Ostracods vs. Cladocerans
Examining Competition Between Two Vernal Pool Crustaceans
Russell Croel  BIO 260  Spring 2012
INTRODUCTION
ABSTRACT
DISCUSSION
Ostracods and cladocerans are crustaceans that inhabit vernal pools. Both
taxa feed on planktonic algae, which
suggests that they compete with each
other for food. I used data from a
nutrient-addition experiment (Fig. 1) to
model competition between populations of these taxa. My goal was to
predict whether the populations would
coexist in the long term (> 8 weeks).
IWATER
used empirical
data to model competition between cladocerans and
CHEMISTRY
ostracods.
I estimated
population
carrying
capacities,
competition
In the high-dung
mesocosms,
water sizes,
chemistry
showed
classic signs
of
coefficients,
and intrinsic growth rates for both taxa, and plotted their
eutrophication.
population isoclines in Excel. My data revealed an unstable equilibrium,
with cladocerans owning the majority of phase-space. The populations’
combined trajectory extended into cladoceran phase-space after 8 weeks,
suggesting that ostracods were entering an extinction vortex. Subsequent
modeling in Mathematica showed that as long as their intrinsic growth rate
was greater than 0.95, cladocerans always drove ostracods to extinction.
This result may be related to cladocerans’ high parthenogenetic fecundity.
Although I calculated a stronger
competition coefficient for ostracods
(6.7, vs. 1.5 for cladocerans), these
models suggest that cladocerans are
the stronger competitor. This may be
due to the rapidity with which
cladocerans can reproduce parthenogenetically. For most of the season in
vernal pools, only female cladocerans
are present. Each female can live for
~ 50 days and produce weekly
clutches of young (~ 12 young/clutch;
Fig. 5). Ostracods also reproduce
parthenogenetically but not as quickly
as cladocerans.
RESULTS
Competition between the taxa was governed by an unstable equilibrium,
and the majority of phase-space belonged to cladocerans (Fig. 2).
Fig. 2 Isoclines for cladocerans (red line) and ostracods (blue line). The
dashed line represents the
boundary between “cladocerans only” and “ostracods
only” phase-space. The solid
black line shows the populations’ combined trajectory
and suggests that ostracods
were entering an extinction
vortex at the 8-week mark.
Fig. 1 Population growth curves from nutrient-addition
experiment. Growth was monitored for 8 weeks. With no
added nutrients, both taxa grew similarly. At high nutrient
levels, cladoceran density increased 800%, while ostracod
density doubled.
METHODS
In the experiment, I added cattle
waste to vernal pool mesocosms and
monitored cladoceran and ostracod
population growth for 8 weeks. I used
these data to:
Modeling in Mathematica suggested that cladocerans should quickly drive
ostracods to extinction (Fig. 3). Further, as long as r ≥ 0.95 for cladocerans,
ostracods were always driven to extinction (Fig. 4).
•Estimate N, K, α, and r for each
taxon
•Plot population isoclines in Excel
•Model long-term population dynamics in Mathematica
I assumed full competitive release of
both taxa in the high-nutrient treatment; i.e., I considered this treatment
the equivalent of monocultures.
Fig. 3 Population dynamics according to experimental data. Cladoceran r = 2.4.
Fig. 4 Population dynamics when cladoceran r =
0.95. Below this value, ostracods drove
cladocerans to extinction.
Fig. 5 Female cladoceran
carrying juveniles within her
carapace. The black dots in
her dorsal carapace are the
eyespots of juveniles. At
least 12 eyespots are visible
in this photo.
Both taxa suspension-feed on phytoplankton but ostracods also feed on
epiphytic algae and on other animals.
Phytoplankton are likely the first to
convert added nutrients to biomass,
giving an advantage to taxa adapted
to feeding solely on phytoplankton.
The models showed that ostracods
outcompeted cladocerans when the
latter had a relatively low r. However,
a low r is unlikely for natural cladoceran populations for the reasons
outlined above.
Cladocerans are commonly found in
high densities in many freshwater
bodies.
Their
high
fecundity
and
possibly
superior
competitive ability may be behind
their cosmopolitan distribution.