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Biological Invasions 01/2013; · 2.90 Impact Factor
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ABSTRACT: Cooperative breeding is generally associated with increased philopatry and sedentariness, presumably because short-distance dispersal facilitates the maintenance of kin groups. There are, however, few data on long-distance dispersal in cooperative breeders-the variable likely to be important for genetic diversification and speciation. We tested the hypothesis that cooperative breeders are less likely to engage in long-distance dispersal events by comparing records of vagrants outside their normal geographic range for matched pairs (cooperatively vs. non-cooperatively breeding) of North American species of birds. Results failed to support the hypothesis of reduced long-distance dispersal among cooperative breeders. Thus, our results counter the conclusion that the lower rate of speciation among cooperative breeding taxa found in recent analyses is a consequence of reduced vagility.
PLoS ONE 01/2013; 8(3):e58624. · 4.09 Impact Factor
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ABSTRACT: Seed production that varies greatly from year to year, known as "masting" or "mast-fruiting" behavior, is a population-level phenomenon known to exhibit geographic synchrony extending, at least in some cases, hundreds of kilometers. The two main nonexclusive hypotheses for the driver of such geographically extensive synchrony are (1) environmental factors (the Moran effect), and (2) the mutual dependence of trees on outcrossed pollen (pollen coupling). We tested 10 predictions relevant to these two hypotheses using 18 years of acorn production data on two species of California oaks. Data were obtained across the entire ranges of the two species at 12 sites (10 for each species) separated by up to 745 km. In general, our results provided strong support for the importance of the Moran effect as a driver of spatial synchrony in and between these two species. Particularly compelling was evidence of close concordance between spatial synchrony in acorn production and key environmental factors extending over the range of both species and significant spatial cross-synchrony between the two species, despite considerable differences in their geographical ecology. Because oaks are monoecious, female flowers are not necessarily related to pollen production, and thus, our tests do not address the role of pollen coupling in bisexual species where pollen and flower production are necessarily correlated. For the oak species considered here, however, the Moran effect is a key driver of large-scale spatial synchrony in acorn production.
Ecology 01/2013; 94(1):83-93. · 4.85 Impact Factor
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ABSTRACT: Abstract The extraordinarily long life cycles, synchronous emergences at 13- or 17-year intervals, and complex geographic distribution of periodical cicadas (Magicicada spp.) in eastern North America are a long-standing evolutionary enigma. Although a variety of factors, including satiation of aboveground predators and avoidance of interbrood hybridization, have been hypothesized to shape the evolution of this system, no empirical support for these mechanisms has previously been reported, beyond the observation that bird predation can extirpate small, experimentally mistimed emergences. Here we show that periodical cicada emergences appear to set populations of potential avian predators on numerical trajectories that result in significantly lower potential predation pressure during the subsequent emergence. This result provides new support for the importance of predators in shaping periodical cicada life history, offers an ecological rationale for why emergences are synchronized at the observed multiyear intervals, and may explain some of the developmental plasticity observed in these unique insects.
The American Naturalist 01/2013; 181(1):145-9. · 4.72 Impact Factor
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ABSTRACT: Life-history traits interact in important ways. Relatively few studies, however, have explored the relationships between life-history traits in long-lived taxa such as trees. We examined patterns of energy allocation to components of reproduction and growth in three species of California oaks (Quercus spp.) using a combination of annual acorn censuses, dendrometer bands to measure radial increment, and litterfall traps. Our results are generally consistent with the hypothesis that energy invested in reproduction detracts from the amount of energy available for growth in these long-lived taxa; i.e., there are trade-offs between these traits. The relationships between reproduction and growth varied substantially among specific trait combinations and tree species, however, and in some cases were in the direction opposite that expected based on the assumption of trade-offs between them. This latter finding appears to be a consequence of the pattern of resource use across years in these long-lived trees contrasting with the expected partitioning of resource use within years in short-lived taxa. Thus, the existence and magnitude of putative trade-offs varied depending on whether the time scale considered was within or across years. Collectively, our results indicate that negative relationships between fundamental life-history traits can be important at multiple levels of modular organization and that energy invested in reproduction can have measurable consequences in terms of the amount of energy available for future reproduction and both current and future growth.
Oecologia 06/2012; · 3.41 Impact Factor
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ABSTRACT: Introduction
We examined the functional relationship between seed size and seedling performance in the valley oak (Quercus lobata Née) by means of a 13-year common garden experiment.
Materials and methods
Acorns were collected from five localities throughout the range of valley oak in autumn 1997, weighed and measured, and planted
at Sedgwick Reserve, Santa Barbara County, California, USA.
Results
In the short term, larger acorns produced larger seedlings that had lower survival than seedlings from smaller acorns. In
the longer term, large seeds correlated positively with both seedling size and survival, with path analyses indicating that
the latter effect was primarily indirect via initial seedling size. The longer-term relative growth rate was only weakly related
to seed size, being a combination of a slight positive direct influence of seed size on relative growth rate and a comparable
negative indirect effect via larger initial seedling size.
Discussion
These results generally matched the predictions of the “seedling size effect hypothesis” (larger seeds yield larger seedlings
with greater competitive abilities), the only one of the three hypotheses we examined that predicts an inverse relationship
between seed size and initial survival and a positive relationship between seed size and longer-term relative growth rate.
The factors influencing the relationships between seed size and seedling performance are complex and may involve both direct
effects of seed size and indirect effects mediated through initial seedling size. Although the seedling size effect was the
most important in our study, other factors may be important under different environmental conditions and/or at different growth
stages.
KeywordsAcorn size–Fagaceae–
Quercus lobata
–Reserve effect–Seed size–Seedling size effect–Valley oak
Annals of Forest Science 04/2012; 68(3):477-484. · 1.79 Impact Factor
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ABSTRACT: Social dominance confers potential advantages in terms of access to superior resources, habitats, and breeding opportunities.
In the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), within-brood dominance among juveniles is correlated with relative body size as nestlings. Capitalizing on this relationship,
we investigated the fitness consequences of dominance by means of paired comparisons of broodmates. We found that (1) larger
fledglings retained at least some of their size advantage as adults; (2) overwinter survival of larger, dominant fledglings
was significantly greater than subordinates, but was not relatively greater when resources were poor than when they were good;
(3) among birds surviving their first winter, there were no differences vis-à-vis dominance in terms of the proportion of
birds acting as helpers or inheriting their natal territory. However, larger, dominant males were present in the study area
longer than subordinates, suggesting that they either survived better or were more successful at gaining reproductive opportunities;
(4) if only one male broodmate became a helper instead of dispersing, he was significantly more likely to be the smaller subordinate,
consistent with the view that helping is a best-of-a-bad-job strategy; and (5) there were no significant differences in reproductive
success among pairs of male broodmates that cobred together as adults, consistent with prior work failing to detect a phenotypic
correlation of reproductive skew. Our results indicate that within-brood dominance relationships established as juveniles
have significant effects on first-year survivorship and at least some aspects of adult fitness.
KeywordsAcorn woodpecker–Cooperative breeding–Dominance–Helping-at-the-nest–
Melanerpes formicivorus
–Reproductive skew
Behavioral Ecology and Sociobiology 04/2012; 65(12):2229-2238. · 3.18 Impact Factor
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ABSTRACT: Cases of mast seeding, involving highly variable seed production synchronized over large geographic areas, provide dramatic
examples of resource pulses that have been documented for most major world land masses. Here, we compare the dynamic behavior
of two of these systems, with the goal of understanding differences in the long-term consequences of masting events to their
respective communities. Responses to mast events in deciduous oak forests in eastern North America are characteristically
complex and of low resilience. That is, each event produces long-lasting cascading effects in the community, ultimately influencing
not only seed consumers such as rodents and deer, but also the parasites and prey of those consumers, diseases transmitted
by those parasites, and outbreaks of insect herbivores. In contrast, despite more extreme resource pulsing in New Zealand
Nothofagus forests, responses to mast events there are less complex and more resilient, i.e., producing only short-term (<2years),
smoothly damped, numerical responses by a few species, except after (rare) ‘double-mast’ events in consecutive years. A detailed
examination of the two systems suggests some tentative explanations for the strongly contrasting dynamics of these systems.
Firstly, the higher number of species involved in North America seems to reduce resilience by increasing food chain length,
lags, and alternative prey, all of which increase the dynamic complexity compared with that in New Zealand. Secondly, lack
of a shared evolutionary history among species did not necessarily reduce resilience. Some exotic species showed well-damped
fluctuations (e.g., stoats (Mustela erminea L.) in New Zealand), while other exotics showed complex dynamics (e.g., gypsy moths (Lymantria dispar L.) in North America). Thirdly, recent extinctions of species such as the passenger pigeon (Ectopistes migratorius L.), once the dominant acorn predator in eastern North American forests, have likely produced qualitative changes in system
dynamics in both communities. Fourthly, the North American community has more lags and hysteresis, which probably contribute
to the greater dynamic complexity in eastern North America than in New Zealand Nothofagus forests. However, positive feedback loops present in North America seem to have little influence on system dynamics. Because
the massive perturbations induced by masting events are major recurring challenges to an ecosystem, disentangling the causes
of different system responses is likely to lead us to a better understanding of ecosystem function, resilience and stability.
Population Ecology 04/2012; 50(4):329-342. · 2.29 Impact Factor
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ABSTRACT: Trade-offs in sex resource allocation are commonly inferred from a negative correlation between male and female reproduction. We found that for three California oak species, aboveground annual net productivity (ANP) differences among individuals were primarily correlated with water availability and soil fertility. Reproductive biomass increased with ANP, but the relative allocation to reproduction was constant, indicating that reproduction tracked productivity, which in turn tracked site quality. Although there was a negative correlation between male and female reproduction, this was not the result of a resource investment trade-off, but rather a byproduct of the positive correlation between female reproductive biomass and ANP combined with the greater overall resource allocation to female, compared to male, function. Thus, we reject the hypothesis of a trade-off between these key life-history components within individuals of these species. For long-lived individuals, a plastic resource tracking response to environmental fluctuations may be more adaptive than directly linking life-history traits through trade-offs.
PLoS ONE 01/2012; 7(8):e43492. · 4.09 Impact Factor
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ABSTRACT: The ecological conditions leading to delayed dispersal and helping behavior are generally thought to follow one of two contrasting scenarios: that conditions are stable and predictable, resulting in young being ecologically forced to remain as helpers (extrinsic constraints and the habitat saturation hypothesis), or that conditions are highly variable and unpredictable, leading to the need for helpers to raise young, at least when conditions are poor (intrinsic constraints and the hard life hypothesis). We investigated how variability in ecological conditions influences the degree to which helpers augment breeder fitness in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), a species in which the acorn crop, territory quality, and prior breeding experience all vary in ways that have important effects on fitness. We found that the relationship between ecological conditions and the probability that birds would remain as helpers was variable but that helpers generally yielded greater fitness benefits when ecological conditions were favorable, rather than unfavorable, for breeding. These results affirm the importance of extrinsic constraints to delayed dispersal and cooperative breeding in this species, despite its dependence on a highly variable and unpredictable acorn crop. Our findings also confirm that helpers can have very different fitness effects, depending on conditions, but that those effects are not necessarily greater when breeding conditions are unfavorable.
The American Naturalist 08/2011; 178(2):145-58. · 4.72 Impact Factor
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ABSTRACT: Examinamos los efectos de la polilla introducida Lymantria dispar sobre siete especies norteamericanas de pájaros carpinteros mediante el cruce de información espacialmente explícita de los eventos de erupción de L. dispar con datos de las poblaciones reproductivas e invernales. En general, detectamos efectos modestos durante las erupciones. En la época reproductiva, la población de una especie (Melanerpes erythrocephalus) se incrementó por encima de los niveles previos a la erupción, mientras que durante el invierno la población de una especie (Sphyrapicus varius) aumentó y la de otra (Picoides pubescens) disminuyó en relación con los niveles previos. Las respuestas posteriores a las erupciones fueron igualmente variables y, en general, no tuvimos éxito en predecir las respuestas poblacionales a las erupciones a partir de conocimiento previo sobre la ecología y comportamiento de los carpinteros. Sin embargo, encontramos evidencia de que la respuesta de al menos la mitad de las especies cambió a lo largo de los 34 años abarcados por el estudio. Con excepción de Colaptes auratus, cuya respuesta a las erupciones durante el invierno disminuyó, las poblaciones generalmente respondieron de forma más positiva a las erupciones con el paso del tiempo. Esta respuesta temporal sugiere que, actualmente, los carpinteros norteamericanos podrían estar aprovechando mejor que antes los pulsos en los recursos y/o los cambios en el hábitat causados por las erupciones de esta peste exótica. Por esto, los futuros efectos de las erupciones de L. dispar sobre estas especies podrían incrementarse.
The Condor 06/2011; · 1.12 Impact Factor
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ABSTRACT: Despite a substantial resource pulse, numerous avian insectivores known to depredate periodical cicadas (Magicicada spp.) are detected less commonly during emergence years than in either the previous or following years. We used data on periodical cicada calls collected by volunteers conducting North American Breeding Bird Surveys within the range of cicada Brood X to test three hypotheses for this observation: lower detection rates could be caused by bird calls being obscured by cicada calls ("detectability" hypothesis), by birds avoiding areas with cicadas ("repel" hypothesis), or because bird abundances are generally lower during emergence years for some reason unrelated to the current emergence event ("true decline" hypothesis). We tested these hypotheses by comparing bird detections at stations coincident with calling cicadas vs. those without calling cicadas in the year prior to and during cicada emergences. At four distinct levels (stop, route, range, and season), parallel declines of birds in groups exposed and not exposed to cicada calls supported the true decline hypothesis. We discuss several potential mechanisms for this pattern, including the possibility that it is a consequence of the ecological and evolutionary interactions between predators of this extraordinary group of insects.
Ecology 03/2011; 92(3):784-90. · 4.85 Impact Factor
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ABSTRACT: 1. Ecologists have long been interested in the role of climate in shaping species' ranges, and in recent years, this relationship has taken on greater significance because of the need for accurate predictions of the effects of climate change on wildlife populations. Bioclimatic relationships, however, are potentially complicated by various environmental factors operating at multiple spatial and temporal scales. Here, we test the hypothesis that climatic constraints on bird distributions are modified by species-specific responses to weather, urbanization and use of supplemental food. 2. Our analyses focused on 18 bird species with data from over 3000 sites across the north-eastern United States and adjacent Canadian provinces. We use hierarchal occupancy modelling to quantify the effects of short-term weather variation and surrounding urbanization on food stress and probabilities of detection, and how these fine-scale changes modify the role that climate has on the distributions of wintering bird populations at regional scales. 3. Examining site occupancy and supplemental food use across the study region, we found that average minimum temperature was an important factor limiting bird distributions, supporting the hypothesis that the occupancy of wintering birds is limited by climatic constraints. We found that 15 of 18 species (83%) were more energetically stressed (had a higher likelihood of visiting a feeder station) as minimum temperature declined from the seasonal average. Because we found these patterns in populations that regularly visit supplemental food sites and were likely not food-limited, we suggest that resource availability is less important than climate in constraining wintering bird distributions. Across a winter season, local within-winter extinction probabilities were lower and colonization probabilities higher at warmer sites supporting the role of climate-mediated range shifts. Importantly, however, these relationships were modified by the degree of urbanization and species' abilities to persist in human-modified landscapes. 4. Our results suggest that urbanization and behavioural adaptation can modify the role of climate on bird ranges and should be included in future analyses of range shifts because of climate change.
Journal of Animal Ecology 03/2011; 80(2):403-13. · 4.94 Impact Factor
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ABSTRACT: With their striking predilection for perching on African ungulates and eating their ticks, yellow-billed (Buphagus africanus) and red-billed oxpeckers (B. erythrorhynchus) represent one of the few potentially mutualistic relationships among vertebrates. The nature of the oxpecker-ungulate relationship remains uncertain, however, because oxpeckers are known to consume ungulate tissues, suggesting that the relationship between oxpeckers and ungulates may also be parasitic. To examine this issue further, we obtained data on oxpecker preferences for different ungulate species, the abundance of ticks on these ungulates, and ungulate hide thickness. In support of the mutualism hypothesis, we found that both species of oxpeckers prefer ungulate hosts that harbor a higher abundance of ticks. We found no evidence that hide thickness-a measure of the potential for parasitism by oxpeckers-predicts oxpecker preferences for different ungulate species. Oxpeckers also prefer larger-bodied ungulates, possibly because larger animals have more ticks, provide a more stable platform upon which to forage, or support more oxpeckers feeding simultaneously. However, the preference for ungulates with greater tick abundance was independent of host body mass. These results support the hypothesis that the relationship between oxpeckers and ungulates is primarily mutualistic.
Evolution 12/2010; 65(5):1297-304. · 5.15 Impact Factor
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ABSTRACT: Landscape characteristics and social behavior can affect the foraging patterns of seed-dependent animals. We examine the movement of acorns from valley oak (Quercus lobata) trees to granaries maintained by acorn woodpeckers (Melanerpes formicivorus) in two California oak savanna-woodlands differing in the distribution of Q. lobata within each site. In 2004, we sampled Q. lobata acorns from 16 granaries at Sedgwick Reserve in Santa Barbara County and 18 granaries at Hastings Reserve in Monterey County. Sedgwick has lower site-wide density of Q. lobata than Hastings as well as different frequencies of other Quercus species common to both sites. We found acorn woodpeckers foraged from fewer Q. lobata seed source trees (K(g) = 4.1 ± 0.5) at Sedgwick than at Hastings (K(g) = 7.6 ± 0.6) and from fewer effective seed sources (N(em)* = 2.00 and 5.78, respectively). The differences between sites are due to a greater number of incidental seed sources used per granary at Hastings than at Sedgwick. We also found very low levels of seed source sharing between adjacent granaries, indicating that territoriality is strong at both sites and that each social group forages on its own subset of trees. We discovered an interesting spatial pattern in the location of granaries. At Sedgwick, acorn woodpeckers situated their granaries within areas of higher-than-average tree density, while at Hastings, they placed them within areas of lower-than-average tree density, with the outcome that granaries at the two sites were located in areas of similar valley oak density. Our results illustrate that landscape characteristics might influence the number of trees visited by acorn woodpeckers and the locations of territories, while woodpecker social behavior, such as territoriality, shapes which trees are visited and whether they are shared with other social groups.
Oecologia 11/2010; 166(1):187-96. · 3.41 Impact Factor
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ABSTRACT: We examined the ecological factors influencing population declines in American crows (Corvus brachyrhynchos) as they were initially exposed to West Nile virus (WNV), a pathogen first detected in the US in 1999 that has since become one of North America's most prevalent vector-borne pathogens. The strongest effects were initial crow population density (denser populations were more likely to suffer declines), avian species diversity (populations in areas with high diversity were less likely to suffer a decline), human population density (populations were more likely to decline in more urban areas), and time since the pathogen's introduction to the US (populations exposed to the pathogen later in its North American sweep were less likely to suffer declines than those exposed earlier). Variables that played only a minor role included rainfall, mean maximum temperature, and total number of birds, used as a proxy for the overall reservoir competence of the community. These findings indicate that WNV declined in virulence during its rapid 5-year sweep and support the importance of the 'dilution effect' whereby a diverse host community dampens pathogen transmission and potentially slows its rate of spread. Results underscore the need for considering the entire community when trying to understand the factors shaping disease risk.
Oecologia 08/2010; 163(4):903-9. · 3.41 Impact Factor
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ABSTRACT: In cooperatively breeding acorn woodpeckers (Melanerpes formicivorus), helper males have a large positive effect on fledging success in good acorn crop years but only a small positive effect in poor acorn crop years, while helper females exhibit the opposite pattern. Based on these findings, we tested the "concealed helper effects" hypothesis, which proposes that laying females reduce investment in eggs (with respect to their size, number, or quality) in a way that confounds helper effects and results in an absence of a relationship between helpers and breeding success. Results generally failed to support the hypothesis. Mean egg size was positively related to temperatures during the 10 days prior to egg-laying and negatively related to the food supply as indexed by the prior fall's acorn crop, but there were no significant differences vis-à-vis helpers except for interactions with the acorn crop that only partly corresponded to those predicted. With respect to clutch size, females laid larger clutches when assisted by female helpers, opposite the pattern predicted. Although our results suggest that egg size is adjusted to particular ecological circumstances, we conclude that neither egg nor clutch size is adjusted in a way that confounds the apparent effects of helpers, as proposed by the concealed helper effects hypothesis.
Behavioral Ecology and Sociobiology 10/2009; 63(11):1659-1665. · 3.18 Impact Factor
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Walter D. Koenig
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ABSTRACT: We report on growth and survivorship of two cohorts of blue oaks Quercus douglasii Hook. & Arn. (Fagaceae) monitored at Hastings Reservation in Monterey County, California, the first growing in an unprotected old field and measured as seedlings in 1965 and the second planted as acorns in 1985 in several sites differing in their degree of protection from grazing. Growth of all individuals was extremely slow: among those surviving in the first cohort, mean (± SD) height in 2006 was only 76.7 ± 45.0 cm for an average growth rate of 1.8 cm yr−1, and only one of the original 73 oaks had grown taller than 1.5 m while one was still a seedling 28 cm in height 41 yr after being first marked. Of the second cohort, mean height 21 yr after planting was 54.3 ± 31.4 cm. None of these latter individuals had grown out of the sapling stage while 25% were still seedlings < 30 cm in height. Growth of this second cohort was significantly greater when protected from grazing and when growing in the open rather than in the shade. Although growth was slow, survivorship of oaks first measured in 1965 was high, indicting that individuals can live for decades despite significant grazing pressure. Our results confirm the difficulties of inferring age from size of blue oaks, since individuals just achieving the height at which they are typically cored may be 50 or more years old. They also indicate that regeneration, although very slow, can occur in open oak savannas in California despite significant grazing pressure. Whether the observed amount of regeneration is sufficient for long-term sustainability will require continued monitoring and modeling of oak demography.
Madroño 09/2009;
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ABSTRACT: We analyzed the relationship between population abundance and variability of western scrub-jays Aphelocoma californica based on 48 yr of Audubon Christmas Bird Counts and the resources on which they depend as indexed by the diversity and abundance of mast-producing oaks and pines and, for California, estimates of acorn production based on a statewide survey. In general, populations of A. c. californica along the Pacific coast were related to oaks, with populations being more abundant and more stable in areas with more oak species and greater oak abundance. In contrast, populations of A. c. woodhouseii in the Great Basin were correlated with pines/conifers, again with higher abundance and greater stability with increased number of pine species and greater abundance of pines/conifers. The presumed driver of these patterns is increased resource abundance with greater habitat abundance and increased resource stability with increasing species diversity due to asynchrony in seed production among different species of trees. Asynchrony in acorn production is particularly high among oaks that require different number of years to produce acorns, but we failed to confirm that populations with access to both types were more stable than those with access to only one type after controlling for oak diversity. However, we did find a strong positive correlation between overall mean scrub-jay abundance in California and overall acorn production one year earlier, suggesting that acorns benefit scrub-jay populations primarily by increasing reproductive success the following year. These patterns demonstrate the strong dependence between population dynamics and resource stability as well as how different these relationships can be within closely related taxa.
Ecography 07/2009; 32(4):561 - 570. · 4.19 Impact Factor
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ABSTRACT: We examined the relationship between acorn mass and number in valley oaks (Quercus lobata) over 4 years in central coastal California. Despite considerable variation in acorn size among both trees and years, trees produced acorns of the same size relative to other trees in different years. Across years, the relationship between acorn mass and acorn crop size was generally positive, even after controlling for environmental conditions and differences in individual tree size and quality. Life-history trade-offs in valley oaks are primarily between current and future reproduction and indirectly between concurrent growth and reproduction, not between seed size and number, and are probably related to this species' mast-seeding behavior. Phenotypic trade-offs in long-lived plants such as oaks exhibit complex patterns of life-history covariation and deserve greater attention, both theoretically and empirically.
The American Naturalist 06/2009; 173(5):682-8. · 4.72 Impact Factor
