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.     

Cryptic lineages of a common alpine mayfly show strong life‐history divergence

Understanding ecological divergence of morphologically similar but genetically distinct species – previously considered as a single morphospecies – is of key importance in evolutionary ecology and conservation biology. Despite their morphological similarity, cryptic species may have evolved distinct adaptations. If such ecological divergence is unaccounted for, any predictions about their responses to environmental change and biodiversity loss may be biased. We used spatio‐temporally replicated field surveys of larval cohort structure and population genetic analyses (using nuclear microsatellite markers) to test for life‐history divergence between two cryptic lineages of the alpine mayfly Baetis alpinus in the Swiss Alps. We found that the more widespread and abundant cryptic lineage represents a ‘generalist’ with at least two cohorts per year, whereas the less abundant lineage is restricted to higher elevations and represents a ‘specialist’ with a single cohort per year. Importantly, our results indicate partial temporal segregation in reproductive periods between these lineages, potentially facilitating local coexistence and reproductive isolation. Taken together, our findings emphasize the need for a taxonomic revision: widespread and apparently generalist morphospecies can hide cryptic lineages with much narrower ecological niches and distribution ranges.     

Fish species-environment and abundance relationships in a Great Plains river system

Fish assemblages in the upper Red River system of southwestern Oklahoma (USA) were predictable along measured environmental gradients Conductivity was the most important variable predicting structure of fish assemblages followed by stream size, alkalinity woody debris and water clarity Classification of abundance data identified four groups each of species and sites Species groups were separated on a habitat template indicating similar environmental responses within groups However, site groups showed considerable overlap on the template Correlations among species environmental preferences were significantly associated with correlations of species abundances Likewise, site correlations on the basis of measured environmental variables and on the basis of species abundances were significantly similar
We tested abundance and distribution data for agreement with the hierarchical model of Kolasa Several testable predictions of the model described our data well specialist species outnumbered generalist species and were less abundant on average, than generalist species Average abundance of species was highly correlated with their ecological ranges and species were clumped along both ecological range and abundance axes     

Ecological preference between generalist and specialist rodents: spatial and environmental correlates of phenotypic variation

Different ecological preferences among species may result in differences in response to similar environmental variation. To test this hypothesis, we assessed the patterns of skull and mandible size and shape variation in three Sigmodontinae mice from agroecosystems of central Argentina with increasing degree of specialization: Calomys musculinus, Akodon azarae and Oxymycterus rufus. Spatial patterns in size and shape were analysed after controlling for allometry and sexual dimorphism using a total of 697 specimens. We then evaluated the covariation between shape, climatic and environmental variables and assessed the contribution of distinct climatic and environmental variables to phenotypic variability. Oxymycterus rufus displayed a marked spatial structure, and there was a high correlation between shape, climatic and environmental variables in this species. Climatic and environmental variables had a moderate effect on the phenotype of A. azarae, and were not correlated with morphological variation in C. musculinus. Our study highlights the difference in phenotypic responses to spatial and environmental gradients across coexisting species, specialist species displaying a more marked spatial structure in morphology than generalist species. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112 , 180–203.     

Field-based insights to the evolution of specialization: plasticity and fitness across habitats in a specialist/generalist species pair

Factors promoting the evolution of specialists versus generalists have been little studied in ecological context. In a large-scale comparative field experiment, we studied genotypes from naturally evolved populations of a closely related generalist/specialist species pair (Polygonum persicaria and P. hydropiper), reciprocally transplanting replicates of multiple lines into open and partially shaded sites where the species naturally co-occur. We measured relative fitness, individual plasticity, herbivory, and genetic variance expressed in the contrasting light habitats at both low and high densities. Fitness data confirmed that the putative specialist out-performed the generalist in only one environment, the favorable full sun/low-density environment to which it is largely restricted in nature, while the generalist had higher lifetime reproduction in both canopy and dense neighbor shade. The generalist, P. persicaria, also expressed greater adaptive plasticity for biomass allocation and leaf size in shaded conditions than the specialist. We found no evidence that the ecological specialization of P. hydropiper reflects either genetically based fitness trade-offs or maintenance costs of plasticity, two types of genetic constraint often invoked to prevent the evolution of broadly adaptive genotypes. However, the patterns of fitness variance and herbivore damage revealed how release from herbivory in a new range can cause an introduced species to evolve as a specialist in that range, a surprising finding with important implications for invasion biology. Patterns of fitness variance between and within sites are also consistent with a possible role for the process of mutation accumulation (in this case, mutations affecting shade-expressed phenotypes) in the evolution and/or maintenance of specialization in P. hydropiper.     

The effect of positional error on fine scale species distribution models increases for specialist species

Species occurrences inherently include positional error. Such error can be problematic for species distribution models (SDMs), especially those based on fine-resolution environmental data. It has been suggested that there could be a link between the influence of positional error and the width of the species ecological niche. Although positional errors in species occurrence data may imply serious limitations, especially for modelling species with narrow ecological niche, it has never been thoroughly explored. We used a virtual species approach to assess the effects of the positional error on fine-scale SDMs for species with environmental niches of different widths. We simulated three virtual species with varying niche breadth, from specialist to generalist. The true distribution of these virtual species was then altered by introducing different levels of positional error (from 5 to 500 m). We built generalized linear models and MaxEnt models using the distribution of the three virtual species (unaltered and altered) and a combination of environmental data at 5 m resolution. The models’ performance and niche overlap were compared to assess the effect of positional error with varying niche breadth in the geographical and environmental space. The positional error negatively impacted performance and niche overlap metrics. The amplitude of the influence of positional error depended on the species niche, with models for specialist species being more affected than those for generalist species. The positional error had the same effect on both modelling techniques. Finally, increasing sample size did not mitigate the negative influence of positional error. We showed that fine-scale SDMs are considerably affected by positional error, even when such error is low. Therefore, where new surveys are undertaken, we recommend paying attention to data collection techniques to minimize the positional error in occurrence data and thus to avoid its negative effect on SDMs, especially when studying specialist species.     

Cryptic lineage diversity,body size divergence,and sympatry in a species complex of Australian lizards (Gehyra)

Understanding the joint evolutionary and ecological underpinnings of sympatry among close relatives remains a key challenge in biology. This problem can be addressed through joint phylogenomic and phenotypic analysis of complexes of closely related lineages within, and across, species and hence representing the speciation continuum. For a complex of tropical geckos from northern Australia—Gehyra nana and close relatives—we combine mtDNA phylogeography, exon‐capture sequencing, and morphological data to resolve independently evolving lineages and infer their divergence history and patterns of morphological evolution. Gehyra nana is found to include nine divergent lineages and is paraphyletic with four other species from the Kimberley region of north‐west Australia. Across these 13 taxa, 12 of which are restricted to rocky habitats, several lineages overlap geographically, including on the diverse Kimberley islands. Morphological evolution is dominated by body size shifts, and both body size and shape have evolved gradually across the group. However, larger body size shifts are observed among overlapping taxa than among closely related parapatric lineages of G. nana, and sympatric lineages are more divergent than expected at random. Whether elevated body size differences among sympatric lineages are due to ecological sorting or character displacement remains to be determined.     

Ecomorphological adaptation of acorn weevils to their oviposition site

Comparisons between closely related species in different habitats provide a window into understanding how biotic factors shape evolutionary pathways. Weevils in the genus Curculio have radiated extensively across the Palearctic, where similar ecomorphs have evolved independently on different hosts. We examined ecological and morphological data for 31 Curculio species using multivariate statistics to determine which morphological traits covary and which correlate with the host seed size. A subset of 15 taxa for which phylogenetic relationships were known were used for comparative analyses and inferring historical patterns of trait evolution. The morphological analyses suggest rostrum size increased proportionately to body size in both males and females and that both rostrum and body size correlate with host seed size but that rostrum shape does not correlate with any of the seed traits used in the analyses. Host shifts from small seeds to considerably larger seeds or vice versa have occurred several times independently and historical trait evolution indicates that these host shifts were accompanied by morphological changes in rostrum size. These patterns suggest that seed size is an important selective agent for changes in rostrum length along with body size and thus may be a key factor promoting morphological differentiation in the genus Curculio.     

Conserved phenotypic variation patterns, evolution along lines of least resistance, and departure due to selection in fossil rodents

Within a group of organisms, some morphologies are more readily generated than others due to internal developmental constraints. Such constraints can channel evolutionary changes into directions corresponding to the greatest intraspecific variation. Long-term evolutionary outputs, however, depend on the stability of these intraspecific patterns of variation over time and from the interplay between internal constraints and selective regimes. To address these questions, the relationship between the structure of phenotypic variance covariance matrices and direction of morphological evolution was investigated using teeth of fossil rodents. One lineage considered here leads to Stephanomys, a highly specialized genus characterized by a dental pattern supposedly favoring grass eating. Stephanomys evolved in the context of directional selection related to the climatic trend of global cooling causing an increasing proportion of grasslands in southwestern Europe. The initial divergence (up to approximately 6.5 mya) was channeled along the direction of greatest intraspecific variation, whereas after 6.5 mya, morphological evolution departed from the direction favored by internal constraints. This departure from the "lines of least resistance" was likely the consequence of an environmental degradation causing a selective gradient strong enough to overwhelm the constraints to phenotypic evolution. However, in a context of stabilizing selection, these constraints actually channel evolution, as exemplified by the lineage of Apodemus. This lineage retained a primitive diet and dental pattern over the last 10 myr. Limited morphological changes occurred nevertheless in accordance with the main patterns of intraspecific variation. The importance of these lines of least resistance directing long-term morphological evolution may explain parallel evolution of some dental patterns in murine evolution.     

Host specialist clownfishes are environmental niche generalists

Why generalist and specialist species coexist in nature is a question that has interested evolutionary biologists for a long time. While the coexistence of specialists and generalists exploiting resources on a single ecological dimension has been theoretically and empirically explored, biological systems with multiple resource dimensions (e.g. trophic, ecological) are less well understood. Yet, such systems may provide an alternative to the classical theory of stable evolutionary coexistence of generalist and specialist species on a single resource dimension. We explore such systems and the potential trade-offs between different resource dimensions in clownfishes. All species of this iconic clade are obligate mutualists with sea anemones yet show interspecific variation in anemone host specificity. Moreover, clownfishes developed variable environmental specialization across their distribution. In this study, we test for the existence of a relationship between host-specificity (number of anemones associated with a clownfish species) and environmental-specificity (expressed as the size of the ecological niche breadth across climatic gradients). We find a negative correlation between host range and environmental specificities in temperature, salinity and pH, probably indicating a trade-off between both types of specialization forcing species to specialize only in a single direction. Trade-offs in a multi-dimensional resource space could be a novel way of explaining the coexistence of generalist and specialists.     

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.     

Competition promotes the evolution of host generalists in obligate parasites

Ecological theory traditionally predicts that interspecific competition selects for an increase in ecological specialization. Specialization, in turn, is often thought to be an evolutionary ‘dead end,’ with specialist lineages unlikely to evolve into generalist lineages. In host–parasite systems, this specialization can take the form of host specificity, with more specialized parasites using fewer hosts. We tested the hypothesis that specialists are evolutionarily more derived, and whether competition favours specialization, using the ectoparasitic feather lice of doves. Phylogenetic analyses revealed that complete host specificity is actually the ancestral condition, with generalists repeatedly evolving from specialist ancestors. These multiple origins of generalists are correlated with the presence of potentially competing species of the same genus. A competition experiment with captive doves and lice confirmed that congeneric species of lice do, in fact, have the potential to compete in ecological time. Taken together, these results suggest that interspecific competition can favour the evolution of host generalists, not specialists, over macroevolutionary time.     

Diet and habit explain head-shape convergences in natricine snakes

The concept of ecomorphs, whereby species with similar ecologies have similar phenotypes regardless of their phylogenetic relatedness, is often central to discussions regarding the relationship between ecology and phenotype. However, some aspects of the concept have been questioned, and sometimes species have been grouped as ecomorphs based on phenotypic similarity without demonstrating ecological similarity. Within snakes, similar head shapes have convergently evolved in species living in comparable environments and/or with similar diets. Therefore, ecomorphs could exist in some snake lineages, but this assertion has rarely been tested for a wide-ranging group within a single framework. Natricine snakes (Natricinae) are ecomorphologically diverse and currently distributed in Asia, Africa, Europe and north-central America. They are primarily semiaquatic or ground-dwelling terrestrial snakes, but some are aquatic, burrowing or aquatic and burrowing in habit and may be generalist or specialist in diet. Thus, natricines present an interesting system to test whether snakes from different major habit categories represent ecomorphs. We quantify morphological similarity and disparity in head shape among 191 of the ca. 250 currently recognized natricine species and apply phylogenetic comparative methods to test for convergence. Natricine head shape is largely correlated with habit, but in some burrowers is better explained by dietary specialism. Convergence in head shape is especially strong for aquatic burrowing, semiaquatic and terrestrial ecomorphs and less strong for aquatic and burrowing ecomorphs. The ecomorph concept is useful for understanding natricine diversity and evolution, though would benefit from further refinement, especially for aquatic and burrowing taxa.     

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.     

Evidence for Frozen-Niche Variation in a Cosmopolitan Parthenogenetic Soil Mite Species (Acari,Oribatida)

Parthenogenetic lineages may colonize marginal areas of the range of related sexual species or coexist with sexual species in the same habitat. Frozen-Niche-Variation and General-Purpose-Genotype are two hypotheses suggesting that competition and interclonal selection result in parthenogenetic populations being either genetically diverse or rather homogeneous. The cosmopolitan parthenogenetic oribatid mite Oppiella nova has a broad ecological phenotype and is omnipresent in a variety of habitats. Morphological variation in body size is prominent in this species and suggests adaptation to distinct environmental conditions. We investigated genetic variance and body size of five independent forest - grassland ecotones. Forests and grasslands were inhabited by distinct genetic lineages with transitional habitats being colonized by both genetic lineages from forest and grassland. Notably, individuals of grasslands were significantly larger than individuals in forests. These differences indicate the presence of specialized genetic lineages specifically adapted to either forests or grasslands which coexist in transitional habitats. Molecular clock estimates suggest that forest and grassland lineages separated 16-6 million years ago, indicating long-term persistence of these lineages in their respective habitat. Long-term persistence, and morphological and genetic divergence imply that drift and environmental factors result in the evolution of distinct parthenogenetic lineages resembling evolution in sexual species. This suggests that parthenogenetic reproduction is not an evolutionary dead end.     

When sources become sinks: migrational meltdown in heterogeneous habitats

We consider the evolution of ecological specialization in a landscape with two discrete habitat types connected by migration, for example, a plant-insect system with two plant hosts. Using a quantitative genetic approach. we study the joint evolution of a quantitative character determining performance in each habitat together with the changes in the population density. We find that specialization on a single habitat evolves with intermediate migration rates, whereas a generalist species evolves with both very low and very large rates of movement between habitats. There is a threshold at which a small increase in the connectivity of the two habitats will result in dramatic decrease in the total population size and the nearly complete loss of use of one of the two habitats through a process of "migrational meltdown." In some situations, equilibria corresponding to a specialist and a generalist species are simultaneously stable. Analysis of our model also shows cases of hysteresis in which small transient changes in the landscape structure or accidental demographic disturbances have irreversible effects on the evolution of specialization.     

Modelling invasion for a habitat generalist and a specialist plant species

Predicting suitable habitat and the potential distribution of invasive species is a high priority for resource managers and systems ecologists. Most models are designed to identify habitat characteristics that define the ecological niche of a species with little consideration to individual species' traits. We tested five commonly used modelling methods on two invasive plant species, the habitat generalist Bromus tectorum and habitat specialist Tamarix chinensis , to compare model performances, evaluate predictability, and relate results to distribution traits associated with each species. Most of the tested models performed similarly for each species; however, the generalist species proved to be more difficult to predict than the specialist species. The highest area under the receiver-operating characteristic curve values with independent validation data sets of B. tectorum and T. chinensis was 0.503 and 0.885, respectively. Similarly, a confusion matrix for B. tectorum had the highest overall accuracy of 55%, while the overall accuracy for T. chinensis was 85%. Models for the generalist species had varying performances, poor evaluations, and inconsistent results. This may be a result of a generalist's capability to persist in a wide range of environmental conditions that are not easily defined by the data, independent variables or model design. Models for the specialist species had consistently strong performances, high evaluations, and similar results among different model applications. This is likely a consequence of the specialist's requirement for explicit environmental resources and ecological barriers that are easily defined by predictive models. Although defining new invaders as generalist or specialist species can be challenging, model performances and evaluations may provide valuable information on a species' potential invasiveness.     

Specificity and specialization of congeneric monogeneans parasitizing cyprinid fish

Patterns and likely processes connected with evolution of host specificity in congeneric monogeneans parasitizing fish species of the Cyprinidae were investigated. A total of 51 Dactylogyrus species was included. We investigated (1) the link between host specificity and parasite phylogeny; (2) the morphometric correlates of host specificity, parasite body size, and variables of attachment organs important for host specificity; (3) the evolution of morphological adaptation, that is, attachment organ; (4) the determinants of host specificity following the hypothesis of specialization on more predictable resources considering maximal body size, maximal longevity, and abundance as measures of host predictability; and (5) the potential link between host specificity and parasite diversification. Host specificity, expressed as an index of host specificity including phylogenetic and taxonomic relatedness of hosts, was partially associated with parasite phylogeny, but no significant contribution of host phylogeny was found. The mapping of host specificity into the phylogenetic tree suggests that being specialist is not a derived condition for Dactylogyrus species. The different morphometric traits of the attachment apparatus seem to be selected in connection with specialization of specialist parasites and other traits favored as adaptations in generalist parasites. Parasites widespread on several host species reach higher abundance within hosts, which supports the hypothesis of ecological specialization. When separating specialists and generalists, we confirmed the hypothesis of specialization on a predictable resource; that is, specialists with larger anchors tend to live on fish species with larger body size and greater longevity, which could be also interpreted as a mechanism for optimizing morphological adaptation. We demonstrated that ecology of host species could also be recognized as an important determinant of host specificity. The mapping of morphological characters of the attachment organ onto the parasite phylogenetic tree reveals that morphological evolution of the attachment organ is connected with host specificity in the context of fish relatedness, especially at the level of host subfamilies. Finally, we did not find that host specificity leads to parasite diversification in congeneric monogeneans.     

Evolution of skull and body shape in Triturus newts reconstructed from three‐dimensional morphometric data and phylogeny

To explore the relationship between morphological change and species diversification, we reconstructed the evolutionary changes in skull size, skull shape, and body elongation in a monophyletic group of eight species that make up salamander genus Triturus. Their well‐studied phylogenetic relationships and the marked difference in ecological preferences among five species groups makes this genus an excellent model system for the study of morphological evolution. The study involved three‐dimensional imagery of the skull and the number of trunk vertebrae, in material that represents the morphological, spatial, and molecular diversity of the genus. Morphological change largely followed the pattern of descent. The reconstruction of ancestral skull shape indicated that morphological change was mostly confined to two episodes, corresponding to the ancestral lineage that all crested newts have in common and the Triturus dobrogicus lineage. When corrected for common descent, evolution of skull shape was correlated to change in skull size. Also, skull size and shape, as well as body shape, as inferred from the number of trunk vertebrae, were correlated, indicating a marked impact of species' ecological preferences on morphological evolution, accompanied by a series of niche shifts, with the most pronounced one in the T. dobrogicus lineage. The presence of phylogenetic signal and correlated evolutionary changes in skull and body shape suggested complex interplay of niche shifts, natural selection, and constraints by a common developmental system. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 243–255.