Study of biological invasions can provide insights into ecological processes sustaining biological diversity and community assembly. In this study we use data collected to study invasions to ask questions about the biotic resistance hypothesis (BRH), which says susceptibility to invasion is related to native species richness, and the niche variance hypothesis (NVH), which states niche breadth and competition are inversely related. Species richness alone may overlook networks of consumptive interactions captured in food webs and network connectedness may be uncorrelated with species richness. Increased network complexity could increase system stability, decreasing invasion risk. Measures of trophic niche and how they vary across habitats and seasons – gradients of competition – can be used to test network complexity and the NVH. To test these hypotheses, we quantified the trophic niche of native and invasive taxa using stomach content data generating ellipse areas for native (26) and non-native (5) fishes across habitats and seasons. These data were used to ask questions testing the BRH and NVH. An NMDS (k = 2) was conducted to reduce the dimensionality of stomach content data. The resulting axes were analyzed in the SIBER package in R to generate probability distributions of ellipse areas.
Results/Conclusions
In testing the BRH, we found species richness was highest in ponds (29), the habitat with the most invasive species (5). Between seasons there was a decrease in both native and invasive species richness, with only one invasive taxon found in the dry season. To test the NVH, we asked if elevated levels of competition led to smaller trophic niches. Competition is thought to be highest when fish are concentrated in ponds in the dry season. Our results supported this. We found the pond was the habitat where a species most often had the smallest trophic niche and spikerush was most often the habitat that had the largest trophic niche. Pond and spikerush had the same number of species with seasonal changes (15), while sawgrass had the fewest (10). We did not find support for the BRH among habitats. Ponds had the highest species richness, but also had increased nutrient availability that creates a broader range of basal resources, potentially increasing invasibility. Our results generally supported the NVH with habitat, season trends suggesting the competition gradient is not in the same direction among habitats between seasons.