The invasion process involves overcoming certain barriers with negative effects on the biodiversity and ecosystem function. This is the case of the African dung beetle Digitonthophagus gazella, which was introduced in the United States and other countries in Central and South America to help in dung removal activity. The distribution of D. gazella has been documented in the Americas, but in Venezuela there is little information. Trying to understand the process of invasion of the Colombian-Venezuelan savannas and measure their potential impact, the information stored in collections and literature was collected. Five new state records were reported for Venezuela, as well as four scenarios of the process of introduction and establishment of this species in the region. Although the amount of records has increased, the number of individuals sampled is still incipient. The possible areas of invasion and the potential negative effects that the presence of this species exerts in these regions are also discussed. In addition, potential areas of invasion and negative effects of this species are discussed. Results emphasize the importance of generating models of the invasive process in order to design programs that monitor the effects incurred by the presence of D. gazella on native species and propose control and mitigation mechanisms.
The invasive process of a foreign species is made up of successive steps that consist in its overcoming environmental, geographical, and demographical barriers in a way that it is able to disperse extensively. Thus, invasive species with a stable population can generate threats to local species [
Since the anthropic introduction of the African dung beetle Digitonthophagus gazella (Fabricius, 1787) [
Within South America, D. gazella has been recorded due to both dispersion and unofficial introduction events in Colombia [
Although there are detailed records of the invasion process in North America, particularly for Mexico [
The information from public and private collections was gathered, field expeditions to specific locations were implemented, and all bibliographic material was validated in order to construct a detailed map of the current distribution of D. gazella in the savannas of Venezuela and Colombia. With all these information we try to understand the biogeographic patterns of the introduction and establishment of the species’. The zoological collections examined in Venezuela were: Entomological Collection of the Andean Entomology Foundation (CEFUNEA), Tachira Joffre Blanco Entomology Collection (METJB), and Museo del Instituto de Zoología Agrícola “Dr. Francisco Fernández Yépez” (MIZA). Distribution maps were generated using ArcView GIS version 3.2.
New records
Five new state reports have been established for Venezuela from the information gathered in the zoological collections, constituting the first official record for this species in the country and expanding its known distribution range (
Examined material: Venezuela. Apure: 1♀, 1♂, Municipio Rómulo Gallegos, Hato La Yeguera, Caño Guaritico, Jun 2006, A. Orellana, leg., METJB. 2♂, Fundo La Florida cr. Quitaparo, 42 m, 7˚05'N - 68˚36'W, Jun 1999, E. Osuna & A. Chacón, leg., MIZA. Bolívar: 4♀, 1♂, Guri, 200 m, Jun-Jul 1998, L.J. Joly, J.L. García & Y. Zavala, leg., light trap, MIZA. Cojedes: 6♀, 6♂, Hacienda El Palmar, Sep 2005, MIZA. Gúarico: 12♀, 22♂, Nicolasito, 08˚08'20"N - 66˚24'32"W, Jan 2000, J. Clavijo, M. Gaiani, R. Briceño, Q. Arias & A. Chacón, leg., MIZA. Táchira: 1♀, 1♂, Municipio García de Hevia, La fría, Feb 2007, J. Blanco, leg., METJB. 2♀, 2♂, Municipio Jáuregui, La Quinta, road la grita, Mar 2008, 1400 m, J. Blanco, leg., METJB.
Records of the dispersal process of D. gazella in Venezuela
The following reports constitute the records of the processes of introduction, establishment, and invasion of D. gazella in Venezuela.
1) Maracaibo Lake―Zulia (1990’s-1996)
In Venezuela at the start of the 90’s, there is information to suppose that a group of Englishmen unofficially liberated D. gazella individuals in cattle ranches to increase the recycling rate of dung (J. Blanco unpub. data). This liberation coincides in time with the liberations in Brazil and Chile. The first sampling of D. gazella in Venezuelan territory was carried out in 1996 within the State of Zulia, municipality of Colon, in the Santa Ana Ranch on introduced pastures [
2) Bolívar, Apure, and Guárico―Venezuela midland (1998-2000)
The following records in Venezuela correspond to the States of Bolívar in 1998 (n = 5 ind.), Apure in 1999 (n = 2 ind.), and Guárico in 2000 (n = 34 ind.). Although these records indicate low population densities, this is because the surveys carried out were not of systematic character nor were intended to calculate the species’ population densities in the area. The survey carried out in the State of Bolivar is of special interest because this region is located in the south portion of the Orinoco River, a potential geographical barrier for the dispersal of the species that may be related to the growing livestock industries.
3) Cojedes and Táchira (2005-2008)
Between the years 2005 and 2008 new records were obtained for D. gazella in Venezuela. New individuals were captured in the State of Cojedes in 2005 (n = 12 ind.) and in the State of Tachira in 2007 (n = 2 ind.) and in 2008 (n = 4 ind.), however still obtaining low population densities. In addition, new localities were registered for the State of Apure.
Increase in the records and establishment of D. gazella
A general trend of increase is observed in the records and presence of D. gazella for the new localities. In this study the presence of D. gazella is reported for the first time in five States of Venezuela: Apure, Bolivar, Cojedes, Guárico and Táchira. The analysis of these records demonstrates an increase in the distribution range and establishment of this species in Venezuelan territory. The number of individuals detected in these new records is still low when compared to other dominant native species such as Canthidium euchalceum Balthasar, Canthon acutus Harold, Onthophagus landolti Harold and Onthophagus marginicollis Harold. It’s demonstrating that D. gazella is still in a stage of establishment in various localities and hasn’t yet become an invasive species at a regional scale. However, the reversal of this relationship is possible within a short timespan according to the records in the State of Zulia where show that D. gazella is a competitive species, with high dispersal, and ability to exploit the local resources of domestic and native animals ( [
Hypothetical scenarios for the processes of arrival and dispersion of D. gazella
Using the new records for Venezuela in this paper and all the available records for Colombia, we propose four hypothetical scenarios for the arrival and dispersal of D. gazella in the Venezuelan and Colombian savannas. The first two (A and B) consider that the species arrived through natural dispersion processes, and the last two (C and D) consider their arrival to be a consequence of anthropic liberation processes (
A) In this first scenario D. gazella initially comes from the colonization through Central America, despite the fact that the nearest country where the species has been recorded is in Nicaragua [
The species would have initially entered through the Caribbean region of Colombia, constituting two dispersion fronts: the first front towards the inter-Andean valleys of the Magdalena and Cauca Rivers in Colombia, and the second front towards the vicinities of Lake Maracaibo in Venezuela. In this scenario, the Andes mountain range would have constituted a strong geographical barrier [
B) In this second scenario D. gazella would arrive through the Antilles, origin that is strongly supported by records both in the Greater Antilles ( [
C) In this third scenario the arrival of D. gazella would have been by means of an unofficial anthropic introduction in Venezuelan territory (J. Blanco unpub. data). It would establish two dispersion fronts from the location of its introduction: the first front going towards the Caribbean region of Colombia, and the second front towards the Venezuelan midland States of Bolivar, Apure, and Guárico. Similarly, in this scenario, the Andes mountain range would have constituted a strong geographical barrier [
D) In this fourth scenario the arrival of D. gazella would have been by means of two unofficial anthropic introductions occurring in a parallel way in Venezuela and Colombia, although for Colombia there is no record of an anthropic introduction of the species before the one carried out in the state of Antioquia by the Wyeth Laboratories Inc. in 2007 [
Potential invasion areas of D. gazella
Digitonthophagus gazella possess a broad distribution range within the Colombian and Venezuelan territories, occupying mainly open habitats, savannas, pastures, and altered landscapes, especially those for livestock use that are situated under 1500 m. Although the species hasn’t been registered in any regions of high endemicity yet, the existence of a steady flow of livestock could facilitate their entry to more susceptible areas [
In Venezuela, it is probable that the species has already colonized the complete vicinities of Lake Maracaibo and entered the States of Anzoátegui, Barinas and Portuguesa, as well as occupied the introduced pastures in the Southern regions of the Lake corresponding to the States of Mérida and Trujillo. Similarly, it is probable that it will disperse throughout the plains region, which covers 200,000 km2 of Venezuelan territory [
The limited records in Venezuelan territory may be the result of a lack of surveys in more relevant regions or because of low population densities. Additionally, it should be noted that until recently in Venezuela the species had been inaccurately identified as Onthophagus sp. [
On top of all this, the added effect of anthropic activity could potentiate its dispersal capacities. This is especially true when considering liberations by the hands of institutions such as the EMBRAPA in Brazil [
Potential negative effects of D. gazella invasion
Given that D. gazella presents a high dispersal capacity [
In one of the localities in Texas where D. gazella was originally introduced, tracking of its population numbers has registered an increase in the species’ abundance to values constituting up to 30% of the beetle species composition in the ecosystem (n = 11,709 of 40,079 ind.; 21 spp.) [
In Colombia (J.A. Noriega unpub. data) and Brazil (C. Flechtmann pers. comm.) similar negative effects have been observed in native ecosystems’ species composition, resulting in sharp declines in their population numbers and presumed local extinctions of native species that have not been recorded again since D. gazella’s establishment. On the other hand, in other localities these effects are not as apparent, making it seem as though their native species assemblages are less susceptible to invasive species negative effects [
It’s imperative to generate biogeographical models using the available information on the natural history and barriers of this species to clearly understand the process of introduction, establishment, and invasion in areas with endemic species [
To the Zoology and Aquatic Ecology Laboratory (LAZOEA). To Andrés Orellana for collecting some of the samples in the State of Apure. To the directors and curators of the entomological collections: the Entomological Collection of the Andean Entomology Foundation (CEFUNEA), the Tachira Joffre Blanco Entomology Collection (METJB), and the Museo del Instituto de Zoología Agrícola “Dr. Francisco Fernández Yépez” (MIZA). To Ricardo Botero-Trujillo, José Ignacio Barrera, Ingrid Quintero, Cesil Solís, and Carolina Vizcaíno for their valuable comments and insights to the various versions of the manuscript. To Luis J. Joly, Carlos A. Flechtmann, Fernando Vaz-de-Mello, Bert Kohlmann and Angel Solís for the information provided and valuable contributions to the study. To Javier Santos and David Morris for their valuable help in the correction and revision of the English version of the manuscript.
Noriega, J.A., Delgado, O., Blanco, J.I., Gámez, J. and Clavijo, J. (2017) Introduction, Establishment, and Invasion of Digitonthophagus gazella (Fabricius, 1787) (Coleoptera: Scarabaeinae) in the Savannas of Venezuela and Colombia. Natural Resources, 8, 370- 381. https://doi.org/10.4236/nr.2017.85023