Turtle species in serious decline: Broad ecological impacts
U. of California Davis-Sept 12, 2018
Approximately 61 percent of the world's 356 turtle species are threatened or already extinct, and the decline could have ecological consequences.
These findings are according to a paper in the journal BioSciencesynthesizing the global status of turtles and their ecological roles by scientists from the U.S. Geological Survey, Tennessee Aquarium Conservation Institute, University of California, Davis, and the University of Georgia.
Turtles are now among the most threatened groups of vertebrate animals on earth, more so than birds, mammals, fish or amphibians. These animals outlived the dinosaurs and have roamed the earth for more than 200 million years. Reasons for the decline of turtles worldwide include habitat destruction, over-exploitation for pets and food, disease and climate change.
The gopher tortoise (Gopherus polyphemus) is a species of the Gopherus genus native to the southeastern United States. The gopher tortoise is seen as a keystone species because it digs burrows that provide shelter for at least 360 other animal species. They are threatened by predation and habitat destruction.
The gopher tortoise (Gopherus polyphemus) is a species of the Gopherus genus native to the southeastern United States.Their burrows provide shelter for over 350 vertebrates and invertebrates (Johnson et al. 2017), many of which cannot dig burrows on their own. Included are burrowing owls (Athene cunicularia), various species of lizards, gopher frogs (Lithobates capito), red foxes (Vulpes vulpes), bobcats (Lynx rufus), rodents, rabbits, and snakes, including other declining species, such as eastern indigo snakes (Drymarchon couperi) and diamondback rattlesnakes (Crotalus adamanteus).
"Our goal is to provide resource managers with a full picture of the state of these iconic animals worldwide, and what long-term impacts our environment might experience if populations continue to decrease and species loss continues," said USGS scientist and lead author of the study Jeffrey Lovich. "Turtles contribute to the health of many environments, including desert, wetland, freshwater and marine ecosystems, and their decline may lead to negative effects on other species, including humans, that may not be immediately apparent."
"Our goal is to provide resource managers with a full picture of the state of these iconic animals worldwide, and what long-term impacts our environment might experience if populations continue to decrease and species loss continues," said USGS scientist and lead author of the study Jeffrey Lovich. "Turtles contribute to the health of many environments, including desert, wetland, freshwater and marine ecosystems, and their decline may lead to negative effects on other species, including humans, that may not be immediately apparent."
Scientists synthesized existing published studies to bring attention to the status of turtles and identify what may be lost from an ecological perspective if current trends hold and they continue to decline and disappear.
Biomass is an important measure in ecology, because it reflects the amount of available and stored energy in the plants and animals occupying an ecosystem. High biomass of a species or group of species within an ecosystem commonly results in a greater impact on ecosystem processes. Many turtle populations make or made significant contributions to the biomass of their environment, as first noted by Iverson (1982). Turtle standing crop biomass is generally very high in comparison with other vertebrates (figure 1), often surpassing that of all other reptiles. Biomass values for turtles are typically at least an order of magnitude higher than those of other reptiles and similar to those of large schools of fish. The high biomass of turtles is related largely to the high population densities they are capable of achieving. For example, two centuries ago, sea turtles were estimated to number in the tens of millions in the Caribbean Sea alone. More recently, their numbers were estimated in the tens of thousands (Jackson et al. 2001). In the last 40 some years alone, various major groups of marine vertebrate populations, including sea turtles, have declined an estimated 22% on average (McCauley et al. 2015), further diminishing their impacts on that ecosystem.
aximum biomass estimates for several major vertebrate assemblages (represented with generic silhouettes), subdivided into dietary groups, such as herbivorous, omnivorous, and carnivorous species. The top image is an inset displaying biomass estimates that were less than 10 kilograms per hectare (kg per ha) and difficult to display on the bottom image. (a) large carnivorous mammals (0.24 kg per ha), (b) birds (approximately 1.0 kg per ha), (c) small carnivorous mammals (approximately 3.0 kg per ha), (d) herbivorous lizards (4.1 kg per ha), (e) snakes (4.58 kg per ha), (f) carnivorous lizards (5.5 kg per ha), (g) frogs (27.3 kg per ha), (h) carnivorous turtles—e.g., Sonora mud turtle—(100.3 kg per ha), (i) aquatic salamanders (99.3 kg per ha), (j) tuataras (250 kg per ha), (k) large herbivorous mammals (280 kg per ha), (l) fish (477 kg per ha), (m) herbivorous turtles—e.g., Aldabra giant torotise—(583.5 kg per ha), and (n) omnivorous turtles—e.g., red-eared slider turtle—(877 kg per ha). All estimates are from Iverson (1982), except for omnivorous turtles (Congdon et al. 1986) and aquatic salamanders (Peterman et al. 2008).
This paper provides the first major review of the various functional roles that large populations and diverse communities of turtles provide from an ecological perspective. This includes maintaining healthy food webs, dispersing seeds and creating habitats necessary for other species.
"Our purpose is to inform the public of the many critical ecological roles turtles perform on a global scale and bring awareness to the plight of these emblematic animals whose ancestors walked with the dinosaurs," said professor emeritus and senior ecologist Whit Gibbons, at the University of Georgia's Savannah River Ecology Laboratory and Odum School of Ecology. "These modern descendants of an ancient lineage are touchstones for how human influences are causing the decline of so much of the world's wildlife. Our hope is that everyone will be encouraged to engage in concerted efforts to conserve their well-earned legacy as part of our natural habitats."
Turtles can be major players in ecosystem food webs because they can be herbivores, omnivores or carnivores. They range from specialists that feed on one to a few food sources, to generalists, feeding on a wide range of items. Their diverse feeding habits allow them to influence the structure of other communities in their habitat. Some turtle species occur in dense numbers that can yield hundreds of pounds of turtles per acre, making them ecologically important by virtue of their mass alone. Such large masses of turtles equate to large amounts of potential food for organisms that feed on turtles or their eggs.
Energy in nutrients transferred from marine to terrestrial environments by nesting sea turtles is now greatly diminished with worldwide reductions in sea turtle populations. This figure shows the projected transfer of energy from 14,305 loggerhead sea turtle (Caretta caretta) nests with an estimated 1.6 million eggs on a 21-kilometer stretch of Florida beach in 1996 (Bouchard and Bjorndal 2000). Note that only about one quarter of total energy transferred to beaches returned to the marine environment in hatchlings.
Turtles can be important for dispersing the seeds of dozens of plant species. Some turtle species may even be the primary seed dispersal agents for specific plants. Not all seeds are destroyed by the digestive tract. In fact, there are specific seeds that exhibit higher rates of germination after being eaten and passed by turtles.
Some turtles, like Agassiz's desert tortoise in the American Southwest and the gopher tortoise in the American Southeast, dig deep burrows creating habitat for other species. For example, the gopher tortoise can dig burrows over 30 feet long. The mounds of soil near the entrance of the burrows can create new habitat for some plant species, increasing overall plant diversity near burrow entrances. The burrows are used by hundreds of other species including spiders, insects, snakes, amphibians, other reptiles, rabbits, foxes and even bobcats.
"The ecological importance of turtles, especially freshwater turtles, is underappreciated, and they are generally understudied by ecologists," says Josh Ennen, research scientist at the Tennessee Aquarium Conservation Institute. "The alarming rate of turtle disappearance could profoundly affect how ecosystems function as well as the structure of biological communities around the globe."
"We must take the time to understand turtles, their natural history, and their importance to the environment, or risk losing them to a new reality where they don't exist," said UC Davis scientist Mickey Agha. "Referred to as a shifting baseline, people born into a world without large numbers of long-lived reptiles, such as turtles, may accept that as the new norm."
Story Source:
Materials provided by University of California - Davis. Note: Content may be edited for style and length.
Journal Reference:
- Jeffrey E Lovich, Joshua R Ennen, Mickey Agha, J Whitfield Gibbons. Where Have All the Turtles Gone, and Why Does It Matter? BioScience, 2018; DOI: 10.1093/biosci/biy095
- September 12, 2018
- University of California - Davis
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