1920 



1945 



Figure 58. Annual muskrat harvest from a 

 52,200-hd brackish Scirpus ojneyi marsh in 

 the Mississippi Delta (O'Neil 1949). 



kill much vegetation digging for the 

 preferred roots. In addition, their 

 house-building activity, underground runs, 

 and surface trails (Figure 59) destroy 

 much more marsh than is directly eaten. 

 For example, in a ID-ha brackish marsh 

 area that contained 24 active and 30 

 inactive houses in April 19S2, 31 new 

 houses were built and 10 "refurbished" 

 during the next year (Table 23). Sixty 

 percent of the active houses and 57 

 percent of the inactive ones simply 

 disappeared. 



When muskrat populations are dense, 

 all this activity can decimate a marsh, 

 creating large "eat-outs" especially in 

 the favored brackish marsh three-corner 

 grass (Sci rpus olneyi ) (Figure 60). 

 Subsequently the local popul ation, with no 



Figure 59. Ground plan of a typical 

 muskrat house with underground runways and 

 surface trails (barred lines) (Arthur 

 1931). 



food, crashes. If water levels are low 

 for a year or two to allow regrowth of the 

 vegetation, the marsh may recover (and the 

 muskrat population with it), but often the 

 damage extends so deeply into the marsh 

 that recovery is poor at best. Severe 

 storms may reset this cycle by destroying 

 nests and burrows and drowning the 

 predatory disease organisms they harbor. 

 The muskrat population often comes back 

 strongly after these storms (O'Neill 

 1949). 



It is interesting that "eat-outs" are 

 seldan found outside of brackish marshes 

 and are always attributed to muskrats, not 

 nutria (O'Neil 1949). The nutria has a 

 much longer gestation period (130 days 

 compared to 28 days for the muskrat) so 

 that its potential for response to 

 environmental change is much slower than 

 the muskrat' s. Consequently, its 



population is more stable. Muskrat 

 "eat-outs" in fresh marshes have been 

 recorded (O'Neil 1949) but the preference 

 for brackish marsh makes this a more 

 likely site. "Eat-outs" are much rarer 

 today than in the 20's and 30's because 

 trapping keeps the population down to 

 nondamaging levels. 



In light of the apparent local 

 importance of plant-eating furbearers and 

 the earlier discussion of the relative 

 lack of herbivory in marshes, it is 

 informative to reconsider the importance 



Table 23. Muskrat house-building activity 

 in 10-ha brackish and salt marsh areas in 

 Barataria basin (Sasser et al. 1982). 



Ac 1 1 ve 

 Inactive 



Total 



Status change 



24 

 30 



54 



47 

 22 



69 



26 

 12 



38 



40 



48 



Brackish 



Salt 



Active to active 

 Active to inactive 

 Active to gone 



Inactive to active 

 Inactive to inactive 

 Inactive to gone 



New active 

 New inactive 



6 (25%) 



3 (12*) 



15 (62%) 



10 (33%) 



3 (10%) 



17 (57%) 



31 



16 



19 (73%) 



3 (12%) 



4 (15%) 



1 ( 8%) 



( 0%) 



11 (92%) 



20 



5 



66 



