Rapid colonization and turnover of birds in a tropical forest treefall gap

Ecological disturbance is an important factor that influences the abundance and distribution of species. Treefalls are a prominent source of disturbance in tropical forests, but robust characterization of community change after treefalls requires baseline data that are often not available. We capitalized on 25 yr of avian mark–recapture data from a lowland moist forest in central Panama to investigate the timescale of colonization and persistence of birds in a newly formed treefall gap. We compared bird species assemblages pre- and post-treefall to explore how the disturbance affected specific foraging guilds and overall assemblage structure (abundance and alpha diversity). We documented rapid colonization (i.e., within five months post-treefall) of the treefall gap by birds. Abundance and alpha diversity increased following the treefall, but both remained relatively constant in a nearby control plot. At the guild level, frugivores spiked in abundance and nectarivores (i.e., hummingbirds) increased in alpha diversity following the treefall. These results are in agreement with those of previous spatial studies of gap dynamics and suggest that certain tropical frugivores and nectarivores have a remarkable ability to rapidly find and exploit preferred resources and microhabitats embedded in a landscape matrix. Assemblage abundance and alpha diversity decreased back to pre-treefall levels within 1 and 4 yr of the treefall, respectively. Thus, even large gaps may provide only ephemeral benefits, highlighting the importance of periodic disturbance for landscape-level persistence of species that use gaps.     

Host‐plant genotype mediates supply and demand of animal food in an omnivorous insect

1. Omnivorous predators can protect plants from herbivores, but may also consume plant material themselves. Omnivores and their purely herbivorous prey have previously been thought to respond similarly to host‐plant quality. However, different responses of omnivores and herbivores to their shared host plants may influence the fitness, trophic identity, and population dynamics of the omnivores. 2. The aim of the present study was to show that an omnivorous heteropteran (Anthocoris nemorum L.) and two strictly herbivorous prey species respond differently to different genotypes of their shared host plant, Salix. Some plant genotypes were sub‐optimal for the omnivore, although suitable for the herbivores, and vice versa. 3. The contrasting patterns of plant suitability for the omnivore and the herbivores highlight an interaction between plant genotype and omnivores' access to animal food. Plant genotypes that were sub‐optimal for the omnivore when herbivores were experimentally excluded became the best host plants when herbivores were present, as in the latter situation additional prey became available. By contrast, the quality of plant genotypes that were intrinsically suitable for omnivores, did not improve when herbivores were present as these plant genotypes were intrinsically sub‐optimal for herbivores, thus providing omnivores with almost no additional animal food. 4. The differential responses of omnivores and their prey to the same host‐plant genotypes should allow omnivores to colonise sub‐optimal host plants in their capacity as predators, and to colonise more suitable host plants in their capacity as herbivores. It may thus be difficult for Salix to escape herbivory entirely, as it will rarely be unsuitable for both omnivores and pure herbivores at the same time.     

Management for oak regeneration: Short-term effects on the bird community and suitability of shelterwood harvests for canopy songbirds

Interest in regenerating oaks (Quercus spp.) has promoted use of partial harvesting techniques that create an open forest structure. From 2007 to 2009, we studied songbirds in mixed-oak forests in southeastern Ohio, comparing shelterwoods recently harvested to 50% stocking and closed-canopy mature second-growth. We surveyed birds using distance-based methods (56 line transects in 18 stands at 4 forests). We intensively investigated suitability of shelterwoods for canopy-nesting species by examining habitat preferences, as measured by settlement patterns, age distributions, and site fidelity; we also examined nesting success. Several midstory and ground-nesting species were 26–73% less abundant in shelterwood than unharvested stands, whereas shrub-nesting species increased >100% several years post-harvesting. Canopy-nesting species were 31–98% more abundant in shelterwoods, but cerulean warbler (Setophaga cerulea) responses varied by forest. Patterns of settlement and site fidelity were generally similar among stands. Proportions of young males were actually greater for several species in shelterwood than unharvested stands, which may have been a consequence of young birds colonizing newly created (or improved) habitat. Even in our predominantly forested study system, nesting success (>700 nests) was low, ranging from 15% to 19% for yellow-throated vireos (Vireo flavifrons) and cerulean warblers, to 27–36% for scarlet tanagers, blue-gray gnatcatchers (Polioptila caerulea) and eastern wood-pewees (Contopus virens). However, nest survival did not differ between shelterwood and unharvested stands, possibly because numbers of avian predators did not change with harvesting. Despite increased numbers of brown-headed cowbirds (Molothrus ater) in shelterwoods, only 2% of canopy nests in which young could be identified were parasitized. Although these results suggest shelterwood harvests containing abundant overstory trees can provide short-term breeding habitat for canopy songbirds, long-term responses of birds to partial harvesting may differ from those documented here depending on different management options employed. Management for oak regeneration will typically remove all overstory trees later in the cutting cycle, initially resulting in loss of nesting substrates and hence breeding habitat for canopy songbirds. © 2011 The Wildlife Society.     

Alewife,rainbow smelt and native fishes in Lake Michigan: competition or predation?

Synopsis Several native fish species in Lake Michigan became rare or locally extinct during the increase of rainbow smelt (1930s) and alewife (1950s–1960s). These particular native species have pelagic eggs or larvae which were large relative to the zooplankton and which co-occurred with feeding alewife, smelt or both. Alewife, smelt and most of the other planktivores in the lake probably consumed eggs, and at least alewife and smelt are known to consume larvae. For each species, I comment on the probable role of predation on early life history stages in their decline. Several species which currently co-exist with alewife and smelt have shown large scale declines during the increase of the exotics. Recruitment declines were often dramatic and the probability of predation on these species during their early life history is evaluated.     

Habitat use and feeding behavior of thirteen species of benthic stream fishes

Synopsis The densities, habitat use, and feeding behaviors of 13 fish species belonging to the benthic invertebrate-feeding guild were studied by snorkeling at three localities in the Little River of eastern Tennessee, U.S.A. Resource partitioning occurred by habitat, feeding behavior and time of activity. Differences were also found at the generic level.Cottus was a nocturnal feeder, whereasPercina andEtheostoma were, for the most part, diurnally active.Percina moved about rapidly and spent most of its time above the bottom. In contrast,Etheostoma varied considerably in the amount of time spent under cover, spent little time above the bottom, and exhibited low levels of swimming activity. Nearly all species sought cover at night, suggesting they may be particularly sensitive to predation at night. Species with small adult sizes (Etheostoma, Cottus andP. evides) were concentrated in shallow water habitats, whereas species with large adult sizes (Percina) were more abundant in deep water habitats. The habitat use data are consistent with the hypothesis that size-selective predation by centrarchid bass may cause smaller fish to avoid deep water areas. Large species should have a lower risk of predation due to their size and behavior.