MAYA PROJECT RESEARCH
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Forest Fragmentation
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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.
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Chrotopterus auritus, one of
three large bats
at Tikal that feed partly on lizards, rodents,
and other small vertebrates.
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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
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One conservation option is to
"high-grade"
the forest's huge, old mahoganies once
and then convert the logged area into a
protected area.
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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.
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The Tawny-crowned Greenlet
(Hylophilus ochraceiceps)
is one
forest bird that appeared negatively
affected by mahogany logging.
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We conducted a preliminary study of logging impacts on birds
at Cooperativa Bethel, in the Sierra del Lacandón of western
Petén. 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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