Scale dependent effects of native prey diversity, prey biomass and natural disturbance on the invasion success of an exotic predator

Reduced vertebrate diversity independent of spatial scale following feral swine invasions

Biological invasions often have contrasting consequences with reports of invasions decreasing diversity at small scales and facilitating diversity at large scales. Thus, previous literature has concluded that invasions have a fundamental spatial scale-dependent relationship with diversity. Whether the scale-dependent effects apply to vertebrate invaders is questionable because studies consistently report that vertebrate invasions produce different outcomes than plant or invertebrate invasions. Namely, vertebrate invasions generally have a larger effect size on species richness and vertebrate invaders commonly cause extinction, whereas extinctions are rare following invertebrate or plant invasions. In an agroecosystem invaded by a non-native ungulate (i.e., feral swine, Sus scrofa), we monitored species richness of native vertebrates in forest fragments ranging across four orders of magnitude in area. We tested three predictions of the scale-dependence hypothesis: (a) Vertebrate species richness would positively increase with area, (b) the species richness y-intercept would be lower when invaded, and (c) the rate of native species accumulation with area would be steeper when invaded. Indeed, native vertebrate richness increased with area and the species richness was 26% lower than should be expected when the invasive ungulate was present. However, there was no evidence that the relationship was scale dependent. Our data indicate the scale-dependent effect of biological invasions may not apply to vertebrate invasions.

Aquatic invasive species: challenges for the future

Humans have effectively transported thousands of species around the globe and, with accelerated trade; the rate of introductions has increased over time. Aquatic ecosystems seem at particular risk from invasive species because of threats to biodiversity and human needs for water resources. Here, we review some known aspects of aquatic invasive species (AIS) and explore several new questions. We describe impacts of AIS, factors limiting their dispersal, and the role that humans play in transporting AIS. We also review the characteristics of species that should be the greatest threat for future invasions, including those that pave the way for invasions by other species ("invasional meltdown"). Susceptible aquatic communities, such as reservoirs, may serve as stepping stones for invasions of new landscapes. Some microbes disperse long distance, infect new hosts and grow in the external aquatic medium, a process that has consequences for human health. We also discuss the interaction between species invasions and other human impacts (climate change, landscape conversion), as well as the possible connection of invasions with regime shifts in lakes. Since many invaders become permanent features of the environment, we discuss how humans live with invasive species, and conclude with questions for future research.

Can nutrients mask community responses to insecticide mixtures?

The ecological effect of simultaneous exposure to two nutrient gradients, three insecticides and different predator intensities was investigated over a 3-week period in 80 outdoor, artificial streams using field-collected benthic invertebrates. The experimental design consisted of a 2 × 5 factorial structure with two nutrient levels (oligotrophic or mesotrophic) and five concentrations of the ternary insecticide mixture consisting of the insecticides (chlorpyrifos, dimethoate and imidacloprid). Equivalent toxic unit doses were summed to create a ternary insecticide dose (e.g., 0.1 + 0.1 + 0.1 = 0.3 TU) resulting in a range of ternary insecticide mixture toxicity (i.e., control groundwater, 0.3, 0.6, 0.9 and 1.2 TU). Two genera of insect predators, Gomphus spp. (Odonata) and Agnetina spp. (Plecoptera) were also added into each replicate stream, at densities and sizes comparable to those found at our collection site, to evaluate how the contribution of predators may change in nutrient limited (oligotrophic) versus amended (mesotrophic) systems. We describe a causal mechanism whereby the combined action of nutrients and insecticides reshaped aquatic community structure by interacting through multiple pathways. Specifically, mesotrophic conditions reduced the toxic effects of ternary insecticide mixtures for aquatic insects which, in some cases, appeared to increase abundance of aquatic insects. However, higher levels of insecticides in mesotrophic streams negated this effect and were even more toxic; for example, to aquatic insect grazers than the same insecticide doses in oligotrophic treatment levels. Effects of predators were only significant in oligotrophic streams. Evidence is provided as to how nutrient and contaminant interactions can greatly complicate the assessment of community level responses to insecticide mixtures due to direct and indirect effects of the resulting changes in the density of different genera and functional feeding groups within a community.

Comparing the invasibility of experimental "reefs" with field observations of natural reefs and artificial structures

Natural systems are increasingly being modified by the addition of artificial habitats which may facilitate invasion. Where invaders are able to disperse from artificial habitats, their impact may spread to surrounding natural communities and therefore it is important to investigate potential factors that reduce or enhance invasibility. We surveyed the distribution of non-indigenous and native invertebrates and algae between artificial habitats and natural reefs in a marine subtidal system. We also deployed sandstone plates as experimental 'reefs' and manipulated the orientation, starting assemblage and degree of shading. Invertebrates (non-indigenous and native) appeared to be responding to similar environmental factors (e.g. orientation) and occupied most space on artificial structures and to a lesser extent reef walls. Non-indigenous invertebrates are less successful than native invertebrates on horizontal reefs despite functional similarities. Manipulative experiments revealed that even when non-indigenous invertebrates invade vertical "reefs", they are unlikely to gain a foothold and never exceed covers of native invertebrates (regardless of space availability). Community ecology suggests that invertebrates will dominate reef walls and algae horizontal reefs due to functional differences, however our surveys revealed that native algae dominate both vertical and horizontal reefs in shallow estuarine systems. Few non-indigenous algae were sampled in the study, however where invasive algal species are present in a system, they may present a threat to reef communities. Our findings suggest that non-indigenous species are less successful at occupying space on reef compared to artificial structures, and manipulations of biotic and abiotic conditions (primarily orientation and to a lesser extent biotic resistance) on experimental "reefs" explained a large portion of this variation, however they could not fully explain the magnitude of differences.