Effects of local processes and spatial isolation on freshwater community assembly: a simulation of land-use intensification

• Main Author: Pelinson, Rodolfo Mei
• Publication Date: 2020
• Format: Doctoral thesis
• Language: eng
• Source: Biblioteca Digital de Teses e Dissertações da USP
• Download full: http://www.teses.usp.br/teses/disponiveis/41/41134/tde-22062020-093533/
• topic: Agroquímicos; Cascatas Tróficas; Impactos da Agricultura; Impactos da Aquacultura; Metacomunidade

Introduction: Freshwater environments are among the most affected by land-use change, particularly by the introduction of exotic fish species for aquaculture and contamination by agrochemicals such as fertilizers and pesticides. However, we essentially ignore whether and how land-use change interacts with spatial processes to affect local communities, and, therefore, biodiversity. In this Thesis we aimed at experimentally understanding how the introduction of exotic predatory fish for aquaculture, and contamination by agrochemicals, can affect macroinvertebrate and amphibian community structure in different spatial contexts using an explicit metacommunity framework. Methods: We constructed 1,200-L artificial ponds (24 in the first experiment and 45 in the second) in a savanna landscape at three different distances from a source wetland (30 m, 120 m, and 480 m). Ponds were spontaneously colonized by semiaquatic insects and amphibians dispersing in the terrestrial landscape. In the first experiment, we manipulated the presence and absence of the exotic omnivorous fish, Tilapia. In the second experiment, we manipulated agrochemical intensification simulating the conversion of savannas into managed pastures (ponds treated with fertilizers) and sugarcane fields (ponds treated with fertilizers and a single pulse of the insecticide fipronil and the herbicide 2,4-D following realistic dosages and application schedules). Main Findings: We generally found that spatial isolation can reduce the abundance of dispersal-limited predatory insects, with few exceptions (i.e., Pantala and Orthemis dragonflies), thus increasing community size (i.e., total abundance of individuals) by favoring insect consumers, which have higher dispersal rates. Spatial isolation can also increase community-to-community variability (i.e., beta-diversity within treatments), by increasing the effects of historical contingency on species with similar dispersal rates. However, these effects can drastically change when predatory fish is present. The stocking of fish can have a strong negative effect on large-bodied predatory insects, and a milder negative effect on insect consumers, dampening the indirect positive effects of isolation on most of them. Fish also appear to override the effects of historical contingency, making community variability decrease because of the increase in community size. Shifting to agrochemicals, fertilization in \'pasture\' ponds caused a slight increase in the abundance of few predatory insects via bottom-up effects. The insecticide pulse in \'sugarcane\' ponds caused a very strong but temporary negative effect on insect populations, followed by an increase in the abundance of generalist larval amphibians. Different from the effects of fish, the effects of fertilization and pesticide pulses do not change with spatial isolation, likely because they have equally acute effects on all invertebrate taxa across isolation treatments, and their indirect effects are mostly on non-dispersal-limited taxa (i.e., dragonflies and amphibians). Therefore, we show that the potential of local environmental processes to interact with the effects of spatial isolation is highly dependent on the type of land management. More importantly, we show that the interspecific variation in dispersal rates and the multi-trophic nature of freshwater communities must be considered if we seek to understand the consequences of environmental change on community structure