Investigating Realistic Scenarios of Biodiversity Loss on Ecosystem Functioning: Extirpation of Rare Species and Food Web Collapse in Tropical Floodplain Lagoons Page: 28

                                
Experimental Design
As described in chapter 2, the same LTER fish assemblage data and chosen 18 fish
species were used to establish realistic diversity treatments based on species loss by ordered
trophic position. Starting from the top of the food web, species richness was sequentially reduced
by four species with each treatment, ranging from 18 to 2 species (Table 7). A sixth treatment
was established as a control with no fish species. For species with the same trophic position,
rarer species based on their summed rank abundance were excluded first. To isolate the effect of
biodiversity loss, I controlled for abundance (65 individuals per mesocosm) and evenness at each
treatment level by proportionally distributing individuals from those species excluded to
remaining species such that the slope of the rank-abundance curve (i.e. evenness) was
maintained constant (Figure 4). Thus, these experimental assemblages exhibit natural patterns of
dominance and rarity observed in an average lagoon, while isolating the effect of sequential loss
species by trophic position. Due to the wide variety of body sizes of species included, I was
unable to maintain biomass constant at the same time as abundance and evenness, therefore
accounted for potential effects of biomass among treatments by including it as a covariate in
analyses. The experiment ran for 18 days and each treatment was replicated four times.
The experimental venue (described in chapter 2) consisted of twenty-four 1000 L
cylindrical mesocosms which were used for experimental units representing isolated lagoons. As
previously mentioned, to better simulate conditions found in isolated lagoons, 0.5 g of fertilizer
and a slurry of phytoplankton (450 mL) and zooplankton (380 mL) taken from backwater
lagoons were added to each mesocosm. Following these additions, average initial concentrations
of variables of interest among mesocosms resembled natural isolated lagoons: total phosphorus
(15 [g/L + 10), total nitrogen (850 [g/L + 92), phytoplankton (3.9 [g/L + 2.8), and zooplankton

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