Global distribution of a key trophic guild contrasts with common latitudinal diversity patterns

Most hypotheses explaining the general gradient of higher diversity toward the equator are implicit or explicit about greater species packing in the tropics. However, global patterns of diversity within guilds, including trophic guilds (i.e., groups of organisms that use similar food resources), are poorly known. We explored global diversity patterns of a key trophic guild in stream ecosystems, the detritivore shredders. This was motivated by the fundamental ecological role of shredders as decomposers of leaf litter and by some records pointing to low shredder diversity and abundance in the tropics, which contrasts with diversity patterns of most major taxa for which broad-scale latitudinal patterns haven been examined. Given this evidence, we hypothesized that shredders are more abundant and diverse in temperate than in tropical streams, and that this pattern is related to the higher temperatures and lower availability of high-quality leaf litter in the tropics. Our comprehensive global survey (129 stream sites from 14 regions on six continents) corroborated the expected latitudinal pattern and showed that shredder distribution (abundance, diversity and assemblage composition) was explained by a combination of factors, including water temperature (some taxa were restricted to cool waters) and biogeography (some taxa were more diverse in particular biogeographic realms). In contrast to our hypothesis, shredder diversity was unrelated to leaf toughness, but it was inversely related to litter diversity. Our findings markedly contrast with global trends of diversity for most taxa, and with the general rule of higher consumer diversity at higher levels of resource diversity. Moreover, they highlight the emerging role of temperature in understanding global patterns of diversity, which is of great relevance in the face of projected global warming.     

Genetic composition of a green turtle (Chelonia mydas) feeding ground population: evidence for multiple origins

Migratory marine turtles are extremely difficult to track between their feeding and nesting areas, and the link between juvenile and adult habitats is generally unknown. To assess the composition of a feeding ground (FG) population of juvenile green turtles (Cheloniamydas Linnaeus), mitochondrial DNA control region sequences were examined in 80 post-pelagic individuals (straight carapace length = 31 to 67 cm) sampled in September 1992 from Great Inagua, Bahamas, and compared to those of 194 individuals from nine Atlantic and Mediterranean nesting colonies. Evidence from genetic markers, haplotype frequencies, and maximum likelihood (ML) analyses are concordant in indicating that multiple colonies contribute to the Bahamian FG population. ML analyses suggested that most Bahamian FG juveniles originated in the western (79.5%) and eastern (12.9%) Caribbean regions, and these proportions are roughly comparable to the size of candidate rookeries. These data support a life-cycle model in which individuals become pooled in post-hatchling (pelagic) and juvenile (benthic) habitats as a consequence of ocean currents and movement among FGs. A substantial harvest of immature turtles on their feeding pastures will influence the reproductive success of contributing nesting populations over a wide geographic scale. Received: 1 April 1997 / Accepted: 14 October 1997     

A comparative study of the costs of alternative mayfly oviposition behaviors

Oviposition behavior of insects has associated fitness costs related to the probability that females survive to oviposit. During summer 2003, we observed the oviposition behavior and compared the mortality rates of females of 17 mayfly species in one western Colorado watershed. We dissected adult females collected on terrestrial sticky traps, in drift nets submerged in streams, and in stomachs of brook trout to determine whether the mayflies had oviposited before capture, drowning, or consumption. Females oviposited by either splashing on the water surface releasing all their eggs (splashers), dropping their eggs from the air (bombers), dipping their abdomens multiple times releasing a few eggs at a time (dippers), landing on rocks and ovipositing on the undersides (landers), or floating downstream while releasing their eggs (floaters). Almost 100% of lander and 50% of dipper females had not oviposited when captured on sticky traps, increasing their vulnerability to preoviposition mortality by aerial predators compared to mayflies with other behaviors. In contrast, most females had laid their eggs before drowning or being eaten by a fish (50–90%). However, groups with oviposition behaviors most exposed to the water surface (floaters, then splashers, dippers, and landers) were more vulnerable to drowning before completing oviposition. In addition, splashers and floaters were most vulnerable to predation by brook trout before ovipositing. These data suggest that fitness costs associated with preoviposition mortality may be considerable depending on mayfly oviposition behavior. Furthermore, previously demonstrated benefits of low predation rates on eggs of lander species may be offset in part by costs to female survival. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Population structure of loggerhead turtle (Carettacaretta) nesting colonies in the Atlantic and Mediterranean as inferred from mitochondrial DNA control region sequences

Mitochondrial (mt) DNA control region sequences were analyzed for 249 Atlantic and Mediterranean loggerhead turtles (Carettacaretta Linnaeus, 1758) to elucidate nesting population structure and phylogeographic patterns. Ten haplotypes were resolved among individuals sampled between 1987 and 1993, from ten major loggerhead nesting areas in the region. Two distinct phylogenetic lineages were distinguished, separated by an average of 5.1% sequence divergence. Haplotype frequency comparisons between pairs of populations showed significant differentiation between most regional nesting aggregates and revealed six demographically independent groups, corresponding to nesting beaches from: (1) North Carolina, South Carolina, Georgia and northeast Florida, USA; (2) southern Florida, USA; (3) northwest Florida, USA; (4) Quintana Roo, Mexico; (5) Bahia, Brazil; and (6) Peloponnesus Island, Greece. The distribution of mtDNA haplotypes is consistent with a natal homing scenario, in which nesting colonies separated by a few hundred kilometers represent isolated reproductive aggregates. However, a strong exception to this pattern was observed in the first group defined by mtDNA data (North Carolina to northeast Florida), which included samples from four nesting locations spread across thousands of kilometers of coastline. These locations were characterized by a single haplotype in 104 out of 105 samples, providing inadequate resolution of population divisions. In view of the subdivisions observed elsewhere, we attribute the lack of differentiation between North Carolina and northeast Florida to recent colonization of these warm temperate coastlines (after the Wisconsin glaciation) not to ongoing gene flow among spatially distinct nesting locations. The relationships among observed haplotypes suggest a biogeographic scenario defined by climate, natal homing, and rare dispersal events. The redefined relationships among nesting aggregations in the western Atlantic region (southeastern USA and adjacent Mexico) prompt a reconsideration of management strategies for nesting populations and corresponding habitats in this region. Received: 28 October 1996 / Accepted: 24 October 1997