Combined effects of interspecific competition and hybridization impede local coexistence of Ficedula flycatchers

Competitive Displacement and Agonistic Character Displacement, or the Ghost of Interference Competition

AbstractInterference competition can drive species apart in habitat use through competitive displacement in ecological time and agonistic character displacement (ACD) over evolutionary time. As predicted by ACD theory, sympatric species of rubyspot damselflies (Hetaerina spp.) that respond more aggressively to each other in staged encounters differ more in microhabitat use. However, the same pattern could arise from competitive displacement if dominant species actively exclude subordinate species from preferred microhabitats. The degree to which habitat partitioning is caused by competitive displacement can be assessed with removal experiments. We carried out removal experiments with three species pairs of rubyspot damselflies. With competitive displacement, removing dominant species should allow subordinate species to shift into the dominant species' microhabitat. Instead, we found that species-specific microhabitat use persisted after the experimental removals. Thus, the previously documented association between heterospecific aggression and microhabitat partitioning in this genus is most likely a product of divergence in habitat preferences caused by interference competition in the evolutionary past.

Interspecific behavioural interference and range dynamics: current insights and future directions

Novel biotic interactions in shifting communities play a key role in determining the ability of species' ranges to track suitable habitat. To date, the impact of biotic interactions on range dynamics have predominantly been studied in the context of interactions between different trophic levels or, to a lesser extent, exploitative competition between species of the same trophic level. Yet, both theory and a growing number of empirical studies show that interspecific behavioural interference, such as interspecific territorial and mating interactions, can slow down range expansions, preclude coexistence, or drive local extinction, even in the absence of resource competition. We conducted a systematic review of the current empirical research into the consequences of interspecific behavioural interference on range dynamics. Our findings demonstrate there is abundant evidence that behavioural interference by one species can impact the spatial distribution of another. Furthermore, we identify several gaps where more empirical work is needed to test predictions from theory robustly. Finally, we outline several avenues for future research, providing suggestions for how interspecific behavioural interference could be incorporated into existing scientific frameworks for understanding how biotic interactions influence range expansions, such as species distribution models, to build a stronger understanding of the potential consequences of behavioural interference on the outcome of future range dynamics.

Assortative Mating in an Ecological Context: Effects of Mate Choice Errors and Relative Species Abundance on the Frequency and Asymmetry of Hybridization

AbstractThe frequency and asymmetry of mixed-species mating set the initial stage for the ecological and evolutionary implications of hybridization. How such patterns of mixed-species mating, in turn, are influenced by the combination of mate choice errors and relative species abundance remains largely unknown. We develop a mathematical model that generates predictions for how relative species abundances and mate choice errors affect hybridization patterns. When mate choice errors are small (<5%), the highest frequency of hybridization occurs when one of the hybridizing species is at low abundance, but when mate choice errors are high (>5%), the highest hybridization frequency occurs when species occur in equal proportions. Furthermore, females of the less abundant species are overrepresented in mixed-species matings. We compare our theoretical predictions with empirical data on naturally hybridizing Ficedula flycatchers and find that hybridization is highest when the two species occur in equal abundance, implying rather high mate choice errors. We discuss ecological and evolutionary implications of our findings and encourage future work on hybrid zone dynamics that take demographic aspects, such as relative species abundance, into account.

Bird Communities in a Changing World: The Role of Interspecific Competition

Significant changes in the environment have the potential to affect bird species abundance and distribution, both directly, through a modification of the landscape, habitats, and climate, and indirectly, through a modification of biotic interactions such as competitive interactions. Predicting and mitigating the consequences of global change thus requires not only a sound understanding of the role played by biotic interactions in current ecosystems, but also the recognition and study of the complex and intricate effects that result from the perturbation of these ecosystems. In this review, we emphasize the role of interspecific competition in bird communities by focusing on three main predictions derived from theoretical and empirical considerations. We provide numerous examples of population decline and displacement that appeared to be, at least in part, driven by competition, and were amplified by environmental changes associated with human activities. Beyond a shift in relative species abundance, we show that interspecific competition may have a negative impact on species richness, ecosystem services, and endangered species. Despite these findings, we argue that, in general, the role played by interspecific competition in current communities remains poorly understood due to methodological issues and the complexity of natural communities. Predicting the consequences of global change in these communities is further complicated by uncertainty regarding future environmental conditions and the speed and efficacy of plastic and evolutionary responses to fast-changing environments. Possible directions of future research are highlighted.

Reproductive interference and Satyrisation: mechanisms, outcomes and potential use for insect control

Reproductive Interference occurs when interactions between individuals from different species disrupt reproductive processes, resulting in a fitness cost to one or both parties involved. It is typically observed between individuals of closely related species, often upon secondary contact. In both vertebrates and invertebrates, Reproductive Interference is frequently referred to as 'Satyrisation'. It can manifest in various ways, ranging from blocking or reducing the efficacy of mating signals, through to negative effects of heterospecific copulations and the production of sterile or infertile hybrid offspring. The negative fitness effects of Satyrisation in reciprocal matings between species are often asymmetric and it is this aspect, which is most relevant to, and can offer utility in, pest management. In this review, we focus on Satyrisation and outline the mechanisms through which it can operate. We illustrate this by using test cases, and we consider the underlying reasons why the reproductive interactions that comprise Satyrisation occur. We synthesise the key factors affecting the expression of Satyrisation and explore how they have potential utility in developing new routes for the management and control of harmful insects. We consider how Satyrisation might interact with other control mechanisms, and conclude by outlining a framework for its use in control, highlighting some of the important next steps.

Genomic Evidence for Rare Hybridization and Large Demographic Changes in the Evolutionary Histories of Four North American Dove Species

Introductions and invasions provide opportunities for interaction and hybridization between colonists and closely related native species. We investigate this phenomenon using the mitochondrial DNA COI and 81,416 base-pairs of overlapping nuclear variation to examine the evolutionary histories and signatures of hybridization among introduced feral Rock Pigeon and Eurasian Collared-Dove and native White-winged and Mourning doves in southwestern North America. First, we report all four species to be highly divergent across loci (overall pair-wise species Φ_ST range = 0.17-0.70) and provide little evidence for gene flow at evolutionary timescales. Despite this, evidence from multiple population genetics analyses supports the presence of six putative contemporary late-stage hybrids among the 182 sampled individuals. These putative hybrids contain various ancestry combinations, but all involve the most populous species, the Mourning Dove. Next, we use a novel method to reconstruct demographic changes through time using partial genome sequence data. We identify recent, species-specific fluctuations in population size that are likely associated with changing environments since the Miocene and suggest that these fluctuations have influenced the genetic diversity of each dove species in ways that may impact their future persistence. Finally, we discuss the importance of using multiple marker types when attempting to infer complex evolutionary histories and propose important considerations when analyzing populations that were recently established or of domestic origins.

Reproductive interference hampers species coexistence despite conspecific sperm precedence

Negative interspecific mating interactions, known as reproductive interference, can hamper species coexistence in a local patch and promote niche partitioning or geographical segregation of closely related species. Conspecific sperm precedence (CSP), which occurs when females that have mated with both conspecific and heterospecific males preferentially use conspecific sperm for fertilization, might contribute to species coexistence by mitigating the costs of interspecific mating and hybridization. We discussed whether two species exhibiting CSP can coexist in a local environment in the presence of reproductive interference. First, using a behaviorally explicit mathematical model, we demonstrated that two species characterized by negative mating interactions are unlikely to coexist because the costs of reproductive interference, such as loss of mating opportunity with conspecific partners, are inevitably incurred when individuals of both species are present. Second, we experimentally examined differences in mating activity and preference in two Harmonia ladybird species known to exhibit CSP. These behavioral differences may lead to local extinction of H. yedoensis because of reproductive interference by H. axyridis. This prediction is consistent with field observations that H. axyridis uses various food sources and habitats whereas H. yedoensis is confined to a less preferred prey item and a pine tree habitat. Finally, by a comparative approach, we observed that niche partitioning or parapatric distribution, but not sympatric coexistence in the same habitat, is maintained between species with CSP belonging to a wide range of taxa, including vertebrates and invertebrates living in aquatic or terrestrial environments. Taken together, it is possible that reproductive interference may destabilize local coexistence even in closely related species that exhibit CSP.

Differences in incubation behaviour and niche separation of two competing flycatcher species

Abstract

Food availability sets the stage for incubation behaviour of a female bird and thereby indirectly determines the nest temperature, which in turn affects development and metabolism of avian embryos. Changes in development and metabolism in turn are known to influence offspring’s ability to adjust to environmental changes later in life. However, few studies have investigated the role of interspecific differences in incubation behaviour in relation to niche separation between competing sibling species. We studied the effects of habitat quality (in terms of caterpillar availability) on incubation behaviour of two ecologically similar and closely related species, collared and pied flycatchers (Ficedula albicollis and F. hypoleuca), in their hybrid zone on the island of Öland, Sweden. Even though both species prefer caterpillar-rich deciduous forests as nesting sites, collared flycatchers, whose nestlings have higher energetic demands, are able to nest only in deciduous forests, whereas pied flycatchers have more flexible habitat requirements. Overall, higher food availability was associated with increased nest attendance, higher incubation temperature and a lower number of foraging trips across species. In addition, collared flycatchers had more frequent and shorter foraging trips across habitat types, allocated more heat to eggs and therefore maintained higher nest temperatures compared to pied flycatchers. We argue that the higher heat allocation or the need to maintain a higher nest temperature for embryo development may constrain collared flycatchers to focus on relatively more profitable prey. Our results highlight the importance of considering incubation behaviour in the context of understanding species differences in niche use.

Significance statement

Niche separation plays an important role in mitigating effects of competition between closely related species. Whether species differences in incubation behaviour relate to differences in niche use remains unknown. We compared incubation behaviour of two sympatric flycatcher species that differ in sensitivity to food availability. The competitively more dominant and larger species, the collared flycatcher, whose nestlings are more sensitive to food shortages, made more frequent foraging trips but allocated more heat to eggs, leading to higher nest temperature despite lower nest attendance, compared to pied flycatchers. These interspecific differences may be a result of differences in embryo sensitivity or female physiology and contribute to the niche separation between the species, which in turn can facilitate coexistence.

Electronic supplementary material

The online version of this article (10.1007/s00265-020-02883-4) contains supplementary material, which is available to authorized users.

To accept or reject heterospecific mates: behavioural decisions underlying premating isolation

Premating isolation in animals involves decision-making processes that affect whether individuals accept or reject heterospecific mates. An integrative understanding of the behavioural processes underlying heterospecific acceptance can clarify the conditions under which premating isolation evolves. As an illustration, we review how Reeve's (Reeve 1989 Am. Nat. 133, 407-435. (doi:10.1086/284926)) acceptance threshold model can help make sense of patterns of premating isolation in nature. This model derives a threshold trait value for acceptance for rejection of recipients of an action (e.g. mating) based on the fitness consequences of these decisions. We show that the maintenance of partial reproductive isolation can be an outcome of optimal acceptance thresholds, even in the face of reinforcement. We also use this model to clarify how the composition of multispecies communities can shape premating isolation. The acceptance threshold model can also be viewed as the behavioural underpinning of reproductive character displacement and cascading reinforcement. Finally, we highlight potential limitations of the acceptance threshold model with respect to investigating the role of sexual selection in speciation, and we propose that integration of behavioural models in speciation research will help us gain a full picture of the mechanisms underlying premating isolation. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Time spent together and time spent apart affect song, feather colour and range overlap in tinkerbirds

Most studies on the processes driving evolutionary diversification highlight the importance of genetic drift in geographical isolation and natural selection across ecological gradients. Direct interactions among related species have received much less attention, but they can lead to character displacement, with recent research identifying patterns of displacement attributed to either ecological or reproductive processes. Together, these processes could explain complex, trait-specific patterns of diversification. Few studies, however, have examined the possible effects of these processes together or compared the divergence in multiple traits between interacting species among contact zones. Here, we show how traits of two Pogoniulus tinkerbird species vary among regions across sub-Saharan Africa. However, in addition to variation between regions consistent with divergence in refugial isolation, both song and morphology diverge between the species where they coexist. In West Africa, where the species are more similar in plumage, there is possible competitive or reproductive exclusion. In Central and East Africa, patterns of variation are consistent with agonistic character displacement. Molecular analyses support the hypothesis that differences in the age of interaction among regions can explain why species have evolved phenotypic differences and coexist in some regions but not others. Our findings suggest that competitive interactions between species and the time spent interacting, in addition to the time spent in refugial isolation, play important roles in explaining patterns of species diversification.

Reproductive Costs for Hybridizing Female Anasa tristis (Hemiptera: Coreidae), but No Evidence of Selection Against Interspecific Mating

Individuals of different species sometimes mate in nature, and such behavior often carries costs, such as wasted gametes and inviable offspring. One context in which interspecific mating commonly occurs is when closely related species come into secondary contact. Here, we tested whether reproductive isolation is greater in an area of recent secondary contact than in allopatry for two closely related insect species, and we examined whether mating between individuals of these two species constitutes reproductive interference. In Florida, two species of squash bugs (Hemiptera: Coreidae: Anasa tristis DeGeer and Anasa andresii Guérin-Méneville) have been secondarily sympatric for ≥80 generations, and male A. andresii copulate with female A. tristis. Because hybridization is often costly for females, we predicted that secondarily sympatric females would be less likely to mate with heterospecifics than would allopatric females. We found no evidence of recent selection on reproductive isolation: females from both populations were equally likely to mate with heterospecifics, and heterospecific males did not make more mating attempts than conspecifics to achieve copulations. However, female A. tristis paired with heterospecifics produced many fewer eggs and offspring than females paired with conspecifics, and this did not differ according to whether females were from allopatric or sympatric populations. Our findings show that reproductive barriers between these species existed before secondary contact. We suggest that habitat use may limit encounter frequency, and that female choice, multiple mating, and postcopulatory processes may reduce costs for females. Consequently, we suggest that mating systems and ecological factors mediate the effects of reproductive interference.

Difference in plasticity of resting metabolic rate - the proximate explanation to different niche breadth in sympatric Ficedula flycatchers

Variation in relative fitness of competing recently formed species across heterogeneous environments promotes coexistence. However, the physiological traits mediating such variation in relative fitness have rarely been identified. Resting metabolic rate (RMR) is tightly associated with life history strategies, thermoregulation, diet use, and inhabited latitude and could therefore moderate differences in fitness responses to fluctuations in local environments, particularly when species have adapted to different climates in allopatry. We work in a long‐term study of collared (Ficedula albicollis) and pied flycatchers (Ficedula hypoleuca) in a recent hybrid zone located on the Swedish island of Öland in the Baltic Sea. Here, we explore whether differences in RMR match changes in relative performance of growing flycatcher nestlings across environmental conditions using an experimental approach. The fitness of pied flycatchers has previously been shown to be less sensitive to the mismatch between the peak in food abundance and nestling growth among late breeders. Here, we find that pied flycatcher nestlings have lower RMR in response to higher ambient temperatures (associated with low food availability). We also find that experimentally relaxed nestling competition is associated with an increased RMR in this species. In contrast, collared flycatcher nestlings did not vary their RMR in response to these environmental factors. Our results suggest that a more flexible nestling RMR in pied flycatchers is responsible for the better adaptation of pied flycatchers to the typical seasonal changes in food availability experienced in this hybrid zone. Generally, subtle physiological differences that have evolved when species were in allopatry may play an important role to patterns of competition, coexistence, or displacements between closely related species in secondary contact.

Causes and Consequences of Behavioral Interference between Species

Behavioral interference between species, such as territorial aggression, courtship, and mating, is widespread in animals. While aggressive and reproductive forms of interspecific interference have generally been studied separately, their many parallels and connections warrant a unified conceptual approach. Substantial evidence exists that aggressive and reproductive interference have pervasive effects on species coexistence, range limits, and evolutionary processes, including divergent and convergent forms of character displacement. Alien species invasions and climate change-induced range shifts result in novel interspecific interactions, heightening the importance of predicting the consequences of species interactions, and behavioral interference is a fundamental but neglected part of the equation. Here, we outline priorities for further theoretical and empirical research on the ecological and evolutionary consequences of behavioral interference.

A generalized population dynamics model for reproductive interference with absolute density dependence

Interspecific mating interactions, or reproductive interference, can affect population dynamics, species distribution and abundance. Previous population dynamics models have assumed that the impact of frequency-dependent reproductive interference depends on the relative abundances of species. However, this assumption could be an oversimplification inappropriate for making quantitative predictions. Therefore, a more general model to forecast population dynamics in the presence of reproductive interference is required. Here we developed a population dynamics model to describe the absolute density dependence of reproductive interference, which appears likely when encounter rate between individuals is important. Our model (i) can produce diverse shapes of isoclines depending on parameter values and (ii) predicts weaker reproductive interference when absolute density is low. These novel characteristics can create conditions where coexistence is stable and independent from the initial conditions. We assessed the utility of our model in an empirical study using an experimental pair of seed beetle species, Callosobruchus maculatus and Callosobruchus chinensis. Reproductive interference became stronger with increasing total beetle density even when the frequencies of the two species were kept constant. Our model described the effects of absolute density and showed a better fit to the empirical data than the existing model overall.

Competition-driven build-up of habitat isolation and selection favoring modified dispersal patterns in a young avian hybrid zone

Competition-driven evolution of habitat isolation is an important mechanism of ecological speciation but empirical support for this process is often indirect. We examined how an on-going displacement of pied flycatchers from their preferred breeding habitat by collared flycatchers in a young secondary contact zone is associated with (a) access to an important food resource (caterpillar larvae), (b) immigration of pied flycatchers in relation to habitat quality, and (c) the risk of hybridization in relation to habitat quality. Over the past 12 years, the estimated access to caterpillar larvae biomass in the habitat surrounding the nests of pied flycatchers has decreased by a fifth due to shifted establishment possibilities, especially for immigrants. However, breeding in the high quality habitat has become associated with such a high risk of hybridization for pied flycatchers that overall selection currently favors pied flycatchers that were forced to immigrate into the poorer habitats (despite lower access to preferred food items). Our results show that competition-driven habitat segregation can lead to fast habitat isolation, which per se caused an opportunity for selection to act in favor of future "voluntarily" altered immigration patterns and possibly strengthened habitat isolation through reinforcement.

Climate adaptation and speciation: particular focus on reproductive barriers in Ficedula flycatchers

Climate adaptation is surprisingly rarely reported as a cause for the build‐up of reproductive isolation between diverging populations. In this review, we summarize evidence for effects of climate adaptation on pre‐ and postzygotic isolation between emerging species with a particular focus on pied (Ficedula hypoleuca) and collared (Ficedula albicollis) flycatchers as a model for research on speciation. Effects of climate adaptation on prezygotic isolation or extrinsic selection against hybrids have been documented in several taxa, but the combined action of climate adaptation and sexual selection is particularly well explored in Ficedula flycatchers. There is a general lack of evidence for divergent climate adaptation causing intrinsic postzygotic isolation. However, we argue that the profound effects of divergence in climate adaptation on the whole biochemical machinery of organisms and hence many underlying genes should increase the likelihood of genetic incompatibilities arising as side effects. Fast temperature‐dependent co‐evolution between mitochondrial and nuclear genomes may be particularly likely to lead to hybrid sterility. Thus, how climate adaptation relates to reproductive isolation is best explored in relation to fast‐evolving barriers to gene flow, while more research on later stages of divergence is needed to achieve a complete understanding of climate‐driven speciation.

Asymmetric dominance and asymmetric mate choice oppose premating isolation after allopatric divergence

Assortative mating promotes reproductive isolation and allows allopatric speciation processes to continue in secondary contact. As mating patterns are determined by mate preferences and intrasexual competition, we investigated male–male competition and behavioral isolation in simulated secondary contact among allopatric populations. Three allopatric color morphs of the cichlid fish Tropheus were tested against each other. Dyadic male–male contests revealed dominance of red males over bluish and yellow-blotch males. Reproductive isolation in the presence of male–male competition was assessed from genetic parentage in experimental ponds and was highly asymmetric among pairs of color morphs. Red females mated only with red males, whereas the other females performed variable degrees of heteromorphic mating. Discrepancies between mating patterns in ponds and female preferences in a competition-free, two-way choice paradigm suggested that the dominance of red males interfered with positive assortative mating of females of the subordinate morphs and provoked asymmetric hybridization. Between the nonred morphs, a significant excess of negative assortative mating by yellow-blotch females with bluish males did not coincide with asymmetric dominance among males. Hence, both negative assortative mating preferences and interference of male–male competition with positive assortative preferences forestall premating isolation, the latter especially in environments unsupportive of competition-driven spatial segregation.