Long‐term responses of zooplankton to invasion by a planktivorous fish in a subarctic watercourse
Summary (2 min read)
INTRODUCTION
- Of alien species represents one of the greatest threats to global biodiversity (Gido & Brown, 1999; Davis, 2003).
- Thus, based on short-term studies, it is not possible to conclude which mechanisms drive the changes in the prey community or to quantify the rates of change, precluding in-depth understanding and predictions.
- In addition to the numerical, direct effects of predation, prey populations may be affected by invasive predators via trait-mediated interactions (Bolker et al., 2003).
- Accordingly, in several cladoceran species a smaller size at first reproduction has been demonstrated to occur with increasing predation pressure from fish (Stibor, 1992; Vonder Brink & Vanni, 1993; Weber & Declerck, 1997).
METHODS
- The Pasvik water system (69oN, 29oE) originates from Lake Inari (1102 km2) in Finland, runs into Russia and then defines the border between Norway and Russia for a length of about 120 km.
- The lake is dimictic, oligotrophic and humic with a Secchi-depth from 2-4 m.
- From each sampling occasion, the body size of Daphnia spp. was measured under a binocular microscope with 40 X magnification on 50 females and, when possible, on up to 40 females with subitaneous eggs.
- Binary logistic regressions were performed with the number of eggs (one or two) as the dependent variable, and body length and year as independent variables.
RESULTS
- The total density of crustacean zooplankton showed a strong negative trend during the study period, including a sharp decrease from 1991 to 1993, followed by a recovery in abundance in 1995, and thereafter a steep decline towards 2002 (Fig. 2a).
- Excluding this species from the analyses gave a significant overall decline in the total density of other zooplankton species over the time period from 1991 to 2002 (Linear regression; P<0.05, R2=0.44).
- The most abundant cladocerans were Bosmina longirostris, Bosmina longispina and Daphnia cristata, whereas Daphnia longiremis, Daphnia longispina (O.F.M), Ceriodaphnia quadrangula (O.F.M), Holopedium gibberum Zaddach and Leptodora kindtii occurred more occasionally.
- The size at first reproduction decreased significantly over the study period for the two Bosmina species (Fig. 5; Table 2; logistic regression: P<0.001 for both species), with the largest decrease being observed in B. longispina.
- The size at first reproduction also decreased significantly in Daphnia spp. during the study, but with a less clear-cut trend than for the Bosmina species (Table 2; logistic regression: P<0.05).
DISCUSSION
- Throughout the whole study, the zooplankton community in Ruskebukta was dominated by small cladocerans, in particular D. cristata, B. longispina and B. longirostris.
- The present findings confirm that efficient and selective predation by vendace may have very strong effects on the density and composition of the zooplankton community, and also demonstrate the impact of an invading predator on its prey community.
- The size at first reproduction also decreased during the study period in all the examined cladoceran species, and B. longispina seemed to be the most affected.
- Whatever the actual mechanisms are in the present system, the observed decreases in size at first reproduction demonstrate severe impacts of the alien predator even on life history traits of the prey populations.
- The authors also acknowledge two anonymous referees for their helpful comments and suggestions.
Legends to figures:
- Relative contribution of vendace in gillnet catches from the pelagic fish community in Ruskebukta during the time period from 1991 to 2002.
- Stippled line represents asymptotic power curve fitted to the data.
- Fig. 2. Total density (a), species diversity (b) and relative species composition (c) of the crustacean zooplankton community in Ruskebukta during the study period.
- For equations and statistics of regression lines, see Table 1. Fig.
- Size at first reproduction (body length with 50% egg-bearing females, EL50) of B. longispina and B. longirostris during the study period.
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Citations
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Cites background from "Long‐term responses of zooplankton ..."
...In this study, we found that zooplankton body size and density decreased with increasing coregonid diversity, a pattern commonly observed in zooplankton communities when the number of specialized planktivorous fish species increases (Nilsson and Pejler 1973; Post et al. 2008; Amundsen et al. 2009)....
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Cites background from "Long‐term responses of zooplankton ..."
...(Bøhn and Amundsen 1998; Amundsen et al. 2009)....
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Cites background from "Long‐term responses of zooplankton ..."
...Long-term studies of fish and zooplankton following the vendace invasion have demonstrated a strong impact of the invader on the size-distribution and abundance of zooplankton [42], which in turn have resulted in a diet shift of the DR whitefish from zooplankton to zoobenthos combined with a competitive relegation from the pelagic to the littoral habitat [35,38,43]....
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References
366 citations
"Long‐term responses of zooplankton ..." refers background in this paper
...In Daphnia, maternal size strongly influences size at first reproduction: small-sized females produce small eggs hatching into small neonates, which 16 again are small when they reproduce for the first time (Ebert, 1993; Lampert, 1993; Tollrian, 1995)....
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...16 again are small when they reproduce for the first time (Ebert, 1993; Lampert, 1993; Tollrian, 1995)....
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351 citations
"Long‐term responses of zooplankton ..." refers background in this paper
...Cues associated with predation risk from fish are known to anticipate the onset of reproduction in order to increase the chance of reproducing successfully before being eaten (Taylor & Gabriel, 1992; Larsson & Dodson, 1993; Lass & Spaak, 2003)....
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...Reduced size at maturity is an effective way to cope with increased predation pressure from planktivorous fish, allowing individuals to start reproducing before they are exposed to strong size-selective predation (Taylor & Gabriel, 1992; Larsson & Dodson, 1993; Lass & Spaak, 2003)....
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348 citations
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"Long‐term responses of zooplankton ..." refers background in this paper
...In addition to the numerical, direct effects of predation, prey populations may be affected by invasive predators via trait-mediated interactions (Bolker et al., 2003)....
[...]
325 citations