On the heels of Kansas City’s improbable postseason run in 2014 and now successful start to 2015, baseball has been further exposed to the importance of team defense. With an electrifying outfield headlined by 2014 Fielding Bible Award winners Lorenzo Cain and Alex Gordon, the Royals have owned the American League’s best defense since the beginning of 2014 by BIS’s Defensive Runs Saved—and they’re winning ballgames because of it.

After saving 34 runs in 2014—third-best in the AL—Kansas City has managed to be even better defensively in 2015. Their 37 Defensive Runs Saved (DRS) through 54 games puts them on a ridiculous pace of 111 across a full season, which would top the DRS record of 97 set by the 2005 Philadelphia Phillies (DRS has been tracked back to 2003).

The Royals’ gain represents one of baseball’s best defensive improvements this season:

* includes BIS’s Shift Runs Saved

However, the very best improvement belongs to one of KC’s division rivals, the Detroit Tigers. There have been a few key factors in Detroit’s defensive turnaround; first and foremost, Nick Castellanos has resembled a competent third baseman. A year removed from being the league’s worst defender by costing the Tigers a whopping 30 runs, Castellanos has actually been a tick above average in 2015. After losing the entire 2014 season to injury, the return of slick-fielding Jose Iglesias at shortstop will surely help in the long run—and it doesn’t hurt that he’s contributing at the plate as well. Furthermore, the additions of Yoenis Cespedes and Anthony Gose have shored up the team’s outfield defense.

The Blue Jays’ two major offseason splashes are paying dividends in 2015, both at the plate and in the field. Josh Donaldson has been a consistent run-saver at the hot corner in his career, while Russell Martin brought his excellent defensive reputation to his home country. In addition, Kevin Pillar has already made a number of highlight-reel grabs in 2015, saving the Jays a total of 13 runs between left and center field.

The Rays’ improved defense has helped keep them competitive in the AL East, as they currently sit in second place, 1.5 games back of the New York Yankees. One of their biggest defensive improvements has actually come on the mound; their pitchers cost the club 13 runs on defense last season but have saved them three in 2015. While they’ve been average at shortstop, shipping Yunel Escobar (and his -23 DRS in 2014) to Washington this past winter has improved their defense at the position. Meanwhile, Kevin Kiermaier’s 11 Runs Saved ties him for second-most among outfielders this season. Tampa Bay has also saved eight runs by shifting alone, third-best in the American League behind Houston’s 13 and Baltimore’s 10.

The aforementioned Royals are looking even better in the field and their outfielders are showing off once again. However, improvements in the infield may be the primary reason for their 2015 defensive success. Last year, Kansas City’s infielders were all below average in converting batted balls into outs; this year, it’s a completely different story:

The biggest swing has come at third base, where Mike Moustakas is finally reaching the potential he showed in 2012 when he saved 14 runs. In all, the Royals have been average or better at eight of the nine positions. The lone exception is on the pitcher’s mound, where their staff has cost them one run defensively.

Although the first two months of the Marlins’ season proved to be a disappointment, they did have one thing going for them: better defense. Second base was a major liability for Miami in 2014; that was corrected, however, with the addition of Dee Gordon. Despite being a below-average defender in his time with Los Angeles, Gordon has been the best defensive second baseman by DRS in 2015. With the help of respected infield instructor Perry Hill, both Gordon and shortstop Adeiny Hechavarria have been more focused on positioning this season and are seeing positive results in the process.

While those five teams have seen the largest defensive gains in 2015, these five have experienced the most drastic declines:

Two of the most significant drops belong to teams who added offense at the expense of defense over the winter. The Padres’ outfield defense, as expected, has been awful, already costing the team a total of 19 runs. A recent Stat of the Week article wondered if the Friars’ outfielders were hitting enough to offset their terrible defense.

Meanwhile, Boston added the powerful bats of Hanley Ramirez and Pablo Sandoval while forfeiting defense at their respective positions. The Ramirez experiment in left field has been a costly one, with his -12 DRS proving to be the worst in the big leagues to this point. Sandoval, meanwhile, has contributed to the team’s -8 total DRS at third base.

The Phillies continue to play poor baseball; on pace for -128 Runs Saved, they could challenge the record-low of -120 set by the 2005 Yankees.

Defense is clearly an important piece of the puzzle, and it could dictate a few important divisional races over the final four months. In the AL Central, the Royals have used it to their advantage in the past, but the Tigers are now turning the corner defensively. In the tight AL East, the Blue Jays and Rays are seeing great defensive improvements, while the Red Sox are trending in the wrong direction. Each team is looking for wins at the margins, and making plays on the defensive side of the ball can be a key difference maker.