Macroinvertebrate abundance

More than five times the abundance of macroinvertebrates was found in bromeliad compared to forest floor litter samples with the most abundant groups being (in order): ants, cockroach larvae, millipedes and fly larvae. Large numbers of wood lice, harvestmen, beetles and spiders were also present. The forest floor samples contained a very low abundance, the most abundant groups being ants, millipedes and beetle larvae. Samples from the unlogged forest floor contained a much higher abundance than those from the logged forest floor. However, there was very little difference between the abundance of macroinvertebrates found in bromeliad samples from the logged and unlogged plots.

Macroinvertebrate diversity

Almost three times the diversity of macroinvertebrates were present in bromeliad compared to forest floor samples. The most diverse groups in bromeliads were (in order): beetles, spiders, ants and millipedes. The most diverse groups in forest floor samples were spiders, ants, millipedes and beetles. Bromeliads from the unlogged forest were found to have a slightly higher diversity compared with those from the logged forest.

Environmental variables

Rosette width and the presence of an ants nest were found to exert a significant effect upon the macroinvertebrate assemblages present in bromeliads. The composition of samples from the logged and unlogged plots was not found to be significantly different when bromeliad and forest floor samples were combined.


Discussion

A clear difference between the litter macroinvertebrate communities present in bromeliad samples compared to forest floor samples was found. Bromeliads were found to be 'hotspots' of macroinvertebrate abundance and diversity, a high abundance of larvae in bromeliad samples suggests that they play an important role as invertebrate 'nurseries'. Possible explanations for these observations could be:

-Bromeliads offer shelter from some predators present on the forest floor such as reptiles, rodents and small mammals. Only one vertebrate was discovered in a bromeliad during the study, which was a small Cloudy Snail-eating Snake (Sibon nebulata).

-Bromeliads may play an important role as refuges during the dry season. Humidity is an extremely important condition for invertebrates and they may seek our bromeliads for their damper microhabitats during dry conditions.

-The large numbers of prey, especially larvae present in bromeliads provide an easily exploitable food source for invertebrate predators such as ants, harvestmen and spiders.

The lower diversity and abundance of macroinvertebrates present in forest floor samples from the logged forest suggests that disturbance may have had a negative impact on the macroinvertebrate community. This is alarming as litter invertebrates play a very important role in the functioning of the ecosystem, helping to free nutrients necessary for the continued growth of the forest. It appears that bromeliads were somewhat buffered against the decrease in diversity and abundance that was apparent in samples from the forest floor. It may be that conditions within the bromeliad rosette are less susceptible to change as a result of logging than those on the forest floor. If this is true, bromeliads may offer a refuge in the logged forest for species unable to persist in the altered conditions on the forest floor. Thus bromeliads may help to sustain diversity that might otherwise be lost from the forest in the event of logging.