PROGRESS REPORT – 18 SEPTEMBER 2012
II. Mesocosms
We have continued to use our 16 artificial streams to perform relatively short term
(usually 28 days) experiments that are designed to address specific hypotheses associated
with the ongoing work in the focal streams. The distinction between the two settings is
that the focal streams provide a natural setting for evaluating the impact of guppies on
their ecosystem and the interaction between ecology and evolution, but provide a
complex time series of dependent variables and associated environmental complexities,
such as the wet and dry season. The artificial streams can only look at evolution
retrospectively, in the form of differences among guppy populations adapted to different
environments, but they enable us to perform replicated experiments that address specific
hypotheses. In the past year, we have completed experiments that address the role of
density and Rivulus in shaping the relative fitness of guppies from high versus low
predation environments.
One of the anomalies in the research that lead up to the FIBR project is that all
available data suggested that the high predation phenotype (HP) was unconditionally
superior to the low predation phenotype (LP), such that the latter should never evolve.
The strongest result behind this argument came from a laboratory-based, comparative
study of senescence in guppies from paired HP and LP localities from two rivers.
Traditional theory for the evolution of senescence predicts that guppies from HP
environments should show evidence of senescence at an earlier age and have shorter life
spans. The opposite was true. They matured at an earlier age, produced more offspring
throughout their lives, continued to reproduce into more advanced ages, had a delayed
acceleration in mortality rate and longer life expectancy than guppies from LP
environments [1-2]. None of the anticipated tradeoffs associated with the evolution of
earlier maturity or higher investment in reproduction were evident. However, this
experiment was executed on fish reared one per aquarium on quantified food availability.
Somewhere in the gulf that separates the conditions of this experiment and the setting in
which these life histories evolved might lie the explanation for why the LP phenotype has
higher fitness in low predation environments. Two possible explanations, based on earlier
work, might be adaptation to the higher guppy population densities and lower resource
availability typical of low
predation environments or to
adaptations to life with high
population densities of
Rivulus.
Population density:
We have recently published
two papers that document
that natural populations of
guppies in low predation
environments are indeed
density regulated (Reznick et
al., 2012, Evolution 66:
2903-2915) and that the
fitness difference between
Relative fitness of
guppy phenotype
guppies from HP and LP environments disappears when they are compared at population
densities that are typical of low predation environments (Bassar, R. D., A. LopezSepulcre, D. Reznick and J. Travis. 2012. American Naturalist, in press). The first article
was based on density manipulation experiments performed in natural streams. The second
article was based on a combination of such experiments and experiments done in our
artificial streams. The artificial stream experiment was a two-way design in which
guppies from HP and LP environments were kept at either high or low population
densities. The chosen densities represent the averages observed in an earlier study of the
comparative ecology of guppies from high and low predation environments [3]. Here we
illustrate a key result from that experiment, which was that the differences in fitness
between HP and LP guppies disappear at high population densities.
Here we illustrate the population growth rate (an index of fitness since it is a
function of survival, growth rate and fecundity) of HP and LP guppies from mesocosms
where they were kept at either high or low population densities. The actual values of
population growth rate are derived from the application of integral population growth
models [4]. The upper panels are calculated using the survival estimates from the density
manipulations in natural streams, assuming equal survivorships for HP and LP guppies.
The lower panels are based on survival estimated in the artificial streams. Panels A and
C are for guppies from the Guanapo River. Panels B and D are for guppies from the
Aripo River. Error bars are 1 standard error.
Note that adaptation to density was Pimentel’s original conceptualization of an
interaction between evolution and ecology, as encapsulated in his “population regulation
and genetic feedback” hypothesis [5]. At that stage, Pimentel was treating the ecosystem
as a black box and was implicitly assuming that high population densities were, in some
unspecified way, modifying their environment and hence the kind of selection the
organism experienced.
Guppy-Rivulus Interactions: Palkovacs et al. [6] reported on an earlier run of this
experiment. Here we designed an experiment that mimics the sequence of events
associated with the invasion of a previously guppy free headwater stream by guppies
derived from a high predation
1.25
environment. One treatment thus pairs
1.20
Rivulus from such a guppy free stream
(Rivulus only, or RO) with HP guppies.
1.15
The next treatment pairs RO Rivulus
1.10
with guppies derived from LP
1.05
environment and hence accommodates
1.00
guppy adaptation to the new setting. The
0.95
third treatment pairs LP guppies with
0.90
Rivulus from an environment where they
Rivulus Phenotype
RO
RO
LP
natural co-occur with guppies (LP
Guppy Phenotype
HP
LP
LP
Rivulus) and hence accommodates
adaptation of Rivulus to life with guppies. We already know from the earlier work by
Matt Walsh that Rivulus life histories evolve in response to interactions with guppies and
that the best explanation for this response is the indirect effect of guppies [7]. Guppies
reduce the population density of Rivulus and indirectly increase per capita resource
availability. Rivulus adapt to greater resource availability. Our key result, illustrated
above, is that LP guppies gain a decisive fitness advantage over LP guppies when they
are paired with Rivulus from an LP environment, where guppies and Rivulus are coadapted to one another. We presume that this result is a consequence of some form of
ecological character divergence between guppies and Rivulus. We hope to test this
hypothesis in the future.
Literature Cited
1.
2.
3.
4.
5.
6.
7.
Reznick, D., M. Bryant, and D. Holmes, The evolution of senescence and postreproductive lifespan in guppies (Poecilia reticulata). Plos Biology, 2006. 4(1): p.
136-143.
Reznick, D.N., et al., Effect of extrinsic mortality on the evolution of senescence
in guppies. Nature, 2004. 431(7012): p. 1095-1099.
Reznick, D.N., M.J.I. Butler, and F.H. Rodd, Life history evolution in guppies 7:
The comparative ecology of high and low predation environments. American
Naturalist, 2001. 157: p. 126-140.
Easterling, M.R., S.P. Ellner, and P.M. Dixon, Size-specific sensitivity: Applying a
new structured population model. Ecology, 2000. 81(3): p. 694-708.
Pimentel, D., Animal Population Regulation by Genetic Feedback Mechanism.
American Naturalist, 1961. 95(885): p. 65-&.
Palkovacs, E.P., et al., Experimental evaluation of evolution and coevolution as
agents of ecosystem change in Trinidadian streams. Philosophical Transactions of
the Royal Society B-Biological Sciences, 2009. 364(1523): p. 1617-1628.
Walsh, M.R. and D.N. Reznick, Experimentally induced life-history evolution in a
killifish in response to introduced guppies. Evolution, 2011. 65(4): p. 1021-1036.