Abstract
ABSTRACT Food webs are not static entities; consumer resource interactions vary in both time and space, which complicates depiction and comparisons of food web structures. We estimated fish assemblage structure and diets in two clear-water streams in the Venezuelan Llanos region (Charcote and Charcotico) and constructed trophic networks (sub-webs defined by fishes as the principal consumers) during four periods of the annual flood pulse. As stream conditions changed from high-water to low-water, we hypothesized that: 1) the piscivore-non-piscivore ratio would increase; 2) dietary diversity would decrease; 3) interspecific dietary overlap would decline; 4) fewer allochthonous food items would be consumed; and 5) food-web connectance would increase. The piscivore-non-piscivore abundance ratio was higher in both streams during the low-water period. Dietary diversity declined as water levels dropped and availability of aquatic habitats and resources declined, but interspecific dietary overlap was not lower. Contrary to our hypothesis, average interspecific dietary overlap increased at Charcote as the dry season progressed, even though dietary overlap among species was significantly lower than expected by chance. We did not find strong support for our hypotheses regarding seasonal patterns of consumption of allochthonous resources and food web connectance, both of which revealed little seasonal variation.
Highlights
Over the past four decades, considerable effort has been expended to describe empirical food webs (Cohen et al, 1993) and create models that can describe their features (Williams, Martinez, 2000); this has not been without controversy (Rooney et al, 2008)
During the four survey periods, 20,347 fish specimens representing 69 species were collected from the two streams (S1 - Available only as online supplementary file accessed with the online version of the article at http://www.scielo. br/ni), and most species occurred in both streams (65.2 %)
canonical-correlation analysis (CCA) axis 1 was associated with seasonal variation (Fig. 2; (S2 - Available only as online supplementary file accessed with the online version of the article at http://www.scielo. br/ni), explained 31.7% of variation in assemblage structure and was statistically significant (F(1,6) = 2.78 P = 0.004)
Summary
Over the past four decades, considerable effort has been expended to describe empirical food webs (Cohen et al, 1993) and create models that can describe their features (Williams, Martinez, 2000); this has not been without controversy (Rooney et al, 2008). The great majority of published food webs are based on information obtained by unspecified methods and time periods (Polis, 1991; Yodzis, Winemiller, 1999; McMeans et al, 2015). Food web structure (the architecture of networks of consumer-resource interactions) varies in both space and time (Winemiller, 1990; McCann, 2011). Given the considerable challenges to describe a network of predator-prey interactions, even one involving a subset of species within a local community, few studies have attempted to document food web structure across gradients of space or time (McMeans et al, 2015)
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