Comparison of the ecology and evolution of plants with a generalist bird pollination system between continents and islands worldwide

Thousands of plant species worldwide are dependent on birds for pollination. While the ecology and evolution of interactions between specialist nectarivorous birds and the plants they pollinate is relatively well understood, very little is known on pollination by generalist birds. The flower characters of this pollination syndrome are clearly defined but the geographical distribution patterns, habitat preferences and ecological factors driving the evolution of generalist‐bird‐pollinated plant species have never been analysed. Herein I provide an overview, compare the distribution of character states for plants growing on continents with those occurring on oceanic islands and discuss the environmental factors driving the evolution of both groups. The ecological niches of generalist‐bird‐pollinated plant species differ: on continents these plants mainly occur in habitats with pronounced climatic seasonality whereas on islands generalist‐bird‐pollinated plant species mainly occur in evergreen forests. Further, on continents generalist‐bird‐pollinated plant species are mostly shrubs and other large woody species producing numerous flowers with a self‐incompatible reproductive system, while on islands they are mostly small shrubs producing fewer flowers and are self‐compatible. This difference in character states indicates that diverging ecological factors are likely to have driven the evolution of these groups: on continents, plants that evolved generalist bird pollination escape from pollinator groups that tend to maintain self‐pollination by installing feeding territories in single flowering trees or shrubs, such as social bees or specialist nectarivorous birds. This pattern is more pronounced in the New compared to the Old World. By contrast, on islands, plants evolved generalist bird pollination as an adaptation to birds as a reliable pollinator group, a pattern previously known from plants pollinated by specialist nectarivorous birds in tropical mountain ranges. Additionally, I discuss the evolutionary origins of bird pollination systems in comparison to systems involving specialist nectarivorous birds and reconstruct the bird pollination system of Hawaii, which may represent an intermediate between a specialist and generalist bird pollination system. I also discuss the interesting case of Australia, where it is difficult to distinguish between specialist and generalist bird pollination systems.     

Relationships between bird morphology and prey selection in two sympatric Great Cormorant Phalacrocorax carbo subspecies during winter

Variation in bird morphology (notably sex size dimorphism) has been suggested to contribute to differences in food use between individuals. We explore the hypothesis of food partitioning (diet overlap and prey size selection) in two sympatric subspecies of the Great Cormorant Phalacrocorax carbo with respect to bird morphology (subspecies and sex) in inland French waters. These areas represent a recent and increasing contact zone used as a common non-territorial winter feeding area by the continental P. c. sinensis and the marine P. c. carbo subspecies. A high dietary overlap between subspecies and sexes was found at the major sites studied. Prey size selection was found to be site-specific and generally related to a gradient of structural size and body mass (male P. c. carbo  > male P. c. sinensis  > female P. c. carbo  > female P. c. sinensis ). With respect to bird morphology, differences in prey size consumed by Great Cormorants were more pronounced between sexes in freshwater habitats than between subspecies. This was reinforced by the fact that P. c. carbo birds entering inland areas were smaller than birds on the coast. These results also suggest that bird morphology is an important determinant of dietary differences. Our study demonstrated that P. c. carbo is an efficient generalist forager in continental areas, and food partitioning in type and size of prey between the two subspecies is low.     

Area mediated shifts in bird community composition: a study on a fragmented Mediterranean grassland

The effects of habitat fragmentation on birds have often been studied in forest specialist species. Here we aimed at comparing the response of open habitat birds within a range of habitat specialization. The study area was a Mediterranean pseudo-steppe, designated as important for conservation yet fragmented by tree encroachment. We defined bird species dependency on steppe-like habitat by a correspondence analysis, allowing us to distinguish between specialists, generalists and scrubland species. We studied species abundance in relation to fragment area, testing whether species representation in fragments differed from those in continuous habitat. This analysis showed a contrasted response to fragment size between “open habitat” specialist species and generalist ones. Open habitat species were under-represented in the smallest fragments, while generalist were over-represented in small fragments in comparison to their distribution in continuous habitats. We discuss how these results can be linked to species habitat requirements. We find that scrubland species seem to be favoured by encroachment of woody vegetation, as they are able to explore and use the wooded matrix; however specialist species are restricted to open patches and are sensitive to a reduction in patch size. This allows us to predict how different species can exhibit a different sensitivity to habitat fragmentation.     

Food utilization of aphidophagous hoverfly larvae (Diptera: Syrphidae, Chamaemyiidae) on herbaceous plants in an urban habitat

We examined food utilization in a community of aphidophagous hoverfly larvae (Diptera: Syrphidae and Chamaemyiidae) in open lands in an urban habitat in central Japan for 3 years. The community consisted of 17 hoverfly species feeding on 20 aphid species occurring on 14 species of dominant herbaceous plants. In terms of larval prey preference, the dominant eight species of hoverfly were categorized into three groups: a polyphagous ‘generalist’ group consisting of four species,Episyrphus balteatus, Betasyrphus serarius, Syrphus vitripennis andSphaerophoria sp.; an oligophagous ‘specialist’ group consisting of three species,Metasyrphus hakiensis, Dideoides latus andParagus hemorrhous; andLeucopis puncticornis, which showed a preference for two aphid species on the plantTorilis scabra. The prey aphids of the second group have behavioral or morphological defense mechanisms that are effective for preventing attacks by generalist hoverflies; two prey aphids are aggressive toward generalist predators and the others are protected by ant-attendance. The specialist hoverflies seem to be adapted to overcome these defense mechanisms. The prey ranges overlapped little between the generalist and the specialist groups, while those within the generalist group overlapped greatly.     

Sex-specific developmental plasticity of generalist and specialist predatory mites (Acari: Phytoseiidae) in response to food stress

We studied developmental plasticity under food stress in three female-biased size dimorphic predatory mite species, Phytoseiulus persimilis, Neoseiulus californicus, and Amblyseius andersoni. All three species prey on two-spotted spider mites but differ in the degree of adaptation to this prey. Phytoseiulus persimilis is a specialized spider mite predator, N. californicus is a generalist with a preference for spider mites, and A. andersoni is a broad generalist. Immature predators were offered prey patches of varying density and their survival chances, dispersal tendencies, age and size at maturity measured. Amblyseius andersoni dispersed earlier from and had lower survival chances in low density prey patches than N. californicus and P. persimilis. Age at maturity was not affected by prey density in the generalist A. andersoni, whereas both the specialist P. persimilis and the generalist N. californicus accelerated development at low prey densities. Species-specific plasticity in age at maturity reflects opposite survival strategies when confronted with limited prey: to prematurely leave and search for other food (A. andersoni) or to stay and accelerate development (P. persimilis, N. californicus). In all species, size at maturity was more plastic in females than males, indicating that males incur higher fitness costs from deviations from optimal body size.     

Functional consequences of trophic specialization in pufferfishes

1. Trophic breadth may be correlated with behavioural and functional versatility or flexibility (Liem 1984), such that species with limited diets would be predicted to be less versatile or flexible in their feeding capabilities than would trophic generalists. The relationship was examined between the trophic breadth of a species and the degree to which muscle activity is influenced by prey type in two species of pufferfishes: Chilomycterus schoepfi, a specialist on hard-shelled prey, and Sphoeroides nephalus, a generalist predator. It was predicted that the specialist would show a decreased ability to alter its motor pattern in response to prey type and an increased amount of motor-pattern variability when feeding on different prey.
2. Electromyographic recordings of five feeding muscles were made from five individuals per species feeding on four prey types that differed in their hardness and escape abilities. Muscle-activity duration, relative onset and integrated rectified area were analysed for prey capture and manipulation behaviours for each prey type.
3. Prey-type effects on mean motor-pattern variables for capture were limited to one of 14 variables in each species. Prey effects on buccal manipulation were not significant for any of the 14 variables for C. schoepfi, but S. nephalus exhibited prey effects for seven out of 14 variables. No differences in motor-pattern variation were found for the strike, but species differed significantly in overall motor-pattern variability during buccal manipulation; C. schoepfi, the dietary specialist, exhibited greater motor variability than S. nephalus, the dietary generalist.
4. The results therefore support a direct relationship between the trophic breadth of a species and its functional versatility for buccal manipulation. In contrast, prey capture was a relatively stereotyped behaviour for both species, as few prey-type effects were found.     

The difference between generalist and specialist: the effects of wide fluctuations in main food abundance on numbers and reproduction of two co-existing predators

Specialist individuals within animal populations have shown to be more efficient foragers and/or to have higher reproductive success than generalist individuals, but interspecific reproductive consequences of the degree of diet specialisation in vertebrate predators have remained unstudied. Eurasian pygmy owls (hereafter POs) have less vole-specialised diets than Tengmalm's owls (TOs), both of which mainly subsist on temporally fluctuating food resources (voles). To test whether the specialist TO is more limited by the main prey abundance than the generalist PO, we studied breeding densities and reproductive traits of co-existing POs and TOs in central-western Finland during 2002–2019. Breeding densities of POs increased with augmenting densities of voles in the previous autumn, whereas breeding densities of TOs increased with higher vole densities in both the previous autumn and the current spring. In years of vole scarcity, PO females started egg-laying earlier than TOs, whereas in years of vole abundance TO females laid eggs substantially earlier than PO females. The yearly mean clutch size and number of fledglings produced of both POs and TOs increased with abundance of voles in the current spring. POs laid large clutches and produced large broods in years of both high and low vole abundance, whereas TOs were able to do so only in years of high vole abundance. POs were able to raise on average 73% of the eggs to fledglings whereas TOs only 44%. The generalist foraging strategy of POs including flexible switching from main prey to alternative prey (small birds) appeared to be more productive than the strictly vole-specialized foraging strategy of TOs. In contrast to earlier studies at the individual-level, specialist predators at the species level (in this case TOs) appear to be less effective than generalists (POs), but diet specialisation was particularly costly under conditions when scarcity of main foods limited offspring production.     

Diet breadth variability in larval blue whiting as a response to plankton size structure

Diet breadth (measured as the S.D. of the log of prey size per larvae; SLH) of blue whiting micromesistius poutassou larvae followed a quadratic equation with larval size. In small larvae, diet breadth in terms of size (SLH), the mean and the maximum of the log of prey size per larvae (MLH and XLH, respectively) increased with larval size as prey size selection shifted to larger prey. In contrast, large larvae tended to reduce diet breadth of prey sizes ingested, focusing on the larger prey that were abundant, instead of raising the upper limit of prey sizes because of the low abundance of larger prey. Except for larvae at the onset of first feeding, number of prey stayed constant or decreased in relation to larval size. Both patterns (in small and large larvae) maintained a constant rate of increase of gut carbon content with increase in larval size. Large larvae appear to maintain the increase in gut carbon content during ontogenetic development by reducing diet breadth (SLH) and increasing selection towards the larger prey that are abundant.     

Predator diet breadth influences the relative importance of bottom-up and top-down control of prey biomass and diversity

We investigated the effects of predator diet breadth on the relative importance of bottom-up and top-down control of prey assemblages, using microbial food webs containing bacteria, bacterivorous protists and rotifers, and two different top predators. The experiment used a factorial design that independently manipulated productivity and the presence or absence of two top predators with different diet breadths. Predators included a "specialist" predatory ciliate Euplotes aediculatus, which was restricted to feeding on small prey, and a "generalist" predatory ciliate Stentor coeruleus, which could feed on the entire range of prey sizes. Both total prey biomass and prey diversity increased with productivity in the predator-free control and specialist predator treatments, a pattern consistent with bottom-up control, but both remained unchanged by productivity in the generalist predator treatment, a pattern consistent with top-down control. Linear food chain models adequately described responses in the generalist predator treatment, whereas food web models incorporating edible and inedible prey (which can coexist in the absence of predators) adequately described responses in the specialist predator treatment. These results suggest that predator diet breadth can play an important role in modulating the relative strength of bottom-up and top-down forces in ecological communities.     

Distinct feeding strategies of generalist and specialist spiders

1. Feeding behaviour of generalist and specialist predators is determined by a variety of trophic adaptations. Specialised prey‐capture adaptations allow specialists to catch relatively large prey on a regular basis. As a result, specialists might be adapted to exploit each item of prey more thoroughly than do generalists. 2. It was expected that obligatory specialist cursorial spiders would feed less frequently than generalists but for a longer time and, thus, that their foraging pause would be longer. First, the feeding frequencies of three generalist spider species (Cybaeodamus taim, Harpactea hombergi, Hersiliola sternbergsi) were compared with those three phylogenetically related specialist species: myrmecophagous Zodarion rubidum, and araneophagous Nops aff. variabilis and Palpimanus orientalis. 3. Generalists captured more prey, exploited each item of prey for a significantly shorter time, and had a shorter foraging pause than was the case for specialists. Generalists also gained significantly less relative amount of prey mass than did specialists. 4. Second, the study compared the prey DNA degradation rate in the gut of generalists and specialists by means of PCR. The degradation rate was not significantly different between specialists and generalists: the detectability half‐life was estimated to exist for 14.3 days after feeding. 5. This study shows that the feeding strategies of cursorial generalist and obligatory specialist spiders are different. Obligatory specialists have evolved a feeding strategy that is based on thorough exploitation of a few large prey, whereas generalists have evolved a strategy that is based on short exploitation of multiple small items of prey.     

Prey species and prey diet affect growth of invertebrate predators

1. The effects of prey species and leaf age used by prey on performance of two generalist invertebrate predators were studied. The focal plant was Plantago lanceolata , which contains iridoid glycosides.
2. Diet of the herbivorous prey influenced their growth rate.
3. The generalist herbivore ( Vanessa cardui ) and the novel-plant feeder ( Manduca sexta ) contained very low levels of iridoid glycosides in their haemolymph, whereas the specialist ( Junonia coenia ) levels were 50–150-fold higher.
4. Predatory stinkbugs ( Podisus maculiventris ) fed either the novel-plant feeder or the specialist exhibited similar developmental rates. However, stinkbugs ate less of the generalist but grew faster. The growth rate of the stinkbugs was higher when the caterpillar species were raised on the new-leaf powder diet, which contained twice as much protein and iridoid glycosides as the mature-leaf powder diet.
5. Jumping spiders ( Phidippus audax ) ate more mealworms ( Tenebrio molitor ) than specialist J. coenia caterpillars, fed either new- or mature-leaf powder diets, and could not gain weight when fed J. coenia.
6. These results indicate that prey quality was not determined solely by the iridoid glycoside concentration in the diet.     

Feeding ecology of thirteen syntopic species of anurans in a seasonal tropical environment

Summary Thirteen species of anurans belonging to three families forage diurnally for arthropods in the leaf litter of the lowland rainforest at the Río Llullapichis in Amazonian Perú. This paper investigates the diets and patterns of coexistence in this group of ecologically similar species. All thirteen species use the forest floor habitat without apparent differentiation. Most species take prey in proportions significantly different from those occurring in the leaf litter and comprise two specialist guilds: dendrobatids and bufonids that eat hard-bodied, slow-moving arthropods such as ants and mites; and leptodactylids that eat soft-bodied, mobile arthropods, primarily orthopterans and large spiders. Dendrobates femoralis (Boulenger) is a generalist, taking prey in proportions not significantly different from those in the leaf litter. Within specialist guilds, body sizes of species vary and are correlated with the size of prey taken. Foraging behavior and predator defense also correlate with the type and sizes of prey taken. Ant specialists tend to be poisonous and active searchers, taking many small prey per day. Non-ant specialists are cryptic, sit-and-wait foragers that take few large prey per day. Similarity in diet within guilds tends tobe lowest in the dry season when food is less abundant, suggesting that food is in short supply in the dry season.     

Reinforcement learning for a stochastic automaton modelling predation in stationary model-mimic environments

In this paper we propose a mathematical learning model for the feeding behaviour of a specialist predator operating in a random environment occupied by two types of prey, palatable mimics and unpalatable models, and a generalist predator with additional alternative prey at its disposal. A well known linear reinforcement learning algorithm and its special cases are considered for updating the probabilities of the two actions, eat prey or ignore prey. Each action elicits a probabilistic response from the environment that can be favorable or unfavourable. To assess the performance of the predator a payoff function is constructed that captures the energetic benefit from consuming acceptable prey, the energetic cost from consuming unacceptable prey, and lost benefit from ignoring acceptable prey. Conditions for an improving predator payoff are also explicitly formulated.     

Suitability of woodlice prey for generalist and specialist spider predators: a comparative study

1. Predators select a prey according to its energetic and nutritional composition. Generalist predators avoid, whereas specialists often specialise on well‐defended prey. The aim of this study was to find the suitability of woodlice prey for generalist and specialist predators by comparing their handling efficiency. 2. Laboratory experiments were performed in which specialist and generalist predators were reared on monotypic diets comprising one or other of two woodlice species that differ in their defensive strategies: rollers (Armadillidium) and clingers (Porcellio). A control group was reared on a mixture of arthropods (excluding woodlice). Three spider predators were used that differ in their adaptations to deal with woodlice prey: a woodlice specialist, Dysdera crocata; an oligophagous generalist, Pholcus phalangioides, that also captures woodlice; and a euryphagous generalist, Tegenaria domestica, that does not feed on woodlice. The frequency of capture was recorded and various fitness parameters were measured, namely survival, growth rate, and ontogenetic development. 3. It was found that the specialist, D. crocata, performed best on the Porcellio diet, and similarly well on Armadillidium and mixed diets. The two generalists, P. phalangioides and T. domestica, had poor performance on both woodlice diets but performed well on the mixed diet. 4. The results show that woodlice are unsuitable prey for both oligophagous and euryphagous generalist predators.     

Food webs in tropical Australian streams: shredders are not scarce

1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low‐order streams in the Queensland wet tropics.     

Context-dependent effects of predator removal from experimental microcosm communities

The loss of a predator from an ecological community can cause large changes in community structure and ecosystem processes, or have very little consequence for the remaining species and ecosystem. Understanding when and why the loss of a predator causes large changes in community structure and ecosystem processes is critical for understanding the functional consequences of biodiversity loss. We used experimental microbial communities to investigate how the removal of a large generalist predator affected the extinction frequency, population abundance and total biomass of its prey. We removed this predator in the presence or absence of an alternative, more specialist, predator in order to determine whether the specialist predator affected the outcome of the initial species removal. Removal of the large generalist predator altered some species' populations but many were unaffected and no secondary extinctions were observed. The specialist predator, though rare, altered the response of the prey community to the removal of the large generalist predator. In the absence of the specialist predator, the effects of the removal were only measurable at the level of individual species. However, when the specialist predator was present, the removal of the large generalist predator affected the total biomass of prey species. The results demonstrate that the effect of species loss from high trophic levels may be very context-dependent, as rare species can have disproportionately large effects in food webs.     

Numerical and dietary responses of a predator community in a temperate zone of Europe

The generalist predation hypothesis predicts that the functional responses of generalist predator species should be quicker than those of specialist predators and have a regulating effect on vole populations. New interpretations of their role in temperate ecosystems have, however, reactivated a debate suggesting generalist predators may have a destabilizing effect under certain conditions (e.g. landscape homogeneity, low prey diversity, temporary dominance of 1 prey species associated with a high degree of dietary specialization). We studied a rich predator community dominated by generalist carnivores ( Martes spp., Vulpes vulpes, Felis catus ) over a 6 yr period in farmland and woodland in France. The most frequent prey were small rodents (mostly Microtus arvalis , a grassland species, and Apodemus spp., a woodland species). Alternative prey were diverse and dominated by lagomorphs ( Oryctolagus cuniculus, Lepus europeus ). We detected a numerical response among specialist carnivores but not among generalist predators. The dietary responses of generalist predators were fairly complex and most often dependent on variation in density of at least 1 prey species. These results support the generalist predation hypothesis. We document a switch to alternative prey, an increase of diet diversity, and a decrease of diet overlap between small and medium-sized generalists during the low density phase of M. arvalis . In this ecosystem, the high density phases of small mammal species are synchronous and cause a temporary specializing of several generalist predator species. This rapid functional response may indicate the predominant role of generalists in low amplitude population cycles of voles observed in some temperate areas.     

Niche width impacts vertebrate diversification

Aim The size of the climatic niche of a species is a major factor determining its distribution and evolution. In particular, it has been proposed that niche width should be associated with the rate of species diversification. Here, we test whether species niche width affects the speciation and extinction rates of three main clades of vertebrates: amphibians, mammals and birds. Location Global. Methods We obtained the time‐calibrated phylogenies, IUCN conservation status, species distribution maps and climatic data for 2340 species of amphibians, 4563 species of mammals and 9823 species of birds. We computed the niche width for each species as the mean annual temperature across the species range. We estimated speciation, extinction and transition rates associated with lineages with either narrow (specialist) or wide (generalist) niches using phylogeny‐based birth–death models. We also tested if current conservation status was correlated with the niche width of species. Results We found higher net diversification rates in specialist species than in generalist species. This result was explained by both higher speciation rates (for the three taxonomic groups) and lower extinction rates (for mammals and birds only) in specialist than in generalist species. In contrast, current specialist species tended to be more threatened than generalist species. Main conclusions Our diversification analysis shows that the width of the climatic niche is strongly associated with diversification rates and may thus be a crucial factor for understanding the emergence of diversity patterns in vertebrates. The striking difference between our diversification results and current conservation status suggests that the current extinction process may be different from extinction rates estimated from the whole history of the group.     

Responses of great tits Parus major to small tortoiseshells Aglais urticae in feeding trials; evidence of aposematism

Abstract.  1. The small tortoiseshell Aglais urticae (Lepidoptera, Nymphalidae) is one of several red-coloured butterflies and moths that are active in early spring in Norway. It has been suggested that tortoiseshells may be warningly coloured and unpalatable to birds, however no experiments have been carried out to test this hypothesis.
2. The reactions of wild-caught great tits Parus major (Paridae) when offered dead specimens of the tortoiseshell and four palatable control species were studied. Two experiments were carried out. In expt 1, only intact prey was presented to the birds. In expt 2, the birds were offered only experimentally de-winged prey. Hence, in expt 2, it was possible to explore to what extent the birds' reaction to the prey offered to them in expt 1 could be explained as a reaction to visual stimuli from the wings of the prey.
3. In both experiments, tortoiseshells were attacked hesitantly, eaten slowly, and rejected often compared with controls, providing evidence that tortoiseshells are mildly distasteful to great tits. Moreover, all birds were more hesitant to attack intact than de-winged tortoiseshells, while there was no such effect for control prey, indicating a warning effect associated with the butterfly wings. There were also indications of avoidance learning in the birds to the presented tortoiseshells. These results are consistent with the idea of aposematism.     

Camouflage that is spot on! Optimization of spot size in prey-background matching

Camouflage is a key predation prevention mechanism. The most intuitive camouflage strategy is background matching, where a prey resembles the colour and pattern of its background. Spots (e.g. geometric ‘elements’ that constitute a pattern) are a common feature on the body of prey, but the effect of their size in relation to elements in the background is rarely examined in camouflage studies. Here, we test the survivorship of computer generated prey possessing patterns of elements grouped into eight size classes on backgrounds composed of elements of the same eight size classes. All 64 possible combinations were presented to human volunteers (n = 10) acting as ‘predators’ in a computer simulation. As hypothesised, prey survivorship was high when the prey-background element size difference was low. We found significant asymmetry, however, in survivorship pattern: prey morphs with elements larger than those of the backgrounds were harder to detect than morphs with elements smaller than those of the backgrounds. When testing for potential trade-offs we determined that, in a habitat consisting of an equal proportion of two backgrounds with a large element size difference between them, net survivorship of specialist prey morphs (same element size as one of the backgrounds) was similar to that of generalist morphs (intermediate element size). When the two backgrounds were less distinctive, i.e. reduced element size difference between the backgrounds, generalist prey morphs achieved significantly better net survivorship than specialists.