Here's the long and the short of it. Your original message was that it could not find the ECOSCode field. I've run a test to go to the hyperlink using the same dataset for ECOSCode that your provided both by button clicking and clicking on an image (I used something local to my machine) and your code worked
Code:
txtURL = "http://ecos.fws.gov/speciesProfile/profile/speciesProfile.action?spcode=" & Me.ECOScode
DoCmd.Hourglass True
FollowHyperlink txtURL
DoCmd.Hourglass False
So, the problem is that on your form (you said report, but I can only imagine this is a form) does not contain a field NAMED ecoscode, the CONTROL SOURCE may be but it's named something other than ECOSCODE
rhewitt.zip
Here's my example. If the picture is not coming up for you just put in a local jpg.