Responses of stream communities
to deforestation in eastern Madagascar.
Abstract: Madagascar
has been recently identified as a global hotspot for freshwater biodiversity.
Loss of most of its eastern rainforest habitat, combined with a high incidence
of micro-endemism and specialization to forest stream habitats, has likely
led to extinction of many of the island's stream insect species.
However, our understanding of this process and the mechanisms driving it
is limited by a paucity of studies. Here we present the first stream
community data from Madagascar. We compared habitat and macroinvertebrate
community structure in three streams draining protected rainforest within
Ranomafana National Park in eastern Madagascar and three agriculture streams
draining the park's largely deforested peripheral zone. Multivariate
analyses showed that macroinvertebrate communities differed between stream
types (P=0.01). Forest streams were characterized by species-rich,
diverse communities composed primarily of collector-gatherers and collector-filterers
belonging to the orders Trichoptera, Ephemeroptera, Plecoptera, and Diptera.
In contrast, simplified agriculture stream communities were dominated by
generalist collector-gatherer taxa mostly belonging to the order Ephemeroptera.
Agriculture streams had 0-14% canopy cover, significantly higher mean and
maximum water temperatures, and significantly lower benthic organic matter
storage than forest streams (71-83% canopy cover). We found no evidence
for higher sedimentation rates or nutrient concentrations in agriculture
streams. Differences in thermal regime may be of paramount importance
in the declines of certain taxa in agriculture streams (e.g., Plecoptera).
However, our data support the hypothesis that differences in stream communities
are caused mainly by differences among taxa in ability to track shifts
in basal resources (i.e., terrestrial detritus vs. in situ algal
production) caused by deforestation. Community simplification and
loss of endemic specialist taxa will continue if projections of future
deforestation are proved correct. We recommend that stream biodiversity
be considered in future conservation planning and propose the creation
and maintenance of multi-use riparian forests that could serve both ecological
and economic functions. Given that more than 5 x 10^5 km of stream
and river channel are affected by tropical deforestation annually, our
results have potential applications in other tropical regions.
Benstead, J. P., M. Douglas, and C. M. Pringle. 2003. Responses of stream communities to deforestation in eastern Madagascar. Ecological Applications 13: 1473-1490.
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