Community Consequences of Biotic Invasions

We used multivariate autoregressive models with detailed time-series data from largely freshwater and brackish regions of the upper San Francisco Estuary to assess the topology, direction and strength of biotic interactions following major invasions and establishment of non-native zooplankton in the early 1990s. We found changes in the networks of biotic interactions in both regions after the major zooplankton invasions. Our results imply an increased pressure on native herbivores; intensified negative interactions between herbivores and omnivores; and stronger bottom-up influence of juvenile copepods but weaker influence of phytoplankton as a resource for higher trophic levels following the invasions. We identified salinity intrusion as a primary pressure but showed relatively stronger importance of biotic interactions for understanding the dynamics of entire communities. Our findings highlight the dynamic nature of biotic interactions and provide evidence of how simultaneous invasions of exotic species may alter interaction networks in diverse natural ecosystems over large spatial and temporal scales (Kratina et al. 2014).

We also studied the nutritional value of resources for pelagic fishes of the upper San Francisco Estuary to understand whether recent fish declines are liked to shifts in zooplankton nutritional conditions due to the establishment of invasive species (Kratina and Winder 2015). Using stable isotopes, elemental stoichiometry and fatty acid analyses for all dominant invasive and native zooplankton taxa and seston, we showed that zooplankton zooplankton taxa differ in their trophic position, energy sources, and biochemical and nutritional composition. Although we found that the shifts in taxonomic composition due to invasions were associated with long-term changes in nutritional status of the zooplankton community, the proportion of the essential fatty acids remained relatively high. This suggests that nutritional prey availability for fish remained unchanged with the shift in species composition (Kratina and Winder 2015).

Climate Change and Food Webs

Anthropogenic climate warming poses grave threats to ecosystems. In addition to its direct impacts, warming will modify ecosystems via synergistic or antagonistic interactions with other perturbations. Currently, it is unknown whether warming will counteract or intensify the impact of other anthropogenic stressors on stability of populations and communities. In a recent field study that simultaneously manipulated temperature, eutrophication, and the presence of fish predators, we showed that warming substantially strengthens trophic cascading effects of predatory fish on primary producers (Kratina et al. 2012). Importantly, warming also reduced the effects of eutrophication and reduced temporal variability of phytoplankton communities, in contrast with the current paradigm of synergistic interactions between warming and eutrophication. Warming, fish, and nutrients all independently enhanced the whole-system rates of net production despite their distinct impacts on the distribution of biomass among different food web compartments (Shurin et al. 2012).

We also investigated how these major global change drivers modify flow of biomass between aquatic and terrestrial ecosystems. We found that fish acted as an effective barrier to exchange of materials between the aquatic and terrestrial spheres while warming accelerated decomposition of terrestrial plant detritus and the emergence of amphibians and flying insects with aquatic larvae (Greig, Kratina et al. 2012). Furthermore, the effects of aquatic predators transcend beyond biotic boundaries and alter the emissions of carbon dioxide into the atmosphere (Atwood et al. 2013). These results indicate that warming exerts a host of indirect effects on global aquatic ecosystems mediated through shifts in the magnitudes of top-down and bottom-up forcing.

Stabilizing / Destabilizing Ecological Mechanisms

Predation occurs in a context defined by both prey species and associated taxa. As species outside of the predator-prey relationship are not directly involved in a particular trophic interaction, they have often been overlooked in food web theory. I experimentally showed that even a low density of a single non-prey species in the environment reduced the maximum rate at which predators attack and consume prey (Kratina et al. 2007). Interestingly, increasing non-prey diversity decreased predator feeding efficiency more strongly than the same number of non-prey individuals of a single species. This effect of diversity-modified predation is a novel mechanism that could enhance the prevalence of weak interaction strengths in nature and explain how diversity itself contributes to the stability and persistence of diverse communities. My colleagues and I also found that this weakening of interaction strengths caused by non-prey translates into long-term dynamics and stabilizes experimental food web modules (Hammill et al. 2015).

Inducible antipredator defenses occur across a wide variety of taxa and theory predicts that they stabilize predator-prey dynamics. We demonstrated empirically that more plastic prey species with greater ability to induce defensive morphology persist significantly longer than prey species with more fixed phenotypes (Kratina et al. 2010). This provides empirical evidence that stronger inducible defenses confer stability, and shows that the degree of plasticity is critical for ecological interactions. We also investigated multiple inducible defenses simultaneously and provided evidence of a co-specialised relationship between morphological and behavioral defenses (Hammill et al. 2009). Behavioral defenses are often quickly induced to rapidly reduce predation risk; however, they are relaxed as morphology changes are realized to avoid paying the cost of expressing both types of defense (Hammill et al. 2010). Incorporation of such biological realism into predator-prey models may improve our ability to predict complex food web dynamics.

Intraguild predation is a form of omnivory where a predator both consumes an intermediate prey and competes with it for shared resources. Although the double threat of competition and predation can have strong negative effects on intermediate prey, intraguild predation is common in nature (Kratina et al. 2012). I showed that more plastic species with a stronger tendency to induce an antipredator morphology also have a greater chance of persistence in food webs with intraguild predation (Kratina et al. 2010). Together with my colleagues, we also demonstrated in pond mesocosms and lake surveys that threespine stickleback (intermediate prey) evolve antipredator morphology and shift their diet to alternative resources in response to predation and competition from prickly sculpin (Ingram et al. 2012). These results show that both phenotypic plasticity and evolution of traits of intermediate prey dampen the strong unstable interactions with an intraguild predator. Furthermore, we found that these changes in response to intraguild predation dramatically altered the structure of whole food web, indicating that micro-evolutionary changes can have repercussions at the ecosystem level.