Multiple routes to interspecific territoriality in sister species of North American perching birds

Behavioral interference between species can influence a wide range of ecological and evolutionary processes. Here, we test foundational hypotheses regarding the origins and maintenance of interspecific territoriality, and evaluate the role of interspecific territoriality and hybridization in shaping species distributions and transitions from parapatry to sympatry in sister species of North American perching birds (Passeriformes). We find that interspecific territoriality is pervasive among sympatric sister species pairs, and that interspecifically territorial species pairs have diverged more recently than sympatric noninterspecifically territorial pairs. None of the foundational hypotheses alone explains the observed patterns of interspecific territoriality, but our results support the idea that some cases of interspecific territoriality arise from misdirected intraspecific aggression while others are evolved responses to resource competition. The combination of interspecific territoriality and hybridization appears to be an unstable state associated with parapatry, whereas species that are interspecifically territorial and do not hybridize are able to achieve extensive fine- and coarse-scale breeding range overlap. In sum, these results suggest that interspecific territoriality has multiple origins and impacts coexistence at multiple spatial scales.     

Joint effect of phylogenetic relatedness and trait selection on the elevational distribution of Rhododendron species

Congeneric species may coexist at fine spatial scales through niche differentiation, however, the magnitude to which the effects of functional traits and phylogenetic relatedness contribute to their distribution along elevational gradients remains understudied. To test the hypothesis that trait and elevational range overlap can affect local speciesʼ coexistence, we first compared phylogenetic relatedness and trait (including morphological traits and leaf elements) divergence among closely related species of Rhododendron L. on Yulong Mountain, China. We then assessed relationships between the overlap of multiple functional traits and the degree of elevational range overlap among species pairs in a phylogenetic context. We found that phylogeny was a good predictor for most functional traits, where closely related species showed higher trait similarity and occupied different elevational niches at our study site. Species pairs of R. subgen. Hymenanthes (Blume) K. Koch showed low elevational range overlap and some species pairs of R. subgen. Rhododendron showed obvious niche differentiation. Trait divergence is greater for species in R. subgen. Rhododendron, and it plays an important role between species pairs with low elevational range overlap. Trait convergent selection takes place between co-occurring closely related species that have high elevational range overlap, which share more functional trait space due to environmental filtering or ecological adaptation in more extreme habitats. Our results highlight the importance of evolutionary history and trait selection for species coexistence at fine ecological scales along environmental gradients.     

Niche overlap in allotopic and syntopic populations of sexually interacting ground‐hopper species

Abstract There is accumulating evidence that sexual interactions among species (reproductive interference) could have dramatic effects for species’ coexistence. It has been shown that the fitness of individuals can be substantially reduced as a consequence of reproductive interference. This might subsequently lead to displacement of a species (sexual exclusion). On the other hand, some evolutionary and ecological mechanisms might enable species to coexist, such as the divergence of mate recognition systems (reproductive character displacement), habitat partitioning, clumped dispersion patterns or different colonization capabilities. We have previously shown that the two ground‐hopper species Tetrix subulata and Tetrix ceperoi interact sexually in the laboratory as well as in the field. At sites where both species co‐occur niche overlap was high, suggesting that coexistence is maintained by different niche breadths rather than by habitat partitioning. To test the hypothesis that habitat partitioning does not contribute to species’ coexistence, we examined whether allotopic and syntopic populations of these two species differ in niche overlap (competitive release). Our results show that niche overlap is higher in syntopic than in allotopic populations, suggesting that the site‐specific habitat structure (heterogeneity) has a stronger influence on microhabitat utilization than the presence of heterospecifics. Hence, our data do not support the hypothesis that habitat partitioning plays a substantial role for the coexistence of these sexually interacting species.     

The importance (or lack thereof) of niche divergence to the maintenance of a northern species complex: the case of the long‐toed salamander (Ambystoma macrodactylum Baird)

The relative importance of ecological vs. nonecological factors for the origin and maintenance of species is an open question in evolutionary biology. Young lineages – such as the distinct genetic groups that make up the ranges of many northern species – represent an opportunity to study the importance of ecological divergence during the early stages of diversification. Yet, few studies have examined the extent of niche divergence between lineages in previously glaciated regions and the role of ecology in maintaining the contact zones between them. In this study, we used tests of niche overlap in combination with ecological niche models to explore the extent of niche divergence between lineages of the long‐toed salamander (Ambystoma macrodactylum Baird) species complex and to determine whether contact zones correspond to (divergent) niche limits. We found limited evidence for niche divergence between the different long‐toed salamander lineages, substantial overlap in the predicted distribution of suitable climatic space for all lineages and range limits that are independent of niche limits. These results raise questions as to the importance of ecological divergence to the development of this widespread species complex and highlight the potential for non‐ecological factors to play a more important role in the maintenance of northern taxa.     

Demography and habitat availability in territorial occupancy of two competing species

Although metapopulation dynamics have become the focus of considerable theoretical research, little attention has been paid to its role when examining the coexistence of species. When two or more species live in the same patch network, interspecific interactions may affect their dispersal, colonization and extinction rates, and it may be possible to incorporate competition affecting these parameters in metapopulation models. Here, we extend the territorial occupancy model proposed by Lande to competing species. Our model estimates an equilibrium proportion of habitat occupancy as a function of life‐history parameters, dispersal behavior, habitat suitability and interspecific interactions. Moreover, it could prove to be useful as a tool in the assessment of potential management decisions. We apply the model to the golden Aquila chrysaetos and the Bonelli's eagle Hieraaetus fasciatus, two territorial raptors that coexist in the Mediterranean region, sharing food and nesting habitats. Over the last twenty years, while the golden eagle has maintained and, in some cases, increased its breeding numbers, Bonelli's eagle has suffered a marked decline, with many territories abandoned by the latter now occupied by the former. This suggests that the dynamics of these species could be influenced by interspecific competition. The model identified the relative importance of competition (stable equilibrium that allows long‐term coexistence) and predicted that, when habitat overlap is slight as in the study area, intraspecific dynamics are much more important for the persistence of each species than interspecific ones. Our results suggest that the improvement of territorial bird survival and productivity are the most urgently needed actions to be undertaken in the case of the golden eagle, while for Bonelli's eagle efforts should be focused on improving territorial and non‐territorial bird survival. As habitat conservation measures, the proportion of suitable exclusive habitat should be increased for both species.     

Phylogeography of a widespread lizard complex reflects patterns of both geographic and ecological isolation

A primary challenge for modern phylogeography is understanding how ecology and geography, both contemporary and historical, shape the spatial distribution and evolutionary histories of species. Phylogeographic patterns are the result of many factors, including geology, climate, habitat, colonization history and lineage‐specific constraints. Assessing the relative influences of these factors is difficult because few species, regions and environments are sampled in enough detail to compare competing hypotheses rigorously and because a particular phylogeographic pattern can potentially result from different evolutionary scenarios. The silky anoles (Anolis sericeus complex) of Central America and Mexico are abundant and found in all types of lowland terrestrial habitat, offering an excellent opportunity to test the relative influences of the factors affecting diversification. Here, we performed a range‐wide statistical phylogeographic analysis on restriction site‐associated DNA (RAD) markers from silky anoles and compared the phylogeographic patterns we recovered to historical and contemporary environmental and topographic data. We constructed niche models to compare niche overlap between sister lineages and conducted coalescent simulations to characterize how the major lineages of silky anoles have diverged. Our results revealed that the mode of divergence for major lineage diversification events was geographic isolation, resulting in ecological divergence between lineages, followed by secondary contact. Moreover, comparisons of parapatric sister lineages suggest that ecological niche divergence contributed to isolation by environment in this system, reflecting the natural history differences among populations in divergent environments.     

Interspecific territoriality in gibbons (Hylobates lar and H. pileatus) and its effects on the dynamics of interspecies contact zones

We investigated the ecology and interspecific interactions of the two gibbon species (Hylobates lar and H. pileatus) that overlap in distribution within a narrow zone of contact in the headwaters of the Takhong River at Khao Yai National Park in central Thailand. The zone is about 10-km wide, with phenotypic hybrids comprising 6.5% of the adult population. We compared species with respect to diet, territory size, intra- and interspecific group encounters, and territory quality. The two gibbon species exploited the same types of resources within their territories despite variation in the relative abundance of food-plant species between territories. The gibbons were interspecifically territorial, and males of both species displayed aggressive behaviors at both intraspecific and interspecific territorial boundaries. There was no marked difference in the amount of overlap between territories of conspecific and heterospecific pairs of groups. Although the habitat was not homogeneous, territory quality did not vary significantly between species. The species have not diverged in habitat preference or in diet. Neither species dominated in interspecific encounters, and both were reproducing well in the contact zone. We analyzed the potential consequences of several types of interspecific interactions on individual dispersal options and on the structure of the contact zone. Interference competition through interspecific territoriality affects the dispersal of individuals into the range of the other species. In general, territorial competition coupled with limited hybridization leads to predictions of a narrow contact zone or parapatry between species; thus, behavioral and ecological interactions between species need to be considered as potential factors in explaining range borders of primate species.     

Pollination outcomes reveal negative density‐dependence coupled with interspecific facilitation among plants

Pollination is thought to be under positive density‐dependence, destabilising plant coexistence by conferring fitness disadvantages to rare species. Such disadvantage is exacerbated by interspecific competition but can be mitigated by facilitation and intraspecific competition. However, pollinator scarcity should enhance intraspecific plant competition and impose disadvantage on common over rare species (negative density‐dependence, NDD). We assessed pollination proxies (visitation rate, pollen receipt, pollen tubes) in a generalised plant community and related them to conspecific and heterospecific density, expecting NDD and interspecific facilitation due to the natural pollinator scarcity. Contrary to usual expectations, all proxies indicated strong intraspecific competition for common plants. Moreover interspecific facilitation prevailed and was stronger for rare than for common plants. Both NDD and interspecific facilitation were modulated by specialisation, floral display and pollinator group. The combination of intraspecific competition and interspecific facilitation fosters plant coexistence, suggesting that pollination can be a niche axis maintaining plant diversity.     

Ecological traits influence the phylogenetic structure of bird species co‐occurrences worldwide

The extent to which species’ ecological and phylogenetic relatedness shape their co‐occurrence patterns at large spatial scales remains poorly understood. By quantifying phylogenetic assemblage structure within geographic ranges of >8000 bird species, we show that global co‐occurrence patterns are linked – after accounting for regional effects – to key ecological traits reflecting diet, mobility, body size and climatic preference. We found that co‐occurrences of carnivorous, migratory and cold‐climate species are phylogenetically clustered, whereas nectarivores, herbivores, frugivores and invertebrate eaters tend to be more phylogenetically overdispersed. Preference for open or forested habitats appeared to be independent from the level of phylogenetic clustering. Our results advocate for an extension of the tropical niche conservatism hypothesis to incorporate ecological and life‐history traits beyond the climatic niche. They further offer a novel species‐oriented perspective on how biogeographic and evolutionary legacies interact with ecological traits to shape global patterns of species coexistence in birds.     

How competitive intransitivity and niche overlap affect spatial coexistence

Competitive intransitivity is mostly considered outside the main body of coexistence theories that rely primarily on the role of niche overlap and differentiation. How the interplay of competitive intransitivity and niche overlap jointly affects species coexistence has received little attention. Here, we consider a rock–paper–scissors competition system where interactions between species can represent the full spectra of transitive–intransitive continuum and niche overlap/differentiation under different levels of competition asymmetry. By comparing results from pair approximation that only considers interference competition between neighbouring cells in spatial lattices, with those under the mean-field assumption, we show that 1) species coexistence under transitive competition is only possible at high niche differentiation; 2) in communities with partial or pure intransitive interactions, high levels of niche overlap are not necessary to beget species extinction; and 3) strong spatial clustering can widen the condition for intransitive loops to facilitate species coexistence. The two mechanisms, competitive intransitivity and niche differentiation, can support species persistence and coexistence, either separately or in combination. Finally, the contribution of intransitive loops to species coexistence can be enhanced by strong local spatial correlations, modulated and maximised by moderate competition asymmetry. Our study, therefore, provides a bridge to link intransitive competition to other generic ecological theories of species coexistence.     

Nest site availability and niche differentiation between two cavity‐nesting birds in time and space

Niche differentiation is a key concept in the field of ecology and refers to the process by which competing species within an ecological community partition utilization of environmental resources to achieve coexistence. The existence of niche differentiation is uniquely difficult to prove on account of the fact that historical interaction among species, which plays a key role in elucidating the current state of coexistence among species, is not well known. We created continuous niche gradients in nest‐site resources between two sympatric secondary cavity‐nesting birds, the green‐backed tit (Parus monticolus) and the russet sparrow (Passer cinnamomeus), and investigated whether nesting site is a factor contributing to limiting breeding overlap by regular inspection and 388,160 min of film recording. Our results indicate that although we manipulated nest site availability to be uniformly high along the habitat gradient, the two bird species have little overlap in nest sites and rarely compete for them. Furthermore, the green‐backed tit possessed a wide range of fundamental niche that covered that of the russet sparrow, while their reproductive time was largely segregated. The sparrow was more aggressive and outcompeted the tit in their overlapped range. These results suggest that even though nesting sites are crucial to the reproduction of cavity‐nesting birds, some other factor plays a more important role in limiting niche overlap between sparrows and tits in space and time. Given that these two cavity‐nesting birds continued to use different habitats and breed in segregated time after our manipulation, their relationship is better explained by the ghost of competition past theory.     

How variation between individuals affects species coexistence

Although the effects of variation between individuals within species are traditionally ignored in studies of species coexistence, the magnitude of intraspecific variation in nature is forcing ecologists to reconsider. Compelling intuitive arguments suggest that individual variation may provide a previously unrecognised route to diversity maintenance by blurring species‐level competitive differences or substituting for species‐level niche differences. These arguments, which are motivating a large body of empirical work, have rarely been evaluated with quantitative theory. Here we incorporate intraspecific variation into a common model of competition and identify three pathways by which this variation affects coexistence: (1) changes in competitive dynamics because of nonlinear averaging, (2) changes in species’ mean interaction strengths because of variation in underlying traits (also via nonlinear averaging) and (3) effects on stochastic demography. As a consequence of the first two mechanisms, we find that intraspecific variation in competitive ability increases the dominance of superior competitors, and intraspecific niche variation reduces species‐level niche differentiation, both of which make coexistence more difficult. In addition, individual variation can exacerbate the effects of demographic stochasticity, and this further destabilises coexistence. Our work provides a theoretical foundation for emerging empirical interests in the effects of intraspecific variation on species diversity.     

The ecological and evolutionary stability of interspecific territoriality

Interspecific territoriality may play an important role in structuring ecological communities, but the causes of this widespread form of interference competition remain poorly understood. Here, we investigate the phenotypic, ecological and phylogenetic correlates of interspecific territoriality in wood warblers (Parulidae). Interspecifically territorial species have more recent common ancestors and are more similar phenotypically, and are more likely to hybridise, than sympatric, non‐interspecifically territorial species. After phylogenetic corrections, however, similarity in plumage and territorial song are the only significant predictors of interspecific territoriality besides syntopy (fine‐scale geographic overlap). Our results do not support the long‐standing hypothesis that interspecific territoriality occurs only under circumstances in which niche divergence is restricted, which combined with the high incidence of interspecific territoriality in wood warblers (39% of species), suggests that this interspecific interaction is more stable, ecologically and evolutionarily, than commonly assumed.     

Spatial evolutionary and ecological vicariance analysis (SEEVA), a novel approach to biogeography and speciation research,with an example from Brazilian Gentianaceae

Aim Spatial evolutionary and ecological vicariance analysis (SEEVA) is a simple analytical method that evaluates environmental or ecological divergence associated with evolutionary splits. It integrates evolutionary hypotheses, phylogenetic data, and spatial, temporal, environmental and geographical information to elucidate patterns. Using a phylogeny of Prepusa Mart. and Senaea Taub. (Angiospermae: Gentianaceae), SEEVA is used to describe the radiation and ecological patterns of this basal gentian group across south‐eastern Brazil. Location Latin America, global. Methods Environmental data for 151 geolocated botanical collections, associated with specimens from seven species, were compiled with Arc GIS, and were matched with geolocated base layers of eight climatological variables, as well as one each of geological, soil type, elevational and vegetation variables. Sister groups were defined on the basis of the six nested nodes that defined the phylogenetic tree of these two genera. A (0, 1)‐scaled divergence index (D) was defined and tested for each of 12 environmental and for each of the six phylogenetic nodes, by means of contingency analyses. We contrast divergence indices of nested clades, allopatric and sympatric sister clades. Results The level of ecological divergence between sister clades/species, defined in terms of D measures, was substantial for five of six nodes, with 21 of 72 environmental comparisons having D > 0.75. Soil types and geological age of bedrock were strongly divergent only for basal nodes in the phylogeny, by contrast with temperature and precipitation, which exhibited strong divergence at all nodes. There has been strong divergence and progressive occupation of wetter and colder habitats throughout the history of Prepusa. Nodes separating allopatric sister clades exhibited larger niche divergence than did those separating sympatric sister clades. Main conclusions SEEVA provides a multi‐source, direct analysis method for correlating field collections, phylogenetic hypotheses, species distributions and georeferenced environmental data. Using SEEVA, it was possible to quantify and test the divergence between sister lineages, illustrating both niche conservatism and ecological specialization. SEEVA permits elucidation of historical and ecological vicariance for evolutionary lineages, and is amenable to wide application, taxonomically, geographically and ecologically.     

Competition drives trait evolution and character displacement between Mimulus species along an environmental gradient

Closely related species may evolve to coexist stably in sympatry through niche differentiation driven by in situ competition, a process termed character displacement. Alternatively, past evolution in allopatry may have already sufficiently reduced niche overlap to permit establishment in sympatry, a process called ecological sorting. The relative importance of each process to niche differentiation is contentious even though they are not mutually exclusive and are both mediated via multivariate trait evolution. We explore how competition has impacted niche differentiation in two monkeyflowers, Mimulus alsinoides and M. guttatus, which often co‐occur. Through field observations, common gardens, and competition experiments, we demonstrate that M. alsinoides is restricted to marginal habitats in sympatry and that the impacts of character displacement on niche differentiation are complex. Competition with M. guttatus alters selection gradients and has favored taller M. alsinoides with earlier seasonal flowering at low elevation and floral shape divergence at high elevation. However, no trait exhibits the pattern typically associated with character displacement, higher divergence between species in sympatry than allopatry. Thus, although character displacement was unlikely the process driving initial divergence along niche axes necessary for coexistence, we conclude that competition in sympatry has likely driven trait evolution along additional niche axes.     

Ecological gradient of sexual selection: elevation and song elaboration in finches

Ecological gradients in natural and sexual selection often result in evolutionary diversification of morphological, life history, and behavioral traits. In particular, elevational changes in habitat structure and climate not only covary with intensity of sexual selection in many taxa, but may also influence evolution of mating signals. Here we examined variation in courtship song in relation to elevation of breeding across cardueline finches-a subfamily of birds that occupies the widest elevational range of extant birds and shows extensive variation in life histories and sexual selection along this range. We predicted that decrease in sexual selection intensity with elevation of breeding documented in this clade would result in a corresponding evolutionary reduction in elaboration of courtship songs. We controlled for the effects of phylogeny, morphology, and habitat structure to uncover a predicted elevational decline in courtship song elaboration; species breeding at lower elevations sang more elaborated and louder songs compared to their sister species breeding at higher elevations. In addition, lower elevation species had longer songs with more notes, whereas frequency components of song did not vary with elevation. We suggest that changes in sexual selection account for the observed patterns of song variation and discuss how elevational gradient in sexual selection may facilitate divergence in mating signals potentially reinforcing or promoting speciation.     

Frequency-dependent community dynamics driven by sexual interactions

Research in community ecology has tended to focus on trophic interactions (e.g., predation, resource competition) as driving forces of community dynamics, and sexual interactions have often been overlooked. Here we discuss how sexual interactions can affect community dynamics, especially focusing on frequency-dependent dynamics of horizontal communities (i.e., communities of competing species in a single ecological guild). By combining mechanistic and phenomenological models of competition, we place sexual reproduction into the framework of modern coexistence theory. First, we review how population dynamics of two species competing for two resources can be represented by the Lotka–Volterra competition model as well as frequency dynamics, and how niche differentiation and overlap produce negative and positive frequency-dependence (i.e., stable coexistence and priority effect), respectively. Then, we explore two situations where sexual interactions change the frequency-dependence in community dynamics: (1) reproductive interference, that is, negative interspecific interactions due to incomplete species recognition in mating trials, can promote positive frequency-dependence and (2) density-dependent intraspecific adaptation load, that is, reduced population growth rates due to adaptation to intraspecific sexual (or social) interactions, produces negative frequency-dependence. We show how reproductive interference and density-dependent intraspecific adaptation load can decrease and increase niche differences in the framework of modern coexistence theory, respectively. Finally, we discuss future empirical and theoretical approaches for studying how sexual interactions and related phenomena (e.g., reproductive interference, intraspecific adaptation load, and sexual dimorphism) driven by sexual selection and conflict can affect community dynamics.     

Effectiveness of size frequency distributions in regulating intraspecific spatial overlap of stream insects

Spatial niche breadth and overlap of size groups of four species of epilithic stream insects were studied to determine if broad size frequency distributions serve to reduce intraspecific competition for space. Mean niche overlap decreased with greater breadth of size frequency distributions for only one species (Baetis intercalaris) and did not vary in relation to population density. Similarly, the ratio of the probabilities of intergroup and intragroup encounter did not vary as a function of the breadth of size frequency distributions. Therefore, it was concluded that changes in size frequency distribution have a minimal effect on intraspecific competition for space among epilithic species. It was suggested that observed size frequency distributions may be more a function of extant environmental conditions (temperature and flow) than of previous or extant intraspecific competition.     

The evolution of floral signals in relation to range overlap in a clade of California Jewelflowers (Streptanthus s.l.)

Because of their function as reproductive signals in plants, floral traits experience distinct selective pressures related to their role in speciation, reinforcement, and prolonged coexistence with close relatives. However, few studies have investigated whether population‐level processes translate into detectable signatures at the macroevolutionary scale. Here, we ask whether patterns of floral trait evolution and range overlap across a clade of California Jewelflowers reflect processes hypothesized to shape floral signal differentiation at the population level. We found a pattern of divergence in floral scent composition across the clade such that close relatives had highly disparate floral scents given their age. Accounting for range overlap with close relatives explained additional variation in floral scent over time, with sympatric species pairs having diverged more than allopatric species pairs given their age. However, three other floral traits (flower size, scent complexity and flower color) did not fit these patterns, failing to deviate from a null Brownian motion model of evolution. Together, our results suggest that selection for divergence among close relatives in the composition of floral scents may play a key, sustained role in mediating speciation and coexistence dynamics across this group, and that signatures of these dynamics may persist at the macroevolutionary scale.     

Ecological and phenotypic divergence in Iberian shrews (Soricidae)

There are examples of coexisting species with similar morphology and ecology, in apparent contradiction to competition theory. Shrews (Soricidae) are a paradigmatic example of this because members of this group exhibit a conserved body form, relatively low variability in lifestyle, and, in many cases, a sympatric distribution. Here, we combined geometric morphometrics and ecological niche modeling to test whether diversification of soricid species inhabiting the Iberian Peninsula has been driven by niche divergence or, conversely, whether niche conservatism has played a paramount role in this process. We also examined whether pairwise morphological distances increase as the degree of niche overlap between species becomes greater, as would be expected if interspecific competition promotes morphological differentiation. Our results showed that water shrews (Neomys), white‐toothed shrews (Crocidurinae), and red‐toothed shrews (Soricinae) are clearly differentiated in terms of both skull shape and mandible shape. However, we found a lack of phylogenetic signal in most morphological traits, indicating that closely related species are not more similar than expected by random chance. Notably, water shrews show a more “triangular” or sharp skull than white‐toothed and red‐toothed shrews, probably as an adaptation to their semiaquatic lifestyle. In agreement with the phenotypic data, climatic traits (mean annual temperature and annual precipitation) were highly labile and sister taxa showed extensive differentiation in their realized niche space. Finally, we found that phenotypic distances between species tend to increase as the degree of niche overlap increases, suggesting that interspecific competition is an important factor in determining the level of morphological resemblance among relatives. Overall, our results indicate that the existence of limited morphological disparity in a given group does not necessarily imply the existence of a niche conservatism signature.