To our readers: Mike Evans' 2016 NFL Catch of the Year and the big Bucs' offseason catches — DeSean Jackson, O.J. Howard and Chris Godwin — inspired us to mark the start of the 2017 season with a series of stories celebrating "The Catch." We hope you enjoy them.This offseason, the Buccaneers signed DeSean Jackson, picked up an option on Mike Evans' contract and drafted O.J. Howard and Chris Godwin.In theory, they should field one of the NFL's most potent offenses for years to come.And, oh, by the way, they'll have a defense, too, probably.About that defense … it'll be a good one, right?It depends on how you cut the numbers.It allowed 137 explosive plays last season, second worst to only San Francisco. No team allowed more pass plays of 40 yards. Yikes.Buuuuuuut the defense did get better as the season wore on. After allowing the fourth most points in the first half, it allowed the fourth fewest in the second half.Plus, the Bucs retained the man who helped engineer that turnaround, coordinator Mike Smith. So maybe that ugly first half is ancient history, the cost of implementing a new system."When you're installing a new system, the first year is probably the toughest simply because it's new to the guys," Smith said. "You hope that you make some progress through Year 1, which I feel like we did in the second half of the season. In Year 2, you should be able to expand your inventory."Continuity matters, and the data proves it.When Smith was hired as the Jaguars defensive coordinator in 2003, he transformed Jacksonville from a bottom 10 unit to a top 10 unit, according to Football Outsiders' efficiency ratings. The improvement continued into 2004, albeit modestly. The Jaguars got stronger again in Smith's third season then peaked in his fourth.The Falcons defense followed a similar trajectory when Smith became the head coach in 2008.While there's a reasonable chance the Bucs defense will be better this season, such an improvement might look more like a step rather than a leap. There are signs it's due for some regression.Here are the red flags worth monitoring:Third downsTampa Bay held opponents last season to a third-down conversion rate of 34.4 percent, an NFL best. Of course that achievement would inspire optimism — that's a nearly 12-point decrease over 2015.That improvement isn't entirely random. Third downs were a point of emphasis for the Bucs, Smith said."Third down is like a turnover — that's what we try to preach," he said. "We know that in most situations if we win on third down, we're giving the ball back to the offense."It's unlikely that Tampa Bay will repeat its success. Since 2000, no team has led the NFL in third-down defense in back-to-back seasons. The top third-down defenses regressed by an average of 7 percentage points. Only the 2004-05 Bears and 2011-12 Cardinals came close.Top third-down defenses, since 2000SeasonTeamThird down %Next seasonDifference2016Bucs34.4--2015Texans28.538.5102014Bills188.8.131.52013Lions30.337.26.92012Broncos30.638.17.52011Cardinals31.432.91.52010Giants31.738.26.52009Jets31.5375.52008Steelers31.442.310.92007Chiefs31.347.416.12006Ravens28.836.67.82005Falcons30.24211.82004Bears30.531.91.42003Titans27.733.35.62002Eagles3135.34.32001Giants30.336.56.22000Titans30.834.94.1One could assume these teams suddenly got worse on third down. But we risk overlooking one simple fact: Third-down conversion rates fluctuate because of a smaller sample size — teams face fewer third downs.To better assess a defense, we also must consider its performance on earlier downs. Defenses that perform well on first and second down but poorly on third down tend to improve the next season, Football Outsiders research shows. The inverse is also true: Defenses that perform poorly on first and second but well on third down tend to decline.So how did the Bucs do on early downs last season? On first down and second downs combined, they allowed 5.9 yards per play, fifth most in the NFL. Opponents gained another first down three out of every 10 plays. Only the Saints and Colts allowed first downs at a higher rate."I think our guys had a very good understanding on third down of what we were defending," Smith said. "We didn't have the comfort within the system with the calls on first and second down to really understand the nuances of the defense that would make it be a more efficient defense."• • •TakeawaysTampa Bay's second-half turnaround had little to do with yardage allowed. Credit a league-high 18 turnovers down the stretch. Safety Keith Tandy led the way with four interceptions, and cornerback Brent Grimes had three, returning one for a touchdown.Overall, the Bucs forced a turnover on 1.7 percent of their plays, the fourth-best rate. As encouraging as that might be, it's not sustainable. Like third-down conversion rates, takeaway rates fluctuate. The Panthers had the NFL's best rate in 2015, while the Ravens had the worst. Last season, Baltimore surged to second and Carolina slid to ninth.Best turnover rates, 2015Team2015 TO%2016 TO%Increase/decline1. Panthers2.31.6-0.72. Chiefs2.11.7-0.43. Bengals2.01.6-0.44. Cardinals1.91.4-0.55. Jets1.81.6-0.2Worst turnover rates, 2015Team2015 TO%2016 TO%Increase/decline32. Ravens0.61.8+1.231. Cowboys0.80.9+0.130. Bears0.80.80.029. 49ers0.81.1+0.328. Jaguars0.80.7-0.1• • •AgeThere isn't much to glean from preseason games, but if Tampa Bay's lack of depth at defensive end and cornerback doesn't trouble you by now, it should.The Bucs will again count on Robert Ayers, who turns 32 this week, and Brent Grimes, 34, to play key roles. How much longer can they defy time?Ayers' 61/2 sacks last season tied Gerald McCoy for the team lead. Ayers played in only 12 games and hasn't played in more than 12 since 2013.Grimes is projected to be the NFL's oldest starting cornerback. He shined in his return to Smith's system (he played for him in Atlanta from 2008 through 2012) and became the first cornerback age 33 or older to start 16 games, deflect at least 20 passes and intercept at least four passes.Contact Thomas Bassinger at [email protected] Follow @tometrics.