Density-dependent effects on hatching success of the olive ridley turtle, Lepidochelys olivacea

Abstract

Historically, the olive ridley arribada at Playa Nancite, Costa Rica, was one of the largest olive ridley arribadas in the eastern Pacific with 70,000 nesting females in a year. Recently the Nancite arribada drastically declined. We hypothesized that the population decline at Playa Nancite could have been due to low hatching success as a result of the high density of nests on the beach, such that recruitment to the population was insufficient to balance losses. To test this hypothesis, we examined density-dependent effects on hatching success and their underlying mechanisms by experimentally manipulating nest densities in experimental plots on the nesting beach. We set up four nest-density treatments in five experimental blocks. We measured effects of density on hatching success, CO(2) and O(2) concentrations and temperature both within nests and in sand adjacent to nests frequently during incubation. Experimental nest densities affected hatching success with the highest density having the lowest hatching success. Higher nest density led to lower O(2) levels and higher CO(2) levels in the nest with greater changes in the latter part of the incubation. Highest temperatures occurred in high-density areas. Temperatures were lower in sand surrounding the nest than in the nest. Effects of density on temperature, CO(2) and O(2) were confirmed at a naturally high-density nesting beach, Playa La Flor, Nicaragua. Long-term failure in production of hatchlings due to historic high densities may have contributed to the decline of arribadas on Playa Nancite. Thus, density-dependent population control would have operated at the embryonic life stage in this population of olive ridley turtles.

Estimating carrying capacity at the green turtle nesting beach of East Island, French Frigate Shoals

“…With the absence of major predators on the islands, the main factors regulating hatchling output would be intraspecific destruction of eggs, as suggested by other researchers (Bustard & Tognetti 1969, Girondot et al 2002, Tiwari et al 2006, Mazaris et al 2009), increased microbial activity resulting from a higher nutrient load after a certain nest density threshold is reached (Cornelius et al 1991, Tiwari et al 2006, as well as changes in the incubation environment. Honarvar et al (2008) demonstrated experimentally that increasing the density of olive ridley Lepidochelys olivacea nests resulted in lower hatching success possibly due to higher temperatures, higher carbon dioxide levels, and lower oxygen levels. Tiwari et al (2006) found that below-beach-surface predation, erosion, and beach flooding affected green turtle hatchling production at Tortuguero, Costa Rica, in a density-independent manner.…”

“…Increased nesting densities on East Island would result in density-dependent changes in the nesting environment (Honarvar et al 2008) and in densitydependent factors such as nest destruction by nesting females and disease regulating population size. Females have been observed to destroy incubating nests during their nesting process on East Island (Balazs 1980).…”

Intraspecific competition affects population size and resource allocation in an ant dispersing by colony fission

“…One experimental approach to testing the effect of intraspecific competition on population growth comprises removing (or adding) individuals and monitoring subsequent population variations (Murdoch 1970, 1994, Massot et al 1992, Adams and Tschinkel 2001, Honarvar et al 2008. A population regulated by intraspecific competition is predicted to be resilient to this kind of manipulation and to return rapidly to its initial state.…”

Hatching and emergence success in green turtle Chelonia mydas nests in the Galápagos Islands

“…The probability of this mortality source is highly dependent on nest density. As the density of nesting females increases, the probability of one female excavating another nest increases (Bustard & Tognetti 1969, Tiwari et al 2006, Honarvar et al 2008). …”

“…Nesting begins in early December on Bahía Barahona and Quinta Playa, and about a month later on Las Bachas and Las Salinas. Early nests are more vulnerable to subsequent destruction from other nesting turtles.Nest destruction by later nesting sea turtles has been observed in high nest density populations such as in green turtles from Tortuguero, Costa Rica (Tiwari et al 2006), and the southern Great Barrier Reef, Australia (Bustard & Tognetti 1969, Limpus et al 2003, in leatherbacks from French Guiana (Giron dot et al 2002), and olive ridleys in Costa Rica (Honarvar et al 2008). Nest destruction rate from nesting females in Tortuguero, Costa Rica, is 4 times higher (Ti wari et al 2006) than our estimates from Bahía Barahona and Quinta Playa, which probably reflects differences in population density.…”