That depends on their claims in question. For example, should any evolutionary biologist believe that the vertebrate eye is so "complex" that it couldn't of formed through nature? If it is so "complex", then why do we have blind spots? The blood vessels and nerves are in front of the photoreceptors which screens some of the incoming light. What about our genome? How is it complex when much of our DNA has no purpose at all, less then 2% of our DNA codes for proteins. Transposons are hazardous DNA sequences with no known function, however, if they insert themselves into functional genes, it may become damaged. Transposons are known for causing disease in humans and mice.
It is biological evidence like this that cast doubt on a creator and especially "intelligent design". In science, you take all the facts together and try to make a theory. To say earths biodiversity is to complex to have evolved isn't a very good theory. A better explanation is that life on earth evolved from a common ancestor, gradually evolving novel characteristics with different functions and design. Some of these characters are selected, because they give an advantage to an organism. Other traits appear in future generations by chance alone, this is genetic drift, another mechanism of modern evolutionary synthesis.
