Nicholas L. Rodenhouse

Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.

SITE‐DEPENDENT REGULATION OF POPULATION SIZE:A NEW SYNTHESIS

The nature and extent of population regulation remains a principal unanswered question for many types of organisms, despite extensive research. In this paper, we provide a new synthesis of theoretical and empirical evidence that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability. The sites may be territories, refuges from predation, oviposition sites, etc. The mechanism, which we call site dependence, is not an alternative to density dependence; rather, site dependence is one of several mechanisms that potentially generate the negative feedback required for regulation. Site dependence has two major features: (1) environmentally caused heterogeneity among sites in suitability for reproduction and/or survival; and (2) preemptive site occupancy, with the tendency for individuals to move to sites of higher quality as they become available. Simulation modeling shows that these two features, acting in concert, generate nega...

Results of Experimental and Natural Food Reductions for Breeding Black-Throated Blue Warblers

We examined effects of natural and experimentally created reductions in food abundance on the reproductive ecology of Black-throated Blue Warblers (Dendroica caerulescens). The study was carried out between 1982 and 1985 on four 30-ha plots of temperate deciduous forest within and near the Hubbard Brook Experimental Forest, New Hampshire, USA. Experimental food reduction using aerially sprayed Bacillus thuringiensis created significant differences in caterpillar biomass between one sprayed and two unsprayed plots throughout the breeding season in 1983. Where caterpillar abundance was reduced, Black-throated Blue Warblers made significantly fewer nesting attempts and diets of nest- lings included fewer caterpillars. Clutch size, hatching success, and number of young fledging per nest did not differ among the food reduction site and controls. Also, the reduced number of nesting attempts per pair on the food reduction site in 1983 did not significantly lower production of young per pair. No detectable differences in caterpillar biomass between sprayed and unsprayed sites were created in 1984 or 1985 because natural caterpillar abundances were already low. Natural declines in food abundance for Black-throated Blue Warblers occurred from 1982 through 1985, primarily because of a decrease in caterpillar abundances. Significant reductions in number of young fledging per nest, nestling growth rates and survival, and number of nests attempted per pair corresponded with the natural decline in food abun- dance. Of those measures, a reduced number of nesting attempts per pair lowered annual production the most. When effects of food limitation were calculated separately from those of nest predation, food limited annual breeding productivity to below that needed to balance annual mortality in at least one of the four years of this study. These findings indicate that neotropical migrant bird species are probably limited periodically by food when breeding in north-temperate habitats.

Social information trumps vegetation structure in breeding-site selection by a migrant songbird.

To maximize fitness, organisms must assess and select suitable habitat. Early research studying birds suggested that organisms consider primarily vegetation structural cues in their habitat choices. We show that experimental exposure to singing in the post-breeding period provides a social cue that is used for habitat selection the following year by a migrant songbird, the black-throated blue warbler (Dendroica caerulescens). Our experimental social cues coerced individuals to adopt territories in areas of very poor habitat quality where individuals typically do not occur. This indicates that social information can override typical associations with vegetation structure. We demonstrate that a strong settlement response was elicited because post-breeding song at a site is highly correlated with reproductive success. These results constitute a previously undocumented, but highly parsimonious mechanism for the inadvertent transfer of reproductive (public) information from successful breeders to dispersers. We hypothesize that post-breeding song is a pervasive and reliable cue for species that communicate vocally, inhabit temporally autocorrelated environments, produce young asynchronously and/or abandon territories after reproductive failure.

Multiple density-dependence mechanisms regulate a migratory bird population during the breeding season.

The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black–throated blue warbler (Dendroica caerulescens), is regulated by multiple density–dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969–2002, strong density dependence in time–series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997–1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high–quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site–dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site–dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site–dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population.

EXPERIMENTALLY REDUCING NEIGHBOR DENSITY AFFECTS REPRODUCTION AND BEHAVIOR OF A MIGRATORY SONGBIRD

Because populations of territorial birds are relatively stable compared to those of other animal taxa, they are often considered to be tightly regulated. However, the mechanisms that produce density-dependent feedbacks on demographic rates and thus regulate these populations are poorly understood, particularly for migratory species. We conducted a three-year density-reduction experiment to investigate the behavioral mechanisms that regulate the abundance of a Nearctic–Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding season. We found that the number of young fledged per territory, territory size, and the proportion of time males spent foraging were significantly greater on territories around which neighbor density was experimentally reduced compared to control territories. Territory quality, proportion of nests depredated per territory, and male countersinging rates were not statistically different between treatments. These results indicate that i...

Long-Term Integrated Studies Show Complex and Surprising Effects of Climate Change in the Northern Hardwood Forest

Evaluations of the local effects of global change are often confounded by the interactions of natural and anthropogenic factors that overshadow the effects of climate changes on ecosystems. Long-term watershed and natural elevation gradient studies at the Hubbard Brook Experimental Forest and in the surrounding region show surprising results demonstrating the effects of climate change on hydrologic variables (e.g., evapotranspiration, streamflow, soil moisture); the importance of changes in phenology on water, carbon, and nitrogen fluxes during critical seasonal transition periods; winter climate change effects on plant and animal community composition and ecosystem services; and the effects of anthropogenic disturbances and land-use history on plant community composition. These studies highlight the value of long-term integrated research for assessments of the subtle effects of changing climate on complex ecosystems.

Climate change effects on native fauna of northeastern forests

We review the observed and potential effects of climate change on native fauna of forests in northeastern North America by focusing on mammals, birds, amphibians, and insects. Our assessment is placed in the context of recent regional-scale climate projections. Climate change, particularly in recent decades, has affected the distribution and abundance of numerous wildlife species. Warming temperatures, alterations to precipitation regimes, seasonality, and climatic extremes are projected to affect species directly or indirectly in each of the focal taxa. Greatest climate change will occur during winter, and the survival of winter-active species as well as the survival, distribution, and abundance of hibernating mammals, amphibians, resident birds, and diapausing insects may be altered. Even under low emissions scenarios, effects on native fauna may be profound, affecting iconic species, endangered species, and species that provide economically valuable services, such as pollination and regulation of insec...

EFFECTS OF WOODLAND SHAPE ON THE SPATIAL DISTRIBUTION AND DENSITY OF 17-YEAR PERIODICAL CICADAS (HOMOPTERA: CICADIDAE)

-We tested the hypothesis that the shape of woodland fragments influences the spatial distribution and density of three species of 17-year periodical cicadas (Homoptera: Cicadidae: Magicicada spp.). These characteristics were quantified for periodical cicadas emerging in four rectangular woodlots (mean size = 3.4 ? 0.7 ha), four linear wooded fencerows (>400 m long by mean width = 15.9 + 3.9 m), and the last remaining stand (81 ha) of primary beech-maple forest in southwestern Ohio, during the emergence of Brood X, May-June 1987. The shape of woodland fragments influenced spatial distribution and density of cicadas, although other factors, particularly the disturbance history of the sites, were equally important. Emergence began earlier in linear wooded fencerows and rectangular woodlot edges than in the interior of woodlots. Density of emerging individuals was greatest in the least disturbed edge areas and was low within the interior of rectangular woodlots. The relative abundances of the cicada species differed significantly between the woodland fragments and the primary forest. The smaller species, M. cassini (Fisher) and M. septendecula (Alexander and Moore), constituted over 93% of all individuals in all the linear and rectangular forest fragments, but the larger species, M. septendecim (L.), constituted 68% of individuals in the interior of the primary forest. We concluded that woodland shape influenced the spatial distribution of cicadas within the fragments, mainly due to the concentration of cicadas in edge areas. Our data also indicated that forest fragmentation shifted the relative abundances of these cicada species to the detriment of M. septendecim.

Effects of uncultivated corridors on arthropod abundances and crop yields in soybean agroecosystems

Abstract Differences in arthropod populations and crop yields between soybean monocultures with either corridors planted to grasses (grassy), corridors allowed to enter secondary succession (successional), no corridors without an insecticide treatment (controls), or no corridors and treated with Carbaryl (insecticide) were examined. The presence of corridors in soybeans suppressed populations of leaf and stem sucking pests, particularly Empoasca fabae Harris, but not defoliators which were often more abundant in plots with corridors than in the controls. Predaceous arthropods were more abundant in successional than grassy corridors, and more numerous in plots with either type of corridor than in plots without corridors from late August until mid-September. Despite these differences among treatments in arthropod abundances, soybean yields per meter of crop row did not differ significantly between controls and any other treatment. However, because of the multiple benefits of landscape corridors identified by this and other studies, uncultivated corridors should be established within croplands for integrated management purposes.