title: why american surrogates are in demand for chinese families
text: world wildlife falls by  in  years  
  bbc news global wildlife populations have fallen by  since  a report says 
  the living planet assessment by the zoological society of london zsl and wwf suggests that if the trend continues that decline could reach twothirds among vertebrates by  
  the figures suggest that animals living in lakes rivers and wetlands are suffering the biggest losses 
  human activity including habitat loss wildlife trade pollution and climate change contributed to the declines 
  dr mike barrett head of science and policy at wwf said its pretty clear under business as usual we will see continued declines in these wildlife populations but i think now weve reached a point where there isnt really any excuse to let this carry on 
  we know what the causes are and we know the scale of the impact that humans are having on nature and on wildlife populations  it really is now down to us to act 
  however the methodology of the report has been criticised image copyright carlos drews  wwf image caption the report looked at data collected on  species of vertebrates over the last  years 
  the living planet report is published every two years and aims to provide an assessment of the state of the worlds wildlife 
  for freshwater species alone the decline stands at  since  dr mike barrett wwf 
  this analysis looked at  different species of birds fish mammals amphibians and reptiles  about  of the total number of vertebrate species in the world 
  the team collected data from peerreviewed studies government statistics and surveys collated by conservation groups and ngos 
  any species with population data going back to  with two or more time points to show trends was included in the study 
  the researchers then analysed how the population sizes had changed over time 
  some of this information was weighted to take into account the groups of animals that had a great deal of data there are many records on arctic and near arctic birds for example or very little data tropical amphibians for example the report authors said this was to make sure a surplus of information about declines in some animals did not skew the overall picture 
  the last report published in  estimated that the worlds wildlife populations had halved over the last  years 
  this assessment suggests that the trend has continued since  populations have declined by an average of  
  dr barrett said some groups of animals had fared worse than others 
  we do see particularly strong declines in the freshwater environment  for freshwater species alone the decline stands at  since  this is related to the way water is used and taken out of fresh water systems and also the fragmentation of freshwater systems through dam building for example 
  it also highlighted other species such as african elephants  which have suffered huge declines in recent years with the increase in poaching and sharks which are threatened by overfishing 
  if pressures  overexploitation illegal wildlife trade for example  increase or worsen then that trend may be worse dr robin freeman zsl 
  the researchers conclude that vertebrate populations are declining by an average of  each year and warn that if nothing is done wildlife populations could fall by  below  levels by the end of the decade 
  dr robin freeman head of zsls indicators  assessments unit said but thats assuming things continue as we expect if pressures  overexploitation illegal wildlife trade for example  increase or worsen then that trend may be worse 
  but one of the things i think is most important about these stats these trends are declines in the number of animals in wildlife populations  they are not extinctions by and large they are not vanishing and that presents us with an opportunity to do something about it image copyright scott dickerson image caption there are still many gaps in our knowledge of the worlds vertebrates 
  however living planet reports have drawn some criticisms 
  there are some numbers in the report that are sensible but there are some numbers that are very very sketchy stuart pimm duke university 
  stuart pimm professor of conservation ecology at duke university in the united states said that while wildlife was in decline there were too many gaps in the data to boil population loss down to a single figure 
  there are some numbers in the report that are sensible but there are some numbers that are very very sketchy he told bbc news 
  for example if you look at where the data comes from not surprisingly it is massively skewed towards western europe 
  when you go elsewhere not only do the data become far fewer but in practice they become much much sketchier there is almost nothing from south america from tropical africa there is not much from the tropics period any time you are trying to mix stuff like that it is is very very hard to know what the numbers mean 
  theyre trying to pull this stuff in a blender and spew out a single number its flawed 
  but dr freeman said the team had taken the best data possible from around the world 
  its completely true that in some regions and in some groups like tropical amphibians for example we do have a lack of data but thats because there is a lack of data 
  were confident that the method we are using is the best method to present an overall estimate of population decline 
  its entirely possible that species that arent being monitored as effectively may be doing much worse  but id be very surprised if they were doing much better than we observed