The demography of a newly established Osprey Pandion haliaetus population in France

Using long‐term mark–resighting data acquired over 27 years in continental France, we estimated demographic parameters and modelled the dynamics of a newly established population of Ospreys Pandion haliaetus using a life‐history model. We then performed prospective and retrospective analyses to estimate the sensitivity of the population growth rate to demographic parameters, and to quantify their contribution to the observed variation in abundance. The observed population growth rate was estimated at 1.150 (from one to 38 pairs in the period 1985–2011), and the stochastic population growth rate was estimated at 1.156. The number of fledglings per nest made the largest contribution to the variance of the observed population growth rate. Breeding productivity was stable across years. In contrast, the prospective analysis indicated that the sensitivity of the population growth rate was greatest for immigration and adult survival. Our results suggest that the increase of a new and recently established breeding population of Ospreys was mainly driven by local dynamics (high productivity and high proportion of breeding individuals), with no sign of density‐dependence except for juvenile survival. This probably reflects highly favourable conditions for breeding. Our results show that productivity can be a major driver in recovering raptor populations, and conservation work should aim to protect occupied nest‐sites and their surrounding habitat and to maintain highly favourable foraging areas in the vicinity of breeding sites.     

Density dependence forces divergent population growth rates and alters occupancy patterns of a central place foraging Antarctic seabird

Density‐dependent regulation is an important process in spatio‐temporal population dynamics because it can alter the effects of synchronizing processes operating over large spatial scales. Most frequently, populations are regulated by density dependence when higher density leads to reduced individual fitness and population growth, but inverse density dependence can also occur when small populations are subject to higher extinction risks. We investigate whether density‐dependent regulation influences population growth for the Antarctic breeding Adélie penguin Pygoscelis adeliae. Understanding the prevalence and nature of density dependence for this species is important because it is considered a sentinel species reflecting the impacts of fisheries and environmental change over large spatial scales in the Southern Ocean, but the presence of density dependence could introduce uncertainty in this role. Using data on population growth and indices of resource availability for seven regional Adélie penguin populations located along the East Antarctic coastline, we find compelling evidence that population growth is constrained at some locations by the amount of breeding habitat available to individuals. Locations with low breeding habitat availability had reduced population growth rates, higher overall occupancy rates, and higher occupancy of steeper slopes that are sparsely occupied or avoided at other locations. Our results are consistent with evolutionary models of avian breeding habitat selection where individuals search for high‐quality nest sites to maximize fitness returns and subsequently occupy poorer habitat as population density increases. Alternate explanations invoking competition for food were not supported by the available evidence, but strong conclusions on food‐related density dependence were constrained by the paucity of food availability data over the large spatial scales of this region. Our study highlights the importance of incorporating nonconstant conditions of species–environment relationships into predictive models of species distributions and population dynamics, and provides guidance for improved monitoring of fisheries and climate change impacts in the Southern Ocean.     

Density dependence in a recovering osprey population: demographic and behavioural processes

1. Understanding how density-dependent and independent processes influence demographic parameters, and hence regulate population size, is fundamental within population ecology. We investigated density dependence in growth rate and fecundity in a recovering population of a semicolonial raptor, the osprey Pandion haliaetus [Linnaeus, 1758], using 31 years of count and demographic data in Corsica. 2. The study population increased from three pairs in 1974 to an average of 22 pairs in the late 1990s, with two distinct phases during the recovery (increase followed by stability) and contrasted trends in breeding parameters in each phase. 3. We show density dependence in population growth rate in the second phase, indicating that the stabilized population was regulated. We also show density dependence in productivity (fledging success between years and hatching success within years). 4. Using long-term data on behavioural interactions at nest sites, and on diet and fish provisioning rate, we evaluated two possible mechanisms of density dependence in productivity, food depletion and behavioural interference. 5. As density increased, both provisioning rate and the size of prey increased, contrary to predictions of a food-depletion mechanism. In the time series, a reduction in fledging success coincided with an increase in the number of non-breeders. Hatching success decreased with increasing local density and frequency of interactions with conspecifics, suggesting that behavioural interference was influencing hatching success. 6. Our study shows that, taking into account the role of non-breeders, in particular in species or populations where there are many floaters and where competition for nest sites is intense, can improve our understanding of density-dependent processes and help conservation actions.     

Factors affecting population regulation of a colonial vulture

Two hypotheses have been proposed to link population regulation to density‐dependent changes in demographical parameters: the habitat heterogeneity hypothesis (HHH) states that, as population density rises, an increasing proportion of individuals are forced to occupy low‐quality territories, which provokes a decline in average per‐capita survival and/or productivity although some individuals show no decline in fecundity; and the individual adjustment hypothesis (IAH), which suggests that increased densities lead to reductions in survival and/or fecundity by enhancing agonistic interactions, which affect all individuals to a similar extent. However, density‐dependent effects can be affected by density‐independent factors (DIF), such as weather. We test the effects of density dependence on annual reproductive success in Griffon Vultures Gyps fulvus at four spatial scales, nest‐site, cliff, colony and metacolony, in northern Spain from 2008 to 2015. Our results showed most support for the HHH at all scales. At the colony and cliff scale, IAH and DIF had similar importance, whereas there was little evidence of IAH at the metacolony and the nest scale. The best protected eyries (caves, potholes and sheltered ledges) produced the most fledglings and were used preferentially, whereas low‐quality eyries (exposed ledges or open crevices) were used only when the number of breeders increased. The significant interaction between breeding failure and density found for the more exposed eyries suggests that at higher densities, breeding pairs are forced to use poorer nesting areas, and the negative effect of density at the cliff scale could be due to the combined effect of a higher proportion of pairs using low‐quality eyries and the negative effect of rainfall.     

Density‐dependent effects on reproductive performance in a recovering population of White‐tailed Eagles Haliaeetus albicilla

Understanding the mechanisms that shape density‐dependent processes and population dynamics is often essential for species conservation. Two key mechanisms of density‐dependent reductions in reproductive performance are a limited access to foraging habitats (the habitat heterogeneity hypothesis) and territorial aggression towards conspecifics (the interference competition hypothesis) at high population densities. Disentangling the relative importance of these mechanisms within populations below their carrying capacity is important for the evaluation of the success of conservation measures. However, relatively few studies have attempted to quantify the relative importance of both mechanisms for the reproductive performance of a population. Many raptor populations are ideal model systems to investigate density‐dependent effects because they are currently recovering from human‐induced reductions during the last decades. Using a 14‐year dataset, we combined analyses of individual reproductive performance with a mechanistic population model to investigate early signs of density‐dependent regulation in a population of White‐tailed Eagles Haliaeetus albicilla in north‐east Germany. We found a negative effect of the number of neighbouring breeding pairs and a positive effect of water surface area (as a proxy for the availability of favourable foraging habitat) on breeding success and on the average number of nestlings. The mean nearest neighbour distance between breeding pairs has decreased, and the mean distance of nests to the nearest water body has increased over the last 14 years. Moreover, the population model indicates that even though the population is still growing, carrying capacity could be reached at about 500–950 territorial pairs. These results suggest that the selection of nesting sites is determined by a trade‐off between the distance to favourable foraging habitat and the distance to neighbouring breeding pairs. To avoid increasing competition with conspecifics, due to continued population growth, breeding pairs seem to select increasingly suboptimal habitats. Therefore, our results suggest that the habitat heterogeneity and interference competition hypotheses are not necessarily mutually exclusive as mechanisms of density‐dependent population regulation, but can determine the reproductive performance of a raptor population simultaneously. Thus, a future decline in breeding success does not necessarily reflect a decrease in habitat quality but may rather be a consequence of density‐dependent mechanisms. This information may be useful for the interpretation of population trends and for the development of appropriate management strategies for recovering raptor populations.     

Implications of nest-site limitation on density-dependent nest predation at variable spatial scales in a cavity-nesting bird

Nest‐site limitation may have different implications in the spatial distribution of breeding pairs depending on the availability of suitable habitat and the types of nest‐sites. Distribution of cavities suitable as nest sites may allow circumstantial aggregation or active choice of colonial nesting, which may have different implications on breeding performance through effects on breeding density, with variable costs and benefits depending on the consequences of intraspecific competition, social interactions and predation. We evaluated the effects of breeding density derived from nesting site limitation on breeding performance and predation at different spatial scales and considering multiple social, population and environmental limiting factors in the red‐billed chough Pyrrhocorax pyrrhocorax. The results indicate that variable breeding density may arise within the population depending on the availability and spatial distribution of nest‐sites. Nest‐site availability and distribution may also determine social breeding systems (isolated or aggregated) at variable densities, thus resembling differences found at different spatially distant populations under contrasting environmental conditions. Breeding performance was related to density‐dependent processes of population regulation, especially density‐dependent nest predation due to predator attraction to nest clusters. Results also indicate that predation pressure depend on density patterns at large scales. This suggest that predation may have important consequences on population dynamics of spatially structured populations depending on the strength of this kind of density dependence, which in turn may depend on habitat features affecting the prey but also the spatially variable guild of predators. Because habitat and nesting site availability may vary spatially depending on multiple human influences, understanding the strength and form in which breeding density and nest predation at different spatial scales may influence the size and persistence of populations can help to manage them more adequately.     

Density,nest site characteristics and breeding rates of the osprey (Pandion haliaetus) in the southern limit of its range in the Western Palearctic (Boa Vista,Cape Verde Islands)

Between 2004 and 2007, we studied density, habitat features and breeding parameters of the osprey (Pandion haliaetus) population in Boa Vista Island (Cape Verde). A total of 79 nest structures were identified, 37 of which were occupied for at least 1 year during the study period. The osprey population ranged between 14 and 18 pairs, and the mean density and distance between neighbouring occupied nests were 2.58 pairs per 100 km² and 3089 m, respectively. Occupied nests were found to be significantly further from the coastline and roads than unoccupied nests, but the distances from villages were similar. The majority (81.1%) of the 37 occupied nests were easily accessible to humans. Mean clutch size was 2.59, average productivity was 0.76 young/active nest, and breeding success was 58.8% [Correction added on 13 May 2013, after first online publication: the average productivity was changed from 0.72 to 0.76]. Density in Boa Vista was higher than that in other sedentary island populations in the Western Palearctic, whereas the productivity was the lowest of this region. Clutch size did not vary among Western Palearctic populations, but the differences observed in productivity were likely influenced by local factors that in Boa Vista are attributed to nest depredation by the brown‐necked raven (Corvus ruficollis) and to direct human persecution.     

Optimal conditions for breeding of captive humboldt penguins (Spheniscus humboldti): A survey of British zoos

We surveyed 16 British zoos and bird gardens to assess the optimal conditions for breeding of captive Humboldt penguins (Spheniscus humboldti). We obtained information on population, enclosure, and husbandry characteristics and related these variables to three measures of per capita breeding success, namely, per capita egg productivity, chick productivity, and hatching success (measured as the proportion of eggs laid that hatched). All three fitness measures increased with an increasing number of breeding pairs and total population size but were not related to population density. Once the effect of number of breeding pairs was removed statistically, chick productivity was found to be highest when nesting boxes were lined with sand and gravel instead of alternative substrata such as twigs or vegetation. Hatching success increased with increasing pool size and was highest in enclosures with concrete floors. Adult mortality in zoos was generally low and appeared related to the use of chlorine in freshwater pools and to the presence of other penguin species in Humboldt penguin displays. Several enclosure and husbandry parameters were not variable enough to assess their impact on reproduction of captive Humboldt penguins. Recommendations for optimising conditions for captive breeding of Humboldt penguins include keeping as large a population as possible in a concrete enclosure with a large pool area, while providing sand and gravel as nesting material. Bird density may be important but we did not detect detrimental effects on breeding for densities up to 0.25 birds m^–2. Adult mortality can be minimised by exhibiting Humboldt penguins in single‐species display and avoiding chlorination of pool water. An experimental approach is recommended to confirm the results of this correlational study. Zoo Biol 20:545–555, 2001. © 2002 Wiley‐Liss, Inc.     

Is the large‐scale decline of the starling related to local changes in demography?

The decline of one farmland bird, the migratory European starling, has been attributed to both agricultural intensification and farmland abandonment and to factors operating both during the winter and during the breeding season. We analysed population data from thirty‐three Swedish nestbox colonies over more than two decades to determine if the national decline was caused by a common factor affecting all colonies or by local changes in the breeding grounds affecting starling colonies. We found that numbers of breeding starling had declined significantly, but at different rates in different colonies. The local population sizes were affected by previous years’ productivity at both national and local scales, suggesting that changes in habitat quality at both scales could affect local population trends. There were no long‐term trends in reproductive output, but fledgling production was lowest at intermediate years. The local population changes were positively related to local changes in reproductive output, but only when including complete nest‐failures. A relationship between population declines and low mean local productivity was the result of the association between population sizes and reproductive success over time, since decline rates of starlings were not related to the average success during the first part of the study, but to the average success during the later part of the study. The relationship between population change and changes in reproductive output was evident, but fledgling production showed negative density‐dependence. In conclusion this study suggests that the decline of the starling population in Sweden has been affected by processes at small spatial scales during the breeding season affecting reproductive success, but does not exclude an additional role for processes at large spatial scales or outside the breeding season.     

The cumulative effects of management on the population dynamics of the Double‐crested Cormorant Phalacrocorax auritus in the Great Lakes

Wildlife species have been subject to control efforts throughout human history due to real or alleged human–wildlife conflicts. The Double‐crested Cormorant Phalacrocorax auritus in the interior of North America is no exception, with recent population growth leading to increased conflicts and consequently the development of many control programmes. These control programmes are usually conducted at local scales, often with little or no effort to assess their cumulative effects at the population level. We attempted the first comprehensive assessment of the cumulative effects of control at various spatio‐temporal scales, focusing on 199 colonies of Double‐crested Cormorant monitored during a 29‐year period. Linear models were used to assess the relationship between colony‐specific growth rates and a set of candidate factors using an information‐theoretic approach. Colony‐level density‐dependent effects and local control efforts had the greatest influences on population growth. We detected a cumulative effect of management, whereby (i) the reduction in population growth rate was generally stronger when different control activities such as culling or egg oiling were combined, and (ii) past control operations tended to have a pervasive impact on growth rates, especially egg oiling and nest destruction, which negatively affected local recruitment. However, our results also suggest that catastrophic events and the culling of breeding adults that occurred at least 2 years previously could fuel subsequent recruitment or natural immigration from nearby colonies, for instance if the breeding success of remaining pairs was increased through a diminution of density‐dependent regulatory processes. Density‐dependence at the metapopulation level constituted a third source of regulation, as local growth rates were reduced with increasing number or proximity of active neighbouring colonies. We also found evidence that the culling of Double‐crested Cormorants wintering in the southeastern USA could negatively impact the population growth of individual breeding colonies in the Great Lakes, although further research integrating models of migratory connectivity is needed to reach more definitive conclusions. Finally, despite previous studies emphasizing its importance, the net effect of management‐induced dispersal appeared small at large spatial scales. We show that this can be explained in part by control strategies (e.g. spatially clustered operations). The continuation of Cormorant management efforts will provide an opportunity to refine the present assessment of the relative importance of density‐dependence, breeding vs. non‐breeding season management and dispersal, particularly if population models are coupled with monitoring programmes within an adaptive management framework.     

Retrospective investigation of captive red wolf reproductive success in relation to age and inbreeding

The critically endangered red wolf (Canis rufus) has been subject to a strictly managed captive breeding program for three decades. A retrospective demographic analysis of the captive population was performed based on data from the red wolf studbook. Data analyses revealed a decrease in the effective population size relative to the total population size, and changes in age structure and inbreeding coefficients over time. To varying degrees, the probability of successful breeding and litter sizes declined in association with increasing dam age and sire inbreeding coefficients. Neonate survival also declined with increasing dam age. Recent changes in strategies regarding breed-pair recommendations have resulted in moderate increases in reproductive success. Zoo Biol 28:214–229, 2009. © 2009 Wiley-Liss, Inc.     

Comparative breeding performance of Marsh Harriers Circus aeruginosus along a gradient of land‐use intensification and implications for population management

Assessing variation in breeding performance in relation to habitat characteristics may provide insights into predicting the consequences of land‐use change on species ecology and population dynamics. We compared four Marsh Harrier Circus aeruginosus populations subject to similar environmental conditions, but which differed in habitat composition, ranging from natural habitats to intensively cultivated areas. Using a 6‐year dataset, we characterized breeding habitat and diet in these four study sites, and analysed breeding performance in relation to this gradient of land‐use intensification. There was minimal variation in breeding performance between study years but consistent variation between study sites. Unexpectedly, Marsh Harriers breeding in intensively cultivated habitats had higher reproductive success than those breeding in more natural habitats, which, however, hosted higher breeding densities, so overall net population productivity (fledglings per unit area) was similar across sites. This resulted from combined effects of density‐dependence and different predation rates between study sites. The colonization of intensive farmland habitats may not necessarily impact negatively on population sustainability when breeding success and population density are traded against each other. However, our findings should not mask longer‐term conservation issues for populations breeding in these intensively managed areas, and further studies should assess potential long‐term negative effects of occupancy of human‐altered habitat.     

Breeding ecology of a suburban population of Woodpigeons Columba palumbus in northwest England

Capsule Between 1981 and 2008 population size was stable, but there were negative trends in breeding parameters.

Aims To determine the current status and long‐term population trend of an isolated breeding population of Bearded Vultures Gypaetus barbatus (Corsica, Mediterranean).

Methods The total Bearded Vulture population was monitored between 1981 and 2008.

Results The current effective breeding population size of Bearded Vultures in Corsica is ten pairs/trios with a slight increase of one to two pairs since 1983. The population is currently estimated at 25 individuals. Breeding parameters (laying rate, breeding success and productivity) have decreased significantly over the full 28‐year study period, although the decrease was not significant when the data set was restricted to 1988–2008. A mean of 60.3% (n = 204) of pairs have laid, but this proportion is highly variable between years. Productivity has been very low (0.16 young/pair/year, n = 233). Breeding parameters of the Corsican population of Bearded Vultures are very low compared with those of other western European populations in the Pyrenees.

Conclusions This isolated insular population is of small size (eight to ten pairs/trios) but shows a stability of distribution and numbers, but low (and decreasing) breeding rates, making this insular population one the most threatened in Europe.     

Invertebrate prey availability limits reproductive success but not breeding population size in suburban House Sparrows Passer domesticus

Factors affecting avian demography and abundance in urban landscapes are poorly understood and this hinders attempts to manage urban bird communities. Several recent studies indicate that lack of invertebrate prey in urban landscapes may constrain avian productivity and fitness relative to that in other habitats. House Sparrow Passer domesticus populations have undergone large declines in many European urban centres and inadequate reproductive success linked to invertebrate availability has been postulated as a potential cause of these declines. We conducted a replicated supplementary feeding experiment to test whether the availability of invertebrate prey limits the breeding success and adult abundance (colony size) of House Sparrows in suburban London, where House Sparrow populations declined by 60% during the decade preceding our study. Daily mealworm provision over two successive breeding seasons, sufficient to provide 82% of chick energy requirements of House Sparrow pairs nesting within 50 m of feeders, had a large positive impact on the abundance of recently fledged birds (+62%), but only a small positive impact, limited mainly to small colonies, on the overall abundance of territorial males. Colony growth was only weakly related to fledgling abundance in the previous year and did not appear to be constrained by nest‐site availability. Conservation interventions that enhance invertebrate availability for suburban House Sparrows may increase reproductive success but are unlikely, on their own, to lead to population growth or recovery.     

Effects of extrinsic and intrinsic factors on breeding success in a long lived seabird

There is growing concern over the impacts of climate change on animal species. Many studies have demonstrated impacts of climate change at the population level, and density dependent effects of climate are frequently reported. However, there is an increasing recognition of the differential impact of such factors on individuals since there is marked variation in individual performance. We investigated the relationships between breeding success and environmental conditions (winter NAO and one year lagged winter NAO) and intrinsic effects (colony size, pair bond duration, past breeding success rate) in the northern fulmar Fulmarus glacialis , using data from a long-term study commenced in 1950. There was a negative trend in breeding success over time, and a negative relationship with winter NAO and lagged winter NAO, which themselves had shown positive increases over the study period. The effects of lagged winter NAO remained after accounting for the linear trend. There was no evidence of density dependence, with breeding success positively related to colony size. We found strong evidence that breeding success was negatively related to pair bond duration but positively related to past breeding success rate. There was also an interaction between these two intrinsic effects such that those pairs that had historically been successful maintained success with increasing pair bond duration, whereas less successful pairs showed a decline. The prediction that there would be a differential impact of extrinsic factors among pairs was supported by an interaction between past breeding success rate and winter NAO, such that pairs with low past success rate exhibited a sharp decline in breeding success with increasing winter NAO, whereas more successful pairs did not. It is critically important to understand interactions between extrinsic factors and individual heterogeneity since a differential impact on individuals will affect population structure, and hence population dynamics.     

Brood sex ratio varies with diet composition in a generalist raptor

While sex allocation has been investigated productively at both population and family levels, as yet no general theory has been developed that is capable of linking processes at these two ecological scales, and very few empirical studies have examined cross‐scale patterns. In Finnish northern goshawks (Accipiter gentilis), nestling sex ratio of local subpopulations is related to the spatial and temporal variation in the abundance of their principal avian prey, woodland grouse. Using data from an urban breeding population in Hamburg, Germany, I investigated: (1) whether brood sex ratio of goshawks varies with diet composition at the family level; (2) whether such variation could reflect adaptive adjustment; and (3) how family‐level allocation can drive population‐level patterns, such as those observed in Finland. Feral pigeons (Columba livia) were the most important prey species, with a pooled contribution to total diet of 36%. Brood sex ratio varied significantly with the proportion of pigeons in the breeding‐season diet of pairs (increasing male bias). However, there was no evidence for sex‐differential effects of diet composition, so it remains unclear whether the observed sex‐ratio variation was an adaptive response. As all study pairs inhabited an (urban) environment where pigeons were unusually abundant, family‐level sex‐ratio adjustment caused a marked male bias in offspring sex ratio at the population level (male‐biased nestling sex ratio in four of five years; pooled data: 60% males). This suggests that the large‐scale variation observed in Finnish goshawk populations mirrors sex‐ratio adjustment shown by individual families in response to small‐scale environmental conditions. Apart from linking patterns empirically across ecological scales, this study is, to my knowledge, the first to demonstrate that family‐level brood sex ratio varies with realized resource use (diet composition) in a raptor species. Previous studies either failed to find significant associations or, more commonly, violated theoretical assumptions by measuring environmental prey abundance (often integrated over large areas) rather than realized prey use of individual breeding pairs. I conducted a meta‐analysis of offspring sex‐ratio data from 17 goshawk populations across Europe to put my results into perspective. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 937–951.     

Inter‐annual variability and long‐term trends in breeding success in a declining population of migratory swans

Population declines among migratory Arctic‐breeding birds are a growing concern for conservationists. To inform the conservation of these declining populations, we need to understand how demographic rates such as breeding success are influenced by combinations of extrinsic and intrinsic factors. In this study we examined inter‐annual variation and long‐term trends in two aspects of the breeding success of a migratory herbivore, the Bewick's swan Cygnus columbianus bewickii, which is currently undergoing a population decline: 1) the percentage of young within the wintering population and 2) mean brood size. We used an information‐theoretic approach to test how these two measures of productivity were influenced over a 26 yr period by 12 potential explanatory variables, encompassing both environmental (e.g. temperature) and intrinsic (e.g. pair‐bond duration) factors. Swan productivity exhibited sensitivity to both types of explanatory variable. Fewer young were observed on the wintering grounds in years in which the breeding period (May to September) was colder and predator (Arctic fox) abundance was higher. The percentage of young within the wintering population also showed negative density‐dependence. Inter‐annual variance in mean swan brood size was best explained by a model comprised of the negative degree days during the swan breeding period, mean pair‐bond duration of all paired swans (i.e. mean pair duration), and an interaction between these two variables. In particular, mean pair duration had a strong positive effect on mean brood size. However, we found no long‐term directional trend in either measure of breeding success, despite the recent decline in the NW European population. Our results highlight that inter‐annual variability in breeding success is sensitive to the combined effects of both intrinsic and extrinsic factors.     

Breeding population size and trends of American Oystercatchers on small islands of Bahía Santa María‐La Reforma,Sinaloa, Mexico

The islands of Bahía Santa María‐La Reforma, Sinaloa, Mexico, host the largest breeding population of the western race of American Oystercatchers (Haematopus palliatus frazari). This resident shorebird has a small breeding range restricted to northwestern Mexico and, although listed as endangered, little is known about their breeding biology. We estimated the number of pairs on the El Rancho and Melendres islands, the detection rate of pairs in different habitats, and the relationship between habitat length and number of pairs during three breeding seasons (2016–2018). In 2016 and 2017, the number of pairs peaked in mid‐April (97 and 95 pairs, respectively) at El Rancho, and in early May and early April (50 and 42 pairs, respectively) at Melendres. In 2018, both islands had the maximum number of pairs during mid‐May, with 109 pairs at El Rancho and 42 at Melendres. Detectability of breeding pairs in plots with sandy beach and dune habitats was higher (86–100%) than in those with mangrove habitat (22%). Detectability was associated with habitat type and should be considered in estimating the population size of this subspecies. We estimated that ~ 11% of the total frazari American Oystercatcher population breeds on these two small islands. The density of breeding pairs varied between habitats (range = 0–13 pairs/km). We also found a positive relationship between the number of pairs and plot length, suggesting that habitat availability might be a limiting factor for populations of American Oystercatchers in Mexico.     

Spatially explicit population responses of crayfish Procambarus alleni to potential shifts in vegetation distribution in the marl marshes of Everglades National Park,USA

Hydropattern disturbance has had wide-ranging impacts on wetland communities of the Florida Everglades, especially on the habitats and the aquatic biota of the seasonally flooded marl marshes. We used the Everglades crayfish Procambarus alleni as a model to study the associations among hydrology, vegetation distribution, and population dynamics to assess the potential impacts of hydrological changes on the aquatic faunal community in Everglades National Park. To classify benthic habitats as sources or sinks for the crayfish population, we quantified vegetation community structure using GIS maps in which dominant vegetation types were weighted by local hydroperiod (length of inundation). Regression analysis showed that this habitat classification was associated with crayfish density distribution. We then used a spatially explicit, stage-structured population model to describe crayfish population fluctuations under current environmental conditions and to simulate the potential population-level responses to habitat changes that might occur following hydrological restoration. In habitat that was initially saturated with crayfish, the crayfish population size declined under current environmental conditions and then stabilized at about 13% of the initial density over a 50-year period. A 4-month increase in hydroperiod was then simulated by converting shorter-hydroperiod Muhlenbergia-dominated marsh habitat to longer-hydroperiod Cladium-dominated marshes. The model predicted a rapid 7-fold increase in crayfish density following the simulated habitat restoration. This indicated that several functional effects may result from the restoration of historical hydropatterns in marl marshes: (1) the areal extent of habitat sinks will be reduced to isolated patches, whereas the spatial distribution of aquatic source habitats will expand; (2) crayfish population size will increase and persist over time; (3) the minimum threshold needed to increase secondary aquatic productivity may be a 7-month hydroperiod over 90% of the marl marsh landscape. Restoration of historical hydropatterns could thus have cascading positive effects throughout the Everglades aquatic food web.     

Climate,trophic interactions,density dependence and carry‐over effects on the population productivity of a migratory Arctic herbivorous bird

Several driving forces can affect recruitment rates in bird populations. However, our understanding of climate‐induced effects or bottom–up vs top–down biological processes on breeding productivity typically comes from small‐scale studies, and their relative importance is rarely investigated at the population level. Using a 31‐year time series, we examined the effects of selected environmental parameters on the annual productivity of a key Arctic herbivore, the greater snow goose Anser caerulescens atlanticus. We determined the extent to which breeding productivity, defined as the percentage of juveniles in the fall population, was affected by 1) climatic conditions, 2) fluctuations in predation pressure caused by small rodent oscillations, and 3) population size. Moreover, we took advantage of an unplanned large‐scale manipulation (i.e. management action) to examine the potential non‐lethal carry‐over effects caused by disturbance on spring staging sites. The most parsimonious model explained 66% of the annual variation in goose productivity. The spring North Atlantic Oscillation and Arctic snow depth were the primary climatic parameters inversely affecting the production of juveniles, likely through bottom–up processes. Indirect trophic interactions generated by fluctuations in lemming abundance explained 18% of the variation in goose productivity (positive relationship). Mean temperature during brood‐rearing and disturbance on staging sites (carry‐over effects) were the other important factors affecting population recruitment. We observed a strong population increase, and found no evidence of density‐dependent effects. Spatially restricted studies can identify factors linking environmental parameters to local bird reproduction but if these factors do not act synchronously over the species range, they may fail to identify the relative importance of mechanisms driving large‐scale population dynamics.