The makeup of modern ecological communities is determined by three factors, speciation, migration, and extinction. The first two of these can be largely understood through the examination of extant species, but our knowledge of past extinction events is entirely derived from the fossil record. Extinction events can create opportunities for surviving species to adapt into vacant niches, but can also lead to the vanishing of those niches, and of co-dependent niches and the organisms that occupy them. Island ecosystems are particularly vulnerable to extinction events, and their ecological communities are therefore particularly prone to being modified in this way. This makes the fossil history of island ecosystems, which are likely to have been shaped by numerous extinction events. of particular interest to evolutionary ecologists, although very few islands have fossil deposits that record past faunas in sufficient detail to be useful.
The island of Hispaniola in the Greater Antilles is home to 126 indigenous Ant species, drawn from a wide range of Neotropical groups which occupy different ecological roles. The island is also home to an excellent Fossil Lagerstätte, the Dominican Amber deposits, from which over 1100 species of Early Miocene Insects, including 86 species of Ants. The fauna recorded in this amber deposit is very similar to that of the modern Neotropics, with 84 of the described Ant species being placed in extant Neotropical genera. However, about a third of the genera found in the fossil fauna, while still present in the wider Neotropics, are absent from not just Hispaniola, but the wider Greater Antilles island group, implying that they have suffered local extinctions over the 16 million years since the amber was deposited. The ancient and modern faunas were studied by biologist Edward O. Wilson in the 1980s, who concluded that larger species and those with more specialize ecologies were more likely to have gone extinct. Since Wilson carried out his study, considerably more has been learned about both the fossil and living Hispaniolan Ant faunas, but Wilson's observations remain unchallenged.
In a paper published in the journal BMC Biology on 8 February 2023, Gianpiero Fiorentino of the Federated Department of Biological Sciences at the New Jersey Instituteof Technology, John Lattke of the Departamento de Zoologia at the Universidade Federal Do Paraná, Adrian Troya of the Departamento de Biología at the Escuela Politécnica Nacional, Christine Sosiak, also of the Federated Department of Biological Sciences at the New Jersey Institute of Technology, Minsoo Dong of the Applied Biology Program at Kangwon National University, and Phillip Barden, again of the Federated Department of Biological Sciences at the New Jersey Institute of Technology, and of the Division of Invertebrate Zoology at the American Museum of Natural History, describe a new species of Ponerine Ant from Miocene Dominican Amber, and discuss its implications for the evolution of the ecological niches occupied by Ants in the Greater Antilles.
The new species is assigned to the Subfamily Ponerinae, one of the largest and most diverse Ant families, both in ecological and morphological terms, and to the genus Neoponera, which is one of the more diverse genera of Neotropical Ponerine Ants, with 58 extant species, divided among seven species groups. The majority of Neoponera species are arboreal in habit, which is unusual in Ponerine Ants, although the genus is quite ecologically variable, with individual species having lifestyle strategies which vary from generalist ground-dwelling hunters, to specialist Termite raiders, to mutuaslistic relationships with certain Trees, which they protect from herbivores in return for food and nesting spaces. Despite this ecological adaptability, the genus Neoponera does not appear to do well in island ecosystems, with a few species in the Lesser Antilles but none in the Greater Antilles or on other Neotropical islands.
The new species is named Neoponera vejestoria, where 'vejestoria' derives from the Spanish 'vejestorio' meaning an old person or thing. The new species is placed in the genus Neoponera on the basis of its eyers, which are convex eyes placed at about head mid-length (found in all members of the genus), its well developed aroliae (a lobe on the leg to which the pretarsus attaches, again well developed in all members of the genus), and its slit-shaped propodeal spiracle (breathing opening, found in most members of the genus). It resembles members of the foetida and aenescens species groups, being assigned to the former group on the basis of its well developed malar carinae (ridges on its 'cheeks') and the positioning of its eyes.
Based upon its morphology, Neoponera vejestoria is thought to have been a ground-dwelling generalist predator. It is the first known fossil species within the genus Neoponera, as well as the first known fossil member of the Pachycondyla genus group of Ponerine Ants from the Neotropics. It is also the largest predatory Ant known from the island of Hispaniola, either in the Miocene Amber deposits or today, although it would only be considered a medium-sized compared to a wider sampling of Ants; most modern Ants on Hispaniola are quite small, and while the size-range present in the Miocene Ants is larger, none of them are exceptionally large.
Living Ponerine Ants are less morphologically variable and, on average, smaller than fossil species. This is less true if only Ants from Hispaniola are considered, where the largest Ponerine Ants other than Neoponera vejestoria, livening or fossil, all belong to the genus Odontomachus, which is the only Ant genus on the island today containing medium-sized species. However, it is likely that the Dominican Amber deposits do not represent the full range of Miocene Ants on the island, and that other larger Ants may have existed on the island at that time.
Fiorentino et al.'s study suggests that the fauna of Hispaniola has been shaped not just by the extinction of lineages of Ants, but also the loss of the ecological niches which they once occupied. Predatory Ants on the island have always tended to be on the small side, but the range of sizes was clearly greater in the Early Miocene than it is today. Furthermore, Ants with more specialized feeding strategies, such as the Blind Army Ants of the genus Neivamyrmex, the Trap-jaw Ants, Acanthognathus spp., and the subterranean predatory Ants, Acanthostichus spp., have also died out on the island. The discovery that the island was once occupied by the Ponerine Ant Neoponera vejestoria provides further evidence of the modification of the island's ecological community through the extinction of certain ecological traits. Neoponera vejestoria is at least a third larger than any living predatory Ant on the island, which is a conspicuous difference in size, and likely to have been a ground-nesting generalist predator, which is not in itself unusual.
The Ant fauna of the Miocene of the island of Hispaniola is likely to be severely under-represented in Dominican Amber. The amber was formed from a resin extruded by a Leguminous Tree, and therefore best preserves the Insects which lived on, or at least visited the canopy of these trees, as well as, to a lesser extent, species which lived on the ground beneath. Insects which lived elsewhere on the island are highly unlikely to have been preserved at all. This could well imply that the Miocene size-range of Ants on the island was wider than has been preserved in the fossil record. Other lineages of larger Ants besides Neoponera appear to have been lost on Hispaniola, such as the large-bodied genus Paraponera, which is found in Dominican amber, but absent from the Caribbean region today.
Mammals, and other Vertebrates, often face selective pressures against larger sizes when groups become isolated on islands, but this is much harder to demonstrate in Insects, although it has previously been demonstrated in Carabid Beetles on European islands.
Excluding Neoponera vejestoria, the largest Ant species found on Hispaniola in both the Miocene and modern faunas belong to the genus Odontomachus. These Ants are ground dwelling generalist predators, something they share with Ants of the genus Neoponera. Today, these two Ant genera are seldom encountered on the same island, with Odontomachus being found on Hispaniola and the other islands of the Greater Antilles, as well as Cocos, Barbados, and Tobago, while Neoponera is only found on the island of Margarita. Fiorentino et al. therefore consider it possible that the two types of Ant compete for similar niches, which may have been a driver of the extinction of Neoponera on Hispaniola.
The discovery of Neoponera in Dominican Amber highlights the fact that these deposits still have much to tell us about the Miocene fauna of Hispaniola, and the way in which the island's ecology has changed over time, despite over five decades of study on the subject.
Neoponera vejestoria is clearly a distinct species, but nevertheless is morphologically very similar to extant species in the same genus, something which has been observed in other Ants from Miocene Dominican Amber. Most modern members of the genus Neoponera inhabit specialist niches, and the group is predominantly arboreal today, but some members are still ground-dwelling generalist predators, and it has previously been suggested that this is likely to be the ancestral state for the genus, a hypothesis supported by the discovery of Neoponera vejestoria, although the alternative hypothesis, that Neoponera vejestoria had secondarily adopted a ground-dwelling generalist lifestyle as an adaptation to island life, cannot be excluded.
The discovery of Neoponera vejestoria also sheds light on the history of the wider Pachycondyla genus group (which includes Pachycondyla, Neoponera, and Dinoponera, among others), which are important members of Neotropical ecosystems, being the first fossil Ant confidently placed within the group, and the first fossil member of an extant genus in the group. Other fossils have previously been referred to this group, but none of them with confidence, and Fiorentino et al. suggest that all of these specimens need re-examination, to shed better light on the history of the group. Molecular clock estimates of the age of the genus Neoponera have suggested that it first appeared between 26 and 12 million years ago, while the foetida species group has been thought to be no more than 12 million years old. A better understanding of the fossil record of the Pachycondyla genus group would clearly lead to better calibration of these molecular estimates.
The discovery of Neoponera vejestoria provides a rare example of island extinction in the fossil record. This is almost certainly linked to the body size of the species, which is likely to have placed it at greater risk of extinction in an island ecosystem. As the ecological niche occupied by Neoponera vejestoria is still in existence, this demonstrates that a large size in itself is a threat to Ant species on islands.
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