Effects of physical factors on numbers of individuals
and species
None of the physical factors measured was significantly
related to the total number of individuals or
the species numbers of butterflies. This result is in
contrast with those of Moss and Pollard21 and Pollard
et al,22 which may reflect the differences between the
tropical and temperate climate patterns. They reported
significant effects of weather: rainfall and humidity
were negatively correlated with the numbers of
individuals and species of butterflies.22-23 Some field
studies of tropical butterflies indicate that periods
of very heavy rain may result in increased mortality
of adults,9 thus causing their numbers to decrease.
Our study showed a similar pattern in the families
Satyridae and Amathusiidae. In contrast, individual
numbers of Hesperiidae increased with high
humidity. It is possible that the immature stages of
Hesperiidae usually reside in blotch mines, and that
the leaf is often folded and becomes a shelter from
rain.24 Thus, Hesperiidae show a positive correlation
with humidity. Otherwise, butterflies are most active
as adults in sunny weather9,25-28 when conditions are
suitable for mating and oviposition.
Butterfly populations increased significantly
during the period of high temperature and low
precipitation. Temperature effects the growth of food
plants23, and therefore should be positively correlated
with the numbers of individuals and species. In our
study, however, correlations with temperature were
not significant, probably because the temperature
in the tropics is rather constant. Also, the average
temperature in each month may have differred from
the real temperature during the survey periods.
Temperature may affect butterfly populations in
several ways. High temperature may enhance
courtship behavior, oviposition and larval development.
High temperature weather may even promote
outbreaks in some species. On the other hand, larval
development may be adversely affected if the
humidity of the soil remains low. Finally, the
family–weather correlations are weak; because of the
large number of tests carried out at the 0.05
probability level, it is expected that some (1 in 20)
will be “significant” by chance alone. Overall,
weather variations in the wet tropics are not great
enough to produce many measurable effects on
butterfly numbers.
110 ScienceAsia 26 (2000)
The study period was relatively short for an
analysis of trends which may affect their conservation
status. Longer term monitoring is needed to
identify significant changes in biological diversity,
permitting the timely adjustment of management
activities to reverse or prevent undesired trends.