The results of our productivity–diversity experiments are largely consistent with the experimental and observational results for larger organisms (Mittelbach et al. 2001
), as well as more simplified microcosm communities (Kassen et al. 2000
; Hall & Colegrave 2007
; Benmayor et al. 2008
). Even though bacterial abundance increased across the productivity gradient, bacterial diversity increased slightly and then decreased with increasing productivity. That bacterial assemblages at high and low productivity are largely independent suggests a fitness trade-off between high and low productivity environments. At intermediate productivities, diversity is maximized because these assemblages start to cross over.
We have also demonstrated that predators of bacteria can have a wide variety of effects on bacterial diversity and community composition (). Such a result is unsurprising because previous studies have shown differential effects of protist predators (Simek et al. 1997
), but the result does emphasize the need to think critically about how different kinds of predator affect bacterial communities. Some of the principal ideas of how predators influence competing prey species revolve around whether particular prey taxa are removed preferentially. Assessing the preferences of protists feeding on diverse bacterial communities containing hundreds or thousands of taxa is problematic, especially as many of the bacteria cannot be isolated and cultured. Feeding trials with individual bacterial strains are therefore not possible in practice, so we determined which bacterial taxa were removed compared with a predator-free control.
The consequence of a uniform per capita attack rate is that the addition of the predator will not alter the relative abundance of the prey species in the short term because each species is equally affected by the predator. The data therefore suggest that Bodo and Cyclidium are acting as generalist predators, at least in our experimental communities (a,b). By contrast, Spumella removed the more dominant bacterial species to a much greater extent than the rarer taxa (c), apparently allowing some previously rare bacterial species to proliferate. Generalist predators would decrease the abundance of every prey species resulting in overall lower diversity but a similar pattern of diversity across a productivity gradient is unaltered. Specialist predators simply allow predator-resistant taxa to proliferate resulting in a relatively flat relationship. This suggests a trade-off between predator resistance and competitive ability.
The experiment is similar to a recent study that constructed microcosm communities with three trophic levels containing bacteria, bacteriovores and either a specialist or a generalist predator (Jiang & Morin 2005
). In apparent contrast to the current work, generalists reduced the variance in diversity across productivity levels, but the specialist had no effect. It is unclear how far these results can be generalized because the specialist predator had no apparent effect on community diversity, structure or density, whereas the generalist also consumed the prey's resource (bacteria). However, contrasting the two studies further emphasizes the importance of biological details in determining the impact of predators on community diversity.
The observations that predators had little effect on bacterial abundance at high productivity, and that predator abundance declined at high productivity despite higher bacterial abundance (electronic supplementary material, figure S1), strongly imply that the bacterial communities were able to withstand predation pressure. The results lend support to the hypothesis that the cost of resistance is lower at high productivity, resulting in bacterial communities that are able to resist protist predation. Such a conclusion is consistent with observations of the development of grazing-resistant forms across productivity gradients (Corno & Jürgens 2008
). A recent chemostat experiment also investigated the effect of nutrient and predator limitation (Corno & Jürgens 2008
). Although their experiment documented different patterns of predator and prey abundance across the productivity gradient (both increased linearly), they also concluded that the observed patterns of composition and diversity hinged on a trade-off between predator resistance and growth rate.
Overall, our findings are consistent with relatively simple underlying mechanisms: that there exist life-history trade-offs in natural bacterial communities, which allows predator-resistant prey to increase with resources availability. That relatively few assumptions are required suggest that these findings are likely to be relevant to a wide range of ecological communities and not only for bacterial communities. Moreover, we have shown that the interactive effects of key ecological variables can be studied in a meaningful way in natural microbial communities, containing largely ‘unculturable’ bacteria.