Increases in disturbance and reductions in habitat size interact to suppress predator body size

Food webs are strongly size‐structured so will be vulnerable to changes in environmental factors that affect large predators. However, mechanistic understanding of environmental controls of top predator size is poorly developed. We used streams to investigate how predator body size is altered by three fundamental climate change stressors: reductions in habitat size, increases in disturbance and warmer temperatures. Using new survey data from 74 streams, we showed that habitat size and disturbance were the most important stressors influencing predator body size. A synergistic interaction between that habitat size and disturbance due to flooding meant the sizes of predatory fishes peaked in large, benign habitats and their body size decreased as habitats became either smaller or harsher. These patterns were supported by experiments indicating that habitat‐size reductions and increased flood disturbance decreased both the abundance and biomass of large predators. This research indicates that interacting climate change stressors can influence predator body size, resulting in smaller predators than would be predicted from examining an environmental factor in isolation. Thus, climate‐induced changes to key interacting environmental factors are likely to have synergistic impacts on predator body size which, because of their influence on the strength of biological interactions, will have far‐reaching effects on food‐web responses to global environmental change.     

Regulation of bacterial assemblages in oligotrophic plankton systems: results from experimental and empirical approaches

Bacteria are relevant members of planktonic food webs, both in terms of biomass and production share. The assessment and comprehension of the factors that control bacterial abundance and production are, thus, necessary to understand how carbon and nutrients circulate in planktonic food webs. It is commonly believed that bacterial abundance, activity and production are either determined by the available nutrient levels (‘bottom-up’ control) or by the effect of predators (‘top-down’). These factors have also been shown to regulate the internal structure (the physiological and phylogenetic structure) of the bacterioplankton black box. We present here different empirical and experimental ways in which the factors that control bacterial communities are assessed, among them, the direct comparison of the rates of bacterial growth and losses to grazing. Application of several of these methods to open ocean data suggests that bacteria are regulated by resources at the largest scales of analysis, but that this overall regulation is strongly modulated by predators in all types of systems. In the most oligotrophic environments, bacterial abundance and growth are regulated by predators, while in the richest environments it is bacterial (phylogenetic, size, activity) community composition that is most affected by protist predators, while abundance can be influenced by metazoans. Because changes in bacterial community composition require that bacteria have enough nutrient supply, the overall effect of these regulations is that bacterial growth appears to be top-down regulated in the most nutrient-poor environments and bottom-up regulated in the richer ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Critical indirect effects of climate change on sub‐Antarctic ecosystem functioning

Sub‐Antarctic islands represent critical breeding habitats for land‐based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub‐Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; “island mass effect”; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long‐term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long‐term decline in enhanced phytoplankton productivity at the islands in response to a climate‐driven shift in the position of the sub‐Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore‐feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of offshore‐feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.     

Ecological effects of longline fishing and climate change on the pelagic ecosystem off eastern Australia

Pelagic longline fisheries target (or catch incidently) large apex predators in the open ocean (e.g. tunas, billfish and sharks) and have the potential to disrupt the ecosystem functionality if these predators exert strong top–down control. In contrast, warming of oceans from climate change may increase bottom–up effects from increases in primary productivity. An ecosystem model of a large pelagic ecosystem off eastern Australia was constructed to explore the potential ecological effects of climate change and longlining by Australia’s Eastern Tuna and Billfish Fishery. The model reproduced historic biomass and fishery catch trends from 1952 to 2006 for seven functional groups. Simulated changes in fishing effort and fishing mortality rate on individual target species from 2008 to 2018 resulted in only modest (<20%) changes in the biomass of target species and their direct predators or competitors. A simulated increase in phytoplankton biomass due to climate change resulted in only small increases (<11%) in the biomass of all groups. However, climate-related changes to the biomass of micronekton fish (−20%) and cephalopods (+50%) resulted in trophic cascades. Our results suggest there may be ecological redundancy among high trophic level predators since they share a diverse suite of prey and collectively only represent <1% of the total system biomass. In contrast, micronekton fishes and cephalopods have high biomasses and high production and consumption rates and are important as both prey and predators. They appear to exert ‘wasp–waist’ control of the ecosystem rather than top–down or bottom–up processes reported to drive other pelagic systems.     

The influence of habitat on the size distribution of groupers in the upper Florida Keys

Synopsis The influence of habitat on the size distribution of groupers was examined at sites in the middle and upper Florida Keys. Transects were used to quantify the size distribution of groupers at study sites. There were significant differences in the size distribution of groupers within and among reef community types related to differences in species composition and patch reef size. Groupers with a giant life-history style (Sullivan & de Garine 1994) were more abundant, but smaller, on inshore patch reefs than offshore reef community types. However, grouper species with a dwarf life-history style (Sullivan & de Garine 1994) showed an opposite pattern with a lower abundance, but larger size, inshore than offshore. The length category of groupers constituting the majority of individuals observed on patch reefs was inversely related to reef size. Graysby Epinephelus cruentatus were shown to recruit to deeper (15–20 m), low-relief habitats offshore. Several factors influenced the size distribution of groupers in the study sites including habitat type, spear-fishing, competition, predation, and recruitment.     

Climate change and heat-related mortality in six cities Part 1: model construction and validation

Heat waves are expected to increase in frequency and magnitude with climate change. The first part of a study to produce projections of the effect of future climate change on heat-related mortality is presented. Separate city-specific empirical statistical models that quantify significant relationships between summer daily maximum temperature (T _max) and daily heat-related deaths are constructed from historical data for six cities: Boston, Budapest, Dallas, Lisbon, London, and Sydney. ‘Threshold temperatures’ above which heat-related deaths begin to occur are identified. The results demonstrate significantly lower thresholds in ‘cooler’ cities exhibiting lower mean summer temperatures than in ‘warmer’ cities exhibiting higher mean summer temperatures. Analysis of individual ‘heat waves’ illustrates that a greater proportion of mortality is due to mortality displacement in cities with less sensitive temperature–mortality relationships than in those with more sensitive relationships, and that mortality displacement is no longer a feature more than 12 days after the end of the heat wave. Validation techniques through residual and correlation analyses of modelled and observed values and comparisons with other studies indicate that the observed temperature–mortality relationships are represented well by each of the models. The models can therefore be used with confidence to examine future heat-related deaths under various climate change scenarios for the respective cities (presented in Part 2).     

The climate of the Younger Dryas as a boundary for Einkorn domestication

The domestication of the Neolithic founder crops of the Near East has recently been a topic of debate particularly with respect to how rapidly the domestication of these crops occurred. One school of thought maintains that these processes lasted several thousand years (‘protracted model’ with ‘gathering’, ‘cultivation’ and ‘domestication’ as three stages of a continuum, each taking up to several thousand years), while another favors the view that domestication occurred much more quickly, over several hundred years at most (‘rapid transition’). Our study focuses on one of these crops, 1-grained domesticated Einkorn wheat, incorporating data from geology, vegetation history, and climate. In the Karacadağ region of southeastern Turkey, where 1-grained Einkorn wheat was domesticated, the climate was severe (i.e. cold and dry) during the Younger Dryas. The climate of the Younger Dryas acted as boundary during which a) conditions were not suitable for ‘cultivation’, and b) Einkorn would have retreated to refugia exhibiting more suitable moisture-bearing soils that would have made ‘gathering’ difficult. Around 11600 cal. b.p., the Younger Dryas ended and a very rapid climatic amelioration commenced at the beginning of the Early Holocene, enabling grasses to spread throughout the region. A ritual PPNA/PPNB site (G?bekli Tepe) and associated PPNB settlements such as Nevali ?ori and ?ay?nü were established at this time. In the settlements of Nevali ?ori and Cafer H?yük, the oldest domesticated Einkorn was found in the earliest archaeological layers. This confirms that the inhabitants made use of domesticated 1-grained Einkorn from the very beginning of settlement activity, although they continued to practice a mixed lifestyle as hunter-gatherers and farmers. For ?ay?nü the issue is more complex, but here domesticated Einkorn also appears around the same time. In summary, by 10400 cal. b.p. domesticated 1-grained Einkorn was present in large quantities at a variety of sites. This would give a maximum window of time lasting approximately 1,200 years and is therefore not in agreement with the ‘protracted’ model but would be consistent with a ‘rapid transition’. It is improbable that the ‘cultivation’ of wild Einkorn was practiced in the Karacadağ region, since wild Einkorn was plentiful during the favorable growing conditions following the Younger Dryas, making it more likely that ‘gathering’ would have been practiced. Because Einkorn has not been found in early settlements in the southern Levant, this crop cannot have been ‘gathered’ and ‘cultivated’ there but instead was ‘domesticated’ independently and solely in southeastern Turkey. Therefore, the YD acts as a boundary, providing a maximum time frame for 1-grained Einkorn domestication.     

The feeding habits of six pelagic and predatory teleosts in eastern Cape coastal waters (South Africa)

A four-year study of the feeding habits of six pelagic teleost species in eastern Cape coastal waters of South Africa is analysed. These predators are Lichia amia, Pomatomus sallatrix, Seriola lalandi, Atractoscion aequidens, Katsuwonus pelamis and Thunnus albacares. All species occur in inshore coastal water (< 1 -8 km from shore) for at least part of the year. The tunas were also sampled from the edge of the continental shelf (40–50 km from shore). The use of squid 'beaks' and fish otoliths allowed accurate prey identification and size estimates. Variations in prey taken are attributed to differences in locality, predator species and size. Three pelagic fish species are important to all the predators: Sardinops ocettata, Etrumeus teres and Engraulis capensis. The inshore squid, Loligo reynaudi, is another major prey item. Offshore tuna samples revealed a different prey spectrum, viz. oceanic squid and saury, Scomberesox saurus, being taken frequently.     

Predator-induced plasticity in guppy (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>) life history traits

A number of invertebrates show predator-induced plasticity in life-history and morphological traits that are considered adaptive. Evidence is accumulating that vertebrates may also adjust their life-history traits in response to predators; however, some of the patterns of plasticity, which appear to be an adaptive response specifically to the risk of size-selective predation, may instead result from reduced foraging in response to predator presence. Here, we describe a study of predator-induced plasticity in guppies (Poecilia reticulata). We have predicted that the plastic response to cues from a small, gape-limited, natural predator of guppies, the killlifish (Rivulus hartii), would be the opposite of that caused by reduced food intake. We have found that male guppies increased their size at maturity, both length and mass, in response to the non-lethal presence of this predator. This pattern of plasticity is the opposite of that observed in response to reduced food intake, where male guppies reduce size at maturity. The increase in size at maturity that we observed would likely reduce predation on adult male guppies by this native predator because it is gape-limited and can only eat juvenile and small adult guppies. This size advantage would be important especially because male guppies grow very little after maturity. Therefore, the pattern of plasticity that we observed is likely adaptive. In contrast, female guppies showed no significant response in size at first parturition to the experimental manipulation; however, we did find evidence suggesting that females may produce more, smaller offspring in response to cues from this predator.     

Re-assessing current extinction rates

There is a widespread belief that we are experiencing a mass extinction event similar in severity to previous mass extinction events in the last 600 million years where up to 95% of species disappeared. This paper reviews evidence for current extinctions and different methods of assessing extinction rates including species–area relationships and loss of tropical forests, changing threat status of species, co-extinction rates and modelling the impact of climate change. For 30 years some have suggested that extinctions through tropical forest loss are occurring at a rate of up to 100 species a day and yet less than 1,200 extinctions have been recorded in the last 400 years. Reasons for low number of identified global extinctions are suggested here and include success in protecting many endangered species, poor monitoring of most of the rest of species and their level of threat, extinction debt where forests have been lost but species still survive, that regrowth forests may be important in retaining ‘old growth’ species, fewer co-extinctions of species than expected, and large differences in the vulnerability of different taxa to extinction threats. More recently, others have suggested similar rates of extinction to earlier estimates but with the key cause of extinction being climate change, and in particular rising temperatures, rather than deforestation alone. Here I suggest that climate change, rather than deforestation is likely to bring about such high levels of extinction since the impacts of climate change are local to global and that climate change is acting synergistically with a range of other threats to biodiversity including deforestation.     

Why get big in the cold? Towards a solution to a life-history puzzle

The temperature–size rule (TSR), which states that body size increases at lower developmental temperatures, appears to be a near-universal law for ectotherms. Although recent studies seem to suggest that the TSR might be adaptive, the underlying developmental mechanisms are thus far largely unknown. Here, we investigate temperature effects on life-history traits, behaviour and physiology in the copper butterfly Lycaena tityrus in order to disentangle the mechanistic basis for the above rule. In L. tityrus the larger body size produced at a lower temperature was proximately due to a greater increase in mass, which was caused by both behavioural and physiological mechanisms: a much-increased food intake and a higher efficiency in converting ingested food into body matter. These mechanisms, combined with temperature-induced changes at the cellular level, may provide general explanations for the TSR. Body fat and protein content increased in butterflies reared at the higher temperature, indicating favourable growth conditions. As predicted from protandry theory, males showed reduced development times, caused by higher growth rates compared to females. The latter was itself related to a higher daily food consumption, while the total food consumption (due to the females’ longer developmental period) and assimilation was higher in females and may underly the sexual body size dimorphism.     

Boundary nature induces greater group size and group density in habitat edges: an agent-based model revealed

Apart from the amount and distribution of food resources and predation pressure, we sought to clarify habitat spatial features that affect group size and group density. To simulate animal groups interacting with one another, we made an agent-based model in a computational space. Assuming no predation pressure, we change the resource conditions between the habitat (comprising ‘interior’ and ‘edges’) and the ‘exterior’. The results are as follows: when the group density is high throughout the habitat, large groups tend to frequent the edges (i.e., the boundaries between the interior and exterior) even if food resources are scarce in the exterior. Additionally, when the group density is high and the group size variation is small, both group size and group density increase at the edges. These findings are discussed with reference to primates, particularly the Japanese macaque (Macaca fuscata) as an example of group-living animal.     

Coexistence of predator and prey in intraguild predation systems with ontogenetic niche shifts

In basic intraguild predation (IGP) systems, predators and prey also compete for a shared resource. Theory predicts that persistence of these systems is possible when intraguild prey is superior in competition and productivity is not too high. IGP often results from ontogenetic niche shifts, in which the diet of intraguild predators changes as a result of growth in body size (life-history omnivory). As a juvenile, a life-history omnivore competes with the species that becomes its prey later in life. Competition can hence limit growth of young predators, while adult predators can suppress consumers and therewith neutralize negative effects of competition. We formulate and analyze a stage-structured model that captures both basic IGP and life-history omnivory. The model predicts increasing coexistence of predators and consumers when resource use of stage-structured predators becomes more stage specific. This coexistence depends on adult predators requiring consumer biomass for reproduction and is less likely when consumers outcompete juvenile predators, in contrast to basic IGP. Therefore, coexistence occurs when predation structures the community and competition is negligible. Consequently, equilibrium patterns over productivity resemble those of three-species food chains. Life-history omnivory thus provides a mechanism that allows intraguild predators and prey to coexist over a wide range of resource productivity.     

Are fish early growth and condition patterns related to life-history strategies?

Life-history studies provide a global framework for comparison of fish species responses and trade-offs facing ecological and environmental constrains. A broad comparison among fishes’ early growth and condition traits is performed in order to determine ecological patterns of early development regarding latitudinal distribution, habitat use and life-history strategies. Based on Winemiller and Rose (1992) classification of life-history strategies, data on early growth and condition indices of 46 fish species worldwide was analysed. Available information on fishes’ early features, namely first year length percentage (relative to species maximum theoretical length), age at maturation and Fulton’s condition index (K), provided a good segregation of species by latitudinal distribution and habitat use, and evidenced the categories of the three-endpoint model. Higher larvae and juvenile growth rates and condition indices (K, mean RNA–DNA ratios and protein contents) were associated with tropical and temperate fish species that occur in complex or variable habitats (respectively coral reefs and estuaries). These species selected for the opportunistic and periodic strategies, investing highly in rapid growth in order to increase survival probability to counter high mortality rates during early stages or unstable habitat conditions. Later age at maturation, slower larvae and juvenile growth as well as lower mean condition indices were consistent with fish species from more stable or predictable environments, as polar regions and freshwater habitats, which selected for the equilibrium strategy. Nonetheless, differences in energy allocation strategies during early stages were not observed, evidencing the scarcity of available data regarding condition indices and/or the importance of integrating life-history intermediate strategies. Future research into condition indices and other physiological processes, for a broader set of species and for a wider latitudinal and habitat range including seasonal variability (particularly for species from tropical and polar regions), is essential to better understand or test current theories of species ecological patterns. The use of direct quantitative measures of young fishes’ metabolic investment and fitness constitutes a new approach for life-history studies, and should be fundamental for predicting species’ responses to acute environmental or human constrains, especially in a global climate change scenario that is expected to affect distribution and abundance of fish species worldwide.     

Nitrogen deposition and herbivory affect biomass production and allocation in an annual plant

If environmental conditions vary, plasticity in life-history traits is predicted. A recent model indicates that males and females should differ in life-history traits, because sexes differ in optimal attributes depending on species ecology. In this study we test the impact of two biotic factors in combination (presence/absence of predators and low/high food level) on gender specific life-history traits in the damselfly Coenagrion puella (Odonata). Results show that predator presence and low food density decreased activity in both sexes. Additionally, individuals with less food grew more slowly, emerged later, remained smaller and had a higher mortality. At low food densities, however, and in contrast to former investigations, individuals from treatments with predator presence were the same size or larger than individuals without predators. Gender had a strong impact on larval activity and life-history traits and sexes differed in development. Females were less active and took longer to complete development, but emerged at a larger size, weight and fat content. This study highlights the importance of gender specific approaches in life-history research.     

A new direct method of stock assessment of the loliginid squid

Hydroacoustic research conducted on chokka squid (Loligo reynaudi d’Orbigny, 1845), off the east coast of South Africa from 1994–2005, has led to the development of an innovative stock assessment technique, perhaps applicable to all loliginids that migrate inshore to spawn. This technique combines hydroacoustic biomass estimates made on the spawning concentrations inshore, and minimum biomass estimates made both inshore and offshore using demersal surveys employing the swept-area method. The hydroacoustic estimate uses an improved method to obtain target strength measurements, and squid concentrations are individually mapped from a small boat with a towed transducer. This method may be used even during intense fishing operations because of the manoeuvrability of the small boat inside a tight cluster of fishing vessels. Biomasses of the individual concentrations are then summed. The inshore biomass, also includes dispersed, mature squid migrating between concentrations, this is assessed using a concentration stability factor. The biomass of dispersed squid offshore is again calculated using the swept-area method, a well known demersal survey methodology. The biomass of concentrated (spawning) squid offshore is calculated using the same proportions between concentrated and dispersed squid which were found inshore. All four components are then summed to calculate the total biomass. The result obtained is subject to the effect of complex temporal dynamics, as new animals are recruited to the adult pool and those recently assessed migrate to other sectors of the distribution area.     

Male reproductive pattern in a polygynous ungulate with a slow life-history: the role of age, social status and alternative mating tactics

According to life-history theory age-dependent investments into reproduction are thought to co-vary with survival and growth of animals. In polygynous species, in which size is an important determinant of reproductive success, male reproduction via alternative mating tactics at young age are consequently expected to be the less frequent in species with higher survival. We tested this hypothesis in male Alpine ibex (Capra ibex), a highly sexually dimorphic mountain ungulate whose males have been reported to exhibit extremely high adult survival rates. Using data from two offspring cohorts in a population in the Swiss Alps, the effects of age, dominance and mating tactic on the likelihood of paternity were inferred within a Bayesian framework. In accordance with our hypothesis, reproductive success in male Alpine ibex was heavily biased towards older, dominant males that monopolized access to receptive females by adopting the ‘tending’ tactic, while success among young, subordinate males via the sneaking tactic ‘coursing’ was in general low and rare. In addition, we detected a high reproductive skew in male Alpine ibex, suggesting a large opportunity for selection. Compared with other ungulates with higher mortality rates, reproduction among young male Alpine ibex was much lower and more sporadic. Consistent with that, further examinations on the species level indicated that in polygynous ungulates the significance of early reproduction appears to decrease with increasing survival. Overall, this study supports the theory that survival prospects of males modulate the investments into reproduction via alternative mating tactics early in life. In the case of male Alpine ibex, the results indicate that their life-history strategy targets for long life, slow and prolonged growth and late reproduction.     

A hypothesis for the evolution of androdioecy: the joint influence of reproductive assurance and local mate competition in a metapopulation

In a subdivided population with recurrent local extinction and re-colonisation, competition amongst related pollen or sperm to fertilise ovules or eggs (‘local mate competition’) is expected to select for female-biased sex allocation. Population turnover should also select against unisexuality in favour of self-fertile cosexuality, because males and females are unable to establish new populations on their own (‘Baker's Law’). Here I argue that androdioecy, a rare breeding system in which males co-occur with hermaphrodites, may evolve in a metapopulation under the joint action of local mate competition and Baker's Law if rates of self-fertilisation decrease with increasing population size. The hypothesis makes several predictions regarding patterns of life-history and sex allocation that are borne out by recent observations of androdioecious species in several unrelated lineages of plants and animals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mismatch in microbial food webs: predators but not prey perform better in their local biotic and abiotic conditions

Understanding how trophic levels respond to changes in abiotic and biotic conditions is key for predicting how food webs will react to environmental perturbations. Different trophic levels may respond disproportionately to change, with lower levels more likely to react faster, as they typically consist of smaller‐bodied species with higher reproductive rates. This response could cause a mismatch between trophic levels, in which predators and prey will respond differently to changing abiotic or biotic conditions. This mismatch between trophic levels could result in altered top‐down and bottom‐up control and changes in interaction strength. To determine the possibility of a mismatch, we conducted a reciprocal‐transplant experiment involving Sarracenia purpurea food webs consisting of bacterial communities as prey and a subset of six morphologically similar protozoans as predators. We used a factorial design with four temperatures, four bacteria and protozoan biogeographic origins, replicated four times. This design allowed us to determine how predator and prey dynamics were altered by abiotic (temperature) conditions and biotic (predators paired with prey from either their local or non‐local biogeographic origin) conditions. We found that prey reached higher densities in warmer temperature regardless of their temperature of origin. Conversely, predators achieved higher densities in the temperature condition and with the prey from their origin. These results confirm that predators perform better in abiotic and biotic conditions of their origin while their prey do not. This mismatch between trophic levels may be especially significant under climate change, potentially disrupting ecosystem functioning by disproportionately affecting top‐down and bottom‐up control.