[MLS] Heading into the 15th MLS season, Soccer America ranks each team at each position. In this installment, Soccer America's Ridge Mahoney explains why the Houston Dynamo has the toughest backline and which teams are the most vulnerable.

The defensive units are evaluated by performance and experience, along with depth and such factors as whether they can improve or might regress this season.

1. HOUSTON. Defender of the Year finalist Geoff Cameron is moving back into midfield, yet a solid unit of veterans – Chad Barrett, Bobby Boswell, Eddie Robinson, Richard Mulrooney -- and reliable backups remains. Houston is two deep at each position.

2. COLUMBUS. The Defender of the Year winner, Chad Marshall, anchors the toughest ‘D’ in the Eastern Conference as Andy Iro pushes for a starting spot. Can Gino Padula get into the attack more often this year from his left back spot?

3. LOS ANGELES. Rookie Omar Gonzalez and veteran Gregg Berhalter quickly formed a solid partnership in the middle, and young Brazilian Alex joins domestic products Todd Dunivant, Sean Franklin and A.J. DeLaGarza.

4. SEATTLE. One of last year’s top defensive units has capable replacements in Pat Ianni and Tyson Wahl along with linchpins Tyrone Marshall, Jhon Kennedy Hurtado and James Riley.

5. CHIVAS USA. Co-captain Jonathan Bornstein and Costa Rican newcomer Michael Umana head a strong starting unit, but building enough depth might take some time.

6. NEW ENGLAND. The loss of veteran Jay Heaps is mitigated by the arrival of veteran Cory Gibbs and an additional year of experience for Emmanuel Osei, Darrius Barnes and Kevin Allston.

7. REAL SALT LAKE. Central defenders Jamison Olave and Nat Borchers anchor a good group that includes outside back Chris Wingert, and which might see Tony Beltran strengthen his case for a starting spot.

8. FC DALLAS. Heath Pearce and Kyle Davies arrived in mid-season last year to beef up this defensive corps; Jair Benitez is already one of the league’s better outside backs, and veteran Ugo Ihemelu can handle the center.

9. D.C UNITED. The pieces seem to be in place; how soon will Coach Curt Onalfo find the right formula? A fully fit Bryan Namoff would help, and Dejan Jakovic has to prove his mettle in the middle.

10. KANSAS CITY. If Pablo Escobar adjusts to MLS and playing alongside Jimmy Conrad, Coach Peter Vermes should have enough options to figure out the rest.

11. CHICAGO. Veterans C.J. Brown, Tim Ward and Wilman Conde have been joined by Julio Martinez and Krzysztof Krol. Salvadoran international Deris Umanzor joined the team for preseason but isn’t yet signed. He could fill a big hole left by the departure of Gonzalo Segares.

12. SAN JOSE. If Ramiro Corrales makes the grade at left back, the Quakes should improve greatly on the 50 goals they gave up last season. Right back Chris Leitch and central defender Bobby Burling need more help than they got in 2009.

13. PHILADELPHIA. Talent is present in Danny Califf, Michael Orozco and Shavar Thomas, and there’s upside to Jordan Harvey. How the unit will function and where the depth will come from are unknowns.

14. NEW YORK. Costa Rican veteran Roy Miller is an upgrade, but injuries affected several players in preseason and head coach Hans Backe is new to this whole MLS thing. Carlos Mendes and Mike Petke are what they are.

15. TORONTO. The defense looked anything but solid in preseason as trialists shuffled in and out. Veterans Jim Brennan, if he plays in the back, Nick Garcia, and Marvell Wynne give a core of experience, at least.

16. COLORADO. Oscar Murillo must adjust to MLS, Julian Baudet didn’t look secure very often last year, and another newcomer, Danny Earls, is getting first nod at left back. Right back Kosuke Kimura isn’t a star but he’s at least a known quantity.