Richness of plant–insect associations in Eocene Patagonia: A legacy for South American biodiversity

Abstract

South America has some of the most diverse floras and insect faunas that are known, but its Cenozoic fossil record of insects and insect herbivory is sparse. We quantified insect feeding on 3,599 leaves from the speciose Laguna del Hunco flora (Chubut, Argentina), which dates to the early Eocene climatic optimum (52 million years ago) and compared the results with three well preserved, rich, and identically analyzed early-and middle-Eocene floras from the following sites in North America: Republic, WA; Green River, UT; and Sourdough, WY. We found significantly more damage diversity at Laguna del Hunco than in the North American floras, whether measured on bulk collections or on individual plant species, for both damage morphotypes and feeding groups. An ancient history of rich, specialized plant-insect associations on diverse plant lineages in warm climates may be a major factor contributing to the current biodiversity of South America.paleobotany ͉ Argentina ͉ herbivory ͉ Laguna del Hunco ͉ paleoecology S outh America is well known for its highly diverse** floras (1-3) and associated insect faunas (4-7). Observations in South America and elsewhere show strong positive linkage between plant and insect-herbivore diversity (8)(9)(10)(11)(12)(13)(14). Proposed mechanisms include dependency of insect diversity on plant diversity (8, 9, 13), coevolution of plants and insects (15-21), herbivore selection against host density (22-25), and herbivore intensification of abiotic factors that select for habitat specialization (26,27). Paleontological data can be used to test whether plant and herbivore diversity were correlated in the past (28-30), although data bearing on the primary mechanisms are not typically available from fossils. However, throughout South America, insect body fossils have a limited Cenozoic record (31), and insect damage on fossil plants has rarely been reported (32,33). Here, we seek to determine whether a diverse fossil flora from South America has an elevated level of plant-insect associations. We quantify insect-feeding damage on one of the most speciose known assemblages of fossil plants, from earlyEocene Patagonia and, for comparison, on three well preserved, diverse early-and middle-Eocene floras from comparable absolute latitudes of North America. Study Areas, Specimens, and MethodsThe Patagonian flora comes from tuffaceous lake beds exposed at Laguna del Hunco (LH; S42.5°, W70°; paleolatitude, Ϸ47°S) in Chubut, Argentina (34-36). These sediments were deposited 52 million years ago (35,36), during the early Eocene global climatic optimum (37). At this time, thermophilic organisms reached middle and high latitudes of both hemispheres (38-42), and the LH flora contains a large proportion of tropical as well as temperate lineages (34)(35)(36)39). Our unbiased macrofloral sample, in which all material was collected or tallied, totals 6,521 specimens from 25 quarries, of which 4,303 specimens (66%) are identifiable to 186 species of plant organs. These include 152 leaf species, of which 132 are ma...

A review of paleobotanical studies of the Early Eocene Okanagan (Okanogan) Highlands floras of British Columbia, Canada, and Washington, USA

“…Smith et al (2012) showed that the Falkland site may be of even higher diversity and proposed that patch dynamics and succession due to landscape-scale disturbance (e.g., area volcanism) may have played a role in generating this diversity. Republic and McAbee were used as the proxy for Northern Hemisphere temperate floras in comparative studies of the highly diverse tropical floras of Early Eocene Patagonia and their relationship to insect diversity (Wilf et al 2003(Wilf et al , 2005Archibald et al 2010Archibald et al , 2013. 3.…”

“…While primarily focused on insects in his study of McAbee, Archibald et al (2010) used rarefaction analysis of a small collection (<200 leaves) to show plant diversity in the Early Eocene comparable to tropical rainforest sites, corroborating an extensive and detailed assessment of insect diversity. These data, in combination with data from Republic (Wilf et al 2003(Wilf et al , 2005 and Falkland (Smith et al 2012) have demonstrated high plant and insect diversity for the Okanagan Highlands fossil sites, which Archibald et al (2010Archibald et al ( , 2013 proposed was caused by low temperature seasonality rather than high tropical-character temperatures.…”

“…Early studies by Berry (1929) and Brown (1935Brown ( , 1937Brown ( , 1939 described Republic fossils, and the site was regarded as conifer-dominated, relatively depauperate, and of Oligocene to Miocene age. Wilf et al (2003Wilf et al ( , 2005 and Radtke et al (2005) considered the Republic flora to be Early Eocene, the latter authors citing an unpublished 40 Ar-39 Ar date of 49.4 ± 0.5 Ma (Fig. 3).…”

Fossil insect folivory tracks paleotemperature for six million years

“…Damage diversity on a per leaf basis, as done here and in previous studies, is a core measure of how many ecological feeding types are present on a certain amount of foliage and, because fossil leaves are the units excavated, from a particular amount of sampling effort. This metric is frequency dependent to some extent because a variable proportion of leaves have damage (Wilf et al 2005). However, if undamaged leaves are excluded from diversity analyses to compensate for the frequency dependence, the critical relationship between observed insect damage and the amount of foliage resource available would be lost.…”

Fossil leaf economics quantified: calibration, Eocene case study, and implications

“…Insect herbivory can be measured directly from leaf fossils (Beck and Labandeira 1998;Labandeira 1998;Wilf and Labandeira 1999;Wilf et al 2001Wilf et al , 2005, but the fundamental leaf economic traits that influence herbivory have been difficult to quantify from fossils. Several methods for estimating LL for fossil species exist, but three of these, comparison with nearest living relatives (Chaloner and Creber 1990), leaf thickness (Chaloner and Creber 1990), and presence of leaf mats (Spicer and Parrish 1986), are qualitative; at best they can distinguish deciduous from evergreen leaf habits (e.g., Wolfe 1987;Wolfe and Upchurch 1987).…”