High densities of the sea urchin Diadema savignyi on a coral reef in French Polynesia. Image: Sven UtichkeIn some cases these species are so crucial to their environments that if their populations either crash or proliferate, catastrophic "phase shifts" to degraded seascapes may result. The evolutionary progress of the species may also be affected, potentially leading to extinctions.
Stars of the sea falling to human-induced perils
AIMS media release 24 Feb 09;
Sea stars, sea urchins, sea cucumbers and other similar creatures, many of them "keystone species" that are essential for keeping ecosystems healthy, are falling prey to a range of disturbances caused by human activity.
In some cases these species are so crucial to their environments that if their populations either crash or proliferate, catastrophic "phase shifts" to degraded seascapes may result. The evolutionary progress of the species may also be affected, potentially leading to extinctions.
The first-ever large scale review of current research to investigate broad trends in populations of these marine invertebrate animals, known as echinoderms, has been carried out by AIMS scientists Dr Sven Uthicke and Dr Britta Schaffelke, in collaboration with Professor Maria Byrne from the University of Sydney. Their paper has been published in the journal Ecological Monographs*.
The researchers chose to review 28 species that are well-represented in current scientific literature, out of about 7,000 echinoderm species. The choice was made on the basis of strong existing data about large population fluctuations.
"Each of these 28 cases was experiencing difficulties because of human activity," Dr Uthicke said. "This involves the whole suite of human activities, including over-fishing, nutrient run-off from the land, species introductions and climate change."
While populations of these species may experience natural boom and bust cycles, Dr Uthicke and colleagues have discovered that human-induced change is intervening. All species examined in the paper displayed population anomalies inconsistent with natural cycles.
These species are crucial to the structure of many marine ecosystems and are known to undergo large population density variations such as die-offs and outbreaks. The most notorious of the outbreak events are those of the crown-of-thorns starfish which regularly ravages the Great Barrier Reef. Outbreaks are defined in the paper as a rapid four-fold increase in a population. Starfish outbreaks have been linked both to excessive nutrients from agricultural run-off and overfishing.
Die-offs, defined as a drop to less than 25 per cent of the previous population, can be just as devastating and may be the final straw for some ecosystems. One example was the die-off of sea urchins throughout the Caribbean. In cases where the ecosystem was already degraded due to overfishing, the loss of sea urchins has tipped some ecosystems into phase shift in which algae rather than coral has come to dominate. On the GBR, at least one species of sea cucumber is overfished and its stocks have not recovered even after the fishery has been closed for at least six years. It is feared that the ecological consequences of this may also be severe.
The review examined species from a wide range of marine environments, from the inshore regions of the Great Barrier Reef to the deep ocean, from temperate regions to the tropics and from the northern to the southern hemisphere.
The pattern found everywhere, according to Dr Uthicke, is that many echinoderm species are being destabilised by a range of human activities. The effects of climate change are not yet as well understood as those of over-fishing and higher nutrients in the water, although they were present in some of the cases.
"We suggest that human-induced disturbance, through its influence on changes to echinoderm population densities, may go beyond present ecosystems impacts and alter future evolutionary trends," Dr Uthicke said. "Human interference can lead to accelerated change, which doesn’t allow ecosystems time to adapt or compensate.
"Similar to terrestrial environments, diversity in marine ecosystems is crucial for ecosystem functioning, productivity and resilience. For these reasons we suggest focused scientific study and conservation effort is required to protect the natural dynamics of these fluctuating species and their ecosystems," Dr Uthicke said.
The paper by Sven Uthicke, Britta Schaffelke and Maria Byrne, titled "A boom and bust phylum? Ecological and evolutionary consequences of large population density variations in echinoderms", may be found here http://www.esajournals.org/doi/abs/10.1890/07-2136.1
More about Singapore's echinoderms on the wildsingapore wild fact sheets.
