Real-world experience is invaluable. It will help you make career decisions in the future (precisely what do you want to do in ecology? Do you like fieldwork, or office work? Do you want to work for the government, or an NGO? What geography are you most interested in?), and if you decide to go for a Master’s degree later, you’ll have those questions answered, or at least narrowed down.
Experience is also more qualifying for a job than an advanced degree. For many jobs you’ll have to have the MS too, but you’re not likely to get hired if you haven’t worked a relevant job beforehand. So to me, this seems like a great opportunity for you to get experience, which is admittedly challenging to do because you need some experience to get hired for that first job. If you take the placement year, you can always get the MS later, and the experience in your field will almost guarantee that you can get into a program. If you get your MS now, there is no guarantee that you will get hired for your first job in that field.
I’ve worked as a conservation ecologist for 5 years now, after having many term field positions and then getting an MS. I actually just held interviews for a permanent position with my organization a few weeks ago, and while many candidates with advanced degrees were competitive, we ended up hiring someone who only has a Bachelor’s degree but has worked in my field, in my area, for several years.
Of course no path is the same, and this is just my 2 cents, but if you have a chance to get a position, I’d take that opportunity. If you want to talk more about my experience and winding career, feel free to PM me! Best of luck!