Introduction to the Maya Project Introduction to The Selva Maya Principal Agents of Tropical Deforestation The Maya Project Components - Activities and Results


Forest Fragmentation

Description and Results

The following account of our research activities follows the outline given. For each topic, we describe the associated conservation challenges, give some background knowledge on the topic, describe our research activities, and give a brief synopsis of our results.

B. Forest Fragmentation

A question that has attracted much attention is that of how habitat fragmentation affects population viability. As tropical forest landscapes become progressively deforested, these forests also generally become increasingly fragmented. In many cases, small fragments of mature forest remain isolated amidst a sea of pastures, crop fields, and young second-growth. It is often difficult to tease apart the precise roles of forest fragment size, time and distance of isolation from larger areas of forest habitat, and degree of forest fragment degradation, with respect to their effects on the biota using such fragments.

Still, it is highly worthwhile to study the biota of forest fragments in real farming landscapes. In this way we may begin to learn how much conservation may be achievable in such real-world farming landscapes. With this objective in mind, we conducted one specifically fragmentation-oriented study at Tikal.


Chrotopterus auritus, one of three large bats
at Tikal that feed partly on lizards, rodents,
and other small vertebrates.

1. Bats in Forest Fragments

Led by Mark Schulze, we conducted a study comparing the bat fauna of undisturbed, extensive forest at Tikal with that of forest fragments in the nearby farming landscape (Schulze et al. 2000). The goal was to help answer the overarching question of how much biotic conservation might be achieved in such fragmented, farming landscapes. The analysis was restricted to the family Phyllostomidae, as these bats are generally reasonably well sampled by use of mist nests in the forest understory.

We compared 11 sites within Tikal's extensive primary forest, with seven forest fragments in the nearby farming landscape. Forest fragments ranged in size from 3-4 ha to about 200 ha, and were 4-10 km from large areas of continuous forest. Sampling was conducted via a standardized amount of mist net effort.

We made 900 captures of 20 bat species. Of these 20 species, 13 were taken in both habitats, four were taken only in the continuous forest, and three were captured only in the forest fragments. In the forest fragments one species, Sturnira lilium, accounted for 43% of captures, resulting in greater dominance of that sample relative to the continuous forest sample, which in turn had relatively greater "evenness" of representation of the species present.

Patterns of relative abundance were dramatically different between the two habitats, both in terms of individual bat species and feeding guilds. Large frugivores (Artibeus jamaicensis and A. lituratus) accounted for a higher proportion of total captures in the continuous forest, whereas small frugivores (seven species) made up more captures in the forest fragments.

We found that the relative abundance of large frugivores (which feed on large fruits of tree species characteristic of mature forest interior) and of small frugivores (which feed on small-fruited plants occurring in young second-growth), provides a useful indicator of the degree of forest disturbance.

The number of captures for insectivorous and carnivorous bats was small, but suggested an interesting pattern. Of the three large, vertebrate-eating bats occurring at Tikal, two species were captured only in the extensive forests of the park. The only large, vertebrate-eating bat captured in the farming landscape, a single Trachops cirrhosus, was captured in the largest forest remnant, about 200 ha in area. In addition, six of the seven rarest bat species in the forest fragment sample were captured in the two largest forest remnants, both greater than 50 ha in area.

Hence it appeared that large, carnivorous bats were less common in forest fragments, especially in the smaller fragments, than they were in large expanses of mature forest. This pattern suggests the value of paying further attention to the ability of such large, vertebrate-eating bats to persist in fragmented forest landscapes. The fact that most captures of several uncommon species took place in the larger forest fragments further suggests that size of forest remnants may have significant effects on the species composition of bat communities in landscapes deforested and fragmented to varying degrees.

Literature Cited, Forest Fragmentation

One conservation option is to "high-grade"
the forest's huge, old mahoganies once
and then convert the logged area into a
protected area.

C. Tropical Hardwood Logging

Does selective logging have a place in enlightened conservation and development planning in the New World tropics? Opinions on this topic are divided, even within the conservation community.

Can mahogany logging achieve the three kinds of needed sustainability--silvicultural, economic, and ecological sustainability? Research on many different topics is needed in order to answer these questions.

The Tawny-crowned Greenlet 
(Hylophilus ochraceiceps) is one 
forest bird that appeared negatively 
affected by mahogany logging.

We conducted a preliminary study of logging impacts on birds at Cooperativa Bethel, in the Sierra del Lacand�n of western Pet�n. We detected some indications of impacts on birds, especially a probable decline of Tawny-crowned Greenlets (Hylophilus ochraceiceps) in logged areas, but results of this pilot study were not definitive. We expanded our efforts by conducting a far more thorough study at Tikal, comparing the unlogged forest within Tikal National Park to similar nearby forest logged 12 years previously.

Results of this study, conducted by Mark Schulze, are not yet published, hence we do not present details here. Mark found detectable but subtle differences in the bird communities of logged and unlogged forest. Among the most novel findings was that the effect of logging on the bird community hinged on the type of forest that was logged. Logging of "hill-base" forest had little effect on the bird community, whereas logging of upland forest had more pronounced effects.

This outcome likely resulted from the fact that logging alters the structure of upland forest more than it does that of hill-base forest. Pristine hill-base forest shares some structural features with logged forest, hence the bird species that are common in pristine hill-base forest are probably less effected by logging than are the birds that reach peak abundance in upland forest.

Mark also studied the silvicultural sustainability of mahogany logging at Tikal, and the effects of logging on the regeneration of several species of important forest trees.



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