Incumbency, Diversity, and Latitudinal Gradients

Paleobiology, 34(2), 2008, pp. 169–178 MATTERS OF THE RECORD Incumbency, diversity, and latitudinal gradients James W. Valentine, David Jablonski, Andrew Z. Krug, and Kaustuv Roy James W. Valentine. Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley, California 95720. E-mail: [email protected] David Jablonski and Andrew Z. Krug. Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637. E-mail: [email protected], [email protected] Kaustuv Roy. Section of Ecology, Behavior and Evolution, Division of Biological Sciences, University of California, San Diego, 9500 Gillman Drive, La Jolla, California 92093. E-mail: [email protected] Accepted: 5 November 2007 Physical environmental factors have been number or proportion of genera that have ex- seen as paramount in determining many panded from the Tropics (the ratio is generally large-scale biodistributional patterns in time ϳ3:1), and even this smaller number is prob- and space. Although this is probably correct ably an overestimate, because the extratropical for many situations, this view has become so post-Paleozoic fossil record is so much better pervasive that it has led to the neglect of the sampled than that of the Tropics (Allison and role of biotic interactions in setting large-scale Briggs 1993; Jablonski 1993; Jackson and John- diversity patterns. (In this paper diversity de- son 2001; Jablonski et al. 2006; Valentine et al. notes taxonomic richness.) New approaches to 2006). The result is a gradient wherein the ma- this perennial debate on the roles of physical jority of taxa in each latitudinal bin is shared and biotic forces in paleoecology and macro- with the Tropics. For genera that originate ex- evolution are needed, and here we explore an tratropically, expansion into the Tropics is vir- argument for the role of incumbency or pri- tually unknown, at least at shelf depths, and ority effects in the dynamics behind the most given the anti-tropical bias in the evidence of dramatic spatial pattern in biodiversity, the first occurrences, most apparent instances latitudinal diversity gradient. may be artifacts (Jablonski et al. 2006; see also A global analysis of the fossil record of liv- Vermeij 2005a). Over the past few centuries, ing marine bivalve genera and subgenera successful species invasions have been less (hereafter simply genera) of the continental frequent in tropical than extratropical regions, shelves provides perhaps the strongest evi- at least for mainland terrestrial communities, dence for the Out of the Tropics (OTT) dy- even though several studies have shown a pos- namic associated with the formation of the itive relationship between the number of ex- present marine latitudinal diversity gradient otic and native species within climatic zones (LDG) (Jablonski et al. 2006). The marine LDG (Rejmańek 1996; Sax 2001; Fine 2002; Sax and appears to be driven primarily by the origin Gaines 2006; Fridley et al. 2007). Fewer data of novel lineages in the Tropics, some of which are available for marine tropical settings, but then expand their ranges into higher, extra- success rates of invasions from the temperate tropical latitudes (see Jablonski 1993, 2005; zones also appear to be low, outside of severe- Clark and Crame 2003; Goldberg et al. 2005; ly disturbed or novel habitats (e.g., Hewitt Jablonski et al. 2006; Martin et al. 2007). Sup- 2002; Paulay et al. 2002; Fridley et al. 2007). port for this pattern comes from the over- Here we explore some features that may reg- whelmingly tropical first fossil occurrences of ulate this evolutionary and biogeographic dy- living bivalve genera and their subsequent ap- namic, and advance a counterintuitive hy- pearances in higher latitudes. Some genera are pothesis in hopes of stimulating research into first found in the extratropical fossil record, the neglected role of biotic factors in shaping but these never match, at any latitude, the the LDG. ᭧ 2008 The Paleontological Society. All rights reserved. 0094-8373/08/3402-0001/$1.00 170 JAMES W. VALENTINE ET AL. ly, a definitive global data set of the ages of marine bivalve species is not available. How- ever, because only one species need reach higher latitudes to create a record of its genus there, these categories are linked to some ex- tent. High-latitude species might be older than tropical ones, although active recent high-latitude speciation is known or inferred in several terrestrial groups (see Mittelbach et al. 2007 and Wier and Schluter 2007 for refer- FIGURE 1. The median ages of living marine bivalve ences). However, the species/genus ratio is genera (solid line, diamonds) increases with latitude as low for extant high-latitude bivalves globally the number of genera (dashed lines) decreases. Modified with added data from supporting online material in Ja- (Polar S/G: 1.9; Tropical S/G: 5.1, suggesting blonski et al. 2006. sluggish per-taxon rates of marine speciation in polar regions [Krug et al. 2008]). Related to the OTT dynamic is the increase Possible Causes of Latitudinal in median ages of bivalve genera with latitude Diversity Dynamics as the number of genera decreases (Fig. 1). The tropical fauna retains most of the older genera The genus-level diversity gradient must be as they expand, but its high origination rate explained by some combination of origination, imparts a younger average age to the fauna extinction, and dispersal rates, and recent there (see Foote 2001 on the dynamical inter- models have explored interactions among pretation of taxon ages). This interpretation is these parameters (see Roy and Goldberg supported by Crame’s (2000) observation that 2007). As noted above, origination rates are diversity gradients are steepest for the bivalve highest in the more diverse regions and taxa clades dominated by younger genera. The few are exported to higher latitudes without eras- genera residing in higher latitudes are thus bi- ing latitudinal diversity trends, so either there ased toward those that evolved earliest, many is a gradient in extinction or young genera are demonstrably at low latitudes (Jablonski et al. being prevented from dispersing, or both. As 2006). Even the few groups with peak diver- shown below, paleontological estimates of ge- sity outside the Tropics, such as the anomal- neric extinction rates in the Pliocene and Pleis- odesmatan bivalves, share the same funda- tocene (ϳ0.012–5 Myr ago) are no higher (and mental relationship between taxon age and di- may be lower) in polar regions than in tem- versity, with minimum genus ages roughly co- perate latitudes, so differential extinction inciding with their extratropical diversity alone cannot account for the temperate to po- maxima, and age maxima coinciding with di- lar decline in diversity (we use polar here sim- versity minima (Krug et al. 2006 and unpub- ply to denote regions poleward of 60Њ lati- lished data). In theory, patterns of species-lev- tude). Thus one or more factors not directly el origination might differ from those of line- related to origination and extinction rates per age formation at the genus level. Unfortunate- se must impede the cross-latitudinal expan- TABLE 1. The qualitative pattern of factors associated with an incumbency model that proposes a filtering of the poleward spread of lineages, chiefly arising in the Tropics, to shape the latitudinal diversity gradient. Factors 1 to 4 are based on observations whereas factors 5 and 6 are inferential. Factor Tropical zone Temperate zone Polar zone 1. Diversity High Intermediate Low 2. Median age of genera Low Intermediate High 3. In-situ origination rate High Low Low 4. Extinction rate Low Higher? Low 5. Invasibility Low Higher? Low 6. Niche breadth Narrow Intermediate Broad INCUMBENCY, DIVERSITY, LATITUDE 171 sion of lower-latitude genera through restric- istence of a diverse deep-sea fauna at low tem- tions on their contained species. These poten- peratures indicates that evolution is quite ca- tial constraints fall into three general catego- pable of overcoming physiological problems ries. associated with the low end of the marine 1. Time. Many genera show a latitudinal temperature range. The fact that most of the array of species along shelves, with poleward marine bivalve families absent from the deep spread presumably involving speciation sea are suspension feeders (Knudsen 1979; across latitudes. Perhaps, then, insufficient Gage and Tyler 1991; Valentine et al. 2006) time has elapsed for the accumulation of pole- suggests that the attenuation of trophic re- ward speciations in many lineages. ‘‘Age and sources may be a more important barrier than area’’ arguments, with geographic expansion low temperature (see Rex et al. 2005 for a sim- as a positive function of taxon age, have a long ilar argument for species richness in the abys- pedigree (e.g., Willis 1922; Miller 1997; Jones sal environment). Furthermore, temperature et al. 2005; see Mittelbach et al. 2007 for a re- variability does not correlate with the LDG, view). However, the LDG has been at least a for it is greatest in middle and least at low and semi-permanent feature of the marine bio- high latitudes (Valentine 1973). Air tempera- sphere for much of the geologic past and tures can certainly be very low in high lati- seems to have been present in something like tudes, affecting intertidal forms, and intertid- its current form not just in the Neogene, but al and shallow subtidal zones may be ice cov- since the Mesozoic breakup

Load more