32 4Gregory H. Adler
and provide environmental cues that allow organ-
isms to prepare physiologically and behaviorally
in advance of the impending seasonal changes
or to respond quickly once those changes have
occurred.
In tropical forests that experience a sea-
sonal climate, most plants reproduce seasonally
(e.g., Foster 1982a, Van Schaiket al. 1993) and
commonly use irradiance or some measure of
moisture availability, such as drought, as a cue to
initiate reproduction (e.g., Van Schaiket al. 1993,
Wrightet al. 1999). An 18-year record from old-
growth forest in central Panama reveals that fruit
productionvariesgreatlythroughouttheyearand
is greatest in April at the end of the dry season and
least in November, December, and January at the
end of the rainy season and beginning of the dry
season (Wright and Calderon 2006).
The seasonal availability of fruits and seeds
thatresultsfromplantreproductiongreatlyaffects
frugivorousandgranivorousanimals.Thus,popu-
lationsof suchanimalsappeartobelimitedbysea-
sonalshortageof fruitsandseeds(e.g.,VanSchaik
et al. 1993), and there are abundant descrip-
tive data on mammals to support this proposition
(e.g., Russell 1982, Smytheet al. 1982, Terborgh
1986). During the season of resource scarcity,
fruit production is insufficient to support the
biomass of frugivorous mammals (Smytheet al.
1982,Terborgh 1986), and frugivores often man-
ifest signs of food deprivation, including weight
loss, increased time spent foraging over wider
areas, increased foraging risk, and reproductive
quiescence (Foster 1982b, Russell 1982, Smythe
et al. 1982). When fruits and seeds are most
abundant during the year, much of the fruit
that is produced is not consumed by mammals
(Hladik and Hladik 1969, personal observations).
In contrast, fruits and seeds that fall to the for-
est floor during the season of resource scarcity
are consumed nearly as rapidly as they fall. Diets
of frugivorous and granivorous mammals often
overlap extensively when resources are abundant,
but overlap is reduced when resources are scarce
(e.g., Smythe 1978, Smytheet al. 1982, Terborgh
1983, 1986).
The demography of frugivorous and granivo-
rous mammals also reflects the impact of seasonal
changes in resource abundance. Reproduction is
often seasonal, and mammals time their maxi-
malreproductiveefforttocoincidewiththeseason
of greatest resource abundance (e.g., Fleming
1971,Bonaccorso1979,Glanzet al.1982,Milton
1982, Russell 1982, Gliwicz 1984). When fruit is
scarce, mortality rates are higher (Smythe 1978,
Milton 1982). Because birth and mortality rates
are not balanced throughout a year, popula-
tion densities fluctuate widely and demonstrate
an annual pattern of peaks and nadirs. Thus,
density reaches a peak following the season of
resourceabundance,declinesasmortalityexceeds
fecundity during the season of resource scarcity,
reaches a nadir at the end of that season, and
begins to increase again when fruit is abundant
and fecundity again exceeds mortality.
Irregular fluctuations such as storms and
El Niño Southern Oscillation (ENSO) events fre-
quently are superimposed on seasonal fluctua-
tions and may impose additional constraints on
the activities of organisms. Such fluctuations can-
not be anticipated in advance of the impending
changes by organisms using environmental cues
because of their irregular periodicity and conse-
quent unpredictability. In seasonally dry tropical
forests, such as in central Panama, an abnormally
wet dry season apparently causes famine con-
ditions for frugivores and granivores, apparently
because many plants require a protracted dry
period or increased irradiance (such as normally
occurs in the dry season) to initiate flowering
(e.g., Foster 1982b, Wright 1999). Thus, few
resources are available for frugivores and grani-
vores later in the year because of poor plant
reproduction. In contrast, ENSO events, which
contribute most to inter-annual variation in trop-
ical climates (Wright and Calderon 2006), cause
abnormally dry conditions in much of the trop-
ics, which apparently stimulate massive plant
reproduction, thereby providing hyperabundant
resources (e.g., Wrightet al. 1999). Although
it is not clear why plant reproduction increases
during ENSO events, the most likely expla-
nation is the increase in irradiance (Wright
and Calderon 2006). Frugivores and grani-
vores respond to the increased abundance of
resources by reaching extremely high but unsus-
tainable densities and subsequently decline, some-
times catastrophically, when fruit production