Changes in Arthropod Assemblages along a Wide Gradient of Disturbance in Gabon

Abstract: Searching for indicator taxa representative of diverse assemblages, such as arthropods, is an important objective of many conservation studies. We evaluated the impacts of a wide gradient of disturbance in Gabon on a range of arthropod assemblages representing different feeding guilds. We examined 4 × 10⁵ arthropod individuals from which 21 focal taxa were separated into 1534 morphospecies. Replication included the understory of 3 sites in each of 4 different stages of forest succession and land use (i.e., habitats) after logging (old and young forests, savanna, and gardens). We used 3 complementary sampling methods to survey sites throughout the year. Overall differences in arthropod abundance and diversity were greatest between forest and open habitats, and cleared forest invaded by savanna had the lowest abundance and diversity. The magnitude of faunal differences was much smaller between old and young forests. When considered at this local scale, anthropogenic modification of habitats did not result in a monotonous decline of diversity because many herbivore pests and their associated predators and parasitoids were abundant and diverse in gardens, where plant productivity was kept artificially high year‐round through watering and crop rotation. We used a variety of response variables to measure the strength of correlations across survey locations among focal taxa. These could be ranked as follows in terms of decreasing number of significant correlations: species turnover > abundance > observed species richness > estimated species richness > percentage of site‐specific species. The number of significant correlations was generally low and apparently unrelated to taxonomy or guild structure. Our results emphasize the value of reporting species turnover in conservation studies, as opposed to simply measuring species richness, and that the search for indicator taxa is elusive in the tropics. One promising alternative might be to consider “predictor sets” of a small number of taxa representative of different functional groups, as identified in our study.     

Susceptibility of non-indigenous ascidian species in British Columbia (Canada) to invertebrate predation

Non-indigenous ascidians are known to significantly alter the structure and composition of benthic communities and adversely affect shellfish aquaculture by fouling both the cultured species and the infrastructure. The ability of these species to persist in new locations and their current and potential distributions are dependent upon physiological tolerances to environmental factors and biotic resistance to competition and predation. Despite significant data on global invasion patterns, potential biotic resistance to non-indigenous ascidians is poorly understood. We identified potential predators of four non-indigenous ascidians (Styela clava, Botryllus schlosseri, Botrylloides violaceus, and Didemnum vexillum) in British Columbia (BC), Canada in order to: (1) assess the potential for biotic interference to limit the establishment and/or spread of these ascidian species in BC, and (2) identify candidate species to be used as ascidian biofouling control agents in shellfish aquaculture. Using a series of single- and multiple-choice laboratory experiments, potential benthic predators (including various species of molluscs, echinoderms, and arthropods) were offered non-indigenous ascidians as prey. The sea urchins Strongylocentrotus droebachiensis and Strongylocentrotus franciscanus, the sea stars Dermasterias imbricata and Evasterias troschelii, the nudibranch Hermissenda crassicornis, and the crabs Cancer productus and Carcinus maenas were found to consume one or more species of non-indigenous ascidians in single-choice experiments. However, when provided a choice, all predators chose their respective preferred food over ascidians. Thus, predation alone is unlikely to prevent large-scale establishment and spread of non-indigenous ascidians in BC, but it may have the potential to significantly reduce localized populations of ascidians. Green sea urchins, S. droebachiensis, were found to be efficient grazers of all four ascidian species, consuming 12.7 ± 5.14 cm² (mean ± SD) of adult B. violaceus over a 3-day period, 15 ± 3.7 juvenile colonies of B. violaceus over a 2-day period, and 63 ± 28.8 juvenile colonies of B. schlosseri over a 2-day period. Using sea urchins as biological control organisms may significantly reduce ascidian fouling in shellfish aquaculture.     

Bachelor and paired male rufous-and-white wrens use different singing strategies

To attract a breeding partner, males may behave differently when they are bachelors compared to when they are paired. Comparisons between groups of paired males versus groups of unpaired males in temperate-breeding animals have revealed such differences in signalling behaviour. Few studies, however, have explored how individual males alter their signalling behaviour with changes in pairing status, and very few investigations have explored paired versus unpaired male behaviour in tropical animals. During a 5-year study in Costa Rica, we analysed changes in the singing behaviour of male rufous-and-white wrens (Thryothorus rufalbus) when they were paired and when they were bachelors. We compared three aspects of male vocal behaviour: gross differences in song output, variation in repertoire use and differences in song structure. Males as bachelors had significantly higher song output and switched song types less frequently. Contrary to our expectation, bachelors sang significantly fewer song types from their repertoire compared to when those same males had a breeding partner. Songs sung by bachelor males were higher in syllabic diversity and had broader-bandwidth terminal syllables than the songs those males sang only when paired. Within song types, the fine structure of songs remained consistent across pairing status. Our results demonstrate that males change their singing behaviour with pairing status, delivering songs at a higher rate but with less variety when they are bachelors. Rufous-and-white wrens are renowned for their vocal duets, and we discuss the pattern of repertoire use in light of their duetting behaviour. These results enhance our understanding of how male behaviour varies with pairing status and the importance of vocal signalling behaviour in socially monogamous tropical animals.