Constraints on Succession 375(Osunkoya 1994, Sarmiento 1997, Duncan and
Duncan 2000) confirm a moderate to high level of
seed removal though these rates can vary widely
amon gsites, species, seed sizes, and distance from
forest edge (Nepstadet al. 1996, Holl and Lulow
1997, Pena-Claros and De Boo 2002, Joneset al.
2003, Myster 2003b). Regardless, it appears likely
that seed predation may strongly limit rates of
woody species colonization and potentially filter
out species whose seeds are highly preferred. We
caution that these results are based primarily
on seed removal trials where seed disappearance
may not always equal seed death; future stud-
ies should document the fates of removed seeds
(e.g., Forget 1997).
Recruitmentlimitationcausedbyseedling
predation
Small mammals are fairly abundant in tropical
pastures (Nepstadet al. 1996, Joneset al. 2003)
and the few studies available suggest they can
cause substantial seedlin gmortality. For example,
Holl and Quiros-Nietzen (1999) found that rabbits
clipped more than 50% of transplanted seedlings
of four native tree species in a lower montane
pasture in southern Costa Rica. Predation did
not differ between open pasture and under-
tree locations (Holl and Quiros-Nietzen 1999)
and some species were clipped more than oth-
ers (high inSideroxylon portoricenseandVochysia
allenii,lowinOcotea galucosericeaand Ocotea
whitei). In Honduras, Zahawi and Augspurger
(2006) found levels of seedlin gpredation simi-
lar to those of Holl and Quiros-Nietzen (1999),
although only at one of three sites. In Puerto
Rico, Zimmermanet al. (2000) observed substan-
tial herbivory on three small-seeded species and
suggested that small-seeded species were more
vulnerable than large-seeded species. Duncan
and Duncan (2000) found that rodent preda-
tion caused at least 29% mortality amon gwoody
seedlings introduced as seeds into East African
pastures.
Leaf-cuttin gants can be major seedlin gpreda-
tors and herbivores. For example,Atta sexdens
removed more than one third of the foliage within
16 days of plantin gof seedlin gs of four tree
species in eastern Amazonia (Nepstadetal. 1996).
In a 2-year-old Brazilian pasture Vasconcelos
and Cherrett (1997) found that small seedlings
were particularly vulnerable to leaf-cutters and at
least 65% of the individuals of six species were
attacked. Because some species are far more vul-
nerable than others, it is likely that leaf-cutter
attacks, like seed predation, can delay succession
as well as alter species composition.Recruitmentlimitationcanbeameliorated
byresidenttreesandshrubsSeed input can increase dramatically beneath
pasture trees and shrubs, regardless of whether
these pasture trees were remnants or new
colonists (e.g., Sarmiento 1997, Slocum and
Horvitz 2000, Carriereet al. 2002). For example,
both Nepstadet al. (1996) and Hollet al. (2000)
found seed deposition for animal-dispersed seeds
was more than two orders of magnitude higher
beneath resident trees compared with open pas-
ture (see also Slocum and Horvitz 2000). Not
surprisingly, wind-dispersed seeds show similar
rates of input both beneath trees and in open
pasture (Hollet al. 2000).
Seedlin gabundance is much greater beneath
isolated trees. In the southern Costa Rican pre-
montane zone, tree seedlings of animal-dispersed
species were more concentrated beneath shrubs
and trees than in open pasture (Holl 1999);
similar trends were documented in lowland pas-
tures of northeastern Costa Rica (Cusack and
Montagnini 2004). In lower montane Ecuado-
rian pastures, woody recruitment was essen-
tially nil in open pastures, whereas recruitment
steadily increased in pastures with abundant
guava trees (Zahawi and Augspurger 1999).
Zanne and Chapman (2001) found that grass-
lands converted to tree plantations had greater
understory woody stem density and species rich-
ness than unplanted grasslands. The identity of
resident trees appears to influence colonization.
For example, in pastures in Costa Rica, Slocum
(2001) found more recruits were present beneath
CordiaandCecropiatrees than beneathFicustrees.
Also, the woody species composition beneath both
CordiaandCecropiatrees was distinct from that
underFicusorPentaclethratrees.