EASONSBY ANGELICA LORENZOART BY ANMEI LITTLEONTHEHOW SEASONAL DIET CHANGESIMPACT POPULATION SIZES OFSAVANNA HERBIVORESAVANNA
EcologyFOCUSconsume, and nonmigratory grazers sufferfrom the lack of plant abundance. Not onlyis grass less abundant during the dry season,but it is also less nutritious, leading speciesthat switch between grass and tree diets tobe more likely to obtain their food fromtrees during the dry season. Because of theirreluctance to change diets, nonmigratorygrazers experience a substantial declinein population sizes during droughts. “Thetwo types of herbivores that actually dobecome abundant—mixed feeders andmigratory grazers—do so by… actively[having] strategies to ensure that theyhave a better dry season than they wouldotherwise,” Staver said.Theoretical ModelsA breathtaking landscape of the Kruger National Park.If you have ever seen Disney’s timehonoredfamily feature The LionKing, you likely have a good pictureof what an African savanna looks like.The African savanna ecosystem is hometo a large and diverse community ofmegafauna, or large terrestrial mammals,whose populations have undergoneserious declines as a result of manycomplex factors, such as predator-preyrelationships, disease, and drought.While the impact of these variables onthe population dynamics of savannaherbivores have been well-studied byecologists, the dietary strategies of thesecommunities have only just recently beeninvestigated as a determining factor ofpopulation abundances.Seeking to better understand thisdeterminant, Carla Staver, AssociateProfessor of Ecology and EvolutionaryBiology at Yale University, and GarethP. Hempson, a postdoctoral fellowat the University of WitwatersrandJohannesburg, examined the effects thatseasonal dietary changes have on thepopulations of savanna herbivore species.After composing two different theoreticalmathematical models and evaluatingdata from several African savanna parks,Staver and Hempson found that speciesthat switch their diets seasonally, inwww.yalescientific.orgaddition to species that migrate to findbetter forage, have increased populationsizes and dominate the savannas.Grazing and BrowsingIMAGE COURTESY OF WIKIMEDIA COMMONSThe savannas of Africa, characterizedby trees and grasslands, boast biodiversehabitats that support herbivores such asbrowsers, who feed on tree leaves andshrubs, grazers, who feed on grass andother low-lying vegetation, and mixedfeeders, who alternate between browsingand grazing depending on the season.Browsers, such as giraffes, and grazers, suchas zebras and wildebeests, are consideredspecialists because of their efficiency ateating a particular, albeit limited, diet.On the other hand, mixed feeders, suchas impala and elephants, are defined asgeneralists because of their ability to adaptand survive off a varied diet.Savannas are highly seasonal systems,meaning they alternate between a wet seasonwith heavy rainfalls and a dry season withlittle to no rainfall. During the wet season,grazers enjoy bountiful plant growth. Butin the dry season, when grass is sparse,herbivores of the savanna tend to undertakethree different practices: mixed feeders altertheir food source and switch to browsing,migratory grazers locate new foliage toReinforcing their theory that migratorygrazers and mixed feeders maintainhigher population abundances comparedto nonmigratory specialists, Staver andHempson presented two mathematicalmodels in a paper published in ScienceAdvances that illustrate the role thatdietary strategies play in populationdynamics. The first model, a discretetimepopulation model, takes into accountthe variance of population sizes betweenwet and dry season and finds the overallrate that the populations of herbivoresincrease by. Specifically, the model usesthe geometric mean—a form of averagingfound by multiplying rather than addingnumbers—of wet and dry season growthrates to establish the comprehensivegrowth rate of a population.PHOTOGRAPH COURTESY OF PROFESSOR STAVERThis photograph captures a group of impala, aclassic example of a mixed feeder herbivore.December 2020 Yale Scientific Magazine 23
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