Gregory H. Adler

IMPACTS OF RESOURCE ABUNDANCE ON POPULATIONS OF A TROPICAL FOREST RODENT

Populations of Proechimys semispinosus (Central American spiny rat) were provisioned with fresh native fruits for 6 mo to test the hypothesis that populations of frugivorous rodents in seasonal Neotropical forests are not limited by food during the season of greatest resource abundance. Islands in the Panama Canal were used as experimental systems so that results would not be confounded by individuals commuting from the fringes of the study areas. Populations isolated on four islands were censused by monthly live-trapping and served as unmanipulated controls, while populations isolated on four additional islands were provisioned with fruit at 315.2 kg/ha and censused by similar methods. Natural fruit abundance was concurrently censused by counting numbers of fruiting trees and lianas to account for naturally available resources. Means of overall spiny rat density and density of known births were compared between treatment groups using repeated-measures analysis of covariance, with the density of fruiting trees and lianas as the covariate. Both variables showed a treatment effect, with higher-than-expected densities and densities of births within experimental populations based on natural resource abundance. Numbers of known births per adult female were compared between treatment groups by constructing a log-linear model. This model also revealed a treatment effect, with per capita production of young being higher within all experimental populations. Monthly survival rates of young and adults were compared between treatment groups by constructing separate linear models for young and adults, and no treatment effect was evident. Adult male body mass was compared among islands by analysis of variance and similarly showed no treatment effect. Results showed that these spiny rat populations were limited by food even during the period of high resource abundance, and increased densities were due to increased production and recruitment of young rather than to increased immigration or survival.

Removal of black palm (Astrocaryum standleyanum) seeds by spiny rats (Proechimys semispinosus).

A seed-removal experiment was conducted to assess the role of Proechimys semispinosus (Central American spiny rat) as potential predator and disperser of Astrocaryum standleyanum (black palm) seeds. One hundred fresh ripe A. standleyanum fruits were placed in semipermeable cages on each of 14 small islands in Panama, and seed disappearance rates were calculated for each island. Spiny rat density and biomass were determined by live-trapping on each island for 5 consecutive days and 4 nights. Censuses of fruiting trees were conducted on each island to control for effects of food availability on removal of palm fruits. Disappearance rates were related positively to total spiny rat density, density of adult and subadult spiny rats, and spiny rat biomass, but the density of fruiting trees accounted for very little variation. Spiny rats evidently prey heavily on A. standleyanum seeds, based on the high removal rates and on feeding observations of captive individuals. Since spiny rats may scatterhoard A. standleyanum seeds, they may also function as effective seed dispersers if seeds are removed to favourable germination sites unavailable to other seed predators. Results indicate that spiny rats, because of their abundance and wide distribution, may be important but overlooked predators and dispersers of A. standleyanum seeds and of other large-seeded tree species.

Fruit and Seed Exploitation by Central American Spiny Rats, Proechimys semispinosus

Central American spiny rats (Proechimys semispinosus) maintained in outdoor enclosures were fed fruits from 97 species of native and naturalized forest plants to examine patterns of fruit and seed exploitation. Individuals had very broad diets and consumed soft parts or seeds of all but three species of fruits included in the trials. Fruits and seeds were eaten even though individuals were provided with abundant alternative food. For most species of fruits that contained a fleshy aril or mesocarp, spiny rats generally consumed both seeds and fleshy parts. Spiny rats often exploited seeds with hard endocarps even when those seeds were surrounded by an abundant pulpy mesocarp or aril that was also eaten. Spiny rats also preferentially ate seeds of several large‐seeded species and primarily peeled off the mesocarp or aril. Seeds of species that contained no mesocarp or aril were also quickly eaten. Fruits from trees in the Moraceae were consumed entirely, including the seeds. Results indicate that, because o...

The island syndrome in isolated populations of a tropical forest rodent

I examined population traits of eight isolated populations of a tropical forest rodent (Proechimys semispinosus, the Central American spiny rat) for 1 year in central Panamá. Populations were sampled by monthly live-trapping, and seven traits (density, population growth rate, adult survival, reproductive effort, age structure, sex ratio, and body mass) were compared among populations. I also compared results with published data from nearby mainland populations. Each isolated population showed characteristics typical of island populations when compared with mainland populations, including higher and more stable densities, reduced reproductive effort, and greater body mass. Densities were the highest yet recorded for this species, and biomass of these island populations was among the highest of any tropical rodent yet studied. Population traits varied not only between island and mainland populations but also among island populations. P. semispinosus have traits that allow individuals in a population to rapidly respond to temporal changes in habitat quality or resource abundance. These traits include a high reproductive rate and an ability to adjust reproductive effort to changes in density. P. semispinosus are therefore able to quickly reach and maintain high densities under favorable conditions, thereby allowing close tracking of temporally and spatially varying resources. This flexibility is predicted for habitat generalists and presumably promotes abundance and persistence in temporally and spatially heterogeneous environments. P. semispinosus, often the most abundant and widely distributed species of rodent in forests throughout their geographic range, therefore have traits that are similar to those of generalist rodents in temperate forests.

Consumption of arbuscular mycorrhizal fungi by terrestrial and arboreal small mammals in a Panamanian cloud forest.

Abstract Fecal pellets collected from 10 small-mammal species captured in a Panamanian cloud forest were examined for presence of spores of arbuscular mycorrhizal fungi. Fifty-two percent of the 94 fecal samples examined contained spores of ≥1 of 6 arbuscular mycorrhizal fungi species, including Sclerocystis coremioides, Glomus fasciculatum, G. rubiforme, G. geosporum, and 2 unidentified Glomus species. G. fasciculatum was the most frequently encountered species, occurring in 87% of the fecal samples that contained spores and occurring in diets of 7 small-mammal species occupying terrestrial and arboreal habitats. Peromyscus mexicanus and Oryzomys devius frequently consumed arbuscular mycorrhizal fungi and may be important spore dispersers in terrestrial habitats. Arbuscular mycorrhizal fungal spores also were common in diets of the primarily arboreal rodent Reithrodontomys mexicanus, suggesting a potentially important role of this species in the dispersal of arbuscular mycorrhizal fungal spores to epiphytes of Neotropical cloud forests.

Impact of habitat degradation on phlebotominae (Diptera: Psychodidae) of tropical dry forests in Northern Colombia.

Abstract We examined changes in the phlebotomine fauna resulting from human intervention in a tropical dry forest of Northern Colombia where visceral and cutaneous leishmaniases are endemic. A natural forest reserve (Colosó) and a highly degraded area (San Andrés de Sotavento [SAS]) were sampled monthly for 8 mo using Shannon traps, sticky traps, and resting-site collections. Overall abundances were higher in Colosó (15,988) than in SAS (2,324), and species richness of phlebotomines was greater in the forest reserve (11 species) than in the degraded habitat (seven species). Fisher alpha, a measure of diversity, reinforced this trend. Both sand fly communities were dominated by Lutzomyia evansi (Nuòez-Tovar), vector of Leishmania chagasi (Cunha & Chagas), representing 92 and 81% of all captures in Colosó and SAS, respectively. Lutzomyia longipalpis (Lutz & Neiva), the common vector of visceral leishmaniasis, accounted for 4–7% of the sand fly community. Lutzomyia panamensis (Shannon) and Lutzomyia gomezi (Nitzulescu), putative vectors of Leishmania braziliensis (Vianna), had low abundances at both study sites. The zoophilic species Lutzomyia cayennensis (Floch & Abonnenc) and Lutzomyia trinidadensis (Newstead) were present in variable numbers according to trapping methods and site. Habitat degradation negatively affected sand fly communities, but medically important species were able to exploit modified environments, thereby contributing to Leishmania endemicity.

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.

Dynamics of Leishmania chagasi infection in small mammals of the undisturbed and degraded tropical dry forests of northern Colombia

The infection rate with Leishmania chagasi and the population dynamics of small mammals were studied in an undisturbed forest reserve (Colosó) and an area of highly degraded forest (San Andrés de Sotavento [SAS]) in northern Colombia, both endemic for visceral leishmaniasis. Live trapping of mammals was done every month, and species, age, sex and reproductive status determined. L. chagasi was detected in samples of skin or spleen by the polymerase chain reaction, after extraction of deoxyribonucleic acid using specific primers (DB8/AJS3), and dot blood hybridization. Didelphis marsupialis was found to be infected in Colosó (3/21, 14.3%) and SAS (13/137, 9.5%); its relative abundance was higher in SAS (93/113, 82% of the captures). Although Proechimys canicollis was also found to be infected in Colosó (3/34, 8.8%) and SAS (2/4), its relative abundance was much lower (4%) in SAS than in Colosó (56% of 77 animals captured). Sciurus granatensis, Marmosa robinsoni, Heteromys anomalus, Zygodontomys brevicauda and Metachirus nudicaudatus were less common, and no L. chagasi infection was detected in them.

Reproductive Phenology of a Tropical Canopy Tree, Spondias mombin1

Abstract We studied the reproductive phenology of isolated populations of Spondias mombin to determine the degree of flowering and fruiting synchrony among populations and to examine spatial and temporal variability in fruit production. The study was conducted on six small islands (1.8–3.5 ha) in the Panama Canal. All individuals (10 cm in diameter at breast height (DBH) were marked in 1991 and censused each year through 1994 to record recruits and deaths. All marked trees were censused monthly for flowering and fruiting activity from April 1992 through December 1995. Spondias mombin was seasonal and highly synchronous in flower and fruit production among islands and across years. Proportions of individuals fruiting varied among islands, years, and tree size classes. There was a positive relationship between probability of fruit production and tree size. Synchronous reproductive activity in S. mombin probably was due to responses to proximate environmental cues such as fluctuations in irradiance, but other factors must have been responsible for temporal and spatial variation in reproductive performance. We suggest that this variation may have been due partly to temporal and spatial differences in pollinator abundance.

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.