Whitney Holmes was the program coordinator at the Depression & Bipolar Support Alliance National Headquarters in Chicago when a local university student committed to volunteering with the non-profit for three months. Holmes appreciated the help: She spent a morning training the new volunteer, bought her lunch, assigned her tasks, and gave her feedback throughout the afternoon. But the next time the new volunteer came to the office, she announced that she had accepted a job and wouldn’t be returning.

Holmes, who also works with volunteers now as the executive director of Switchback Books, was discouraged but not surprised. She has regularly seen volunteers drop out even though significant time was spent recruiting and training them.

“I essentially lost a full day’s work. Even though she came to help, she ended up hurting the organization by diverting our resources,” says Holmes. “But for her, there were no repercussions to quitting.”

Few non-profits are immune to the negative effects of unreliable volunteers. The problem is well documented: The Corporation for National and Community Service reports that one out of three volunteers don’t return the next year. That costs both time and money.

Leaders of non-profits are always looking for ways to mitigate their losses: The 2014 State of the Nonprofit Sector Survey reveals pinched budgets across the third sector. One method is to replace traditional volunteers with interns, whose obligation to follow through with their work to receive college credits or resume references can make them more reliable.

“By using interns instead of traditional volunteers, non-profits can attract higher talent and better accomplish their goals,” says Nathan Parcells, Vice President of Consumer Marketing at InternMatch. The website matches students with internships at both non-profit and for-profit organizations, but Parcells originally conceived of the site to address the issue that volunteers doing less skilled work quit at a higher rate.

“It’s a ‘catch-22’ situation,” says Parcells, who saw non-profits shifting toward hiring more interns. “Non-profits would love to have volunteers doing higher-level tasks, such as website design and marketing collateral. However, volunteers drop out at such a high rate that they can’t be entrusted with those duties.” Instead, they are given menial, low-level tasks that don’t engage them—which makes them less likely to return.

When volunteers sign up to help, but fail to follow through on their end of the non-contractual bargain—it's the nonprofit that absorbs the loss. Interns, on the other hand, are more likely to face repercussions if they drop out. Students often have an academic advisor or internship coordinator at their university to enforce their internship commitments.

Interns are also interested in building skills and good references in preparation for their career. InternMatch’s 2014 State of the Internship Report surveyed more than 9,000 students and found that nearly 20 percent are most interested in working for a non-profit. “That represents a great recruiting opportunity for non-profits with strong intern programs,” says Parcells.

The study also shows student demand for internships outpacing availability. Completing an internship has been shown to give graduates a valuable edge in the job market, one that students are eager to take advantage of: More than 93 percent of those surveyed are seeking some form of internship this year. Consider that against national volunteer rates, which are at a 10-year low, and it’s easy to see why so many non-profits are replacing volunteers with interns.

But creating an internship program comes with its own set of challenges. “Shifting volunteer opportunities to internships requires a different mentality,” says Parcells. “It takes time and commitment. It’s not a switch you can flip on.” Besides having to carve out a role with dedicated hours and responsibilities, organizations must be versed on legal considerations. Non-profits are exempt from U.S. Department of Labor guidelines that might otherwise require them to pay interns minimum wage—they can pay a stipend, or nothing at all. Non-profits are the top source of unpaid internships, though the practice has been criticized for putting financial strain on students and promoting inequality by only giving opportunities to those who can afford to work without pay.

Regardless of whether an internship is paid, limiting opportunities to interns narrows the pool of candidates who can contribute to an organization’s cause. Volunteers can come from a range of ages and backgrounds, but the majority of interns are young college students.

“It’s unfortunate to think that other people who aren’t students or don’t have the hours during the day to commit to an internship can’t contribute,” says Holmes. Go on websites from New York’s Henry Street Settlement to San Francisco’s Playwrights Foundation, and you’ll find that the only open opportunities listed for volunteers are labeled as internships with six- to 15-hour minimum weekly commitments.

Using interns to replace unreliable volunteers isn’t an option for every organization, whether it's due to their small size or the nature of the work. For those non-profits, the challenge of poor volunteer retention is countered by providing opportunities that volunteers will find fulfilling and meaningful.

Shaharris Beh is the founder of HackerNest, a three-year-old nonprofit that organizes global tech communities to build tools that alleviate poverty. With only two full-time staff, the organization has learned some hard lessons about managing volunteers.

“Many volunteers come in with the attitude of ‘if I don’t do it, I’m not hurting anyone,’ but that’s not true,” says Beh. “Because of volunteers failing to do their work, we have lost sponsors. We have lost venues. Trusting volunteers is critical for your work but can also shoot you in the foot.”

Beh has learned to approach potential volunteers in a more circumspect way. “We’ve become wary of people who want to help. I want to make sure this is win-win and isn’t charity for them. We’re not interested in taking help from someone who we can’t help back, whether that’s resume references, fulfilling volunteer hour requirements or just socializing.” While this may limit potential volunteers, it means those who sign up are more invested.

Bob Holdsworth has also learned that keeping volunteers starts with knowing what they hope to get out of the experience. Holdsworth is a volunteer paramedic who consults for ambulance services and fire departments. The training alone takes a year and costs departments as much as $4,000 per volunteer. He has seen volunteers make it completely through training, only to drop out before ever making it onto the field.

“Every volunteer has a reason for wanting to be there, whether it’s social, a personal connection to the cause, or a patriotic desire to give back to the community,” says Holdsworth, who says committed volunteers are a vanishing breed. He recommends buffering recruitment by assigning new volunteers a mentor, and giving them easy tasks before making larger commitments. According to Holdsworth, putting extra effort into instilling a sense of involvement and accomplishment at the beginning can have a big payoff.

But sometimes even the best recruiting and training strategies can’t compensate for unproductive volunteers. Both Beh and Holdsworth have had to “fire” volunteers who were draining resources from their organizations. Beh says cutting ties with a volunteer is “the worst feeling in the world,” but a necessary part of process. Other organizations have even shifted the liability to volunteers—for instance, some organizations require volunteers to pay a fee for any hours they don’t end up working.

Holdsworth stresses the importance of setting firm expectations early, and holding everyone in an organization to the same rules whether they are paid staff, interning students, or volunteers who consider themselves free labor.

“There’s a saying I learned from a friend and often use,” says Holdsworth. “You volunteered to join. The rest is mandatory.”