Error messages

I keep getting these 2 error messages. I tried to load a new template but that was not successful. So, I used another template. But I am still getting these two error messages:
Warning: include_once(/home/backtoba/public_html/wp-content/plugins/nature/) [function.include-once]: failed to open stream: Success in /home/backtoba/public_html/wp-settings.php on line 473

Warning: include_once() [function.include]: Failed opening ‘/home/backtoba/public_html/wp-content/plugins/nature/’ for inclusion (include_path=’.:/usr/local/php52/pear’) in /home/backtoba/public_html/wp-settings.php on line 473

How can I resolve them??? Thank you!