Predator complementarity dampens variability of phytoplankton biomass in a diversity-stability trophic cascade

Effects of protection and temperature variation on temporal stability in a marine reserve network

Understanding the drivers of ecosystem stability has been a key focus of modern ecology as the impacts of the Anthropocene become more prevalent and extreme. Marine protected areas (MPAs) are tools used globally to promote biodiversity and mediate anthropogenic impacts. However, assessing the stability of natural ecosystems and responses to management actions is inherently challenging due to the complex dynamics of communities with many interdependent taxa. Using a 12-year time series of subtidal community structure in an MPA network in the Channel Islands (United States), we estimated species interaction strength (competition and predation), prey species synchrony, and temporal stability in trophic networks, as well as temporal variation in sea surface temperature to explore the causal drivers of temporal stability at community and metacommunity scales. At the community scale, only trophic networks in MPAs at Santa Rosa Island showed greater temporal stability than reference sites, likely driven by reduced prey synchrony. Across islands, competition was sometimes greater and predation always greater in MPAs compared with reference sites. Increases in interaction strength resulted in lower temporal stability of trophic networks. Although MPAs reduced prey synchrony at the metacommunity scale, reductions were insufficient to stabilize trophic networks. In contrast, temporal variation in sea surface temperature had strong positive direct effects on stability at the regional scale and indirect effects at the local scale through reductions in species interaction strength. Although MPAs can be effective management strategies for protecting certain species or locations, our findings for this MPA network suggest that temperature variation has a stronger influence on metacommunity temporal stability by mediating species interactions and promoting a mosaic of spatiotemporal variation in community structure of trophic networks. By capturing the full spectrum of environmental variation in network planning, MPAs will have the greatest capacity to promote ecosystem stability in response to climate change.

Fungal pathogens increase community temporal stability through species asynchrony regardless of nutrient fertilization

Natural enemies and their interaction with host nutrient availability influence plant population dynamics, community structure, and ecosystem functions. However, the way in which these factors influence patterns of community stability, as well as the direct and indirect processes underlying that stability, remains unclear. Here, we investigated the separate and interactive roles of fungal/oomycete pathogens and nutrient fertilization on the temporal stability of community biomass and the potential mechanisms using a factorial experiment in an alpine meadow. We found that fungal pathogen exclusion reduced community temporal stability mainly through decreasing species asynchrony, while fertilization tended to reduce community temporal stability by decreasing species stability. However, there was no interaction between pathogen exclusion and nutrient fertilization. These effects were largely due to the direct effects of the treatments on plant biomass and not due to indirect effects mediated through plant diversity. Our findings highlight the need for a multitrophic perspective in field studies examining ecosystem stability.

Will a large complex system be productive?

While the relationship between food web complexity and stability has been well documented, how complexity affects productivity remains elusive. In this study, we combine food web theory and a data set of 149 aquatic food webs to investigate the effect of complexity (i.e. species richness, connectance, and average interaction strength) on ecosystem productivity. We find that more complex ecosystems tend to be more productive, although different facets of complexity have contrasting effects. A higher species richness and/or average interaction strength increases productivity, whereas a higher connectance often decreases it. These patterns hold not only between realized complexity and productivity, but also characterize responses of productivity to simulated declines of complexity. Our model also predicts a negative association between productivity and stability along gradients of complexity. Empirical analyses support our predictions on positive complexity-productivity relationships and negative productivity-stability relationships. Our study provides a step forward towards reconciling ecosystem complexity, productivity and stability.

Beyond the fish-Daphnia paradigm: testing the potential for pygmy backswimmers (Neoplea striola) to cause trophic cascades in subtropical ponds

Trophic cascades, or indirect effects of predators on non-adjacent lower trophic levels, are a classic phenomenon in ecology, and are thought to be strongest in aquatic ecosystems. Most research on freshwater trophic cascades focused on temperate lakes, where fish are present and where Daphnia frequently dominate the zooplankton community. These studies identified that Daphnia often play a key role in facilitating trophic cascades by linking fish to algae with strong food web interactions. However, Daphnia are rare or absent in most tropical and subtropical lowland freshwaters, and fish are absent from small and temporary water bodies, where invertebrates fill the role of top predator. While invertebrate predators are ubiquitous in freshwater systems, most have received little attention in food web research. Therefore, we aimed to test whether trophic cascades are possible in small warmwater ponds where Daphnia are absent and small invertebrates are the top predators. We collected naturally occurring plankton communities from small fishless water bodies in central Texas and propagated them in replicate pond mesocosms. We removed zooplankton from some mesocosms, left the plankton community intact in others, and added one of two densities of the predaceous insect Neoplea striola to others. Following an incubation period, we then compared biomasses of plankton groups to assess food web effects between the trophic levels, including whether Neoplea caused a trophic cascade by reducing zooplankton. The zooplankton community became dominated by copepods which prefer large phytoplankton and exhibit a fast escape response. Perhaps due to these qualities of the copepods and perhaps due to other reasons such as high turbidity impairing predation, no evidence for food web effects were found other than somewhat weak evidence for zooplankton reducing large phytoplankton. More research is needed to understand the behavior and ecology of Neoplea, but trophic cascades may generally be weak or absent in fishless low latitude lowland water bodies where Daphnia are rare.