Food Supply Regimes and Individual Growth Histories
81
Threshold Food Level for Positive Individual Growth. Lowering the food
concentration from the incipient level and downwards will lead to a
proportional reduction in the ingestion rate. When the food concentration
is so low that assimilation is just adequate to cover the costs of basal
metabolism, an animal is said to have reached the threshold food level
[Crt; (mg C) rl] for positive individual growth (Lampert 1977). For a set of
zooplankton individuals competing for a common food resource, the individual with the lowest Crt will be superior, because it is able to lower the
food concentration to a level where the competitors are unable to capture
enough food to cover their maintenance costs (DeMott 1989). From the
model of Hebert (1982), it is expected that competition among zooplankton
will increase with increasing genetic similarity, and that it will be most
intense among individuals of the same species sharing the same niche.
Assimilation is just adequate to cover the costs of basal metabolism when
& I =r, or by inserting C = C' in Eq. (4.11) under the necessary assumption
that Crt S C',when
C N
el'-=r.
C'
( 4.12)
With C' being assumed independent of body size, Eq. (4.12) implies that
C'should increase with body size. Solving Eq. (4.12) with respect to Crt for
C' = 0.17 (mg C) r 1 and I' = 1'1 gives a threshold food concentration for
positive neonate growth C"} = 0.05 (mg C) r 1 with a l::110% increase from
neonate to primiparous adult. Lampert (1977) found a similar size dependence for D. pulex feeding on Scenedesmus, while the trend was reversed
when Stichococcus was used as food source. Gliwicz and Lampert (1990)
found higher threshold food levels for egg production than for positive
juvenile growth in several Daphnia species, indicating that juveniles might
be competitively superior to their mothers.
Estimates based on short-term radiotracer experiments (Lampert 1977),
egg production in natural popUlations (Lampert and Schober 1980), and
growth experiments under constant food conditions (Lampert and Muck
1985) are fairly consistent in indicating a threshold food concentration for
individual growth around 0.05 (mg C) r l for several Daphnia species
(D. pulex, D. longispina, and D. rosea). On the other hand, species like D.
pulicaria and D. hyalina seem to have threshold levels significantly below
0.05 (mg C) r
1 under optimal conditions (Gliwicz and Lampert 1990).
Food-Limited Daphnia Growth in Transfer Culture - a Case Study. Most
growth experiments with Daphnia under limiting food conditions have
been performed with transfer-culture methods; that is, animals are grown
individually in small vessels with transfer to fresh food preparations at
1-2-day intervals (Richman 1958; Paloheimo et a1. 1982; Lynch et a1. 1986;
Lynch 1989). A culture volume of 10 ml (as used by Richman 1958) might
be expected to be swept clear in a few hours by an adult Daphnia pulex,
81
Threshold Food Level for Positive Individual Growth. Lowering the food
concentration from the incipient level and downwards will lead to a
proportional reduction in the ingestion rate. When the food concentration
is so low that assimilation is just adequate to cover the costs of basal
metabolism, an animal is said to have reached the threshold food level
[Crt; (mg C) rl] for positive individual growth (Lampert 1977). For a set of
zooplankton individuals competing for a common food resource, the individual with the lowest Crt will be superior, because it is able to lower the
food concentration to a level where the competitors are unable to capture
enough food to cover their maintenance costs (DeMott 1989). From the
model of Hebert (1982), it is expected that competition among zooplankton
will increase with increasing genetic similarity, and that it will be most
intense among individuals of the same species sharing the same niche.
Assimilation is just adequate to cover the costs of basal metabolism when
& I =r, or by inserting C = C' in Eq. (4.11) under the necessary assumption
that Crt S C',when
C N
el'-=r.
C'
( 4.12)
With C' being assumed independent of body size, Eq. (4.12) implies that
C'should increase with body size. Solving Eq. (4.12) with respect to Crt for
C' = 0.17 (mg C) r 1 and I' = 1'1 gives a threshold food concentration for
positive neonate growth C"} = 0.05 (mg C) r 1 with a l::110% increase from
neonate to primiparous adult. Lampert (1977) found a similar size dependence for D. pulex feeding on Scenedesmus, while the trend was reversed
when Stichococcus was used as food source. Gliwicz and Lampert (1990)
found higher threshold food levels for egg production than for positive
juvenile growth in several Daphnia species, indicating that juveniles might
be competitively superior to their mothers.
Estimates based on short-term radiotracer experiments (Lampert 1977),
egg production in natural popUlations (Lampert and Schober 1980), and
growth experiments under constant food conditions (Lampert and Muck
1985) are fairly consistent in indicating a threshold food concentration for
individual growth around 0.05 (mg C) r l for several Daphnia species
(D. pulex, D. longispina, and D. rosea). On the other hand, species like D.
pulicaria and D. hyalina seem to have threshold levels significantly below
0.05 (mg C) r
1 under optimal conditions (Gliwicz and Lampert 1990).
Food-Limited Daphnia Growth in Transfer Culture - a Case Study. Most
growth experiments with Daphnia under limiting food conditions have
been performed with transfer-culture methods; that is, animals are grown
individually in small vessels with transfer to fresh food preparations at
1-2-day intervals (Richman 1958; Paloheimo et a1. 1982; Lynch et a1. 1986;
Lynch 1989). A culture volume of 10 ml (as used by Richman 1958) might
be expected to be swept clear in a few hours by an adult Daphnia pulex,