The authors evaluated for the first time how anthropogenic habitat fragmentation can affect food chain length in highly diverse food webs. The authors found that bottom-up mass effects are important but also network structural changes driven by habitat modification have the potential to affect chain length.

Abstract

The food chain length represents how much energy reaches different trophic levels in food webs. Environmental changes derived from human activities have the potential to affect chain length. We explore how habitat area and edges affect chain length through: (1) a bottom-up effect of abundance (‘pyramid hypothesis’); (2) the truncation of the highest trophic level (‘trophic-rank hypothesis’); and (3) changes in species connectivity patterns (‘connectivity hypothesis’). We built plant-leaf miner-parasitoid food webs in 19 remnants of a fragmented Chaco forest from central Argentina. On each remnant, we constructed food webs from different locations at the forest interior and edges. For each food web, we registered the abundance of species, the species richness of each trophic level, estimated the connectivity of their networks, and the average food chain length. We used structural equation models to evaluate the direct and indirect effects of habitat area and edge/interior location on food chain length mediated by species richness, abundance and connectivity. We found no direct effects of habitat area on chain length but chains were longer at forest edges than at their interior. The three mechanisms were supported by our results, although they showed different strengths. First, we found that the interior favours a bottom-up abundance effect from herbivores to parasitoids that positively affected chain length; second, we found that the forest area positively affects plant richness, which has a strong effect on the number of resources used by consumers, with a positive effect on chain length. Third, the remnant area and interior position favoured plant richness with a negative effect on the abundance of parasitoids, which had a positive effect on chain length. In general, the strongest effects on chain length were detected through changes in abundance rather than species richness although abundance was less affected by habitat fragmentation. We evaluated for the first time the effects of human-driven habitat fragmentation on the length of trophic chains in highly diverse plant-herbivore-parasitoid networks. Despite the loss of species, small habitat fragments and edges embedded in the agricultural matrix can support interaction networks, making them conservation targets in managed landscapes.

Resumen

El largo de cadenas tróficas representa cuanta energía alcanza diferentes niveles tróficos en redes tróficas. Los cambios ambientales producto de las actividades humanas tienen el potencial de afectar el largo de las cadenas tróficas. Exploramos como el área de hábitat y los bordes afectan el largo de cadenas tróficas a través de: (1) un efecto ascendente de la abundancia (‘hipótesis pirámide’); (2) el truncamiento del nivel trófico superior (‘hipótesis de ranking trófico’); y (3) cambios en los patrones de conectividad (‘hipótesis de conectividad’). Construimos redes tróficas entre plantas-minadores de hoja-parasitoides en 19 remanentes de bosque Chaqueño serrano altamente fragmentado en el centro de Argentina. Para cada remanente construimos redes tróficas en distintas ubicaciones en el borde e interior del bosque. Para cada red trófica registramos la abundancia media de las especies, la riqueza de cada nivel trófico, estimamos la conectividad de las redes y el largo de cadenas tróficas promedio. Utilizamos modelos de ecuaciones estructurales para evaluar los efectos directos e indirectos del área y la ubicación borde/interior sobre el largo de cadenas tróficas mediado por la riqueza de especies, la abundancia y la conectividad. No encontramos efectos directos del área de hábitat sobre el largo de cadenas, pero las cadenas fueron más largas en los bordes que en el interior. Los tres mecanismos propuestos fueron apoyados por los resultados, pero mostraron distinta fuerza. Primero, encontramos que el interior de los bosques favorece los efectos ascendentes de la abundancia desde los herbívoros a los parasitoides lo que afectó positivamente al largo de las cadenas; segundo, encontramos que el área de bosque afectó positivamente a la riqueza de especies, lo que tuvo un efecto positivo en el largo de cadenas. Tercero, el área de bosque remanente y la ubicación en el interior favorecieron la riqueza de plantas, influyendo negativamente en la abundancia de parasitoides lo que tuvo un efecto positivo en el largo de cadenas. En general, los efectos más fuertes sobre el largo de cadenas se detectaron a través de cambios en la abundancia más que en la riqueza, aunque la abundancia fue menos afectada por la fragmentación del hábitat que la riqueza de especies. En este estudio evaluamos por primera vez los efectos de la fragmentación del hábitat por causas humanas sobre el largo de cadenas tróficas en redes tróficas altamente diversas de plantas, herbívoros y parasitoides. A pesar de la pérdida de especies, los fragmentos pequeños y los bordes de bosque inmersos en una matriz agrícola pueden sostener redes de interacciones, convirtiéndolos en objetivos de conservación en paisajes manejados.

In one of the largest field campaigns of its kind, we found that ecophysiological traits predicted species distributions for 116 species across four taxonomic groups across two elevational transects.

Abstract

Understanding the role of thermal tolerances in determining species distributions is important for assessing species responses to climate change. Two hypotheses linking physiology with species distributions have been put forward—the climatic variability hypothesis and the climatic extreme hypothesis. The climatic variability hypothesis predicts the selection of individuals with broad thermal tolerance in more variable climatic conditions and the climatic extreme hypothesis predicts the selection of individuals with extreme thermal tolerance values under extreme climatic conditions. However, no study has tested the predictions of these hypotheses simultaneously for several taxonomic groups along elevational gradients. Here, we related experimentally measured critical thermal maxima, critical thermal minima and thermal tolerance breadths for 15,187 individuals belonging to 116 species of ants, beetles, grasshoppers, and spiders from mountain ranges in central and northern Pakistan to the limits and breadths of their geographic and temperature range. Across all species and taxonomic groups, we found strong relationships between thermal traits and elevational distributions both in terms of geography and temperature. The relationships were robust when repeating the analyses for ants, grasshoppers, and spiders but not for beetles. These results indicate a strong role of physiology in determining elevational distributions of arthropods in Southern Asia. Overall, we found strong support for the climatic variability hypothesis and the climatic extreme hypothesis. A close association between species' distributional limits and their thermal tolerances suggest that in case of a failure to adapt or acclimate to novel climatic conditions, species may be under pressure to track their preferred climatic conditions, potentially facing serious consequences under current and future climate change.

The authors show that a food web of a symbiont community living in ant nests is not steered by environmental predictors, but by the trophic decisions of the ant host. Symbionts occupied higher trophic positions, and their community had a longer food chain length when the host was more predatory.

Abstract

Food chain length provides key information on the flow of nutrients and energy in ecosystems. Variation in food chain length has primarily been explained by environmental drivers such as ecosystem size and productivity. Most insights are obtained from theory or aquatic systems, but the importance of these drivers remains largely untested in terrestrial systems. We exploited red wood ant nests markedly differing in size as natural experiments to quantify the drivers of trophic structure and food chain length of their symbiont arthropod communities. Using stable isotopes, we explored the variation in the trophic positions of four symbiont species with the trophic position of the top predator as a proxy for food chain length of the symbiont community. Nest size did not affect food chain length, nor trophic distance between the symbionts. Instead, food chain length and the trophic positions of the symbionts were strongly affected by the host's foraging decisions. When the host diet shifted from predominantly herbivorous to more predacious, the trophic position of the symbionts and food chain length strongly increased. We show for the first time that a food web can be structured by biotic interactions with an engineering species rather than by abiotic environmental variables.

In this study, we experimentally test the combined effects of two nest parasites on host fitness in a natural system. We show that the effects of nest flies and brood parasites on breeding birds are additive and severe, and are likely mediated by indirect interactions with nest predators.

Abstract

Co-parasitism is ubiquitous and has important consequences for the ecology and evolution of wild host populations. Studies of parasite co-infections remain limited in scope, with few experimental tests of the fitness consequences of multiple parasites, especially in natural populations. We measured the separate and combined effects of Philornis seguyi nest flies and shiny cowbirds Molothrus bonariensis on the fitness of a shared host, the chalk-browed mockingbird (Mimus saturninus) in Argentina. Using a two-factor experimental approach, we manipulated the presence of nest flies and cowbirds in mockingbird nests and assessed their effects on mockingbird haemoglobin levels, begging and provisioning rates, body size, and fledging success. We also monitored rates of nest predation in relation to parasitism by flies and cowbirds. Nest flies reduced the haemoglobin concentration, body size, and fledging success of mockingbirds, likely because mockingbirds did not compensate for parasitism by begging more or feeding their nestlings more. Cowbirds also reduced the fledging success of mockingbirds, even though they had no detectable effect on haemoglobin or body size. Nests with cowbirds, which beg more than mockingbirds, attracted more nest predators. There was no significant interaction between the effects of flies and cowbirds on any component of mockingbird fitness. The combined effects of nest flies and cowbirds were strictly additive. In summary, we show that nest flies and cowbirds both reduce host fitness, but do not have interactive effects in co-parasitized nests. Our results further suggest that predators exacerbate the effects of nest flies and cowbirds on their hosts. Our study shows that the fitness consequences of co-parasitism are complex, especially in the context of community-level interactions.

RESUMEN

El co-parasitismo es ubicuo y tiene consecuencias importantes para la ecología y la evolución de las poblaciones de hospedadores silvestres. Los estudios de coinfecciones por parásitos poseen un alcance limitado, con pocos trabajos experimentales que analicen las consecuencias del parasitismo múltiple sobre la eficacia biológica, especialmente en poblaciones naturales. Medimos los efectos separados y combinados de las moscas de los nidos Philornis seguyi y el tordo renegrido (Molothrus bonariensis) sobre la eficacia biológica de un hospedador compartido, la calandria grande (Mimus saturninus) en Argentina. Utilizando un enfoque experimental de dos factores, manipulamos la presencia de moscas de nido y tordos en los nidos de calandrias y evaluamos sus efectos sobre los niveles de hemoglobina, las tasas de solicitud de alimento y aprovisionamiento, el tamaño corporal y el éxito de supervivencia de los pichones de calandria. También monitoreamos las tasas de depredación de nidos en relación con el parasitismo de moscas y tordos. Las moscas de los nidos redujeron la concentración de hemoglobina, el tamaño corporal y el éxito de supervivencia de los pichones de calandrias, probablemente porque las calandrias no compensaron el parasitismo solicitando más alimento o alimentando más a sus pichones. Los tordos también redujeron el éxito de supervivencia de las calandrias, aunque no tuvieron un efecto detectable sobre la hemoglobina o el tamaño corporal. Los nidos con tordos, quienes solicitaron más alimento más que las calandrias, atrajeron a más depredadores de nidos. El aumento de la depredación de nidos asociado a la alta intensidad de solicitud de alimento puede explicar por qué las calandrias en Argentina no solicitan más alimento en respuesta al parasitismo de las moscas. No hubo una interacción significativa entre los efectos de las moscas y los tordos en ningún componente de la eficacia biológica de las calandrias. Los efectos combinados de las moscas de los nidos y los tordos fueron estrictamente aditivos. En resumen, mostramos que las moscas de los nidos y los tordos reducen la eficacia biológica del hospedador, pero no tienen efectos interactivos en los nidos co-parasitados. Nuestros resultados sugieren además que los depredadores exacerban los efectos de las moscas de los nidos y los tordos sobre sus hospedadores. Nuestro estudio muestra que las consecuencias del co-parasitismo sobre la eficacia biológica son complejas, especialmente en el contexto de las interacciones a nivel comunitario.

The authors extend transient life table response experiments to decompose variance in population growth rates into contributions from demographic and environmental stochasticity. The contributions from environmental stochasticity are then decomposed into contributions from environmental variables affecting growth via vital rates. The methods are illustrated in a study of northern wheatears.

Abstract

How demographic factors lead to variation or change in growth rates can be investigated using life table response experiments (LTRE) based on structured population models. Traditionally, LTREs focused on decomposing the asymptotic growth rate, but more recently decompositions of annual ‘realized’ growth rates using ‘transient’ LTREs have gained in popularity. Transient LTREs have been used particularly to understand how variation in vital rates translate into variation in growth for populations under long-term study. For these, complete population models may be constructed to investigate how temporal variation in environmental drivers affect vital rates. Such investigations have usually come down to estimating covariate coefficients for the effects of environmental variables on vital rates, but formal ways of assessing how they lead to variation in growth rates have been lacking. We extend transient LTREs to further partition the contributions from vital rates into contributions from temporally varying factors that affect them. The decomposition allows one to compare the resultant effect on the growth rate of different environmental factors, as well as density dependence, which may each act via multiple vital rates. We also show how realized growth rates can be decomposed into separate components from environmental and demographic stochasticity. The latter is typically omitted in LTRE analyses. We illustrate these extensions with an integrated population model (IPM) for data from a 26 years study on northern wheatears (Oenanthe oenanthe), a migratory passerine bird breeding in an agricultural landscape. For this population, consisting of around 50–120 breeding pairs per year, we partition variation in realized growth rates into environmental contributions from temperature, rainfall, population density and unexplained random variation via multiple vital rates, and from demographic stochasticity. The case study suggests that variation in first year survival via the unexplained random component, and adult survival via temperature are two main factors behind environmental variation in growth rates. More than half of the variation in growth rates is suggested to come from demographic stochasticity, demonstrating the importance of this factor for populations of moderate size.

The authors tested whether animals dynamically balance risks and rewards in the context of road crossings. Using extensive field-based and GPS data from elk in Yoho National Park, the authors demonstrate that animals can modulate their selection for foraging resources with the risks they experience relative to roads while travelling.

Abstract

Animal behaviour is shaped by the ability to identify risks and profitably balance the levels of risks encountered with the payoffs experienced. Anthropogenic disturbances like roads generate novel risks and opportunities that wildlife must accurately perceive and respond to. Basic concepts in predator–prey ecology are often used to understand responses of animals to roads (e.g. increased vigilance, selection for cover in their vicinity). However, prey often display complex behaviours such as modulating space use given varying risks and rewards, and it is unclear if such dynamic balancing is used by animals in the context of road crossings. We tested whether animals dynamically balance risks and rewards relative to roads using extensive field-based and GPS collar data from elk in Yoho National Park (British Columbia, Canada), where a major highway completely bisects their range during most of the year. We analysed elk behaviour by combining hidden Markov movement models with a step-selection function framework. Rewards were indexed by a dynamic map of available forage biomass, and risks were indexed by road crossings and traffic volumes. We found that elk generally selected intermediate and high forage biomass, and avoided crossing the road. Most of the time, elk modulated their behaviour given varying risks and rewards. When crossing the highway compared with not crossing, elk selected for greater forage biomass and this selection was stronger as the number of highway crossings increased. However, with traffic volume, elk only balanced foraging rewards when they crossed a single time during a travel sequence. Using a road ecology system, we empirically tested an important component of predator–prey ecology—the ability to dynamically modulate behaviour in response to varying levels of risks and rewards. Such a test articulates how decision-making processes that consider the spatiotemporal variation in risks and rewards allow animals to successfully and profitably navigate busy roads. Applying well-developed concepts in predator–prey theory helps understand how animals respond to anthropogenic disturbances and anticipate the adaptive capacity for individuals and populations to adjust to rapidly changing environments.

Résumé

Le comportement animal est influencé par la capacité des animaux à identifier et minimiser les risques rencontrés, tout en maximisant les gains obtenus. Les perturbations anthropiques, telles que les routes, engendrent de nouveaux risques et opportunités pour la faune. Les concepts de l'écologie prédateur-proie sont fréquemment utilisés pour comprendre les réactions des animaux aux routes (e.g. vigilance accrue, choix de couvert à proximité des routes). Cependant, même s'il est connu que les proies ajustent fréquemment leur utilisation de l'espace de façon à minimiser les risques et maximiser les récompenses, il n'est pas clair si une telle optimisation est utilisée par les animaux lorsqu'ils traversent des routes. Ici, nous avons évalué comment les animaux ajustent leur sélection d'habitat par rapport aux routes en fonction des risques et des récompenses disponibles. Nous avons examiné cette question chez les wapitis du parc national Yoho (Colombie-Britannique, Canada), où une autoroute majeure divise complètement leur domaine vital pendant une majeure partie de l'année. À l'aide d'une analyse de sélection d'habitat à fine échelle, nous avons testé si les wapitis optimisent les risques liés aux traversées d'autoroute et les récompenses alimentaires obtenues lorsqu'ils se déplacent entre des zones d'alimentation. Les récompenses ont été estimées à l'aide d'une carte dynamique de la biomasse végétale disponible pour les wapitis, et les risques ont été estimés en fonction des traversées de route et du trafic automobile rencontré. Nos résultats indiquent que les wapitis sélectionnaient généralement des zones d'alimentation avec une biomasse intermédiaire à élevée, et évitaient de traverser l'autoroute. La plupart du temps, les wapitis ajustaient leur sélection d'habitat en fonction des risques et des récompenses. Les wapitis sélectionnaient des zones d'alimentation avec une biomasse plus élevée lorsqu'ils traversaient l'autoroute, comparé à lorsqu'ils ne traversaient pas. Ils optimisaient également la biomasse végétale obtenue en fonction du nombre de traversées de l'autoroute effectuées durant une séquence de déplacement. Cependant, les wapitis optimisaient uniquement les récompenses alimentaires avec le trafic automobile durant les séquences de déplacement avec une seule traversée. Nous avons testé empiriquement un élément essentiel de l'écologie prédateur-proie, soit la capacité d'ajuster de façon dynamique un comportement en réponse à des niveaux variables de risques et de récompenses, dans le contexte de l'écologie routière. Notre étude permet d'illustrer comment les processus décisionnels considérant à la fois les risques, les récompenses et leur variation spatiotemporelle, permettent aux animaux de naviguer de façon optimale les routes très fréquentées. L'utilisation de concepts bien établis de l'écologie prédateur-proie aide à comprendre comment les animaux réagissent aux perturbations anthropiques, et contribue à anticiper la capacité d'adaptation des individus et des populations face à des environnements en transformation rapide.

The authors found that in times of habitat loss, survival rate of birds was influenced by migration time and fuel load, suggesting the influence of directional selection.

Abstract

Loss and/or deterioration of refuelling habitats have caused population declines in many migratory bird species but whether this results from unequal mortality among individuals varying in migration traits remains to be shown. Based on 13 years of body mass and size data of great knots (Calidris tenuirostris) at a stopover site of the Yellow Sea, combined with resightings of individuals marked at this stopover site along the East Asian-Australasian Flyway, we assessed year to year changes in annual apparent survival rates, and how apparent survival differed between migration phenotypes (i.e. migration timing and fuel stores). The measurements occurred over a period of habitat loss and/or deterioration in this flyway. We found that the annual apparent survival rates of great knots rapidly declined from 2006 to 2018, late-arriving individuals with small fuel stores exhibiting the lowest apparent survival rate. There was an advancement in mean arrival date and an increase in the mean fuel load of stopping birds over the study period. Our results suggest that late-arriving individuals with small fuel loads were selected against. Thus, habitat loss and/or deterioration at staging sites may cause changes in the composition of migratory phenotypes at the population-level.

摘要

迁徙停歇地丧失和/或退化导致许多候鸟的种群数量下降，但这是否是由于不同迁徙表型个体的死亡率差异所造成的，仍有待证明。 根据黄海区域的一个迁徙停歇地大滨鹬(Calidris tenuirostris)13年环志所记录的体重和体型大数据，并结合在东亚-澳大利西亚迁飞区对标记个体的观察记录，我们评估了大滨鹬存活率的年际变化，以及不同迁徙表型(迁徙时间和能量储备)的存活率差异。本研究是在迁飞区发生栖息地丧失和/或退化期间进行的。 我们发现，从2006年到2018年，大滨鹬的年存活率迅速下降，迁徙日程较晚且能量储备较少的个体的存活率最低。在研究期间，大滨鹬到达该迁徙停歇地的日期有所提前，能量储备也有所增加。 研究结果表明，迁徙日程较晚且能量储备较少的个体被选择性地淘汰。因此，迁徙停歇地丧失和/或退化可能会导致迁徙表型的组成在种群水平上发生变化。

Urban organisms tend to present duller colouration in traits based on carotenoid pigmentation. The authors combine correlational, experimental and meta-analytical approaches to comprehensively evaluate the uniformity of this phenomenon. They found that urban individuals are paler but population-specific differences exist in the magnitude of the urbanisation effect on colouration.

Abstract

Urbanisation is accelerating across the globe, transforming landscapes, presenting organisms with novel challenges, shaping phenotypes and impacting fitness. Urban individuals are claimed to have duller carotenoid-based colouration, compared to their non-urban counterparts, the so-called ‘urban dullness’ phenomenon. However, at the intraspecific level, this generalisation is surprisingly inconsistent and often based on comparisons of single urban/non-urban populations or studies from a limited geographical area. Here, we combine correlational, experimental and meta-analytical data on a common songbird, the great tit Parus major, to investigate carotenoid-based plumage colouration in urban and forest populations across Europe. We find that, as predicted, urban individuals are paler than forest individuals, although there are large population-specific differences in the magnitude of the urban-forest contrast in colouration. Using one focal region (Malmö, Sweden), we reveal population-specific processes behind plumage colouration differences, which are unlikely to be the result of genetic or early-life conditions, but instead a consequence of environmental factors acting after fledging. Finally, our meta-analysis indicates that the urban dullness phenomenon is well established in the literature, for great tits, with consistent changes in carotenoid-based plumage traits, particularly carotenoid chroma, in response to anthropogenic disturbances. Overall, our results provide evidence for uniformity in the ‘urban dullness’ phenomenon but also highlight that the magnitude of the effect on colouration depends on local urban characteristics. Future long-term replicated studies, covering a wider range of species and feeding guilds, will be essential to further our understanding of the eco-evolutionary implications of this phenomenon.