Movingbeyondtheresourcedispersionhypothesis
EloyRevilla
Estacio´nBiolo´gicadeDon˜anaCSIC,AvenidadeMarı´aLuisas/n,E-41013Seville,Spain
In a recentarticlein TREE[1], Johnsonet al.review theevidenceshowingthat,whenresources are patchily distributed,the economics of exploitingthese patches enableindividuals toshare resourcesovera common area, satisfying their needs without imposinglarge costson each other.This resourcedispersionhypoth- esis (RDH) is presented as a causal mechanism of groupliving,especially whenindividualsobtainno apparent benefits from such living. There are two pointson whichI would like tocomment.
First, the authors only refer to the evidence in
favouroftheRDH. However,in no singlecase areall theassumptionsand predictions satisfied (Table 1 in [1]). To test the RDH, all the assumptionsmust be fulfilledand,then,only a simultaneouspositivetestof allitspredictionswould validate the hypothesis[1]. However,we need evidence againstonly one ofits assumptionsand/orpredictionsforittobeinvalid.This evidenceexists [2]. For example,in badgers (a RDH model species), there is a high costofliving in groups [3–5], theterritorialityand use of space by different individuals are affectedby different resources [6–8], and decreasesinfoodavailability increaseterritory overlap [9]. These pieces of evidenceinvalidatethe assumptionoftherebeing no cost to groupliving,that the determinant of space use is the same for all individuals,and thatgroup rangesareexclusive, respectively.Neither was therea consistent relation- ship found betweenterritory size and resource dis- persion,nor was groupsize consistently relatedto resourcerichnessin one site[10]. In anotherstudy,a decreaseinfoodavailability wasfollowedbyterritory expansion rather than by the expectedgroupsize reduction [9].Furthermore,inonegroup-livingpopu- lation,territorysize was relatedtoits richnessand not topatchdispersion, whereasan adjacentpopulation, sufferingthesamestrongseasonalresourceheterogen- eity,was solitaryliving [8].
Second, from its conceptual and mathematical
definition,the RDH providesan explanation for why there isasurplusofresourcesinsideoneterritory. Although thisis important,thereis a largeconceptual gap betweenthisexplanationand demonstratingthat thissurplusisthecausal meansofgroupformation. Otherwise, in our heterogeneous world, we would observemanymore speciesliving in groups.
Correspondingauthor:EloyRevilla().
Current evidence shows that resource patchiness alone isnot enoughtoexplaingroupliving.In ecology, the adaptiveprocessofunderstandingincludesthefirst step of developing theory based on empirical infor- mation,followed bytesting oftheoreticalpredictions. When there are conflictsbetween empirical evidence and predictions, thetheoryhas toberefinedto accommodatethe new knowledge. I thinkthat it is now time to move beyond the RDH as a casual mechanismofgroupliving.
References
1 Johnson,D.D.P. etal. (2002)Doesthe resourcedispersionhypothesis explaingroupliving? TrendsEcol.Evol.17,563–570
2Revilla,E.(2003)Doesthe resourcedispersion hypothesisexplain anything?Oikos101,428–432
3 Macdonald,D.W.etal. (2002)Density-dependenceregulationofbody mass and conditioninbadgers(Melesmeles)fromWythamwoods. Ecology83,2056–2061
4 Silva, J.D. et al. (1994) Net cost of group living in a solitary forager, the Eurasian badger (Meles meles). Behav. Ecol. 5,
151–158
5 Woodroffe, R.and Macdonald, D.W.(1995)Female/femalecompetition inEuropeanbadgersMelesmeles:effectsonbreedingsuccess.J.Anim. Ecol.64,12–20
6 Tuyttens, F.A.M. et al. (2000) Spatial perturbation causedby a badger (Meles meles) culling operation: implications for the functionofterritorialityand thecontrol ofbovine tuberculosis (Mycobacteriumbovis). J. Anim. Ecol. 69, 815–828
7 Revilla, E. and Palomares, F. (2001) Differences in key habitat use between dominantand subordinateanimals:intraterritorial dominance payoffs in Eurasian badgers? Can. J. Zool. 79,
165–170
8 Revilla,E.and Palomares,F.(2002)Spatialorganization,groupliving and ecological correlatesinlow-density populationsofEurasian badgers,Melesmeles. J.Anim. Ecol.71,497–512
9 Kruuk,H. and Parish,T.(1987) Changesin the size ofgroupsand rangesofEuropeanBadger(Meles meles L.)in an areain Scotland. J.Anim. Ecol.56,351–364
10 Johnson, D.D.P. etal.(2001)Long-termresource variationand group size:alargesamplefieldtest ofthe ResourceDispersionHypothesis. BMCEcology1,2(