Understanding why and how multiple species manage to coexist represents a primary goal of ecological and evolutionary research. This is of particular relevance for communities that depend on resource rich ephemeral habitats that are prone to high intra- and interspecific competition. Black scavenger flies (Diptera: Sepsidae) are common and abundant acalyptrate flies associated with livestock dung decomposition in human-influenced agricultural grasslands worldwide. Several widespread sepsid species with apparently very similar ecological niches coexist in Europe, but despite their ecological role and their use in evolutionary ecological research, our understanding of their ecological niches and spatio-temporal distribution is still rudimentary. To gain a better understanding of their ecology, we here investigate niche partitioning at two temporal scales. First, we monitored the seasonal occurrence, often related to thermal preference, over multiple years and sites in Switzerland that differ in altitude. Secondly, we also investigate fine-scale temporal succession on dairy cow pastures. In accordance with their altitudinal and latitudinal distribution in Europe, some species were common over the entire season with a peak in summer, hence classified as warm-loving, whereas others were primarily present in spring or autumn. Phenological differences thus likely contribute to species coexistence throughout the season. However, the community also showed pronounced species turnover related to cow pat age. Some species colonize particularly fresh dung and are gradually replaced by others. Furthermore, the correlation between co-occurrence and phylogenetic distance of species revealed significant under-dispersion, indicating that more closely related species are frequently recovered at the same location. As a whole, our data suggests temporal niche differentiation of closely related species that likely facilitates the rather high species diversity on Swiss cattle pastures. The underlying mechanisms allowing close relatives to co-occur however require further scrutiny.

climate distribution, Diptera , ecology, seasonality, sepsid, dung flies, thermal niche

The mechanisms driving species diversity and its persistence are of paramount interest in ecological research. In spite of longstanding and continuing scientific scrutiny, the phenomenal species diversity observed on earth, particularly within ecological guilds, remains puzzling (Tilman 1982, Levene and HilleRisLambers 2009). This is in part due to the challenges in uncovering the mechanisms driving species co-occurrence, because environmental filtering and interspecific competition (among other processes) can act in all aspects of an organism’s life including what individuals eat, where and when they develop and reproduce, and what (abiotic) environmental conditions they prefer (Chesson 2000). To better understand the ecology and evolution of any species, descriptive studies of their population biology, meaning their spatio-temporal distribution and habitat preferences, are therefore indispensable.

Seasonality is a paramount environmental factor that systematically affects the population biology and distribution of organisms. Systematic latitudinal or altitudinal variation in climate and seasonality mediates prominent macro-ecological gradients that strongly contribute to the distribution of entire species assemblages (Tauber et al. 1986, Danks 1987, Addo-Bediako et al. 2000, Deutsch et al. 2008, Blanckenhorn et al. 2018, Gaston and Blackburn 2000, Gaston et al. 2008, Rohner et al. 2015, 2018, Roy et al. 2018). Such spatial patterns may affect the phenotype of co-existing species in a similar fashion, but equally often contribute to geographic species replacements along latitude or altitude (Loboda et al. 2018). In addition, species exhibit temporal, i.e. both seasonal and diurnal patterns of occurrence, which equally reflect their thermal niches and competitive interactions (e.g. Cárdenas et al. 2016). For example, the heat-sensitive yellow dung fly abounds on pastures in early spring and late autumn but largely disappears from their mating sites (dung pats) during the hottest summer months, and also shows corresponding diurnal activity patterns, being active midday on cool days but in the late afternoon on warm days (Parker 1970, Jann et al. 2000, Blanckenhorn 2009). The intrinsic physiological temperature tolerance thus underlies mostly plastic (but sometimes genetic) behavioural variation that phenomenologically gives rise to seasonal and diurnal patterns of species occurrence.

Similarly, insect communities, in particular those that inhabit ephemeral and resource rich habitats, are often characterized by pronounced species turnover (e.g. Hanski and Combefort 1991, Jiri et al 2017). While this may be driven by environmental filtering (e.g. via diurnal patterns of activity), it can also be influenced by differences in the resources individuals forage for and how they reproduce. Such patterns of succession may help individuals evade direct competition for food and/or space by colonizing the same habitat at different times. Thus, spatio-temporal gradients, both along seasons and substrate colonization, co-define the ecological niche of closely related species that belong to the same ecological guild, and as such aid in understanding the mechanisms of species coexistence.

Closely related species are often found to be ecologically and phenotypically more similar compared to more distantly related taxa (Neeson and Mandelik 2014). Hence, the phylogenetic relationship among species can affect species interactions and community composition. If environmental filtering (e.g. via thermal adaptation) is the main driver of species assembly, closely related species are expected to co-occur more often than expected by chance because they are more likely to share an adaptation that enables them to colonize a given niche (that is, they are phylogenetically underdispersed or clustered: Webb et al. 2002). In contrast, if competition for resources is strongly limiting co-occurrence, close relatives are expected to co-occur less often than expected by chance due to competitive exclusion (i.e. they are phylogenetically overdispersed: Elton 1946, Webb et al. 2002). Incorporating the phylogenetic relationship among species can therefore aid in understanding the putative processes driving species assembly.

Black scavenger flies (Diptera: Sepsidae) are common worldwide (Ozerov 2005, Ang et al. 2013; see http://sepsidnet-rmbr.nus.edu.sg/), and are used as model organisms in a broad range of research fields ranging from classic behavioural and evolutionary ecology to developmental biology and ecotoxicology (e.g. Eberhard 2001, Bowsher and Nijhout 2007, Ingram et al. 2008, Puniamoorthy et al. 2009, 2012, 2014, Berger et al. 2013, Blanckenhorn et al. 2013, Roy et al. 2018). These flies generally depend on decaying organic matter for reproduction and larval development. There are broad differences between genera in terms of substrate specialization limiting direct competition (e.g. Orygma on rotting brown algae, Zuskamira and Ortalischema on horse dung, or Themira on waterfowl dung), but often the same species can dwell on very different sources ranging from vertebrate dung to rotting plant matter. Members of the genus Sepsis are most commonly found near livestock faeces (Pont and Meier 2002), a ubiquitous habitat in human-influenced agricultural grasslands, and hence are among the most common sepsids in Central Europe. Several widespread species of Sepsis with apparently very similar ecological niches coexist in Europe, which are somewhat, but not very clearly separated along latitude and altitude (Pont 1987, Ozerov 2005, Rohner 2015, Rohner et al. 2015). Our understanding of the spatio-temporal distribution of these closely related species is still rudimentary at the local, regional, and global scale. For example, all 12 Sepsis species occurring in Switzerland were found in a single sample from one pasture in Lenzerheide, Switzerland (Rohner et al. 2014), even though so far unpublished laboratory rearing experiments suggest direct larval resource competition between species. Based on the principle of competitive exclusion, these closely related species are expected to inhabit different diurnal, seasonal or foraging niches, facilitating their coexistence (Holt 2009, Shimadzu et al 2013). But so far, the ecological differentiation between species received little attention despite its immediate relevance for ecological and evolutionary studies (but see Püchel 1993, Bährmann and Bellstedt 2012, Jiri et al. 2017).

To further our faunistic and ecological understanding of sepsid flies in central Europe, we here systematically monitor the seasonal occurrence of the entire family over 3 years at multiple Swiss sites that differ in altitude, and complement these broad, seasonal patterns with detailed observations on species occurrence and turnover relating to cow dung age. We also investigate seasonal variation in species diversity and community structure and combine these faunistic data with information on the relatedness among species to test for phylogenetic under- or over-dispersion. If thermal adaptation contributes to temporal variation, we expect species to differ in their phenology, and that those taxa common at high latitudes and altitudes are more abundant in spring and/or autumn while warm adapted species should peak in summer.

Temporal niche partitioning represents a major axis of species differentiation that can allow for and maintain species diversity, particularly in ephemeral habitats such as vertebrate dung (Hanski and Cambefort 1991). We here found seasonal variation in species occurrence, diversity and community composition, suggesting temporal, presumably thermal niche differentiation among closely related species. Additionally, detailed longitudinal data show pronounced species turnover of sepsids on individual cowpats, suggesting temporal segregation on a much finer scale relating to desiccation, in addition to broad phenological differences. We in the following discuss the implications of temporal and thermal niche differentiation for our understanding of the natural history of black scavenger flies.

The seasonal distribution patterns of a total of 20 (16 of them common) Swiss sepsid fly species north of the Alps (Mittelland) agree well with their distribution and thermal niches previously inferred from their spatial distribution in Switzerland (Rohner et al. 2015) and Central Europe as a whole (Pont and Meier 2002, Ozerov 2005). Along with Nemopoda nitidula, which we only found in early summer at one northern Swiss low altitude site (Siglistorf), and Saltella nigripes, which only occurred at the Zürich site in late summer, Sepsis nigripes, S. luteipes, S. biflexuosa, and most Themira species were generally very rare around cattle pastures. Only T. annulipes was found in low numbers at most sites throughout the season (April – September). Themira spp. are known to be often specialized on bird (waterfowl) excrements (Pont and Meier 2002), so our pasture sites may simply be the wrong habitats to find them.

Saltella sphondylii is cosmopolitan in Switzerland, though generally more common at lower altitudes. Of the 12 Sepsis spp. reported in Switzerland (Haenni 1998; Rohner et al. 2014, Rohner 2015; Rohner and Bächli 2016), which are also common in northern Germany (Bielefeld: Püchel 1993), 9 were common at low altitudes, with only 5 of them also common at higher altitudes above 1000 m. Sepsis punctum, S. violacea and S. thoracica were almost exclusively present at low altitude sites (ca. 500 m) but rarely if ever observed above 1000 m. At the special site Le Cachot in the Swiss Jura (ca. 1100 m) sampled with a Malaise trap in 1973 (Haenni and Matthey 1984), S. orthocnemis was extraordinarily common, for unknown reasons, and S. punctum and S. violacea also regularly occurred, whereas S. thoracica, S. nigripes and S. biflexuosa were definitely absent. The Malaise trap, however, was not placed directly on a pasture, and is therefore not directly comparable to the other samples, as this type of trap likely catches moving or migrating rather than reproducing or foraging individuals (Bährmann and Bellstedt 2012).

For the common species, some seasonal patterns emerged in the lowlands: S. cynipsea, S. orthocnemis, S. punctum, S. thoracica, and S. duplicata, as well as Saltella sphondylii, were all commonly present over the entire season with a peak in summer. Presupposing that these phenological patterns reflect thermal preferences, the latter can therefore be classified as warm-loving or warm-adapted (as is definitely the case for S. thoracica: Busso and Blanckenhorn 2018). S. neocynipsea, S. fulgens and S. violacea peaked in spring, and S. flavimana (and lowland Saltella sphondylii) in autumn (Fig. 1). Save S. violacea, the latter species are also common at highland sites in the Alps and can therefore be classified as cold-loving or cold-adapted (Rohner et al. 2015). Correspondingly, S. fulgens, S. violacea and S. flavimana also abound at higher latitudes (Bährmann and Bellstedt 2012; Pont and Meier 2002). As S. orthocnemis and S. punctum, S. violacea appears to be found most everywhere, though in low numbers, some local abundance notwithstanding (viz. Le Cachot); the Swiss Mittelland may actually mark S. violacea’s southern distribution edge (Pont and Meier 2002). S. neocynipsea, in contrast, is known to be rare in Europe and restricted to higher altitudes, whereas this species is common in North America, even in warm lowlands such as Arizona (pers. obs.), where it apparently takes the ecological niche that S. cynipsea has in Europe (Pont 1979, Rohner et al. 2015, 2016, Giesen et al. 2017). Though repeatedly reported in Britain even in the lowlands, but not in Scandinavia (Pont 1979, Pont and Meier 2002), our own experience is that S. neocynipsea is difficult if not impossible to find in Europe except in mountainous regions such as Asturias (Spain), Cevennes (France), or the (Swiss) Alps, where it can be locally abundant (Rohner et al. 2014, 2015, 2016), a somewhat puzzling distribution pattern. The sister species S. neocynipsea and S. cynipsea coexist and meet regularly at our Sörenberg site and throughout the Swiss Alps (e.g. Rohner et al 2015, Rohner 2015, Rohner and Bächli 2016). These two species hybridize readily in the laboratory, though hybridization is apparently prevented in nature by behavioural incompatibility (Giesen et al. 2017, 2018).

Rohner et al. (2015) demonstrated greater sepsid species richness at higher altitudes in the Swiss Alps. Although diversity tended to be lower at high altitudes here, species diversity was highest in spring and summer and dropped off towards the end of the season in autumn (Fig. 2). This result is unsurprising given that S. flavimana is the only species with a phenology peaking late in the season (see also Püchel 1993), and knowing that Sepsis spp. in general seem to overwinter as adults, thus being ready to reproduce early in the season (Blanckenhorn 1998, Zeender 2015).

Given that members of the same taxonomic group are expected to use similar resources, and thus to compete most intensely, such high species diversity is intriguing, particularly in an ephemeral habitat that is characterized by severe intra- and interspecific competition for food and space (e.g. Jiri et al 2017, Papp 1992, 2007, Hanski and Cambefort 1991). Intraspecific resource competition has been demonstrated repeatedly in sepsids (Rohner et al 2016, Busso and Blanckenhorn 2018), and yet unpublished laboratory data provide direct evidence for some larval competition between S. cynipsea and S. punctum. Still, we found here and elsewhere (Rohner et al. 2015; Rohner and Bächli 2015) that more closely related species are more prone to co-occur. In order to evade direct competition, species are therefore expected to differ in aspects other than phenology. In this context, temporal succession in the colonization of dung pats has been suggested, as some species seem to be associated with particularly fresh (e.g. S. cynipsea) or older cow dung (e.g. S. duplicata or S. fulgens; Jiri et al. 2017, Pont and Meier 2002, Rohner et al. 2015). Based on Püchel’s (1993) work, we could here confirm this (Fig. 4). Niche differentiation may thus be revealed at much finer temporal scales. Furthermore, specialization on specific substrates (i.e. cattle dung versus excrements of other large vertebrates), or spatial separation within substrates, may account for the co-occurrence of species within the genus Sepsis. However, even if species dwell and reproduce in the same substrate simultaneously, it is unclear whether larvae actually feed on the same compounds. Sepsids are presumed to filter-feed on small particles such as fungi and bacteria (Pont and Meier 2002), but whether different species or different larval stages feed on different nutrients is unknown. Further anecdotal evidence suggests that some species use a very broad range of substrates (e.g. S. fulgens is found on cow, pig, chicken dung as well as compost: Pont and Meier 2002), while other are highly specialized (Sepsis duplicata is mostly found on dry cow dung). In addition to different substrate preferences, such a specialist – generalist continuum may further facilitate coexistence. Both niche differentiation by substrate choice and specialization, as well as fine-scale temporal succession within cattle pastures, hence should be studied further.

Our finding that species that regularly co-occur tend to be more closely related than expected by chance can also be interpreted as evidence against a strong influence of competitive exclusion, i.e. that other mechanisms, such as environmental filtering, may be more important. However, this assumes phylogenetic inertia of those traits related to competition. Although body size and development times show phylogenetic signals in Sepsidae (e.g. Rohner et al. 2016), behavioural traits evolve very rapidly across species (Ang et al. 2008, Puniamoorthy et al. 2009, Puniamoorthy 2014, Tan et al. 2011). Given our lack of data on larval physiology and nutritional ecology, we cannot yet link co-occurrence to interspecific variation (or population differentiation within species) in the larval ecology of sepsids, but this clearly warrants further scrutiny.

The seasonal distribution patterns of Swiss sepsid fly species on Swiss cow pastures north of the Alps agree well with their previously documented spatial distribution patterns (Ozerov 2005, Pont and Meier 2002, Rohner et al. 2014, 2015). Nemopoda nitidula, Saltella nigripes, most Themira spp. (except T. annulipes), Sepsis nigripes, S. luteipes and S. biflexuosa are generally rare on Swiss dairy cow pastures (see also Rohner et al. 2015). Sepsis punctum, S. violacea and S. thoracica were absent to rare at high altitude sites >1000 m (except the Jura). Abundances of the more common and widespread species throughout Europe (S. cynipsea, S. orthocnemis, S. punctum, S. thoracica, S. duplicata, Saltella sphondylii) peaked in summer, whereas species also common at high latitudes peaked in spring (S. neocynipsea, S. fulgens, S. violacea) or autumn (S. flavimana). Phenology and thermal adaptation alone are however unlikely to explain the high levels of species diversity and coexistence. As closely related species are more likely to be present in a sample, temporal succession and substrate specialization may limit direct competition and enable coexistence even of close relatives.

This work was supported over the years by the University of Zurich, the Zoological Museum of Zurich, and several grants from the Swiss National Science Foundation, most recently grant no. 31003A_143787.