Shane A. Blowes

Heterospecific Aggression and Dominance in a Guild of Coral-Feeding Fishes: The Roles of Dietary Ecology and Phylogeny

Interspecific competition mediates biodiversity maintenance and is an important selective pressure for evolution. Competition is often conceptualized as being exploitative (indirect) or involving direct interference. However, most empirical studies are phenomenological, focusing on quantifying effects of density manipulations, and most competition theory has characterized exploitation competition systems. The effects on resource use of traits associated with direct, interference competition has received far less attention. Here we examine the relationships of dietary ecology and phylogeny to heterospecific aggression in a guild of corallivorous reef fishes. We find that, among chaetodontids (butterflyfishes), heterospecific aggression depends on a synergistic interaction of dietary overlap and specialization: aggression increases with dietary overlap for interactions between specialists but not for interactions involving generalists. Moreover, behavioral dominance is a monotonically increasing function of dietary specialization. The strong, positive relationship of dominance to specialization suggests that heterospecific aggression may contribute to the maintenance of biodiversity where it promotes resource partitioning. Additionally, we find strong phylogenetic signals in dietary overlap and specialization but not behavioral dominance. Our results support the use of phylogeny as a proxy for ecological similarity among butterflyfishes, but we find that direct measures of dietary overlap and specialization predict heterospecific agression much better than phylogeny.

Risk spreading, connectivity, and optimal reserve spacing.

Two important processes determining the dynamics of spatially structured populations are dispersal and the spatial covariance of demographic fluctuations. Spatially explicit approaches to conservation, such as reserve networks, must consider the tension between these two processes and reach a balance between distances near enough to maintain connectivity, but far enough to benefit from risk spreading. Here, we model this trade-off. We show how two measures of metapopulation persistence depend on the shape of the dispersal kernel and the shape of the distance decay in demographic covariance, and we consider the implications of this trade-off for reserve spacing. The relative rates of distance decay in dispersal and demographic covariance determine whether the long-run metapopulation growth rate, and quasi-extinction risk, peak for adjacent patches or intermediately spaced patches; two local maxima in metapopulation persistence are also possible. When dispersal itself fluctuates over time, the trade-off changes. Temporal variation in mean distance that propagules are dispersed (i.e., propagule advection) decreases metapopulation persistence and decreases the likelihood that persistence will peak for adjacent patches. Conversely, variation in diffusion (the extent of random spread around mean dispersal) increases metapopulation persistence overall and causes it to peak at shorter inter-patch distances. Thus, failure to consider temporal variation in dispersal processes increases the risk that reserve spacings will fail to meet the objective of ensuring metapopulation persistence. This study identifies two phenomena that receive relatively little attention in empirical work on reserve spacing, but that can qualitatively change the effectiveness of reserve spacing strategies: (1) the functional form of the distance decay in covariance among patch-specific demographic rates and (2) temporal variation in the shape of the dispersal kernel. The sensitivity of metapopulation recovery and persistence to how covariance of vital rates decreases with distance suggests that estimating the shape of this function is likely to be as important for effective reserve design as estimating connectivity. Similarly, because temporal variation in dispersal dynamics influences the effect of reserve spacing, approaches to reserve design that ignore such variation, and rely instead on long-term average dispersal patterns, are likely to lead to lower metapopulation viability than is actually achievable.

Indirect benefits of high coral cover for non-corallivorous butterflyfishes

Extensive coral loss often leads to pronounced declines in the abundance of fishes, which are not necessarily limited to those fishes that are directly reliant on live coral for food or shelter. This study explored changes in the abundance of two non-corallivorous butterflyfish, Chaetadon auriga and Chaetodon vagabundus, during declines in coral cover at Lizard Island, northern Great Barrier Reef, caused by localised outbreaks of crown-of-thorns starfish (COTS). At North Reef, where COTS caused significant coral depletion, the abundance of C. auriga declined from 1995–1996 to 1997–1999, whereas abundance was unchanged at Washing Machine Reef, which was relatively unaffected by COTS. Abundance of C. vagabundus did not vary through the course of this study at either site. To better understand inter-specific differences in the responses of non-corallivorous butterflyfishes, feeding rates of C. auriga and C. vagabundus were quantified across sites with varying coral cover. Feeding rates of C. auriga were significantly and positively correlated with live coral cover. In contrast, feeding rates of C. vagabundus did not differ among sites with varying levels of live coral cover. This study shows that C. auriga is negatively affected by localised coral depletion, possibly because its prey is more abundant in coral-rich habitats. C. vagabundus, meanwhile, is generally unaffected by changes in coral cover. This study stresses the need for more detailed research in light of current and predicted declines in coral cover to elucidate specific differences in the dietary composition of C. auriga versus C. vagabundus, and the extent to which their prey is actually reliant on live coral.

Embracing scale-dependence to achieve a deeper understanding of biodiversity and its change across communities

Because biodiversity is multidimensional and scale-dependent, it is challenging to estimate its change. However, it is unclear (1) how much scale-dependence matters for empirical studies, and (2) if it does matter, how exactly we should quantify biodiversity change. To address the first question, we analysed studies with comparisons among multiple assemblages, and found that rarefaction curves frequently crossed, implying reversals in the ranking of species richness across spatial scales. Moreover, the most frequently measured aspect of diversity - species richness - was poorly correlated with other measures of diversity. Second, we collated studies that included spatial scale in their estimates of biodiversity change in response to ecological drivers and found frequent and strong scale-dependence, including nearly 10% of studies which showed that biodiversity changes switched directions across scales. Having established the complexity of empirical biodiversity comparisons, we describe a synthesis of methods based on rarefaction curves that allow more explicit analyses of spatial and sampling effects on biodiversity comparisons. We use a case study of nutrient additions in experimental ponds to illustrate how this multi-dimensional and multi-scale perspective informs the responses of biodiversity to ecological drivers.

Global reef fish richness gradients emerge from divergent and scale-dependent component changes

Biodiversity varies from place to place due to environmental and historical factors. To improve our understanding of how history and the environment influence observed patterns, we need to address the limitations of the most commonly used biodiversity metric, species richness. Here, we show that scale-dependent dissections of species richness into components of total abundance, species relative abundances and spatial aggregations of species reveal that two well-known biogeographic reef fish species richness gradients emerge from very different underlying component patterns. Latitudinal richness is underpinned by scale-independent patterns of total and relative abundances, suggesting ecological constraints scale up to determine abundances within communities. In contrast, the longitudinal gradient of species richness typically attributed to historical biogeography only emerges at the largest scale and is accompanied by a similar pattern of relative abundances, suggesting that site-to-site compositional variation leading to species aggregation (i.e. a component of β-diversity) underlies this gradient. Examining relationships among the components that underpin biodiversity gradients reveals new patterns that can better identify processes influencing patterns of biodiversity.

Macroecology to Unite All Life, Large and Small

Macroecology is the study of the mechanisms underlying general patterns of ecology across scales. Research in microbial ecology and macroecology have long been detached. Here, we argue that it is time to bridge the gap, as they share a common currency of species and individuals, and a common goal of understanding the causes and consequences of changes in biodiversity. Microbial ecology and macroecology will mutually benefit from a unified research agenda and shared datasets that span the entirety of the biodiversity of life and the geographic expanse of the Earth.

MoB (Measurement of Biodiversity): a method to separate the scale‐dependent effects of species abundance distribution, density, and aggregation on diversity change

1Biology Department, College of Charleston, Charleston, South Carolina; 2School of Biology and Ecology, and Senator George J. Mitchell Center of Sustainability Solutions, University of Maine, Orono, Maine; 3Leuphana University Lüneburg, Lüneburg, Germany; 4German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Leipzig, Germany; 5Department of Biology, University of Vermont, Burlington, Vermont; 6Institute of Biology, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany; 7Department of Community Ecology, Helmholtz Centre for Environmental Research – UFZ, Halle (Saale), Germany and 8Department of Computer Science, Martin Luther University, Halle-Wittenberg, Leipzig, Germany

Aggression, interference, and the functional response of coral-feeding butterflyfishes

Functional responses describing how foraging rates change with respect to resource density are central to our understanding of interspecific interactions. Competitive interactions are an important determinant of foraging rates; however, the relationship between the exploitation and interference components of competition has received little empirical or theoretical consideration. Moreover, little is known about the relationship between aggressive behavioural interactions and interference competition. Using a natural gradient of consumer and resource densities, we empirically examine how aggressiveness relates to consumer–consumer encounter rates and foraging for four species of Chaetodon reef fish spanning a range of dietary niche breadths. The probability of aggression was most strongly associated with both total consumer and resource densities. In contrast, total encounter rates were best predicted by conspecific consumer density, and were highest for the most specialised consumer (Chaetodon trifascialis), not the most aggressive (Chaetodon baronessa). The most specialised consumer, not the most aggressive, also exhibited the largest reduction in foraging rates with increasing consumer density. Our results support the idea of a positive link between the exploitation and interference components of competition for the most specialised consumer. Moreover, our results caution against inferring the presence of ecological interactions (competition) from observations of behaviour (aggression and agonism) alone.

No change in subordinate butterflyfish diets following removal of behaviourally dominant species

Direct interference interactions between species are often mediated by aggression and related to resource use. Interference interactions are frequently asymmetric, whereby one species wins the majority of interactions; however, the effect of this asymmetry on the diet of subordinate species has not received the same attention as the impact of interference on habitat use. Here we experimentally evaluated whether release from asymmetric interference led to increased use of a preferred dietary resource by subordinate species, using coral-feeding butterflyfishes as a model system. Following experimental removal of the behaviourally dominant species, we found no change in diet breadth or foraging on the preferred resource by subordinate species. Our results suggest that release from asymmetric interspecific interference does not necessarily result in changes to subordinate species’ diets, at least not over the course of our study. Rather, consistently asymmetric interactions may contribute to behavioural conditioning of subordinate species, meaning that even in the absence of dominants, subordinate individuals maintain established feeding patterns. Additionally, our results suggest that antagonistic interactions between butterflyfishes may have contributed to niche partitioning and conservatism over evolutionary time scales.

Remarkable size-spectra stability in a marine system undergoing massive invasion

The Mediterranean Sea is an invasion hotspot, with non-indigenous species suspected to be a major driver behind community changes. We used size spectra, a reliable index of food web structure, to examine how the influx of Red Sea fishes into the Mediterranean Sea has impacted the indigenous species community. This is the first attempt to use changes in the size spectra to reveal the effect of biological invasions. We used data from trawl catches along Israels shoreline spanning 20 years to estimate changes in the community size spectra of both indigenous and non-indigenous species. We found that the relative biomass of non-indigenous species increased over the 20 years, especially for small and large species, leading to a convergence with the indigenous species size spectra. Hence, the biomass of indigenous and non-indigenous species has become identical for all size classes, suggesting similar energetic constraints and sensitivities to fishing. However, over this time period the size spectrum of indigenous species has remained remarkably constant. This suggests that the wide-scale invasion of non-indigenous species into the Mediterranean may have had little impact on the community structure of indigenous species.