OIKOS 89: 115–122. Copenhagen 2000

Are predators rare?

Matthew Spencer

Spencer, M. 2000. Are predators rare? – Oikos 89: 115–122.

Predators are usually thought to be rarer (in the sense of having lower populationdensities) than non-predators. Recent analyses have suggested that this is not the casebecause a decline in species richness compensates for the well-known decline innumber of individuals with increasing trophic rank. I show that a variety ofinvertebrate communities contain more species of predators than would be expectedfrom the number of predatory individuals. This is not due to differences in domi-nance or taxonomic resolution between predatory and non-predatory guilds, andimplies that predators are indeed relatively rare. I suggest that patterns of energy flowand body size make it likely that there will be a higher proportion of predatoryspecies than individuals in a community, provided that predators have moderatelyspecialized diets.

M. Spencer, Community Ecology Laboratory, Inst. of E6olution, Uni6. of Haifa, Haifa31905, Israel (spencer@research.haifa.ac.il).

Species feeding at high trophic ranks are often assumedto be relatively rare (used throughout this paper in thesense of having low population densities), because pop-ulation density decreases with increasing trophic rank(where ‘trophic rank’ is some measure of the length offood chains through which a species obtains energy).Thermodynamic constraints usually mean that thenumber of consumer individuals is lower than the num-ber of individuals of the resource on which they feed(e.g. Farlow 1993), unless the turnover of resources ismuch more rapid than the turnover of consumers.However, species richness also declines with increasingtrophic rank, and Rosenzweig and Lomolino (1997)suggested that these two patterns cancel each other out.They assumed that a species obtains most of its energythrough the shortest food chain leading to it, becauseavailable energy declines with each link in a food chain.They used the length of this shortest food chain (‘mini-mum trophic level’) as a measure of trophic rank.Species richness in arthropod-dominated food websdeclines by about 86% for every unit increase in mini-mum trophic level (Rosenzweig and Lomolino 1997). Ifavailable energy also declines by about 85–90% perunit increase in minimum trophic level (Rosenzweigand Lomolino 1997), the number of individuals of a

species is proportional to available energy, and averagebody size does not decline very much with increasingminimum trophic level, there should be little relation-ship between trophic rank and rarity. The argument isinteresting, but does not constitute direct evidence thatthere is no relationship between trophic rank and rar-ity. I will present simpler evidence that predators(which by definition feed at higher trophic ranks thannon-predators) are rarer than non-predators, based onthe proportions of predatory individuals and species inpublished community studies.

Maiorana and Van Valen (1991) give examples (with-out citing original sources) where predators make up ahigher proportion of species than individuals (or insome cases biomass) in assemblages including:arthropods in decaying logs (5% of individuals, 50% ofspecies), collard plants (2% of individuals, 54% of spe-cies) and beech canopies (15% of individuals, 35% ofspecies); large mammals in Serengeti bushland (1% ofindividuals or biomass, 27% of species) and Venezuelabushland (1% of individuals, 50% of species); Oligocenelarge mammals and Cretaceous and Jurassic dinosaurs(8% of individuals, 2% of biomass, 17% of species); andPermian reptiles (21% of individuals, 47% of species). Ifthese data are representative, it appears at first sight

Accepted 13 September 1999

Copyright © OIKOS 2000ISSN 0030-1299Printed in Ireland – all rights reserved

OIKOS 89:1 (2000) 115

that predators are rarer than non-predators. However,Maiorana and Van Valen (1991) argued that herbivoreguilds will be dominated by the few species that aremost successful in dealing with the antiherbivore de-fences of plants, and most herbivores will be as rare aspredators. This assumes that predators do not facesimilar difficulties in dealing with their prey, and thatthe less successful herbivores remain rare but do notbecome extinct.

A critical test is that non-predatory guilds should bedominated by individuals of only a few species, whilepredatory guilds should have more even abundance-fre-quency distributions. Furthermore, the argumentshould not hold for non-living resources such as de-tritus and carrion, which are important energy sourcesin many communities. Defensive chemicals and struc-tures are gradually degraded by microbial and physico-chemical processes, leaving dead organic matter as arelatively undefended resource, so the proportion ofpredatory species should not be higher than the propor-tion of predatory individuals in communities in whichdead organic matter is the dominant energy source.

Differences in taxonomic resolution could also ex-plain differences in the proportions of predatory speciesand individuals. Predators are often large, conspicuousand interesting to ecologists. If more effort is expendedon identifying predatory than non-predatory species,one would expect to see more (but rarer) species for agiven number of predatory than non-predatory individ-uals. If this is important, one would also expect to seemore taxa resolved to species level in predatory thannon-predatory guilds.

Here, I will: (1) compare the proportions of preda-tory species and individuals in a collection of publishedcommunity studies, in order to determine whetherpredators appear to have lower population densitiesthan non-predators; (2) test for systematic differences indominance or taxonomic resolution between predatoryand non-predatory guilds; and (3) test whether thesepatterns differ between communities based on plantsand dead organic matter.

Methods

Data sources

Many data sets were found from the bibliographies ofJeffries and Lawton (1984, 1985), Schoenly et al. (1991)and Warren and Gaston (1992). The remainder werelocated by searches of Biological Abstracts, journalcontents and other sources. I only included papers thatstudied a variety of guilds (predators and herbivores,predators and saprophytes, etc.). I rejected studies ofjust a single taxonomic group (e.g. nematodes, poly-chaetes), and studies that reported a known samplingbias against a particular guild. All studies were re-

stricted to macroinvertebrates, as no suitable paperswhich included vertebrates were found. Microorgan-isms were included in one study (Schaeffer andSchauermann 1990), but I excluded them from thisanalysis as their feeding habits were not recorded. Notrue parasites were included in any study, althoughthree studies (Chapman and Sankey 1955, Oatman andPlatner 1969, Root 1973) included parasitoids. In total,I found sixteen suitable studies, of which nine describedfreshwater, six terrestrial, and one a marine benthiccommunity (Appendix). Only one of these (Root 1973)is among the studies discussed by Maiorana and VanValen (1991). The bias introduced by ignoring phyloge-netic relationships is unlikely to be large, as the speciesfound in different studies are very taxonomically di-verse. I do not attempt to correct for differences inbody size between predatory and non-predatory spe-cies, but I later discuss the consequences of thesedifferences.

Proportions of predatory species and individuals

For each study, I compiled species lists with numbers ordensities, sorted by guilds into predators and non-predators using information given in the paper, exceptthat I obtained dietary information for one species inChapman and Sankey (1955) from Chinery (1986).‘Predators’ here means species feeding primarily onliving metazoa (Jeffries and Lawton 1985, Warren andGaston 1992). ‘Non-predators’ here means all otherspecies. I excluded two guilds in one study (‘flower,pollen and miscellaneous feeders’ and ‘cryptozoa’ inWiegert et al. 1967) which did not appear to consist oforganisms with similar feeding habits. I counted omni-vores as non-predators in the one study in which theywere listed (Minshall 1967). In one study (Rosenberg1973) a taxon (Nemertini) was reported as belonging toboth the deposit feeder and carnivore guilds. The analy-ses were repeated counting the Nemertini either in bothguilds, or as deposit feeders only. The results were verysimilar, and only the version counting Nemertini asdeposit feeders is reported here. Saprophytes werecounted as non-predators. I followed Jeffries and Law-ton (1985) in counting two species for taxonomicgroups which were not resolved to species level andwere represented by more than one individual, unlessthe original source explicitly indicated that only onespecies was present. This should reduce the bias causedby uneven taxonomic resolution. If juvenile forms werenot resolved to species level but adults were alsopresent and resolved to species level, I assumed thejuveniles were of the same species as the adults. I usedthese data to calculate the proportions of predatoryspecies and individuals in the community. I treated eachstudy as an independent data point. Thus, where stud-ies contained several samples taken at different times

116 OIKOS 89:1 (2000)

(e.g. Chapman and Sankey 1955, Root 1973), I usedcumulative species richness and abundance data. Whereseveral similar sites in the same area were studied (e.g.different sampling locations in a river: Marchant et al.1985), or several types of similar habitat (e.g. differentspecies of pitcher plants: Clarke and Kitching 1993), Iaveraged the proportions of predatory species and indi-viduals across sites or habitats. I used a Wilcoxonsigned-ranks test to determine whether the proportionsof predatory species and individuals were significantlydifferent.

Primary resource type and taxonomic resolution

I determined the primary resource type for each com-munity (living or non-living) from information given inthe original papers, and compared the differences be-tween proportions of predatory species and individualsin communities based on different resource types usinga Wilcoxon two-sample test. I calculated the proportionof taxonomic groups in each guild that were not pre-sented at the species level (‘aggregated groups’). I didthis for all guilds in all studies used to compare theproportions of predatory species and individuals, exceptthose that presented only the numbers of species andindividuals as opposed to species lists. I compared theproportions of aggregated groups between predatoryand non-predatory guilds using a Wilcoxon two-sampletest, treating each guild as an independent data point.

Dominance

I measured dominance as the proportional abundanceof the most abundant species in each guild. This is theBerger-Parker index, a simple dominance measure (rec-ommended by Magurran 1988) which is relatively in-sensitive to poor taxonomic resolution or evenexclusion of rare species. I used cumulative abundanceand species richness over time, and averaged acrosssampling sites or similar habitats within studies, asabove. I excluded any guilds for which the dominanttaxon was a group of several species in at least one ofthe sites studied, and sites which had no representativesof a guild, leaving 43 guilds for analysis. I tested for adifference in dominance between predatory and non-predatory guilds using Kendall’s partial tb, a non-para-metric partial correlation (Gibbons 1993). Thisaccounts for the relationship between dominance andguild species richness and for the probably substantialmeasurement error in both variables. Dominance isconstrained to be between 1/(guild species richness) andone, which would make it difficult to fit a parametricmodel. Because dominance is constrained to be onewhen there is only one species in the guild, there can beno ‘‘main effect’’ of trophic type (predatory or non-

predatory), only an interactive effect of trophic typeand guild species richness on dominance. Thus thecoefficient of interest is the partial correlation betweendominance and the product of guild species richnessand trophic type (coded as 1 for predatory guilds and 0for non-predatory guilds), after accounting for the ef-fect of species richness. I compared this coefficient withtabulated quantiles of Kendall’s partial tb in Magh-soodloo and Pallos (1981; Table 2), using a one-tailedtest because Maiorana and Van Valen’s (1991) hypothe-sis is directional.

Results

The mean proportion of predatory species was signifi-cantly higher than the mean proportion of predatoryindividuals (Fig. 1a). There were only two studies(Chapman and Sankey 1955, Louton et al. 1996) inwhich the proportion of predatory individuals washigher than the proportion of predatory species. Theproportion of aggregated taxonomic groups was notsignificantly different between predatory guilds (mean0.09, standard error 0.04, N=13) and non-predatoryguilds (mean 0.06, standard error 0.02, N=25),(Wilcoxon two-sample test: Z=0.44, P=0.66). Domi-nance and guild species richness were negatively corre-lated (Fig. 1b: partial tb= −0.58, PB0.001), but thedifference in dominance between predator and non-predator guilds (measured as the partial correlationbetween dominance and the product of trophic typeand guild species richness) was not significant (partialtb= −0.11, 0.15BPB0.20). The difference betweenproportions of predatory species and individuals wasnot significantly greater for communities based on liv-ing than on non-living resources (Table 1).

Discussion

The proportion of predatory species was almost alwayshigher than the proportion of predatory individuals(Fig. 1a), with two exceptions. In rabbit carcasses(Chapman and Sankey 1955), all species fed on eitherlive or dead animal tissue, so there may be fewerdifferences than usual in nutrition between predatorsand non-predators. In bamboo internode communities,the presence of a recently hatched predatory mosquitoegg raft resulted in a high number of predatory individ-uals (Louton et al. 1996). There was no evidence fordifferences in taxonomic resolution between predatoryand non-predatory guilds. This suggests that the au-thors of the original papers did not systematically paymore attention to either predatory or non-predatoryspecies (a bias which may be less likely among studiesof invertebrates than vertebrates). There was no signifi-

OIKOS 89:1 (2000) 117

cant difference in dominance between predatory andnon-predatory guilds (Fig. 1b). Maiorana and VanValen (1991) suggested that the chemical and structuraldefences of plants are difficult to overcome, but the fewspecies of non-predatory animals that do overcomethese defences can become very abundant, leading tovery uneven species-abundance distributions in herbi-vore guilds. This does not appear to be true for the dataI compiled. Furthermore, if Maiorana and Van Valen(1991) were correct, the difference between the propor-tions of predatory species and individuals should be less

Table 1. Mean proportions of predatory species and individu-als in communities based on living (N=5) and non-living(N=11) resources. Standard errors are given in parentheses.The excess of predatory species relative to individuals was notsignificantly different between resource types (Wilcoxon two-sample test, Z=0.57, P=0.57). Four of the five communitiesbased on living resources were terrestrial, and only one terres-trial community was based on non-living resources.

Proportion preda-Proportion preda-tory species tory individuals

Living resources 0.43 (0.11) 0.18 (0.05)0.10 (0.04)0.23 (0.03)Non-living re-

sources

Fig. 1. (a) Proportions of predatory species and individuals in16 terrestrial, freshwater and marine communities. The meanproportion of predatory species (0.29, standard error 0.04) wassignificantly higher than the mean proportion of predatoryindividuals (0.13, standard error 0.03): Wilcoxon signed-rankstest statistic=59, P=0.001. Open circles represent data fromcommunities based primarily on non-living resources, andfilled circles are from communities based primarily on livingresources. The line shows equal proportions of predatoryspecies and individuals. (b) Dominance (the relative abun-dance of the most common species) in predatory (filled circles)and non-predatory (open circles) guilds. The broken linesshow the constraints on possible values of dominance.

in communities based on non-living resources, whichare unlikely to be well-defended. I found no evidencefor this, although the distinction between living andnon-living resources was largely confounded with thatbetween terrestrial and other habitat types. If neithertaxonomic biases nor differences in dominance causedby the difficulties of overcoming plant defences canexplain the higher proportion of predatory species thanindividuals in macroinvertebrate assemblages, thenpredators must be rarer than non-predators. Rosen-zweig and Lomolino’s (1997) finding to the contrarywas based on an indirect test with no data on thenumber of individuals per species, and their conclusionthat this pattern does not exist is premature.

Why should there be many rare species of predators(with the result that predators make up a higher pro-portion of species than of individuals)? Why not a fewcommon species? A single species of predator is un-likely to efficiently capture all the production of non-predators, due to morphological, biochemical andbehavioural constraints (Stenseth 1985, Warren 1996).Optimal foraging theory suggests that individual preda-tors will often maximize their fitness by concentratingon the most profitable subset of the available prey(Schoener 1971). Predatory species with different mor-phological, biochemical and behavioural characteristicsmay then evolve to specialize on different kinds of prey.

Only a moderate amount of dietary specialization bypredators is needed for predators to make up a higherproportion of species than individuals (I am grateful toan anonymous reviewer for this idea). Assume thatthere are r different non-animal resources (such as plantspecies, or size fractions of detritus) in a habitat. Fur-ther assume that on average, n different species ofnon-predator can coexist on a single non-animal re-source, and p different species of predator can coexiston a single species of non-predator (where n and p maybe less than one, if a single species of consumer needsmany different species of resource). Assume that non-predators and predators form two discrete trophic lev-els. For the moment, also assume that there are nopredators on predators (I later relax this assumption).In total, there will be rn species of non-predator and

118 OIKOS 89:1 (2000)

rnp species of predator. The proportion of predatoryspecies will be p/(1+p). Let e be the trophic transferefficiency for predators (the proportion of non-predatorproduction that is converted into predator production).Then

Pp=ePn (1)

where Pp and Pn are the production of predators andnon-predators, respectively. Across species, the ratio ofproduction P to average biomass B scales with bodymass W (Peters 1983):

P

B=aW −k (2)

where a and k are constants, assumed to be the samefor predators and non-predators. Combining Eqs 1 and2 and rearranging, the total biomasses Bp and Bn ofpredators and non-predators, respectively, are

Bp=ePn

aWp

k

Bn=Pn

aWn

k

(3)

where Wi is the (k−1)th root of the mean of the(k−1)th power of the mass of each species j on trophiclevel i (wij), weighted by the proportion of availableenergy captured by each species (fij):

Wi=�%

j

fijw ijk−1�1/(k−1)

(4)

Dividing Eqs 3 by the Wi gives the numbers of individ-uals Np and Nn for predators and non-predators:

Np=ePn

aWp

k−1

Nn=Pn

aWn

k−1 (5)

The proportion of predatory individuals is

Np

Np+Nn

=eWp

k−1

eWpk−1+Wn

k−1 (6)

For the proportion of predatory species (p/(1+p)) tobe greater than the proportion of predatory individuals,

p\e�Wp

Wn

�k−1

(7)

Begon et al. (1996: 733–735) summarize information onthe components of trophic transfer efficiency for inver-

tebrates, which suggest that e is usually likely to bearound 0.1. Wp/Wn is unlikely to be less than one ifparasites are ignored, because predators tend to belarger than their prey (Vezina 1985, Warren and Law-ton 1987, Cohen et al. 1993). Estimates of k for inverte-brates range from about 0.2 to 0.4 (Peters 1983). Thismeans that p\e is sufficient to satisfy Inequality 7 ifpredators and non-predators are the same size on aver-age, and is more than sufficient if predators are largerthan non-predators on average. In other words, iftrophic transfer efficiency (e) is about 0.1, there willprobably be a higher proportion of predatory speciesthan individuals so long as it takes an average of fewerthan about ten species of prey to support each speciesof predator. In systems where trophic transfer efficiencyis unusually high, the number of species of prey re-quired to support each predator would have to be lowerfor the proportion of predatory species to be higherthan the proportion of predatory individuals.

Predators on predators (hyper-predators) are unlikelyto qualitatively affect this result. Assume that the num-ber of species of hyper-predator that can coexist oneach species of predator is the same as the number ofspecies of predator that can coexist on each species ofnon-predator (p). This implies a linear relationshipbetween log species richness and trophic level, which issupported by some data (Rosenzweig and Lomolino1997). Assume that the trophic transfer efficiency is thesame for hyper-predators as for predators. This isreasonable (provided that one is not mixing ectothermsand endotherms) because both hyper-predators andpredators are carnivores. Let Wh be the (k−1)th rootof the weighted mean of hyper-predator masses raisedto the (k–1)th power, as defined in Eq. 4. By argumentsanalogous to those above, the condition for the totalproportion of predatory species (predators and hyper-predators) to be greater than the proportion of preda-tory individuals is

p+p2\e�Wp

Wn

�k−1

+e2�Wh

Wn

�k−1

(8)

If e is small, the second term on the RHS of Inequality8 is unimportant unless Wh5Wn and Wp�Wn. Inother words, the contribution of hyper-predators to theproportion of predatory individuals will be negligibleunless hyper-predators are smaller than predators,which does not seem likely (ignoring parasites). ThusInequality 8 will probably be satisfied whenever In-equality 7 is satisfied. The contribution of hyper-preda-tor species richness to the LHS (p2) will tend to makeInequality 8 easier to satisfy, although this will berelatively unimportant if p is much less than one.Strictly, Inequality 7 is a condition for average abun-dance to be lower on trophic level i+1 than trophiclevel i, while Inequality 8 (or its obvious extension to ntrophic levels) is a condition for average abundance to

OIKOS 89:1 (2000) 119

be lower on trophic levels i+1 to i+n−1 than ontrophic level i.

Eqs 1–8 formalize the verbal model of Rosenzweigand Lomolino (1997). What led Rosenzweig and Lo-molino to conclude that there is unlikely to be anyrelationship between trophic rank and rarity? One dif-ference between Rosenzweig and Lomolino’s data andmine is that they were able to separate predators bytrophic rank, using information from binary food webs(Schoenly et al. 1991). I combined all carnivores intothe group ‘predators’. If there is a weak negative rela-tionship between trophic rank and average number ofindividuals per species, but predators exist at a widerange of trophic ranks, the average predatory speciesmight still have substantially lower abundance than theaverage non-predatory species. However, if organismsabove trophic rank 3 (all predators on predators) makea small contribution both to the number of species andto the number of individuals (p and e small for alltrophic ranks), this difference will not be important.Rosenzweig and Lomolino (1997) suggest that both pand e are small. They assume:

(1) Trophic transfer efficiency (e) is about 10–15%,which seems quite reasonable (Begon et al. 1996).

(2) The shortest food chain through which energyflows to a species is the best measure of trophic rank. Aspecies that acquires energy through several food chainsof different lengths is likely to obtain most of its energythrough the shortest food chain if it consumes prey inproportion to their abundances (Yodzis 1984). Usingthe length of the shortest food chain as a measure oftrophic rank gives p about 0.14 in Rosenzweig andLomolino’s data (the antilog of the slope of the rela-tionship between log species richness and trophic rankin their Fig. 5.1). This implies little relationship betweenabundance and trophic rank if e is close to 0.1 and thesize differences between species of different trophicranks are small (see below). At the other extreme, apredator might select prey (perhaps on the basis ofbody size) so as to maximize its fitness, in such a waythat most of its energy is obtained through the longestfood chain leading to it. Assuming that predators be-have in this way gives p about 0.35 in Rosenzweig andLomolino’s data (their Fig. 5.1). This implies thatabundance is likely to decline faster than species rich-ness with increasing trophic rank, if e is close to 0.1.My results are not concordant with the first alternative(that the shortest food chain leading to a species is thebest measure of trophic rank), although the other ex-treme is probably equally unrealistic. To make realisticestimates of trophic rank, one needs quantitative dataon the relative energetic importance of food chains ofdifferent lengths to polyphagous predators. Stable iso-tope measurements (France 1997) may provide thesedata.

(3) Body size does not change very much with in-creasing trophic rank. This may be a reasonable as-

sumption for invertebrates. Siemann et al. (1999) foundthat among grassland arthropods, trophic group (para-site, predator, herbivore or detritivore) was not a goodpredictor of body size, although the interaction betweentrophic group and taxonomic order was a good predic-tor of body size. If there is any tendency for predatorsto be larger than non-predators, it will increase thetendency for predators to have low abundances relativeto non-predators.

There may be many other ecological differences be-tween predators and non-predators, including: lowerminimum viable population sizes for predators (inmammals: Silva and Downing 1994); either higher colo-nizing success for predators (in beetles: Becker 1975) orlower colonizing success for predators (in birds andmammals: Wolf et al. 1998); greater sensitivity ofpredators to disturbance (in tree-hole invertebrates:Jenkins et al. 1992); a diversity peak at higher primaryproductivity for predators than for non-predators (inmammals: Owen 1988); and a higher turnover rate ofnon-predatory than predatory species in evolutionarytime (in mammals: Van Valkenburgh and Janis 1993).These differences may affect the proportions of preda-tory species and individuals in communities. However,diet breadth, energy flow and body size patterns areprobably the major explanations for the low abundanceof predatory species compared to non-predatoryspecies.

Acknowledgements – I am grateful to Avigdor Beiles, LeonBlaustein, Kevin Gaston, Donald Libby, Natasha Loder andPhil Warren for advice and criticism. Part of this work wassupported by United States-Israel Binational Science Founda-tion grant c95/0035 to Leon Blaustein and Joel E. Cohen.

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OIKOS 89:1 (2000) 121

122 OIKOS 89:1 (2000)

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