PS 65-86
Effects of vertebrate exclusion on secondary forest dynamics during old-field succession in a Neotropical rainforest
An increasing number of studies have documented chronosequence and dynamic patterns of forest regeneration and secondary succession in old fields in the tropics. However, there is a lack of knowledge about the driving ecological mechanisms, especially regarding plant-vertebrate interactions. Here, we explored the role played by vertebrate herbivores in determining structural changes of regenerative tree communities along secondary succession in abandoned agricultural fields in the La Selva Lacandona, southeastern Mexico. In February of 2005, seven abandoned cornfields, with fallow ages between 0 and 22 years were selected to establish a vertebrate exclusion experiment. In each field a 10 x 50 m plot was delimited where 20 subplots (1 x 2.5 m each) were randomly established. Ten subplots were used as controls and ten were surrounded with metallic mesh (1 m height, 0.5 inch aperture) to exclude vertebrates. All seedlings and saplings 10-200 cm height were tagged, measured, and identified. New censuses were conducted at approximately three-four months intervals during five following years, recording new recruits and the height of survivors.
Results/Conclusions
In total, more than 3,000 individual plants were monitored. Relative to controls, exclosures significantly changed plant density, species density, species replacement rate, and reduced height growth. On average, in absence of vertebrates, plant density increased 82%, while plant height showed a decrease of 211%. These results show that, by consuming seeds and or seedlings, vertebrate herbivores can reduce possible effects of thinning of the community that could happen for competition or interference when plant density is high. The increase in plant abundance in the exclosures was followed by an increase of 216% en species density and by a faster change in species composition, indicating that vertebrate herbivores shape species diversity, filtering the species that can participate along succession. The exclosure effects changed with fallow age, generally becoming stronger in older secondary forests. Our results show that vertebrate herbivores play a critical role in determining the structure and species composition over the forest secondary succession in abandoned tropical fields.