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O R I G I N A LA R T I C L E

Untangling latitudinal richness gradients

at higher taxonomic levels: familial

perspectives on the diversity of New

World bat communities

Richard D. Stevens

Department of Biological Sciences, Program in

Ecology, Texas Tech University, Lubbock, TX,

and National Center for Ecological Analysis

and Synthesis, University of California, Santa

Barbara, CA, USA

Correspondence: Richard D. Stevens, National

Center for Ecological Analysis and Synthesis,

735 State Street Suite 300, Santa Barbara, CA

93101-5504, USA.

E-mail: [email protected]

ABS T RACT

Aims (i) To describe at the level of local communities latitudinal gradients in the

species richness of different families of New World bats and to explore the

generality of such gradients. (ii) To characterize the relative effects of changes in

the richness of each family to the richness of entire communities. (iii) To

determine differences in the rate and direction of latitudinal gradients in species

richness within families. (iv) To evaluate how differences among families

regarding latitudinal gradients in species richness influence the latitudinal

gradient in species richness of entire communities.

Location Continental New World ranging from the northern continental

United States (Iowa, 42 N) to eastern Paraguay (Canindeyu, 24 S).

Methods Data on the species composition of communities came from 32

intensively sampled sites. Analyses focused on species richness of five of nine New

World bat families. Multivariate analysis of variance and discriminant function

analysis determined and described differences among temperate, subtropical, and

tropical climatic zones regarding the species richness of bat families. Simple linear

regression described latitudinal gradients in species richness of families. Path

analysis was used to describe: (i) the direct effect of latitude on species richness of

communities, (ii) the indirect effects of latitude on the species richness of

communities through its effect on the species richness of each family, (iii) the

relative effects of latitude on the species richness of bat families, and (iv) the

relative contribution of each family to variation in the species richness of

communities.

Results Highly significant differences among climatic zones existed primarily

because of a difference between the temperate zone and the tropical and

subtropical zones combined. This difference was associated with the high number

of vespertilionids in the temperate zone and the high number of phyllostomids in

the tropical and subtropical zones. Latitudinal gradients in species richness were

contingent on phylogeny. Although only three of the five families exhibited

significant gradients, all families except for the Vespertilionidae exhibited

indistinguishable increases in species richness with decreases in latitude. The

Emballonuridae, Phyllostomidae and Vespertilionidae exhibited significantlatitudinal gradients whereby the former two families exhibited the classical

increase in species richness with decreasing latitude and the latter family exhibited

the opposite pattern. Variation in species richness of all families contributed

significantly to variation in the species richness of entire communities.

Nonetheless, the Phyllostomidae made a significantly stronger contribution to

changes in species richness of communities than did all other families. Much of the

Journal of Biogeography ( J. Biogeogr .) (2004) 31, 665–674

ª

2004 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 665



INT RO DUCT IO N

One of the most widely documented biogeographic patterns

describing the spatial distribution of organisms is the latitudi-

nal gradient in species richness. In general, species richness

increases from the polar regions to the tropics and this has been

demonstrated regardless of the taxonomic affiliation of species

(i.e. vertebrates, invertebrates, or plants) or the particular

continent on which they reside (see reviews by Gaston, 1994;

Brown, 1995; Rosenzweig, 1995; Willig, 2001). Mammals have

received much attention regarding latitudinal gradients

(Fischer, 1960; Fleming, 1973; Wilson, 1974; McCoy & Connor,

1980; Mares & Ojeda, 1982; Willig & Selcer, 1989; Arita et al.,

1990; Currie, 1991; Meserve et al., 1991; Pagel et al., 1991;

Willig & Sandlin, 1991; Mares, 1992; Willig & Gannon, 1997;

Lyons & Willig, 1999, 2002; Stevens & Willig, 2002). When all

mammals are considered as a single group, they exhibit strong

latitudinal gradients in which species richness increases towards

the equator in the New World as a whole as well as in both

North and South America when considered separately (Simpson, 1964; Fleming, 1973; Kaufman & Willig, 1998).

Unfortunately, latitudinal gradients in species richness across

mammalian orders or any other taxon have not been

quantitatively examined in a comprehensive fashion. Thus,

the relative degree to which different groups of mammals

contribute to the overall gradient remains unclear. Nonetheless,

Kaufman (1995) graphically demonstrated that while 11 of 12

New World mammalian orders exhibit a latitudinal gradient in

species richness, much variation existed with respect to the

strength and even direction of gradients.

The greatest contribution to latitudinal gradients in species

richness of mammals overall is perhaps thestrong increase in the

number of bat species as one moves towards the equator

(Simpson, 1964; Fleming, 1973; Kaufman, 1995; Willig, 2001).

The faunal diversity (regional species richness or gamma

diversity estimated from geographical distribution maps) of

bats has received perhaps the most attention (Willig & Selcer,

1989; Willig & Sandlin, 1991; Lyons& Willig, 1999, 2002; Stevens

& Willig, 2002),whereaspatternsmeasured at thelocal level have

received much less attention. Latitudinal gradients in the

diversity of bats exist not only across a number of spatial scales

(Lyons & Willig, 2002),but also at thelevel of local communities

and when characterized by a number of indices of taxonomic

diversity that incorporate species abundances (Stevens & Willig,

2002). However, there are significant differences between

latitudinal gradients measured at the local and regional levels

(Stevens & Willig, 2002). Because beta diversity also exhibits a

latitudinal gradient, the disparity in diversity between the

regional and local levels increases towards the equator. The effect

of latitude on the species richness of bats is so strong that it hasbeen claimed to be the impetus behind the pattern for mammals

in general (Fleming, 1973). This is further suggested by the

observation that the species richness of bats exhibits absolute

rates of latitudinal change that are either stronger or indistin-

guishable from the rate of change in species richness of all other

terrestrial mammal orders combined (Kaufman & Willig, 1998).

While concordance among levels in a taxonomic hierarchy

among lower and higher taxa of a particular clade regarding

latitudinal species richness gradients suggest their generality in

some cases (Macpherson & Duarte, 1994), the disproportion-

latitudinal gradient in species richness of communities could be accounted for by

the effects of latitude on the species richness of constituent families.

Main conclusions Ecological and evolutionary differences among higher

taxonomic units, particularly those differences involving life-history traits,

predispose taxa to exhibit different patterns of diversity along environmental

gradients. This may be particularly true along extensive gradients such as latitude.

Nonetheless, species rich taxa, by virtue of their greater absolute rates of change,

can dominate and therefore define the pattern of diversity at a higher taxonomiclevel and eclipse differences among less represented taxa in their response to

environmental gradients. This is true not only with respect to how bats drive the

latitudinal gradient in species richness for all mammals, but also for how the

Phyllostomidae drives the latitudinal gradient for all bats in the New World.

Better understanding of the mechanistic basis of latitudinal gradients of diversity

may come from comparing and contrasting patterns across lower taxonomic

levels of a higher taxon and by identifying key ecological and evolutionary traits

that are associated with such differences.

Keywords

Chiroptera, environmental gradient, latitudinal gradient, path analysis, species

richness, taxonomic diversity.

R. D. Stevens

666 Journal of Biogeography 31, 665–674, ª 2004 Blackwell Publishing Ltd



ate influence of a particular lower taxonomic group on the

strength of a latitudinal richness gradient characterizing a

higher taxon has also been demonstrated in a number of cases.

For example, Turner (1981) found a significant, negative

relationship between latitude and the number of copepod

species in the estuarine waters of eastern North America.

Nonetheless, when these species were divided into their

respective calanoid and cyclopoid suborders it was found that

the calanoid pattern was weak and non-significant and the

cyclopoid pattern was strong and highly significant. Thus, the

pattern for copepods as a group was driven by the pattern for

cyclopoids and unrepresentative of the pattern for calanoids. A

similar pattern holds true for bivalves (Crame, 2000). More

specifically, the global distribution of bivalves is characterized

by a strong latitudinal gradient in which the greatest number

of species is found at low latitudes. Nonetheless, when species

were divided into seven subclades comprised by the Bivalvia,

the same pattern did not obtain in many situations. When

patterns were different than for the Bivalvia as a whole they

were most frequently non-significant, but significant increases

in species richness with increases in latitude were alsodescribed. Moreover, those clades that exhibited the same

relationships as the Bivalvia were among those with the highest

species richness.

At the greatest extreme, such a taxonomic bias could create a

situation in which variation in diversity in one or a relatively

few species-rich taxa dominates variation in diversity of the

higher taxon. This would create the appearance of generality of

a particular pattern of diversity at the higher taxonomic level,

even when a number of relatively rare taxa at the lower level

fail to evince the pattern. While it may seem unrealistic to

expect that all lower taxa within a clade exhibit the same

response in diversity along an environmental gradient, the

prevailing pattern for a higher taxon may grossly misrepresent

patterns for many constituent lower taxa. Moreover, because

clades within a higher taxon represent closely related groups of

species, a better understanding of the mechanistic basis of

gradients in diversity may come from understanding, which

ecological and evolutionary characteristics are associated with

different patterns of diversity. Although much has been learned

about latitudinal gradients characterizing the class Mammalia,

little is known regarding the differences in quantitative

characteristics of these gradients among orders. Moreover,

even for bats, the best-studied mammalian order in terms of

latitudinal gradients, no quantitative assessment of differences

in latitudinal gradients in species richness within genera oreven families has been conducted [however see Willig & Selcer

(1989) for a qualitative assessment conducted at a larger spatial

scale].

Members of nine of the 17 extant families of bats can be

found in the New World (Koopman, 1993). Six of these

families are endemic to North and South America and their

surrounding islands, one family is cosmopolitan, and two

exhibit tropical affinities in both the Old and New World.

Inter-familial differences in geographical distribution suggest

that families respond differently to environmental gradients.

Because a number of environmental gradients coincide with

latitude, different families should exhibit unique latitudinal

gradients. The nature of these differences and how they affect

the structure of local communities is poorly understood.

Herein, latitudinal gradients in species richness of five of the

most species-rich families of bats (Vespertilionidae, Phyllos-

tomidae, Molossidae, Mormoopidae and Emballonuridae)

occurring in the New World were evaluated. Moreover,

differences among bat families regarding the strength of

latitudinal gradients in species richness were evaluated. Finally,

the differential effects of variation in species richness of these

different families on the species richness of entire communities

were determined.

M AT E RIALS AND M E T HO DS

Data on the structure of 32 bat communities were amassed

from the literature (Fig. 1, Table 1). A more comprehensive

description of the diversity of these communities and criteria

used in their selection from the many available in the literature

can be found in Stevens (2002) and Stevens & Willig (2002).Familial distinctions follow Koopman (1993). In total, nine

families of bats occur in the New World. Four tropical families

were excluded from analyses due to the small number of

species they comprise (i.e. four Thyropteridae, two Furipter-

Figure 1 Location of 32 New World bat communities used to

evaluate geographical patterns of species richness within families.

(a), (b) and (c) represent temperate, subtropical and tropical

zones, respectively.

New World bat communities

Journal of Biogeography 31, 665–674, ª 2004 Blackwell Publishing Ltd 667



idae, two Noctilionidae and five Natalidae species). Thus, these

analyses focused on the Emballonuridae (c. 20 species in the

New World), Mormoopidae (c. eight species), Phyllostomidae

(c. 145 species), Molossidae (c. 29 species in the New World),

and Vespertilionidae (c. 70 species in the New World).

Latitudinal variation in species richness within families may

not be represented by continuous linear changes. Accordingly,

communities were assigned to three geographical zones [Fig. 1,

tropical (12 N to 12 S), subtropical (13–23.45 N and

13–23.45 S), or temperate (> 23.45 N or 23.45 S)] and

significant differences among zones with regard to species

richness within families was evaluated using a Multivariate

Analysis of Variance (manova). A discriminant functionanalysis was used to graphically illustrate significant differences

indicated by manova and to determine the relative contribu-

tion of each family to differences among geographical zones

regarding species richness within families. The location

of significant differences was determined by conducting

three pair-wise manovas between geographical zones. In all

manovas, geographical zone was the independent variable and

species richness of each family per site were the dependent

variables. Experiment-wise error rate for pair-wise manovas

was held at 5% by application of a Bonferonni sequential

adjustment (Rice, 1989). All manovas and the discriminant

function analysis were conducted in Matlab (Math Works,

1995).

Simple linear regression (Sokal & Rohlf, 1995) was used to

determine the relationship between species richness and

latitude for each bat family across the 32 communities.

Regression analyses were conducted in SPSS (SPSS Inc., 1990)

and experiment-wise error rate was held at 5% by application

of a Bonferonni sequential adjustment (Rice, 1989).

A path diagram was constructed to explore two interrelated

questions: (i) what are the relative effects of latitude on familial

species richness and are there significant differences? and (ii)

what are the relative contributions of increases in speciesrichness within families to increases in total species richness

within communities and are there significant differences? First,

a multiple regression in which total species richness for the

community was the dependent variable and latitude as well as

the richness of each family were independent variables

determined the relative effects of each of these variables on

increases in total species richness of the entire community.

Then, independent simple regressions between latitude and

the species richness of each of the bat families determined the

relative effects of latitude on species richness within each of the

Table 1 Geographical and environmental

characteristics of each of 32 bat communities

used to evaluate patterns of diversity in the

New World

Community Country Latitude R eference

Iowa USA 42.3 N Kunz 1973

California USA 36.5 N Suprenant (1977)

Nevada USA 36.2 N O’Farrell & Bradley (1970)

New Mexico USA 33.9 N Blac k (1974)

Big Bend Ranch USA 29.8 N Yancey (1996)

Queretaro Mexico 21.1 N Navarro & Leon-Paniagua (1995)

Manantlan Mexico 19.3 N Iniguez Davalos (1993)Ixtapan del Oro Mexico 19.3 N Alvarez & Alvarez-Castaneda (1996)

Los Tuxtlas Mexico 18.4 N Estrada et al. (1993)

Chiapas Mexico 16.1 N Medellin (1993)

Guanacaste-1 Costa Rica 10.4 N LaVal & Fitch (1977)

Guanacaste-2 Costa Rica 10.4 N Fleming et al. (1972)

Puntarenas Costa Rica 10.0 N LaVal & Fitch (1977)

Heredia Costa Rica 10.5 N LaVal & Fitch (1977)

Sherman Panama 9.3 N Fleming et al. (1972)

Rodman Panama 9.0 N Fleming et al. (1972)

BCI Panama 9.2 N Handley et al. (1991)

Paracou French Guiana 5.3 N Simmons & Voss (1998)

Zabelitas Colombia 4.0 N Thomas (1972)

Marcarena Colombia 3.3 N Sanchez-Palomino et al. (1993)

Pance Colombia 3.0 N Thomas (1972)

Hormiguero Colombia 3.0 N Thomas (1972)

Manaus Brazil 3.0 S Dos Reis (1984)

Edaphic Cerrado Brazil 7.2 S Willig (1982)

Caatinga Brazil 7.6 S Willig (1982)

Linhares Brazil 19.0 S Peracchi & Albuquerque (1993)

Panga Brazil 19.3 S Pedro & Taddei (1997)

Minas Gerais Brazil 19.8 S Moura de Souza Aguiar (1994)

Jenaro Herrera Peru 4.9 S Gorchov & Ascorra (unpublished data)

Manu Peru 11.9 S Ascorra et al. (1996)

Mbaracayu Paraguay 24.1 S Stevens & Willig (in press)

Rio Verde Paraguay 23.5 S Stevens & Willig (in press)

R. D. Stevens




families. Path diagrams were constructed by combining this

suite of simple and multiple regressions according to themethods outlined in Nie et al. (1975). Accordingly, standard-

ized regression coefficients were used to compare the magni-

tude of effects. An advantage of using standardized regression

coefficients is that differences among groups regarding the

magnitude of species richness are accounted for thereby

making more meaningful comparisons of rates of change

possible. Approximate standard errors of the standardized

regression coefficients were determined in SPSS (SPSS Inc.,

1990) using the methods of Meyer & Younger (1976). All

regression analyses were performed in SPSS.

RE S ULT S

New World bats exhibited impressive spatial variation

regarding the number of families and the number of species

per family found in communities (Fig. 2). Temperate com-

munities were the most species poor and possessed the fewest

families of bats, whereas subtropical and tropical communities

were more species rich and exhibited the greatest number and

variability of bat families. Members of the New World family

Phyllostomidae exhibited the greatest range and magnitude of

species richness. Members of the cosmopolitan family Vesper-

tilionidae exhibited their greatest species richness in temperate

communities, especially in North America. The primarily

tropical Emballonuridae were widely distributed among sub-tropical and tropical communities, but were never represented

by a large number of species. Although the New World tropical

Mormoopidae were primarily distributed throughout Central

America, northern South America and southern North Amer-

ica, they too were relatively species poor throughout their

range. Finally, the cosmopolitan family Molossidae exhibited a

patchy distribution and moderate to low levels of species

richness throughout the New World. The correlations among

families regarding spatial variation in species richness was

moderate ranging from )0.39 to 0.74.

Significant differences among temperate, subtropical, and

tropical communities existed regarding the species richnesswithin families of New World bats (Table 2). Inspection of the

results from the discriminant function analysis allowed for

interpretation of significant differences inferred from the

manova (Fig. 3). Most of the differences among geographical

zones were related to variation in the number of vespertilionid

and phyllostomid species whereby in the temperate zone

vespertilionids were common and phyllostomids were rare and

in the tropical and subtropical zones the opposite pattern

obtained. Moderate differences among zones existed for

mormoopids and emballonurids. Finally, the species richness

of molossids contributed little to the differences among

geographical zones. By plotting the communities in a space

defined by the two discriminant functions it was visually

apparent that most differences among geographical zones were

related to differences between the temperate zone and both the

subtropical and tropical zones (Fig. 3b). Moreover, these two

figures (Fig. 3a,b) together suggest that the differences among

geographical zones were primarily due to high species richness

Figure 2 Familial composition of 32 New

World bat communities. Communities are in

latitudinal order from the most northerly

community in North America (Iowa) to the

most southerly community in South America

(Mbaracayu).

Table 2 Results from multivariate analysis of variance deter-

mining significant differences among geographical regions

regarding species richness within families of New World bats

H0 F P d.f.

Temp ¼ Sub-trop ¼ Trop 5.500 < 0.001*** 10, 50

Sub-trop ¼ Trop 1.610 0.206 5, 19

Temp ¼ Sub-trop 7.266 0.006*** 5, 9

Temp ¼ Trop 16.072 < 0.001*** 5, 18

H0 refers to the particular null hypothesis under examination. F , P and

d.f. refer to the F -statistic, type 1 error rate, and degrees of freedom

associated with a particular hypothesis test. Asterisks refer to a signi-

ficant difference after controlling for experiment-wise error rate among

the three pairwise comparisons using a Bonferonni sequential adjust-

ment (Rice, 1989).





of the Vespertilionidae in the temperate zone and high species

richness of the Phyllostomidae in the subtropical and tropical

zones. This pattern was statistically corroborated by pair-wise

manovas (Table 2) that together indicated that species

richness of families in the temperate zone was significantly

different than both the subtropical and tropical zones, but that

species richness within families was indistinguishable between

the subtropical and tropical zones.

Three of the five families of bats exhibited significant

latitudinal gradients in species richness (Table 3, Fig. 4).

Species richness of the Vespertilionidae increased with increa-ses in latitude, whereas species richness of the families

Phyllostomidae and Emballonuridae increased towards the

equator. No consistent pattern with latitude existed for the

families Molossidae and Mormoopidae. Significant coefficients

of determination ranged from 0.24 to 0.48.

The path diagram summarized the inter-relationships

among latitude and the species richness of each of the families

that determined the total species richness of local communities

(Fig. 5). All families contributed significantly and positively to

variation among communities regarding their species richness.

Phyllostomids exhibited the greatest contribution, whereas the

Mormoopidae contributed the least to total species richness at

the community level. Moreover, latitude strongly affected the

species richness of the Vespertilionidae, Phyllostomidae and

Emballonuridae but not the Molossidae or Mormoopidae.

Finally, once the effect of latitude on each of the families was

accounted for, it had little residual effect on the total species

richness of communities.

The comparison of 95% confidence intervals (CI) around

standardized regression coefficients from the path diagram

indicates that families exhibit significant qualitative differences

regarding latitudinal gradients in species richness (Fig. 6a) as

well as quantitative differences regarding the degree to which

they contributed to variation in the total species richness of

communities (Fig. 6b). Although phyllostomids and embal-lonurids exhibited significant latitudinal gradients and molos-

sids and mormoopids exhibited non-significant gradients, all

four of these families appeared to exhibit the same underlying

relationship with latitude. In contrast, vespertilionids exhibited

a significantly different latitudinal gradient in which the number

of species increased with latitude as opposed to decreasing with

latitude as with other families. Changes in the species richness of

the Mormoopidae contributed the least to variation in the

species richness of communities, whereas vespertilionids, mol-

ossids, and emballonurids made intermediate contributions.

Table 3 Results of simple linear regression analyses between

latitude and species richness of New World bat families (Table 1).

Experiment-wise error rate was held constant at 5% by application

of a Bonferonni sequential adjustment (Rice, 1989)

Family Intercept Slope r 2 P

Vespertilionidae 2.06 0.20 0.42 < 0.001*

Phyllostomidae 33.46 )0.87 0.48 < 0.001*

Molossidae 2.78 )

0.04 0.03 0.331Emballonuridae 3.32 )0.10 0.24 0.005*

Mormoopidae 1.18 )0.02 0.02 0.426

An asterisk indicates significant regressions between latitude and a

particular measure of diversity.

Figure 3 Results from discriminant function analysis describing

the significant difference in species richness of families between

temperate communities and subtropical and tropical communities

as a group. (a) Dispersion of the 32 communities in a two-dimen-

sional space defined by two discriminant functions. Temperate

communities are represented by black squares, subtropical com-

munities by grey diamonds, and tropical communities by black

circles. (b)Correlations of variationin species richness of each of the

five families (VESP, Vespertilionidae; MORM, Mormoopidae;

PHYL, Phyllostomidae; EMBA, Emballonuridae; MOLO, Molossi-

dae) with the two discriminant functions. Arrows represent the

correlation (i.e. loadings of variables onto a particular discriminant

function) of species richness within a particular family with a par-

ticular discriminant function (a). Arrows that are long and parallel

with a particular discriminant function identify families that are

highly correlated with that function and contribute much to the

difference defined by that discriminant axis. Thus, vespertilionidsand phyllostomids are most highly associated with the difference

represented by the first discriminant function whereas mormoopids

and emballonurids are most highly associated with the difference

represented by the second discriminant function.

R. D. Stevens




Figure 4 Scatter plots describing latitudinal gradients in species

richness of each of the five bat families. Significant regressions

existed regarding the species richness of the Vespertilionidae,

Phyllostomidae, and Emballonuridae and are indicated by a black

line. Black lines indicate significant linear regressions.

Figure 5 Path diagram describing the direct and indirect effects

of latitude on local species richness, the effects of latitude on the

species richness of five families of bats, and the contributions of

species richness of the five families to total local species richness.

Construction of the path diagram follows the methods of Nie et al.

(1975). Arrows represent the assumed direction of relationships.

Values represent the magnitude of standardized regression coef-

ficients. Asterisks indicate standardized regression coefficients that

are significantly different from zero.

Figure 6 Diagrammatic representation of the variability (95% CI) and significant differences of standardized regression

coefficients used to summarize relationships among latitude,

familial species richness, and total local species richness.

(a) Regression coefficients used to describe the effects of latitude

on the species richness of five bat families. The Vespertilionidae

exhibits a significant and qualitatively different latitudinal gradient

in species richness. (b) Effect of increases in species richness within

families on the total species richness of communities. Changes

in species richness of the Phyllostomidae have a significantly

greater effect on the species richness of entire communities than

do changes in the species richness of the other four families.





Variation in the number of phyllostomid species made the

greatest contribution to variation in species richness of

communities, and this was significantly greater than for all

other families.

DIS CUS S IO N

Although the latitudinal gradient in species diversity is fre-

quently documented, it is most often evaluated only at the

regional scale and only from the perspective of entire assem-

blages (see reviews by Gaston, 1994; Brown, 1995; Rosenzweig,

1995; Willig, 2001). This study documents interfamilial differ-

ences in the strength and direction of latitudinal variation in

species richness measured at the local level for New World bats.

A significant difference between the temperate zone and the

tropical and subtropical zones together exists regarding the

species richness of families. Moreover this is primarily the result

of large number of species of the Vespertilionidae in commu-

nitiesof thetemperate zone andlargenumber of members of the

Phyllostomidae in communities of the tropical and subtropical

zones. Monotonic latitudinal gradients in species richness existwithin at least three of the five New World families evaluated.

Nonetheless, considerable variation exists among families

regarding the rate and direction of latitudinal gradients. The

Phyllostomidae exhibits the strongest increase in the number of

species towards the equator followed by the Emballonuridae.

The Molossidae and Mormoopidae exhibited no consistent

trend with latitude. Finally, the Vespertilionidae exhibited a

positive relationship between species richness and latitude. All

families contributed significantly to the species richness of bat

communities in the New World. Nonetheless, the greatest

influence on species richness of communities was changes in the

species richness of the Phyllostomidae.

The richness of all families except the Vespertilionidae

declines to zero within the range of latitudes characterizing

these communities; this is true at both local (presented here)and

regional levels (Hall, 1981). The theoretical basis to family-

specific latitudinal termini are firmly suggested by the works of

McNab (1969, 1980, 1986, 1988a,b) that highlight the physio-

logical consequences of mammalian trophic strategies. Because

resources available to bats vary greatly both within and among

seasons in the temperate zone, only those families possessing

species that readily exhibit facultative or obligate torpor can

persist at high latitudes. Hibernation torpor (Ransome, 1990)

helps to mitigateseasonaleffects whereas daily torpor (Ransome,

1990) helps to mitigate the effects of variation within seasons.Accordingly, those communities at relatively high latitudes are

composed exclusively of vespertilionids, which exhibit perhaps

the greatest plasticity regarding the precision of their metabolic

rates (McNab, 1969; Ransome, 1990). Members of other

insectivorous families such as the Molossidae and Mormoopi-

dae, by virtue of their insectivorous diets enjoy relatively lower

metabolic rates and thus are able to persist at higher latitudes

than families comprising other trophic groups (McNab, 1969).

Finally, because of their diets, relatively large size, and precise

thermoregulation,the distributions of phyllostomidsare limited

to the lowest latitudes. Thus, the physiological consequences of

thermoregulatory strategies, body size, and diet likely define

specific termini in the geographical distribution of species

within families, and this results in family-specific latitudinal

gradients.

Although variation regarding the strength of latitudinal

gradients in diversity may reflect a physiological basis,

differences might be enhanced by ecological characteristics as

well. For example, phyllostomids primarily consume fruit,

nectar, or glean vertebrates and invertebrates from foliage

found within forest canopies. Frugivores and nectarivores may

respond more directly to changes in the year-round availability

of food supplies than species from other trophic groups

(Fleming, 1973). Frugivores and nectarivores rely on resources

found primarily in subtropical and tropical forests, and the

attenuation in diversity of these trophic groups with latitude

may be amplified by the latitudinal attenuation of the diversity

of plants that provide fleshy fruit and nectar (Findley, 1993).

Likewise, gleaning animalivores are dependent on diverse

substrates that can maintain sufficiently abundant vertebrates

and invertebrates to allow the persistence of species of thistrophic group. Optimal substrates such as these likely exist

primarily in subtropical and tropical forest. Although some

species of phyllostomid gleaning animalivores do persist [e.g.

Macrotus californicus (Hall, 1981)] in areas that lack structur-

ally diverse forests, the attenuation of the number of species of

this trophic group with latitude may reflect the latitudinal

decrease in the vertical complexity of plant communities.

In contrast to gleaning animalivores, families comprised

primarily by aerial and high-flying insectivores need not rely on

terrestrial substrates in order to obtain their prey items; they

consume them in flight. Thus, weaker latitudinal gradients in

these groups may reflect a greater degree of independence of

latitudinally varying resources. Clearly, other environmental

factors such as seasonality and temperature vary latitudinally,

and these likely affect the life-history attributes of species from

the Vespertilionidae, Emballonuridae, Mormoopidae, and

Molossidae; it would be naive to suggest that these groups are

unaffected by latitudinally varying environmental gradients.

Nonetheless, these families may be affected by fewer environ-

mental gradients that vary latitudinally and may for this reason

exhibit weaker latitudinal gradients.

Contrary to the spatial distribution of species from other

families, and in contrast to findings regarding species richness

gradients measured using geographical quadrats and bands

(Willig & Selcer, 1989), species richness of the Vespertilionidaewithin communities increased towards the poles. The family

Vespertilionidae is highly diverse, exhibiting physiological and

behavioural (e.g. torpor, hibernation, and migration) adapta-

tions that allow a cosmopolitan distribution (Findley, 1993;

Koopman, 1994). Moreover, their unique life-history strategies

including delayed fertilization and sometimes multiple off-

spring, combined with their abundance in desert habitats

(Feldhammer et al., 1999), have enhanced their success in

temperate areas. Indeed, the positive latitudinal gradient in

vespertilionid species richness found at the level of local

R. D. Stevens




communities may reflect the success of those species in the

temperate zone relative to other New World bat families.

Such disparity between the results described here and those of

Willig & Selcer (1989) presumably has to do with the scale at

which the data were collected and highlights the beta diversity of

vespertilionid species. The negative latitudinal gradient de-

scribed by Willig & Selcer (1989) is primarily the result of the

diminution of species richness at extreme northerly latitudes,

especially those above 50. In fact, their data demonstrates that

below 45 barely, if any, relationship with latitude exists

regarding species of Vespertilionidae. My data span only c. 45

latitude and likely are not of sufficient geographical extent to

detect the attenuation of the geographical distribution of the

Vespertilionidae. Nonetheless, the results of this work and those

of Willig & Selcer (1989) still differ in that the species richness of

vespertilionids measured at the local level is consistently lower

than that measure by latitudinal bands and quadrats. Moreover,

this difference appears to increase towards the equator. As a

group, bats exhibit an increase in beta diversity towards the

equator (Stevens & Willig, 2002). This is likely true forspecies of

the Vespertilionidae as well. Thus, increase in beta diversity would give rise to a situation where there is little change in

species richness when measured at large spatial scales and a

decrease in species richness towards the equator when measured

at the local level.

Over the last half century we have learned much about

variation in the spatial distribution of mammalian species.

Indeed, much of the latitudinal variation in mammalian

species richness is due to the latitudinal gradient in bat species

richness (Simpson, 1964; Fleming, 1973). Moreover, Fleming

(1973) suggested that understanding the question ‘Why are

there more species of mammals in tropical habitat than in a

similar habitat at higher latitudes?’ could be greatly enhanced

by understanding the question ‘Why are there more bats in

tropical habitats?’. Indeed, there are more bats in tropical

habitats because at lower latitudes more ecological opportun-

ities have facilitated the diversification of bats into a number of

ecological groups (i.e. frugivores, insectivores, nectarivores,

piscivores, sangunivores, etc.) and this is primarily the result of

the wide ecological radiation of the family Phyllostomidae.

Nonetheless, understanding the effects of latitude on the

composition of bat communities may require better under-

standing of the contrasting latitudinal gradients of the

Vespertilionidae and Phyllostomidae. While our understand-

ing of the effects of latitude on variation in the geographical

distributions of bats has been improved by addressingquestions such as those posed by Fleming (1973), improved

understanding almost always leads to more questions. Perhaps

deeper understanding of the effects of latitude on the

distribution of bat species and ultimately spatial variation in

the composition of communities may come from answering

the question ‘Why is it that vespertilionids and phyllostomids

exhibit such disparate latitudinal gradients at the local level,

and how much do the biological differences between these

families of bats determine spatial variation in the communities

that they form?’ Discordant latitudinal gradients among lower

taxa within a higher taxon may be a relatively unappreciated

yet general biogeographic phenomenon. It has now been

demonstrated for vertebrates and invertebrates as well as in

terrestrial, marine, and estuarine environments (Turner, 1981;

Crame, 2000; this study). Moreover, often the latitudinal

gradient characteristic of the higher taxon is simply a reflection

of one or a few of the most abundant taxa within the clade.

Although the generality of latitudinal gradients, in which

species richness increases towards the equator, is reflected by

involvement by a majority of species in these studies,

contrasting patterns of diversity can be quite informative,

especially when they indicate important life-history charac-

teristics that may cause contrasting patterns. Future studies

should evaluate the ubiquity of such contrasting patterns

among closely related groups of species in an effort to

better understand when and under what circumstances

traditional latitudinal gradients in species richness are not

likely to obtain.

ACK NO W LE DGM E NT S

Support was provided in the form of teaching and research

assistantships from the Department of Biological Sciences,

Texas Tech University. D. Vazquez, S. K. Lyons and an

anonymous referee provided valuable comments on a previous

version of this manuscript. D. Brandts provided much

appreciated computer assistance. D. Gorchov and C. Ascorra

provided unpublished data for the Jenaro Herrera community.

RE FE RE NCE S

Arita, H.T., Robinson, J.G. & Redford, K.H. (1990) Rarity in

Neotropical forest mammals and its ecological correlates.

Conservation Biology , 4, 181–192.

Brown, J.H. (1995) Macroecology . University of Chicago Press,

Chicago, IL.

Crame, J.A. (2000) Evolution of taxonomic diversity gradients

in the marine realm: evidence from the composition of

recent bivalve faunas. Paleobiology , 26, 188–214.

Currie, D.J. (1991) Energy and large-scale patterns of animal-

and plant-species richness. American Naturalist , 137, 24–49.

Feldhammer, G.A., Drickamer, L.C., Vessey, S.H. & Merrit, J.F.

(1999) Mammalogy: adaptation, diversity, and ecology .

McGraw-Hill, Boston, MA.

Findley, J.S. (1993) Bats: a community perspective. Cambridge

University Press, Cambridge.Fischer, A.G. (1960) Latitudinal variations in organic diversity.

Evolution, 14, 64–81.

Fleming, T.H. (1973)Numbersof mammal species in Northand

Central American forest communities. Ecology , 54, 555–563.

Gaston, K.J. (1994) Rarity . Chapman and Hall, London.

Hall, E.R. (1981) The mammals of North America, 2nd edn.

John Wiley and Sons, New York.

Kaufman, D.M. (1995) Diversity of New World mammals:

universality of the latitudinal gradient of species and bau-

plans. Journal of Mammalogy , 76, 322–334.





Kaufman, D.M. & Willig, M.R. (1998) Latitudinal patterns of

mammalian species richness in the New World: the effects of

sampling method and faunal group. Journal of Biogeograpy ,

25, 795–805.

Koopman, K.F. (1993) Order Chiroptera. Mammal species of

the World: a taxonomic and geographic reference (ed. by D.E.

Wilson and D.M. Reeder), pp. 137–242. Smithsonian

Institution Press, Washington, DC.

Koopman, K.F. (1994) Chiroptera: systematics. Handbook of

zoology , Vol. VIII (ed. by J. Neithammer, H. Schliemann and

D. Starck), pp. 1–217. Walter de Gruyter Publishers, Berlin.

Lyons, S.K. & Willig, M.R. (1999) A hemispheric assessment of

scale dependence in latitudinal gradients of species richness.

Ecology , 80, 2483–2491.

Lyons, S.K. & Willig, M.R. (2002) Species richness, latitude,

and scale sensitivity. Ecology , 83, 47–58.

McCoy, E.D. & Connor, E.F. (1980) Latitudinal gradients in

the species diversity of North American mammals. Evolu-

tion, 34, 193–203.

McNab, B.K. (1969) The economics of temperature regulation

in Neotropical bats. Comparative Biochemistry and Physiol-ogy , 31, 227–268.

McNab, B.K. (1980) Food habits, energetics, and the population

biology of mammals. American Naturalist , 116, 106–124.

McNab, B.K. (1986) The influence of food habit on the

energetics of eutherian mammals. Ecological Monographs,

56, 1–19.

McNab, B.K. (1988a) Food habits and the basal rate of

metabolism in birds. Oecologia, 77, 343–349.

McNab, B.K. (1988b) Complications inherent in scaling the

basal rate of metabolism in mammals. Quarterly Review of

Biology , 63, 25–34.

Macpherson, E. & Duarte, C.M. (1994) Patterns of species

richness, size, and latitudinal range of east Atlantic fishes.

Ecography , 17, 242–248.

Mares, M.A. (1992) Neotropical mammals and the myth of

Amazonian biodiversity. Science, 255, 976–979.

Mares, M.A. & Ojeda, R.A. (1982) Patterns of diversity and

adaptation in South American hystricognath rodents.

Mammalian biology in South America, Vol. 6 (ed. by M.A.

Mares and H.H. Genoways), pp. 393–432. Special Publica-

tions Series, Pymatuning Laboratory of Ecology, University

of Pittsburgh, Pittsburgh, PA.

MathWorks (1995) Matlab reference guide. MathWorks, Natick.

Meserve, P.L., Kelt, D.A. & Martinez, D.R. (1991) Geo-

graphical ecology of small mammals in continental ChileChico, South America. Journal of Biogeography , 18, 179–187.

Meyer, L.S. & Younger, M.S. (1976) Estimation of standar-

dized coefficients. Journal of the American Statistical Asso-

ciation, 71, 154–157.

Nie, N.H., Hull, C.H., Jenkins, J.G., Steinbrenner, K. & Bent,

D.H. (1975) Statistical package for the social sciences, 2nd

edn. McGraw-Hill Book Co., New York.

Pagel, M.D., May, R.M. & Collie, A.R. (1991) Ecological

aspects of the geographical distribution and diversity of

mammalian species. American Naturalist , 137, 791–815.

Ransome, R.D. (1990) The natural history of hibernating bats.

Chrostoher Helm Press, London.

Rice, W.R. (1989) Analyzing tables of statistical tests. Evolu-

tion, 43, 223–225.

Rosenzweig, M.L. (1995) Species diversity in space and time.

Cambridge University Press, Cambridge.

Simpson, G.G. (1964) Species density of North American

recent mammals. Systematic Zoology , 13, 57–73.

Sokal, R.R. & Rohlf, F.J. (1995) Biometry: the principles and

practice of statistics in biological research. W.H. Freeman and

Co., New York.

SPSS Inc. (1990) The SPSS base system user’s guide . SPSS, Inc.,

Chicago, IL.

Stevens, R.D. (2002) Taxonomic, functional, and phenetic

components of biodiversity: perspectives on the community

ecology of New World bats. Dissertation, Texas Tech Uni-

versity, Lubbock, TX.

Stevens, R.D. & Willig, M.R. (2004) Comparative community

ecology of bats from eastern paraguay: taxonomic, ecologi-

cal, and biogeographic perspectives. Journal of Mammalogy ,

in press.Stevens, R.D. & Willig, M.R. (2002) Geographical ecology at

the community level: perspectives on the diversity of New

World bats. Ecology , 83, 545–560.

Turner, J.T. (1981) Latitudinal patterns of calanoid and

cyclopoid copopod diversity in estuarine waters of eastern

North America. Journal of Biogeography , 8, 369–382.

Willig, M.R. (2001) Latitude, common trends within. Encyc-

lopedia of biodiversity , Vol. 3 (ed. by S.A. Levin), pp. 701–

714. Academic Press, San Diego, CA.

Willig, M.R. & Gannon, M.R. (1997) Gradients of species

density and turnover in marsupials: a hemispheric per-

spective. Journal of Mammalogy , 78, 756–765.

Willig, M.R. & Sandlin, E.A. (1991) Gradients in species

density and species turnover in New World bats: a com-

parison of quadrat and band methodologies. Latin American

mammals: their conservation, ecology, and evolution (ed. by

M.A. Mares and D.J. Schmidly), pp. 81–96. University of

Oklahoma Press, Norman, OK.

Willig, M.R. & Selcer, K.W. (1989) Bat species diversity gra-

dients in the New World: a statistical assessment. Journal of

Biogeography , 16, 189–195.

Wilson, J.W. (1974) Analytical zoogeography of North

American mammals. Evolution, 28, 124–140.

B I O S K E T C H

Richard Stevens received a PhD in biology from Texas Tech

University in 2002 and is currently a postdoctoral research

associate at the National Center for Ecological Analysis and

Synthesis, University of California, Santa Barbara. His research

interests include the community ecology, biogeography and

macroecology of mammals.

R. D. Stevens


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