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Resource selection specialization may increase vulnerability of populations to environmental change. One environmental change that may negatively impact some populations is the broad decline of quaking aspen Populus tremuloides, a preferred nest tree of cavity‐nesting organisms who are commonly limited by nest‐site availability. However, the long‐term consequences of this habitat change for cavity‐nesting bird populations are poorly studied. I counted densities of woody plants and eight cavity‐nesting bird species over 29 years in 15 high‐elevation riparian drainages in Arizona, USA. I also studied nest‐tree use and specialization over time based on 4946 nests across species. Aspen suffered a severe decline in availability over time, while understorey woody plants and canopy deciduous trees also declined. The decline of plants resulted from increased elk Cervus canadensis browsing linked to declining snowfall. Woodpeckers exhibited very high specialization (>95% of nests) on aspen for nesting, and densities of all six species declined with aspen over time. Mountain chickadees Poecile gambeli and house wrens Troglodytes aedon exhibited increasingly less specialization on aspen. Chickadees strongly increased in density over time, despite a relatively high specialization on aspen. House wren densities declined moderately over time, but nest‐box addition experiments demonstrated that nest‐site availability was not limiting their population. House wren densities increased with understorey vegetation recovery in elk exclosures via increased generality of nest‐site use, demonstrating that the decline in understorey vegetation on the broader landscape was the cause of their population decline. Synthesis and applications. Management should target species that specialize in resource selection on a declining resource. Species with greater resource selection generalization can reduce population impacts of environmental change. Resource generalization can allow a species like the wren to take advantage of habitat refuges, such as those provided by the elk exclosures. Yet, resource generalization cannot offset the negative impacts of broad‐scale declines in habitat quality on the landscape, as demonstrated by the general decline of wrens. Ultimately, aspen is an important habitat for biodiversity, and land management programmes that protect and aid recovery of aspen habitats may be critical.

Many factors drive the organization of communities including environmental factors, dispersal abilities, and competition. In particular, ant communities have high levels of interspecific competition and dominance that may affect community assembly processes. We used a combination of surveys and nest supplementation experiments to examine effects of a dominant ground-nesting ant (Pheidole synanthropica) on (1) arboreal twig-nesting, (2) ground-foraging, and (3) coffee-foraging ant communities in coffee agroecosystems. We surveyed these communities in high-and low-density areas of P. synanthropica over 2 years. To test for effects on twig ant recruitment, we placed artificial nesting resources on coffee plants in areas with and without P. synanthropica. Thefirst sampling period revealed differences in ant species composition on the ground, in coffee plants, and artificial nests between high-and low-density sites of P. synanthropica. Highdensity sites also had significantly lower recruitment of twig ants and had species-specific effects on twig ant species. Prior to the second survey period, abundance of P. synanthropica declined in the high-density sites, such that P. synanthropica densities no longer differed. Subsequent sampling revealed no difference in total recruitment of twig ants to artificial nests between treatments. Likewise, surveys of ground and coffee ants no longer showed significant differences in community composition. The results from the first experimental period, followed by survey results after the decline in P. synanthropica densities suggest that dominant ants can drive community assembly via both recruitment and establishment of colonies within the community.

Tree cavities are a critical resource for many animals, especially as nesting sites for birds. Patterns of cavity distribution in temperate forests are well studied, yet little is known of cavities in tropical forests, despite a hypothesized decrease in cavity availability with decreasing latitude. We studied cavity density and distribution in a wet lowland tropical forest in Costa Rica and compared our results with estimates from forests around the world. Cavities at our site were common, occurred frequently in living trees, and were often formed by damage or decay rather than by woodpeckers. Most cavities had small openings, and woodpecker-created cavities were nonrandomly oriented. Contrary to prediction, cavity density appears to increase from the poles to the tropics. We suggest potential mechanisms to explain these patterns.

An important question in evolutionary biology is to identify the mechanisms that control the number of reproductives in social groups. Ants are appropriate models to address this question because of the variety of their social structures both within and between species, making this taxon suitable for initiating comparative studies to examine the drivers of this diversity. In this study, we developed a comparative approach between populations of the ponerine ant Odontomachus hastatus from Brazil and French Guiana. In Brazil, monogynous and polygynous colonies coexist in the same populations, whereas only monogynous colonies are present in French Guiana. We combined ecological, behavioural and chemical analyses to identify the factors associated with the expression of this social polymorphism. In Brazil, nest densities were higher than in French Guiana, but nesting sites were available in large quantities in both areas, indicating that habitat saturation is probably not the cause of social polymorphism. We did not detect any difference in queen size, regardless of the social structure of the colonies, suggesting that dispersal strategies may be similar between monogynous and polygynous populations. We found no influence of social structure on aggression intensity in dyadic encounters between workers. Last, we showed that the level of aggression increased with both geographical and chemical distance, but we did not find any difference in cuticular profiles between monogynous and polygynous colonies. Overall, the determinism of social polymorphism in O. hastatus still eludes us and calls for further field experiments coupled with genetic approaches.

Assumptions that populations of cavity-nesting birds are limited by access to nest sites have largely been based on anecdotal reports or correlative data. Nest-box-addition experiments or tree-cavity-blocking experiments are potentially rigorous ways to investigate how densities of breeding birds are affected by access to nest cavities. Experimental evidence indicates that natural tree holes are limited in human-altered landscapes, but the possibility that cavity nests are limited in old growth (unmanaged) forests is less clear. I reviewed 31 nest-cavityremoval or addition experiments conducted with 20 species of cavity-nesting birds in mature forests. Of these 31 experiments conducted with a variety of different species of birds, only 19% reported statistically significant changes in breeding densities. However, none of these studies included data about the reproductive history of individuals colonizing the boxes (i.e., whether birds using the boxes would have otherwise been floaters or that birds excluded from blocked cavities on the plots did not simply move elsewhere), so they provided no strong evidence that the number of breeding pairs was limited by availability of nest sites at the population scale. Although some studies indicate that nest sites are limited at local (plot) scales in old growth forests, there is still little empirical evidence for nest-site limitation at the population- and landscape-level in mature, unmanaged forests. I review the challenges in designing and interpreting box-addition experiments and highlight the main gaps in knowledge that should be targeted in the future. El asumir que poblaciones de aves que anidan en cavidades están limitadas por el acceso a lugares para anidar ha estado basado mayormente en informes anecdóticos o datos correlativos. Los experimentos en donde se suplen cajas para anidar o se obstruyen cavidades en árboles son formas potenciales y rigurosas para determinar como la densidad de aves reproductivas es afectada por el acceso a cavidades para el anidamiento. La evidencia experimental indica que las cavidades naturales para anidar están limitadas en paisajes alterados por los humanos, pero la posibilidad que las mismas están limitadas en bosques maduros esta menos clara. Revisé 31 experimentos donde se proveían o se removían cavidades, llevados a cabo en 20 especies de aves que anidan en cavidades en bosques maduros. De estos 31 experimentos, llevados a cabo con una gran variedad de especies, solo el 19% informaron cambios estadísticamente significativos en la densidad reproductiva. Sin embargo, ninguno de estos estudio incluyó información sobre la historia reproductiva de individuos colonizando cajas de anidamiento (ej. si las aves utilizando dichas cajas eran individuos flotadores o si las aves excluidas de cavidades no se movían a áreas nuevas), y no proveyeron evidencia contundente de que el número de parejas reproductivas estuviera limitada por la disponibilidad de lugares para anidar a escala poblacional. Aunque algunos estudios indican que los lugares de anidamiento están limitados a nivel local o en parcelas particulares, en bosques maduros todavía existe poca evidencia empírica referente a limitación de lugares de anidamiento al nivel poblacional o del paisaje en bosques maduros no manejados. Revise los retos en el diseño e interpretación de experimentoas donde se añaden cajas de anidamiento y señale las lagunas principales en la documentación que debe ser el objetivo de estudios futuros.

Nest sites are a limiting resource for arboreal twig-nesting ants, and nest sites may be increasingly limited with habitat modification. One such habitat modification is the conversion of traditional coffee farms, where coffee is cultivated under a dense, diverse shade canopy, to more intensive production systems with reduced canopy cover and lower diversity, height, and density of shade trees. As a result of such management intensification, ant diversity declines. We ask here if: (1) nest sites are a limiting resource for arboreal twig-nesting ants in coffee farms, especially in intensively managed systems and (2) nest-site limitation is a mechanism causing loss of ant diversity with coffee management intensification. During 2000-2003, we investigated occupancy, species richness, and species composition of arboreal twig-nesting ants using natural (hollow coffee twig) and artificial (bamboo stem) nests in farms either with high or low diversity and density of shade trees. In both high- and low-shade sites ants occupied a majority (>55%) of natural nests and occupied some (>15%) artificial nests, and significantly more artificial nests were occupied in low-shade sites. In both high- and low-shade sites, ant richness was higher in artificial than in natural nests. More species occupied natural nests in low-shade sites, and more species occupied artificial nests in high-shade sites. Furthermore, species composition differed between nest types, with more ant species found more often or only in artificial nests. These results indicate that, although ants are not strongly nest-site limited in coffee agroecosystems, nest limitation increases somewhat with increasing management intensification. Reductions in numbers of nest sites may be a mechanism causing ant diversity loss with coffee management intensification. Interestingly, because relatively fewer species colonized artificial nests in the low-shade site, ants may be recruitment limited in the low-shade sites, possibly maintaining low ant richness in these sites.

Resource selection plasticity and behavioral dominance may influence the ability of a species to respond to changes in resource availability, particularly if dominant species have highly specialized resource requirements. We examined the response of several dominant and subordinate cavity-nesting species to a reduction in the availability of an essential resource (nesting cavities) using the novel experimental approach of blocking the entrances to high-quality cavities. We monitored nest abundance on seven treatment and 13 control sites (aspen groves in a grassland matrix) in British Columbia, Canada, for two years pretreatment (2000—2001), two years during treatment (2002—2003), and two years posttreatment (cavities reopened; 2004—2005). At the community level, nest abundance declined by 50% on treatment sites following cavity blocking and returned to pretreatment levels following cavity reopening. Nest abundance of European Starlings (Sturnus vulgaris), a dominant secondary cavity-nester (SCN), declined by 89% and failed to recover posttreatment. Conversely, nest abundance of Mountain Bluebirds (Sialia currucoides; a subordinate SCN) increased following cavity blocking and remained high following reopening. Tree Swallows (Tachycineta bicolor) were unaffected by cavity blocking. We suggest that starlings, while being the dominant SCN, may be limited by availability of suitable nest sites, whereas bluebirds may be limited by starling abundance. We propose that plasticity in nest site preferences of subordinate cavity-nesters may enable them to contend with natural variation in availability of critical resources, such as nest cavities and food, in addition to coping with interspecific competition. This is the first community-level, multiyear study involving manipulation of nest site availability via experimental cavity blocking.

Nest-site availability limits cavity-using populations in many harvested forests; however, little is known about the extent of nest-site limitation in mature forests with a full complement of excavator species and intact processes of cavity creation and loss. To examine the role of nest-site availability in limiting cavityusing populations in mature mixed conifer forests in central British Columbia, Canada, we conducted an 11-year before-after control-impact experiment in which we increased nest-site availability via nest box addition. Our 7 sites (3 treatments, 4 controls) had low cavity densities (<2/ha) prior to treatment and cavity occupation rates were also low (<10%/yr), which is a relationship often cited in the literature as evidence of non-limitation in cavity-nesting populations. Following nest box addition at our treatment sites, which tripled the availability of cavities, total density of bird and mammal nests more than tripled. Density of mountain chickadee (Poecile gambeli) nests increased 9-fold on treatment sites and returned to pre-treatment levels following box removal, suggesting that chickadee populations were limited by cavity availability at our study sites. Density of red squirrel (Tamiasciurus hudsonicus) and northern flying squirrel (Glaucomys sabrinus) nests and roosts also increased significantly at treatment sites following box addition and declined following box removal. We noted little change in chickadee or squirrel nest density at control sites monitored concurrently. Squirrels preferred large-sized over small-sized boxes, and significantly enlarged the entrance areas of small boxes by chewing, suggesting that there may have been a shortage of suitable nest and roost sites for them in our study area. We contend that low cavity occupancy rates may not accurately reflect nest-site availability for cavity nesters in mature forests, and that cavity size may influence the true availability of cavities on the landscape.

Myrmecophyte plants house ants within domatia in exchange for protection against herbivores. Ant-myrmecophyte mutualisms exhibit two general patterns due to competition between ants for plant occupancy: (i) domatia nest sites are a limiting resource and (ii) each individual plant hosts one ant species at a time. However, individual camelthorn trees (Vachellia erioloba) typically host two to four ant species simultaneously, often coexisting in adjacent domatia on the same branch. Such fine-grain spatial coexistence brings into question the conventional wisdom on ant-myrmecophyte mutualisms. Camelthorn ants appear not to be nest-site limited, despite low abundance of suitable domatia, and have random distributions of nest sites within and across trees. These patterns suggest a lack of competition between ants for domatia and contrast strongly with other ant-myrmecophyte systems. Comparison of this unusual case with others suggests that spatial scale is crucial to coexistence or competitive exclusion involving multiple ant species. Furthermore, coexistence may be facilitated when co-occurring ant species diverge strongly on at least one niche axis. Our conclusions provide recommendations for future ant-myrmecophyte research, particularly in utilizing multispecies systems to further our understanding of mutualism biology.

Local guilds define groups of species that share common resources and coexist in space and time. Local guilds have historically been a major focus of community ecology; however, studies of local guilds rarely measure consequences of coexistence for fitness-related traits or test predictions of alternative hypotheses for how species may interact. We studied consequences of coexistence for Orange-crowned Warblers (Vermivora celata) and Virginia's Warblers (V. virginiae), which have overlapping breeding territories in central Arizona. We used reciprocal removal experiments to examine (1) whether co-existence results in ecological consequences with respect to access to nest sites, access to food resources, nest predation, and adult female predation, and (2) whether ecological consequences result in fitness consequences with respect to reproductive success (clutch size, number of young fledged per nest), or adult female survival (within a breeding season). When we removed Virginia's Warblers, Orange-crowned Warblers experienced reduced nest predation rates compared with control plots where Virginia's Warblers were present. When we removed Orange-crowned Warblers, Virginia's Warblers (1) shifted their nest sites to sites indistinguishable from Orange-crowned Warbler nest sites, (2) increased feeding rates during both the incubation and nestling periods, and (3) suffered reduced nest predation rates, compared with control plots where Orange-crowned Warblers were present. When the two species coexist, increased nest predation rates for both species appear to result from density-dependent functional shifts in nest predator behavior (short-term apparent competition). Reduced access to preferred nest sites for Virginia's Warblers coexisting with Orange-crowned Warblers appears to result from both Orange-crowned Warbler interference during nest site selection and building periods, and from Orange-crowned Warbler preemption of nest sites preferred by both species. The mechanisms whereby Orange-crowned Warblers may reduce access to food resources for coexisting Virginia's Warblers, however, are not yet fully understood. Both Orange-crowned and Virginia's Warblers fledged between 78% and 129% more young per nest on plots where the opposite species had been removed, indicating that both species suffer substantial fitness costs of coexistence. Overall, results illustrate that (1) Orange-crowned and Virginia's Warblers do not coexist independently of each other, (2) interactions between the two species are complex and asymmetrical, (3) interactions between the two species result in substantial fitness costs of coexistence for both species, and (4) ecological interactions between the two species extend far beyond competition for food resources which has dominated studies of terrestrial vertebrate communities.