Se suponía que la complejidad de las redes ecológicas en general (por ejemplo, las cadenas tróficas) había aumentado conforme avanzaba la evolución desde el origen de la vida. La doctrina estándar sostenía que tal hecho, como también que tal proceso había  sido acompañado por un incremento de la biodiversidad. En muy pocas semanas parece ser que todo este entramado teórico se ha venido abajo. Desde hace más de 500 millones de años, tanto la topología de les mencionadas redes de interacción como el número de especies no parecen haber sufrido modificaciones relevantes. El trabajo sobre la topología de las redes tróficas del Cámbrico ha sido recientemente publicado por PloS Bioloy en acceso abierto, por lo que os podéis bajar el material pinchando el siguiente enlace (al final del post os incluimos el resumen).

Redes ecológicas del presente y pasado

Fuente: Dune et al. (2008) Plos One. Enlace en el texto

El estudio fue realizado haciendo uso del material paleontológico preservado en dos yacimientos excepcionales y de modelos de computación. No entraré en detalles, por cuanto lo que interesa a los jóvenes es tal regularidad, mientras que los profesionales disponen del artículo, así como de los dos resúmenes que incluyo al final de este post.

Propiedades de las Redes ecológicas del presente y pasado

Fuente: Dunne et al. (2008) Plos One. Enlace en el texto.

Leyenda al final de la última figura

 Ya hemos venido hablando en varios post de que las redes de los sistemas complejos (ya sean biológicas, sociales o mentales) y artefactos tecnológicos, se encuentran sujetos a una serie de leyes que condicionan su estructura y dinámica. Lo que pudiera sorprender a primera vista deviene de que tales estructuras se alcanzaran relativamente pronto a lo largo de la historia de la vida (al menos desde la aparición de los animales marinos, hace aproximadamente 500 millones de años). Sin embargo, tal descubrimiento no nos debería deparar asombro alguno. Hemos venido mostrando y demostrando que tales arquitecturas emergen espontáneamente en los más disparatados ámbitos, por ser consecuencia de la autoorganización de los elementos de los sistemas abiertos a los flujos de energía, materia e información. Si las redes sociales en Internet se han autoorganizado en pocos años, no resulta insólito que en los ecosistemas naturales se alcanzaran las mismas regularidades en millones de años.

 Propiedades de las Redes ecológicas del presente y pasado

Fuente: Dunne et al. (2008) Plos One. Enlace en el texto.

Leyenda al final de la última figura

Uno debe tener cuidado al generalizar las repercusiones de tales hallazgos por cuanto los patrones detectados no refutan que la vida haya ido ganando en complejidad, como lo demuestra la emergencia de animales cada vez más complejos y sofisticados. La lección e extraer resultaría ser que la estructura invariante de las redes complejas emerge rápidamente debido a las leyes de la física de los sistemas complejos.  ¿Porqué hemos tardado tanto tiempo en percatarnos? De los textos incluidos más abajo se infiere rápidamente que la razón estriba en la carencia de las bases de datos adecuadas. No nos cansaremos de repetir que obtener buenos inventarios es una de las bases fundamentales para extraer conclusiones acertadas en casi todos los ámbitos de las ciencias de la tierra y de la Vida. Sin embargo, en la ciencia moderna se desprecia tal actividad en aras de investigaciones más sofisticadas. ¡Error de bulto!. Por mucha instrumentación y modelización que apliquemos, todo el edificio se viene abajo si no se dispone de buenas bases de datos (trabajo muy arduo, ingrato y despreciado). Los pilares del conocimiento deben hundir sus raíces en los inventarios. Una vez disponiendo de buenos productos todo lo demás viene por añadidura. 

Propiedades de las Redes ecológicas del presente y pasado

Fuente: Dunne et al. (2008) Plos One. Enlace en el texto.

Leyenda al final de la última figura

En cualquier caso, nuestro conocimiento del pasado es muy limitado quedando mucho por estudiar. Lo que lo que hoy nos ofrece la ciencia bien pudiera ser refutado en poco tiempo.       

Juan José Ibáñez

Otros post Previos Relacionados con el Tema

Redes Complejas: Redes Sociales y Redes Ecológicas (Los Mundos Pequeños)

Conectividad, Redes Sociales y Redes Ecológicas

Especies Clave y Nodos Clave: Redes Ecológicas, Redes Sociales

Evolución de la Biodiversidad a lo largo de la Historia de la Tierra

Continuará…………

Juan José Ibáñez

Find "terradaily "food webs"" on Tribe.net

by Staff Writers; Washington DC (SPX) May 01, 2008

Similarities between half-billion-year-old and recent food webs point to deep principles underpinning the structure of ecological relationships, as shown by researchers from the Santa Fe Institute, Microsoft Research Cambridge and elsewhere It was an Anomalocaris-eat-trilobite world, filled with species like nothing on today's Earth. But the ecology of Cambrian communities was remarkably modern, say researchers behind the first study to reconstruct detailed food webs for ancient ecosystems.  Their paper, published this week in the open-access journal PLoS Biology, suggests that networks of feeding relationships among marine species that lived hundreds of millions of years ago are remarkably similar to those of today.

Food webs depict the feeding interactions among species within habitats--like food chains, only more complex and realistic. The discovery of strong and enduring regularities in how such webs are organized will help us understand the history and evolution of life, and could provide insights for modern ecology--such as how ecosystems will respond to biological extinctions and invasions.

A multidisciplinary group of scientists led by ecologist Jennifer Dunne of the Santa Fe Institute in Santa Fe, New Mexico and the Pacific Ecoinformatics and Computational Ecology Lab in Berkeley, California, studied the food webs of sea creatures preserved in rocks from the Cambrian, when there was an explosion of diversity of multicellular organisms--including early precursors to today's species as well as many strange animals that were evolutionary dead ends.

Report co-author Richard Williams of Microsoft Research in Cambridge, UK, developed the cutting edge "Network3D" software that was used for analysis and visualization of the food webs. The researchers compiled data from the 505 million-year-old Burgess Shale in British Columbia, Canada and the even earlier Chengjiang Shale of eastern Yunnan Province, China, dating from 520 million years ago. Both fossil-rich assemblages are unusual because they have exquisitely preserved soft-body parts for a wide range of species. They determined who was eating whom by piecing together a variety of clues.

There was the occasional smoking gun, such as fossilized gut contents in the carnivorous, cannibalistic priapulid worm Ottoia prolifica. However, in most cases, feeding interactions were inferred from where species lived and what body parts they had. For example, grasping claws, swimming lobes, big eyes, and toothy mouthparts suggest that Anomalocaris canadensis, a large, unusual organism with no modern descendents, was a formidable predator of trilobites and other arthropods, consistent with bite marks found on some fossils.

To compare the organization of Cambrian and recent ecosystems, the team used methods for studying network structure, including new approaches for analyzing uncertainty in the fossil data. "Paleontologists have long known that food webs were important but we have lacked a rigorous method for studying them in deep time," comments co-author and paleontologist Doug Erwin of the Santa Fe Institute and the Smithsonian Institution.

"We have shown that we can reconstruct ancient food webs and compare them to modern webs, opening up new avenues of paleoecology. We were surprised to see that most aspects of the basic structure of food webs seem to have become established during the initial explosion of animal life." The Cambrian food webs share many similarities with modern webs, such as how many species are expected to be omnivores or cannibals, and the distribution of how many types of prey each species has. Such regularities, and any differences, become apparent only when variation in the number of species and links among webs is accounted for.

"There are a few intriguing differences with modern webs, particularly in the earlier Chengjiang Shale web. However, in general, it doesn't seem to matter what species, or environment, or evolutionary history you've got, you see many of the same sorts of food-web patterns," explains Dunne. "What we don't know," Dunne adds, "is why food webs from different habitats and across deep time share so many regularities. It could be that species-level evolution leads to stable community-level patterns, for example by limiting the number of species with many predators through selective pressures that result in extinctions or development of predator defences. Or, patterns may reflect dynamically persistent configurations of many interacting species, or fundamental physical constraints on how resources flow through ecological networks."

Answering such questions will break new ground at the intersection of ecology, evolution and physics. And it may provide valuable insights into present-day ecology. As Williams points out, "This research is an excellent example of how computational methods can be used as part of an inter-disciplinary study to help produce novel results. By getting a better idea of how ecosystems behaved in the past, we may better comprehend and mitigate what is happening to ecosystems today and in the future."

www.terradaily.com/reports/

Compilation and Network Analyses of Cambrian Food Webs

Jennifer A. Dunne1,3*, Richard J. Williams2,3, Neo D. Martinez3,4, Rachel A. Wood5,6, Douglas H. Erwin1,7

1 Santa Fe Institute, Santa Fe, New Mexico, United States of America, 2 Microsoft Research Limited, Cambridge, United Kingdom, 3 Pacific Ecoinformatics and Computational Ecology Lab, Berkeley, California, United States of America, 4 National Center for Ecological Analysis and Synthesis, Santa Barbara, California, United States of America, 5 Grant Institute, School of GeoSciences, University of Edinburgh, Edinburgh, United Kingdom, 6 Edinburgh Collaborative of Subsurface Science and Engineering, University of Edinburgh, Edinburgh, United Kingdom, 7 Department of Paleobiology, National Museum of Natural History, Washington, D.C., United States of America

A rich body of empirically grounded theory has developed about food webs—the networks of feeding relationships among species within habitats. However, detailed food-web data and analyses are lacking for ancient ecosystems, largely because of the low resolution of taxa coupled with uncertain and incomplete information about feeding interactions. These impediments appear insurmountable for most fossil assemblages; however, a few assemblages with excellent soft-body preservation across trophic levels are candidates for food-web data compilation and topological analysis. Here we present plausible, detailed food webs for the Chengjiang and Burgess Shale assemblages from the Cambrian Period. Analyses of degree distributions and other structural network properties, including sensitivity analyses of the effects of uncertainty associated with Cambrian diet designations, suggest that these early Paleozoic communities share remarkably similar topology with modern food webs. Observed regularities reflect a systematic dependence of structure on the numbers of taxa and links in a web. Most aspects of Cambrian food-web structure are well-characterized by a simple “niche model,” which was developed for modern food webs and takes into account this scale dependence. However, a few aspects of topology differ between the ancient and recent webs: longer path lengths between species and more species in feeding loops in the earlier Chengjiang web, and higher variability in the number of links per species for both Cambrian webs. Our results are relatively insensitive to the exclusion of low-certainty or random links. The many similarities between Cambrian and recent food webs point toward surprisingly strong and enduring constraints on the organization of complex feeding interactions among metazoan species. The few differences could reflect a transition to more strongly integrated and constrained trophic organization within ecosystems following the rapid diversification of species, body plans, and trophic roles during the Cambrian radiation. More research is needed to explore the generality of food-web structure through deep time and across habitats, especially to investigate potential mechanisms that could give rise to similar structure, as well as any differences.

Se ha venido dando por cierto que la biodiversidad ha incrementado desde el origen de La Tierra. En principio, el patrón más aceptado por los científicos fue denominado Curva de Sepkoski El ya fallecido J. John Sepkoski, fue Catedrático de Paleontología de la Universidad de Chicago, y recopiló los datos que dieron nombre al patrón que detectó. Abajo os mostramos su famosa curva. Sin embargo, como podréis observar, recientes investigaciones publicadas en la revista Science y recogidas en dos notas de prensa por Terra Daily en este y también en este enlace ofrecen un cuadro totalmente distinto. Abajo incluimos las dos.  Al menos desde hace cientos de millones de años, no parece haber existido incremento de biodiversidad alguno en la faz del planeta, como mínimo en los océanos.

Diversidad de los fondos oceánicos durante el cretácico

No se trata de un descubrimiento intrascendente, ni mucho menos. De ser corroborado nos indicaría que la capacidad de carga de especies de invertebrados en los océanos de La Tierra parece haberse saturado, o al menos estabilizado, desde antes de finales del Cretácico. Hasta la fecha se suponía que la variedad de organismos que yacen en la tierra creció lentamente al principio, para adoptar luego un crecimiento exponencial. Por el contrario, los nuevos resultados corroboran los previamente compilados por Spekoski en lo que concierne a que los trópicos han sido más ricos en especies de invertebrados marinas desde hace 450 millones de años que las latitudes elevadas.  Resumiendo, de haber existido un incremento de diversidad a lo largo del Fanerozoico, este ha sido poco significativo.

Curva de Sepkoski Fuente Bulletin of American Paleonlology

Tal hecho no significa que la biodiversidad global no se alterara. Muy por el contrario, la nueva curva detecta más bien una curva ondulante alrededor de un valor medio, con épocas de pérdidas y ganancias. Nadie duda de que se dieran extinciones masivas seguidas por súbitos incrementos del número de especies (radiaciones). Todavía se desconoce si los orígenes de tales cambios fueron causados por eventos catastróficos (impactos de grandes meteoritos, gigantescas erupciones volcánicas, fragmentación y fusión de los continentes) y/o cambios climáticos de gran magnitud y/o por la propia dinámica interna de la biosfera, como sistema complejo. Eso si, los ensamblajes locales en las comunidades locales se asemejan mucho  a lo largo de los últimos cientos de millones de años. ¿Ocurrió lo mismo en las masas de tierra emergidas?. No lo sabemos a “ciencia cierta”.

Extinciones causadas por catástrofes naturales

Fuente Nacional Science Fundation

El nuevo estudio parte de mejores bases de datos, potencia computacional, colaboración de numerosos investigadores de varios países etc. Empero el punto clave son los inventarios más completos. De nuevo vuelve a la palestra la necesidad de inventariar adecuadamente los recursos naturales, una actividad denostada, que se traduce en escaso número de publicaciones y el descrédito ante sus colegas de científicos que se han afanado durante años, casi en silencio, en recopilar evidencias. Como ya argumentamos en otro post, la crisis de la taxonomía ha generado que la masa crítica de taxónomos y paleontólogos sí decreciera exponencialmente (en el sentido amplio del término). Tal hecho no se encuentra justificado por criterios lógicos. De seguir así, a medio plazo pocos descubrimientos más de esta guisa podrán salir a la palestra. Nos topamos de nuevo con la irrazonable racionalidad de la ciencia. Más valen 100 papers insustanciales que un buen trabajo de investigación. ¡Falso!.          

Huella de un Dinosaurio Jurásico

Species Have Come And Gone At Different Rates Than Previously Believed

by Staff Writers; Santa Barbara CA (SPX) Jul 09, 2008

Diversity among the ancestors of such marine creatures as clams, sand dollars and lobsters showed only a modest rise beginning 144 million years ago with no clear trend afterwards, according to an international team of researchers.  This contradicts previous work showing dramatic increases beginning 248 million years ago and may shed light on future diversity. "Some of the time periods in the past are analogies for what is happening today from global warming," says Jocelyn Sessa of Penn State. "Understanding what happened with diversity in the past can help us provide some prediction on how modern organisms will fare. If we know where we have been, we know something about where it will go."

J. John Sepkoski

Using contemporary statistical methods and a paleobiology database, the researchers report in the July 4 issue of Science, a new diversity curve that shows that most of the early spread of invertebrates took place well before the Late Cretaceous, and that the net increase through the period since, is proportionately small relative to the 65 million years that elapsed. The research team was led by John Alroy of the University of California at Santa Barbara.

One key to the new curve is the Paleobiology Database, housed at the National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara. Previous research was based on databases of marine invertebrate fossils that recorded only the first occurrence of an organism and the last occurrence of the organism. There was no information in between for the organism. "Over 30 years ago, researchers looked at the curve they had and considered that perhaps diversity did not increase at all," says Mark E. Patzkowsky of Penn State. "What researchers saw was the diversity curve leveled off for quite some time and then took off exponentially. However, diversity results are strongly controlled by sampling techniques."

The new database allows researchers to standardize sample size because it includes multiple occurrences of each fossil. Researchers can randomly choose equal samples from equal time spans to create their diversity curve. This new curve uses 11 million-year segments, but the researchers hope to reduce the time intervals to 5 million years to match the interval of the previous curve, known as Sepkoski. The data for this study contains 284,816 fossil occurrences of 18,702 genera that equals about 3.4 million specimens from 5384 literature sources. The old curve, developed by J. John Sepkoski Jr., used a database that contained only about 60,000 occurrences.

The researchers also looked at evenness in diversity. If there are 100 specimens divided into 10 time intervals, they could be divided with 10 individual specimens in each interval, or 91 specimens could be in one interval with one each in the remainder. The more even the distribution, the higher the evenness.

"Evenness says something about resource distribution," says Patzkowsky. "Much of invertebrate diversity has been attributed to diversity increase in the tropics, but the curve is not driven by that totally. It seems that 450 million years ago was not so different from today because it also contained more diversity in the tropics. "The major points of the Sepkoski curve are still seen in the new curve. Some things that are not seen, such as the decrease in diversity due to the Cretaceous Tertiary (KT) extinction 65 million years ago are not visible because of the scale of the intervals used. The extinction and recovery in the KT took less than 11 million years and so do not show.

Some things not seen on the Sepkoski curve include a peak in the Permian. Also unexpected is that the diversity in the Jurassic (206 to 144 million years ago) is lower than diversity in the Triassic (248 to 206 million years ago), indicating a dip and rise in the diversity curve.  The curve then rises in the Cretaceous and remains more or less flat after that. The previously thought exponential increase in diversity is not there.

"Comparing diversity through time is about how our world works, about the origin of species and how diversity changes with temperature," says Sessa. "If we think that the net increase over time will not get much greater, things are very different from if the diversity increases exponentially."

Species Diversity Less Dramatic Than Believed

by Misha Kidambi;College Station TX (SPX) Jul 10, 2008

The analysis helped the researchers conclude that the increase in species diversity through the Phanerozoic Era was much less dramatic than previously believed.

A study published in the current issue of Science challenges the long-held belief that diversity of marine species has been increasing continuously since the origin of animals. Dr. Thomas D. Olszewski, a geology and geophysics professor at Texas A and M University, has been a part of the international team that carried out this decade-long study, which concludes that most of the diversification occurred early on - relatively speaking.

"The general understanding for many decades has been that since the rise of the modern major groups of animals about 545 million years ago (i.e., since the beginning of the Phanerozoic Era), the diversity of animal life in the seas has undergone a roughly four-fold exponential increase," says Olszewski. A steep increase in the diversity was believed to have occurred only between 145 million and 60 million years ago. But many paleontologists were doubtful about the accuracy of this theory, which was derived using older methods.

Olszewski explains that the older methods did not account for many important occurrences in the history of the Earth, including changes in the geography of Earth due to continental drift and variations in the state of global climate. Collaborative efforts of 35 researchers from the U.S., Germany, the UK, France and Slovakia resulted in a more accurate interpretation of the prehistoric data. Olszewski says that the researchers used a "fundamentally new analysis, which differs in several important aspects from the previous [methods used for] understanding of the history of marine diversity."

The analysis helped the researchers conclude that the increase in species diversity through the Phanerozoic Era was much less dramatic than previously believed. "Diversity levels comparable to the present day appear to have been reached after a few tens of millions of years following the first appearance of modern animal groups," says Olszewski. The new fossil data also indicate that the current pattern of distribution of life - with low species diversity in the poles and a very high diversity in the tropics - was established some 450 million years ago. With the huge amount of data that was used for the analysis, (fossil occurrences representing nearly 3.5 million specimens) it also became possible to assess the diversity changes in local ecological communities as well as in that of the global total.

Again, the researchers concluded that local ecological communities have changed relatively little since the establishment of marine animal ecosystems during the Phanerozoic Era. So what bearing do the study conclusions have on the life on Earth today? Maybe a great deal, Olszewski says. "As global climatic conditions change, either naturally or anthropogenically, (animal) life responds, which in turn can influence human life," says Olszewski. "Understanding what life was like under different conditions can help us assess and prepare for the consequences of this ongoing change," he adds.

 Juan José Ibáñez