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BIRD SPECIES RESPONSES TO DROUGHT IN TWO SOUTHERN CALIFORNIA COASTAL SAGE SCRUB COMMUNITIES
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Microhabitat matters
Understanding how our warming climate affects vulnerable species is of paramount importance. However, predicting responses is complicated because species are complex and may adapt or respond in distinct ways. Riddell et al. compared a century-old dataset on species richness in the Mojave against modern surveys to measure climate-related changes in bird and small mammal communities. They found little change in mammal richness or occupancy but large declines across birds. They attribute these differences to differences in microclimate opportunities: Specifically, mammals can mitigate temperature impacts through burrowing, whereas birds are generally more exposed.
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Birds respond rapidly to changes in both habitat and climate conditions and thus are good indicators of the ecological effects of a changing climate, which may include warmer temperatures, changing habitat conditions, and increased frequency and magnitude of extreme events like drought. We investigated how a widespread tree mortality event concurrent with a severe drought influenced the avian community of the Sierra Nevada mountain range in California. We assessed and compared the separate effects of climate stresses and altered habitat conditions on the avian community and used this information to evaluate the changes that are likely to occur in the near future. We built tree mortality maps from freely available Landsat imagery with Google Earth Engine. We analyzed avian point counts from 2010 to 2016 in the southern Sierra Nevada, to model temperature, water deficit, and tree mortality effects on the abundances of 45 bird species, and then used these models to project abundances into the future based on three climate projections. A large portion of the avian community, 47%, had a positive relationship with temperature increase, compared to 20% that responded negatively. More species (36%) declined with drier conditions than increased (29%). More species declined in response to high tree mortality (36%) than increased (9%). A preponderance of species adapted to colder temperatures (higher elevation) had negative responses to high tree mortality and water deficit, but positive responses to increasing temperature. We projected the highest total bird abundances in the future under the warmest climate scenario that we considered, but habitat modification (e.g., tree mortality) and water deficit could offset the positive influence of temperature for many species. As other studies have shown, climate warming may lead to substantial but idiosyncratic effects on wildlife species that could result in community composition shifts. We conclude that future climate conditions may not have a universally negative effect on biodiversity in the Sierra Nevada, but probable vegetation changes and increased likelihood of extreme events such as drought should be incorporated into climate‐smart forest and wildlife management decisions.
One of the most important direct effects of climate change is the continuous increasing of droughts, particularly in arid and semiarid regions. By the use of temporal climatic datasets of fourteen years (1991-2004) and information arisen from ringing captures of small reed passerines it is demonstrated that drought cycles in a wetland of SE Spain, immerse in a semiarid landscape, had not effect on the two small passerines that conform the gross of the reed-bed bird’s assemblage, in terms of abundance and biomass. Weak breeding seasons caused by drastic environmental events may have a synergic effect on the different flexibility of the moult strategies of these species and morphological adaptations of well adapted different body-sizes and hence not affecting quantitatively. Finally, novel studies focused on genomic sequence and environmental change in non-model species like birds could offer future prospects to find drought-resistant genes in small individual birds associated to terrestrial aquatic bodies in especially sensitive areas as the semiarid landscapes.
An unprecedented era of climatic volatility is altering ecosystems across our planet1. The potential scale, pace and consequences of this global change have been modelled extensively2, yet little empirical research has quantified the impacts of extreme climate events on the composition of contemporary ecological communities. Here, we quantified the responses of 423 sympatric species of plants, arthropods, birds, reptiles and mammals to California’s drought of 2012–2015—the driest period in the past 1,200 years3 for this global biodiversity hotspot. Plants were most responsive to one-year water deficits, whereas vertebrates responded to longer-term deficits, and extended drought had the greatest impact on carnivorous animals. Locally rare species were more likely to increase in numbers and abundant species were more likely to decline in response to drought, and this negative density dependence was remarkably consistent across taxa and drought durations. Our system-wide analysis reveals that droughts indirectly promote the long-term persistence of rare species by stressing dominant species throughout the food web. These findings highlight processes that shape community structure in highly variable environments and provide insights into whole-community responses to modern climate volatility.
The historical and presettlement relationships between drought and wildfire are well documented in North America, with forest fire occurrence and area clearly increasing in response to drought. There is also evidence that drought interacts with other controls (forest productivity, topography, fire weather, management activities) to affect fire intensity, severity, extent, and frequency. Fire regime characteristics arise across many individual fires at a variety of spatial and temporal scales, so both weather and climate - including short- and long-term droughts - are important and influence several, but not all, aspects of fire regimes. We review relationships between drought and fire regimes in United States forests, fire-related drought metrics and expected changes in fire risk, and implications for fire management under climate change. Collectively, this points to a conceptual model of fire on real landscapes: fire regimes, and how they change through time, are products of fuels and how other factors affect their availability (abundance, arrangement, continuity) and flammability (moisture, chemical composition). Climate, management, and land use all affect availability, flammability, and probability of ignition differently in different parts of North America. From a fire ecology perspective, the concept of drought varies with scale, application, scientific or management objective, and ecosystem.
Climate change elevates conservation concerns worldwide because it is likely to exacerbate many identified threats to animal populations. In recent decades, grassland birds have declined faster than other North American bird species, a loss thought to be due to habitat loss and fragmentation and changing agricultural practices. Climate change poses additional threats of unknown magnitude to these already declining populations. We examined how seasonal and daily weather conditions over ten years influenced nest survival of five species of insectivorous passerines native to the shortgrass prairie and evaluate our findings relative to future climate predictions for this region. Daily nest survival (n = 870) was best predicted by a combination of daily and seasonal weather variables, age of nest, time in season and bird habitat guild. Within a season, survival rates were lower on very hot days (temperatures ≥ 35° C), on dry days (with a lag of one day), and on stormy days (especially for those species nesting in shorter vegetation). Across years, survival rates were also lower during warmer and drier breeding seasons. Clutch sizes were larger when early spring temperatures were cool and the week prior to egg-laying was wetter and warming. Climate change is likely to exacerbate grassland bird population declines because projected climate conditions include rising temperatures, more prolonged drought, and more intense storms as the hydrological cycle is altered. Under varying realistic scenarios, nest success estimates were halved compared to their current average value when models both increased the temperature (3˚ C) and decreased precipitation (two additional dry days during a nesting period), thus underscoring a sense of urgency in identifying and addressing the current causes of range-wide declines. This article is protected by copyright. All rights reserved.
The U.S. Government created the Kofa National Wildlife Refuge (Kofa NWR) in 1939 in response to a citizen campaign to improve desert bighorn sheep populations in Arizona.The Kofa NWR is mountainous and remote, and its management by the U.S. Fish and Wildlife Service (FWS) keeps anthropogenic disturbance levels low. As such, Partners In Flight (PIF) listed the Kofa NWR as one of its Sonoran Desert portfolio sites in its Desert Bird Conservation Plan (McCreedy and others,2009). Research presented here demonstrates that bird communities within a well-managed and remote Sonoran Desert portfolio site can nonetheless be negatively affected by human-caused stressors like fire and Brown-headed Cowbird (Molothrus ater) parasitism, which originate from beyond the Kofa NWR’s boundaries.
In chapter 1, we examine how avian productivity can be influenced by timing of nest initiation. We demonstrate that(1) late nesting dates are correlated with low winter precipitation levels, a condition expected to occur in greater frequency in coming decades due to climate change (Seager and others, 2007) and that (2) late nesting dates result in decreased productivity, due to higher rates of nest depredation and,in the case of an open-cup nesting species, higher rates of brood parasitism experienced later in the breeding season. The brood parasite, the Brown-headed Cowbird, appears to forage and roost on agricultural lands north of the Kofa NWR’s boundary. From that location, they commute to the refuge to parasitize other passerine bird nests. Drought and subsequent loss in primary production have been correlated with decreased productivity for birds that breed in arid habitats (Preston and Rotenberry, 2006; Chase and others, 2005; Johnson and others, 2002; Morrison and Bolger, 2002; Brown and Li, 1996; Anderson and Anderson,1973), yet drought’s standing as a threat to bird populations has been underestimated in recent regional conservation planning efforts (McCreedy and others, 2009; Latta and others,1999).
In chapter 2, we examine the effects of the King Valley fire on breeding and migrant birds within the Kofa NWR. This fire was caused by incendiary weapons testing within Yuma Proving Ground, south of the Kofa NWR boundary (Esque and others,2013). We found large differences in spring migrant and breeding species abundance and richness between bird count stations within the 2005 King Valley fire zone and bird count stations immediately outside the fire perimeter. Habitat loss to fire, and the subsequent slow regeneration of a Sonoran Desert flora that is not well adapted to fire disturbance,is a recognized threat to bird populations (McCreedy and others, 2009; Latta,1999), and of all Sonoran Desert wildlife, birds may be the most impacted by loss of perennial Sonoran Desert vegetation to fire (Esque and Schwalbe, 2002). We conclude that decreases in both breeding and migrant use of washes within burned areas will likely persist into the long term (>25 years) due to slow return rates of xeroriparian woodlands lost in the fire.
Spatially weighted averages of Palmer Drought Severity Index (PDSI) over central and southern California show that the 1-year 2014 drought was not as severe as previously reported, but it still is the most severe in the 1895–2014 instrumental record. Using the typical adjustment procedure that matches the mean and standard deviation of tree-ring PDSI values to those of instrumental data shows over ten droughts from 800–2006 that were more severe than the 1-year 2014 drought, with the 2014 drought having a return period of 140–180 years. Quantile mapping allows for a closer correspondence between instrumental and tree-ring PDSI probability distributions and produces return periods of 700–900 years for the 1-year 2014 drought. Associated cumulative 3- and 4-year droughts, however, are estimated to be much more severe. The 2012–2014 drought is nearly a 10,000-year event while the 2012–2015 drought has an almost incalculable return period and is completely without precedent.
Supplemental feeding is a key component to many conservation efforts. Because the effect of feeding on a target population's breeding biology interacts with weather in complex ways, supplemental feeding programs may be more influential on target populations in some years than in others. We examined the effect of supplemental feeding on breeding characteristics of San Clemente loggerhead shrike (Lanius ludovicianus mearnsi) during both relatively dry and wet years. From 2001 through 2007, the mean number of independent young produced by supplementally fed shrike pairs was 30% higher than for non-supplementally fed pairs. Shrike breeding phenology was affected by both rainfall and supplemental feeding, with the beneficial effects of supplemental feeding most evident in dry years. In dry years, supplementally fed pairs initiated breeding an average of 12 days earlier, were nearly a third again as likely to renest after successfully fledging young, and attempted a third again as many nests as non-fed pairs. None of these effects were observed following relatively wet winters. Supplemental feeding did not affect nest success, nor did it result in a statistically significant increase in independent young production regardless of winter rainfall. These observations indicate that supplemental feeding reduces the negative impacts of food limitation imposed by periodic drought, but does not mitigate predation pressure on nesting shrikes. Our study suggests that supplemental feeding efforts should vary from year to year based on winter rainfall, with greater efforts put forth in drought years than during relatively wet years. This study demonstrates that evaluating the effects of provisioning on breeding components can provide a sensitive indicator of whether or not a program is working and that understanding the proximate mechanisms through which supplemental feeding acts is an important step to optimizing adaptive management of endangered species.
AimClimate change is expected to increase the frequency and intensity of extreme climatic events, such as severe droughts and intense rainfall periods. We explored how the avifauna of a highly modified region responded to a 13-year drought (the ‘Big Dry’), followed by a two-year period of substantially higher than average rainfall (the ‘Big Wet’).LocationTemperate woodlands in north central Victoria, Australia.Methods
We used two spatially extensive, long-term survey programmes, each of which was repeated three times: early and late in the Big Dry, and in the Big Wet. We compared species-specific changes in reporting rates between periods in both programmes to explore the resistance (the ability to persist during drought) and resilience (extent of recovery post-drought) of species to climate extremes.ResultsThere was a substantial decline in the reporting rates of 42–62% (depending on programme) of species between surveys conducted early and late in the Big Dry. In the Big Wet, there was some recovery, with 21–29% of species increasing substantially. However, more than half of species did not recover and 14–27% of species continued to decline in reporting rate compared with early on in the Big Dry. Species' responses were not strongly related to ecological traits. Species resistance to the drought was inversely related to resilience in the Big Wet for 20–35% of the species, while 76–78% of species with low resistance showed an overall decline across the study period.Conclusions
As declines occurred largely irrespective of ecological traits, this suggests a widespread mechanism is responsible. Species that declined the most during the Big Dry did not necessarily show the greatest recoveries. In already much modified regions, climate extremes such as extended drought will induce on-going changes in the biota.
Results are presented from point-counts at six sites in the Kalahari in Botswana. Counts were repeated three times: during a dry season following good rains (1991), during the next wet season when rains were far below average, and the following dry season (1992) when the area became drought-stricken. Compared to the wet season, bird numbers decreased during the drought by 37–81% and species by 8–52%; compared to the previous dry season, birds decreased by 5–71% and species by 2–47%. Bird diversity (relative to numbers) tended to increase during the wet season but was little affected by drought, except in the northern Kalahari, where a greater proportion of birds moved out in response to drought. This gave the northern Kalahari the most distinct bird community during a wet cycle, but it became again typically Kalahari during the drought. Thus, the typical Kalahari bird communities expanded their range during drought into the moister periphery. Changes in numbers, most probably resulting from (local) movements were found in many species. Most confirmed earlier reports on their nomadic nature but some, like Red-crested Lophotis ruficrista and Northern Black Afrotis afraoides Korhaan, Chestnut-vented Tit-Babbler Parisoma subcaeruleum, Ant-eating Chat Myrmecocichla formicivora, Tinkling Cisticola Cisticola rufilatus, Black-chested Prinia Prinia flavicans, Marico Flycatcher Bradornis mariquensis and Brown-crowned Tchagra Tchagra australis have not been or are not widely recognised as mobile species.
The 2013-14 California drought was accompanied by an anomalous high-amplitude ridge system. The anomalous ridge was investigated using reanalysis data and the Community Earth System Model (CESM). It was found that the ridge emerged from continual sources of Rossby wave energy in the western North Pacific starting in late summer, and subsequently intensified into winter. The ridge generated a surge of wave energy downwind and deepened further the trough over the northeast U.S., forming a dipole. The dipole and associated circulation pattern is not linked directly with either ENSO or Pacific Decadal Oscillation; instead it is correlated with a type of ENSO precursor. The connection between the dipole and ENSO precursor has become stronger since the 1970s, and this is attributed to increased GHG loading as simulated by the CESM. Therefore, there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge during winter 2013-14, the associated drought and its intensity.
We studied the effect of a severe drought on the population dynamics and community structure of grassland birds in western North Dakota. During the spring and summer of 1988 the northern Great Plains suffered one of the warmest, driest periods in its recorded history. We compared the changes in bird populations and nesting productivity over a 3-yr period before, during, and after the drought. Total grassland bird density declined 61% (P < .05) between June 1987 and June 1988. Densities of six of eight common species declined significantly during the drought. Populations of all but two species recovered in 1989 and total bird density in June 1989 did not differ significantly from June 1987. Species richness and species diversity both declined significantly during the drought and recovered to predrought levels in 1989. Species richness declined more on fair condition than on good condition range during the drought. Vesper Sparrow (Pooecetes gramineus) hatching success, number of young fledged per success
Few long-term studies describe population dynamics of tropical avian communities during normal or abnormal periods. Karr et al. (1982) re- ported comparable measurements of bird density and community structure in Panamanian forest over a period of nearly a decade and Willis (1974) has long-term data for selected species on Barro Colorado Island, Panama. Although Diamond (1971) and Terborgh and Faaborg (1973) reported sup- posed cases of turnover (change in species composition over time without change in the equilibrium species number (see Diamond and May 1977) in island communities (cases which have been questioned (Lynch and Johnson 1974)), no one has examined the role of actual population fluc- tuations in the turnover process. Here I report on the population dynamics of a West Indian bird com- munity over a 5-year period that included severe drought conditions. The effects of these conditions on total populations, populations of foraging guilds, and winter resident populations are shown. Possible reasons for differences among species are discussed, along with the way that such "ecological crunches" (Wiens 1977) may have led to patterns found in the Guanica Forest bird community. This study was done in seasonally dry scrub in the Guanica Forest of southwestern Puerto Rico (for detailed habitat description see Terborgh and Faaborg 1973). The vegetation is situated on a limestone platform and is generally short and sclerophyllous, with many ar- boreal cacti. Population size was determined by mist netting, as described earlier (Terborgh and Faa- borg 1973). Net lines were operated from dawn to dusk, usually for 3 days. This allowed us to construct regressions of the capture rate of birds over time and to predict a total capture. On a couple of occasions only 2 days of netting were done; thus 2-day totals are used. Addition of the third day only accentuates the difference between good and bad periods, as the third day of netting during drought periods often yielded fewer than 10 birds. In all cases, the number of unbanded birds netted declined throughout the sample period, suggesting a resident population; during the drought period drastic drops in capture rates often occurred after the first day. One line of 16 nets was operated in January of 1972. In 1973 a second line was erected and both lines were operated subsequently in January or early February of each year. The area was visited and netted in June of 1973, but due to possible seasonal differences in territorial behavior, these data are not presented here. All birds captured were banded, measured and released (for longevity records see Faaborg and Winters 1979, 1980), and age and sex were recorded when possible.
It has been hypothesized that clutch size in bird species occurring in arid habitats is influenced by annual rainfall. We propose an alternative hypoth- esis that avian clutch size in resident species should be more strongly associated with rainfall in the short-term (during egg-formation) than during the long-term (cu- mulative across the season). We tested this hypothesis with museum egg-sets for California Gnatcatcher (Po- lioptila californica) nests taken in southern California combined with rainfall data from the geographically closest weather station to nest collection site. Clutch size was independent of laying date. Seasonal rainfall was not a good predictor of clutch size; rainfall during egg formation was a better predictor. Using isotonic regression, we detected a strong positive trend in the association between clutch size and cumulative rain- fall, with cumulative rainfall across 1 month prior to the estimated month of clutch completion having the strongest positive association. These data support the hypothesis that smaller clutches result from more im- mediate conditions, not from the wet-year/dry-year di- chotomy.
[1] We use a multi-model, multi-scenario climate model ensemble to identify climate change hotspots in the continental United States. Our ensemble consists of the CMIP3 atmosphere-ocean general circulation models, along with a high-resolution nested climate modeling system. We test both high (A2) and low (B1) greenhouse gas emissions trajectories, as well as two different statistical metrics for identifying regional climate change hotspots. We find that the pattern of peak responsiveness in the CMIP3 ensemble is persistent across variations in GHG concentration, GHG trajectory, and identification method. Areas of the southwestern United States and northern Mexico are the most persistent hotspots. The high-resolution climate modeling system produces highly localized hotspots within the basic GCM structure, but with a higher sensitivity to the identification method. Across the ensemble, the pattern of relative climate change hotspots is shaped primarily by changes in interannual variability of the contributing variables rather than by changes in the long-term means.
Anthropogenic climate change is predicted to be a major cause of species extinctions in the next 100 years. But what will actually cause these extinctions? For example, will it be limited physiological tolerance to high temperatures, changing biotic interactions or other factors? Here, we systematically review the proximate causes of climate-change related extinctions and their empirical support. We find 136 case studies of climatic impacts that are potentially relevant to this topic. However, only seven identified proximate causes of demonstrated local extinctions due to anthropogenic climate change. Among these seven studies, the proximate causes vary widely. Surprisingly, none show a straightforward relationship between local extinction and limited tolerances to high temperature. Instead, many studies implicate species interactions as an important proximate cause, especially decreases in food availability. We find very similar patterns in studies showing decreases in abundance associated with climate change, and in those studies showing impacts of climatic oscillations. Collectively, these results highlight our disturbingly limited knowledge of this crucial issue but also support the idea that changing species interactions are an important cause of documented population declines and extinctions related to climate change. Finally, we briefly outline general research strategies for identifying these proximate causes in future studies.
Many South American hotspots of bird endemism are found in Polylepis dominated forests. Although the avifauna of Polylepis forests has been relatively well studied in the tropical Andes, little is known in Argentina. In this study, we characterize the Argentine avifauna of Polylepis australis forests along their entire latitudinal gradient of distribution and provide a first systematic bird list and their conservation status. Fieldwork was carried out from 1,500 to 2,800 m a.s.l. in three study sites: North (Jujuy province), Centre (Tucumán province) and South (Córdoba province); we surveyed 30 point counts per site. We recorded 543 individuals belonging to 50 bird species; two subspecies are endemic to the South site and four species are typical Polylepis forest birds of the North and Centre sites. We identified four species of conservation concern; one globally threatened and three declining at regional level. Bird richness and abundance decreased with latitude; and equitability showed an inverse pattern. Sorenson's similarity index ranged from 12% to 40% showing a large latitudinal turnover in avian communities. Disturbance-sensitive species were the most diverse group in the North site. Variations in species richness, evenness and guild composition may be a response to latitude, but also to differences in habitat complexity and food availability, which appear to be a consequence of forest degradation at the South site. We recommend: (1) the enlargement of Calilegua National Park to include the Polylepis australis belt to assure the conservation of many disturbance-sensitive species which could decline or disappear in degraded areas; (2) management of Quebrada del Condorito National Park to increase the forest structure complexity and therefore provide habitat for disturbance-sensitive species.
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This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO2 or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life-history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.
Understanding the mechanisms by which climate change will affect animal populations is vital for adaptive management. Many studies have described changes in the timing of biological events, which can produce phenological mismatch. Direct effects on prey abundance might also be important, but have rarely been studied. We examine the likely importance of variation in prey abundance in driving the demographics of a European golden plover (Pluvialis apricaria) population at its southern range margin. Previous studies have correlated plover productivity with the abundance of their adult cranefly (Tipulidae) prey, and modelled the phenology of both plover breeding and cranefly emergence in relation to temperature. Our analyses demonstrate that abundance of adult craneflies is correlated with August temperature in the previous year. Correspondingly, changes in the golden plover population are negatively correlated with August temperature 2 years earlier. Predictions of annual productivity, based on temperature-mediated reductions in prey abundance, closely match observed trends. Modelled variation in annual productivity for a future scenario of increasing August temperatures predicts a significant risk of extinction of the golden plover population over the next 100 years, depending upon the magnitude of warming. Direct effects of climate warming upon cranefly populations may therefore cause northward range contractions of golden plovers, as predicted by climate envelope modelling. Craneflies are an important food source for many northern and upland birds, and our results are likely to have wide relevance to these other species. Research into the potential for habitat management to improve the resilience of cranefly populations to high temperature should be an urgent priority.
Droughts are expected to become more frequent under global climate change. Avifauna depend on precipitation for hydration, cover, and food. While there are indications that avian communities respond negatively to drought, little is known about the response of birds with differing functional and behavioural traits, what time periods and indicators of drought are most relevant, or how response varies geographically at broad spatial scales. Our goals were thus to determine (1) how avian abundance and species richness are related to drought, (2) whether community variations are more related to vegetation vigour or precipitation deviations and at what time periods relationships were strongest, (3) how response varies among avian guilds, and (4) how response varies among ecoregions with different precipitation regimes. Using mixed effect models and 1989–2005 North American Breeding Bird Survey data over the central United States, we examined the response to 10 precipitation- and greenness-based metrics by abundance and species richness of the avian community overall, and of four behavioural guilds. Drought was associated with the most negative impacts on avifauna in the semiarid Great Plains, while positive responses were observed in montane areas. Our models predict that in the plains, Neotropical migrants respond the most negatively to extreme drought, decreasing by 13.2% and 6.0% in abundance and richness, while permanent resident abundance and richness increase by 11.5% and 3.6%, respectively in montane areas. In most cases, response of abundance was greater than richness and models based on precipitation metrics spanning 32-week time periods were more supported than those covering shorter time periods and those based on greenness. While drought is but one of myriad environmental variations birds encounter, our results indicate that drought is capable of imposing sizable shifts in abundance, richness, and composition on avian communities, an important implication of a more climatically variable future.
Our goal was to evaluate how avian assemblages varied along a gradient of urbanization in the highly fragmented landscape of coastal southern California. We measured species richness and abundance of birds within continuous blocks of habitat, within urban habitat fragments that varied in landscape and local habitat variables, and within the urban matrix at different distances from the wildland interface. These comparisons allowed us to characterize patterns of avifaunal response to a gradient of urban fragmentation. At the fragment scale, we found that fragment area was a strong, positive predictor of the total number of breeding species detected per fragment; total bird abundance per point count also increased with fragment size. Tree cover was higher in small fragments, as was the abundance of birds that typically occupy wooded habitats. Comparisons between core, fragment, and urban transects revealed differing patterns of response of individual bird species to urbanization. In unfragmented habitat, we recorded a relatively high diversity of urbanization-sensitive birds. In urban transects, these species were rare, and a relatively few species of non-native and anthropophilic birds were common. These urbanization-enhanced birds were also recorded in previous urban gradient studies in northern California and Ohio. Bird communities along the urban gradient reached their highest richness and abundance in fragments. The marked difference in vegetation structure between urban and natural landscapes in this arid shrubland system likely contributed to this pattern; the presence of native shrubs and exotic trees in fragments enabled both shrub and arboreal nesters to co-occur. As is characteristic of biotic homogenization, urban fragmentation in coastal southern California may increase local diversity but decrease overall regional avifaunal diversity.
Climate models robustly predict that the climate of southwestern North America, defined as the area from the western Great Plains to the Pacific Ocean and from the Oregon border to southern Mexico, will dry throughout the current century as a consequence of rising greenhouse gases. This regional drying is part of a general drying of the subtropics and poleward expansion of the subtropical dry zones. Through an analysis of 15 coupled climate models it is shown here that the drying is driven by a reduction of winter season precipitation associated with increased moisture divergence by the mean flow and reduced moisture convergence by transient eddies. Due to the presence of large amplitude decadal variations of presumed natural origin, observations to date cannot confirm that this transition to a drier climate is already underway, but it is anticipated that the anthropogenic drying will reach the amplitude of natural decadal variability by midcentury. In addition to this drop in total precipitation, warming is already causing a decline in mountain snow mass and an advance in the timing of spring snow melt disrupting the natural water storage systems that are part of the region's water supply system. Uncertainties in how radiative forcing will impact the tropical Pacific climate system create uncertainties in the amplitude of drying in southwest North America with a La Niña-like response creating a worst case scenario of greater drying.
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.
Recently, climate change research has emphasized the potential increase in the frequency and severity of climatic extremes. We compared the reproductive effort and output among four species of passerine birds in coastal southern California, USA, a semi-arid region, during a normal precipitation year (2001) and the driest year in a 150-year climate record (2002). Both reproductive effort and output differed dramatically between years. Mean reproductive output among the four species was 2.37 fledglings/pair in 2001 and 88.4% of all pairs observed attempted at least one nest. The birds attempted a mean of 1.44 nests per pair and were successful in 47.7% of those attempts. In 2002, only 6.7% of the pairs even attempted a nest and only 1.8% were successful, for a total output of 0.07 fledglings per pair. The abundance of suitable arthropod prey items in the environment was also much lower in 2002, suggesting that low food availability was the proximal cause of the reproductive failure. The data for one of these species, the rufous-crowned sparrow (Aimophila ruficeps), were combined with reproductive and rainfall data from a previous 3-year study (1997-1999) in the same sites. The combined data sets suggest that the response of reproduction to rainfall variation is linear, and that the low end of the precipitation range brings the population near reproductive failure. Any change in climate that would increase the frequency of extreme dry conditions would likely endanger populations of these species.
We evaluated the influences of several ecological, biological, and methodological factors on post-fledging survival of a shortgrass prairie bird, the Lark Bunting (Calamospiza melanocorys). We estimated daily post-fledging survival (n = 206, 82 broods) using radiotelemetry and color bands to track fledglings. Daily survival probabilities were best explained by drought intensity, time in season (quadratic trend), ages < or = 3 d post-fledging, and rank given drought intensity. Drought intensity had a strong negative effect on survival. Rank was an important predictor of fledgling survival only during the severe drought of 2002 when the smallest fledglings had lower survival. Recently fledged young (ages < or = 3 d post-fledging) undergoing the transition from nest to surrounding habitat experienced markedly lower survival, demonstrating the vulnerable nature of this time period. Survival was greater in mid and late season than early season, corresponding to our assumptions of food availability. Neither mark type nor sex of attending parent influenced survival. The model-averaged product of the 22-d survival calculated using mean rank and median value of time in season was 0.360 +/- 0.08 in 2001 and 0.276 +/- 0.08 in 2002. Survival estimates that account for age, condition of young, ecological conditions, and other factors are important for parameterization of realistic population models. Biologists using population growth models to elucidate mechanisms of population declines should attempt to estimate species-specific of post-fledging survival rather than use generalized estimates.
I investigated the causes and consequences of adult breeding-site fidelity, territory fidelity, and natal philopatry in Willow Flycatchers (Empidonax traillii) in southeastern Oregon over a 10-year period, testing the general hypothesis that fidelity and dispersal distances are influenced by previous breeding performance. Willow Flycatchers adhered to the generally observed tendencies of passerine birds for low natal philopatry and high breedingsite fidelity. Site fidelity (return to the study area) of adult males (52.0%) and females (51.3%), and median dispersal distances between seasons (16 m vs. 19 m) were similar. Previous breeding performance and residency (age-experience), but not study-site quality, explained site fidelity in females. Site fidelity of females rearing 4–5 young (64.4%) exceeded that of unsuccessful females (40.0%), breeding dispersal was less (successful: 15 m; unsuccessful: 33 m), and novice residents were more site-faithful than former residents. Probability of site fidelity was higher for previously successful females (odds ratio = 4.76), those with greater seasonal fecundity (odds ratio = 1.58), novice residents (odds ratio = 1.41), and unparasitized females (odds ratio = 2.76). Male site fidelity was not related to residency, site quality, or previous breeding performance. Territory fidelity (return to the previous territory) in females was best explained by previous breeding performance, but not by site quality or residency. Previously successful females were more likely to return to their territory of the previous season than either unsuccessful (odds ratio = 14.35) or parasitized birds (odds ratio = 6.38). Male territory fidelity was not related to residency, site quality, or previous breeding performance. Natal philopatry was low (7.8%) and similar for males and females. Site quality appeared to influence philopatry, given that no birds reared at a low-quality study site returned there to breed, and birds reared there dispersed farther than birds reared at two other study sites. My results partially support the hypothesis that site fidelity is an adaptive response: (1) previously successful females that switched territories underperformed those that did not switch (P = 0.01); and (2) previously unsuccessful females that switched territories outperformed those that did not switch, but not significantly (P = 0.22).
A bird survey in the Wanjarri Nature Reserve in the Goldfields of Western Australia at the end of a year long drought revealed two responses to the short-term dry spell. Firstly, many bird populations were present in very low numbers. Secondly, areas of groving and creekline mulga supported a significantly higher number of bird species and individuals than other habitats. This was thought to be due to the greater foliage cover in groving and creekline mulga, probably as a consequence of soil structure, greater water availability and higher soil fertility. These areas of groving and creekline mulga may be important as drought refugia for resident bird species. Our results need to be confirmed by a more comprehensive study but the identification of drought refugia is of fundamental importance in the conservation of birds in the arid-zone of Australia, so we believe that the conservation of these groving and creekline mulga areas should be given a high priority.
From 1983 to 2003 I examined the effects of fire on the bird community of two 1.25-ha tracts of coastal sage scrub, Box Canyon and F Canyon, 0.4 km apart in a 31-ha reserve in Los Angeles County, California. Wildfire burned Box Canyon in 1981 and both sites in 1989.1 observed 90 species in F Canyon, 80 in Box Canyon, of which 73 were seen in both. The same species were common throughout the 20-year period. F Canyon had more species per count than Box Canyon in both summer and winter. Immediately after the 1989 wildfire, observations of some species of open habitat increased, and observations of some species confined to shrubs decreased. Effects of the 1989 fire on the sites were of short duration. Differences between the sites in number of species attributed to the 1981 fire in earlier studies are confounded by differences between the sites.
Avian communities of arid ecosystems may be particularly vulnerable to global climate change due to the magnitude of projected change for desert regions and the inherent challenges for species residing in resource limited ecosystems. How arid-zone birds will be affected by rapid increases in air temperature and increased drought frequency and severity is poorly understood because avian responses to climate change have primarily been studied in the relatively mesic northern temperate regions. We studied the effects of increasing air temperature and aridity on a Burrowing Owl (Athene cunicularia) population in the southwestern USA from 1998-2013. Over 16 years, the breeding population declined 98.1%, from 52 pairs to 1 pair, and nest success and fledgling output also declined significantly. These trends were strongly associated with the combined effects of decreased precipitation and increased air temperature. Arrival on the breeding grounds, pair formation, nest initiation, and hatch dates all showed significant delays ranging from 9.4 to 25.1 days over 9 years, which have negative effects on reproduction. Adult and juvenile body mass decreased significantly over time, with a loss of 7.9% mass in adult males and 10.9% mass in adult females over 16 years, and a loss of 20.0% mass in nestlings over 8 years. Taken together, these population and reproductive trends have serious implications for local population persistence. The southwestern USA has been identified as a climate change hotspot, with projections of warmer temperatures, less winter precipitation, and an increase in frequency and severity of extreme events including drought and heat waves. An increasingly warm and dry climate may contribute to this species’ decline, and may already be a driving force of their apparent decline in the desert southwest.This article is protected by copyright. All rights reserved.
A suite of climate datasets and multiple representations of atmospheric moisture demand are used to calculate many estimates of the self-calibrated Palmer Drought Severity Index, a proxy for near-surface soil moisture, across California from 1901–2014 at high spatial resolution. Based on the ensemble of calculations, California drought conditions were record-breaking in 2014, but probably not record-breaking in 2012–2014, contrary to prior findings. Regionally, the 2012–2014 drought was record-breaking in the agriculturally important southern Central Valley and highly populated coastal areas. Contributions of individual climate variables to recent drought are also examined, including the temperature component associated with anthropogenic warming. Precipitation is the primary driver of drought variability but anthropogenic warming is estimated to have accounted for 8–27% of the observed drought anomaly in 2012–2014 and 5–18% in 2014. Although natural variability dominates, anthropogenic warming has substantially increased the overall likelihood of extreme California droughts.
Achieving long-term persistence of species in urbanized landscapes requires characterizing population genetic structure in order to understand and manage the effects of anthropogenic disturbance on connectivity. Urbanization over the past century in coastal southern California has caused both precipitous loss of coastal sage scrub habitat and declines in populations of the cactus wren (Campylorhynchus brunneicapillus). Using 22 microsatellite loci, we found that remnant cactus wren aggregations in coastal southern California comprised 20 populations based upon strict exact tests for population differentiation, and 12 genetic clusters with hierarchical Bayesian clustering analyses. Genetic structure patterns largely mirrored underlying habitat availability, with cluster and population boundaries coinciding with fragmentation caused primarily by urbanization. Using a habitat model we developed, we detected stronger associations between habitat-based distances and genetic distances than Euclidean geographic distance. Within populations, we detected a positive association between available local habitat and allelic richness and a negative association with relatedness. Isolation by distance patterns varied over the study area, which we attribute to temporal differences in anthropogenic landscape development. We also found that genetic bottleneck signals were associated with wildfire frequency. These results indicate that habitat fragmentation and alterations have reduced genetic connectivity and diversity of cactus wren populations in coastal southern California. Management efforts focused on improving connectivity among remaining populations may help to ensure population persistence. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
For the past three years (2012-2014), California has experienced the most severe drought conditions in its last century. But how unusual is this event? Here we use two paleoclimate reconstructions of drought and precipitation for Central and Southern California to place this current event in the context of the last millennium. We demonstrate that while 3-year periods of persistent below-average soil moisture are not uncommon, the current event is the most severe drought in the last 1200 years, with single year (2014) and accumulated moisture deficits worse than any previous continuous span of dry years. Tree-ring chronologies extended through the 2014 growing season reveal that precipitation during the drought has been anomalously low but not outside the range of natural variability. The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures.
Understanding demographic processes will be essential to construct robust models of population responses to climate change. We show that survival is related to the strength of the North Atlantic Oscillation in five out of ten British resident passerine species, and explore the importance of biologically more specific variables (duration of winter frosts and snow periods; occurrence of cold, wet days; spring temperature; and summer drought). The most important variables differed between species in relation to differences in foraging strategy. In almost all cases, first-year survival was influenced by weather more than was the survival of adult birds. Particularly vulnerable species, such as the Wren Troglodytes troglodytes, may exhibit a 25% reduction in juvenile survival rates due to adverse weather within the range experienced in the last 30 years; variation in survival by 10% or more is commonplace in most species. Thus, climate influences on food availability may provide the mechanism by which populations will alter under changed climatic conditions, though the presence of density dependence may reduce the impact of this on long-term population trajectories.
The relative influence of local versus landscape heterogeneity on species abundance is an important consideration in the design and management of nature reserves in heterogeneous landscapes. We described the spatial patterns of resident breeding bird abundance at a landscape scale in an urbanizing landscape gradient in coastal San Diego County, California (USA). The gradient spanned approximately 260 km2 and included a large, mostly contiguous block of undeveloped shrub habitat mosaic which graded into an area of internally fragmented habitat and finally into an area characterized by discrete patches of habitat in an urban matrix. We quantified the distribution of resident bird species in this landscape with point counts and used stepwise logistic regression and canonical correspondence analysis to explore the relative strengths of association between bird abundance and local habitat variation versus landscape variation for the 20 most common bird species. We measured local habitat conditions in a 100-m radius around each point count station. Landscape variables were extracted from a GIS database of this region and included habitat patch size, distance to developed edge, and two indices of urban exposure. The addition of landscape variables to stepwise logistic regression analyses substantially improved the predictive power of the resulting models in 14 of 20 species. Model concordance (the ability of the model to correctly classify all pairs of divergent sites) increased from a mean of 56% without landscape variables to 76% when one or more landscape variables were selected for inclusion in the final model. Response of the 20 focal species to the landscape gradient differed markedly among species. Canonical correspondence analysis was used to rank species along a landscape response gradient from edge/fragmentation reduced species to edge/fragmentation enhanced species. The four species exhibiting the most edge/fragmentation reduced pattern were sparrows: Sage (Amphispiza belli), Rufous-crowned (Aimophila ruficeps), Lark (Chondestes grammacus), and Black-chinned sparrows (Spizella atrogularis). Costa’s Hummingbird (Calypte costae) also showed a pattern of edge/fragmentation reduction. The opposite end of the gradient was occupied by species whose densities are elevated near edges and in fragmented areas. The middle of the gradient was occupied by species that were abundant, widely distributed, and showed no sensitivity to landscape position or shrub habitat type. The abundance of edge/fragmentation reduced species appears to be depressed within 200 to 500 m of an edge and the abundance of the edge/fragmentation enhanced species is elevated up to 1000 m from an edge, depending on the species. Conservation analyses, including GAP analysis, often assume that the relationship between species abundance and habitat is invariant with landscape position. In heterogeneous landscapes these analyses may underestimate the conservation risk to species in the edge/fragmentation sensitive category because their abundance varies strongly with landscape location.
La influencia relativa de heterogeneidad local y de paisaje en abundancia de especies es de importante consideración en el diseño y manejo de reservas naturales en paisajes heterogéneos. Describimos los patrones espaciales de aves reproductoras residentes a escala de paisaje en un paisaje con gradiente de urbanización en la costa del municipio de San Diego, California (USA). El gradiente se extiende por approximadamente 260 km2 e incluye un bloque grande casi contiguo de mosaicos de hábitat arbustivo poco desarrollado que atravieza un área de hábitat internamente fragmentado y finaliza en un área caracterizada por parches discretos de hábitat en una matríz urbana. Cuantificamos la distribución de especies de aves residentes en este paisaje con puntos de conteo y usamos una regresión logistica por pasos y un análsis de correspondencia canónica para explorar el grado relativo de asociación entre la abundancia de aves y la variación local del hábitat contra la variación del paisaje para las 20 especies de aves mas comúnes. Medimos las condiciones locales de hábitat en un radio de 100 m alrededor de cada estación de conteo. Las variables de paisaje fueron extraídas de una base de datos del Sistema de Información Geográfico (GIS) de la región y que incluye el tamaño del parche, distancia hasta el borde desarrollado y dos índices de exposición urbana. La adición de variables del paisaje al análisis de regresión logística por pasos mejoró sustancialmente el poder predictivo de los modelos resultantes en 14 de las 20 especies. La concordancia del modelo (habilidad del modelo para clasificar correctamente todos los pares de sitios divergentes) incrementó de un promedio de 56% sin variables del paisaje a un 76% donde una o mas variables del paisaje fueron seleccionadas para su inclusión en el modelo final. La respuesta de las 20 especies al gradiente del paisaje varió considerablemente entre especies. El análisis de correspondencia canónica fue usado para ubicar las especies en rangos a lo largo de un gradiente de respuesta al paisaje a partir de especies con reducido borde/fragmentación hasta especies con mayor borde/fragmentación. Las cuatro especies que exhibieron los patrones de reducción de borde/fragmentación mas fuerte fueron: Amphispiza belli, Aimophila ruficeps, Chondestes grammacus y Spizella atrogularis. Calypte costa tambien mostró un patron de reducción de borde/fragmentación. El sitio opuesto del gradiente fue ocupado por especies cuyas densidades son elevadas cerca de los bordes y en áreas fragmentadas. El punto medio del gradiente fue ocupado por especies que son abundantes, ampliamente distribuídas y no muestran sensitividad a la posición del paisaje o tipo de hábitat arbustivo. La abundancia de especies reducidas en borde/fragmentación aparentemente disminuye dentro de los 200 y 500 m de un borde y la abundancia de las especies con mejor borde/fragmentación se eleva mas alla de los 1000 m de un borde, dependiendo de la especie. Análisis de conservación, incluyendo GAP análisis frecuentemente asumen que la relación entre abundancia de especies y hábitat no cambia con la posición del paisaje. En paisajes heterogeneos estos análisis pueden desestimar el riesgo de conservación para especies en una categoria sensitiva de borde/fragmentación debido a que su abundancia varía fuertemente con la ubicación del paisaje. Modelo de metapoblación basado en el hábitat de Polioptila c. californica.
I investigated the causes and consequences of adult breeding-site fidelity, territory fidelity, and natal philopatry in Willow Flycatchers (Empidonax traillii) in southeastern Oregon over a 10-year period, testing the general hypothesis that fidelity and dispersal distances are influenced by previous breeding performance. Willow Flycatchers adhered to the generally observed tendencies of passerine birds for low natal philopatry and high breedingsite fidelity. Site fidelity (return to the study area) of adult males (52.0%) and females (51.3%), and median dispersal distances between seasons (16 m vs. 19 m) were similar. Previous breeding performance and residency (age-experience), but not study-site quality, explained site fidelity in females. Site fidelity of females rearing 4–5 young (64.4%) exceeded that of unsuccessful females (40.0%), breeding dispersal was less (successful: 15 m; unsuccessful: 33 m), and novice residents were more site-faithful than former residents. Probability of site fidelity was higher for previously successful females (odds ratio = 4.76), those with greater seasonal fecundity (odds ratio = 1.58), novice residents (odds ratio = 1.41), and unparasitized females (odds ratio = 2.76). Male site fidelity was not related to residency, site quality, or previous breeding performance. Territory fidelity (return to the previous territory) in females was best explained by previous breeding performance, but not by site quality or residency. Previously successful females were more likely to return to their territory of the previous season than either unsuccessful (odds ratio = 14.35) or parasitized birds (odds ratio = 6.38). Male territory fidelity was not related to residency, site quality, or previous breeding performance. Natal philopatry was low (7.8%) and similar for males and females. Site quality appeared to influence philopatry, given that no birds reared at a low-quality study site returned there to breed, and birds reared there dispersed farther than birds reared at two other study sites. My results partially support the hypothesis that site fidelity is an adaptive response: (1) previously successful females that switched territories underperformed those that did not switch (P = 0.01); and (2) previously unsuccessful females that switched territories outperformed those that did not switch, but not significantly (P = 0.22).
Aims Biogeographical evidence suggests a strong link between climate and patterns of species diversity, and climate change is known to cause range shifts. However, there is little understanding of how shifts affect community composition and we lack empirical evidence of recent impacts of climate change on the diversity of vertebrates. Using a long‐term comprehensive dataset on bird abundance, we explore recent patterns of change in different components of species diversity and avian communities, and postulate a process to explain the observed changes in diversity and specialization.
Location Britain.
Methods We used Breeding Bird Survey data for Britain from 1994 to 2006 to calculate site‐specific diversity and community specialization indices. We modelled these indices using generalized additive models to examine the relationship between local climate and spatial and temporal trends in community metrics and the relationship between changes in diversity and specialization.
Results Local temperature was positively associated with alpha diversity, which increased over the study period, supporting empirical and theoretical predictions of the effect of climate warming. Diversity increased in all habitats, but the rate of increase was greatest in upland areas. However, temperature was negatively associated with community specialization indices, which declined over the same period. Our modelling revealed a nonlinear relationship between community specialization and species diversity.
Main conclusions Our models of diversity and specialization provide stark empirical evidence for a link between warming climate and community homogenization. Over a 13‐year period of warming temperatures, diversity indices increased while average community specialization decreased. We suggest that the observed diversity increases were most likely driven by range expansion of generalist species and that future warming is likely to increase homogenization of community structure. When assessed in combination, diversity and specialization measures provide a powerful index for monitoring the impacts of climate change.
From 1997 to 1999, we monitored the reproductive success of individual rufous-crowned sparrows (Aimophila ruficeps) in coastal sage scrub habitat of southern California, USA. Annual reproductive output of this ground-nesting species varied strongly with annual variation in rainfall, attributed to the El Nio-Southern Oscillation. Birds fledged 3.0 young per breeding pair in 1997, when rainfall was near the long-term mean, 5.1 offspring per pair in 1998, a wet El Nio year, and 0.8 fledglings per pair in 1999, a dry La Nia year. Variation in many components of reproductive output was consistent with the hypothesis that food availability was positively correlated with rainfall. However, the factor most responsible for the high reproductive output in 1998 was low early season nest predation which, combined with favorable nesting conditions, enabled more pairs to multiple-brood. Cool, rainy El Nio conditions may have altered the activity of snakes, the main predator of these nests, in the early season of 1998. Overall, more of the annual variation in fecundity was attributable to variation in within-season components of reproductive output (mean number of nests fledged per pair) than to within-nest components (mean brood size). Annual variation in rufous-crowned sparrow fecundity appears to be driven primarily by food resource-mediated processes in La Nia years and by predator-mediated processes in El Nio years.
I used doubly labeled water to study the field metabolic rate (FMR) of Tree Swallows (Tachycineta bicolor) on Kent Island, New Brunswick, Canada, during the breeding season. I tested the hypothesis that aerially foraging species have higher energy requirements than other species.
For incubating female swallows, carbon dioxide production averaged 201.4 ± 15.8 ml CO2/h. While feeding 5 young, male and female swallows expired CO2 at a rate of 211.6 ± 23.3 and 231.0 ± 26.4 ml CO2/h, respectively. During this period males worked at similar levels to females, but the power consumption of females that fed young was significantly higher than incubating females. For both parents together, the mean number of visits to the nest/h was correlated with CO2 production: ml CO2/h = 201.6 + 2.49 (visits/h).
On Kent Island, Tree Swallows had a higher FMR than Savannah Sparrows (Passerculus sandwichensis), suggesting that aerial-foraging birds have a higher FMR than ground-foraging species. For 7 species of hirundines, energy expenditure was associated positively with body mass; log(kJ/d) = 1.34 + 0.53 log(body mass, g). This relationship differed from one for species which use alternative foraging modes (ground foraging and flycatching, n = 11), log(kJ/d) = 0.89 + 0.75 log(body mass, g). Aerial foragers expend from 16-38% more energy/day than do other birds of similar size that spend less time flying.
We focus on the implications of movement, landscape variables, and spatial heterogeneity for food web dynamics. Movements of nutrients, detritus, prey, and consumers among habitats are ubiquitous in diverse biomes and can strongly influence population, consumer-resource, food web, and community dynamics. Nutrient and detrital subsidies usually increase primary and secondary productivity, both directly and indirectly. Prey subsidies, by movement of either prey or predators, usually enhance predator abundance beyond what local resources can support. Top-down effects occur when spatially subsidized consumers affect local resources by suppressing key resources and occasionally by initiating trophic cascades. Effects on community dynamics vary with the relative amount of input, the trophic roles of the mobile and recipient entities, and the local food web structure. Landscape variables such as the perimeter/area ratio of the focal habitat, permeability of habitat boundaries, and relative productivity of trophically connected habitats affect the degree and importance of spatial subsidization.
Conservationists are far from able to assist all species under threat, if only for lack of funding. This places a premium on priorities: how can we support the most species at the least cost? One way is to identify 'biodiversity hotspots' where exceptional concentrations of endemic species are undergoing exceptional loss of habitat. As many as 44% of all species of vascular plants and 35% of all species in four vertebrate groups are confined to 25 hotspots comprising only 1.4% of the land surface of the Earth. This opens the way for a 'silver bullet' strategy on the part of conservation planners, focusing on these hotspots in proportion to their share of the world's species at risk.
Movements and habitat selection of male Rio Grande wild turkeys during drought in south Texas
- B A Collier
- J D Guthrie
- J B Hardin
COLLIER, B. A., J. D. GUTHRIE, J. B. HARDIN. AND K. L. SKOW. 2017.
Movements and habitat selection of male Rio Grande wild
turkeys during drought in south Texas. Journal of the
Southeastern Association of Fish and Wildlife Agencies
4:94-99.
California's most significant droughts: comparing historical and recent conditions
- J Jones
JONES, J. 2015. California's most significant droughts: comparing
historical and recent conditions. California Department of
Water Resources, Sacramento.