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Termites, not lions, may be kings in African grassland

May 25, 2010
Courtesy of Public Library of Science
and World Science staff

The ma­jes­tic an­i­mals most closely as­so­ci­at­ed with the Af­ri­can grass­land – li­ons, ele­phants and gi­raffes – aren’t ne­ces­sarily the ma­jor play­ers in ac­tu­ally shap­ing the land­scape, ecol­o­gists say.

In­stead, they ar­gue, the real king of the sa­van­na, or flat grass­land, may be the lowly ter­mite.

A sat­el­lite im­age, us­ing in­fra­red light, show­ing (in red) ter­mite mounds and as­so­ci­at­ed ar­eas of high pro­duc­tiv­i­ty. Larger red zones are aban­doned cat­tle cor­rals. The white rec­tan­gle en­com­passes the ar­ea, about 600 me­ters on a side, mapped for the anal­y­sis. (Im­age cour­te­sy Pringle et al., P­LoS Bi­ol­o­gy)

In a new study, Rob­ert M. Pringle of Har­vard Uni­vers­ity and col­leagues found that this in­sect con­tri­butes mightily to a grass­land in cen­tral Ken­ya. Ter­mite mounds greatly en­hance plant and an­i­mal ac­ti­vity at a lo­cal lev­el, while their even dis­tri­bu­tion over a larg­er ar­ea max­i­mizes ec­o­sys­tem-wide pro­duc­ti­vity, or bi­o­mass.

The find­ings, to appear next week in the on­line re­search jour­nal PLoS Bi­ol­o­gy, af­firm a coun­ter­in­tu­itive ap­proach to ecol­o­gy: of­ten it’s the small things that have the most im­pact.

Pri­or re­search on the Ken­ya dwarf geck­o in­i­tially drew Pringle’s at­ten­tion to the pe­cu­liar role of grassy ter­mite mounds, which in this part of Ken­ya are some 10 me­ters (yards) wide and spaced some 60 to 100 me­ters apart. Each mound teems with ter­mites, who build the mounds over cen­turies. 

Af­ter ob­serv­ing un­ex­pectedly high num­bers of lizards near mounds, Pringle and his col­leagues be­gan to meas­ure ec­o­log­i­cal pro­duc­ti­vity rel­a­tive to mound dens­ity. They found that each mound sup­ported dense ag­grega­t­ions of plants and animals: plants grew more rap­idly the clos­er they were to mounds, and an­i­mal popula­t­ions and repro­duc­tive rates fell off ap­pre­ciably with great­er dis­tance. 

What was ob­served on the ground was even clear­er in sat­el­lite im­age­ry, the sci­en­tists said. Each mound – fairly in­con­spic­u­ous on the Ken­yan grass­land – stood at the cen­ter of a burst of flo­ral pro­duc­ti­vity. More im­por­tant­ly, these bursts were highly or­gan­ized in rela­t­ion to one an­oth­er, evenly dis­persed as if squares on a check­er­board. The re­sult, said Pringle, is an op­ti­mized net­work of plant and an­i­mal out­put closely tied to the or­dered dis­tri­bu­tion of ter­mite mounds.

“In es­sence, the highly reg­u­lar spa­tial pat­tern of fer­tile mounds gen­er­at­ed by ter­mites ac­tu­ally in­creases overall lev­els of ec­o­sys­tem pro­duc­tion. And it does so in such a pro­found way,” said Todd M. Palm­er, Uni­vers­ity of Flor­i­da bi­ol­o­gist and an af­fil­i­ate of the Mpala Re­search Cen­tre in Nanyuki, Ken­ya. “Seen from above, the grid-work of ter­mite mounds in the sa­van­na is not just a pret­ty pic­ture. The over-dispersion, or reg­u­lar dis­tri­bu­tion of these ter­mite mounds, plays an im­por­tant role in el­e­vat­ing the ser­vic­es this ec­o­sys­tem pro­vides.”

Just how ter­mite ac­ti­vity is trans­formed in­to far-reach­ing ef­fects on the ec­o­sys­tem is com­plex, the re­search­ers said. Pringle and Palm­er sus­pect ter­mites im­port coarse par­t­i­cles in­to the oth­er­wise fi­ne soil in the vicin­ity of their mounds. These coars­er par­t­i­cles pro­mote wa­ter in­filtra­t­ion of the soil, even as they dis­cour­age dis­rup­tive shrink­ing and swell­ing of top­soil in re­sponse to pre­cipita­t­ion or drought. The mounds al­so show high lev­els of nu­tri­ents such as phos­pho­rus and ni­tro­gen. All this ben­e­fi­cial soil al­tera­t­ion seems to di­rectly and indi­rectly mold ec­o­sys­tem ser­vic­es far be­yond the im­me­di­ate vicin­ity of the mound.

“Ter­mites are typ­ic­ally viewed as pests,” Pringle said. “But pro­duc­ti­vity – of both wild and human-dominated land­scapes – may be more in­tri­cately tied to the pat­tern-generating or­gan­isms of the larg­er nat­u­ral land­scape than is com­monly un­der­stood.”

The find­ings al­so have im­por­tant im­plica­t­ions for con­serva­t­ion, Palm­er said. “As we think about re­stor­ing de­grad­ed ec­o­sys­tems, as we think about re­stor­ing cor­al reefs, or re­stor­ing plant com­mun­i­ties, this over-dis­persed pat­tern is teach­ing us some­thing,” he said. “It’s say­ing we might want to think about do­ing our cor­al restora­t­ion or plant restora­t­ion in a way that takes ad­van­tage of this ec­o­sys­tem pro­duc­ti­vity-enhancing phe­nomenon.”

* * *

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The majestic animals most closely associated with the African grassland – lions, elephants and giraffes – aren’t the major players in actually shaping the web of relationships among organisms and the landscape, ecologists say. Instead, they argue, the real king of the savanna, or flat grassland, may be the lowly termite. Ecologists have found that this insect contributes mightily to a grassland in central Kenya. Robert M. Pringle of Harvard University and colleagues found that termite mounds greatly enhance plant and animal activity at a local level, while their even distribution over a larger area maximizes ecosystem-wide productivity, or biomass. The findings, publishing next week in the online research journal PLoS Biology, affirm a counterintuitive approach to ecology: often it’s the small things that have the most impact. Prior research on the Kenya dwarf gecko initially drew Pringle’s attention to the peculiar role of grassy termite mounds, which in this part of Kenya are some 10 meters (yards) wide and spaced some 60 to 100 meters apart. Each mound teems with termites, who build the mounds over centuries. After observing unexpectedly high numbers of lizards near mounds, Pringle and his colleagues began to measure ecological productivity relative to mound density. They found that each mound supported dense aggregations of flora and fauna: plants grew more rapidly the closer they were to mounds, and animal populations and reproductive rates fell off appreciably with greater distance. What was observed on the ground was even clearer in satellite imagery, the scientists said. Each mound – relatively inconspicuous on the Kenyan grassland – stood at the center of a burst of floral productivity. More importantly, these bursts were highly organized in relation to one another, evenly dispersed as if squares on a checkerboard. The result, said Pringle, is an optimized network of plant and animal output closely tied to the ordered distribution of termite mounds. “In essence, the highly regular spatial pattern of fertile mounds generated by termites actually increases overall levels of ecosystem production. And it does so in such a profound way,” said Todd M. Palmer, University of Florida biologist and an affiliate of the Mpala Research Centre in Nanyuki, Kenya. “Seen from above, the grid-work of termite mounds in the savanna is not just a pretty picture. The over-dispersion, or regular distribution of these termite mounds, plays an important role in elevating the services this ecosystem provides.” The mechanism through which termite activity is transformed into far-reaching effects on the ecosystem is complex, the researchers said. Pringle and Palmer suspect termites import coarse particles into the otherwise fine soil in the vicinity of their mounds. These coarser particles promote water infiltration of the soil, even as they discourage disruptive shrinking and swelling of topsoil in response to precipitation or drought. The mounds also show high levels of nutrients such as phosphorus and nitrogen. All this beneficial soil alteration seems to directly and indirectly mold ecosystem services far beyond the immediate vicinity of the mound. While further studies will explore the mechanism through which these spatial patterns of termite mounds emerge, Pringle and Palmer suggest that the present work has implications beyond the basic questions of ecology. “Termites are typically viewed as pests, and as threats to agricultural and livestock production,” Pringle said. “But productivity – of both wild and human-dominated landscapes – may be more intricately tied to the pattern-generating organisms of the larger natural landscape than is commonly understood.” The findings also have important implications for conservation, Palmer said. “As we think about restoring degraded ecosystems, as we think about restoring coral reefs, or restoring plant communities, this over-dispersed pattern is teaching us something,” he said. “It’s saying we might want to think about doing our coral restoration or plant restoration in a way that takes advantage of this ecosystem productivity-enhancing phenomenon.”