Extending Rapoport's rule to Pacific marine fishes

Journal of Biogeography (1996) 23, 149–154

Extending Rapoport’s rule to Pacific marine fishes

G C. S Department of Biology, University of New Mexico, Albuquerque, NM 87131, U.S.A.

Abstract. The mean bathymetric (depth) range of other Rapoport’s rules. Species richness is found to beassemblages of marine fishes is negatively correlated with correlated with Rapoport phenomena, consistent withthe depth at which the assemblage is found and negatively previous studies. Moreover, local species richness is foundcorrelated with species richness. These findings support the to be strongly influenced by regional patterns of speciescreation of a new biogeographic rule patterned after the richness (so-called macro-ecological factors), not just localterrestrial Rapoport’s Rules for Latitude and Elevation. influences such as productivity in the immediate area.Previous work on Rapoport’s rule is extended to include

Key words. Rapoport’s rule, bathymetric range, depththis bathymetric rule and the basis for all known Rapoport’srules is generalized in hopes of broadening the search for range, latitude, marine fishes.

has been severed. The Rapoport rules mentioned aboveINTRODUCTION

provide a viable alternative explanation for the correlationbetween latitude and elevation or ocean depth and rangeRapoport’s rule (Stevens, 1989, 1992) is a newsize. More recently the link between species richness andbiogeographical principle that may shed light on globalRapoport’s rule has been questioned (Rohde, Heap & Heap,patterns of species richness. It is a correlation between the1993) and deserves special attention here. The purpose ofwidths of overlapping distributions of species found atthis paper is to review the data from the marine environmentpoints along an ecological gradient (such as latitude orand present new data that help to clarify the issues relatingelevation) and the position of the sample point along theRapoport phenomena to local species richness.gradient. That tropical organisms tend to have smaller

Given that Rapoport’s Latitudinal Rule (Stevens, 1989)latitudinal ranges than boreal organisms is Rapoport’shas recently been extended to elevation (Stevens, 1992) aLatitudinal Rule (Stevens, 1989). Here, as the latitude ofmore generalized definition of the rule is needed. There isthe sampling point increases, the mean latitudinal range ofno reason to suppose, a priori, that Rapoport phenomenaco-occurring species increases. In a conceptually similarwould not be detected along soil moisture gradients, salinityway, Rapoport’s Elevation Rule has been confirmed byor toxicity gradients, gradients in disturbance, or alongobserving that low elevation organisms tend to have smallerother biogeographically important gradients unrelated toelevational ranges than high elevation organisms (Stevens,latitude or elevation. A definition of a Rapoport rule that1992).allows it to be extended to a wider variety of gradients is:Declines in the species richness of communities along

both latitudinal and elevational gradients tend to mirror There exists a correlation between the mean geographicalthe mean range size, being higher in both low latitude and breadth of taxa occurring at any particular point alonglow elevation sites and lower in high latitude and high a biogeographical gradient and the relative position ofelevation areas. This link between biogeographical patterns the point along the gradient.and community organization may be spurious, but it hasgenerated speculation that processes external to the local The ‘mean geographical breadth’ has heretofore beencommunity might strongly influence community defined as the mean latitudinal or mean elevational extentmembership (Stevens, 1989, 1992). It is this link to species of species found together at particular latitudes or elevationsrichness that attracts the interest of community ecologists (Stevens, 1989, 1992). Other measures of the breadth ofto Rapoport’s biogeographical pattern. distribution could be imagined (e.g. range in the number of

Several distinct causal mechanisms have been proposed thermal degree days over the geographical range of thefor Rapoport’s rule and will be discussed shortly. Previously, species, range in millibars of soil water potential over thethe relationship between age of taxa and range size has geographical range, etc.). Other taxonomic units might alsobeen explored (Willis, 1922), but with the revolution in be considered (subpopulations of species [Stevens & Enquist,biogeography created by the Equilibrium Theory of Island in press], genera, families). The sign of the correlationBiogeography (MacArthur & Wilson, 1963, 1967) and depends on how the gradient is plotted, but the magnitudeTaxon Cycles (Ricklefs & Cox, 1972; Wilson, 1961) this of the correlation reflects the degree to which the range

(measured in whatever units chosen) reflects the tolerancescausative link between age and geographical distribution

1996 Blackwell Science Ltd 149

150 George C. Stevens

‘annual variations in temperature decrease with depth andare rarely perceptible below 100–300 m’ (Pickard & Emery,1982: 40). As a result, at high latitudes surface temperaturesare consistently cooler than at low latitudes, but deep-watertemperatures are about the same over all latitudes.

As a consequence, one might expect that marine fisheswould exhibit no relationship between their geographicalrange size as measured by mean latitudinal extent and thelatitudinal zone in which they are found, since one musttake into account both the relation between depth andtemperature fluctuations. If fish divided their world intodistinct communities based on depth, with no species movingbetween depth intervals, then one might expect the fishcommunity at the surface of the ocean to show a clear

FIG. 1. Annual variation in water temperature as a function of correlation between latitude and the mean latitudinal rangedepth for a site in the eastern North Pacific (50°N, 145° W). Data of the species found at that latitude. If only the fish of deeperfrom Pickard & Emery (1983). water were studied under the same restrictive situation, no

such correlation would be found since deep water variesof the taxonomic units that make up the range. In the case little in temperature over the seasons or with latitude.of species and latitude, when ecotypic variation within the Given that real marine fish assemblages do not divide thegeographical distribution of a species is high, an individual’s depths of the oceans in this way, Rapoport’s Latitudinaltolerances are unlikely to predict the tolerances of the species Rule is unlikely to be found in the marine environment (asas a whole and so the magnitude of the Rapoport correlation reported by Rohde et al., 1993). However, freshwater fishesis likely to be weak. do show an unambiguous Latitudinal Rapoport Rule

There are at least three hypotheses to account for the (Stevens, 1989; Rohde et al., 1993). Perhaps this is becauseorigin of Rapoport’s rules. They are the: (1) Seasonal they live in a relatively much thinner layer of water thanVariability Hypothesis, (2) Differential Extinction do marine fishes. Not only do marine fishes found at theHypothesis, and (3) Competition Hypothesis for the origin surface of the ocean migrate up and down (migratory birdsof Rapoport’s rule. Each will be considered in turn as a also do not show Latitudinal Rapoport’s Rule [Stevens,means of introducing the analysis of Pacific Ocean fish 1989]), but even if they did not mix with fish of deeperdistributions. water, the conflicting expectations for Rapoport phenomena

for surface and deep water fishes clouds any possibleTHE SEASONAL VARIABILITY HYPOTHESIS correlation in simple latitudinal surveys that do not factor

out depth as a co-variate (such as Rohde et al., 1993).I have proposed (Stevens, 1989) that contemporary seasonalvariation between sites separated by latitude or elevation If the bathymetric distribution of fish is taken into

account, Rapoport’s rule should appear (provided thedrives Rapoport phenomena. The seasonal variation withineach sample location sets the minimum breadth of tolerances seasonal change hypothesis for origin of the rule is correct).

In high latitudes, surface waters will vary the most inrequired by individuals that are found to reside in each site.A non-migratory individual of a species found at high temperature over the course of the seasons. The variation

in water surface temperature at high latitude is so large thatlatitude must be able to tolerate the full range of climaticconditions imposed on it by seasonal change. Individuals it will include the range of temperature found in the deepest,

high latitude waters. In low latitudes surface waters will beof such a broadly tolerant species can reside in more sites,hence the geographical ranges of high latitude species are much less varying and, moreover, will be more distinctly

different than deeper waters at low latitude. In high latitudes,larger than those species of low latitudes whose individualssurvive with narrower tolerances. Note that this does not fish that can survive at the surface will be able to tolerate

the whole range of temperature conditions found in thepreclude broadly tolerant species from sites where thosetolerances are not challenged. Low elevation or low latitude water column since surface water temperature can be as

cold as that found near the ocean bottom at that latitude.sites may have species with broad geographical ranges, buthigh elevation or high latitude sites are unlikely to support In low latitudes, shallow and deep water temperatures are

always very different. As a result, if the seasonal variationspecies with narrow tolerances.In the marine environment (the main focus of this study) hypothesis for the origin of Rapoport’s rule is correct,

narrow depth ranges are expected in the tropics and broadorganisms are exposed to different climatic gradients thanare terrestrial organisms. Water has a higher specific heat depth ranges in temperate and arctic waters.than air, meaning that it is relatively slow to respond toseasonal changes in the sun’s angle. Water also acts as a

THE DIFFERENTIAL EXTINCTIONmore effective insulating layer than air, meaning that deeper

HYPOTHESISwaters show less variation in temperature than water at thesurface. Surface waters show the most extreme shifts in A completely different way of viewing Rapoport’s rule is

offered by J. H. Brown (pers. comm.). He proposes thattemperature with season (see Fig. 6, Rhode et al., 1993),some seasonal change occurs below the surface (Fig. 1), but differential extinction due to glaciation and climate change

Blackwell Science Ltd 1996, Journal of Biogeography, 23, 149–154

Rapoport’s bathymetric rule 151

may account for the ordering of tolerances along latitudinal larger, more fecund, source populations are elsewhere. Theaccidentals do, however, disrupt competitive interactions,or elevational gradients. Given the past fluctuations in

climatic conditions, species with narrow tolerances that once predator/herbivore feeding, etc. through their use of localresources. No matter how poorly individuals of a speciesoccurred at high latitude or high elevation have become

extinct. Thus Rapoport’s rule becomes the outcome of do in a ‘sink’ (Pulliam, 1988), the constant rain of newcolonists of their species means that their local populationhistory and not the outcome of immediate (and more easily

measured) processes in operation today. The importance of density does not reflect their lack of competitive ability,predator avoidance or herbivore resistance, but ratherthis differential extinction model can be tested by comparing

terrestrial Rapoport phenomena with those in marine reflects both this and the rate at which new colonists arrive.This explanation, however, is based on the belief that speciesenvironments. In the latter, the impact of the earth’s history

of climatic variation is expected to be less important than richness and Rapoport phenomena are not linked throughthe effects of competition on geographical range size (asin the terrestrial environment and so Rapoport effects should

be less visible. If any sort of Rapoport’s rule is found it will discussed in the previous paragraph).To clarify these differences in interpretation, bio-severely weaken arguments based on extinction during past

ice ages as the cause of Rapoport phenomena. geographical information from the marine environmentwere analysed with regard to both latitudinal and depthranges. The interaction of these two ecologically important

THE COMPETITION HYPOTHESISdimensions highlight:

Several others (Rosenzweig, 1975; Pianka, 1989; J. H. Brown,(1) the role of past extinctions in producing Rapoport1995) have asked whether the competitive environment

effects;(measured here by local species richness) might have a direct(2) the response of individual organisms to changes in theireffect on geographical range size. Teasing out cause and

immediate environment and its cumulative effect oneffect in the positive correlation between species richnesstheir geographical range dimensions; andand geographical range size along latitudinal and elevational

(3) local (resource-based) v. regional (biogeographical)gradients is not possible. However, in the marineprocesses and their effects on species richness.environment, tropical latitudes that have the highest species

richness may or may not have the narrowest depth rangesat all depths (since deep waters vary little seasonally) as a METHODSvery strict application of the Competition hypothesis wouldrequire. If this competitive explanation for the origin of As in earlier studies of Rapoport phenomena, data for

this analysis were gathered from a previously publishedRapoport’s rule is correct, then mean depth range wouldbe expected to be consistently smallest in the depths with compilation of geographical ranges (Eschmeyer & Herald,

1983) chosen for its completeness both in terms of taxonomythe greatest species richness. This line of reasoning wouldrequire that species would evolve the tolerances that set and geography. In this particular case, both the distribution

of Pacific Ocean fishes over latitude and depth weretheir range size in areas of their lowest population (giventhe negative correlation between community richness and available. These data are the result of thousands of captures

of animals through the use of commercial fishing nets setaverage population density) and most diffuse competitiveenvironment (given the larger number of potential at particular depths. Many of the fish included in this

analysis are of economic importance and so theircompetitors in rich sites). Moreover, since any particularorganism is likely to be found in many different communities distributions are well known. The ranges of others are only

extrapolated from scattered collections.(some rich, some poor) this explanation is difficult tosupport. What little evidence that has been applied to As before (Stevens, 1989, 1992) the mean ranges (either

latitude or depth) were calculated over fixed intervals (5°this question has yielded ambiguous results (Anderson &Koopman, 1981). Given this, a fair evaluation of this for latitude, 50 m for depth). For example, the mean depth

range of fish found at the surface includes fish from 0 tohypothesis is difficult. A weak test of this can be made byasking if tropical latitudes have species with narrow depth 1 m as well as those found from 0 to 3000 m. However, fish

found in waters 50 m or deeper would not be included inranges at all depths, relative to other latitudes.The connection of Rapoport phenomena to species the sample of ‘surface’ (0–49 m depth) fish. This method

means that the values of mean depth range are notrichness is problematical. I have used a non-equilibrialexplanation for the observed patterns of species richness independent of one another in that many of the points share

fish. This method emphasizes the community of fish that(Stevens, 1989, 1992) called the Rapoport-rescueHypothesis. Sites that are bombarded by propagules that interact at the surface rather than placing emphasis on

individual species. Unfortunately, this method precludes theare slightly out of their range (i.e. at the edge of theirnarrow tolerances) are more likely to be decoupled from use of simple correlation statistics on the data points since

they are not independent. As a result, lines are drawn onthe ecological processes that limit the number of co-existingspecies in the community. In rich communities, accidents of the figures to ease the eye in connecting points, not to imply

statistical significance. When correlation statistics are useddispersal (the propagules themselves are called ‘accidentals’)both inflate local species richness and limit the effectiveness they are only used to provide a simple metric for comparison

within this dataset, not to imply strength of statisticalof species exclusion processes (Stevens, 1989). Theaccidentals do not evolve broader tolerances because their inference in relation to other studies.



FIG. 2. Mean depth range of Pacific Ocean fishes as a function of FIG. 4. Species richness of Pacific Ocean fishes as a function ofboth latitude (x axis) and survey depth (z axis). Note the general ocean depth (x axis) and latitude (z axis). Data as in Fig. 2. Thedecline toward origin indicative of Rapoport’s rule for both depth slight drop-off in species richness at the far southern latitude (30°N)range and depth, and depth range and latitude. Note also the is due to incomplete sampling. Note the shift in peak speciesweakening of Rapoport’s Bathymetric Rule with increasing depth richness with latitude.in Pacific Ocean fishes.

ranges between different latitudes (Fig. 2). At the surfaceand in near-surface waters the correlation between latitudeand depth range is nearly perfect (at 0 m depth rs=1; 25 mrs=0.93; 50 m rs=0.83) and the magnitude of the differencesbetween latitudes are relatively large. With increasing depthgreater variability is seen (75 m rs=NS; 100 m rs=NS; 200 mrs=0.88; 500 m rs=NS; 1000 m rs=0.95; 2000 m rs=NS)and the differences between latitudes are relatively slight.This is consistent with the Seasonal Variation hypothesisand an application of the Rapoport-rescue hypothesis tothese fishes since the magnitude of the species richnessgradient also declines with depth (Fig. 4).

Since fish can easily move between depths, the DifferentialExtinction Hypothesis is unlikely to explain the differencesseen between depths with regard to depth range. By analogy,FIG. 3. Mean depth range and species richness plotted as a functionthe link between past glaciations and contemporaryof latitude for Pacific Ocean fishes (data from Eschmeyer & Herald,

1983) using methodology of Stevens (1989). Correlation between distributions of terrestrial organisms is not likely to bespecies richness and mean depth range is statistically significant useful in explaining the marine situation where Rapoport(rs=−0.93, P<0.01). effects are clearly evident despite the absence of such severe

climatic challenges in the past. No glaciers forced fish outof the deepest depths, yet an indisputable Rapoport’s rule

RESULTSfor depth appears.

The weak test of the Competition Hypothesis also lendsRapoport’s rule can be seen in the depth ranges of marinefish (Fig. 2) at all latitudes. When comparisons are made support only to the Seasonal Variability Hypothesis. The

expectation of narrower depth ranges at all depths inbetween latitudes, with regard to depth range and speciesrichness, a pattern entirely consistent with that seen in the latitudes that support large numbers of fishes is not seen.

As mentioned above, in middle depths the latitudinalterrestrial environment (Stevens, 1989, 1992) appears (Fig.3). This latter comparison is not latitudinal extent with variation in depth range is virtually non-existent, or at the

very least is near the limit of resolution of the analysis (50 mlatitude, but depth range (a measure of geographical extent)with latitude. As mentioned in the introduction, no depth intervals).

There should be no doubt that Rohde et al. (1993) werecorrelation between latitudinal range and latitude is expectedin the marine environment if depths are ignored as a co- correct in their observation that no meaningful correlation

between latitudinal range and latitude exists for marinevariate and none is found (either here or in Rohde et al.,1993). fishes. However, that does not mean that Rapoport’s

Bathymetric Rule does not play a role in the latitudinalFinding narrow depth ranges for fishes near the surfaceof the ocean and broader ranges in deeper waters supports gradient in species richness as those authors claim (compare

Figs 4 and 2). Changes in species richness do mirror changesthe Seasonal Variation Hypothesis, but the strongestevidence for the role of seasonal change in producing in mean depth range.

Furthermore, the claim that Rapoport’s rule is simplyRapoport’s Bathymetric Rule comes from comparing depth


Rapoport’s bathymetric rule 153

depth range. The reasons for this relate to the thermallayering of ocean waters.

Near the equator the temperature of the water may dropfrom 25°C at the surface to 5°C at a depth of 1 km, butit may be necessary to go 5000 km north or south fromthe equator to reach a latitude where the surfacetemperature has fallen to 5°C. The average verticaltemperature gradient (change of temperature per unitdistance) in this case is about 5000 times the horizontalone. (Pickard & Emery, 1982: 29)

While surface waters do show shifts in temperature withseason (Fig. 1), seasonal variation in temperature attenuateswith depth. This attenuation, and the weakening of the

FIG. 5. Frequency histogram of depth ranges for Pacific Ocean latitudinal component of Rapoport’s Bathymetric Rule forfishes found at 30°N latitude v. depth of transect. Mid-points of marine fishes (Fig. 2), is the strongest evidence to daterange of depths (0–100, 100–200, etc.) are plotted on the x axis. for the Seasonal Variability Hypothesis outlined in theNote the shift in dominance by narrow-ranged species at the surface

introduction. Neither the Differential Extinction nor theto larger-ranged species at greater depth. This shift in frequencyCompetition hypotheses explain the marine data as well.distribution produces Rapoport’s Bathymetric Rule for marine

The link between species richness and Rapoportfishes. Compare with Fig. 6.phenomena remains firmly in place despite claims to thecontrary (Rohde et al., 1993). The presence of a clearRapoport Depth Rule for marine fishes allows the non-equilibrium hypothesis proposed in terrestrial systems(Stevens, 1989, 1992) still to function. In low latitudesthere is a high potential for species to spill-over fromtheir preferred depth range into non-preferred depths. Theincreased potential for this spill-over in low latitudes is dueto the narrower depth ranges and the shorter distancesbetween preferred and non-preferred depths. Since manymarine organisms disperse widely during juvenile stagesthe potential for spill-over species to disrupt competitiveexclusion in local assemblages of species is quite high.Unsuccessful colonists use up local resources and generatediffuse competition for established residents, yet can neverbe excluded no matter how poorly suited they might be tolocal abiotic or biotic conditions. There is a constant rain

FIG. 6. As in Fig. 5 but for 65°N latitude.of these poorly adapted, but disruptive, propagules fromthe depths to which they are well adapted.

On inspection of the relationship between depth andspecies richness (Fig. 4) most biologists would probablydue to statistical inertia (Rohde et al., 1993, their Fig. 11)

is ingenious, but irrelevant. When frequency histograms of relate the species richness gradient to the drop inproductivity with depth. Local productivity may be partiallydepth range v. depth are plotted for different latitudes (Figs

5 and 6) it is evident that there are major differences in the responsible for the decline in species richness but spill-oversmay still play a part. The most productive waters (at thedepth range sizes with different depths and latitudes. There

are very few species with narrow depth ranges at high surface) in high latitudes are not the most species-rich. Thismay be an artefact of the use of regional surveys v. pointlatitude or at great depth. There are very few species with

broad depth ranges at low latitude or near the surface of samples (see Stevens, 1989, 1992 for fuller discussion), butmight also reflect the limited source for colonization bythe ocean. Rapoport’s Bathymetric Rule is not a statistical

artefact that results from the greater weighting of each warm-tolerant species at high latitudes. The warm waterhabitat at high latitudes is too transient for narrow-rangedspecies of low diversity assemblages as compared to

members of rich communities. The underlying frequency species to appear. Productivity alone does not producehigh species richness; colonization from persistent sourcedistributions of depth range are clearly different. All of the

terrestrial assemblages I have analysed (Stevens, 1989, 1992) populations also shapes local marine communities.show analogous patterns.

ACKNOWLEDGMENTSDISCUSSION AND CONCLUSIONS

Discussions and correspondence with J. H. Brown, R. K.Colwell, K. Rohde, P. A. Marquet and an anonymousThe climatic tolerances of open ocean marine organisms is

not as well reflected in their latitudinal range as it is in their reviewer improved the manuscript. Data analysis and



Pulliam, H.R. (1988) Sources, sinks, and population regulation.manuscript preparation were supported by the DepartmentAm. Nat. 132, 652–661.of Biology and Office of the Associate Provost of the

Ricklefs, R.E. & Cox, G.W. (1972) Taxon cycles in the West IndianUniversity of New Mexico and through funding providedavifauna. Am. Nat. 106, 195–219.by the National Science Foundation DEB-9318096.

Rohde, K., Heap, M. & Heap, D. (1993) Rapoport’s rule does notapply to marine teleosts and cannot explain latitudinal gradients

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Extending Rapoport's rule to Pacific marine fishes

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