Conservation Implications of Georaphic Range Size—Body Size Relationships

Conservation Implications .of Geographic Range Size-Body Size Relationships KEVIN J. GASTON* AND TIM M. BLACKBURN

Department of Biology and NERC Centre for Population Biology, Imperial College, Silwood Park, Ascot, Berkshire SL5 7PY, U.K.

Introduction

The oft-cited statement that 'big fierce animals are rare' (Colinvaux 1978) is, like most such generalizations in ecology, only partially correct. Animals with large bod- ies, fierce or otherwise, do have lower densities than smaller species, at least in studies based on compendia of data drawn from the literature (for example, Damuth 1981, 1987, 1993; Peters & Wassenberg 1983; Currie 1993; Silva & Downing 1994); conclusions may be rather different w h e n based on sampling whole assemblages of taxonomically similar animals e.g. Brown & Maurer 1987; Morse et al. 1988; Cotgreave 1993; Black- burn & Lawton 1994). However, a growing body of work has documented positive relationships be tween the geographic range size and the body size of animal species (Table 1). In terms of geographic range size, within taxonomic assemblages (such as North American mammals) big animals are often rather common; geographic range size is a dimension of the rare-common axis equally as valid, and perhaps as widely applied, as density or population size (Rabinowitz 1981; Rabinowitz et al. 1986; Reed 1992; McCoy & Mushinsky 1992; Fiedler & Ahouse 1992; Gaston 1994). Moreover, at least amongst terrestrial mammals, the big fierce species may tend to have particularly large geographic range sizes; carnivores have larger geographic ranges on average than species in other terrestrial mammalian orders (Brown 1981; Rapoport 1982; Pagel et al. 1991; Letcher & Harvey 1994).

The relationship be tween interspecific geographic range size and body size has attracted attention primarily in the context of macroecology and may explain how species partition space and resources (Brown & Maurer 1987, 1989; Gaston & Lawton 1988b; Gaston 1994; Law-

* Current address: Department of Animal and Plant Sciences, Univer- sity of Sheffield, Sheffield $10 2TN, U.K. Paper submitted January 3, 1995; revised manuscript accepted June 8, 1995.

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ton et al. 1994; Taylor & Gotelli 1994). It also has some potentially important consequences for conservation (as do various other macroecological patterns) (Lawton 1993; Gaston 1994; Gaston & Blackburn 1995a,b). We limit ourselves here to consideration of interspecific range size to body size relationships from studies on assemblages at large geographic scales, such that all or most of the geographic ranges of the species in the assemblage are considered. It is on global, rather than local, scales that the conservation status of species is most important. At more restricted scales the form of the range size to body size relationship is less clear (see for example Gaston 1988; Gaston & Lawton 1988a,b).

The Range Size-Body Size Relationship

Although frequently described simply as a positive cor- relation, the relationship be tween geographic range size and body size at large scales is commonly rather more complex than this implies (Fig. 1; the former variable is usually, and often both variables are, logarithmically transformed; Brown & Maurer 1987; Gaston 1994). Typi- caUy, species of all body sizes may have large geographic ranges, with the uppe r limit to range size (Fig. 1, boundary AB) being set by the size of the entire study area (often a continent) under consideration (Brown & Maurer 1987, 1989). However, the minimum geographic range size exhibited by species tends to increase with body size (Fig. 1, BC), such that small species have a variety of range sizes, but large-bodied species have only large ranges. In short, the relationship is approximately triangular (Fig. 1).

The degree of definition of the lower boundary to the range size to body size relationship (the increase in minimum geographic range size with increasing body size) appears rather variable. In some data sets it is reasonably clear cut, but in others it is not and the approximately triangular range size to body size relationship may only reflect the predominant combinations of range size and body size. The statistical significance of the lower

Gaston & Blackburn Range Size-body Size Relationships 639

Table 1. Case studies a of interspecitic geographic range size-body size relationships for animals.

Taxon Relationship b Comments Sottrce

North American terrestrial + mammals

North American Peromyscus +, - & NR mice

Coral-dwelling mantis + shrimps

North American land + mammals

North American Brachinus NR beetles

North American land birds + Neotropical forest mammals +

Australian birds + North American mammals + Amazonian primates +

Afrotropical dung beetles 4- Primates NR Cyprinella minnows +

Relationship is - v e across all species in genus, but + , - and NR found within species-groups

No formal statistics No relationship after controlling

for latitude

Van Valen (1973)

Glazier (1980)

Reaka (1980)

Brown (1981), Brown & Maurer (1989), Brown & Nicoletto (1991)

Juliano (1983)

Brown & Maurer (1987) Arita et al. (1990), [see also Fig. 6.7

in Gaston (1994)] Maurer et al. (1991) Pagel et al. (1991) Ayres & Clutton-Brock (1992)

Cambefort (1994) Gaston (1994) Taylor & Gotelli (1994)

aStudies are only included i f carried ou t a t sufficiently large geographic scales so that all or mos t o f the spatial extents o f the geographic ranges o f the species in the assemblage in quest ion are embraced. Al though beyond the scope o f this paper, a posi t ive relationship between range size a n d body size has also been documen t ed Jbr oak trees (Aizen & Patterson 1990; see Gaston 1994 f o r general discussion o f relationships f o r plants). b +, statistically s ignif icant posi t ive relationship; - , statistically signif icant negative relationship; a n d NR, no statistically signif icant relationship.

b o u n d a r y is difficult to tes t ob jec t ive ly (bu t see Black- bu rn et al. [1992] for some m e t h o d s d e v e l o p e d in an ana logous s i tuat ion) .

F rom a conse rva t ion p e r s p e c t i v e the l o w e r b o u n d a r y is, none the les s , t he mos t i m p o r t a n t fea ture o f t he inter- speci f ic g e o g r a p h i c range size to b o d y size re la t ionsh ip b e c a u s e it appea r s to r e p r e s e n t some l o w e r l imit to the g e o g r a p h i c range size poss ib l e for a spec ies o f a g i v e n b o d y size to attain. Species may lie a long the b o u n d a r y

Figure 1. Idealized interspecific geographic range size to body size relationship.

w h e n a res t r i c t ed geog raph i c range size (for the i r b o d y size) is the i r no rma l state, b e c a u s e the size o f the i r range is in the p roce s s o f increas ing (and will move t h e m away f rom the l o w e r bounda ry ) , o r because they are on a tra- j ec to ry to ex t i nc t i on ( t hey have m o v e d to the l o w e r b o u n d a r y f rom a larger range size). Whicheve r , it seems l ikely that spec ies lying a long this b o u n d a r y w o u l d have the h ighes t p robab i l i ty o f be ing t h r e a t e n e d w i th ext inc- t ion as a result ' o f any fac tor that t e n d e d to dep re s s the i r g e o g r a p h i c range sizes further . Never the less , it wil l be difficult to s ta te even this w i t h o u t answer ing the important ques t ion o f w h y the l o w e r b o u n d a r y exists. Various m e c h a n i s m s have b e e n p ro f fe red to exp la in the overal l re la t ionsh ip b e t w e e n geog raph i c range size and b o d y size (Brown & Maurer 1987; Gas ton & Lawton 1988b; Gas ton 1990, 1994). Wi th r e s p e c t specif ical ly to the l o w e r boundary , t h e r e are t h ree p r inc ipa l possibi l i t ies .

(1) Min imum viable p o p u l a t i o n size (Brown & Maurer 1987): Because la rger -bodied spec ies have, o r re- quire, la rger h o m e ranges (McNab 1963; Arm- s t rong 1965; S c h o e n e r 1968; Clut ton-Brock & Har- vey 1977; Linstedt et al. 1986; Swihar t et al. 1988), t hey may also requi re larger geog raph ic range sizes in o r d e r to main ta in v iable p o p u l a t i o n sizes.

(2) Real ized vs. po ten t i a l geog raph ic range size ratios: Min imum range size may increase w i th b o d y size because , on average, larger-bodied spec ies d isperse

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640 Range Size-body Size Relationships Gaston & Blackburn

(3)

more rapidly and successfully (i.e. manage to establish) than small-bodied or are evolutionarily older and have therefore had longer periods in which to disperse, establish, and attain larger geographic range sizes. Larger-bodied species may therefore occupy a greater proportion of their potential range sizes (the geographic range which could be occupied were all barriers to dispersal overcome; Gaston 1994). Latitudinal gradients (Pagel et al. 1991): Both the average body sizes and geographic range sizes of species in assemblages tend to decline toward lower latitudes (Bergmann's and Rapoport 's rules, respectively; Lindsey 1966; Stevens 1989, 1992a; France 1992; Cushman et al. 1993; Letcher & Har- vey 1994; Taylor & Gotelli 1994). This could result, perhaps incidentally, in an increase in minimum geographic range size with increasing body size.

As is frequently the case in macroecology (Warren & Gaston 1992; Gaston 1994), each of these mechanisms may contribute to the observed geographic range size to body size interaction in greater or lesser part, depending on the taxon and region under consideration. However, the minimum viable population size mechanism must be the strongest contender for having a disproportionately large influence on the pattern, and it has garnered sub- stantial support (Brown & Maurer 1987, 1989; Taylor & Gotelli 1994). This mechanism essentially expands con- ventional population viability arguments to a species' entire geographic range.

There is likely an element of truth in the realized vs. potential range size ratio hypothesis. Cases are known in which larger-bodied species are better and more suc- cessful dispersers or are evolutionarily older (Reaka 1980; Dingle et al. 1980; Derr et al. 1981; Taylor & Go- telli 1994). However, interactions of body size with dispersal and establishment abilities and with evolutionary age often seem to be very complex, with competing trade-offs and dynamics making simple predictions tenu- ous and alternative patterns likely (Southwood 1981; Lawton & Brown 1986; Brown & Maurer 1989).

The observation that there is a potentially confound- ing influence of latitudinal gradients in range size and body size on the interaction between these two variables (Pagel et al. 1991) is important and should be considered in future interspecific analyses of the geographic range size-body size relationship (likewise in other basic macroecological patterns, such as relationships between local density and geographic range size and between local density and body size). In one study at least, it was found that after controlling for the effect of latitude there was no significant interaction between geographic range size and body size (Taylor & Gotelli 1994). How- ever, it seems likely that often this will not be the case, particularly because neither Bergmann's nor Rapoport 's

rules are universal (Geist 1987; Ricklefs & Latham 1992; Rohde et al. 1993; Barlow 1994; Cotgreave & Stockley 1994; Roy et al. 1994; Smith et al. 1994; Hawkins & Law- ton 1995), and even when they are exhibited detailed changes in range size and body size with latitude need not be similar.

Conservation Implications

The overall triangular form of the range size-body size relationship, with the possible mechanisms that struc- ture the lower boundary, has several important consequences.

Body Size as a Predictor of Range Size

A triangular relationship between range size and body size means that body size cannot be used as a simple predictor of geographic range size by generalizing that larger-bodied species have larger geographic range sizes. Such assertions have been used in the past, for example, in defense of estimates of extremely high numbers of insect species in tropical realms (given that many tropical insects are small-bodied; e.g. Erwin 1991). Likewise, it also means that it will often not be possible to remove correctly the effect of interspecific differences in body size from relative measures of geographic range size by using linear models, for example to produce body size-inde- pendent evaluations of rarity (Dobson & Yu 1993). One solution may be to fit polynomial regressions, although it is unclear how the residuals from such relationships should be interpreted (Gaston & Blackburn 1995a).

Missing Range Size-Body Size Combinations

A triangular range size-body size relationship indicates that species with some combinations of these variables do not exist. Developing an understanding of what is the dominant determinant of the increase in minimum geographic range size with body size is important because it would enable us to distinguish whether large-bodied species with small geographic ranges simply tend not to exist or whether they cannot exist. Under the realized to potential geographic range ratio mechanism, large-bodied species with small geographic ranges may be capa- ble of persisting. The same would be true of the latitudinal gradient mechanism if, as has been suggested, the increase in the mean (or median) size of species' geographic ranges toward high latitudes (and altitudes) is a consequence of the greater breadth of environments that species at high latitudes can exploit, as a result of their requiring greater environmental tolerances to sur- vive in any one area (Stevens 1989, 1992b). However, under the favored hypothesis, the minimum viable population size mechanism, such large-bodied species with



Table 2. Case studies of interspeciflc body size/risk of extinction relationships for animals.

Taxon M e a s u r e a Scale ~ Re la t ionsh ip c C o m m e n t s Source

Mammals no. of mountain ranges local + on which spp. persist

Birds local + Birds recorded extinctions local + ?

Birds missing from land-bridge local NS islands

Birds missing from land-bridge local + island

Mammals no. of mountain ranges local +/NS on which spp. persist

Birds recorded extinctions local

Birds missing from habitat local NS islands

Mammals threat listing local +

Waterfowl threat listing global NS

Birds missing from land-bridge local NS islands

Mammals threat listing global + ?

Mammals response to fragmentation local NS Birds est. time to extinction global NS

Birds vulnerability index global +/NS Mammals threat listing local + Birds recorded extinctions local NS

within trophic groups

slight trend within families for spp. presumed extinct to be larger

results vary with trophic group

for a given pop. size, below 7 pairs a large-bodied sp. is less prone to extinction than a small-bodied, above 7 pairs the advantage reverses a

when analysis controls for differences in abundance, larger spp. have lower risk of extinction

none of spp. in largest body size class scored as declined/extinct

slight excess of extant large-bodied spp. on islands

endangered species in all body size classes

minimum time to extinction increased above 100 g

results vary with taxon

analysis based on residual extinction probability, having controlled for differences in population size

Brown (1971)

Leek (1979) Jiirvinen & Ulfstrand

(1980) Terborgh & Winter

(1980) Karr (1982)

Patterson (1984)

Pimm et al. (1988)

Soul6 et al. (1988)

Burbridge & McKenzie (1989)

Laurila & Jiirvinen (1989)

Gotelli & Graves (1990)

Ceballos & Navarro (1991)

Laurance (1991) Maurer et al. (1991)

Kattan(1992) Rebelo(1992) Rosenzwe~ & Clark

(1994)

Fish recorded extinctions local NS Angermeier (1995) Birds threat listing global + Gaston & Blackburn

(1995) Butterflies threat listing local NS Mawdsley & Stork

(1995) Wildfowl threat listing global NS Gaston & Blackburn

(in press)

aMeasure o f risk extinction. °Scale o f extinction (i.e., local v. global; local embraces areas o f wide range o f sizes). c +, statistically significantposttive relationship (larger-bodied spp. at greater risk); - , statistically significant negative relationship; and NR, no statistically significant relationship. dBut see Tracy and George (1992).

small geographic ranges can exist only t ransient ly on route to at taining geographic range sizes large e n o u g h that they have popu la t ion sizes that enable pers is tence or o n route to ext inct ion.

Range Size, Body Size, and Likelihood of Extinction

A n u m b e r of ecological traits have b e e n pos tu la ted as correlates of the l ikelihood that species will b e c o m e ex- t inct (e.g. adult survival rate, body size, dispersal ability,

env i ronmen ta l tolerance, fecundity, geographic range size, habitat specificity, longevity, populat ion size, trophic status; e.g. Terborgh & Win te r 1980; Diamond 1984; Karr 1990; Gaston 1994; Lawton 1995). In the main, such relat ionships appear at best to be weak or inconsis- t en t and of ten almost or ent i rely absent . Nonetheless, animal species of small geographic range size and large body size (especially those species at h igher t rophic lev- els) have repeatedly b e e n claimed to exhibi t high proba- bilities of ext inct ion.


642 Range Size-body Size Relationships aaston & Blackburn

Table 3. Case studies of interspeciflc size of spatial distrilmtion/rlsk of extinction relationships for animals.

T a x o n M e a s u r e a Scale ° R e l a t i o n s h i p c Source

Mangrove island insects recorded extinctions local Leafhoppers recorded extinctions local Freshwater molluscs recorded extinctions local Bracken insects recorded extinctions local Waterfowl threat listing global Waterfowl threat listing global Butterflies threat listing local Wildfowl threat listing global

D

Hanski (1982) Hanski (1982) Hanski (1982) Gaston & Lawton (1989) Laurila & Jfirvinen (1989) Mace (1994) Mawdsley & Stork (1995) Gaston & Blackburn (in press)

aMeasure o f risk o f extinction. aScale o f extinction (i.e. local v. global, local embraces areas o f a wide range o f sizes). c +, statistically significant positive relationship; - , statistically significant negative relationship (more widely distributed species at greater risk); and NR, no statistically significant relationship.

We have collated all those published studies of the interspecific relationships between likelihood of extinction and body size (Table 2) and likelihood of extinction and size of spatial distribution (Table 3) of which we are aware. We extended the collation to all studies of the relationship between risk of extinction and spatial distribution because of the limited number of studies of relationships between risk of extinction and overall geographic range size. Nonetheless, the number of studies considered remains very small. It could be greatly increased were density or population size accepted as a surrogate for size of spatial distribution on the grounds that the two are positively correlated (Hanski 1982; Brown 1984; Gaston & Lawton 1990; Hanski et al. 1993; Gaston 1994). However, such correlations are variable and sometimes rather weak (Gaston 1994), and we do not find this a helpful approach.

There is little evidence that large-bodied species are consistently at greater risk of extinction than small-bodied, even when no attempt is made to control for any broad differences in the densities or population sizes of species of differing body size (Table 2). If larger-bodied species had lower abundances they might be expected to have higher risks of extinction for this reason alone. Rather, as many studies find no significant relationship as find a positive one.

In contrast, there is a reasonably consistent general negative relationship between the size of a species' spatial distribution and its risk of extinction (Table 3). How- ever, some caution is required in interpreting these re- suits. First, most studies concern very small scales and extinctions of species at a few sites. Although it seems reasonable to suppose that at larger scales species with small ranges on average experience higher risks of extinction, it is not inevitably so; species are known that retained large geographic range sizes prior to their re- cent extinction (Vermeij 1993). Second, sampling arti- facts tend to result in greater numbers of local extinctions being recorded for narrowly distributed species; species occurring at smaller numbers of sites tend to

have lower local densities (Hanski 1982; Brown 1984; Hanski et al. 1993; Gaston 1994) and are therefore more likely to be overlooked at any one site. Third, where studies employ listings of the threat status of species as measures of their likelihood of extinction, relationships with the size of a species' spatial distribution may be meaningless if this variable was used to determine the threat status at the outset. Fourth, this difficulty aside, cause and effect often cannot be readily determined. Do species have small distributions because they are experiencing a high risk of extinction or are they experiencing a high risk of extinction because they are narrowly distributed? Documented negative interspecific relationships between the persistence of species in the fossil record and their spatial distribution suffer from many of these same limitations (Jackson 1974; Hansen 1978, 1980; Jablonski 1986, 1987; Russell & Lindberg 1988a,b; Buzas & Culver 1991).

The triangular shape of the interspecific range size to body size relationship suggests a reason why there is no consistent relationship between the risk of extinction faced by a species and its body size and that there need not always be a negative interspecific relationship between risk of extinction and range size. If the species at greatest risk of extinction are those closest to the lower boundary of the range size to body size relationship, they need not belong to any particular body size class or range size class. In assessing risks of extinction where species lie in a range size to body size plot may be more important than whether they are large or small or widespread or narrowly distributed. Note, however, that although a widespread species may have a high risk of extinction, its trajectory to extinction will inevitably involve attaining a smaller and smaller geographic range size; these are two different issues. This is particularly significant because many schemes for categorizing species in terms of their presumed risks of extinction (as a prelude to determining conservation priorities) tend to give disproportionate weight to species with small geographic range sizes.

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aaston & Blackburn Range Size-body Size Relationships 643

Minimum Viable Population Size Mechanism

The minimum viable population size mechanism for the lower boundary to the range size to body size relationships appears to imply at least two things. First, it suggests that species on the lower boundary tend to have open population structures, such that species persistence depends fundamentally on global population size. Because large-bodied species tend to lie on or close to the lower boundary, this means that on average they should tend more often to have open population structures than small-bodied species.

One way to begin to test the validity of the open population assumption may be to look for differences in the racial subdivision of populations of small and large-bodied species. The evolution of phenotypically distinct subspecies suggests isolation of parts of a species' population. If large-bodied species are constrained to have large geographic ranges through a minimum viable population size mechanism, they should also have open populations and hence fewer distinct subspecies than small-bodied species with equivalent geographic range sizes (although other mechanisms--for example, greater dispersal ability of large than small-bodied species-- might also generate this result). A negative relationship between the body size of a species and the number of races into which it is divided has been shown in a different context for Amazonian primates (Ayres & Clutton- Brock 1992).

If population structures are not open, then the effec- tive size of a species' overall population may be considerably smaller than it appears from its position in the interspecific geographic range size to body size relationship. Few conclusions about minimum viable population sizes could be drawn from such plots in this case. However, if the open population assumption is met, the minimum viable population size mechanism suggests a serious consequence of population fragmentation. Frag- mentation of the geographic range of a species near the lower boundary of the geographic range size to body size plot (Fig. 1, BC) into two or more separate populations could easily result in sub-populations smaller than the minimum viable size, with the extinction of possibly all sub-populations as an inevitable consequence.

Second, the minimum viable population size hypothesis implies that large-bodied species are on average closer to their minimum viable population size than are small-bodied species. If species closer to this boundary are more likely to slip below it, this could suggest why, in those studies that have shown a relationship at all, it is generally large-bodied species that have been shown to be at greater risk of extinction (Table 2). However, the situation may be complicated if the tendency for greater fluctuations in the local abundance of small-bodied species is also reflected in greater plasticity in the geographic range sizes of these species. What is effectively a

"safe ~ distance from the lower boundary may not be constant across body sizes.

In a related vein Silva and Downing (1994) noted that minimal mammal densities scaled as the - 0 . 6 8 power of body mass, so that larger-bodied species can sustain lower minimum densities. However, their analyses ex- plicitly include populations of rare and endangered species. A minimum viable population size model for the lower boundary of range size to body size plots implies that many of these species may be below their minimum. Whether an analysis of density involving these species gives any information on how minimum density scales with body size will depend on whether minimum viable density and minimum viable population size are related. If there is a positive relationship, species below the lower range size to body size boundary should be ex- cluded from minimum density analyses. If there is no or a negative relationship, minimum density may not be a good estimator of population sustainability.

Range Size Reduction

The geographic ranges of many large-bodied animal species have been substantially reduced as a product of hu- man activities. If the dominant determinant of the interspecific increase in minimum range size with body size is the minimum viable population size mech~ i sm, then when such species are driven below the lower range size boundary (Fig. 1, BC) they are inevitably on a trajectory to extinction, unless they are managed in such a way as to counteract the fact that they have global populations that would not naturally be viable. The reduction in their geographic ranges has placed them in an area of the geographic range size to body size space in which species do not occur naturally because they suffer selec- tive extinction. It will often be unclear for what proportion of species this is the case in assemblages where studies of interspecific geographic range size to body size relationships are based on data for species historical ranges (e.g. Pagel et al. 1991). Likewise, it will be less obvious when species' range sizes are quantified in terms of extents of occurrence rather than areas of occu- pancy (sensu Gaston 1991) because the latter may potentially be considerably reduced with little change in the former.

The concern that reducing species geographic range sizes so that they fall below the lower range size boundary (Fig. 1, BC) may take species below their minimum viable population size will be true not only for larger- bodied species in an assemblage, but for species of all body sizes if they lie close to the boundary. Further, because geographic range is typically plotted on a logarith- mic scale in these comparisons, a considerable reduction in the range size of a species near the boundary will not necessarily take that species far over it. From the


644 Range Size-body Size Relationships aaston & Blackburn

minimum viable population mechanism, it is unclear h o w the dynamics (such as rapidity) of extinction might change with distance below the lower range size boundary, and hence affect opportunities for conservation ac- tion. Because geographic range size is plotted on a loga- rithmic scale, the probability of a species becoming extinct likely multiplies as distance is added to the gap be tween the species and the boundary; thus, any species noticeably below the boundary probably has a high risk of extinction. This suggests that a wise conservation strategy would be to prevent species from crossing this boundary. Moreover, it also provides a possible criterion for judging one dimension of the conservation status of species using their geographic ranges: Species close to the boundary, or that are fast approaching it, are in greater danger of slipping over the edge.

Acknowledgments

K. J. G. is a Royal Society University Research Fellow. T. M. B. was supported by N.E.R.C. grant GR3/8029. We thank John Lawton and two anonymous referees for comments on this work.

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Conservation Implications of Georaphic Range Size—Body Size Relationships