Latitudinal Richness Gradient Research Articles

Interspecific patterns of species richness, geographic range size, and body size among New World venomous snakes

Many higher taxa exhibit latitudinal gradients in species richness, geographic range size, and body size. However, these variables are often interdependent, such that examinations of univariate or bivariate patterns alone may be misleading. Therefore, I examined latitudinal gradients in, and relationships between, species richness, geographic range size, and body size among 144 species of New World venomous snakes [families Elapidae (coral snakes) and Viperidae (pitvipers)]. Both lineages are monophyletic, collectively span 99° of latitude, and are extremely variable in body size and geographic range sizes. Coral snakes exhibit highest species richness near the equator, while pitviper species richness peaks in Central America. Species – range size distributions were strongly right‐skewed for both families. There was little support for Bergmann's rule or Rapoport's rule for snakes of either family, as neither body size nor range size increased significantly with latitude. However, range area and median range latitude were positively correlated above 15° N, indicating a possible “Rapoport effect” at high northern latitudes. Geographic range size was positively associated with body size. Available continental area strongly influenced range size. Comparative (phylogenetically‐based) analyses revealed that shared history is a poor predictor of range size variation within clades. Among vipers, trends in geographic range sizes may have been structured more by historical biogeography than by macroecological biotic factors.

Correspondence between regional delineations and spatial patterns in macroinvertebrate assemblages of boreal headwater streams

Geographical stratification may provide a useful framework for stream management programs, yet most studies testing the utility of such stratifications have been conducted in temperate regions. We studied the correspondence between regional delineations (5 ecoregions, 11 subecoregions), environmental characteristics, and benthic macroinvertebrate assemblages in 156 boreal headwater streams in Finland, using a combination of principal components analysis, nonmetric multidimensional scaling, and discriminant function analysis (DFA). Both stream characteristics and macroinvertebrate assemblage structure showed a closer correspondence to ecoregions than to subecoregions, a pattern partly attributable to the smaller number of classes at the ecoregion level. Based on environmental variables, DFA classified correctly 84.0% of the streams by ecoregion and 74.4% by subecoregion. For macroinvertebrate assemblages, 64.7% of the streams were correctly classified at the ecoregion, and 51.9% at the subecoregion level. Arctic-alpine and north boreal ecoregions supported the most distinct benthic assemblages. There were significant differences among ecoregions in mean species number, with lowest species richness in the middle boreal and highest in the south boreal streams. However, no clear latitudinal gradients in local species richness were detected. Relative abundances of functional-feeding groups also exhibited regional differences. Shredders dominated the middle boreal brown-water streams, whereas scrapers attained their highest abundances in the arctic-alpine and north boreal clear-water streams. Despite some congruence among ecoregions and macroinvertebrate assemblages, regional delineations alone do not provide a suitable background for stream biomonitoring programs. A combination of regional delineations and local characteristics probably provides a better framework for the classification of boreal headwater streams, and for the prediction of their benthic fauna.

Ecology and biogeography of marine parasites

A review is given of (mainly recent) work on the biodiversity, ecology, biogeography and practical importance of marine parasites. Problems in estimating species numbers have been thoroughly discussed for free-living species, and the main points of these discussions are reviewed here. Even rough estimates of the richness of most parasite groups in the oceans are premature for the following reasons: species numbers of host groups, in particular in the deep sea and the meiofauna, are not known; most host groups have been examined only insufficiently for parasites or not at all; even in some of the best known groups, latitudinal, longitudinal and depth gradients in species richness are only poorly understood or not known at all; effects of hosts on parasite morphology and geographical variation have been studied only in a few cases; there are few studies using techniques of molecular biology to distinguish sibling species. Estimates of species richness in the best known groups, trematodes, monogeneans and copepods of marine fishes, are given. Parasites are found in almost all taxa of eukaryotes, but most parasitic species are concentrated in a few taxa. Important aspects of the ecology of marine parasites are discussed. It is emphasized that host specificity and host ranges should be distinguished, and an index that permits calculation of host specificity is discussed. The same index can be applied to measure site specificity. Central problems in ecology are the importance of interspecific competition and whether equilibrium or non-equilibrium conditions prevail. Marine parasites are among the few groups of organisms that have been extensively examined in this regard. A holistic approach, i.e. application of many methods, has unambiguously shown that metazoan ecto- (and probably endo-) parasites of marine fish live in largely non-saturated niche space under non-equilibrium conditions, i.e. they live in assemblages rather than in communities structured by competition. Nestedness occurs in such assemblages, but it can be explained by characteristics of the species themselves. There is little agreement on which other factors are involved in "structuring" parasite assemblages. Few studies on metapopulations of marine parasites have been made. A new approach, that of fuzzy chaos modelling, is discussed. It is likely that marine parasites are commonly found in metapopulations consisting of many subpopulations, and they are ideally suited to test the predictions of fuzzy chaos. Some recent studies on functional ecology and morphology--especially with regard to host, site and mate finding--are discussed, and attention is drawn to the amazing variety of sensory receptors in some marine parasites. Effects of parasites on hosts, and some studies on the evolution and speciation of marine parasites are discussed as well. A detailed overview of biogeographical studies is given, with respect to latitudinal gradients in species diversity, reproductive strategies and host ranges/specificity. Studies of marine parasites have contributed significantly to giving a non-equilibrium explanation for latitudinal diversity gradients. Recent studies on longitudinal and depth gradients are discussed, as well as parasites in brackish water, parasites as indicators of zoogeographical regions and barriers, and parasites as biological tags. The practical importance of marine parasites in mariculture, as monitors of pollution, agents of human disease, the use of parasites for controlling introduced marine pests, and some related aspects, are also discussed.

Regional-scale assembly rules and biodiversity of coral reefs.

Tropical reef fishes and corals exhibit highly predictable patterns of taxonomic composition across the Indian and Pacific Oceans. Despite steep longitudinal and latitudinal gradients in total species richness, the composition of these key taxa is constrained within a remarkably narrow range of values. Regional-scale variation in reef biodiversity is best explained by large-scale patterns in the availability of shallow-water habitat. Once habitat area is accounted for, there is surprisingly little residual effect of latitude or longitude. Low-diversity regions are most vulnerable to human impacts such as global warming, underscoring the urgent need for integrated management at multinational scales.

Latitudinal gradients and geographic ranges of exotic species: implications for biogeography

AimTo explore the biogeographic patterns of exotic species in order to discriminate between hypotheses postulated to produce several biogeographic phenomena: the latitudinal gradient in species richness, the latitudinal gradient in species geographical range size (Rapoport’s rule), species geographical range boundaries and patterns of invasion in the tropics.LocationData were taken from North and South America, Europe, Asia, Africa and oceanic islands.MethodsThe latitudinal extents of the geographical range of exotic species were recorded to the nearest 1° of latitude from several published sources. Species richness was tabulated by recording the number of naturalized species present within 5° bands of latitude. Rapoport’s rule was calculated by averaging the latitudinal extent of all exotic species present within 5° bands. The low‐latitude boundaries of species’ native and naturalized ranges were compared with the nearest 1° of latitude. These comparisons on continents were contrasted with those on islands.ResultsWithin the tropics, few exotic species have become naturalized and those that are have established large geographical ranges. Outside of the tropics, exotic species of birds, mammals, fishes and plants demonstrate qualitatively similar latitudinal gradients in richness and geographical range size. They show the same patterns as native species, where richness is negatively correlated and range size positively correlated with latitude. Further, the geographical ranges of naturalized species on continents rarely extend to latitudes lower than those in their native ranges. On islands (where biotic pressure is reduced) exotic species are more frequently naturalized at latitudes lower than those in their native ranges.Main conclusionsHypotheses for the latitudinal gradient in species richness and geographical range size that are based on past glaciation events or differential rates of speciation between regions may not be necessary to explain these gradients, as exotic species are recent colonists and their patterns of distribution cannot have been directly affected by past glaciation events or differential rates of speciation between regions. Further, the low‐latitude boundary of species geographical ranges may be set by tolerance for biotic pressure.

Strategy Space and the Disturbance Spectrum: A Life-History Model for Tree Species Coexistence.

The disturbance spectrum consists of disturbance patterns differing in type, size, intensity, and frequency. It is proposed that tree life-history traits are adaptations to particular disturbance regimes. Four independent axes are proposed to define the dominant dimensions of tree strategy space: shade tolerance, tree height, capacity for vegetative reproduction, and seed dispersal distance. A fitness model was developed to elucidate interactions between the proposed life-history traits. The model shows how alternate life-history sets can coexist when disturbance patterns fluctuate in space and time. Variable disturbance regimes were shown, based on data and simulation results, to enhance species coexistence, as predicted. The strategy space model accurately predicts the number of common tree species for the eastern United States, boreal Canada, and southwestern piñon-juniper woodlands. The model also provides an explanation for latitudinal gradients in tree species richness in North America and Europe. The proposed model predicts a relationship between disturbance characteristics and the species composition of a forest that allows for the coexistence of large numbers of species. The life-history traits of size, growth rate, life span, shade tolerance, age of reproduction, seed dispersal distance, and vegetative reproduction are all incorporated into the model.

Strategy Space and the Disturbance Spectrum: A Life-History Model for Tree Species Coexistence

The disturbance spectrum consists of disturbance patterns differing in type, size, intensity, and frequency. It is proposed that tree life‐history traits are adaptations to particular disturbance regimes. Four independent axes are proposed to define the dominant dimensions of tree strategy space: shade tolerance, tree height, capacity for vegetative reproduction, and seed dispersal distance. A fitness model was developed to elucidate interactions between the proposed life‐history traits. The model shows how alternate life‐history sets can coexist when disturbance patterns fluctuate in space and time. Variable disturbance regimes were shown, based on data and simulation results, to enhance species coexistence, as predicted. The strategy space model accurately predicts the number of common tree species for the eastern United States, boreal Canada, and southwestern piñon‐juniper woodlands. The model also provides an explanation for latitudinal gradients in tree species richness in North America and Europe. The proposed model predicts a relationship between disturbance characteristics and the species composition of a forest that allows for the coexistence of large numbers of species. The life‐history traits of size, growth rate, life span, shade tolerance, age of reproduction, seed dispersal distance, and vegetative reproduction are all incorporated into the model.

Weak links: ‘Rapoport's rule’ and large‐scale species richness patterns

SummaryMany hypotheses have been proposed to explain regional species richness patterns. Among these, ‘Rapoport's rule’ has sparked considerable controversy by stating that the latitudinal gradient in species richness can be explained indirectly as a function of narrower geographic ranges for species at low latitudes. Annual climatic variability, or deviation from mean climatic conditions, has been hypothesized to moderate this phenomenon. Furthermore, taxa that avoid much of this seasonality, such as temperate zone insects that enter diapause or species that migrate, were predicted to show reduced latitudinal gradients in richness. I test the suggested link between ‘Rapoport's rule’ and species richness for two higher level insect taxa as well as for the class Mammalia. Although these taxa exhibit the well‐known latitudinal gradient in species richness, simple annual climatic variability and deviation from mean annual climatic conditions provide very poor predictions of species richness in each of them. Potential evapotranspiration, a measurement of ambient climatic energy, explains most of the observed variance in regional species richness patterns for all three taxa, consistent with the species richness‐energy hypothesis. I find no support for an indirect link between ‘Rapoport's rule’ and terrestrial species richness patterns in North America.

Reply from K.J. Gaston, T.M. Blackburn and J.I. Spicer

We are delighted that Cowlishaw and Hacker find our review of Rapoport's rule[1xGaston, K.J., Blackburn, T.M., and Spicer, J.I. Trends Ecol. Evol. 1998; 13: 70–74Abstract | Full Text | Full Text PDF | PubMed | Scopus (200)See all References][1]worthy of extension. In response, we would make two points.First, we agree that if one excludes analyses that do not account for problems of spatial and phylogenetic non-independence and analyses based on geographic range area, rather than latitudinal extent, then those that remain support Rapoport's rule. We also wholeheartedly support the argument, implicit in Cowlishaw and Hacker's observations, that it is desirable to perform statistically rigorous analyses. However, the fact that the most rigorous analyses conducted support Rapoport's rule should not be interpreted either as evidence that the rule may be a general one or as a basis for dismissing conclusions based on other less rigorous analyses. The effect of using geographic range area is, to some extent, ameliorated by the general high correlation between latitudinal and longitudinal extents[2xLutz, F.E. Am. Mus. Novitates. 1922; 5: 335–366See all References, 3xBrown, J.H. and Maurer, B.A. Science. 1989; 243: 1145–1150CrossRef | PubMedSee all References, 4xSee all References].Moreover, failing to account for spatial and phylogenetic non-independence actually biases analyses towards finding evidence for Rapoport's rule when it does not exist; both artificially inflate degrees of freedom, leading to a likelihood of spurious statistical significance. Thus, the observation that a high proportion of studies performed in this way do not support the rule is a conservative basis for rejecting the notion that it is a general one and, therefore, worthy of being termed a `rule' (with the significant caveat that studies of Rapoport's rule are highly biased with respect to taxon and biogeographic region). This seems to us a more compelling argument for rejecting the generality of Rapoport's rule than for accepting it on the basis of the three studies that Cowlishaw and Hacker find suitable.Second, and on a more general note (notwithstanding our comments above), we urge circumspection in the pursuit of ever more statistical rigour in analyses. The history of the study of Rapoport's rule has been one of increasing analytical complexity. Indeed, we note that none of the three studies that Cowlishaw and Hacker regard as acceptable, including their own[5xCowlishaw, G. and Hacker, J.E. Am. Nat. 1997; 150: 505–512CrossRef | PubMedSee all References][5](published after completion of our review), has accounted for the recently highlighted problems of boundedness[6xColwell, R.K. and Hurtt, G.C. Am. Nat. 1994; 144: 570–595CrossRef | Scopus (356)See all References][6], which may be subtle and need not necessitate that ranges abut continental limits[7xLyons, S.K. and Willig, M.R. Oikos. 1997; 79: 568–580CrossRefSee all References][7]. Although we applaud increased rigour, particularly because it can reveal the existence of artefactual patterns[8xBlackburn, T.M. and Gaston, K.J. Am. Nat. 1998; 151: 68–83CrossRef | PubMed | Scopus (124)See all References][8], we do worry that the continual pursuit of the `perfect' analysis may have two undesirable side effects.First, this pursuit may be given greater emphasis than is warranted by the general body of evidence for or against the existence of a particular macroecological pattern. To take one example, no empirical analysis of latitudinal gradients in species richness, of which we are aware, has ever accounted for the non-independence of data points. Technically, all published analyses are, therefore, statistically flawed and, as such, invalidated. However, no one would seriously doubt that the species richness of the vast majority of major taxonomic groups is higher towards lower latitudes.Second, this pursuit might result in enormous variation in how macroecological patterns are tested and reported, seriously constraining, and sometimes precluding, any possibilities for comparison and contrast of the results of different studies. In this vein, we would propose that, alongside more-rigorous analyses, some standard plots and statistics for macroecological analyses should be established and routinely reported.

An Analytical Model of Latitudinal Gradients of Species Richness with an Empirical Test for Marsupials and Bats in the New World

Although latitudinal gradients in species richness are well-documented for a plethora of taxa in terrestrial, freshwater, and marine environments, little consensus exists concerning the predominant biological factor that is responsible for the pattern. We produced an analytical null model to assess the degree to which gradients in species richness could be a consequence of the random determination of the limits of species ranges. The model predicts a parabolic increase in species richness toward the middle of a latitudinal domain in the absence of underlying environmental gradients. Our stochastic model accounted for a significant portion of variation in marsupial and bat species richness for each of three different latitudinal domains in the New World: continental limits, the latitudinal extent of each higher taxon, and the smallest latitudinal extent which comprises 95% of the species in the higher taxon. A unique prediction of the stochastic model, which distinguishes it from all other hypotheses, is that parabolic latitudinal gradients in richness should exist for species wholly contained within random latitudinal subsets. Observed gradients for New World marsupials and bats document that this is true. Regardless of taxon or domain, differences between observed and expected species richness (residuals) were not related appreciably to latitudinal band area (r2 i 0.15). The ubiquity and similarity of latitudinal gradients in species richness for different taxa could be a consequence of pervasive stochastic mechanisms rather than a product of a dominant underlying environmental gradient to which all species respond. Application of our null model to other gradients (e.g., depth, productivity, disturbance) may provide insight into mechanisms affecting patterns of species richness in other ecological or biogeographic settings.

Effects of Sampling Area and Subsampling Procedure on Comparisons of Taxa Richness among Streams

the biotic integrity of communities. In all cases, we make the implicit and sometimes brash assumption that we can really measure the number of taxa in a community. Although measuring taxa richness might appear straightforward, accurate measurement has been extraordinarily difficult; and despite years of effort, no universally accepted methods for its measurement have emerged. The essential problem is that we can never completely census a taxonomic assemblage or entire community; we rely instead on estimates that describe some portion of the real taxa richness of an assemblage. The problem of knowing what percent of the taxa present have been collected is exacerbated when investigators fail to explicitly define their universe of interest (i.e., the spatial bounds of the community or communities in question). Comparisons of taxa richness among studies that used different sampling and subsampling methods are especially difficult and should be viewed skeptically. The difficulty of obtaining accurate measurements of richness is due to the collector's curve

A Sideways Look at Patterns in Species Richness, or Why There Are So Few Species Outside the Tropics

What causes the widely observed latitudinal gradients in species richness remains one of the key unanswered questions in ecology. Despite numerous hypotheses generating a wealth of tests, no consensus has yet been reached. We do not add to the list either of tests or of hypotheses, but instead examine how some of the implicit (and occasionally explicit) assumptions of most research into the problem may be hindering its resolution. We discuss four ideas that could aid progress in this regard. First, we observe that view depends on viewpoint, and that altering the latter often leads to changes in the former. The most common view of richness gradients is that the tropics are unusually species rich. Viewing the richness of the tropics as the norm causes more than a simple rephrasing of the question: rather, it alters the perspective on other assumptions about richness gradients. Second, we suggest that the latitudinal gradient in species richness may actually arise from more than one process. The amount of energy flowing into a region must set an upper bound on the amount of the raw material of life the region can support. However, this raw material can be viewed as first being converted into individuals, and these individuals as subsequently distributed into species. Both processes will contribute to the determination of how many species inhabit a region. Third, we argue that the species richness of a region will not be independent of the mean body size and mean abundance (density) of the species, because all these variables depend on division of the same basic raw material. The energy entering a region may be viewed as being allocated to the number, abundance and size of species, but the allocation to each will not be independent. Spatial variation in mean abundance and mean body size will interact with spatial patterns in species richness. Fourth, we argue that, given the second and third points, it is unlikely that a single cause for species richness gradients will be identified. Effectively, this amounts to trying to give one answer to two quite different questions. © 1996 Blackwell Science Ltd.

Large scale patterns of biodiversity: spatial variation in family richness

Quantitative information on large scale spatial patterns of biodiversity remains poor. This is arguably a consequence of the disproportionate emphasis given to species as units of diversity. In this paper the regional biodiversity of terrestrial and freshwater seed plants, beetles, amphibians, reptiles and mammals is mapped worldwide at the family level. The `hotspots' of family richness are located in southeast Asia for one group, in the New World for two groups, in Africa for one group, and the fifth group has tied values in southeast Asia and the New World. Seed plants, amphibians, reptiles and mammals show classic latitudinal gradients in family richness; beetle family richness essentially lacks a strong latitudinal gradient. A very high proportion of the families of each of the five taxonomic groups can be embraced within a small number of regions.

Rapoport's Rule Does Not Apply to Marine Teleosts and Cannot Explain Latitudinal Gradients in Species Richness

Two methods are used to analyze latitudinal ranges of fish species: means of ranges of all species with a midpoint in the same 5⚬ latitude band plotted against latitude, and means of ranges of all species occurring in the same 5⚬ latitude band plotted against latitude. Application of the first method to data on 894 teleost species from the Indo-Pacific and Atlantic oceans showed that species with midpoints of latitudinal ranges near the equator have, on the average, much greater latitudinal ranges than species with midpoints at higher latitudes. Ranges of species were more or less the same when the second method was used, because of larger ranges and greater species numbers of low-latitude species. Rapoport's rule, according to which high-latitude species have greater latitudinal range than low-latitude species, therefore does not generally apply to marine teleosts. In contrast, the rule applies to North American (720 species) and North European (61 species) freshwater fish above a latitude of approximate...

African Fig Wasp Communities: Undersaturation and Latitudinal Gradients in Species Richness

We examined species richness patterns for the wasps associated with southern and central African figs (Ficus) to determine if fig wasp communities are saturated with species and if the degree of saturation (or undersaturation) differs in temperate and tropical communities. Saturation was tested by regressing local wasp-community richness (alpha diversity) against the regional wasp species richness (gamma diversity) of each fig species, and latitudinal variation in saturation was tested by regressing local community species richness against latitude

Number and proportion of southern bird species in Norway in relation to latitude, spring temperature and respiration equivalent

Numbers and proportions of southern bird species in Norway were analyzed, mainly using data from the literature. Southern, regularly breeding, bird species constituted 18% of water/wetland birds and 45% of landbirds. This difference is related to the opposing latitudinal gradients in species richness. Also, the majority of southern water/wetland bird species in Norway are associated with a habitat type that covers small areas, in contrast to southern landbird species.Correlation analysis showed that May temperature and respiration equivalent (a measure of potential for plant growth and ripening) each could account for about 85% of the variation in proportion of southern passerine species, and about 75% in proportion of southern wader species.Significant second‐order, partial correlations were found between proportion of southern passerines and latitude. May temperature and respiration equivalent. It is suggested that the northern distributional limits of passerines, may be indirectly causally related to May temperature and respiration equivalent, and also directly causally related to May temperature. Evidence of causal relationships between northern distributional limits of waders and May temperature or respiration equivalent was not found. For May temperature this may be because of insufficient number of wader censuses.