"Long before it's in the papers"
May 16, 2011

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Species are to ecosystems as cells are to the body, study asserts

May 16, 2011
Courtesy of Car­los III Uni­vers­ity of Ma­drid
and World Science staff

An ec­o­sys­tem is like a great or­gan­ism, in that the spe­cies mak­ing it up act as cells do with­in the body, three scient­ists claim.

Bas­ing their find­ings on a math­e­mat­i­cal stu­dy, they say the the spe­cies in an ec­o­sys­tem form a per­ma­nent ent­ity though the spe­cies are con­stantly be­ing sub­sti­tut­ed, even to the point of a to­tal change­o­ver.

“The spe­cies change, but the struc­ture does not”—much like the change that oc­curs with the cells in a per­son, said José A. Cues­ta of Car­los III Uni­vers­ity of Ma­drid, one of the au­thors of the stu­dy. The work is pub­lished in the Jour­nal of Theo­reti­cal Biol­ogy.

Cues­ta and col­leagues cre­at­ed a math­e­mat­i­cal mod­el that mimics the be­hav­ior of an ec­o­sys­tem un­der dif­fer­ent situa­t­ions. The model was de­signed to let the re­search­ers watch ideal­ized ec­o­sys­tems form and de­vel­op. The mod­el showed that an ec­o­sys­tem forms as it is in­vad­ed by new spe­cies, the re­search­ers said. But at some point the ec­o­sys­tem be­comes “ro­bust” and no long­er per­mits fur­ther ad­di­tions to its struc­ture, al­though it does al­low an ex­change of parts.

The find­ings al­so in­di­cate that mul­ti­cel­lu­lar be­ings can be seen as ec­o­sys­tems, they added. That is, we are formed by dif­fer­ent types of cells that co­op­er­ate and com­pete for re­sources; we are col­o­nized by di­verse types of bac­te­ria whose ac­ti­vity is linked to oth­er pro­cesses in our or­gan­ism: we are in­vad­ed by vi­rus­es, which can be harm­ful or can take part in pro­cesses that reg­u­late our DNA. 

“These be­ings are con­stantly be­ing changed, in such a way that af­ter a long enough time passes, all of the ent­i­ties that form us have been sub­sti­tut­ed one or more times. Nev­er­the­less, through­out the pro­cess, we con­tin­ue to be our­selves. This is the same thing that hap­pens with ec­o­sys­tems,” said Cues­ta.

The key im­plica­t­ion of the find­ing is that it forc­es us to see ec­o­sys­tems in a dif­fer­ent light, as self-con­tained ent­i­ties rath­er than as col­lec­tions of spe­cies, the au­thors said. They ar­gue that even more im­por­tant than pre­serv­ing in­di­vid­ual spe­cies is pre­serv­ing ec­o­sys­tems. Seen this way, for ex­am­ple, at times it could be ben­e­fi­cial to sub­sti­tute an en­dan­gered spe­cies with anoth­er one – with si­m­i­lar in­ter­ac­tions with the oth­er spe­cies in the ec­o­sys­tems – so that the ec­o­sys­tem won’t be threat­ened, be­cause then we would lose one spe­cies, but we would save the ec­o­sys­tem.

Sci­en­tists of­ten make a dis­tinc­tion be­tween spe­cies and en­vi­ron­ment. But it’s im­por­tant to real­ize, and the study em­pha­sizes, that the spe­cies them­selves form the en­vi­ron­ment, Cues­ta and col­leagues said. “Math­e­mat­i­cal mod­els turn out to be very help­ful for fo­cus­ing on the type of da­ta to be gath­ered in or­der to ver­i­fy hy­pothe­ses,” he added. “In fact, math­e­mat­i­cal ecol­o­gy has a long tra­di­tion in this dis­ci­pline and ecol­o­gists them­selves are do­ing very in­ter­est­ing things by ap­ply­ing math­e­mat­i­cal tech­niques that were de­vel­oped for use with oth­er phe­nom­e­na.”

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An ecosystem is like a great organism, in that the species making it up act as cells do within the body, three mathematicians claim. Basing their findings on a mathematical study, they say the the species in an ecosystem form a permanent entity though the species are constantly being substituted, even to the point of a total changeover. “The species change, but the structure does not”—much like the change that occurs with the cells in a person, said José A. Cuesta of Carlos III University of Madrid, one of the authors of the study. Cuesta and colleagues created a mathematical model that recreates the behavior of an ecosystem under different situations and let them “observe” ecosystems as they form and develop. The model showed that an ecosystem forms as it is invaded by new species, the researchers said. But at some point the ecosystem becomes “robust” and no longer permits further additions to its structure, although it does allow an exchange of parts. The findings also indicate that multiccellular beings can be seen as ecosystems, they added. That is, we are formed by different types of cells that cooperate and compete for resources; we are colonized by diverse types of bacteria whose activity is linked to other processes in our organism: we are invaded by viruses, which can be harmful or can take part in processes that regulate our DNA. “These beings are constantly being changed, in such a way that after a long enough time passes, all of the entities that form us have been substituted one or more times. Nevertheless, throughout the process, we continue to be ourselves. This is the same thing that happens with ecosystems,” said Cuesta. The key implication of the finding is that it forces us to see ecosystems in a different light, as self-contained entities rather than as collections of species, the authors said. They argue that even more important than preserving individual species is preserving ecosystem. Seen this way, for example, at times it could be beneficial to substitute an endangered species with another one – with similar interactions with the other species in the ecosystems – so that the ecosystem won’t be threatened, because then we would lose one species, but we would save the ecosystem. Scientists often make a distinction between species and environment. But it’s important to realize, and the study emphasizes, that the species themselves form the environment, Cuesta and colleagues said. “Mathematical models turn out to be very helpful for focusing on the type of data to be gathered in order to verify hypotheses,” he added. “In fact, mathematical ecology has a long tradition in this discipline and ecologists themselves are doing very interesting things by applying mathematical techniques that were developed for use with other phenomena.”