An initial survey showed that plants that had
received beetle herbivory were smaller, on the average,
than ungrazed plants. Beetle herbivory on mature
Passiflora vitifolia was associated with an
increased proportion of leaf loss in the early wet
season. However, a similar negative response was not
shown in the late wet season.
When responses of plants to herbivory were compared for the experimental plants, a higher proportion of beetle damage was associated with reduced plant growth, shown as a lower number of leaves on individual P. vitifolia plants.
In one experiment, where the proportion of beetle herbivory levels was manipulated, the growth of P. vitifolia was directly affected. Beetle herbivory on the plants reduced their branch extension, but only when previous damage had occurred. The negative effect of beetle herbivory was also detected as a negative change in leaf numbers, regardless of whether the plants had received previous damage.
There is clear evidence of reduced growth of P. vitifolia with increased levels of beetle herbivory. The results show positive associations between beetle damage and net leaf loss, a measure of the balance between leaf formation and leaf abscission. This suggest that beetle damage may cause leaf loss to P. vitifolia. The correlation between beetle herbivory and leaf loss in the early season is an indication that beetle herbivory could be causing leaf abscission.
The beetles only cause partial damage to the leaves,
but this could result in physiological stress or
chemical changes in the leaves. Surprisingly, little
difference was found between plant responses in the
contrasting experimental habitats. However, the greater
impact of herbivory during the sunnier, early half of
the rainy season indicates that seasonal differences in
light levels could affect plant responses to damage.
The seasonally greater impact of beetle herbivory in the early as opposed to the late season is not directly related to higher levels of herbivory at that time. However, it may be partly explained by a relatively greater impact of physical factors in the late as opposed to the early wet season. Seasonal differences could be an indicator of the importance of internal resources for the responses of P. vitifolia to herbivory. After a long dry season, the plants may have inadequate stored reserves to recover after herbivory and this may result in reduced growth or leaf loss. Seasonal fluctuation in resources are likely affecting the response of P. vitifolia to damage.
Plants growing in natural habitats and at the experimental site showed an identical pattern. Both groups showed reduced growth in response to natural levels of beetle herbivory. Plants growing in the forest were much larger than the experimental plants. The similar responses of large and small plants indicate that large and small plants may have the same basic mechanism in their response to herbivory. The size or age of Passiflora may be correlated with plant quality as food for herbivores or herbivore apparency to predators.
There is evidence from the present study that previous damage causes the plants to be more susceptible to later damage. This conclusion indicates that herbivory acts as a physiological stress factor on the plants. It is concluded that beetle herbivory is important for the growth of P. vitifolia in the tropical lowland rainforest.
Author: Soffia Arnthorsdottir
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