Intra and inter-annual variation in seed rain in a secondary dry tropical forest excluded from chronic disturbance

Domestic livestock grazing and firewood extraction are chronic and pervasive disturbances in dry secondary forest that reduce diversity and modify composition. Exclosure of such disturbances may allow the reestablishment of original diversity. To investigate potential vegetation recovery following 30 years of perturbation, seed rain was evaluated monthly during 3 years in 96 0.78 m² seed traps located in eight exclosures established in 2006 in a species-poor secondary dry forest in central Mexico. Twelve additional seed traps were located outside the exclosures in 2010 for comparison purposes. Baseline diversity of seed sources (trees, ?5 cm DBH) was recorded to compare standing tree composition with seed rain. Also, baseline diversity of potential primary (birds) and secondary (ants) dispersal vectors were registered to infer current plant-animal interactions. Total seed rain over 3 years within the exclosures comprised 180,375 seeds from 61 woody species, an average of 1.12 seeds/m²/month. Half of these species (31) and 95% of the seeds falling within the exclosures belonged to trees typical of perturbed sites. Sixty percent of all recorded seeds belonged to an early-successional, bird-dispersed shrub (Hamelia patens Jacq., Rubiaceae). Seed rain at one non-excluded plot was dominated by the wind-dispersed early-successional tree Heliocarpus sp. (Tiliaceae). Richness and seed density increased significantly in the second year of exclusion; however, by the third year, richness remained constant while seed density decreased. Seed rain was richer and more abundant during the dry season and was dominated by wind-dispersed species. Phenology, successional status and the dispersal modes of woody species explained most variance in richness and seed density of seed rain in early-successional dry forest. Seeds of early-successional species were dispersed in equal measure by abiotic and biotic vectors, whereas biotic vectors dispersed a higher density of late-successional seeds. The amount and timing of fall of abiotically dispersed seeds of the three most abundant species in the standing tree community may prolong their current domination of the landscape. However, even when richness in the seed rain within exclosures remained constant through time, composition was gradually shifting towards animal-dispersed species which may start a positive feedback producing an increase in animal visitation and a further increase of richness.     

Short- and long-term implications of clearcut and two-age silviculture for conservation of breeding forest birds in the central Appalachians, USA

Two-age (deferment or leave tree) harvesting is used increasingly in even-aged forest management, but long-term responses of breeding avifauna to retention of residual canopy trees have not been investigated. Breeding bird surveys completed in 1994-1996 in two-age and clearcut harvests in the central Appalachian Mountains of West Virginia, USA allowed us to document long-term changes in these stands. In 2005 and 2006, we conducted point counts in mature unharvested forest stands and in 19-26 year-old clearcut and two-age harvests from the original study and in younger clearcut and two-age stands (6-10 years old). We found differences in breeding bird metrics among these five treatments and temporal differences in the original stands. Although early-successional species are typically absent from group selection cuts, they were almost as common in young two-age stands as clearcuts, supporting two-age harvests as an alternative to clearcutting. Although older harvests had lower species richness and diversity, they were beginning to provide habitat for some species of late-successional forest songbirds that were absent or uncommon in young harvests. Overall, late-successional forest-interior species were more flexible in their use of different seral stages; several species used both age classes and harvest types in addition to mature forest, which may reflect the lack of edges in our heavily-forested landscape. Consequently, two-age management provides habitat for a diverse group of species as these stands mature and may be an ecologically sustainable alternative to clearcutting in landscapes where brown-headed cowbirds (Molothrus ater) are uncommon.     

Effects of management regime on the abundance and nest survival of shrubland birds in wildlife openings in northern New England, USA

Many shrubland bird species are declining in eastern North America and as a result forest managers have used a variety of techniques to provide breeding habitat for these species. The maintenance of permanent “wildlife openings” using prescribed burns or mechanical treatments is a widely used approach for providing habitat for these species, but there have been no studies of the effects of treatment regime on bird abundance and nest survival in managed wildlife openings. We studied shrubland birds in wildlife openings on the White Mountain National Forest (WMNF) in New Hampshire and Maine, USA, during 2003 and 2004. We analyzed bird abundance and nest survival in relation to treatment type (burned versus mowed), treatment frequency, time since treatment, and patch area. We found that wildlife openings provided habitat for shrubland birds that are not present in mature forest. There was relatively modest support for models of focal bird species abundance as a function of treatment regime variables, despite pronounced effects of treatment on habitat conditions. This probably was attributable to the combined effects of complex site histories and bird site fidelity. Overall nest success (52%) was comparable to other types of early-successional habitats in the region, but there were few supported relationships between nest survival and treatment variables. We conclude that wildlife openings provide quality habitat for shrubland birds of high conservation interest as long as managers ensure treatment intervals are long enough to permit the development of woody vegetation characteristic of the later stages of this sere. Also, wildlife openings should be large enough to accommodate the territory sizes of all target species, which was ≥1.2 ha in this study.     

Characteristics of occupied habitats and identification of sites for restoration and translocation of New England cottontail populations

Since 1960 the range occupied by New England cottontails (NEC, Sylvilagus transitionalis) in the northeastern United States has declined dramatically. Populations in some regions are known to be vulnerable to extirpation, but little was known about the status of populations in most areas. A recently conducted (2000-2004) range-wide survey identified five disjunct populations within approximately 14% of the historic range of NEC. We incorporated the results of this survey into a geographic information system to examine habitat features associated with remnant populations of NEC at two spatial scales. The regional scale characterized habitats within our survey sample units, 7.5 min topographic quadrangles (quads, ∼40 × 10 km) that were occupied by NEC or vacant. The landscape scale described habitats within a 1-km radius of occupied patches and an equivalent sample of vacant patches. At the regional scale, northeastern and southeastern populations were associated with human-dominated habitats with a greater abundance of developed and disturbed lands, less forest coverage, more edge habitats, and less snow fall than unoccupied quads. Landscapes occupied by NEC in these regions were characterized by a greater abundance of potential dispersal corridors than unoccupied landscapes. In contrast, quads occupied by NEC in the southwestern portion of the historic range were in rural areas that were dominated by forests and agricultural fields. At the landscape scale, southwestern populations were affiliated patches of habitat surrounded by more agricultural lands than patches that were not occupied by NEC. Logistic regression models were then developed to identify habitats suitable for restoration or translocation within each region. We suggest that initial restoration efforts be directed toward expanding existing populations of NEC. Next, habitat connections should be developed among these populations. Finally, new populations should then be established via translocation in portions of the historic range that are vacant. In addition to promoting New England cottontails, management of early-successional and shrub-dominated habitats in the northeastern United States will benefit other taxa of conservation concern that are dependent on these habitats.     

Investigating area-sensitivity in shrubland birds: Responses to patch size in a forested landscape

Population declines of shrubland birds in the eastern United States have been attributed to loss of early-successional habitat. Given that shrubland habitats are often ephemeral and patchily distributed, understanding the sensitivity of shrubland birds to patch characteristics is important for conservation. We tested the extent to which patch area was related to shrubland bird density, annual survival, and productivity by examining capture rates, apparent annual survival estimates, and juvenile-to-adult-female ratios for six focal species of shrubland birds in southeastern Ohio. Identical 3 × 3 mist-net grid arrays were set at each of 13 clearcut patches ranging in size from 4 to 16 ha and visited once per week between June and August of 2002–2006. Over the five seasons, 1428 juveniles and 2001 adults of six species were banded. Capture rates for all six species increased with patch area (mean of 44% higher in largest than smallest patch) but this relationship was only significant for the Yellow-breasted Chat (Icteria virens; F_1,11 = 34.2, P < 0.001) and the Common Yellowthroat (Geothlypis trichas; F_1,11 = 7.0, P = 0.023). However, after accounting for the effect of bird movements on capture rates, capture rates for only four of the six species increased patch area (mean of 22% higher in largest than smallest patch) and this relationship was only significant for the Yellow-breasted Chat (F_1,11 = 8.9, P = 0.012). Patch area was not a good predictor of apparent annual survival or juvenile-to-adult-female ratios for any species. Overall, we detected limited evidence of area-sensitivity in our system and no evidence that annual survival or productivity differed by patch area.     

Post-breeding bird responses to canopy tree retention, stand size, and edge in regenerating Appalachian hardwood stands

Avian use of even-aged timber harvests is likely affected by stand attributes such as size, amount of edge, and retained basal area, all characteristics that can easily be manipulated in timber harvesting plans. However, few studies have examined their effects during the post-breeding period. We studied the impacts of clearcut, low-leave two-age, and high-leave two-age harvesting on post-breeding birds using transect sampling and mist-netting in north-central West Virginia. In our approach, we studied the effects of these harvest types as well as stand size and edge on species characteristic of both early-successional and mature forest habitats. In 2005-2006, 13 stands ranging from 4 to 10 years post-harvest and 4-21 ha in size were sampled from late June through mid-August. Capture rates and relative abundance were similar among treatments for generalist birds. Early-successional birds had the lowest capture rates and fewer species (∼30% lower), and late-successional birds reached their highest abundance and species totals (double the other treatments) in high-leave two-age stands. Area sensitivity was evident for all breeding habitat groups. Both generalist and late-successional bird captures were negatively related to stand size, but these groups showed no clear edge effects. Mean relative abundance decreased to nearly zero for the latter group in the largest stands. In contrast, early-successional species tended to use stand interiors more often and responded positively to stand size. Capture rates for this group tripled as stand size increased from 4 to 21 ha. Few birds in the forest periphery responded to harvest edge types despite within-stand edge effects evident for several species. To create suitable habitat for early-successional birds, large, non-linear openings with a low retained basal area are ideal, while smaller harvests and increased residual tree retention would provide habitat for more late-successional birds post-breeding. Although our study has identified habitat use patterns for different species in timber harvests, understanding habitat-specific bird survival is needed to help determine the quality of silvicultural harvests for post-breeding birds.     

Habitat use and nest success of scrub-shrub birds in wildlife and silvicultural openings in western Massachusetts,USA

Many conservationists are concerned about reports of declining populations of scrub-shrub birds. Wildlife opening management, involving repeatedly burning or mechanically treating early-successional stands, is one strategy for creating habitat for these species, but this practice is costly. Some silvicultural treatments also create scrub-shrub habitat and have the advantage of potentially generating revenue, but the relative effectiveness of wildlife opening management and silviculture for creating bird habitat is not known. During 2004 and 2005, we compared scrub-shrub bird abundance, nest success, and habitat characteristics between wildlife and silvicultural openings in western Massachusetts to determine whether the habitats created by these practices are equivalent for birds. We recorded 1927 detections of 31 scrub-shrub bird species during the course of the study and monitored 368 nests. We found that although most species were present in both treatments, many differed in abundance between wildlife and silvicultural openings, including several species of high conservation concern. These differences were attributable to differences in habitat characteristics between treatments. The overall probability of a nest surviving to fledging was 0.43, and did not differ between wildlife and silvicultural openings. Our results indicate that the habitat quality of wildlife and silvicultural openings is not equivalent for scrub-shrub birds, but the two management approaches serve as complementary strategies for maintaining these declining species.     

Effects of width, edge and habitat on the abundance and nesting success of scrub–shrub birds in powerline corridors

Concern about declines in scrub–shrub bird populations has resulted in efforts to create and maintain habitat for these species. Vegetation within powerline corridors is managed to prevent contact of vegetation with transmission lines, and comprises approximately 2% of all of habitat for scrub–shrub birds in southern New England. Although previous studies have documented the use of powerline corridors by scrub–shrub birds, important questions remain about the factors affecting the quality of corridors as habitat for these species. We surveyed birds and monitored nests on 15 corridors in western Massachusetts during 2002 and 2003 to determine whether scrub–shrub birds occupy and successfully reproduce in powerline corridors, and to identify the principal factors affecting scrub–shrub abundance and nesting success. We found that corridors were occupied by scrub–shrub birds of high regional conservation priority, however, four of seven focal scrub–shrub bird species were scarce or absent in narrow corridors, and the abundance of these species was highest in corridors of intermediate width. Overall, nest survival was low (0.14) at these sites relative to other types of early successional habitats in the region, however, if we consider only our sites that were wider than the median width (49 m), nest survival in corridors was (0.33), similar to survival rates reported in other studies of scrub–shrub birds. We conclude that powerline corridors provide habitat for early successional birds of conservation concern, with wider corridors (50 m) contributing more to regional conservation of these species.     

The rise and fall of bobcat populations in New Hampshire: Relevance of historical harvests to understanding current patterns of abundance and distribution

Harvest records reveal that populations of bobcats (Lynx rufus) in New Hampshire have undergone substantial changes during the past 200 years. In the 1800s, a nearly continuous bounty program resulted in annual harvests that averaged ∼30 bobcats. Harvests increased in 1915, and fluctuated from 100 to 400 bobcats during the 1920s through the 1950s. In 1959, harvests peaked at 421 and then rapidly declined. By 1970, payment was made on only 10 bobcats, and legal status was changed from nuisance animal to game species in 1973. In 1989, trapping and hunting seasons were closed and bobcats were designated a protected species. After 15 years of protection, populations of bobcats seem to be remaining at modest levels. To understand what factors may have contributed to the remarkable rise and fall of bobcat populations, we compared the temporal distribution of harvests to comments by early naturalists, legislation to control bobcat abundance, and historical changes in land use. We then used two approaches with a geographic information system to identify the environmental features that may affect present-day populations. The empirical approach relied on a comparison of landscape characteristics associated with recent (1990-2004) observations of bobcats to characteristics found at a comparable set of random locations. We also examined the characteristics of townships that yielded the majority of historical bobcat harvests (1931-1965) and developed a process-oriented model to rank present-day habitat suitability. The irruption of bobcat populations coincided with the availability of early-successional habitats as abandoned agricultural lands reverted to second-growth forests during the first half of the 20th century. Likewise, bobcat populations rapidly declined as these forests matured and no longer supported abundant prey, especially New England cottontails (Sylvilagus transitionalis). Our efforts to identify habitat features associated with present-day populations had mixed results. The empirically derived model correctly classified only 52% of recent bobcat locations, whereas the process-oriented model indicated that nearly 88% of recent bobcat observations were associated with sites that were ranked at high suitability. The results of this study demonstrate the utility of information on historical harvests when addressing questions on the status of contemporary wildlife populations.