2023-06-29 15:15:18
The academic journal Science published in March of this year a study that proposed a causal relationship between the body size of island species and their danger of extinction. According to those responsible for the study, the dwarfism or gigantism of these species made them more vulnerable. Now, another study calls into question the validity of that conclusion.
Islands often function as natural laboratories for the study of evolution, since they are less complex systems than continental ones. The characteristics of geographic isolation that hinder species migrations, the limitation of physical space, and the scarcity of food resources in insular ecosystems contribute to common evolutionary patterns in the faunas that live there. Therefore, insular endemisms are forms of life that have evolved in conditions very different from those of the mainland. In contrast, continental ecosystems have a high diversity where terrestrial predators occupy the apex of the trophic pyramid. In these ecosystems, the predator-prey interaction is one of the most important evolutionary drivers. Escape so as not to be hunted and hunt to survive create an evolutionary dynamic that shapes the capabilities (locomotor, sensory, and biological) of prey and predator.
However, terrestrial predators are typically absent on the islands. This fact completely determines the adaptations of the herbivores that inhabit them. They no longer have to escape predators, therefore all the adaptations that on the mainland serve to minimize being hunted are no longer necessary. They reduce their locomotor abilities, sense organs, brain size, change their body mass and other aspects of their biology. Precisely, the changes in body mass follow a well-determined rule known as the island law or island effect (Island rule). This pattern often manifests itself in the phenomenon of dwarfism or gigantism, with species being significantly smaller or larger on islands compared to their mainland counterparts. Currently, the causes of these size changes are not known and their origin remains a mystery.
There are many examples in the fossil record that illustrate this variation in body size. An emblematic species is the Sicilian dwarf elephant (Palaeoloxodon falconeri). It is a species with a body of approximately one meter in height and around 250 kilograms in weight. The size and weight are extremely small compared to the current elephant species.
On the island of Menorca (in the Balearic Islands) we found the giant rabbit Nuralagus rex, which weighed 10 times more than a modern rabbit. Also in the Balearic Islands lived Myotragus balearicus, a dwarf goat that during its evolution underwent important changes that especially affected the locomotor system and size, as well as the nervous and alimentary systems. Dwarfism, the reduction of the brain and changes in the dentition are its most characteristic evolutionary features.
Recreation of the appearance of a specimen of Myotragus balearicus. (Image: Roc Olivé / © Catalan Institute of Paleontology Miquel Crusafont. With the collaboration of the Spanish Foundation for Science and Technology – Ministerio de Economía, Industria y Competitividad)
The study led by Roberto Rozzi (German Center for Integrative Biodiversity Research (iDiv)), and published in March 2023 in the academic journal Science, assessed how the evolution of species body size would increase the extinction susceptibility of taxa. island endemics. The authors of that study establish a causal relationship between the differences in size of island species, according to the Island rule, and their vulnerability to the danger of extinction, through correlations derived from statistical models (GLMM). They conclude that there is a greater probability of extinction and of being endangered in the most extreme dwarf and giant taxa of the islands, but they do not explain the origin of the causality that they propose or any mechanism that explains it.
In an analysis published in the same journal as that study, researchers Meike Köhler and Salvador Moyà-Solà, group leaders at the Miquel Crusafont Catalan Institute of Paleontology (ICP), argue that this relationship is not causal and warn that an interpretation erroneous characteristics of insular endemism can lead to serious errors in estimating their danger of extinction.
Köhler and Moyà-Solà point out that statistical models do not reveal the causal structure of the data, so it is necessary to carry out a prior causal analysis to identify, adjust and take into account possible variables that influence the evolution of island species. For example, Rozzi and his colleagues have not considered as a variable the life-history (maximum body size, longevity, age of maturity and fecundity), despite being fundamental components of the fitness (understood as the proportion of genes of an individual in the total genes of the next generation) of the species that evolve to guarantee reproductive success throughout their lives. These components of the fitness (especially the time to reach sexual maturity and the quantity and quality of offspring) are crucial determinants of a population’s resilience and, therefore, of its vulnerability.
Köhler and Moyà-Solà continue to argue that insular evolutionary models, based on the evolution of life-history (LH), clearly show that size changes are a secondary consequence of changes in growth rate, age at sexual maturity, and reproductive pattern, ie, changes in LH. They propose that the correlation between LH parameters rather than size changes should be investigated, because changes in body size, by themselves, are not a causal mechanism, since they do not incorporate intrinsic biological parameters (components of the fitness) that explain or cause the danger of extinction.
While Rozzi’s team suggests that body size should be included in the parameters used to determine the endangerment of a species by conservation agencies (International Union for Conservation of Nature, IUCN), staff ICP researcher believes that this suggestion, in addition to ignoring the fact that the characteristics of the life-history are already part of the IUCN parameters, it can lead to serious errors in estimating the extinction danger of an insular species, errors that go beyond a merely theoretical discussion.
Currently, the conservation of biodiversity has become an emergency. We see how the impact of human activities on the biosphere is reaching devastating levels. It is estimated that between 150 and 200 species of plants and animals go extinct daily on average. The rapidity of this extinction phase far exceeds the speed at which the other five documented great extinctions in the history of life on Earth have occurred. In the case of islands, this issue is aggravated, since island biota are typically destroyed when they come into contact with human activity. For this reason, Köhler and Moyà-Solà emphasize that any suggestion on how to approach the preservation of biodiversity in international conservation agencies must be based on solid foundations to avoid mistakes in protection measures. (Source: ICP)
#Open #debate #relationship #body #size #extinction #risk #island #species