For a bird, to fly hundreds or thousands of miles each spring and fall is a difficult and dangerous journey, one that not all birds can survive. 【小题1】 There is more than one single reason for different birds to migrate(迁徙), but it all comes down to survival.

Birds are migrating for a meal.

For all birds, one of the main driving forces behind migration is food. If all birds were to stay in the same places year-round, food there would become not enough. 【小题2】 But as the food supplies decrease in the fall, they return to the southern areas.

【小题3】

Over millions of years, birds have developed different migration patterns, times and destinations. This helps birds take advantage of a wide variety of suitable conditions to raise their young, which increases the survival chances of the young birds.

Birds are migrating because of climate changes.

Changes in climate can affect migration. Many birds leave the Arctic for example, when temperatures begin to fall. They need warmer habitats. Similarly, the hottest regions can be an unpleasant environment for raising young birds. 【小题4】

Birds are migrating because of predators.

Habitats that have rich food sources also attract a greater number of predators who may kill birds. 【小题5】 Many birds even migrate to specialized habitats that are nearly inaccessible to predators, such as coastal cliffs or rocky offshore islands.

An absolute description of the threat hanging over the world's mammals,reptiles,amphibians and other life forms has been published by the well-known scientific journal,Nature.A special analysis carried out by the journal indicates that an astonishing 41% of all amphibians on the planet now face extinction while 26% of mammal species and 13% of birds are similarly threatened.

Many species are already critically endangered and close to extinction,including the Sumatran elephant,Amur leopard and mountain gorilla.But also in danger of vanishing（消失）for the wild,it now appears,are animals that are currently rated as merely being endangered.

In each case,the finger of blame points directly at human activities.The continuing spread of agriculture is destroying millions of hectares of wild habitats every year,leaving animals without homes,while the introduction of newly-come species,often helped by humans,is also damaging native populations.At the same time,pollution and overfishing are destroying ocean ecosystems.

"Habitat destruction,pollution or overfishing either kill off wild creatures and plants or leaves them badly weakened," said Derek Tittensor,an ocean ecologist at the World Conservation Monitoring Centre in Cambridge. "The trouble is that in coming decades,the additional threat of worsening climate change will become more and more common and could then kill off these survivors."

The problem,according to Nature,is worsened because of the huge gaps in scientists knowledge about the planet's biodiversity.Evaluations of the total number of species of animals and plants alive vary from 2 million to 50 million.In addition,evaluations of current rates of species' disappearances vary from 500 to 36,000 a year. "That is the real problem we face," added Tittensor. "The scale of uncertainty is huge."

In the end,however,the data indicate that the world is heading cruelly towards a mass extinction-which is defined as one involving a loss of 75% of species or more.This could arrive in less than a hundred years or could take a thousand,depending on extinction rates.

It’s hard to turn down a tasty banana. They taste good and you don’t have to wash them before eating them. However, British scientists say that the fruit may disappear by 2050.

One reason for this is climate change. Scientists at the University of Exeter collected data from 27 countries and regions that produce 86 percent of the world’s bananas. They found that climate change has improved growing conditions in 21 of these countries. In the past 60 years the average yield has reached 1.37 tons of bananas per hectare.

However, if temperatures keep rising, this advantage will disappear. Ten of the countries, including India, the world’s biggest producer of bananas, will produce fewer bananas. By 2050, the fruit may die out.

Shouldn’t bananas grow more easily if it’s hotter outside? In fact, the perfect temperature range for growing bananas is between 24℃ and 32℃. If temperatures get too high, they will stop growing.

Another reason for this phenomenon is diseases. Unlike other crops, the bananas we grow come from shoot cuttings rather than seeds. This means that all banana plants have the same genotype. In other words, if a disease is able to kill one plants it could kill them all.

One serious disease is called Panama. Caused by fungus（真菌）in the soil, it spreads easily. It has spread across South Asia, Africa, the Middle East, and Australia. If one banana plantation suffers from the disease, it will take 30 years until it is able to grow bananas again. Scientists have not found a cure for this disease.