More than 400 dead zones in the world’s seas

It is now estimated that there are now more than 400 dead zones in our seas.

David Perlman, the SF Chronicle Science Editor reports…

Diaz, R. J. & R. Rosenberg, 2008. Spreading dead zones and consequences for marine ecosystems. Science, 321 (5891): 926 – 929. 15 August 2008.

Abstract – Dead zones in the coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning.

The formation of dead zones has been exacerbated by the increase in primary production and consequent worldwide coastal eutrophication fueled by riverine runoff of fertilizers and the burning of fossil fuels. Enhanced primary production results in an accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters.

Dead zones have now been reported from more than 400 systems, affecting a total area of more than 245,000 square kilometers, and are probably a key stressor on marine ecosystems.

Dead zones in South and East Asia
Click to view in Google Maps (from Diaz).

El Nino, local extinction of fig wasps and starving frugivores

In June 2007, Sunbear researcher Wong Siew Te was to be in town for a meeting at the Singapore Zoo. He offered to talk in NUS and I seized the opportunity and quickly set it up. It would turn out to be very impactful. During his excellent talk on “The ecology and conservation of the sun bear in Malaysia,” he told us about the critical role figs trees play in the diet of the sunbear.

In 1997/8, the region experienced a severe El Nino event. This led to the local extinction of fig wasps (due to direct impact and the resultant haze). In the absence of pollinators in 1999, fig trees had aborted their fruits resulting in a famine for frugivores in at least to locations in Sabah and Kalimantan, at least (where there were sun bear reserchers).

While Orang utans managed to resort to other food sources like young shoots and plant sap, animals like sun bears and bearded pigs starved. All of Siew Te’s six radio-collared sun bears were emaciated and two died (see photo below). This also led to starving bearded pigs attempting to raid field centres as well. This cascading event is believed to have contributed to the very low density of large mammals in Bornean rainforests.

Below I provide the abstract and links to the papers behind the story. We have hosted both speakers in the department before. We are lucky to have many visitors passing through so if you want to be alerted about such talks in future, sign up with the Ecotax announcement list that I manage.

Rhett D. Harrison, 2000. Repercussions of El Niño: drought causes extinction and the breakdown of mutualism in Borneo. Proc. R. Soc. B (1996-) – 267 (1446): 911-915 (07 May 2000). [NUS link]

Figs (Ficus spp.) and their species-specific pollinators, the fig wasps (Agaonidae), have coevolved one of the most intricate interactions found in nature, in which the fig wasps, in return for pollination services, raise their offspring in the fig inflorescence. Fig wasps, however, have very short adult lives and hence are dependent on the near-continuous production of inflorescences to maintain their populations.

From January to March 1998 northern Borneo suffered a very severe drought linked to the El Niño Southern Oscillation event of 1997-1998. This caused a substantial break in the production of inflorescences on dioecious figs and led to the local extinction of their pollinators at Lambir Hills National Park, Sarawak, Malaysia. Most pollinators had not recolonized six months after the drought and, given the high level of endemism and wide extent of the drought, some species may be totally extinct.

Cascading effects on vertebrate seed dispersers, for which figs are often regarded as keystone resources, and the tree species dependent on their services are also likely. This has considerable implications for the maintenance of biodiversity under a scenario of climate change and greater climatic extremes.

Photo by Wong Siew Te

Wong, Siew Te, C. Servheen, L. Ambu, A. Norhayati, 2005. Impacts of fruit production cycles on Malayan sun bears and bearded pigs in lowland tropical forest of Sabah, Malaysian Borneo. Journal of Tropical Ecology, 21: 627-639. [NUS link]

We observed a period of famine in the lowland tropical rain forest of Sabah, Malaysia from August 1999 to September 2000. All six Malayan sun bears (Helarctos malayanus) that were captured and radio-collared were in poor physical condition, and two were later found dead. The physical condition of bearded pigs (Sus barbatus) that were captured, observed or photographed by camera traps also revealed that the pigs were in various stages of emaciation and starvation.

We surmise that the famine resulted from prolonged scarcity of fruit during an intermast interval in the study area. These phenomena of emaciated animals and fruit scarcity have also been reported from other areas of Borneo. Lowland tropical rain-forest trees of Borneo display supra-annual synchronized general fruiting.

We believe that the starvation we observed and the generally low density of large animals in Borneo forests is a consequence of a history of prolonged food scarcity during non-general-fruiting years, but may be accentuated by anthropogenic factors such as forest fragmentation, selective logging, and reduced density of fig trees in logged forests.

Top-down ecosystem effect of predator hunting modes

Schmitz, O.J., 2008. Effects of predator hunting mode on grassland ecosystem function. Science, 319(5865): 952 – 954 (15 Feb 2008).

Abstract – The way predators control their prey populations is determined by the interplay between predator hunting mode and prey antipredator behavior. It is uncertain, however, how the effects of such interplay control ecosystem function. A 3-year experiment in grassland mesocosms revealed that actively hunting spiders reduced plant species diversity and enhanced aboveground net primary production and nitrogen mineralization rate, whereas sit-and-wait ambush spiders had opposite effects. These effects arise from the different responses to the two different predators by their grasshopper prey—the dominant herbivore species that controls plant species composition and accordingly ecosystem functioning. Predator hunting mode is thus a key functional trait that can help to explain variation in the nature of top-down control of ecosystems.

Hypothesized predator indirect effects on plant community composition (dashed lines) and on ecosystem functions (dotted lines).

How is this similar (or not) to the trophic cascade hypothesis applied to a terrestrial ecosystem?

New Paradigm On Ecosystem Ecology Proposed.” ScienceDaily, 19 Feb 2008.

‘…the jumping spider (Phidippus rimator) prowls its neighborhood, or ecosystem, and engages in random acts of violence against its plant-eating prey, the garden variety grasshopper (Melanopuls femurrubrum).’

“And much like victims of crime, grasshoppers facing an imminent threat go into a heightened state of alert, taking refuge in the ecosystem’s dominant plant, the goldenrod. Its mobility restricted, the grasshoppers dine on its own shelter, promoting habitat diversity in the process. The goldenrod’s competitors–Asters, Queen Anne’s Lace and a variety of clover and grasses–flourish, but the diversity comes at a price. With the demise of the goldenrod, nitrogen–a key fertilizer in the soil’s renewal–is depleted.

In a separate part of the experiment, nursery web spiders (Pisaurina mira), which Schmitz called “sit-and-wait ambush spiders” because they are coy about their predatory intentions, occupy a certain “bad neighborhood,” enabling grasshoppers to avoid them, roam the ecosystem and eat a wider variety of plants. In this milieu, the goldenrod thrives, ultimately nourishing the soil when its nitrogen-rich tissue decays.

“What’s really cool here is that different spiders have different hunting modes, and it’s those modes that cause grasshoppers to behave differently, which then carries down the chains of the community structure of the plants,” said Schmitz. “So it’s a top-down view…[m]y research shows that carnivores have an indirect effect on plants.”

Finally, see: “Ecology: Green with Complexity,” by Shahid Naeem. Science (Perspectives), 319(5865): 913 – 914 (15 Feb 2008).