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Small mammals, particularly rodents, are often important seed-dispersal agents in Neotropical forests. Directional seed dispersal into tree-fall gaps may enhance seedling survival of light-demanding species and thus influence forest regeneration. To examine this proposition, we tracked seeds of a light-demanding palm ( Attalea butyracea ), with a focus on spiny rats ( Proechimys semispinosus ), the most-likely seed-removal agents. We established seed-removal stations at three distances relative to 28 gaps (gap center, gap edge, and intact forest 10 m from a gap edge) in a lowland forest in Central Panama. We placed five fresh fruits (with their seed) in semi-permeable exclosures to exclude larger mammals at each station and tracked the directions in which seeds were moved and deposited intact. More seeds were moved toward or into gaps when removed from gap center or edge stations; however, seeds dispersed from intact forest stations showed no such directionality. Small mammals may have dispersed seeds into and within tree-fall gaps because they favored caching seeds in areas that offered increased cover, which is typical of gaps, and consequently protection from predation. The lack of directional dispersal from intact forest stations may have been because spiny rats were able to find sufficient cover in the young intact forest that was closer than the gaps. In older forest, the contrast between intact forest and gaps may be greater, resulting in directed dispersal into gaps.

Acer negundo (Boxelder) is a common tree in eastern North America. Fruits are paired samaras that mature in late summer but persist on trees for months. We examined temporal variation in predation rates by small mammals by attaching samaras to sewing bobbins in a forest in northeastern Wisconsin. We placed samaras in groups of 5 along transects and checked them regularly across 2 fall and 2 spring seasons to determine seed fates. Of the 6702 seeds, 15.26% were consumed, 81.41% were left intact, and 3.33% were aborted. Predation rates were higher in fall than in spring. Granivorous mammals did not secondarily disperse seeds but instead acted solely as seed predators. Releasing seeds over a longer period of time than other species of maples allows trees to increase seed survival by experiencing varying soil conditions and predation rates. Retained sterile fruits may act as decoys that decrease predation rates.

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.

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 <3 m and deposited with the fruit eaten and the seed intact. However, some seeds did experience relatively long-distance dispersal (>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.

Echimyid rodents are widely distributed and abundant throughout most of the Neotropics. I review the available information on spacing patterns and mating systems and develop a general framework for understanding variation among populations and species. In most studied species males have larger home ranges than do females, and overlap with either sex can range from 0% to nearly 100%. Males typically display more overlap with both other males and females than do females. Although lack of overlap has been interpreted as territoriality, most such cases have been from populations at low densities. Individuals of those same species show considerable overlap at higher densities. I suggest that individuals typically are not territorial but instead merely space themselves out when vacant space is abundant. Individuals are then facultatively monogamous because multiple mates are not available. As densities increase male home ranges overlap more simply because their home ranges are larger than those of females. Males then can acquire multiple mates, and polygyny becomes evident. At the highest densities multiple mates are available for both males and females, and promiscuity results. The mating system within a given population of many echimyids therefore depends upon density, which in turn is determined primarily by resource availability.

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.

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.

Seed dispersal and predation by spiny rats (Proechimys semispinosus) and red-tailed squirrels (Sciurus granatensis) were studied in five forest gaps and adjacent forest understorey on Barro Colorado Island, Panama, during June and July 2003 and January 2004. Live trapping on grids was used to estimate the abundances of rats in both habitats and was conducted during 10 nights at the beginning of each field season. We captured 70 individual spiny rats (53 in forest gaps and 17 in understorey). To address the removal and dispersal of seeds, semi-permeable wire-mesh exclosures were set at randomly selected stations within the trapping grids. These exclosures allowed the entry of spiny rats, squirrels, and other small rodents but excluded larger granivorous mammals. Seeds were marked with pieces of string that were dusted with fluorescent powder and tracked at night with an ultraviolet light. A total of 304 seeds was removed (159 in forest gaps and 145 in understorey). Spiny rats removed more seeds from gaps, and squirrels removed more seeds from understorey. Both species dispersed more seeds during the rainy season than during the dry season. Squirrels dispersed seeds farther than did spiny rats. Due to their foraging behaviour and association with gaps, spiny rats may disperse seeds from forest understorey into gaps.

Aim We examine the influence of geography on species richness and endemism of mammals on tropical Pacific archipelagos to determine the importance of intra- and inter-archipelago speciation in promoting local and regional species richness. Location Thirty tropical Pacific archipelagos. Methods A distributional list of mammals on 30 archipelagos was compiled, and values for 10 geographical variables were estimated for each archipelago. Mammal species were placed in three different categories (continental, Pacific and endemic) based on their distribution. The total number of species and numbers of species within each category were related to the geographical variables using Poisson regression analysis. Results Species richness was related positively to variables describing land area, numbers of large islands and elevation; and negatively to variables describing isolation. Levels of endemism did not differ between volant and non-volant species, but differed between mega- and microchiropterans. Main conclusions Variation in species richness of mammals in the tropical Pacific region can be accounted for by a combination of intra-archipelago speciation within archipelagos composed of large islands, and inter-archipelago speciation, particularly among more isolated archipelagos. Mammals were less widely distributed throughout the study area than previously found for butterflies, skinks or birds. However, the level of endemism was similar to that of skinks and birds on the same archipelagos, and was higher than that of butterflies.

It is now understood that alterations in the species composition of soil organisms can lead to changes in aboveground communities. In this study, we assessed the importance of spatial scale and forest size on changes in arbuscular mycorrhizal fungal (AMF) spore communities by sampling AMF spores in soils of forested mainland and island sites in the vicinity of Gatun Lake, Republic of Panama. We encountered a total of 27 AMF species or morphospecies, with 17, 8, 1 and 1 from the genera Glomus, Acaulospora, Sclerosystis, and Scutellospora, respectively. At small scales (<100 m₂), we found little evidence for spatial structuring of AMF communities (decay of Morisita-Horn community similarity with distance). However, at large spatial scales, we found that the AMF spore community of a mainland plot was more similar to other mainland plots several kilometers (>5) away than to nearby island plots (within 0.7 km). Likewise, most island plots were more similar to other island plots regardless of geographic separation. There was no decay in AMF species richness (number of species), or Shannon diversity (number of species and their spore numbers) either with decreasing forest-fragment size, or with decreasing plant species richness. Of the six most common species that composed almost 70% of the total spore volume, spores of Glomus \"tsh\" and G clavisporum were more common in soils of mainland plots, while spores of Glomus \"small brown\" and Acaulospora mellea were more abundant in soils of island plots. None of these common AMF species showed significant associations with soil chemistry or plant diversity. We suggest that the convergence of common species found in AMF spore communities in soils of similar forest sizes was a result of forest fragmentation. Habitat-dependent convergence of AMF spore communities may result in differential survival of tree seedlings regenerating on islands versus mainland.