There's no such thing as a free lunch

It is an old adage, but one scientists seem to never learn. If you ramp up biological productivity, by breeding cows to be mere milk-producing machines, by using huge quantities of fertilisers, or by dumping shit in fish ponds, you might get more output, but you'll also get environmental and other side-effects. The latest, quite possibly the spread of bird flu:

Bird flu may be spread by using chicken dung as food in fish farms, a practice now routine in Asia, according to the world's leading bird conservation organisation.
Fertilising fish ponds with poultry faeces, which can dramatically improve fish growth, may set up major new reservoirs of avian influenza infection if the chickens providing the manure are infected themselves, according to BirdLife International, the Cambridge-based umbrella body for bird protection groups in 100 countries.
The suggestion, which has echoes of the BSE outbreak in Britain - when cattle were infected by their food - puts a question mark over a technique firmly backed by the UN's Food and Agriculture Organisation (FAO) as a primary means of providing protein for mushrooming populations in developing countries.
Known as integrated livestock-fish farming, the technique involves transferring the wastes from raising pigs, ducks or chickens directly to fish farms. At the right dosage, the nutrients in the manure give an enormous boost to the growth of plankton in the ponds, which are the main food of fish such as carp and tilapia.

(I've quoted quite a bit of that because it will disappear behind a paywall in a few days.)

But in an early bid for an IgNobel prize, a scientist has calculated Earth could hold a population of 1.3 million billion. That is on the basis of their heat production alone not overheating the planet. Which means?

Assuming that every person emits 120 watts of heat and that it would be uncomfortable if the average temperature at the Earth's surface rose too much, the researchers declared the Earth could sustain 1.3 million billion people without overheating.
Writing in the journal, the researchers acknowledge the Earth's resources could be put under severe strain long before the theoretical population peak is reached. "Constraints like food availability or physiological necessities may become critical in the relatively near future. But they are subjected to a continuous change as a result of the development of human civilisation and technology," Dr Badescu said.

It seems some parts of science have still to recognise the dangers of their own arrogance.