Biological Determinants Of Species Diversity
Biological Determinants of Species DiversityAuthor(s): Avi Shmida and Mark V. WilsonSource: Journal of Biogeography , Jan., 1985, Vol. 12, No. 1 (Jan., 1985), pp. 1-20Published by: WileyStable URL: http://www.jstor.com/stable/2845026JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a widerange of content in a trusted digital archive. We use information technology and tools to increase productivity andfacilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available athttps://about.jstor.org/termsWiley is collaborating with JSTOR to digitize, preserve and extend access to Journal ofBiogeographyThis content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
Journal of Biogeography (1985) 12, 1-20Biological determinants of species diversityAVI SHMIDA and MARK V. WILSON* Department of Botany, The Hebrew University,Jerusalem, Israel, and Section of Ecology and Systematics, Cornell University, Ithaca,New York 14850, U.S.A.ABSTRACT. We consider four categories of biological mechanisms of determinants which cause and maintain species diversity: niche relations, habitat diversity, mass effects and ecological equivalency. Two of these determinants are origi-nal to this paper: mass effect, the establishment of species in sites where theycannot be self-maintaining; and ecological equivalency, the coexistence of specieswith effectively identical niche and habitat requirements. The mode of action andecological implications of each biological determinant are discussed using aschematic method for measuring alpha (community), beta (differentiation), andgamma (regional) diversities. The importance of mass effects and ecologicalequivalency to species richness is documented with several types of field datafrom Israel and California, U.S.A.Floristic richness and, in particular, the richness of floristic transitions, arediscussed and interpreted by use of the biological determinants of diversity.Contact transitions between distinct floras are rich predominantly because ofmass effects. Transitions induced by marked environmental changes are richbecause of the combined influences of habitat diversity and mass effects.The rate at which species richness increases with sample area is related to thecombined effects of all four biological determinants. This complexity explains thefailures of simple species-area models. The relative intensity of each determinantis related to area: niche relations are most important at within-communityscales, habitat diversity most important at both within-community and landscape scales, and ecological equivalency most important at regional scales. Wesuggest that understanding patterns of species diversity will be enhanced by thepartitioning of total species richness into the richness caused by each of the fourecologically distinct determinants of diversity.Introductionand Whittaker, 1972, 1977). No hypothesishas been found to apply well to all bodies ofNumerous hypotheses have been proposed todata. Instead, the hypothesized mechanismsexplain patterns of species diversity (seereviews by Fischer, 1960; MacArthur, 1965;may function together, with their relativeThis work is dedicated to the memory of R. H.Whittaker, who taught us the importance of under-importance dependent on particular circum-stances (Pianka, 1966; Slobodkin & Sanders,1969). In studies of species diversity moststanding natural diversity.attention has been placed on processes within* Present address: Department of Botany andPlant Pathology, Oregon State University, Corvallis,among species. However, no community is aOR 97331-2902, U.S.A.the community, that is, on niche relationsclosed system, isolated from all other sites.1This content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
2 Avi Shmida and Mark V. WilsonWhen communities are viewed as interacting1965, 1972; Sanders, 1969; Terborgh, 1973;locations within an integrated, continuousCody, 1975; Diamond, 1975; Routledge,system several other processes can be1977). We demonstrate that there are addi-examined. We consider three categories:tional biological determinants - mass effecthabitat heterogeneity, mass effect (the estab-and ecological equivalency - that combinelishment of species in sites where they cannotwith niche differentiation and habitat responsebe self-maintaining), and ecological equiva-to produce patterns of diversity.lency (coexistence of species with effectivelyIn order to keep distinct the three aspectsidentical niche and habitat requirements). Forof diversity we follow this convention: Theeach of these, plus the category of niche rela-word diversity alone, or with a prefix of scaletions, we examine the mode of influence on(e.g. alpha-diversity) refers to the phenomenondiversity and the scale of action. Each mech-of diversity; a greek symbol (e.g. a) refers toanism is shown schematically by use of diver-the corresponding measure of diversity; and asity displays. Examples are presenteddeterminant of diversity is represented by thedocumenting the importance of thesymbol D, with a subscript denoting the typemechanisms.of determinant (e.g. DME for mass effect).At least three aspects of diversity problemsThe three fields of classical biogeography,have been emphasized by various workers:quantitative phytosociology (communityphenomena, measurements and mechanisms.analysis), and theoretical population ecologyThe phenomenon of diversity is a characteristicare all concerned, in part, with comparativeof species distributions in communities, and,species diversity. Each of these approaches hasas such, cannot be entirely perceived withoutits own viewpoint and manner of interpretationa complete knowledge of community composi-of the phenomena of diversity. Classicaltion. Most ecological studies have been con-biogeographers are concerned with the relativecerned with the phenomenon of alpha-richnesses of regional faunas or floras (e.g.diversity, the species richness of samplesGrisebach, 1884; Schimper, 1903; Cain, 1944;representing communities (generally 102 -1IOmi2) (Whittaker, 1977). MacArthur (1965)sometimes calculated as number of species perDarlington, 1957; Udvardy, 1969). Richness isand others use the term within-habitat diversityunit area (Wulf, 1950; Good, 1964), leadingas a synonym of alpha-diversity. The diversityto the construction and interpretation ofof landscapes (106 108 M2) is gamma-species-area curves. The similarity betweendiversity. Each level or scale of inventoryregions is calculated by measures of Jaccarddiversity (sensu Whittaker, 1977) is nested(1912), Sorensen (1948) and others (seewithin the higher level diversity. The mannerSimpson, 1949). Classical biogeography stres-in which inventory diversities at one scalesed historical factors (migration, isolation,combine to produce the inventory diversity atspeciation) in the interpretation of diversitythe next larger scale is a differentiation diver-(Udvardy, 1969).sity, the amount of biotic change among units.Quantitative phytosociologists have deve-The differentiation of units on the alpha-loped techniques for assessing diversity asdiversity scale is beta-diversity (Whittaker,dissimilarity or 'ecological distance' between1960), or between-habitat diversitycommunity samples (Goodall, 1952, 1973;(MacArthur, 1965). The measurement ofWhittaker, 1952, 1960, 1977;Williams, 1971;diversity is the manner in which field observa-Gauch, 1973; Mueller-Dombois & El'lenberg,tions of diversity phenomena are summarized.1974). The phytosociological concept of beta-Partly separate from relationships anddiversity or species turnover along environ-measurement is a third aspect, the mechanismsmental gradients has been applied also toor determinants of diversity. The ecologicalanimal communities (MacArthur, 1965; Cody,interpretation of diversity and its measure-1975; Tramer, 1974). Interpretation ofment has historically been simple and clearcut:patterns of diversity in quantitative phytoso-alpha-diversity results from niche differentiation among species and beta-diversity fromspecies responses to a range of habitats(Whittaker, 1960, 1967, 1972; MacArthur,ciology generally consists of the direct correlation of ecological distance with differences inenvironmental conditions, the unexplainedvariance in field data often being labelledThis content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
Biological determinants of species diversity 3'noise' (Whittaker, 1975a). Irregular spatialapproaches. Many island biogeography studiespatterns (Greig-Smith, 1964; Kershaw, 1973),have involved relationships between speciesnon-uniform dispersal (see Harper, 1977), andrichness and area in terms of population-levelhistorical effects (Heinselman, 1973; Loucks,processes (see MacArthur & Wilson, 1967;1970) and sampling error or bias (Bormann,Simberloff, 1974). Other attempts at the syn-1953; Kershaw, 1973) can contribute to thethesis of different approaches to diversityincomplete correlation of vegetation with theinclude Williams' (1964) monumental sum-physical environment.marization of the relationships between speciesTheoretical population ecology has concen-richness and area, and the long-term work oftrated on the role of species interactions,Whittaker (1960, 1969, 1972, 1977) on theespecially competition and predation, in themeasurement and evolution of diversity.explanation of species diversity patterns. NicheBy discussing biological mechanisms, and bytheory (Hutchinson, 1957; MacArthur, 1968;using a single terminology, we attempt toWhittaker, 1969), models of coexistencecombine insights from these traditional fields(MacArthur & Levins, 1967; May & MacArthur,into a more complete understanding of the1972; Vandermeer, 1972), and theories ofphenomena of species diversity.resource partitioning (Shoener, 1974) allfocus on competition of species withincommunities. Because of the concentration byThe diversity displaytheoretical population ecologists on small-scale species interactions, until recently theCertain simplifying assumptions are necessarynotion of noise mostly has not been enter-to facilitate examination of the determinantstained. Conversely, because classical biogeo-of diversity. First, we assume that a true, singlegraphers deal with complete lists of the biotacommunity gradient or coenocline of mono-of large regions, the effects of noise aretonically changing environmental conditionsdecreased and become unimportant. A sum-can be identified for a given set of field data.mary of the differences among these threeAs natural systems typically show the com-approaches to diversity is presented in Tablebined effects of several environmental factors1.the structure of the landscape usually must beAlthough workers in diversity are aware ofsummarized by stratification or compositethe accomplishments in each of the threesamples. Second, we assume that all species inapproaches, the interpretation of diversity isthe system (or a high percentage) are actuallyusually limited to a small set of the possibleobserved in the samples. In practice, thisdeterminants. For example, observations ofassumption is not overly restrictive if the fieldlatitudinal trends in diversity have spawneddata are from exhaustive field studies, typicalnumerous explanations (see reviews by Fischer,of large-scale vegetation analysis or from range1960; MacArthur, 1965, 1969; Pianka, 1966;records, museum specimens and floristicTerborgh, 1973; Rosenzweig, 1975; Osman &manuals that indicate the extremes of speciesWhitiach, 1978) with population ecologistsdistributions.emphasizing species interactions within com-The discussion of diversity relationships ismunities, quantitative phytosociologists em-aided by a pictorial representation or diversityphasizing environmental trends, and biogeo-display (Fig. 1) of species distributions withingraphers concentrating on historical factors.an ecosystem. The abscissa of the diversitySeveral attempts have been made to mergedisplay is a transect along an environmentalthe concepts and terminologies of the differentgradient (or complex-gradient, sensuTABLE 1. Characteristics of three approaches to the study of diversityClassic Phytosociology Theoreticalbiogeography population biologyScale of observation Regional Among-communities mary proposed determinants Historical Environmental Species interactiveRole of noise Irrelevant Important Unobserved. This content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
4 Avi Shm ida and Mark V. Wilsonaand habitat axes, a larger number of species,SixS5and the abundances of these species. TheSBcomplexity prohibits adequate representationin a figure. We choose instead to use a stillS Isimpler version of the diversity display (Fig.lb). Because we are primarily concerned withomeso-scale (among-community) aspects ofS2w S12r S4bdiversity and not the details of species inter-actions within communities, the form ofspecies distributions along the niche axis mayISiJKS8S5sobe reduced to mean values without much lossof important information. Also, because wedeal with single habitat gradients at any onetime and are concerned with the presence orwabsence of species only, the two dimensions59 I.1toiestdsly17) hwna t speiesourneS12of niche and habitat suffice. For the remainderof this paper we use only this simple form ofthe diversity display shown in Fig. l(b). Notethat species distributions along the habitatfodetails.ENVIRONMENTALGRADIENTFIG. 1. Diversity displays showing species occurrencesgradient are in effect summaries of speciesranges in communities in the field,(il5-12) in a hypothetical landscape. (a) Species aredistributed with respect to interacting resource axisand environmental gradient. (b) Simplified diversitydisplay. I, J and K are the sampling points. (See textfor details.)The measurement of diversityDiversities of a system shown in a diversitydisplay are easily measured. Samples areWhittaker, 1967) and the ordinate is a majorresource axis. Many studies use, as leastimplicitly, the framework of the diversitydisplay (e.g. MacArthur & Wilson, 1967: 103).Species' overall occurrences are the enclosedrepresented as vertical segments at pointsalong the environmental gradient. In Fig. l(b),segment J is such a community sample. Theindex of the alpha-diversity of J is simply thenumber of species intercepted by the sampleareas in the field of environmental and resource(Ol 5). The measure of landscape, or gamma-conditions. In the terminology of Whittaker,diversity, is the total number of species in theLevin & Root (1973) the niche of a speciesincludes that range of resources used at adiversity display (ol 12).Measures of beta-diversity with qualitativesingle point along the environmental gradient,(presence/absence) data have been proposed bywhereas the habitat of a species is the range ofWhittaker (1960), Cody (1 975) and Routledgeenvironmental conditions in which the species(1977). We have compared the perfcrmanceis found. The species' ecotope is the intersection of its niche and habitat, or the enclosedof these measures and have proposed a newand superior one (Wilson & Shmida, 1984)area in the diversity diplay of Fig. 1. Fig. 1 (a) that mirrors directly the aspect of beta-diveris an over-simplified pattern of the species dis- sity in which we are most interested here: thetribution within a given community in nature:increase and decrease in species richness alongwe assume no overlapping of ecotopes betweencoexisting species (for a different approachgradients. We will use this new measure, OT,throughout the present paper. OT ('beta-see Shmida & Ellner, 1984). However, it isturnover') is calculated as:important to assume for our idealistic diversitydisplay a monotonic changing environmentalgradient which corresponds with non-disjunctg(H) 1 (H)2clwhere H is the given range of a habitat gradient,species ranges.A more realistic diversity display shouldg(H) is the number of new species encoun-reflect, at the least, more than single nichetered or gained along H, 1 (H) is the numberThis content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
Biological determinants of species diversity 5of species that drop out or are lost along H,1961; Harper, Williams & Sagar, 1965; Cody,and c- is the average number of species found1968; Bratton, 1976), and the temporalin samples along H. 1T can be easily calculatedheterogeneity of resource availabilityfrom species range data. In the diversity display(Whittaker & Niering, 1965; Whittaker, 1972).of Fig. 1(b), OT between samples I and K is:Niche relations influence species diversityat the within-community scale (Whittaker et8 8al., 1973). In a restricted region of more orOT 1.82x4.4less homogeneous environment, niche relationsT can be interpreted as the value that wouldbe obtained in a system of exactly fT com-should not vary systematically through space.That is, niche relations as a determinant ofplete changes in community composition.diversity (denoted DNR) will usually not showThus, in Fig 1(b), an equivalent of 1.8 1 2.8spatial trends, although fluctuations must becommunities are depicted. That is to say thereexpected. Niche characteristics determined byare 1.8 changes and the equivalent of 2.8 com-microsite heterogeneity (Whittaker & Levin,munities. Whenever we mention beta diversi-1977) are possible exceptions to this rule ofties, these are calculated by the T index.independence of niche and space if new kindsof microsites are added along a gradient.The influence on species diversity of alteringDeterminants of diversity: evidence and effectsDNR is shown in the diversity display of Fig.In this section we discuss the contribution ofOl 2.2, ,B 2.05, and y 7. If the combinedeach of four biological determinants to specieseffects of DNR are doubled, with all other2. Fig. 2 represents a landscape system withdiversity. For the well-known determinantsdeterminants of diversity held constant, theniche relations and habitat diversity wenumber of species within a community willexamine only their influence on alpha-, beta-approximately double. (Species richness willand gamma-diversities. For the determinantsnot double exactly because of possible inter-we introduce here, mass effects and ecologicalaction with other determinants of diversity, asequivalency, we also document their impor-explained below.) Fig. 2(b) shows this doublingtance to overall species diversity with fielddata and observations.aSiNiche relationsThe interactions among species and betweenwspecies and environment within a communitydescribe the niches of species. Field observations and theoretical developments on nicherelations have been ably reviewed elsewhere0CI)wdiversity in three broad modes: more speciesvcan be accommodated in communities withhyperspace include the range of food sizesavailable (Cody, 1968; Pianka, 1975; Whittaker,1977), the small-scale structural diversity ofthe environment (MacArthur & MacArthur,S6S7S2of a community have been related to speciesniche overlap. Aspects of the size of the nicheS5S4cn Sigeneral, the niche and resource characteristics-perhaps in communities with high averageS3bColwell & Fuentes, 1975; Pianka, 1975). Inlarger niche hyperspaces, in communities inDS2w(e.g. Levins, 1968; MacArthur, 1972;Whittaker et al., 1973; Schoener, 1974;which average niche breadth is reduced, L GRADIENTFIG. 2. The influence of niche differentiation (DNR)on diversity. Each species occurrence in the systemshown in (a) is duplicated in the system in (b),causing oi and -y to double but leaving ,B unchanged.This content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
6 Avi Shmida and Mark V. Wilsonof DNR by the simple replication of each(MacArthur, 1965) is often used as a synonymspecies by a new species of identical range. Infor beta diversity. Since, as we demonstratethe new system o 4.4, and y 14, twicebelow, phenomena other than habitat hetero-those of Fig. 2(a), while ,B 2.05, unchanged.geneity can cause changes in species composi-Thus DNR affects alpha- and gamma-diversitytions among sites, we prefer to alterbut does not alter beta-diversity.MacArthur's term to 'between-site diversity'with no implication of which determinantscause the observed diversity. We denote theHabitat diversityrole of habitat diversity as a determinant ofDifferences in habitat conditions are widelyspecies diversity as DHD. Systems with lowknown to influence the distribution andand high DHD are shown in Fig. 3. All othercoexistence of species in the landscapedeterminants remain constant. In Fig. 3(a),(Grisebach, 1884; Schimper, 1903; MacArthur,o 5, ,B 0, and y 5. Fig. 3(b) shows a1965; Whittaker, 1975a). Three scales ofsecond system, identical to the first except forhabitat heterogeneity (and correspondinga larger DHD. For this new system o 5differentiation diversities) can be recognized:(unchanged) while ,B 3 and y 20. Thusthe heterogeneity of microsites (pattern), theDHD affects beta- and gamma-diversity butheterogeneity among community samplesdoes not alter alpha-diversity.(beta), and the heteogeneity among regionsThe effects of habitat differences on species(delta diversity; Whittaker, 1977). As the scalediversity can be mediated through competitiveapproaches the size of individual organisms,interactions. Colwell & Fuentes (1975) showspatial (microsite) heterogeneity is morethat displacement by competition (nicheprofitably viewed as a niche factor (MacArthurtruncation) can be different for different sites& MacArthur, 1961; Cody, 1968; Whittakeralong a habitat gradient. Tansley (1917) andet al., 1973). Because differences in speciesByer (1969) have demonstrated that, thoughcomposition owing to differences in habitatplant species may be equal in their ability toare reflected in measurements of beta-diversity,survive a broad portion of the habitat gradientthe term 'between-habitat diversity'when grown in competition-free plots, theirnatural distributions are limited to dissimilarranges of habitat conditions. In such cases, thearesulting increase in beta-diversity fromx SIcompetitive exclusion is properly ascribed to S2the ultimate determinant, habitat diversityoL S3w(DHD).z S40 S5Mass effectc:wIn nature, communities are never closedsystems, exempt from the influences ofadjacent areas. One type of influence is dis-btn SI S6 Sil S16 S2S7S12S17L S3 S8 S13 S18S4S9S14S19o S5 S10 S15 S20Iii.persal. With a high rate of propagule influx,some individuals of a species will becomeestablished in sites in which they cannotmaintain viable populations. This flow ofindividuals from areas of high success (coreareas) to unfavourable areas we call the masseffect (Shmida & Whittaker, 1981; Shmida &Ellner, 1984).ENVIRONMENTAL GRADIENTFI(G. 3. The intluence of habitat diversity (DHD) ondiversity. The five communities shown in (b) eachMass effect always functions to increasealpha-diversity. Note that the species found ina community can be divided into two groups:have the same structure as the single community inthose that are self-maintaining, coexisting(a). DHD does not change oe, but increases ,B and -y.with other species, and those whose presenceThis content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
Biological determinants of species diversity 7ahigh heterogeneity. Consider another hypo-SIjection into surrounding steeply sloping toot-thetical example of a flat valley and its pro-S4ohills (Fig. 5a). Diversity displays are presentedS3ain Fig. 5(b-d) for transects through the valleyS2S50but uninfluenced by the hills (transect 1),along the slope of the hills (transect 2), andthrough the projection of the valley that iswinfluenced by the hills (transect 3). A, B, C, Dand E represent intergrading habitat types.Transect 1 has low beta-diversity (,B 0.6) andbtransect 2 has high beta-diversity (3 2.2)Cfn SiBecause of the chance establishment of speciesX S4 S :0wDoS2.S5S6from the hills along adjacent portions of thebeach, transect 3 takes an intermediate valueof beta-diversity (, 1.7), but one higher thanS7would occur without mass effects.One goal of phytosociology is relatingspecies distributions to environmental factors.ENVIRONMENTAL GRADIENTFIG. 4. The influence of mass effect (DME), in theMass effects, the occurrence of species outsidetheir core habitats, will dilute this relation-form of range extensions only, on diversity. Eachship between species and environment. In factspecies in the sytem shown in (a) has an exactlyit might be that much of the noise, that isdoubled range in (b). Included are species that encroach from outside the original system.unexplained patterns of species distributionsin phytosociology, can be explained in termsof mass effects.Field data from a study area in the Judeandepends entirely on high reproductive successDesert, Israel, have been collected to test ourin adjacent areas. Thus the species richness ofassertion that mass effects can increase overalla community is determined by the processesspecies richness. Species were recorded alongof niche differentiation (DND) and mass effectthree sample transects in a homogeneous(DME)-valley floor (TI) in foothills through fiveFig. 4 is the diversity display for a simplehabitat types (chalk, small-rock, limestone,hypothetical system in which the spatialstone terra-rosa and bare terra-rosa) (T2), anddistribution of species is primarily deter-in the same homogenous valley but adjacentmined by a single widespread gradient. Hereto the foothills (T3). Thus these transectsmass effects result only in range extensions ofduplicate the conditions shown in Fig. 5(a).the species along the gradient. The proportionalFig. 6 shows the cumulative number of speciesincrease in ae (2.0 - 4.0) is in fact equal to therecorded in each of the three Judean desertaverage range extension. The influence oftransects. Relatively few species (44) weremass effect on beta-diversity depends on thefound in transect 1. In contrast, transect 2 hadproperties of the system. In the hypotheticalmany more species (107); intervals of rapidcase of Fig. 4, beta-diversity is decreased (B 1.15 to ,B 0.75) because of the homogenizingtransitions between habitat-types. Transect 3,effects of expanded species ranges. Beta-although in the same habitat as is transect 1,accumulation of species correspond to thediversity can only decrease when mass effectscontains consistently more species (80) thancause the expansion of species ranges withindoes transect 1. Moreover, thirty-one of thethe system (Wilson & Shmida, 1984). Gamma-eighty species of transect 3 are common in thediversity increases (T 5 to y 7) to thefoothill transect (T2), and twenty-nine of thesewere not found at all in transect 1. We concludeextent that species from beyond the extremesof the gradient encroach into the system.from these results that mass effects (DME)An opposite effect on beta-diversity canfrom the foothills to the adjacent valley areobtain if the transect is adjacent to areas ofresponsible for the presence of at least twenty-This content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
8 Avi Shmida and Mark V. Wilsonb(I)x Siw0aoS2TTS3(I)A B C D E cT x -t -F' 2 2S4 S113IS6510wS130 57 59 5 14se h C D E d(I) 59 - - - x Si101256o S 3nS752S14ENVIRONMENTAL GRADIENTFIG. 5. The influence of mass effect (DMrF), in the form of neighbourhood effects, on diversity. In themap in (a), transect 1 (T1)) passes through a single habitat type (A). Transect 2 (T i) passes throughseveral habitat types (B, C, D, E). Transect 3 (T 3) like T1,is in a single habitat type but is adjacent toB, C, D and E. The diversity displays for the three transects are shown at right (transect 1, b; transect 2,c; transect 3, d). Species present in transect 3 only because of mass effect are shown with dotted lines.120 -* Otee T2 107sp.ffi Q)O D------ 80- -o T3 80sp.c-60C340:B*,T144sp.UI)10 20 30 40 50N, contiguous sequential quaFIG. 6. Cumulative number of species recorded in transects in three habitats in the Judean Desert studyarea. Transect 1 (T I) is in a homogeneous valley habitat, transect 2 (T2) is in heterogeneous foothilhabitats, and transect 3 (T3) is in a homogeneous valley habitat adjacent to the foothills. Transect 3 hasmore species than transect 1 because of mass effects (DME) from the foothills to the adjacent valley.Quadrats are 1 m2 in area.This content downloaded from68.202.217.239 on Wed, 19 Aug 2020 18:04:13 UTCAll use subject to https://about.jstor.org/terms
Biological determinants of species diversity 9nine species or 36% of the total species of thehabitat represented by transect 3.example is the species-area curve for vascularplants on the Yizreel valley (Esdraelon plain)The desert washes of the Middle Eastin the Mediterranean region of Israel and theprovide another example of the strong in-Sharm-E-Sheikh, an extreme desert region offluence of mass effect on vegetation. SomeSouth Sinai. The Yizreel Valley is a large, flat,washes have apparently identical environ-alluvial valley. The Sharm-E-Sheikh plain is amental characteristics but often containvast, alluvial aridisol hammada. Because ofdifferent sets of plant species. Although thetheir uniform topography, soils and climate itwashes may provide similar environments
cerned with the phenomenon of alpha-diversity, the species richness of samples representing communities (generally 102 -1IO mi2) (Whittaker, 1977). MacArthur (1965) and others use the term within-habitat diversity as a synonym of alpha-diversity. The diversity of landscapes (106_108 M2) is gamma-diversity. Each level or scale of inventory
tion diversity. Alpha diversity Dα measures the average per-particle diversity in the population, beta diversity Dβ mea-sures the inter-particle diversity, and gamma diversity Dγ measures the bulk population diversity. The bulk population diversity (Dγ) is the product of diversity on the per-particle
diversity of the other strata. Beta (β) Diversity: β diversity is the inter community diversity expressing the rate of species turnover per unit change in habitat. Gamma (γ) Diversity : Gamma diversity is the overall diversity at landscape level includes both α and β diversities. The relationship is as follows: γ
of diversity indices, species abundance models, species accumulation models and beta diversity, extrapolated richness and probability of being a member of the species pool. The document is still incomplete and does not cover all diversity methods in vegan. Keywords: diversity, Shannon, Simpson, R enyi, Hill number, Tsallis, rarefaction, species ac-
Chapter 13:The origin of species Species are independently evolving lineages General lineage species concept: species are metapopulations that exchange alleles frequently enough to comprise the same gene pool Ways to identify species Biological species concept: species are
BIological diversity, or biodiversity, refers to the variety and variability of living organisms on the planet. Ecologists tend to focus on three lev-els of biological diversity: genetic, spe-cies, and ecosystem diversity. Species dive
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alpha, beta, and gamma diversity. Alpha (α) diversity is local diversity, the diversity of a forest stand, a grassland, or a stream. At the other extreme is gamma (γ) diversity, the total regional diversity of a large area that contains several communities, such as the eastern deciduous forests
additif a en fait des effets secondaires nocifs pour notre santé. De plus, ce n’est pas parce qu’un additif est d’origine naturelle qu’il est forcément sans danger. Car si l’on prend l’exemple d’un champignon ou d’une plante toxique pour l’homme, bien qu’ils soient naturels, ils ne sont pas sans effets secondaires.
