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A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders FEATURE ARTICLE Modern Lessons from Ancient Food Webs From the Cambrian Burgess Shale to ancient Egypt, food webs share surprising structural attributes. When redundancy is lost, the threat of extinction grows. Justin D. Yeakel and Jennifer A. Dunne bout 10,000 years ago, one patterns of feeding interactions be- systems is more complete when we could have mistaken the tween and among species (food web unite the detailed but temporally lim- Egyptian landscape for that structure) and changes in species’ ited knowledge of modern ecological of East Africa today. Instead abundance over time (dynamics) of systems with the wealth of informa- Aof vast arid deserts, the region north of the Egyptian mammal community is tion obtained from historical and pale- Aswan held enough annual precipita- providing insight into how this ecosys- ontological contexts. tion to support large herbivores that tem unraveled in the face of both cli- Recent investigations into the struc- are strongly tied to standing bodies of matic and human-induced pressures. ture and dynamics of past and contem- water, including zebra, elephant, and But the observed community collapse porary food webs has shown that there rhinoceros. Lions, wild dogs, giraffes, in Egypt is just a small piece of a much is a certain fixedness in the patterns and wildebeest filled out the savanna- larger story that describes the rise and of species interactions, independent woodland landscape, while the earliest fall of animal communities over the of species identity, habitat, and time. pyramids were still thousands of years last 540 million years of multicellular However, this fixed structure appears in the future. This was Egypt during life on Earth. to be sensitive to external disturbances, the African Humid Period, which end- As with the analysis of ancient in particular large-scale events such as ed roughly 5,500 years ago, when the Egyptian ecosystems, a central goal in mass extinctions. The structural fea- moisture supplied to equatorial Africa ecology is to understand how animal tures common to food webs, as well by seasonal monsoons declined, result- and plant communities arise, change, as their sensitivity to disturbances, are ing in an increasingly arid climate. and respond to disturbances both large relevant to our ability to predict (and It was just after the African Humid and small. To that end, ecologists have perhaps engineer) the fate of modern Period that urban centers began to de- adopted new tools designed to inves- biological systems. Integrating con- velop around the Nile Valley. As hu- tigate ecological networks and help us temporary ecology with observations mans shifted to agriculture and popu- uncover and understand past extinc- of the past will provide key insights lation densities grew, the local animal tions and their community-level con- into the future risks and uncertainties community became increasingly mar- sequences in the hopes of predicting facing the multitude of species with ginalized. The cumulative evidence of those in the future. To do so, we com- which we share the planet. mammalian species present in Egypt pare reconstructions of past ecological before the end of the African Humid networks, such as food webs, to mod- Food Webs in Deep Time Period, taking into account paleonto- ern biological communities in an effort The behaviors of all evolutionarily suc- logical and archeological observations, to identify similar drivers of change. cessful organisms are constrained by reveals 37 larger-bodied species occu- Analysis of the structure and func- two major dictates: They must pass pying the region. Of those 37, only 8 tion of modern and paleontological down their genetic material, and they remain today. This massive loss of bio- ecosystems falls within the bounds must acquire the necessary energy to do diversity likely resulted from a combi- of complex systems science, a bur- so. Plants and other organisms that can nation of factors: habitat loss, hunting, geoning suite of research topics that make their own food generate their own and local and regional climate change. unite many aspects of mathematical, energy; herbivores, carnivores, and om- Reconstructing how extinctions physical, and biological sciences. As nivores must find, capture, and consume throughout the Holocene affected the researchers at the Santa Fe Institute their food to acquire energy. In a commu- in New Mexico — a not-for-profit or- nity of many species, the sum total of the Justin D. Yeakel is an Omidyar Postdoctoral Fellow at ganization centered around the study patterns of interactions that characterize the Santa Fe Institute. Jennifer A. Dunne is a profes- of complex adaptive systems— we these processes can appear to be a chaot- sor and the Vice President for Science at the Santa Fe think that our understanding of the ic tangle. In fact, they comprise a highly Institute. E-mail for Dr. Yeakel: [email protected]. underlying organization of natural organized network of who eats whom. 188 American Scientist, Volume 103 © 2015 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. Griffith Institute/University of Oxford Determining how these communi- Depictions of animals, such as this hunting scene from one side of King Tutankhamun’s Painted ties function requires not only knowing Box, built more than 3,000 years ago, document the mammal community of ancient Egypt over which species are present and when, time. Piecing together ancient food webs can help us understand their fundamental properties. but how they interact. In a relatively re- cent development, ecological research- power grids, research on ecological counts of feeding links for each con- ers are now borrowing tools from networks such as food webs shows sumer are normalized (divided by the graph theory and nonlinear dynamics, that they have nonrandom structure. average number of links per species in analyzing the structure of ecological This property implies that there are its food web). communities as networks, with nodes underlying forces that constrain inter- The remarkable similarity of the representing species or groups of sim- actions between species, much as there structure of modern food webs gives ilar species and links representing a are underlying— though different— rise to a tantalizing question: Have type of interaction. For example, food forces constraining the formation food webs always been this way? In webs are ecological networks where of friendships in social networks or other words, is the fundamental struc- links represent a flow of biomass be- transmission lines in power grids. ture of food webs fixed? If food webs tween two species, and the patterns of More surprising, food webs, indepen- have an underlying structure that is these feeding interactions determine dent of locality, habitat, and species maintained across different ecosys- the structure of the community. composition, appear to share a num- tems over evolutionary time, there This nonrandom organization is in ber of regularities in their organiza- may be fundamental rules driving part responsible for the dynamics of tion. For instance, the distribution of ecological interactions independent of the system: how long the system is ex- feeding links across species (the de- species, habitat, or community. If some pected to persist, whether it is sensitive gree distribution, which characterizes underlying mechanism guiding com- to external disturbances, and to what the relative abundance of specialist munity structure exists, they might re- extent it can recover after a disturbance or generalist feeders) is similar across spond similarly to large perturbations. has ended. In addition to a large and food webs, such that most species are In this case, such knowledge could growing body of work on contempo- quite specialized and a very few are directly inform management of con- rary ecological networks, uncovering highly general feeders. temporary ecosystems in the wake of the structure of paleontological or his- Because food-web structure is sys- current or future perturbations. torical communities provides a means tematically sensitive to the numbers To address these questions, one to understand how they evolve and of species and links represented in the must look into the fossil record, and change over long periods of time, and system, when this scale dependence is the Cambrian- era Burgess Shale fauna to gauge how they respond to known accounted for, the remarkable regular- is about as far back in time as preser- large-scale changes in the environment. ity of the degree distribution is uncov- vation allows for ecosystem-level anal- Much like other kinds of real-world ered. In this case, the general pattern ysis. The Burgess Shale was formed networks including social groups and across food webs only emerges when more than a half a billion years ago, www.americanscientist.org © 2015 Sigma Xi, The Scientific Research Society. Reproduction 2015 May–June 189 with permission only. Contact [email protected]. Great Pyramid constructed aridication urban event ends 2nd 3rd expansion African Humid aridication aridication in the Period event event Mediterranean African Humid Period 11,000 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 thousands of years ago millions of years ago Mass Mass Mass Extinction Extinction Extinction Cambrian explosion Permian Cretaceous Eocene 600 550 500 450 400 350 300 250 200 150 100 50 0 millions of years ago millions of years ago when a muddy embankment col- lucigenia. However, it turns out that in communities emerged early in the lapsed on a shallow sea community there are many ways to infer feeding history of multicellular life.