While attending the 6th annual conference on Ecology and Evolution of Infectious Disease (EEID) last week, I had a chance to listen to the latest research on host-parasite interactions from some of the leading scientists both within and outside USA.

It is, therefore, quite timely to read this new finding from World Science, about a parasite changing the behavior of its host, making it a suicidal bodyguard.

But, first thing first. What is a parasite, and a host?

Host-parasite (and pathogen) interaction

A host-parasite interaction, also called parasitism, is an ecological interaction between two biological species - a host species and a parasite species - where the parasite lives (grows, feeds, shelters and reproduces) on or within the body of its host.

Depending on the species, this is either a win-nothing or a win-lose interaction, where the parasite always benefits from the host, but the host gets nothing in return, and may be even harmed, resulting in a disease.

A pathogen is a parasitic micro-organism (virus or bacteria) that usually lives within its much larger host body (such as us), and a host-pathogen interaction almost always leads to a disease in the host, which can be relatively harmless (for example, common cold) or even fatal.

Every biological species, including parasites, always tries to promote its own persistence in a generally hostile environment (Darwinian struggle for existence). For a parasite that is not free-living (cannot live without a host), this means that it must try to maximize its own transmission from one host to another.

When the transmission is between individuals of the same host species, maximizing transmission will facilitate parasite survival when, for example, the first host individual dies (of disease). And when transmission is from one host species to a new host species, maximizing transmission helps the parasite to survive by completing its life cycle, as we will see in some examples below.

Manipulating host behavior

One of the several strategies that a parasite uses to achieve high host-to-host transmission is by manipulating host behavior. Some of the fascinating examples of such manipulation, including the present case, are as follows:

• Ants parasitized by the trematode Dicrocoelium dendriticum climb up the blades of grass, to be accidentally eaten along with the grass by grazing sheep. The trematode is then transmitted from the ants (intermediate host species) to sheep (definitive host species).
• Killifish (intermediate host) infected by the trematode Euhaplorchis californiensis start swimming near the water surface, and are captured by birds (definitive host) more frequently than uninfected fish.
• Rats (intermediate host) infected with the protozoan Toxoplasma gondii tend to lose their anti-predatory instinct, and are therefore more heavily predated by cats (definitive host).
• Crickets parasitized by the nematomorph Paragordius tricuspidatus exhibit erratic behavior and jump into water (in what appears like suicide) more often than the unparasitized insects, which allow the parasite to access the aquatic environment needed for its reproduction.
• Spider infected with the parasitic wasp Hymenoepimecis sp. is induced to build a special “cocoon web” to protect the wasp pupae from heavy rain.
• The latest example: After the larva of the parasitic wasp Glyptapanteles sp. crawls out of the body of its host caterpillar (Thyrinteina leucocerae) to build a cocoon, the caterpillar becomes a devoted security guard for the cocoon. It stops eating and stays close by the cocoon, wraps it into a protective silk web, and defends it against predators with violent head swings. The caterpillar continues to do this till the wasp emerges from the cocoon, and then it dies.

In the first three cases, the intermediate host species is usually host to the parasitic egg or larva, which then must be transmitted to its definitive host species to develop into an adult, thereby completing its life cycle.

Parasites achieve such manipulative capability often by affecting the host’s central nervous system, for example by forming cysts in the brain of killifish and rat, which then behave abnormally compared to their unparasitized cousins.

Survival of the fittest

Of course, parasites cannot learn to manipulate host behavior by trial and error like we do (you can amuse yourself by trying to guess a worm’s IQ).

Instead, it is a result of evolutionary selection acting on many different interaction patterns between parasites and their hosts over millions of years, which selects the strategy that provides the best chance of the parasite’s survival in its struggle for existence.

Thus, in the Darwinian sense, a smart host manipulation strategy for maximizing transmission is a tool for survival of the fittest parasite. Others that adopted less efficient mechanisms went extinct along the way.

Scientists have already shown significant alteration of the American landscape because of increased CO₂ emission from burning fossil fuel, which causes global warming. But, pressured by various lobbies (mainly oil and auto industries), the US government has been dragging its feet over acknowledging that global warming is indeed a serious threat.

Until now, that is. Bowing to a district court judge’s ruling that the government must produce the latest scientific assessment of global warming by May 31, the Climate Change Science Program, which was commissioned by USDA and integrates the federal research efforts of 13 agencies on climate and global change, released its findings on May 27.

The study, which was carried out by 38 scientists from inside the government and outside (universities, national laboratories and non-governmental organizations), includes no new research, but synthesizes a thousand scientific papers to highlight how the human-generated CO₂ emission has already caused major changes in our environment.

From the press release, the main points of the study are:

1. Increasing temperatures will increase the risk of crop failures, particularly if rainfall decreases and/or becomes more unpredictable.
2. Higher temperatures will reduce productivity of livestock and dairy animals, and greater mortality will offset the reduced mortality in warmer winters.
3. Climate change has already increased the size and frequency of forest fires, insect outbreaks and tree mortality, in the forests of interior West, Southwest and Alaska. These effects will continue.
4. The West and Southwest have also experienced increased draught conditions.
5. Weeds grow more rapidly under high atmospheric CO₂, and they are expected to migrate northward and be resistant to herbicides.
6. Invasion by exotic grass species into arid lands will increase, causing higher fire frequency, which in turn will affect rivers and riparian systems in these areas.
7. Horticultural crops (tomato, onion, fruit) are more sensitive to climate change than grains and oilseed crops.
8. The length of the growing season has increased by 10 to 14 days over the last 19 years across the temperate latitudes, and species distributional ranges have shifted as a result.
9. Arctic snow and ice covers have dramatically declined because of warming, and the resulting habitat loss is threatening the wildlife, such as the polar bear, that depend on ice.

What is Ecology?

The word ecology derives from the Greek word oikos, meaning “home”. In 1870 German zoologist Ernst Haeckel defined ecology as follows:

By ecology, we mean the body of knowledge concerning the economy of nature - the investigation of the total relations of the animal both to its organic and to its inorganic environment; including above all, its friendly and inimical relation with those animals and plants with which it comes directly and indirectly into contact - in a word, ecology is the study of all the complex interrelationships referred to by Darwin as the conditions of the struggle for existence.

This definition still holds today - it is the study of interactions of all organisms (animals including humans, plants bacteria, fungi) with one another and with their environments. In short, ecology is the science of the living and breathing nature.

For example, when a shorebird catches a fish from the water, to an ecologist this event represents the sum-total interactions, up to that instant of time, of the bird and the fish with each other, and with all other biological species within and above the water, as well as with the surrounding environment.

Because ecology includes understanding the relationships between living beings and their environments, predicting the impacts of environmental degradation (such as global warming and habitat loss) on a biological species (such as the polar bear), and on the overall biodiversity and ecosystems, is an important goal of ecological research.

Why should I care about Ecology?

In today’s context of the heightened awareness about global warming and other human abuses on the earth’s environment, ecology is arguably the most important, and also least appreciated, branch of modern science.

Ecology is important, because it is fundamental to what little we know, and so much that we do not know, about maintaining a sustainable world - a world that will not run out of its finite supply of resources (such as food) and energy (such as fossil fuel) that nurture the rapidly growing humanity.

Ecology is under-appreciated, because we take the nature around us so much for granted that we often forget we are an integral part of her, and what is good for Mother Nature is good for us. The march of technology has only deepened this identity crisis, giving us a sense of detachment from, and an unhealthy disregard for, the other biological species that share the world with us.

A healthy biodiversity is key to maintaining the delicate balance of the global ecosystems, which provide essential resources and services for our own survival. When environmental stress drives a species to near extinction, we not only may lose that species for ever, the ecosystem balance itself may be at risk.

In a sense, ecology is to a sustainable earth as physics is to the technology - the latter cannot exist without the former.

So, yes, it pays to care about nature, and about ecology. Maybe next time when we pop that pill in our mouth, we should pause a little before marveling at the technology for saving our lives. Because, first and foremost, it is the ingredients in the pill, which are extracted from nature, that are really saving us.

According to a recent report, increasingly acidified water along the Pacific Coast is posing a danger to the sealife, including microscopic plants and animals at the base of the food chain, to shellfish, corals and the young of some marine species.

This acidification results from the high concentration of dissolved CO₂ in ocean water, from the accumulation of dead organisms and also human activity. CO₂ reacts with water molecule (H₂O) to create carbonic acid (H₂CO₃), which turns the water acidic.

While the corrosive water is not known to be an immediate threat to humans, it can dissolve the shells of clams, oysters and other shellfish species, which may in turn alter marine food webs, resulting in unproductive and undesirable ecosystems.

WWF (World Wildlife Fund) began publishing its Living Planet Reports since 1998, to update us on the changing state of the earth’s biosphere because of human impact.

There are two major components in the latest report, released in 2006. The first one reflects the health of the planet’s ecosystems, which is measured by the living planet index. The second component shows the extent of human demand on these ecosystems, and is measured by the ecological footprint.

Here I summarize the results for the living planet index.

Living Planet Index (LPI)

LPI tracks over 3600 populations of 1313 vertebrate species - fish, birds, reptiles, amphibians and mammals - from all across the globe. Of them, 695 are terrestrial (living on land) species, 274 marine (ocean-dwelling), and 344 freshwater (living in rivers, lakes, streams and wetlands) species.

Accordingly, three separate indices are estimated, for terrestrial, marine and freshwater species, and the overall LPI is then calculated as the average of these three indices. The LPI shows a decline of about 30% over the 33-year period from 1970 to 2003 (see the picture).

Below is a summary of the three component indices.

Terrestrial Species. The terrestrial LPI has declined around 30% between 1970 and 2003. Among the species considered, populations of tropical species showed a 55% decline, whereas the temperate species held relatively steady.

This high rate of decline in tropics is because of the loss of natural habitat to cropland and pasture between 1950 and 1990. By contrast, in temperate zones the agricultural conversion of habitats was mostly over before 1950, and wild populations stabilized soon thereafter.

Marine Species. The marine LPI is divided by the Pacific Ocean, Arctic/Atlantic Ocean, Indian/Southeast Asian Ocean, and Southern Ocean (seas around Antarctic). The index shows over 25% decline across all four ocean basins.

Pacific and Arctic/Atlantic Oceans show relatively stable trends, where increasing populations of sea birds and some mammal species since 1970 mask a decline in many commercially important fish stocks, such as cod and tuna, because of overfishing, and also turtles and other species lost to bycatch.

By comparison, Indian/Southeast Asian and Southern Oceans show dramatic decline. This is because of the rapid loss of Mangrove habitats, which are saltwater-tolerant, inter-tidal forests along tropical shorelines, and provide food and shelter to 85% of the commercial fish species in the tropics.

Freshwater species. The freshwater LPI of 344 species (of which 287 live in temperate zones and 51 in tropics) declined an average 30% between 1970 and 2003. Freshwater bird populations stayed relatively stable, whereas other freshwater species declined by about 50% over this time.

Habitat loss, overfishing, invasive species, pollution, and disruption of river systems for water supply are among the main drivers of this decline. Damming of rivers for industrial and domestic use, irrigation and hydroelectric power have fragmented over half of the world’s river systems, thus affecting the productivity of wetlands, flood plains, and deltas, and disrupting fish migration.