AUTECOLOGY

Assessing the impacts of invasive organisms is a major challenge in ecology. Some widespread invasive species such as crayfish are potential competitors and reciprocal predators of ecologically and recreationally important native fish species. Here, we examine the effects of signal crayfish (Pacifastacus leniusculus) on the growth, diet, and trophic position of the chub (Squalius cephalus) in four rivers in Britain. Growth rates of 0+ chub were typically lower in sympatric populations with signal crayfish compared with allopatric populations, and this effect could be traced through to 2+ chub in one river. However, growth rates of older chub (5+ to 6+) were typically higher in the presence of crayfish. Sympatry with crayfish resulted in lower chub length-at-age and mass-at-age in half of the rivers sampled, with no change detected in the other rivers. Stable isotope analyses (δ13C and δ15N) revealed that both chub and crayfish were omnivorous, feeding at multiple trophic levels and occupying similar trophic positions. We found some evidence that chub trophic position was greater at invaded sites on one river, with no difference detected on a second river. Mixing models suggested crayfish were important food items for both small and large chub at invaded sites. This study provides evidence that invasive species can have both positive and negative effects on different life stages of a native species, with the net impact likely to depend on responses at the population level.

Understanding the limits to species-wide demographic generalizations: the ecology and management of Parkinsonia aculeata. Abstract. The search for generalizations in the face of complex species-environment interactions is particularly important for minimizing the cost of managing populations of species. We tested whether we could generalize, at various nested scales, the species-level demography of the widely invasive plant species Parkin-sonia aculeata (Fabaceae) and whether these generalizations were representative of the demography observed locally. Full demographic surveys at all life stages of the species were conducted in 23 Australian sites during seven years (from 2000 to 2007), across a 1000-km climatic gradient. Sites were nested across four climate regions (arid, semi-arid, semi-wet/dry tropics, wet/dry tropics) and three habitat types (upland, wetland, and riparian). We estimated the vital rates (growth/retrogression, survival, fecundity) at all life stages and size classes and combined them to create 99 site-year demographic matrix population models. With these models , we then estimated site-year-specific asymptotic population growth rates and their corresponding prospective elasticity values to perturbation of the vital rates. We then developed a nested retrospective elasticity analysis (nested LTRE) to test whether and how upscaling the results (i.e., from site to habitat, to climate region, and to the invaded range) produced averaging bias, which could lead to spurious interpretations of the relationships between the retrospective elasticity values. The prospective analysis highlighted that site-year variation in the matrix population models, population growth rates, and corresponding elasticities could not be well summarized by a single species-level analysis and that the spread was as diverse as found in previously reported multispecies-level demographic analyses. The nested LTRE analysis showed that upscaling demographic models introduced for most sites new sources of errors in the estimation (in terms of magnitude and sign of retrospective elasticities), which increased drastically as we progressively aggregate the demographic information between nested scales of observation. Our findings suggest that regardless of the scale, demographic generalizations at the species scale are not always useful for managing P. aculeata across sites in its invaded range, given its plasticity in demography.