Sentence:  This does not of course prevent people from acting strategically — deciding not to contribute in the hope that others will <WSD>take up the slack</WSD>. But it will be clear in these situations that such people are behaving unfairly and that prima facie at least, those who decide to <WSD>take up the slack</WSD> are doing more than justice requires. 
Meaning of the word marked with <WSD> tags:  To provide extra resources that are not met by normal sources.