Living in an urban pod: Seed predation and parasitism of bruchid beetles in a native tree species

1. Urbanisation is one of the main drivers of insect species loss worldwide. However, its impacts on ecological interactions involving insects still deserve further research, especially seed predation and parasitism of seed predators.
2. Here, we evaluated the seed predation rate by the specialist bruchid beetle Pseudopachymerina spinipes and its parasitism rate in the native tree Vachellia caven (Fabaceae) along an urbanisation gradient in Cordoba (Argentina). Since resource availability can influence these ecological interactions, we also investigated whether seed and prey availability could affect seed predation and parasitism rates, respectively.
3. We sampled trees in 10 sampling sites along an urbanisation gradient estimated by the Normalised Difference Vegetation Index (NDVI) within a 100 m radius. In our system, sites with low NDVI, beyond representing the low amount of vegetation cover, also indicate high surface temperature and low availability of host trees.
4. Seed predation in V. caven and the parasitism rate of P. spinipes were significantly reduced with increasing urbanisation. Notably, seed availability at the pod level did not affect seed predation rate, while prey availability was negatively correlated with parasitism rate.
5. These findings suggest a deleterious effect of urbanisation on the studied antagonistic interactions, giving no support to the idea of resource limitation effects.

     

Ecosystem engineering in a high and cold desert: Effect of a subterranean rodent on lizard abundance and behaviour

Subterranean rodents are often considered as ecosystem engineers because they physically modify the surrounding environment due to their burrowing and foraging activities. Understanding the modifications that ecosystem engineering species exert on the environment are of crucial importance in ecology studies, since they may affect the structure and population dynamics of several species, including lizards. Thus, the objective of the present study is to test the effect that Ctenomys mendocinus exert in the abundance of Liolaemus ruibali and its escape behaviour, in a high-elevation desert. Lizard abundance was estimated using observation transects and escape behaviour was studied with an experiment where the observer was considered by lizards as a potential predator and distance before the lizard flees was measured. All the variables were compared between areas disturbed by C. mendocinus and undisturbed ones. We found that L. ruibali was favoured by C. mendocinus activity. By creating burrow systems that serve as refuges for lizards, this rodent species increases the abundance of L. ruibali and reduces its flight distance, thereby improving its escape performance. We may suggest that C. mendocinus, through the construction of burrow systems, would be acting as an ecosystem engineer in Puna desert, affecting L. ruibali ecology.     

Invasion strategies of Cynodon dactylon: Competitive ability under low-nutrient conditions

Cynodon dactylon is one of the five most important invasive alien species worldwide. It is the invasive alien species with the broadest distribution range in Uruguay, and its expansion is frequently associated with disturbances. Since natural grasslands are facing processes of productive intensification, C. dactylon represents a threat as it could displace native species. However, the mechanisms that explain its invasion success remain unclear. The objective of this study was to analyse interspecific interactions under low nutrient conditions between C. dactylon and two species that are native to the Campo grasslands in Uruguay. Specifically, we assessed differences in the components of competitive ability effects and responses (or tolerance) as possible mechanisms involved in C. dactylon invasiveness. We performed a greenhouse experiment in pots with low-nutrient substrate assessing pair-wise interactions between C. dactylon, Mnesithea selloana and Paspalum notatum plus control pots consisting of single individual of each species. The invasive species showed greater competitive ability than both native grasses, as it reduced their below and above-biomass. Conversely, the size of C. dactylon plants interacting with native species was similar to that of single C. dactylon plants growing alone (controls). This reveals that the greater competitive ability of the invasive species was due to a greater tolerance to grow with neighbouring plants. The reason underlying this tolerance was a marked increase in biomass allocation towards stolons and leaves, at the expense of roots. Conversely, native species barely changed their shoot-root allocation pattern when interacting with neighbours. Furthermore, C. dactylon induced reproductive development solely when interacting with neighbours. Along with the fact that the potential growth rate of the invasive and native species was quite similar, these results suggest that sensitive and rapidly triggered shade avoidance responses could be one mechanism involved in the invasion success of C. dactylon.