Size spectra in tropical streams: effects of land use changes
Abstract
Changes in land use have altered the structure and functioning of ecosystems, including trophic
interactions that control the flow of matter and energy. We investigated the differences between
the relationship of log10-biomass and log10-body size classes (size spectrum) of impacted and
preserved streams along a land use gradient, going from almost complete coverage by native
forest to complete agriculture of monocultures. The size spectrum characterizes how many
organisms of each size class are maintained in the communities and is therefore a way of
accessing the energy flow between trophic levels. We used data from macroinvertebrates of 30
streams, located in southeastern Brazil. We predicted that impacted streams could have more or
less basal resources available (i.e. higher or lower size spectra intercept, respectively), and in both
cases we expected a higher energetic cost, sustaining less biomass in large size classes (i.e. steeper
size spectra slope). We also expected more deviations in the model in impacted streams (lower
regression R²), due to the high energy demand and low energy efficiency transfer, causing
organisms to feed outside their ideal predator–prey mass ratio. Our results demonstrated that
impacted streams have fewer small organisms available at lower trophic levels (lower intercepts)
than preserved streams, but with a more efficient energy transfer (shallower slopes). This may be
due to few strong interaction links related to communities with less diversity and the
simplification of the food web in impacted environments, which tends to decrease stability and
enhance vulnerability to stochastic events. Preserved streams are more complex and have more
energy pathways possible, resulting in weaker interaction strengths, which leads to a higher
community stability. We also demonstrated that deviations in the model do not vary
systematically across the land use gradient, as well as its structure is not defined by the land use
type. Our study represents a step forward to understand how anthropogenic impacts affect trophic
interactions and ecosystem functioning in tropical streams.
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