The Natural Regulation of Giant Tortoise Populations on Aldabra Atoll. Movement Polymorphism, Reproductive Success and Mortality

Swingland, Ian R.; Lessells, C. M.; Grey, Edward

doi:10.2307/4184

Cited by 60 publications

(35 citation statements)

References 13 publications

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“…Intraspecific differences in seasonal migration driven by resource partitioning have been shown for various ungulate species such as the Yellowstone pronghorn Antilocapra americana (White et al 2007) and moose Alces alces (Ball et al 2001) and tend to emerge when migrants and non-migrants in a population receive approximately equal payoffs (Swingland & Lessells 1979). While differences in migratory behavior in those species were not sex specific, it is possible that the subtle sexual dimorphism in body size of seven gill sharks can lead to sex specific resource partitioning.…”

Section: Discussionmentioning

confidence: 98%

Intraspecific differences in movement, dive behavior and vertical habitat preferences of a key marine apex predator

Stehfest

Patterson²,

Barnett³

et al. 2014

Mar. Ecol. Prog. Ser.

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Understanding the patterns of large-scale movements of highly mobile marine predators is essential to understanding the impacts of anthropogenic pressures on the animals and the ecosystems they frequent. The broadnose sevengill shark Notorynchus cepedianus is one of the most important apex predators in temperate coastal areas around the world, yet little is known of its seasonal large-scale movements. Five male and five female sevengill sharks were equipped with pop-up satellite archival tags (PSATs) in a coastal embayment in southern Tasmania, that collected depth and temperature data during winter, when the animals leave the coastal embayment, resulting in a dataset covering a total of 818 d. Animal tracks indicated that males moved northwards into warmer waters, whereas females remained in southern waters. Three of the females stayed in the Tasmanian coastal areas while the other two left, with one of them moving into deeper waters of up to 360 m depth at the southern edge of the Tasmanian shelf before returning to the Tasmanian coast. These sex-specific differences in large-scale movement could potentially lead to the differential exploitation of the sexes when the sharks leave the protected areas where they were tagged. Both males and females switched between diel vertical migration and reverse diel vertical migration over the course of their tracks and displayed oscillatory vertical movements, probably linked to foraging. These vertical movements persisted throughout the tracks, suggesting that sevengill sharks foraged continuously during their migration rather than switching between transiting and foraging modes.KEY WORDS: Broadnose sevengill shark · Notorynchus cepedianus · Pop-up satellite archival tag · Biotelemetry · Wavelet analysis · Sex-specific · Hexanchiformes Resale or republication not permitted without written consent of the publisher Editorial responsibility: Matthias Seaman,

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Section: Discussionmentioning

confidence: 98%

Intraspecific differences in movement, dive behavior and vertical habitat preferences of a key marine apex predator

Stehfest

Patterson²,

Barnett³

et al. 2014

Mar. Ecol. Prog. Ser.

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“…Despite evidence that elk benefited from migration from a foraging perspective, however, the decline of migrants in this system ) driven by lower adult female and calf survival of migrants (Hebblewhite 2006) reveals forage does not determine fitness of migratory strategies in isolation. Elk must balance the benefits of migration from a foraging perspective with the costs of mortality from predation (e.g., Swingland and Lessels 1979, Nicholson et al 1997, Testa 2004.…”

Section: Discussionmentioning

confidence: 99%

A Multi‐scale Test of the Forage Maturation Hypothesis in a Partially Migratory Ungulate Population

Hebblewhite¹,

Merrill²,

McDermid³

2008

Ecological Monographs

403

599

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Abstract. The forage maturation hypothesis (FMH) proposes that ungulate migration is driven by selection for high forage quality. Because quality declines with plant maturation, but intake declines at low biomass, ungulates are predicted to select for intermediate forage biomass to maximize energy intake by following phenological gradients during the growing season. We tested the FMH in the Canadian Rocky Mountains by comparing forage availability and selection by both migrant and nonmigratory resident elk (Cervus elaphus) during three growing seasons from [2002][2003][2004]. First, we confirmed that the expected trade-off between forage quality and quantity occurred across vegetation communities. Next, we modeled forage biomass and phenology during the growing season by combining ground and remote-sensing approaches. The growing season started 2.2 days earlier every 1 km east of the continental divide, was delayed by 50 days for every 1000-m increase in elevation, and occurred 8 days earlier on south aspects. Migrant and resident selection for forage biomass was then compared across three spatial scales (across the study area, within summer home ranges, and along movement paths) using VHF and GPS telemetry locations from 119 female elk. Migrant home ranges occurred closer to the continental divide in areas of higher topographical diversity, resulting in migrants consistently selecting for intermediate biomass at the two largest scales, but not at the finest scale along movement paths. In contrast, residents selected maximum forage biomass across all spatial scales. To evaluate the consequences of selection, we compared exposure at telemetry locations of migrant and resident elk to expected forage biomass and digestibility. The expected digestibility for migrant elk in summer was 6.5% higher than for residents, which was corroborated with higher fecal nitrogen levels for migrants. The observed differences in digestibility should increase migrant elk body mass, pregnancy rates, and adult and calf survival rates. Whether bottom-up effects of improved forage quality are realized will ultimately depend on trade-offs between forage and predation. Nevertheless, this study provides comprehensive evidence that montane ungulate migration leads to greater access to higher-quality forage relative to nonmigratory congeners, as predicted by the forage maturation hypothesis, resulting primarily from large-scale selection patterns.

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“…The interplay between thermal ecology and foraging biology is likely to be important for taxa that forage in other ways as well. It is interesting to note that the thermal inertia provided by large body size has been identified as an important influence on foraging tactics of reptiles as diverse as Gartersnakes in coldwater streams of northern California (Lind & Welsh 1994), Giant Terrestrial Tortoises on Aldabra Atoll (Swingland & Lessells 1979) and pythons in eastern Australia (present study). Size-related ontogenetic shifts in foraging biology (involving prey size, prey type, foraging modes and foraging microhabitats) are widespread in reptiles (e.g.…”

Section: Extrapolation To the Fieldmentioning

confidence: 98%

“…Huey & Slatkin 1976). For example, the timing, place and duration of predatory bouts may be constrained by thermal factors if the predator must enter a thermally unfavourable environment to obtain its food, as is the case with Californian Gartersnakes capturing fishes in coldwater streams (Lind & Welsh 1994) and Giant Tortoises searching for edible vegetation in hot, unshaded areas of Aldabra Atoll (Swingland & Lessells 1979). The present paper examines another situation in which thermal biology may constrain foraging in important ways: ambush predation by snakes.…”

Section: Introductionmentioning

confidence: 99%

Thermal influences on foraging ability: body size, posture and cooling rate of an ambush predator, the python Morelia spilota

Ayers

Shine

1997

Functional Ecology

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Summary 1.The interface between thermal biology and foraging mode has attracted little scientific attention, but may be crucially important to the biology of ectothermic predators. Slip & Shine (1988c) suggested that the ability of large heavy-bodied snakes to ambush nocturnally active mammals relied on the snakes' control of cooling rates through their thermal inertia (via body size and postural adjustments) and microhabitat selection. 2. We tested assumptions underlying this hypothesis, using Diamond Pythons (Morelia s. spilota) from southeastern New South Wales. Our laboratory studies confirmed that larger body sizes and coiled postures significantly retarded cooling rates, and that body temperature affected the snakes' ability to detect potential prey items. 3. The magnitude of these effects on cooling rates was great enough to extend the time period substantially over which an adult Diamond Python, lying in ambush in a suitable microhabitat, would be able to detect and capture nocturnally active prey. For example, the times taken for pythons to reach thermal equilibration under our experimental conditions (cooling from 33 to 12°C) were < 1 h for hatchling pythons regardless of posture, 1 h for outstretched juveniles, 2 h for coiled juveniles and outstretched adults, and almost 8 h for coiled adults. 4. The high rates of cooling of juvenile pythons, even when they are tightly coiled, may force them to rely upon diurnally active prey rather than crepuscular or nocturnal species.

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The Natural Regulation of Giant Tortoise Populations on Aldabra Atoll. Movement Polymorphism, Reproductive Success and Mortality

Cited by 60 publications

References 13 publications

Intraspecific differences in movement, dive behavior and vertical habitat preferences of a key marine apex predator

Intraspecific differences in movement, dive behavior and vertical habitat preferences of a key marine apex predator

A Multi‐scale Test of the Forage Maturation Hypothesis in a Partially Migratory Ungulate Population

Thermal influences on foraging ability: body size, posture and cooling rate of an ambush predator, the python Morelia spilota

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