Richard L. Hutto

The forgotten stage of forest succession: early‐successional ecosystems on forest sites

Early-successional forest ecosystems that develop after stand-replacing or partial disturbances are diverse in species, processes, and structure. Post-disturbance ecosystems are also often rich in biological legacies, including surviving organisms and organically derived structures, such as woody debris. These legacies and post-disturbance plant communities provide resources that attract and sustain high species diversity, including numerous early-successional obligates, such as certain woodpeckers and arthropods. Early succession is the only period when tree canopies do not dominate the forest site, and so this stage can be characterized by high productivity of plant species (including herbs and shrubs), complex food webs, large nutrient fluxes, and high structural and spatial complexity. Different disturbances contrast markedly in terms of biological legacies, and this will influence the resultant physical and biological conditions, thus affecting successional pathways. Management activities, such as post-disturbance logging and dense tree planting, can reduce the richness within and the duration of early-successional ecosystems. Where maintenance of biodiversity is an objective, the importance and value of these natural early-successional ecosystems are underappreciated.

Examining historical and current mixed-severity fire regimes in ponderosa pine and mixed-conifer forests of Western North America

There is widespread concern that fire exclusion has led to an unprecedented threat of uncharacteristically severe fires in ponderosa pine (Pinus ponderosa Dougl. ex. Laws) and mixed-conifer forests of western North America. These extensive montane forests are considered to be adapted to a low/moderate-severity fire regime that maintained stands of relatively old trees. However, there is increasing recognition from landscape-scale assessments that, prior to any significant effects of fire exclusion, fires and forest structure were more variable in these forests. Biota in these forests are also dependent on the resources made available by higher-severity fire. A better understanding of historical fire regimes in the ponderosa pine and mixed-conifer forests of western North America is therefore needed to define reference conditions and help maintain characteristic ecological diversity of these systems. We compiled landscape-scale evidence of historical fire severity patterns in the ponderosa pine and mixed-conifer forests from published literature sources and stand ages available from the Forest Inventory and Analysis program in the USA. The consensus from this evidence is that the traditional reference conditions of low-severity fire regimes are inaccurate for most forests of western North America. Instead, most forests appear to have been characterized by mixed-severity fire that included ecologically significant amounts of weather-driven, high-severity fire. Diverse forests in different stages of succession, with a high proportion in relatively young stages, occurred prior to fire exclusion. Over the past century, successional diversity created by fire decreased. Our findings suggest that ecological management goals that incorporate successional diversity created by fire may support characteristic biodiversity, whereas current attempts to “restore” forests to open, low-severity fire conditions may not align with historical reference conditions in most ponderosa pine and mixed-conifer forests of western North America.

The Composition and Social-Organization of Mixed-Species Flocks in a Tropical Deciduous Forest in Western Mexico

I recorded the flocking propensity of birds within a tropical deciduous forest in western Mexico during the nonbreeding season, and determined the species composition of57 mixed-species, canopy insectivore flocks. Each of 27 canopy insectivore species present on the study area was observed foraging in mixed-species flocks on at least half of the occasions that it was detected on bird surveys. The proportion of flocks within which a given species was detected could be predicted on the basis of its index of abundance, as determined from independently derived point count data. Therefore, flocks are not comprised of a special subset of canopy insectivores; rather, the composition of flocks appears to be a product of whichever species co-occur within the foraging range of one or more nuclear species. No two canopy insectivore species were negatively associated among flocks, but 40 of 210 possible pairs (19%) were significantly positively associated. Two long-distance migratory species (Nashville Warbler [ Vermivora ruficapilla] and Bluegray Gnatcatcher [Polioptila caerulea]) shared features that characterize nuclear species, thereby constituting one of the first recorded instances where nonresident species play such a role. Five of the more common flocking species were equally likely to have a foraging neighbor nearby (<3 m away), but the identity of that neighbor differed significantly among the five species. Specifically, the identity of close neighbors of the two nuclear species was a random subset of the species available, while the close associates of three attendant species were a nonrandom subset of (mostly) other attendant species. The independent associations among flocks, which characterize most species, and the observation that nuclear species were not close neighbors more than expected for any of three attendant species suggests that foraging enhancement is not the principal benefit that attendant species derive from flocking.

SENSITIVITY OF SPECIES HABITAT-RELATIONSHIP MODEL PERFORMANCE TO FACTORS OF SCALE

Researchers have come to different conclusions about the usefulness of habitat-relationship models for predicting species presence or absence. This difference frequently stems from a failure to recognize the effects of spatial scales at which the models are applied. We examined the effects of model complexity, spatial data resolution, and scale of application on the performance of bird habitat relationship (BHR) models on the Craig Mountain Wildlife Management Area and on the Idaho portion of the U.S. Forest Services Northern Region. We constructed and tested BHR models for 60 bird species detected on the study areas. The models varied by three levels of complexity (amount of habitat information) and three spatial data resolutions (0.09 ha, 4 ha, 10 ha). We tested these models at two levels of analysis: the site level (a homogeneous area <0.5 ha) and cover-type level (an aggregation of many similar sites of a similar land-cover type), using correspondence between model predictions and species detections ...

THE EFFECTS OF POSTFIRE SALVAGE LOGGING ON CAVITY-NESTING BIRDS

Abstract ABSTRACT We investigated the effects of postfire salvage logging on cavity-nesting birds by comparing nest densities and patterns of nest reuse over a three-year period in seven logged and eight unlogged patches of mixed-conifer forest in the Blackfoot-Clearwater Wildlife Management Area, Montana. We found 563 active nests of 18 cavity-nesting birds; all species were found nesting in the uncut burned forest plots, but only eight nested in the salvage-logged plots. All except one species nested at a higher density in the unlogged areas, and half of the species were significantly more abundant in the unlogged plots. Every timber-drilling and timber-gleaning species was less abundant in the salvage-logged plots, including two of the most fire-dependent species in the northern Rocky Mountains—American Three-toed (Picoides dorsalis) and Black-backed (P. arcticus) Woodpeckers. Lower abundances in salvage-logged plots occurred despite the fact that there were still more potential nest snags per hectare than the recommended minimum number needed to support maximum densities of primary cavity-nesters, which suggests that reduced woodpecker densities are more related to a reduction in food (wood-boring beetle larvae) than to nest-site availability. Because cavities were present in only four of 244 randomly selected trees, and because frequency of cavity reuse by secondary cavity-nesters was higher in salvage-logged than in unlogged plots, nest-site limitation may be a more important constraint for secondary cavity-nesters in salvage-logged areas. These results suggest that typical salvage logging operations are incompatible with the maintenance of endemic levels of most cavity-nesting bird populations, especially populations of primary cavity-nesting species.

AN EAST-WEST COMPARISON OF MIGRATION IN NORTH AMERICAN WOOD WARBLERS

Abstract That western and eastern songbird migration routes are distinct ecological systems has been proposed for over 100 years. Nonetheless, this distinction has not been widely recognized nor have there been any comparative studies that quantitatively evaluate the differences and similarities between western and eastern songbird migration systems. We drew from previously published research on wood warblers to highlight patterns in stopover ecology that suggest fundamental differences between western and eastern migrants. In particular, we compared biogeography, evolutionary relationships, and stopover ecology of wood warblers from western and eastern North America and found: (1) multiple lines of evidence that indicate western wood warblers are geographically isolated from eastern conspecifics or congeners throughout the annual cycle, (2) eastern and western wood warbler taxa are distinct evolutionary units, (3) migrant wood warblers captured in the Southwest tended to carry lower fat loads and be comprised of more after-hatch-year birds than is typical of eastern migrants, (4) frugivory is unknown in wood warblers endemic to the Northwest or Southwest and (5) relative to other regions and seasons, riparian vegetation is heavily used by western wood warblers in the spring. We think that further examination and synthesis of these differences would yield a more mechanistic understanding of Nearctic-Neotropical avian migration. On this basis, we elaborate our view that (1) an improved understanding of western songbird migration ought to be a high priority for science, conservation, and education, and (2) large-scale coordinated research efforts would be the most effective strategy for advancing our knowledge of passerine migration in the West. Una Comparación Este-Oeste de la Migración de las Reinitas de Bosque (Parulidae) de Norte América Resumen. Por más de 100 años se ha propuesto que las rutas de migración de aves paseriformes del este y del oeste son sistemas ecológicos diferentes. Sin embargo, esta distinción no ha sido reconocida ampliamente, ni se han realizado estudios comparativos que evalúen cuantitativamente las diferencias y similitudes entre los sistemas de migración del este y del oeste. Revisamos estudios previamente publicados sobre aves de bosque de la familia Parulidae (Reinitas) para destacar los patrones ecológicos de las paradas migratorias que sugieran diferencias fundamentales entre las aves paseriformes migratorias el este y del oeste. Particularmente, comparamos la biogeografía, las relaciones evolutivas y la ecología de las paradas migratorias de las Reinitas del este y del oeste de Norteamérica y encontramos: (1) múltiples líneas de evidencia que indican que las Reinitas del oeste se encuentran geográficamente aisladas de sus coespecíficos o cogenéricos del este durante todo el ciclo anual, (2) los taxa del este y del oeste son unidades evolutivas independientes, (3) las Reinitas migratorias capturadas en el suroeste tendieron a presentar menores cargas de grasa y una mayor proporción de aves eclosionadas durante el año, que lo es típicamente observado para las aves migratorias del este, (4) la frugivoría no se conoce en las Reinitas endémicas del noroeste o sudoeste y (5) en relación a otras regiones o estaciones, la vegetación riparia es usada intensamente durante la primavera por las Reinitas del oeste. Creemos que estudios adicionales y una síntesis de estas diferencias podrían producir un mejor entendimiento mecanístico de la migración Neártico-Neotropical de aves. Basados en esto, sugerimos que (1) un mejor entendimiento de la migración de aves paserinas del oeste debería tener alta prioridad para la ciencia, conservación y educación y (2) que los esfuerzos de investigación coordinados a gran escala serían la estrategia más efectiva para progresar en nuestro conocimiento sobre la migración de las aves paserinas en el oeste.

IS SELECTIVELY HARVESTED FOREST AN ECOLOGICAL TRAP FOR OLIVE-SIDED FLYCATCHERS?

Abstract ABSTRACT Disturbance-dependent species are assumed to benefit from forestry practices that mimic the appearance of postdisturbance landscapes. However, human activities that closely mimic the appearance but not the fundamental quality of natural habitats could attract animals to settle whether or not these habitats are suitable for their survival or reproduction. We examined habitat selection behavior and nest success of Olive-sided Flycatchers (Contopus cooperi) in a naturally occurring burned forest and an anthropogenically created habitat type—selectively harvested forest. Olive-sided Flycatcher density and nestling provisioning rates were greater in the selectively harvested landscape, whereas estimated nest success in selectively harvested forest was roughly half that found in naturally burned forest. Reduced nest success was probably a result of the relatively high abundance of nest predators found in the artificially disturbed forest. These results are consistent with the hypothesis that selectively harvested forest can act as an “ecological trap” by attracting Olive-sided Flycatchers to a relatively poor-quality habitat type. This highlights the importance of considering animal behavior in biodiversity conservation.

A Description of Mixed-Species Insectivorous Bird Flocks in Western Mexico

Insectivorous bird flocks were observed in all types of forested habitats during the nonbreeding season in western Mexico. The species composition of flocks changed markedly and predictably among five categories of habitat type. The average number of species per flock in lowland habitats was 4.7, while a mean of 18.6 species participated in highland flocks, ranking the latter among the most species-rich flocks in the world. The mean proportion of the local insectivorous pecies that participated in mixed-species flocks was significantly greater in the highlands (6 1.3%) than in the lowlands (24.6%). About half of the flock participants in both undisturbed lowland and highland habitats were north temperate migrants, ranking west Mexican flocks among the most migrant-rich in the world as well. In highland flocks, the maximum number of individuals per attendant species was generally two to three, but there were often six to twelve individuals belonging to each of several nuclear species. The lowland deciduous forest flocks seemed to lack nuclear species.

IS THE EFFECT OF FOREST STRUCTURE ON BIRD DIVERSITY MODIFIED BY FOREST PRODUCTIVITY

Currently, the most common strategy when managing forests for biodiversity at the landscape scale is to maintain structural complexity within stands and provide a variety of seral stages across landscapes. Advances in ecological theory reveal that biodiversity at continental scales is strongly influenced by available energy (i.e., climate factors relating to heat and light and primary productivity). This paper explores how available energy and forest structural complexity may interact to drive biodiversity at a regional scale. We hypothesized that bird species richness exhibits a hump-shaped relationship with energy at the regional scale of the northwestern United States. As a result, we hypothesized that the relationship between energy and richness within a landscape is positive in energy-limited landscapes and flat or decreasing in energy-rich landscapes. Additionally, we hypothesized that structural complexity explains less of the variation in species richness in energy-limited environments and more in energy-rich environments and that the slope of the relationship between structural complexity and richness is greatest in energy-rich environments. We sampled bird communities and vegetation across seral stages and biophysical settings at each of five landscapes arrayed across a productivity gradient from the Pacific Coast to the Rocky Mountains within the five northwestern states of the contiguous United States. We analyzed the response of richness to structural complexity and energy covariates at each landscape. We found that (1) richness had a hump-shaped relationship with available energy across the northwestern United States, (2) the landscape-scale relationships between energy and richness were positive or hump shaped in energy-limited locations and were flat or negative in energy-rich locations, (3) forest structural complexity explained more of the variation in bird species richness in energy-rich landscapes, and (4) the slope of the relationship between forest structural complexity and richness was steepest in energy-limited locations. In energy-rich locations, forest managers will likely increase landscape-scale bird diversity by providing a range of forest structural complexity across all seral stages. In low-energy environments, bird diversity will likely be maximized by managing local high-energy hotspots judiciously and adjusting harvest intensities in other locations to compensate for slower regeneration rates.

Temporal patterns of foraging activity in some wood warblers in relation to the availability of insect prey

SummaryI monitored the temporal pattern of diurnal feeding activity in several wood warbler (Parulidae) species and concomitantly recorded the numbers of active (flying) insects in 2 willow habitats in the western United States. At one site the temporal relationship between the density of active and inactive (nonflying) insects was investigated. The diurnal patterns of insect and bird activity were inversely related and each pattern was significantly nonuniform throughout the day; the wood warblers were largely inactive during the middle of the day when insects were most active.As foliage-gleaning birds, wood warblers depend primarily on the availability of inactive (nonflying) insects that they pick from the foliage, and they appear to be limited in their foraging activity by the unavailability of such insects during midday. Interestingly, the duration of midday inactivity for a given bird species varied inversely with the proportion of time that species spent flycatching. Thus, food availability may play an important role in determining the temporal patterns of feeding activity in these insectivorous bird species.