Size-mediated non-trophic interactions and stochastic predation drive assembly and dynamics in a seabird community

High response diversity and conspecific density-dependence, not species interactions, drive dynamics of coral reef fish communities

Species-to-species and species-to-environment interactions are key drivers of community dynamics. Disentangling these drivers in species-rich assemblages is challenging due to the high number of potentially interacting species (the 'curse of dimensionality'). We develop a process-based model that quantifies how intraspecific and interspecific interactions, and species' covarying responses to environmental fluctuations, jointly drive community dynamics. We fit the model to reef fish abundance time series from 41 reefs of Australia's Great Barrier Reef. We found that fluctuating relative abundances are driven by species' heterogenous responses to environmental fluctuations, whereas interspecific interactions are negligible. Species differences in long-term average abundances are driven by interspecific variation in the magnitudes of both conspecific density-dependence and density-independent growth rates. This study introduces a novel approach to overcoming the curse of dimensionality, which reveals highly individualistic dynamics in coral reef fish communities that imply a high level of niche structure.

A Case for the "Competitive Exclusion-Tolerance Rule" as a General Cause of Species Turnover along Environmental Gradients

AbstractClosely related, ecologically similar species often segregate their distributions along environmental gradients of time, space, and resources, but previous research suggests diverse underlying causes. Here, we review reciprocal removal studies in nature that experimentally test the role of interactions among species in determining their turnover along environmental gradients. We find consistent evidence for asymmetric exclusion coupled with differences in environmental tolerance causing the segregation of species pairs, where a dominant species excludes a subordinate from benign regions of the gradient but is unable to tolerate challenging regions to which the subordinate species is adapted. Subordinate species were consistently smaller and performed better in regions of the gradient typically occupied by the dominant species compared with their native distribution. These results extend previous ideas contrasting competitive ability with adaptation to abiotic stress to include a broader diversity of species interactions (intraguild predation, reproductive interference) and environmental gradients, including gradients of biotic challenge. Collectively, these findings suggest that adaptation to environmental challenge compromises performance in antagonistic interactions with ecologically similar species. The consistency of this pattern across diverse organisms, environments, and biomes suggests generalizable processes structuring the segregation of ecologically similar species along disparate environmental gradients, a phenomenon that we propose should be named the competitive exclusion-tolerance rule.

The morphological allometry of four closely related and coexisting insect species reveals adaptation to the mean and variability of the resource size

The size of organisms may result from various, sometimes antagonistic forces operating on distinct traits, within an evolutionary framework that may also be constraining. Morphological allometry, referring to the way trait size scales with body size, has been shown to reflect ecological adaptation to the mean size of the resource exploited. We examined the allometric relationships between rostrum and body size among four insect (Curculio spp.) specialists of oak acorns. In all four species, weevil females drill a hole with their rostrum prior depositing one or a few eggs inside the seed. The four weevil species, that coexist on the same individual trees, displayed partitioned egg-laying periods in the year, thereby encountering acorns of different size and maturation stage. We found marked differences in the allometric slope among females: species laying eggs late in the season had a steeper slope, leading to increasingly longer rostrum relative to body length, along with the mean size of the growing acorns. Females of the smallest species had the longest oviposition period and also had the steepest slope, which provided them with the most variable rostrum length, thereby matching the variable size of the resource through time. Our work highlights the need to consider not only the average size but also the degree of variability in resource size to understand the adaptive value of allometric relationships.

Long-term demographic dynamics of a keystone scavenger disrupted by human-induced shifts in food availability

Scavenging is a key ecological process controlling energy flow in ecosystems and providing valuable ecosystem services worldwide. As long-lived species, the demographic dynamics of vultures can be disrupted by spatiotemporal fluctuations in food availability, with dramatic impacts on their population viability and the ecosystem services provided. In Europe, the outbreak of bovine spongiform encephalopathy (BSE) in 2001 prompted a restrictive sanitary regulation banning the presence of livestock carcasses in the wild on a continental scale. In long-lived vertebrate species, the buffering hypothesis predicts that the demographic traits with the largest contribution to population growth rate should be less temporally variable. The BSE outbreak provides a unique opportunity to test for the impact of demographic buffering in a keystone scavenger suffering abrupt but transient food shortages. We studied the 42-year dynamics (1979-2020) of one of the world's largest breeding colonies of Eurasian griffon vultures (Gyps fulvus). We fitted an inverse Bayesian state-space model with density-dependent demographic rates to the time series of stage-structured abundances to investigate shifts in vital rates and population dynamics before, during, and after the implementation of a restrictive sanitary regulation. Prior to the BSE outbreak the dynamics was mainly driven by adult survival: 83% of temporal variance in abundance was explained by variability in this rate. Moreover, during this period the regulation of population size operated through density-dependent fecundity and subadult survival. However, after the onset of the European ban, a 1-month delay in average laying date, a drop in fecundity, and a reduction in the number of fledglings induced a transient increase in the impact of fledgling and subadult recruitment on dynamics. Although adult survival rate remained constantly high, as predicted by the buffering hypothesis, its relative impact on the temporal variance in abundance dropped to 71% during the sanitary regulation and to 54% after the ban was lifted. A significant increase in the relative impact of environmental stochasticity on dynamics was modeled after the BSE outbreak. These results provide empirical evidence on how abrupt environmental deterioration may induce dramatic demographic and dynamic changes in the populations of keystone scavengers, with far-reaching impacts on ecosystem functioning worldwide.

Towards a system-level causative knowledge of pollinator communities

Pollination plays a central role in both crop production and maintaining biodiversity. However, habitat loss, pesticides, invasive species and larger environmental fluctuations are contributing to a dramatic decline of pollinators worldwide. Different management solutions require knowledge of how ecological communities will respond following interventions. Yet, anticipating the response of these systems to interventions remains extremely challenging due to the unpredictable nature of ecological communities, whose nonlinear behaviour depends on the specific details of species interactions and the various unknown or unmeasured confounding factors. Here, we propose that this knowledge can be derived by following a probabilistic systems analysis rooted on non-parametric causal inference. The main outcome of this analysis is to estimate the extent to which a hypothesized cause can increase or decrease the probability that a given effect happens without making assumptions about the form of the cause-effect relationship. We discuss a road map for how this analysis can be accomplished with the aim of increasing our system-level causative knowledge of natural communities. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Decrease in social cohesion in a colonial seabird under a perturbation regime

Social interactions, through influence on behavioural processes, can play an important role in populations’ resilience (i.e. ability to cope with perturbations). However little is known about the effects of perturbations on the strength of social cohesion in wild populations. Long-term associations between individuals may reflect the existence of social cohesion for seizing the evolutionary advantages of social living. We explore the existence of social cohesion and its dynamics under perturbations by analysing long-term social associations, in a colonial seabird, the Audouin’s gull Larus audouinii, living in a site experiencing a shift to a perturbed regime. Our goals were namely (1) to uncover the occurrence of long-term social ties (i.e. associations) between individuals and (2) to examine whether the perturbation regime affected this form of social cohesion. We analysed a dataset of more than 3500 individuals from 25 years of monitoring by means of contingency tables and within the Social Network Analysis framework. We showed that associations between individuals are not only due to philopatry or random gregariousness but that there are social ties between individuals over the years. Furthermore, social cohesion decreased under the perturbation regime. We sustain that perturbations may lead not only to changes in individuals’ behaviour and fitness but also to a change in populations’ social cohesion. The consequences of decreasing social cohesion are still not well understood, but they can be critical for the population dynamics of social species.

Breeding transients in capture-recapture modeling and their consequences for local population dynamics

Standard procedures for capture-mark-recapture modelling (CMR) for the study of animal demography include running goodness-of-fit tests on a general starting model. A frequent reason for poor model fit is heterogeneity in local survival among individuals captured for the first time and those already captured or seen on previous occasions. This deviation is technically termed a transience effect. In specific cases, simple, uni-state CMR modeling showing transients may allow researchers to assess the role of these transients on population dynamics. Transient individuals nearly always have a lower local survival probability, which may appear for a number of reasons. In most cases, transients arise due to permanent dispersal, higher mortality, or a combination of both. In the case of higher mortality, transients may be symptomatic of a cost of first reproduction. A few studies working at large spatial scales actually show that transients more often correspond to survival costs of first reproduction rather than to permanent dispersal, bolstering the interpretation of transience as a measure of costs of reproduction, since initial detections are often associated with first breeding attempts. Regardless of their cause, the loss of transients from a local population should lower population growth rate. We review almost 1000 papers using CMR modeling and find that almost 40% of studies fitting the searching criteria (N = 115) detected transients. Nevertheless, few researchers have considered the ecological or evolutionary meaning of the transient phenomenon. Only three studies from the reviewed papers considered transients to be a cost of first reproduction. We also analyze a long-term individual monitoring dataset (1988-2012) on a long-lived bird to quantify transients, and we use a life table response experiment (LTRE) to measure the consequences of transients at a population level. As expected, population growth rate decreased when the environment became harsher while the proportion of transients increased. LTRE analysis showed that population growth can be substantially affected by changes in traits that are variable under environmental stochasticity and deterministic perturbations, such as recruitment, fecundity of experienced individuals, and transient probabilities. This occurred even though sensitivities and elasticities of these parameters were much lower than those for adult survival. The proportion of transients also increased with the strength of density-dependence. These results have implications for ecological and evolutionary studies and may stimulate other researchers to explore the ecological processes behind the occurrence of transients in capture-recapture studies. In population models, the inclusion of a specific state for transients may help to make more reliable predictions for endangered and harvested species.

Immature survival, fertility, and density dependence drive global population dynamics in a long-lived species

Disentangling the influence of demographic parameters and the role of density dependence on species' population dynamics is a challenge, especially when fractions of the population are unobservable. Additionally, due to the difficulty of gathering data at large spatial scales, most studies ignore the global dynamic of a species, which would integrate local heterogeneity dynamics and remove the noise of dispersal. We developed an integrated population model (IPM) at a global scale to disentangle the main demographic drivers of population dynamics in a long-lived species. We used 28 yr of Audouin's Gull demographic data encompassing 69 local patches (comprising 90% of the world population). Importantly, we took into account the unobservable fraction of non-breeders and also assessed the strength of density dependence for this fraction of the population. As predicted by life histories of long-lived organisms, temporal random variation in survival was highest for immature individuals (1.326, 95% credible interval [CRI] 1.290-1.940) and lowest for adults (0.499, 95% CRI 0.487-0.720). Large temporal fluctuations in the probability of taking a reproductive sabbatical would partly explain the consistency in adult survival, with individuals most likely refraining from breeding when environmental conditions were harsh. Immature survival and fertility were the main drivers of population dynamics during the study period (r²  = 0.83, 0.77-0.87 and 0.73, 0.63-0.79, respectively). We found strong evidence of density dependence, not only due to the number of breeders (r²  = -0.34, -0.43 to -0.24) but also due to individuals on sabbatical (r²  = -0.18, -0.33 to -0.01). From a conservation point of view, the species shows a 5% annual global decrease during the last 10 years, and we propose an update of its conservation status. Even though population dynamics of long-lived organisms are very sensitive to changes in adult survival, we show here that, in the absence of strong environmental perturbations affecting this vital rate, fluctuations in population density are mainly driven by variations in survival of immature individuals and fertility. Integrated models based on long-term monitoring at a global scale may enhance our ecological and evolutionary understanding of how demographic drivers influence population dynamics.

Complex demographic heterogeneity from anthropogenic impacts in a coastal marine predator

Environmental drivers, including anthropogenic impacts, affect vital rates of organisms. Nevertheless, the influence of these drivers may depend on the physical features of the habitat and how they affect life history strategies depending on individual covariates such as age and sex. Here, the long-term monitoring (1994-2014) of marked European Shags in eight colonies in two regions with different ecological features, such as foraging habitat, allowed us to test several biological hypotheses about how survival changes by age and sex in each region by means of multi-event capture-recapture modeling. Impacts included fishing practices and bycatch, invasive introduced carnivores and the severe Prestige oil spill. Adult survival was constant but, unexpectedly, it was different between sexes. This difference was opposite in each region. The impact of the oil spill on survival was important only for adults (especially for females) in one region and lasted a single year. Juvenile survival was time dependent but this variability was not synchronized between regions, suggesting a strong signal of regional environmental variability. Mortality due to bycatch was also different between sex, age and region. Interestingly the results showed that the size of the fishing fleet is not necessarily a good proxy for assessing the impact of bycatch mortality, which may be more dependent on the fishing grounds and the fishing gears employed in each season of the year. Anthropogenic impacts affected survival differently by age and sex, which was expected for a long-lived organism with sexual size dimorphism. Strikingly, these differences varied depending on the region, indicating that habitat heterogeneity is demographically important to how environmental variability (including anthropogenic impacts) and resilience influence population dynamics.

How are species interactions structured in species-rich communities? A new method for analysing time-series data

Estimation of intra- and interspecific interactions from time-series on species-rich communities is challenging due to the high number of potentially interacting species pairs. The previously proposed sparse interactions model overcomes this challenge by assuming that most species pairs do not interact. We propose an alternative model that does not assume that any of the interactions are necessarily zero, but summarizes the influences of individual species by a small number of community-level drivers. The community-level drivers are defined as linear combinations of species abundances, and they may thus represent e.g. the total abundance of all species or the relative proportions of different functional groups. We show with simulated and real data how our approach can be used to compare different hypotheses on community structure. In an empirical example using aquatic microorganisms, the community-level drivers model clearly outperformed the sparse interactions model in predicting independent validation data.

Colonisation in social species: the importance of breeding experience for dispersal in overcoming information barriers

Studying colonisation is crucial to understand metapopulations, evolutionary ecology and species resilience to global change. Unfortunately, few empirical data are available because field monitoring that includes empty patches at large spatiotemporal scales is required. We examine the colonisation dynamics of a long-lived seabird over 34 years in the western Mediterranean by comparing population and individual data from both source colony and the newly-formed colonies. Since social information is not available, we hypothesize that colonisation should follow particular dispersal dynamics and personal information must be crucial in decision making. We test if adverse breeding conditions trigger colonisation events, if personal information plays a role in colonisation and if colonisers experience greater fitness. Our results show a temporal mismatch between colonisation events and both density-dependence and perturbations at the source colony, probably because colonisers needed a longer prospecting period to compensate for the lack of public information. Colonisers were mostly experienced individuals gaining higher breeding success in the new colony. Our results highlight the demographic value that experienced individuals can have on metapopulation dynamics of social long-lived organisms.

Consecutive cohort effects driven by density-dependence and climate influence early-life survival in a long-lived bird

Conditions during early life, including maternal cohort effects, can influence the future fitness of individuals. This may be particularly true for long-distance migrating birds, because, apart from conditions experienced by cohorts during rearing, conditions during early life in regions far from breeding grounds may also influence their population dynamics. Very little is known about the fitness consequences of those conditions experienced by juveniles after independence, especially in wild populations and for long-lived birds. We used multi-event capture-recapture-recovery models and a unique 26-year dataset for the Audouin's gull (Larus audouinii) to assess for the first time whether survival was influenced by early conditions, both during the rearing period (i.e. a maternal cohort effect potentially affected by density dependence) and the first winter (i.e. a cohort effect driven by climate when birds disperse to wintering grounds). Our results show that juvenile survival was highly sensitive to early-life conditions and that survival decreased with stronger density dependence and harsh climate. The two consecutive cohort effects were of similar magnitude and they may represent a selection filter. Thus, early-life conditions had a strong impact on survival, and neglecting this complexity may hinder our understanding on how populations of long-lived animals fluctuate and respond to perturbations.

Biological invasions, ecological resilience and adaptive governance

In a world of increasing interconnections in global trade as well as rapid change in climate and land cover, the accelerating introduction and spread of invasive species is a critical concern due to associated negative social and ecological impacts, both real and perceived. Much of the societal response to invasive species to date has been associated with negative economic consequences of invasions. This response has shaped a war-like approach to addressing invasions, one with an agenda of eradications and intense ecological restoration efforts towards prior or more desirable ecological regimes. This trajectory often ignores the concept of ecological resilience and associated approaches of resilience-based governance. We argue that the relationship between ecological resilience and invasive species has been understudied to the detriment of attempts to govern invasions, and that most management actions fail, primarily because they do not incorporate adaptive, learning-based approaches. Invasive species can decrease resilience by reducing the biodiversity that underpins ecological functions and processes, making ecosystems more prone to regime shifts. However, invasions do not always result in a shift to an alternative regime; invasions can also increase resilience by introducing novelty, replacing lost ecological functions or adding redundancy that strengthens already existing structures and processes in an ecosystem. This paper examines the potential impacts of species invasions on the resilience of ecosystems and suggests that resilience-based approaches can inform policy by linking the governance of biological invasions to the negotiation of tradeoffs between ecosystem services.

Living on the edge: demography of the slender-billed gull in the Western Mediterranean

Small and peripheral populations are typically vulnerable to local extinction processes but important for the metapopulation dynamics of species. The Slender-billed gull (Chroicocephalus genei) is a long-lived species breeding in unstable ephemeral coastal habitats. Their Western Mediterranean populations are relatively small and represent the edge of their global geographical distribution. At a local scale, using long-term data (14 years) on annual breeding success and capture-resights of marked individuals, we estimated and compared the vital rates and evaluated the connectivity of two Spanish populations (Ebro Delta and Doñana) varying in their local environmental conditions. At a metapopulation scale, we analyzed 22 years of data on breeding numbers to predict their future prospects by means of population demographic models. Local survival and breeding success of gulls from the Ebro Delta was lower than those from Doñana, which is likely the result of higher permanent emigration and/or winter mortality in the former. Gulls from the Ebro Delta wintered mostly in Mediterranean areas whereas those from Doñana did so in Atlantic coasts, where food availability is higher. Whereas adult local survival was constant, juvenile local survival showed temporal parallel variations between colonies, probably related to natal dispersal to other breeding colonies. Our results suggested that dispersal was higher at the Ebro Delta and gulls emigrating from their natal colonies settled preferentially in close patches. We found large fluctuations in breeding numbers among local populations probably related to the fact that the Slender-billed gull is a species adapted to unstable and unpredictable habitats with high abilities to disperse between suitable patches depending on environmental stochastic conditions during breeding.

Ecological and evolutionary implications of food subsidies from humans

Human activities are the main current driver of global change. From hunter-gatherers through to Neolithic societies-and particularly in contemporary industrialised countries-humans have (voluntarily or involuntarily) provided other animals with food, often with a high spatio-temporal predictability. Nowadays, as much as 30-40% of all food produced in Earth is wasted. We argue here that predictable anthropogenic food subsidies (PAFS) provided historically by humans to animals has shaped many communities and ecosystems as we see them nowadays. PAFS improve individual fitness triggering population increases of opportunistic species, which may affect communities, food webs and ecosystems by altering processes such as competition, predator-prey interactions and nutrient transfer between biotopes and ecosystems. We also show that PAFS decrease temporal population variability, increase resilience of opportunistic species and reduce community diversity. Recent environmental policies, such as the regulation of dumps or the ban of fishing discards, constitute natural experiments that should improve our understanding of the role of food supply in a range of ecological and evolutionary processes at the ecosystem level. Comparison of subsidised and non-subsidised ecosystems can help predict changes in diversity and the related ecosystem services that have suffered the impact of other global change agents.

A bird's eye view of discard reforms: bird-borne cameras reveal seabird/fishery interactions

Commercial capture fisheries produce huge quantities of offal, as well as undersized and unwanted catch in the form of discards. Declines in global catches and legislation to ban discarding will significantly reduce discards, but this subsidy supports a large scavenger community. Understanding the potential impact of declining discards for scavengers should feature in an eco-system based approach to fisheries management, but requires greater knowledge of scavenger/fishery interactions. Here we use bird-borne cameras, in tandem with GPS loggers, to provide a unique view of seabird/fishery interactions. 20,643 digital images (one min(-1)) from ten bird-borne cameras deployed on central place northern gannets Morus bassanus revealed that all birds photographed fishing vessels. These were large (>15 m) boats, with no small-scale vessels. Virtually all vessels were trawlers, and gannets were almost always accompanied by other scavenging birds. All individuals exhibited an Area-Restricted Search (ARS) during foraging, but only 42% of ARS were associated with fishing vessels, indicating much 'natural' foraging. The proportion of ARS behaviours associated with fishing boats were higher for males (81%) than females (30%), although the reasons for this are currently unclear. Our study illustrates that fisheries form a very important component of the prey-landscape for foraging gannets and that a discard ban, such as that proposed under reforms of the EU Common Fisheries Policy, may have a significant impact on gannet behaviour, particularly males. However, a continued reliance on 'natural' foraging suggests the ability to switch away from scavenging, but only if there is sufficient food to meet their needs in the absence of a discard subsidy.

Exploiting uncertain ecological fieldwork data with multi-event capture--recapture modelling: an example with bird sex assignment

1. Sex plays a crucial role in evolutionary life histories. However, the inclusion of sex in demographic analysis may be a challenge in fieldwork, particularly in monomorphic species. Although behavioural data may help us to sex individuals in the field, this kind of data is unlikely to be error free and is usually discarded. 2. Here we propose a multi-event capture-recapture model that enables us to exploit uncertain field observations regarding the sex of individuals based on behavioural or morphological criteria. The multi-event capture-recapture model allows us to account for sex uncertainty not restricting our ability to estimate the parameters of interest. In this case, by adding the confirmed sex of just a few individuals, we greatly improve the efficiency of the optimization algorithm. 3. Using such an approach, we analysed sex differences in demographic parameters (e.g. survival, transience and sex ratio) in a population of Audouin's gulls using observations from long-term fieldwork monitoring (1988-2007). We also assessed the probability of ascertaining sex over time and the probability of error for each field-sexing criterion. 4. We detected no strong effect of sex on either survival or transience probabilities, and both sexes showed a decreasing trend in survival over time and transience probability after recruitment increased with age and over time. The probability of ascertaining sex over time depended on observers' experience. Strikingly, courtship feeding (but not copulation) emerged as the most reliable clue for sexing individuals, which would suggest that Audouin's gulls engage in same-sex sexual behaviour such as same-sex mounting. 5. The present modelling emerged as a reliable method for estimating demographic parameters and state transition parameters in ecological studies in which field observations of sex or other individual states are assigned erroneously and uncertainly. This approach could also be useful for applied ecologists for assessing the reliability of their criteria for assigning sex or other individual covariates in the field, thereby permitting them to optimizing their field ecological protocols.