The importance of tropical rain forest fragments to the conservation of plant species diversity in Los Tuxtlas, Mexico

Of what value are forest fragments to the conservation of the tropical rain forest diversity for a landscape? We compared the changes in composition, diversity, and plant structure of 15 small (1-76 ha) relatively unprotected forest fragments with those of a large (700 ha) well protected fragment (LWPF) in Los Tuxtlas, Mexico. The trees, shrubs, lianas, palms, and herbs with dbh >= 2.5 cm were sampled in 1000 m(2) at each site. For each ecological group (based on light requirements for germination: primary, secondary, and non-secondary light demanding, NSLD, species) and life form, estimates of species diversity, density, and basal area were analyzed using a stepwise multiple regression to determine whether there were any relationships between these variables and fragment characteristics (size, shape index, and isolation). The composition and plant structure of LWPF were different from those of the small fragments and large trees are absent from the canopy of the latter. Fragment size best explained the differences in composition and plant structure. Species composition in the largest fragments was similar to that of LWPF, but there was no significant difference in total richness between LWPF and the fragments, even though both the richness and abundance of secondary and NSLD species did differ. LWPF had more large primary trees in the canopy, and a greater abundance and basal area of palms and herbs. For tropical rain forest conservation, it is important to maintain the greatest possible number of large fragments and establish policies that prevent forest remnants from being further reduced in size and increasingly isolated from each other.