What are some credible sources opposing the idea that global warming is affecting the ecosystem?
Before answering this question, it should be understood that anthropogenic global warming (the theory most commonly referred to as simply "global warming") is a contentious issue. Most scientists and scientific organizations agree that global temperatures are rising, but not all of them agree on the causes or on the eventual effects. Because of the intensive politicization of the issue, pro- and anti-AGW advocates tend to dismiss all evidence presented by the opposition as data cooked for political gain. Be wary when trying to argue one side or the other.
With that said, there are several reasonably credible sources that argue against the idea of global warming as a significant threat to the planet and humanity. During the height of the debates, during the 2000s, it was fashionable to gather groups of scientists to sign documents for one side or another; there have been several such documents signed by credible scientists advocating caution in claiming incredible planetary changes caused by AGW. One letter to NASA boasted the signatures of 50 former NASA scientists and astronauts. A commonly-cited petition involved over 1,000 scientists or science industry workers, although it is important to note that not all of those scientists are studied in climate science.
Essentially, the argument from authority is difficult to promote here. Just because a lot of people don't believe something doesn't make it untrue, with the reverse also applying in this case. One major dispute arose over released emails between AGW scientists and science centers, showing evidence of deliberately cooked data and fraudulent claims. However, while this certainly shows a dishonest side to AGW proponents, it doesn't by itself disprove AGW.
AGW has not been definitively proved as true or untrue just yet. Data from recent years shows a decline in major hurricane strikes, despite the constant refrain that AGW will cause disaster-movie style weather. Exceedingly cold winters have been cited as proof on both sides. Everyone who discusses AGW will have a personal theory and selective evidence to back it up. Most recently, reports in both Forbes and the Wall Street Journal claimed that the Earth is actually cooling.
A final word of warning; dismissing any evidence or report based solely on perceived political leanings is intellectually dishonest. All evidence should be examined on the basis of its quality and claims alone.