Global diversity and distribution of longhorn beetles (Coleoptera: Cerambycidae)

Abstract

Longhorn beetles (Coleoptera: Cerambycidae) is one of the largest, most diverse, ecologically and economically important groups of beetles in the world. In the following paper, through a comprehensive synthesis and review of various sources of data, we have described the general patterns of the distribution of longhorn beetles on the global scale. We found that the vast majority of Cerambycidae diversity is concentrated in two hyper–diverse subfamilies: Lamiinae and Cerambycinae, which together cover 90.5% of all longhorn beetles. Nearly 60% of the global fauna of Cerambycidae is distributed within the Oriental or Neotropical realms. Although the monospecific genera are predominated, most of the taxonomic diversity of longhorn beetles is concentrated on the limited number of large, multi–species genera; only the first 829 richest genera encompass three quarters of the total number of Cerambycidae species. Although the estimated number of described longhorn beetle species (34,490) showed a good agreement with other studies, that figure seems to be far from the actual number, since more than 200 new species are described every year in the recent decade. Most newly–described species originate from Asia and Southern America. The vast majority (88.1%) of the Cerambycidae species were found to be restricted to only one biogeographic realm. The highest number of endemic species can be found in cerambycid fauna of the Australasian, Ethiopian, Madagascan and Neotropical realms. Moreover, our findings highlight the phylogenetic distinctness of the Madagascan fauna of Cerambycidae. More research involving a comprehensive multigene approach is needed to resolve urgent issues regarding both the higher and lower level taxonomy of Cerambycidae and to better understand the factors influencing the distribution and diversification of cerambycids.

Keywords:

• Biodiversity
• cerambycidae
• longhorn beetles
• global
• biogeography

Introduction

Coleoptera (beetles) represents almost one-quarter of all currently known animal species. Hence, the understanding of their global diversity and distribution patterns is crucial, especially in the face of a sixth mass extinction crisis (Barnosky et al. 2011; Ceballos et al. 2015). Within coleopteran families, Cerambycidae (longhorn beetles) is one of the most species–rich, diverse, ecologically, and economically important group, and in addition, many longhorn beetles are considered endangered due to both habitat loss and habitat alterations (Jeppsson & Forslund 2014; Seibold et al. 2015).

According to recent estimations, as many as 35,000 extant cerambycid species subdivided into nearly 4,000 genera were described globally (Švácha & Lawrence 2014; Wang 2017). Longhorn beetles are exclusively phytophagous, and their larvae are mostly internal feeders of living or dead plant tissues (Ślipiński & Escalona 2016). Cerambycid species play a crucial ecological role, especially in forest habitats where they are one of the major groups of decomposers of dead plant matter. Moreover, longhorn beetles pollinate numerous species of herbaceous plants, shrubs and trees, and serve as an important food resource for a large number of vertebrates (Ślipiński & Escalona 2016; Wang 2017; Haddad et al. 2018). On the other hand, some cerambycid species are considered serious pests in agriculture, horticulture and forestry. Cerambycid larvae feeding habits can significantly weaken cultivated plants or even be fatal to the hosts, leading to an enormous economic loss globally (Wang 2017). Some longhorn beetles are also known to be the vectors of dangerous plant pathogens like nematodes and fungi (Jankowiak & Rossa 2007; Akbulut & Stamps 2012). Furthermore, in recent decades of the progressive globalization of global trade, many cerambycid species have been intercepted outside their native ranges, and some of them have established new populations causing serious ecological and economic issues (Haack 2006).

Despite the unquestionable ecological and economical significance of longhorn beetles, a comprehensive, global-scale synthesis on cerambycid distribution and diversity patterns is still lacking. Existing summaries have focused on particular taxonomic groups (Monné 2005a; Wallin et al. 2013; Lingafelter 2015; Huang et al. 2020) or faunae of particular region (Bílý & Mehl 1989; Sama 2002; Monné 2005b; Spomer 2014; Danilevsky 2014; Kariyanna et al. 2019), which prevents a comparison of longhorn beetle diversity patterns at a global scale.

The main aim of this paper was to summarize the current knowledge on the worldwide taxonomic diversity and distribution of longhorn beetles. Comprehensive revision and synthesis of data deposited in databases, catalogues, scientific papers, and checklists allowed us to provide new insights into the general pattern of the distribution and taxonomic diversity longhorn beetles.

Material and methods

Data sources and record selection procedure

As a basic source of information on the global taxonomic diversity of longhorn beetles, we used the “Titan” database (Tavakilian & Chevillotte 2019) founded by the Institut de Recherche pour le Développement (IRD) (MNHN, Paris, France) which constitutes the most comprehensive and up–to–date information about the worldwide cerambycid species (Tavakilian & Chevillotte 2019). For the next step, we critically revised the whole database in order to exclude duplicates, species with invalid taxonomic statuses, identify synonymous species, and to also take into account taxa not included in this database. We have achieved it by confronting the content of the “Titan” database with other sources such as checklists, catalogues, and taxonomic revisions (Monné 2005a, 2005b, 2006; Danilevsky 2012; Ślipiński & Escalona 2016; Wang 2017; Bezark 2019; Pirkl 2019; Roguet 2019; Vitali 2019). In the case of data incompatibility between the compared data sources, each unclear record was manually evaluated prior to inclusion in analyses by using the same record selection protocol (Figure 1). Especially, we checked synonyms and the correctness of the name spelling. In the case of special difficulties in determining the taxonomic status, we relied on the latest revision of the particular taxa published in the peer–reviewed scientific journal. For this reason, we used a simple query in the Google Scholar search engine (https://scholar.google.co.uk): revision “Genus_name species_name”, sorted results by date, and manually evaluated whether the publications found are in fact taxonomic revisions of the species of interest, and if the revision was published in peer–reviewed scientific journal.

Figure 1. Longhorn beetles (Cerambycidae) record selection and evaluation protocol

Given that many aspects of longhorn beetles taxonomy are still controversial (Haddad & Mckenna 2016; Ślipiński & Escalona 2016; Wang 2017) and there are numerous disputes regarding the higher level taxonomy (e.g. subfamilies and tribes classification) of the group, and because the presented study was not aimed at elucidating taxonomic ambiguities within longhorn beetles, we have decided to adopt one of the most recent and widely applied system of the beetles’ subfamilies classification (Bouchard et al. 2011), which recognized up nine subfamilies of Cerambycidae including Apatophyseinae Lacordaire, Cerambycinae Latreille, Dorcasominae Lacordaire, Lamiinae Latreille, Lepturinae Latreille, Necydalinae Latreille, Parandrinae Blanchard, Prioninae Latreille, and Spondylidinae Audinet–Serville. The other groups within the traditional “cerambycoid assemblage’ (Cerambycidae s.l.) (Haddad & Mckenna 2016; Haddad et al. 2018) were not included in the analyses, and only basic information about the number of species in the families: Oxypeltidae Lacordaire, Vesperidae Mulsant, and Disteniidae J. Thomson were given.

Data analysis

Each of the species were assigned to one (or more) biogeographic realms (Newton 2003) and to one (or more) continents (Australia [mainland] and Oceania were separated), based on the distribution data found in the explored databases, catalogues, or scientific papers (Monné 2005a, 2005b, 2006; Danilevsky 2012; Ślipiński & Escalona 2016; Wang 2017; Bezark 2019; Pirkl 2019; Roguet 2019; Tavakilian & Chevillotte 2019; Vitali 2019). In rare cases when a species was found to have occurred directly on the border of two areas (realms or continents), and the data on its distribution were not precise enough to determine its affiliation to one of the specific areas, the species was assigned to both border areas. The year of the description and the name of the discoverer have been assigned for each species. The compliance with the normal distribution was tested using the Shapiro–Wilk normality test (Shapiro & Wilk 1965) in STATISTICA v. 13 software (StatSoft Inc 2017). As a measure of the distinctness of the Cerambycidae fauna of particular realms and continents, we applied the widely–used Jaccard Dissimilarity Index (Real & Vargas 1996; Carvalho et al. 2013; Gergócs & Hufnagel 2015):

${\mathrm{J}}_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{m}.}=1-(\mathrm{a}/\left(\mathrm{a}+\mathrm{b}+\mathrm{c}\right)$

where a is is the number of genera shared by the two units (realms or continents), b is is the number of genera present only in unit 1, and c is is the number of genera occurring only in unit 2. The Jaccard Dissimilarity Index adopts values between 0 and 1; where value ‘0ʹ indicates that the fauna of the two compared units are identical, whereas value ‘1ʹ indicates that the two compared regions do not have any species in common. The matrix of the Jaccard Dissimilarity Index was then used in the cluster analyses with the Unweighted Pair–Group Method with Arithmetic Averages (UPGMA) in MEGA–X v. 10.1.8 software (Kumar et al. 2018).

Results

Taxonomic diversity

According to our estimations, there are 34,490 described species of Cerambycidae s.s. worldwide (March 2019). Whereas their close congeners within the traditional “cerambycoid assemblage” (see Materials and Methods section) encompass only 460 species (March 2019) subdivided into three families: Oxypeltidae (3 species, 1 subfamily), Vesperidae (68 species, 3 subfamilies), and Disteniidae (389 species, 1 subfamily) (Table I).

Table I. The summary of species richness in the families Oxypeltidae, Vesperidae, Disteniidae and Cerambycidae (March 2019)

Family	Subfamily	No. of genera	No. of species	Species per genus
Oxypeltidae
	Oxypeltinae	2	3	1.5
		2	3	1.5
Vesperidae
	Anoplodermatinae	10	27	2.7
	Philinae	5	21	4.2
	Vesperinae	1	20	20
		16	68	4.25
Disteniidae
	Disteniinae	34	389	11.4
		34	389	11.4
Cerambycidae	Parandrinae	16	123	7.7
	Prioninae	277	1058	3.8
	Lepturinae	190	1500	7.9
	Apatophyseinae	88	308	3.5
	Spondylidinae	29	147	5.1
	Necydalinae	12	124	10.3
	Dorcasominae	12	22	1.8
	Cerambycinae	1713	11172	6.5
	Lamiinae	2622	20036	7.6
	Σ	4959	34490

Most of the Cerambycidae s.s. diversity is concentrated in two hyper–diverse subfamilies: Lamiinae – which contains 20,036 species and constitutes as many as 58% of all longhorn beetle species, and Cerambycinae with 11,172 species which covers 33% of cerambycid species diversity. The other subfamilies of Cerambycidae are less numerous (Figure 2, Table I). The subfamily Lepturinae contains 1,500 species which constitutes 4% of all known cerambycids, whereas Prioninae encompasses 1,058 species and covers only 3% of extant longhorn beetle species (Figure 2). The total contribution of the remaining subfamilies is as follows: Parandrinae (123 species), Apatophyseinae (308 species), Spondylidinae (147 species), Necydalinae (124 species), and Dorcasominae (22 species). Given together, aforementioned families do not exceed 3% of the global species diversity of Cerambycide. The analyses of the frequency of distribution of Cerambycidae species richness was conducted separately for each subfamily, and indicated consistently significant, right–skewed distributions (Figure 3) which clearly deviated from the normal distribution with the mode equal to one (Table II).

Table II. Frequency of distribution of Cerambycidae s.s. species richness

Subfamily	Average	Mode	Skweness	SESkewness	Shapiro–Wilk’s test	P
Apatophyseinae	3.50	1	2.84	0.26	0.61	< 0.00001
Cerambycinae	6.52	1	10.33	0.06	0.29	< 0.00001
Dorcasominae	3.50	1	2.98	0.64	0.49	< 0.00001
Lamiinae	7.64	1	14.46	0.05	0.17	< 0.00001
Lepturinae	7.89	1	6.25	0.18	0.39	< 0.00001
Necydalinae	10.33	1	2.80	0.64	0.60	< 0.0001
Parandrinae	7.69	1	1.42	0.56	0.69	< 0.001
Prioninae	3.82	1	4.1	0.15	0.54	< 0.0001
Spondylidinae	5.07	1	2.14	0.43	0.35	< 0.01

Figure 2. The percentage contribution of subfamilies of Cerambycidae s.s. to the total species diversity of this group

Figure 3. Histograms of the frequency of distribution of Cerambycidae s.s. species diversity. The top left histogram shows the overall frequency of distribution of species per genera in the whole Cerambycidae s.s. family, while the others indicate species distribution in 8 out of 9 subfamilies of Cerambycidae s.s (the smallest subfamily – Dorcasominae not included). The y axis indicates the cumulative number of genera containing the particular number of species, while the x axis shows the number of species in the genus (the original values ordered from the poorest to the most species-rich genera)

Although the Cerambycidae family encompasses a large number of genera (4,959), most of the species richness in this group is contained in the first 1,000 of the richest genera (78.32%). As many as 50% of the global species diversity of Cerambycidae accounts for the first 215 richest genera, while the first 829 richest genera contained 75% of the total global richness of longhorn beetles (Figure 4). Although single–species genera were predominant (2253 single–species genera), they contained together only 6.5% of the total species richness of longhorn beetles. The top five of richest genera of Cerambycidae belonged to the Lamiine subfamily, and included: genus Pterolophia Breuning, 1961, Glenea Jordan, 1894, Sybra Hayashi, 1956, Dorcadion Ganglb., 1884, Exocentrus Dejean, 1835 (Table III). The richest genus in the second-largest subfamily of Cerambycidae – Cerambycinae was the Demonax Thomson, 1861 genus. Other Cerambycidae genera did not exceed 200 species (Table III).

Table III. The most species-rich genera of Cerambycidae s.s.

Category	Subfamily	Genus	No. of species	% Cerambycidae	% subfamily
Top 5 richest genera	Lamiinae	Pterolophia	815	2.36	4.07
	Lamiinae	Glenea	710	2.06	3.54
	Lamiinae	Sybra	439	1.27	2.19
	Lamiinae	Dorcadion	400	1.16	2.0
	Lamiinae	Exocentrus	392	1.14	1.96
The richest genus in the subfamily	Lamiinae	Pterolophia	815	2.36	4.07
	Cerambycinae	Demonax	381	1.10	3.41
	Lepturinae	Pidonia	174	0.5	11.60
	Necydalinae	Necydalis	61	0.18	51.61
	Prioninae	Prionus	47	0.14	4.44
	Parandrinae	Acutandra	28	0.08	22.76
	Apatophyseinae	Apatophysis	26	0.08	8.44
	Dorcasominae	Dorcasomus	8	0.02	36.36
	Spondylidinae	Tetropium	27	0.08	18.37

Figure 4. The accumulation curve (solid blue line) of the global Cerambycidae s.s. species richness in the following genera ordered from the richest one (Pterolophia Breuning, 1961; 815 species) to the poorest genera containing single species. As many as 50% of the global species richness of Cerambycidae s.s. accounts for the first 215 richest genera (see green dotted line), while the first 829 richest genera contain 75% of the total global richness of longhorn beetles (see red dotted line)

The rate of Cerambycidae description has changed significantly over time from the low and constant rate up to the 1830s, through the rapid increase that had begun in the 1850s, to the second acceleration in the 1940s (Figure 5). In the twenty-first century (from 2000 up to 2018), the species description ratio was at an average of 257 newly described cerambycidae per year. The species description ratio also varied significantly, both among biogeographic realms and continents (Figure 5). In the year 1900, as many as 55.28% of all currently described longhorn beetles of the Nearctic were already known, whereas in the case of the Oriental or Etiophian realms, the percentage of known species were still less than 20% during that same time. In the following decades, the step increase in the species description ratio can be found especially in the Oriental and the Neotropical realms, and a slightly lower ratio can be found in the Palearctic realm (Figure 5). The rest of studied realms, including the Ethiopian, Australasian, Nearctic, and Madagascan were characterized by more flattened description curves. At the continental-level analysis, Asia and South America were found to be the two continents with the steepest species description curve in the 21st century (Figure 5).

Figure 5. Discovery curves of Cerambycidae s.s. from 1758 up to 20th March 2019: (a) Discovery curves of Cerambycidae s.s. jointly and separately for each of the seven biogeographic realms (Newton 2003); (b) Discovery curves of Cerambycidae s.s. jointly and separately for each of the seven continents (Australia (mainland) and Oceania were separated); (c) Number of Cerambycidae s.s. species described in each of the seven biogeographic realms given in 10 years intervals; (d) Number of Cerambycidae s.s. species described in each of the seven continents given in 10 years intervals (Australia (mainland) and Oceania were separated). The last interval (1998–2018) has been increased by the first decade of 2019 (up to 20th March 2019)

Geographic distribution

Most of the Cerambycidae taxonomic diversity was found to be concentrated in the Oriental and Neotropical realms (30.9%, 28.7%, respectively), however, the Palearctic, Ethiopian, and Australasian regions also contributed significantly to the global longhorn beetle diversity (18.2%, 15.8%, 10.9%, respectively). Only 4.5% of known cerambycid species occurred in the Nearctic, and 3.1% in the Madagascan realm. An analysis at the continental-level showed that Asia and South America, as well as Africa, were the most species–rich continents (37.9%, 21.6%, 19.2%, respectively). A lower contribution was found for North America (12.4%), Oceania (7.5%), Australia (3.5%) and Europe (2.0%).

The percentage contribution of each subfamily of Cerambycidae to the overall species diversity of this group has varied significantly among biogeographic realms (Figure 6 – left) and continents (Figure 6 – right). The subfamily Lamiinae was found to be a major contributor of the beta diversity of longhorn beetles in six out of seven analyzed realms, and its percentage contribution surpassed 50% in these regions (Figure 6 – left). The only exception was the Nearctic, where the Lamiinae contribution (27.8%) was exceeded by Cerambycinae (50.2%) (Figure 6 – left). Some similarities can be found between the Australasian and Ethiopian realms, where Lamiinae constituted around 70% (65.9%, 71.5% – Australasian and Ethiopian, respectively) and the second most abundant subfamily was Cerambycidae with about 30% of the contribution (30.1%, 24.5% – Australasian and Ethiopian, respectively), while other subfamilies did not exceed 5% in total (Figure 6 – left). Another similarity can also be found between the Nearctic and Palearctic realms, where the Lepturinae contribution (14.9%, 14.7% – Nearctic and Palearcic, respectively) was markedly higher than in other realms (Figure 6 – left). A unique pattern can be found in the Madagascan realm, where the endemic Apatophyseinae subfamily constituted as much as 24.5%, the contribution of the subfamily Prioninae (9.7%) was the highest among all realms, and at the same time the contribution of Cerambycinae was the lowest (10.8%) (Figure 6 – left).

Figure 6. The percentage contribution of each of the nine subfamilies of Cerambycidae s.s to the overall species diversity of the family in each of the even biogeographic realms (left), and in each of the seven continents (right). Australia (mainland) and Oceania were separated

The clustering analysis based on the Jaccard Dissimilarity Index revealed three distinct branches representing three geographical assemblies of Cerambycidae (Figure 7), including Pan-American, Palearctic-Oriental (with the Australian sub–branch) and Afrotropical. The vast majority (88.1%) of the Cerambycidae species occurred only in one biogeographic realm. Over 95% of cerambycid fauna of the Australasian, Ethiopian, Madagascan, and Neotropical realms were restricted to the only one biogeographic realm (97.3%, 97.0%, 96.6%, 95.6%, respectively). The higher overlap with the other realms was found in the case of the Palearctic – where only 42.7% of longhorn beetles were endemic, while the same index was 67.0% for the Oriental and 71.7% for the Nearctic realms. The analysis at the continental level showed that over 95% of the longhorn beetle fauna of Africa and Asia was restricted to the only one continent (97.5%, 95.9%, respectively), while other continents with a higher level of endemism were Australia, Oceania, and South America (94.8%, 94.7%, 90.8%, respectively). The same index was lower for Europe and North America (41.5%, 83.7%, respectively). Only 12.2% of longhorn species globally were distributed in at least two different realms. Widespread species that occurred in at least 3, but up to even 5 realms were rare and constituted 0.2% of all cerambycids (Table IV). The most widespread species (distributed in five biogeographic realms) were: Arhopalus rusticus (L., 1758), Callidiellum rufipenne (Motsch., 1862), Phoracantha recurva Newman, 1840, Ph. semipunctata (Fabr., 1775), Xystrocera globosa (Oliv., 1795) and Batocera rufomaculata (DeGeer, 1775). The same analysis at the genus level indicated that 79.2% of Cerambycidae genera are restricted to the only one biogeographic realm (Table IV). Eight genera occurred in more than six biogeographic realms, including: Callidium Fabr., 1775, Ceresium Newman, 1842, Xylotrechus Chevrolat, 1860, Stromatium Audinet–Serville, 1834, Xystrocera Audinet–Serville, 1834, Apomecyna Guerin–Meneville, 1835, Batocera Dejean, 1835, Monochamus Dejean, 1821. The most widespread genera (distributed in seven different biogeographic realms) were Callidium Fabr., Xylotrechus Chevrolat, and Monochamus Dejean.

Table IV. Occurrence of Cerambycidae s.s species and genera in biogeographic realms

SPECIES
No. of realms	1	≥2	≥3	≥4	≥5	≥6	7
Subfamily	No. of species	No. of species	No. of species	No. of species	No. of species	No. of species	No. of species
Parandrinae	112	11	0	0	0	0	0
Prioninae	912	145	1	0	0	0	0
Lepturinae	1100	400	0	0	0	0	0
Apatophyseinae	302	6	0	0	0	0	0
Spondylidinae	130	17	1	1	1	0	0
Necydalinae	104	20	0	0	0	0	0
Dorcasominae	22	0	0	0	0	0	0
Cerambycinae	9697	11172	25	9	4	0	0
Lamiinae	18019	2017	24	4	1	0	0
GENERA
No. of realms	1	≥2	≥3	≥4	≥5	≥6	7
Subfamily	No. of genera	No. of genera	No. of genera	No. of genera	No. of genera	No. of genera	No. of genera
Parandrinae	10	6	3	0	0	0	0
Prioninae	215	62	10	2	0	0	0
Lepturinae	100	90	16	2	0	0	0
Apatophyseinae	83	5	0	0	0	0	0
Spondylidinae	16	13	6	3	1	0	0
Necydalinae	10	2	1	0	0	0	0
Dorcasominae	11	1	0	0	0	0	0
Cerambycinae	1371	342	68	28	12	5	2
Lamiinae	2111	511	77	23	11	3	1

Figure 7. Dendrograms based on the UPGMA clustering method for the seven biogeographic realms (a) and for the seven continents (b) (Australia (mainland) and Oceania were separated) based on the matrix of Jaccard dissimilarity (1– Jaccard index) conducted for longhorn beetles (Cerambycidae s.s.) at the genus level

Discussion

Taxonomic richness

With the 34,490 extant species subdivided into 4,959 genera, (March 2019) the Cerambycidae constitute the largest family within the Chrysomeloidea Latreille superfamily (> 63,000 extant species worldwide (Ślipiński et al. 2011), and one of the largest families of beetles in existence. Although the obtained number of cerambycid species showed a good agreement with the other studies (Švácha & Lawrence 2014; Wang 2017) it should be treated with caution as the collected data is an estimation rather than the actual number of species, which is impossible to determine precisely due to numerous taxonomic uncertainties and the group complexity.

The quantitative contribution of other families to the overall taxonomic richness of the traditional “cerambycoid assemblage” (see Materials and Methods section) is rather marginal and does not surpass 1.5%. Most of the Cerambycidae taxonomic diversity is concentrated in two hyper–diverse subfamilies: Lamiinae and Cerambycinae which together cover 90.5% of all longhorn beetles. Although monospecific genera are predominant, the vast majority of the taxonomic diversity of longhorn beetles is concentrated in the large multi–species genera; only the first 829 richest genera encompass three quarters of the total number of Cerambycidae species. This resulted in a highly–asymmetrical, right–skewed frequency of distribution which is a common pattern also found in other groups of animals (Fisher et al. 1943; Preston 1948; Jetschke & Hubbell 2002; Barber-James et al. 2008; Pincheira-Donoso et al. 2013). These findings suggest that the prominent evolutionary proliferations leading to the formation of a large number of recently radiated species are rather rare events in the recent evolutionary history of Cerambycidae. On the other hand, a large number of monospecific genera might suggest the occurrence of relict lineages of formerly more diversified evolutionary branches, or alternatively, indicate young or even ongoing radiations (Barber-James et al. 2008).

Description ratio

We found that the species description ratio of longhorn beetles differed significantly among biogeographic realms and continents, and has also changed significantly throughout time. Dynamic development of the modern taxonomy in Europe contributed to the description of a significant number of European longhorn beetle species quite early in the eighteenth century (Figure 5). The modern faunistic trends have reached North America relatively quickly and have initiated taxonomic studies on the Nearctic fauna of Cerambycidae. As a result, three-quarters of all currently known Nearctic species were described by the end of the 1950s. Nevertheless, due to political issues and limited study effort, some other areas like Africa remained almost completely unexplored until the end of the nineteenth century. Historical events have also affected the species description curve of Cerambycidae; for example, the number of newly discovered species dropped during the Second World War. Species description curves for some areas e.g. Europe or mainland Australia are currently relatively flat, suggesting they are approaching their asymptotes. This indicates that the vast majority of cerambycid fauna of these areas have been already described. On the other hand, steep curves that can be found for Asia or South America, strongly suggest that the significant number of Cerambycidae species from these areas have not yet been described.

Diversity drivers

The patterns of biodiversity have been driven by a complex interaction of ecological and evolutionary processes acting in the geological time frame. As an example of ecological processes shaping biological diversity, we can count e.g. environmental filtering (Paine et al. 2012; Lamanna et al. 2014) or specific interspecies interactions (both positive and negative) (Elith & Leathwick 2009; Urban 2011; Wiens 2011), while the most prominent evolutionary processes affecting biodiversity patterns are extinction and the radiation of species (Foote et al. 1999; Roelants et al. 2007; Simões et al. 2016). Although the main drivers of organismic diversity are well known, there is still an ongoing debate on the importance of particular ecological and evolutionary factors in the shaping of the diversity patterns of the particular group or organism. Given that Coleoptera (beetles) are the largest group of animals in the world, the understanding of the drivers of their diversification have been a crucial challenge for evolutionary biologists. Several different hypotheses were proposed to explain the extraordinary diversity of Coleoptera (including Cerambycidae) (Hunt et al. 2007). One of the most frequently discussed explanations highlights the key role of co–radiation of Chrysomelidae with the early angiosperms (Farrell 1998; Farrell & Sequeira 2004; Wang et al. 2013). Nevertheless, these mechanisms can only partially explain the diversity of cerambycids (Haddad & Mckenna 2016; Haddad et al. 2018). Numerous species of longhorn beetles are known to be highly polyphagous at the larva stage, and some species may feed on both gymnospermous as well as on angiospermous plants. Even species within one genus may exhibit high diversity in respect to the host plants which strongly suggest that there were multiple host switches between angiospermous and gymnospermous plants during the cerambycid evolution (Haddad et al. 2018). We found that many genera containing exclusive polyphagous species are species–rich and widely distributed, which highlights an adaptive role of host plasticity in Cerambycidae evolution.

Biogeographic patterns

We found that at the global scale, the cerambycid fauna are geographically clustered into three major assemblages, including Pan–American, Afrotropical, and Palearctic-Oriental assemblage (with the Australasian branch). Our results also highlighted the high endemism of the Australasian, Ethiopian, Madagascan and Neotropical realms, and the phylogenetic distinctiveness of Madagascan fauna of Cerambycidae; whereas high species turnover was found for the Palearctic, Oriental and Nearctic realms. Moreover, we found the vast majority of the species are distributed within only one biogeographic realm. Most of the species assigned to two realms were distributed at the transition zones, especially within the Nearctic–Neotropical and Palearctic–Oriental transition zones. A small number of genera with the Trans–Pacific disjunct distribution were found, suggesting that the vicariance events were rather rare in the evolution of Cerambycidae (Kim et al. 2018). Other factors that might have potentially shaped the large–scale distribution patterns of Cerambycidae are continental drift, especially the break–up of Gondwana and the split of Laurasia (Haddad et al. 2018), or climatic oscillations - in particular, glacial events (Hewitt 1996; Shoda et al. 2003; Vitali & Schmitt 2017; Goczał et al. 2020). Some similarities in the distribution patterns of the Cerambycidae subfamilies among realms representing relatively similar environmental conditions, e.g. a significant contribution of Lepturinae to the longhorn beetle fauna of the Northern Hemisphere (Nearctic and Palearctic) might suggest the significant influence of environmental filtering on the distribution patterns of cerambycids. Furthermore, it was shown that live species’ history traits, especially dispersal abilities, might have had a significant influence on the Cerambycidae fauna composition (Vitali & Schmitt 2017). Finally, it should be strongly emphasized that all of the longhorn beetle species that exhibit exceptionally wide geographic distribution (which can be found in six different biogeographic realms) were accidentally introduced outside their native range by humans, which might have a negative effect on the native Cerambycidae fauna (Dearborn et al. 2016), and might affect the evolutionary patterns of this group.

Further challenges

Although Cerambycidae constitutes one of the largest, most diverse, ecologically, and economically important group of beetles in the world, there are still numerous issues and controversy regarding both their higher– and lower–level taxonomy (Haddad & Mckenna 2016; Ślipiński & Escalona 2016; Wang 2017). Moreover, most of the taxa revisions and new species descriptions are based only on morphological characters, rather than a genetic approach. This results in a high number of synonymous species, and hinders the comparison of Cerambycidae assemblages. A significant number of longhorn beetles, especially newly described species, have a dramatically poorly studied distribution; some species are known for only one uncertain location, which does not allow for an analysis of longhorn beetle distribution patterns based on high-resolution data. Finally, due to the limited number of high-quality genetic data, the phylogeography of Cerambycidae is poorly studied, and the main drivers of their diversity are still unclear. We strongly suggest that more research involving a comprehensive, multigene approach is needed to resolve the urgent taxonomy issues of Cerambycidae, and to better the understanding of the drivers of cerambycid diversity.

Acknowledgements

We would like to express our gratitude to Anna Dudzik for her help in data acquisition. We also thank anonymous reviewers for valuable comments on earlier version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The research was supported by the Ministry of Science and Higher Education of the Republic of Poland under the statutory grants SUB/040013/0019. Jakub Goczał was supported by the ETIUDA doctoral scholarship from the National Science Centre (Poland), grant number 2019/32/T/NZ9/00272.