentially depositing seeds in sites with certain characteristics or were randomly depositing seeds. During the study, rodents handled 98 seeds, 85 of which were not predated upon and could potentially germinate. Removal rates were not influenced by rodent abundance or fruit availability. Seeds were most frequently moved 10 m). Rodents preferentially deposited seeds in locations with large logs (>10 cm diameter), dense herbaceous cover, and an intact canopy. The number of large logs was different from random locations. Despite not being able to determine long-term fate (greater than c . 1 y), we show that these small rodents are not primarily seed predators and may in fact be important mutualists by dispersing seeds relatively long distances to favourable germination sites.

Influence of seed height on removal rates by rodents in central Panama

No abstract available

Habitat structure influences refuge use by two sympatric species of Neotropical forest rodents

Terrestrial echimyid rodents typically use subterranean cavities and hollow logs as refuges. We examined habitat characteristics at refuges used by 2 such species of rodents, Proechimys semispinosus and Hoplomys gymnurus, to better understand coexistence of these morphologically similar sympatric species. The study was performed in a lowland moist forest in central Panama from 2009 to 2013. Sewing-bobbin tracking was used to locate refuges. We examined refuge habitat at 2 scales. At a coarser scale (microhabitat), we measured 10 variables within a 5-m radius around each refuge site and an associated random point, which represented available habitat. At a finer scale (refuge entrance), we measured 11 variables within a radius of 0.5 m centered on each refuge and an associated random point. We used discriminant function analysis to search for differences between refuge and available habitats at both scales and for both species separately. We located 70 refuges of both species. Refuges used by P. semispinosus were positively associated with litter depth, canopy openness, numbers and sizes of nearby logs, and proximity to tree-fall gaps, which all indicate forest disturbances. Hoplomys gymnurus refuges did not show any differences from available habitat. Lastly, we found no differences in refuge use between the sexes or age classes for either species. We conclude that interspecific competition for refuges is generally unlikely to be important because small-scale differences in refuge habitats facilitate coexistence.

First record of Alston's Woolly Mouse Opossum (Micoureus alstoni) from the canal area of Central Panama

No abstract available