Spatial Guilds Research Articles

Toward Ecosystem-Based Assessment and Management of Small-Scale and Multi-Gear Fisheries: Insights From the Tropical Eastern Pacific

Small-scale fisheries (SSF) remain a largely under-assessed and overlooked sector by governments and researchers, despite contributing approximately 50% to global fish landings and providing food and income for millions of people. The multi-species, multi-gear and data-poor nature of SSF makes implementation of traditional single-species management approaches - like catch-quotas or size limits - particularly challenging and insufficient. A more holistic approach is thus required, which demands assessment of ecological impacts. Here we carried out an estimation of selected ecological indicators of the impact of fisheries (mean length, maximum body size, mean trophic level, trophic and spatial guilds, threatened species and landed by-catch) based on the nominal catch of different gears in three representative SSF along the Colombian Pacific using landings data collected in multiple years (2011 – 2017). Results showed that taxonomic, size-based, functional and conservation features of the nominal catch vary greatly with geographical location and gear type used. Overall, handlines and longlines tend to select larger sizes and higher trophic levels than nets, but they also catch a higher proportion of intrinsically vulnerable species and species of conservation concern. This challenges the idea that more selective gears have overall lower ecological impacts. In contrast, nets target a wider size range – although focusing on small or medium sized fish - and include a higher diversity of trophic and spatial guilds, which could arguably be considered a more “balanced harvest” type of fishing that retains ecosystem structure and functionality. Bottom-trawls, though, exhibited a relatively high percentage of landed by-catch, an undesirable feature for any fisheries in terms of sustainability. We propose that the assessment of a suite of ecological indicators, like those implemented here, should be included as part of periodic evaluations of multi-gear and multi-species SSF in tropical coastal areas, as a practical step towards ecosystem-based fisheries management.

More than the sum of the parts: annual partitioning within spatial guilds underpins community regulation

To withstand the pressures of a rapidly changing world, resilient ecosystems should exhibit compensatory dynamics, including uncorrelated temporal shifts in population sizes. The observation that diversity is maintained through time in many systems is evidence that communities are indeed regulated and stabilized, yet empirical observations suggest that positive covariance in species abundances is widespread. This paradox could be resolved if communities are composed of a number of ecologically relevant sub-units in which the members compete for resources, but whose abundances fluctuate independently. Such modular organization could explain community regulation, even when the community as a whole appears synchronized. To test this hypothesis, we quantified temporal synchronicity in annual population abundances within spatial guilds in an estuarine fish assemblage that has been monitored for 36 years. We detected independent fluctuations in annual abundances within guilds. By contrast, the assemblage as a whole exhibited temporal synchronicity-an outcome linked to the dynamics of guild dominants, which were synchronized with each other. These findings underline the importance of modularity in explaining community regulation and highlight the need to protect assemblage composition and structure as well as species richness.

Morphology and microchemistry of the otoliths of the inner ear of anuran larvae

To navigate in space most vertebrates need precise positional cues provided by a variety of sensors, including structures in the inner ear, which are exquisitely sensitive to gravity and linear acceleration. Although these structures have been described in many vertebrates, no information is available for anuran larvae. The purpose of our study was to describe, for the first time, the size, complexity and microchemistry of the saccular otoliths of the larva of 13 anuran species from central Argentina, using electron microscopy and X-ray spectroscopy (N = 65). We concluded that a) these structures differ in area, perimeter, otolith relative size and fractal dimension, but are similar in terms of their microchemistry when compared by spatial guilds, b) that nektonic species have larger otoliths than nektonic-benthic and benthic species and c) that benthic species have larger otolith relative size than nektonic-benthic and nektonic species.

Population and community dynamics and habitat selection of rodents in complex desert landscapes

A review of population and community dynamics and habitat selection of rodents in complex desert landscapes is presented. Population dynamics of rodents in all deserts are driven mainly by resource pulses induced by rainfall events. In a long-term perspective, this pattern is sometimes interrupted by rare extreme weather events and by habitat transformation due to wildfire or climate change. Rainfall-population density relations were found to be modified by intrinsic (population autoregulation, habitat selection) and extrinsic (interspecific competition, predation) processes, but the relative importance and geographic specificity of these processes are unclear. Density-dependent habitat selection was found in all studied desert rodent species inhabiting more than one habitat. Density-dependent habitat selection was expressed in different types of population regulation, resulting in density-dependent or density-independent habitat distribution. However, processes of density-dependent habitat selection have been studied in only one region; there are no data yet to analyze how different strategies of habitat selection vary among different desert regions. Dynamics of community composition include one widely distributed pattern, an increase in species richness with density. Another pattern, zero-sum community dynamic, was found in North American deserts. Dynamics of the niche community structure is expressed in the increase of niche overlap with density growth. In Palaearctic deserts, this increase in niche overlap was accompanied by stronger aggregation of niches in spatial guilds. The last phenomenon was not recorded in North American deserts, which reflects differences in expression of spatial guilds between Palaearctic and Nearctic rodent communities.

How selection structures species abundance distributions

How do species divide resources to produce the characteristic species abundance distributions seen in nature? One way to resolve this problem is to examine how the biomass (or capacity) of the spatial guilds that combine to produce an abundance distribution is allocated among species. Here we argue that selection on body size varies across guilds occupying spatially distinct habitats. Using an exceptionally well-characterized estuarine fish community, we show that biomass is concentrated in large bodied species in guilds where habitat structure provides protection from predators, but not in those guilds associated with open habitats and where safety in numbers is a mechanism for reducing predation risk. We further demonstrate that while there is temporal turnover in the abundances and identities of species that comprise these guilds, guild rank order is conserved across our 30-year time series. These results demonstrate that ecological communities are not randomly assembled but can be decomposed into guilds where capacity is predictably allocated among species.

Spatial guilds in the Serengeti food web revealed by a Bayesian group model.

Food webs, networks of feeding relationships in an ecosystem, provide fundamental insights into mechanisms that determine ecosystem stability and persistence. A standard approach in food-web analysis, and network analysis in general, has been to identify compartments, or modules, defined by many links within compartments and few links between them. This approach can identify large habitat boundaries in the network but may fail to identify other important structures. Empirical analyses of food webs have been further limited by low-resolution data for primary producers. In this paper, we present a Bayesian computational method for identifying group structure using a flexible definition that can describe both functional trophic roles and standard compartments. We apply this method to a newly compiled plant-mammal food web from the Serengeti ecosystem that includes high taxonomic resolution at the plant level, allowing a simultaneous examination of the signature of both habitat and trophic roles in network structure. We find that groups at the plant level reflect habitat structure, coupled at higher trophic levels by groups of herbivores, which are in turn coupled by carnivore groups. Thus the group structure of the Serengeti web represents a mixture of trophic guild structure and spatial pattern, in contrast to the standard compartments typically identified. The network topology supports recent ideas on spatial coupling and energy channels in ecosystems that have been proposed as important for persistence. Furthermore, our Bayesian approach provides a powerful, flexible framework for the study of network structure, and we believe it will prove instrumental in a variety of biological contexts.

Linking species abundance distributions in numerical abundance and biomass through simple assumptions about community structure

Species abundance distributions (SADs) are widely used as a tool for summarizing ecological communities but may have different shapes, depending on the currency used to measure species importance. We develop a simple plotting method that links SADs in the alternative currencies of numerical abundance and biomass and is underpinned by testable predictions about how organisms occupy physical space. When log numerical abundance is plotted against log biomass, the species lie within an approximately triangular region. Simple energetic and sampling constraints explain the triangular form. The dispersion of species within this triangle is the key to understanding why SADs of numerical abundance and biomass can differ. Given regular or random species dispersion, we can predict the shape of the SAD for both currencies under a variety of sampling regimes. We argue that this dispersion pattern will lie between regular and random for the following reasons. First, regular dispersion patterns will result if communities are comprised groups of organisms that use different components of the physical space (e.g. open water, the sea bed surface or rock crevices in a marine fish assemblage), and if the abundance of species in each of these spatial guilds is linked to the way individuals of varying size use the habitat. Second, temporal variation in abundance and sampling error will tend to randomize this regular pattern. Data from two intensively studied marine ecosystems offer empirical support for these predictions. Our approach also has application in environmental monitoring and the recognition of anthropogenic disturbance, which may change the shape of the triangular region by, for example, the loss of large body size top predators that occur at low abundance.

Temporal dynamics in spatial organization of a rodent community in the Negev Highlands (Israel)

AbstractDynamics of a rodent community were studied in the Negev Desert Highlands (Israel) during 8 years on 24 permanent 1‐ha plots representing six main habitat types. Based on theoretical models of habitat selection, we hypothesized that niche overlap between species of the same spatial guild should increase with density faster than that between species of different guilds. Data were analysed using multivariate (principal component and discriminant) analyses to test the hypothesis. Density changes in most rodent species followed fluctuations in the amount of rainfall. Habitat niche breadth was density‐dependent in six species and density‐independent in four species. Significant habitat shifts and switches of spatial guild membership were found in three of 10 species. Pairwise values of habitat overlap were significantly and positively correlated with density only in species pairs where both species demonstrated density‐dependent variation of habitat breadth. In most other pairs of species, overlap values were density‐independent. Comparison of changes in average within‐season values of habitat overlap with density within and among spatial guilds directly supported our hypothesis demonstrating faster increase of niche overlap with density between species of the same spatial guild than between species of different guilds.

Spatial structure of community of darkling beetles (Coleoptera: Tenebrionidae) in the Negev Highlands, Israel

The distribution of niches in resource space of darkling beetles of Negev desert, Israel were studied using discriminant analysis. Seventeen environmental variables (substrate structure and vegetation) were measured at 375 plots within 15 grids. The first three discriminant axes account for 77% of the variance and reflect a soil structure gradient from sand to clay (the first axis), a gradient of increasing productivity (the second axis), and a type of vegetation architecture (the third axis). Abundance and biomass of tenebrionids increased linearly along the first, the second and the third axes. Species richness and diversity had a slanting parabola shape along the substrate axis and increased along the productivity axis. There were two distinct spatial guilds of tenebrionid species. One guild was composed by species that preferred sandy‐gravel soils, and another guild consisted of species that preferred clay soils. Niche overlapping of the “sandy‐gravel” species was lower than that of the “clay” species. Niches of the “sandy‐gravel” species were randomly distributed in the resource space. Those of the “clay” species formed a tight cluster with most niches placed in the center of the space, representing the centrifugal type of organization. The conformity of the guild structure with the different types of organization was supported by comparisons between empirical data and stochastic models. The border between these two guilds matches with proposed earlier biogeographical boundary (Ramon‐Zohar line).

Comparison of Niche-Packing and Community Organization in Desert Rodents in Asia and North-America

We compared patterns of species diversity, locomotory morphology, feeding modes, and spatial organisation for rodent communities in four Asian deserts (Kyzylkum, Gobi, ?Thar, Negev) and one North American (Chihuahuan) desert. Deserts were similar in gamma and alpha diversity. A positive relationship between regional species diversity (and biomass) and mean annual precipitation was found. The Asian deserts showed a greater degree of divergence and specialisation between bipedal and quadrupedal forms. The range of feeding modes was similar in deserts on both continents, but the Negev was the only Asian desert in which granivory was as important as in the Chihuahuan. Temperate Asian desert rodents were organised into spatial guilds, separated primarily by characteristics of the soil and perennial vegetation. North American desert rodent species overlapped more extensively in habitat use. The similarities and differences between these deserts can be explained by their biogeographic histories.