Mesocosm Experiments Progress Report. Ron Bassar, Andrés López-Sepulcre and David Reznick. January 2011. I. Summary of Mesocosm studies completed ...
Mesocosm Experiments Progress Report Ron Bassar, Andrés López-Sepulcre and David Reznick January 2011
I. Summary of Mesocosm studies completed as of 12/10 1. Spring 2007. First trial to the Travis-Reznick design: Phenotype x Density (Aripo drainage only), with only 8 mesocosms. Served as a pilot experiment to troubleshoot ecosystem sampling but guppy lifehistory data is valid. 2. Summer 2007. Kinnison-Palkovacs experiment (Palkovacs et al. 2006): simulated stages of guppyRivulus co-evolution. 3. Spring 2008. Travis-Reznick (Aripo + Guanapo) design with electric exclosures which gave the PNAS paper (Bassar et al. 2010) and two more manuscripts so far (direct and indirect effects of phenotype + biodiversity effects) 4. Spring 2009. Phenotype x Size Structure. 5. Spring 2009. HP-LP competition in same mesocosms (50:50): comparison of relative growths 6. Spring 2009. Phenotype x Light 7. Spring 2010. Fussman's lab experiment: Felipe Perez-Jostov. Phenotype x Parasite load 8. Spring 2010. Fraser/Lamphere/Gilliam experiments on guppy-Rivulus interactions 9. Summer 2010. Aripo HP-LP F2s to look at genetic basis of diet. 10.Summer 2010. Guanapo HP-LP and LaLaja year 1 F2s to look at diet and excretion. 11.Summer 2010. Travis design on Phenotype x Density x Frequency. 12.Spring 2011 plan: Kinnison-Palkovacs re-do with electric exclosures. Focus on trophic cascades and indirect effects
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II. Highlights of Mesocosm Results: 1) Effects of the phenotype on the ecosystem We have demonstrated that guppies adapted to different predation regimes alter community and ecosystem properties in different ways (Bassar et al. 2010). For example, mesocosms stocked with low predation guppies have significantly less algae and more invertebrates (fig 1B and D) than mesocosms with guppies from high predation localities. Less algae in low predation mesocosms leads to lower areal-specific gross primary production (fig 1A). Potential causes for these differences in the effect of the phenotype include numerous direct and indirect effects. We have shown that guppies from low predation environments consume more algae and detritus and fewer invertebrates than guppies from high predation localities (Bassar et al. 2010). In addition, guppies from low predation environments excrete nitrogen at lower rates than high predation guppies (Palkovacs et al. 2009, Bassar et al. 2010). Individually, these changes in dietary preferences and potential physiological traits of the two phenotypes can explain the differences in the total effect of the phenotype on the ecosystem via direct (consumptive effects) and indirect (trophic, and nutrient cascades). Together, the combination of these traits produces a balancing act of primary (direct), secondary (first order indirect), and tertiary (second order indirect) effects. Using a unique combination of experimental and modeling techniques, we have shown that not only are each of these pathways important in determining the net effects of the phenotype on the ecosystem, but have also shown that ecological studies likely underestimate indirect effects in general. For example, high predation guppies directly increased algal biomass in mesocosms through a reduction in consumption of algae. While total indirect effects were of a slightly smaller magnitude (fig D), the individual components of this effect were much larger than the direct effect, but because they opposed each other in direction, they canceled each other out. The sum of this work shows that not only does the effect of the phenotype on ecosystem
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properties matter for determining the state of the ecosystem, but that these changes are likely to influence the how the phenotype evolves.
2) The effect of guppy phenotype on diversity: With the differences in the feeding strategies between the low and high predation guppies it is reasonable to expect that the two phenotypes may also alter the members of the primary producer community in addition to their total number or biomass. In the same set of mesocosm experiments, we have found that the changes in chl-a associated with guppy treatments are the result of different underlying changes in the producer community. Decreases in chl-a in the fish vs. no fish treatments was the result of changes in both the overall volume and diversity of producers in the system. The influence of guppy density on chl-a was mostly the result of a decrease in the overall volume (biomass) of producers in the system, indicating that as guppies increase in number they mostly alter the environment by decreasing the total amount of algae. In contrast, increased chl-a in the high predation guppy mesocosms was not associated with changes in overall volume of producers, rather the increase in chl-a was associated with higher producer diversity. These changes in diversity reflected overall changes in the composition of the community. Mesocosms with guppies contained different communities of producers compared to those without guppies (p=0.012). Low predation mesocosms were more similar to mesocosms with no fish and were significantly different than communities with high predation guppies (p=0.015). 3) The influence of population size structure in determining interactions between the phenotype and environment Decreased mortality in low predation guppies increases guppy density (Reznick et al. 1996, Reznick et al. 2001), but it also alters the size-structure of the population via demographic changes due directly to changes in the mortality regime and also due to evolutionary changes in the number and size of offspring (Rodd and Reznick 1997). Low predation populations have a larger average size compared to high predation populations, which is due in part to a reduction in the number of smaller individuals in the population. We have investigated how this type of combined evolutionary and demographic change can influence Mesocosms 3
the ecosystem. We used a factorial design and crossed phenotype with size structure (dominated by small vs large guppies). The overall densities between size structure treatments were held constant, where the biomasses were 2 times higher in the populations dominated by large guppies (similar to the biomass differences due to density in exp 1 above). The results of these experiments reveal an interaction between the individual effects of the phenotype and the structure of the overall population. Contrasting these results with those from exp 1 (no interaction) shows that the way in which guppy biomass changes matters to how the phenotype effects the environment. Low predation populations dominated by larger guppies decrease the amount of chl-a as in exp 1. However, populations dominated by smaller individuals show the opposite pattern. We are currently investigating mechanistic connections that may explain these patterns. 4) The role of density and phenotype frequency in determining the invasibility of the low predation phenotype Traditional demographic life history theory posits that age-specific extrinsic sources of mortality are the primary drivers of life history diversification. These theories generally make simplifying assumptions about how the evolutionary process works. One major assumption is that the fitness of one genotype is not affected by the frequency and density of another phenotype in the environment. We have tested this assumption by using a combination of experimental density manipulations in natural low predation environments and in the common garden mesocosms. Manipulations of densities in wild populations have shown that low predation populations are regulated via density-dependent adjustment of the vital rates. For example, altering guppy densities causes a population growth response that counteracts the direction of the manipulation. Moreover, control populations (1.0 x) do not differ from a population growth rate that indicates no growth or decline in population size. In a series of mesoscosm experiments, we have directly tested the idea that high predation guppies always have higher fitness than low predation guppies (high predation as super guppies) and, in doing so, show that low predation guppies can have higher fitness (measured as phenotype-specific growth rates) than high predation guppies. First, in 2 x 2 factorial experiments with guppy phenotype and density, we have shown that at low densities high predation phenotypes have higher rates of population growth compared to low predation phenotypes. However, at Mesocosms 4
high densities, high predation phenotypes lose this fitness advantage as evidenced by the interaction between phenotype and density (fig Y). This experiment shows that at least part of the adaptation to a low predation environment is due to adaptation to high densities and presumably low resource conditions. One possibility is that under high density conditions, low predation guppies are better at acquiring or making use of more limited or lower quality resources. The results of gut content analysis from these fish show that low predation guppies are more catholic in their diet—consuming all sources of benthic food items, whereas high predation guppies prefer to consume invertebrates. Decomposing the effects of the experimental treatments on lambda into each demographic rate shows that the weighting of demographic rates responsible for this interaction is spread relatively evenly across the vital rates. While these results are promising in showing how the low predation phenotype can evolve, more compelling results come from preliminary results from a recent experiment intended to simulate the conditions of high predation guppies invading a previously guppy free stream—as in our focal stream guppy introduction. Here, we used for guppy treatments that represented a temporal sequence of changes in population density and frequency of phenotypes (high and low predation) that may occur in the focal streams if the density and frequency of phenotypes are important in the evolution of low predation populations. Treatment 1 contains guppies at a low density and a population dominated by high predation phenotypes—this simulates the initial conditions immediately after HP fish are introduced. Owing to the high reproductive rates of the HP fish and low densities, the population can rapidly expand and possibly overshoot the natural densities of LP populations. Treatment 2 simulates the point wherein population density is high, but the frequency of the phenotypes in the population is still dominated by high predation phenotypes. Treatment 3 again simulates the high density condition, but this time the low predation phenotype is numerically dominant in the population. Finally, treatment 4 simulates the condition if the evolution of the low predation phenotype results in drastic decrease in overall population growth such that the population is again at a low density but is now dominated by the low predation phenotype (fig Z). Preliminary results from this experiment show that not only is density important for the evolution of the low predation phenotype, but that the frequency of the phenotypes is also important in first not allowing the low predation Mesocosms 5
phenotype to evolve (higher growth of high predation fish in treatment 1), but also in maintaining the fitness advantage of the low predation phenotype under decreased density conditions (treatment 4).
Literature Cited
Bassar, R. D., M. C. Marshall, A. Lopez-Sepulcre, E. Zandona , S. K. Auer, J. Travis, C. M. Pringle, A. S. Flecker, S. A. Thomas, D. F. Fraser, and D. N. Reznick. 2010. Local adaptation in Trinidadian guppies alters ecosystem processes. Proceedings of the National Academy of Sciences 107:3616-3621. Palkovacs, E. P., M. C. Marshal, B. A. Lamphere, B. R. Lynch, D. J. Weese, D. F. Fraser, D. N. Reznick, C. M. Pringle, and M. T. Kinnison. 2009. Experimental evaluation of evolution and coevolution as agents of ecosystem change in Trinidadian streams. Philosophical Transactions of the Royal Society of London, Series B: Biological Sciences 364:1617-1628. Reznick, D., M. J. Butler, and H. Rodd. 2001. Life-history evolution in guppies. VII. The comparative ecology of high- and low-predation environments. American Naturalist 157:126-140. Reznick, D. N., M. J. Butler, F. H. Rodd, and P. Ross. 1996. Life-history evolution in guppies (Poecilia reticulata) .6. Differential mortality as a mechanism for natural selection. Evolution 50:1651-1660. Rodd, F. H., and D. N. Reznick. 1997. Variation in the demography of guppy populations: The importance of predation and life histories. Ecology 78:405-418.
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