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
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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
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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
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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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