Distributed, dynamic terrorist networks cannot scale like hierarchical networks. The same network design that makes them resiliant against attack puts absolute limits on their size. If so, what are those limits?
A good starting point is to look at limits to group size within peaceful online communities on which we have extensive data -- terrorist networks are essentially geographically dispersed online communities. Chris Allen does a good job analyzing optimal group size with his critique of the Dunbar number.
His analysis (replete with examples) shows that there is a gradual fall-off in effectiveness at 80 members, with an absolute fall-off at 150 members. The initial fall-off occurs, according to Chris, due to an increasing amount of effort spent on "grooming" the group to maintain cohesion. The absolute fall-off at 150 members occurs when grooming fails to stem dissatisfaction and dissension, which causes the group to cleave apart into smaller subgroups (that may remain affiliated).
Al Qaeda may have been able to grow much larger than this when it ran physical training camps in Afghanistan. Physical proximity allowed al Qaeda to operate as a hierarchy along military lines, complete with middle management (or at least a mix of a hierarchy in Afghanistan and a distributed network outside of Afghanistan). Once those camps were broken apart, the factors listed above were likely to have caused the fragmentation we see today (lots of references to this in the news).
This leads us to optimal group size, which according to Chris Allen's online group analysis, can be seen at two levels: both small and medium sized. Small, viable (in that they can be effective at tasks) groups (or cells) are optimized at 7-8 members. A lower boundary can be seen at 5 (with groups less than 5 not having sufficient resources to be effective) and an upper boundary at 9. Medium sized groups are optimal at 45-50 members, with a lower limit of 25 and an upper limit of 80. Between these levels is a chasm that must be surmounted with significant peril to the group. This is due to the need for groups above 9-10 members to have some level of specialization by function. This specialization requires too much management oversight to be effective given the limited number of participants in each function. At 25 members, the group gains positive returns on specialization given the management effort applied (a break-even point).
This chasm (between 9-25 members) nicely matches the problem period in the development of terrorist and guerrilla networks that studies of guerrilla groups refer to. The amount of damage a small (7-8 member) group can do is limited to narrow geographies and therefore does not represent a major threat. Once a network grows to 45-50 members, they can mount large attacks across multiple geographies. They are also very difficult to eliminate due to geographically dispersion of cells. However, during the transition to a larger group they are vulnerable to disruption. This vulnerability necessitates fast counter-terrorist action (this gives credibility to the military strategists who claim we didn't have enough troops in Iraq immediately after the war, nor were we quick enough to establish martial law) during that short period of time a network is transitioning in size.
This size dynamic can also be seen in criminal organizations. The mafia (BBC), despite their widespread influence, has closely mirrored the limits on group size:
- The Genoveses are the largest of the five families in New York and they recruited nine new foot soldiers, bring their total to 152.
- The Gambinos, had a terrible year from 2000-2001, losing 33 members, but they still managed to retain 130, making them the second largest in terms of manpower.
- Meanwhile the Luccheses have initiated three more gangsters, lifting them to third place with a total of 113 hoods on the streets, according to FBI reports.
A recent Washington Post article on Islamic terrorist cells in Iraq says:
Dempsey said he estimated there were only about 100 "foreign terrorists" in Baghdad, organized into about six cells. In Anbar province, which stretches across western Iraq and includes the strife-torn cities of Ramadi and Fallujah, Maj. Gen. Charles H. Swannack Jr. of the 82nd Airborne Division said he believed there were a total of 50 to 80 foreign fighters in eight to 10 cells.
This indicates a cell size (the optimal size of the smallest viable network) of between 5-12 members.
Note: The limits on organizational size does not mean that terrorist or crime organizations can't expand their ranks on a temporary basis. There are plenty of "contract" employees available. Also, there is also the potential for intergroup cooperation (we see this in both crime and terrorism).