We measured the nest density of stingless bees
(Apidae, Meliponini) in undisturbed and logged-over
dipterocarp forests in Sabah, northern Borneo, and evaluated
hypotheses on proximate factors leading to the observed
variation: population control mediated by (1) nest
predation, (2) limitation of nest trees, or (3) food limitation.
Per-area nest density varied twentyfold across 14
forest sites and was significantly affected by locality, but
not by the degree and history of disturbance. Nest density
was generally high in sites located in the Sepilok Forest
fragment (mean 8.4 nests/ha), bordering mangroves
or plantations. In contrast, nest densities in continuous
forests were all low (between 0 and 2.1 nests/ha,
mean 0.5 nests/ha). Yearly nest mortality was low
(13.5–15.0%) over 4 years of observation and did not
vary between forest localities, thus limiting the potential
of nest predation (1) in creating the observed variation in
nest density. The presence of potential nest trees (2),
though positively correlated with nest density, explained
only a minute fraction of the observed variation. Nest
density was best explained by differences in the pollen
resources (3) available to the bees (quantified by analysis
of pollen in bee garbage). Across five selected sites
the amount of nonforest pollen (from mangrove or crop
plants) included in diets of Trigona collina was positively
correlated with T. collina nest density. External pollen
sources are a likely supplement to bee diets at times
when little flowering occurs inside the forest, thus increasing
overall bee carrying capacity. Pollen limitation
was also indicated by direct measurements of pollen import
and foraging activity of T. collina in three selected
sites: Pollen traps installed at nests in high-density Sepilok
captured significantly more corbicular pollen than
colonies in low-density Deramakot. At the same time,
morning foraging activity was also greater in Sepilok, indicating
a regulatory increase in foraging in response to
high pollen availability. We conclude that the abundance
of stingless bees in forests in Sabah is chiefly dependent
on the local availability of food resources. Bee populations
strongly benefit from edge effects and increased
foraging habitat diversity. Electronic supplementary material
to this paper can be obtained by using the Springer
Link server located at http://dx.doi.org/10.1007/s00442-
001-0848-6
Keywords Pollen