The Shurin (2001) study considered the roles of predation and dispersal on the s

Question

The Shurin (2001) study considered the roles of predation and dispersal on the species richness of zooplankton communities. Shurin also measured the abundance of phytoplankton, which is eaten by the zooplankton. The figure below shows the amount of chlorophyll a (a measure of phytoplankton abundance) in the four predation treatments, with and without dispersal, imposed in the experimental ponds: (1) no predators, (2) fish predators only (juvenile bluegill sunfish, Lepomis macrochirus), (3) insect predators only (the backswimmer bug Notonecta undulata), and (4) both fish and insect predators.

Question 1. How did predation alone affect the abundance of phytoplankton within the ponds? Give a plausible explanation for why this occurred. Did fish and insect predators have different effects on phytoplankton abundance without dispersal?

Question 2. How does phytoplankton abundance change with the addition of zooplankton dispersal into the ponds? Without knowing anything about zooplankton abundance in the ponds, can you say what these results suggest about the dual effects of predation and zooplankton dispersal on phytoplankton abundance?

No dispersal Dispersal Control FishInsects Fish and insects (no predators) Predation treatment Note: for the control treatment, there was no statistical difference in phytoplankton abundance in the ponds with and without zooplankton dispersal

Explanation / Answer

The present study suggests that the effects of predators on local diversity also depend on the traits of species in the regional pool and their rates of dispersal among sites. Invasion of predators such as fish may reduce prey diversity in the short term. However, colonization by species from nearby fish ponds may compensate for the initial loss of species and result in a long-term positive effect if there is sufficient dispersal among local habitats. The outcome of other types of local interactions may also depend on spatial heterogeneity and movement of species at broad regional scales. The hypothesis that invasion by members of the regional pool would compensate for the productivity that was lost to predators, and thereby weaken the control of predators over a trophic structure, was not supported. The effects of predators on zooplankton mean body size, community biomass, and phytoplankton biomass were independent of the dispersal treatment. Both predators reduced zooplankton mean body size, although fish had larger effects than notonectids. This result is consistent with a number of studies showing that size-selective predation shifts the size structure of zooplankton communities in favour of smaller species (Brooks and Dodson 1965, Hall et al. 1976, Carpenter and Kitchell 1993, Brett and Goldman 1997). There was a nearly significant (P 5 0.052) positive main effect of the dispersal treatment on zooplankton size. This effect arose because zooplankton in the fish treatment were, on average, 0.14 mg larger with dispersal than without. Zooplankton were very slightly (0.04 and 0.005 mg) smaller with dispersal in the control and notonectid treatments, respectively. This result suggests that the species that were facilitated by fish may have shifted the size structure of the zooplankton community toward larger-bodied species to a small degree. The difference in mean size may also account for the apparent negative effect of the dispersal treatment on phytoplankton biomass. However, the effects of dispersal were slight compared to the reduction in mean body size by fish or notonectids, or the effect of fish on phytoplankton. These results suggest that the species that were negatively affected by predators were functionally distinct from those that were facilitated in that they had greater impacts on zooplankton size structure and phytoplankton biomass. Although both predators consistently reduced mean zooplankton body size, their effects and those of the dispersal treatment on community biomass were somewhat idiosyncratic. This result is similar to those found by other studies, including Soranno et al. (1993:182) who stated, ‘‘More often than not, total biomass responded oppositely to predictions.’’ Soranno et al. propose a number of plausible explanations for the variable effects of planktivores on community biomass, and argue that mean zooplankton body size responds much more predictably to planktivory than biomass, and is a better indicator of grazing (see also Pace 1984). Similarly, in this experiment, both predators reduced zooplankton body size while only notonectids affected total biomass. The effect of notonectids appeared to be driven by the pattern that biomass was greater in the no-dispersal treatment in both the control and fish treatments. There was a nearly significant negative main effect of dispersal on biomass. The reason for the apparent increase in zooplankton biomass in the absence of dispersal is unknown. However, the results of this study support the contention of Soranno et al. (1993) that zooplankton community biomass responds in complex and sometimes counterintuitive ways to changes in predation. Although invasion by members of the regional pool did not weaken the effects of fish on trophic structure, this result depended on both the initial local assemblage and the regional pool from which colonists were drawn. The mean initial local richness of 9.5 crustaceans species per tank was representative of natural fishless ponds in the area (a mean of 8.3 and a range of 4–16 from 11 ponds, Shurin 2000a), while the number of rotifers (2.5 per tank) was relatively low (mean 5 10.9, range 5 4–16). The low rotifer diversity may be due to the absence of major zooplankton predators in the initial assemblage. Drawing species for the high-dispersal treatment from a larger regional pool would have increased the number of potential invaders, and possibly the impact of the dispersal treatment on trophic structure as well (Shurin et al. 2000). Zooplankton for the high-dispersal treatment were collected from a mean of 8.2 ponds within an area of; 100 km. Since the distances over which species disperse is unknown, the spatial extent of the regional pool could not be reliably defined. However, previous work (Shurin 2000a) suggested high rates of dispersal among eleven ponds within a 100 km region. Indirect facilitation of small species by fish took place either through suppression of competitors or intermediate predators, or by some combination of the two. The species that declined in the fish treatment were almost entirely large cladocerans and cyclopoid copepods. Large zooplankton consume a broader size spectrum of particles and have lower threshold food density below which they cannot maintain metabolic demands (Burns 1968, Hall et al. 1976, Gliwicz 1990). Cyclopoids such as Acanthacyclops vernalis and Eucyclops agilis are omnivorous, preying on protozoans and rotifers as adults, however most stages are primarily herbivorous (Adrian and Frost 1993). In addition, fish enhanced phytoplankton biomass, suggesting that more resources were available to invaders in the fish treatment These results indicate that the patterns associated with the dispersal treatment resulted from invasions by new species and not from sink effects due to the individuals added in the inoculum. The two predators influenced planktonic community structure and invasibility differently, indicating that fish and notonectids are functionally distinct in their effects on lower trophic levels. Fish had greater impacts on the densities of large zooplankton and reduced mean body size to a greater extent. These effects resulted in a larger increase in phytoplankton biomass and greater facilitation of invaders in the dispersal treatment. Fish and notonectids had nonadditive effects on zooplankton body size (i.e., the effects of the two predators together were smaller than expected based on their independent effects). This likely reflects a reduced impact of notonectids in the presence of fish, probably due to consumption of juvenile notonectids by fish. Although both predators are size selective, preferentially consuming large-bodied zooplankton, the magnitude of the effects of fish were generally greater, indicating that fish may be more important than notonectids in promoting regional coexistence among zooplankton. This study suggests that regional processes play an important role in determining the outcome of local interactions such as predation in communities. Studies of the effects of species diversity on ecosystem function and response to environmental change have focussed largely on the importance of diversity at the local scale (e.g., Hooper and Vitousek 1997, McGradySteed et al. 1997, Tilman 1999). However, immigration from a pool of species with diverse traits may buffer local communities against environmental changes such as the invasion of predators (Leibold et al. 1997). For instance, Hairston (1996) suggested that zooplankton egg banks represent reserves of genetic and species diversity that may be important for regulating the response of lake ecosystems to environmental fluctuations. Spatial heterogeneity in local conditions, such as refugia from predators, may serve a similar function. In the present study, dispersal by species from the regional pool reversed the negative effects of predators on local zooplankton diversity. Ecologists have long debated the question of what properties of communities influence their response to environmental perturbations. I argue that regional species diversity and composition affect local zooplankton assemblages and may, therefore, be important for determining the long-term effects of changes such as predator invasions. The isolation of local communities, for instance through habitat fragmentation, may, therefore, alter the impact of extrinsic perturbations even if it has no direct effect on the local biota.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.