Secondary Extinctions Research Articles

Managing mangroves with benthic biodiversity in mind: Moving beyond roving banditry

This review addresses mangrove management activities in the broader context of the diversity of the mangrove benthos. Goals for mangrove ecosystem management include silviculture, aquaculture, or ‘ecosystem services’ such as coastal protection. Silvicultural management of mangroves generally neglects the benthos, although benthic invertebrates may affect tree establishment and growth, and community composition of benthic invertebrates may be a reliable indicator of the state of managed mangrove forests. Similarly, mangrove aquaculture focuses on particular species with little attention paid either to impacts on other trophic levels or to feedbacks with the trees. Exploitation of mangrove-associated prawns, crabs, and molluscs has a total economic value > US $4 billion per year. These aquaculture operations still rely on wild-collected stock; world-wide patterns of exploitation fit the well-known process of ‘roving banditry’, where mobile agents move from location to location, rapidly exploiting and depleting local resources before moving on to other, as-yet unprotected grounds. Collection of brood stock and fishing for other external inputs required by aquaculture (e.g., ‘trash fish’) removes intermediate trophic levels from marine food webs, may destabilize them, and lead to secondary extinctions of higher-order predators. Increased attention being paid to the role of mangroves in coastal protection following the 2004 Indian Ocean tsunami provides an opportunity to reassess the relative merits of management focused on short-term economic gains. Managing for ecosystem services may ultimately preserve benthic biodiversity in mangrove ecosystems.

RESPONSE OF COMPLEX FOOD WEBS TO REALISTIC EXTINCTION SEQUENCES

Although an ecosystem's response to biodiversity loss depends on the order in which species are lost, the extinction sequences generally used to explore such responses in food webs have been ecologically unrealistic. We investigate how several extinction orders affect the minimum number of secondary extinctions expected within pelagic food webs from 34 temperate freshwater lakes. An ecologically plausible extinction order is derived from the geographically nested pattern of species composition among the lakes and is corroborated by species' pH tolerances. Simulations suggest that lake communities are remarkably robust to this realistic extinction order and highly sensitive to the reverse sequence of species loss. This sensitivity is not well explained by the known sensitivity of networks to the loss of highly connected species but appears to be better explained by our observation that trophic specialists preferentially consume widely distributed species at low risk of extinction. Our results highlight an important aspect of community organization that may help to maintain biodiversity amidst changing environments.

Cascading extinctions and ecosystem functioning: contrasting effects of diversity depending on food web structure

The consequences of species loss on cascading extinctions in food webs have been the focus of several recent theoretical studies, with differing results. Changes in ecosystem properties consecutive to cascading extinctions have received far less attention even though such dramatic events might strongly alter ecosystem functioning. Here we use various food web models to investigate the effects of species loss and diversity on both secondary extinctions and their associated changes in ecosystem properties. Our analysis shows that diversity has contrasting effects depending on the presence of self‐limiting terms at consumer levels and, to a lower extent, on connectance and interspecific competition. Ecosystems that lose a high proportion of species through cascading extinctions exhibit the most important changes in ecosystem properties. Linking studies on cascading extinctions in food webs with studies that investigate the effects of biodiversity on ecosystem functioning appears crucial for a better understanding of the consequences of species extinctions.

Early onset of secondary extinctions in ecological communities following the loss of top predators

The large vulnerability of top predators to human-induced disturbances on ecosystems is a matter of growing concern. Because top predators often exert strong influence on their prey populations their extinction can have far-reaching consequences for the structure and functioning of ecosystems. It has, for example, been observed that the local loss of a predator can trigger a cascade of secondary extinctions. However, the time lags involved in such secondary extinctions remain unexplored. Here we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. Moreover, in most cases time to secondary extinction increases with increasing species richness. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby. The implications of these results for community persistence and conservation priorities are discussed.

Species loss and secondary extinctions in simple and complex model communities

1. The loss of a species from an ecological community can trigger a cascade of secondary extinctions. Here we investigate how the complexity (connectance) of model communities affects their response to species loss. Using dynamic analysis based on a global criterion of persistence (permanence) and topological analysis we investigate the extent of secondary extinctions following the loss of different kinds of species. 2. We show that complex communities are, on average, more resistant to species loss than simple communities: the number of secondary extinctions decreases with increasing connectance. However, complex communities are more vulnerable to loss of top predators than simple communities. 3. The loss of highly connected species (species with many links to other species) and species at low trophic levels triggers, on average, the largest number of secondary extinctions. The effect of the connectivity of a species is strongest in webs with low connectance. 4. Most secondary extinctions are due to direct bottom-up effects: consumers go extinct when their resources are lost. Secondary extinctions due to trophic cascades and disruption of predator-mediated coexistence also occur. Secondary extinctions due to disruption of predator-mediated coexistence are more common in complex communities than in simple communities, while bottom-up and top-down extinction cascades are more common in simple communities. 5. Topological analysis of the response of communities to species loss always predicts a lower number of secondary extinctions than dynamic analysis, especially in food webs with high connectance.

Extinction cascades and catastrophe in ancient food webs

A model is developed to explore the potential responses of paleocommunities to disruptions of primary production during times of mass extinction and ecological crisis. Disruptions of primary production are expected to generate bottom-up cascades of secondary extinction, and these are predictable given species richnesses, functional diversity, and trophic link distributions. If, however, consumers are permitted to compensate for the loss of trophic resources by increasing the intensities of their remaining biotic interactions, top-down driven catastrophic increases of secondary extinction emerge from the model. Both bottom-up and top-down effects are themselves controlled by the geometry of the food webs. The general Phanerozoic trends of increasing taxonomic and ecological diversities, as well as the varying strengths of biotic interactions, have led to food webs of increasing complexity. The frequency of catastrophic secondary extinction increases as food web complexity increases, but increased complexity also serves to dampen the magnitude of the secondary extinctions. When intraguild competitive interactions are included in the model, competitively inferior taxa are observed to possess greater probabilities of survival if the guilds are embedded in simple subnetworks of the overall food web. The result is the emergence of postextinction guilds dominated by those inferior taxa. These results are congruent with empirical observations of “disaster taxa” dominance after some mass extinction events, and provide a mechanism for the reorganization of ecosystems that is observed after those events. The model makes the testable prediction that dominance by disaster taxa, however, should be observed only when bottom-up disruptions have caused ecosystems to collapse catastrophically.

Secondary extinctions in ecological networks: Bottlenecks unveiled

In ecosystems, a single extinction event could eventually precipitate in a mass extinction, involving species that may be several connections away from the target of the perturbation. This topic has been illuminated by recent studies on network mechanics, thanks to the concepts of hub, error and targeted removal, attack sensitivity, small world, and so forth. To forecast the effects of a species removal one can use an algorithm that unfolds a complex food web into a topologically simpler scheme, called its dominator tree. This structure is simple, elegant, and highly informative; all the bottlenecks and the effects of species removal are clearly traceable. While food web studies are mostly qualitative, in this paper the use of the dominator tree is extended to weighted food webs, in which link magnitude is specified. These structures were obtained from ecological flow networks. In eight of these food webs, the analysis consisted in removing links that were weaker than a threshold of magnitude and building the dominator tree associated to the remaining structure. By progressively increasing the threshold up to the value that would make the graph disconnected, we had the opportunity to investigate patterns of dominance as a function of link magnitude.

Keystone species and vulnerable species in ecological communities: strong or weak interactors?

The loss of a species from an ecological community can trigger a cascade of secondary extinctions. The probability of secondary extinction to take place and the number of secondary extinctions are likely to depend on the characteristics of the species that is lost—the strength of its interactions with other species—as well as on the distribution of interaction strengths in the whole community. Analysing the effects of species loss in model communities we found that removal of the following species categories triggered, on average, the largest number of secondary extinctions: (a) rare species interacting strongly with many consumers, (b) abundant basal species interacting weakly with their consumers and (c) abundant intermediate species interacting strongly with many resources. We also found that the keystone status of a species with given characteristics was context dependent, that is, dependent on the structure of the community where it was embedded. Species vulnerable to secondary extinctions were mainly species interacting weakly with their resources and species interacting strongly with their consumers.

COMMUNITY VIABILITY ANALYSIS: THE RESPONSE OF ECOLOGICAL COMMUNITIES TO SPECIES LOSS

The loss of a species from an ecological community can set up a cascade of secondary extinctions that in the worst case could lead to the collapse of the community. Both deterministic and stochastic mechanisms may be involved in such secondary extinctions. To investigate the extent of secondary extinctions in ecological communities following the loss of a species, we here develop a community viability analysis. We introduce a measure called the “quasi-collapse risk” that is defined as the probability that the number of species in a community falls below some defined value within a fixed period of time following the loss of a species. We develop deterministic and stochastic methods for finding post-extinction communities. We use these methods to investigate the relationship between diversity (species richness) and quasi-collapse risks in model communities. It is shown that, in a deterministic context, communities with more species within trophic levels have a larger fraction of species remaining in post-extinction communities. This benefit of species richness is to a large extent lost in the presence of demographic stochasticity. The reason for this is a negative relationship between population density and species diversity. We also show that communities become increasingly triangular in shape as secondary extinctions take place, due to greater extinction risk of species at higher trophic levels. We argue that this new approach holds some promise for identifying fragile ecosystems and keystone species.

Network structure and robustness of marine food webs

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 273:291-302 (2004) - doi:10.3354/meps273291 Network structure and robustness of marine food webs Jennifer A. Dunne1,4,*, Richard J. Williams2,4, Neo D. Martinez3,4 1Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501, USA 2National Center for Ecological Analysis and Synthesis, University of California Santa Barbara, 735 State St., Suite 300, Santa Barbara, California 93101-3351, USA 3Center for Applied Mathematics, 657 Frank HT Rhodes Hall, Cornell University, Ithaca, New York 14853, USA 4Pacific Ecoinformatics and Computational Ecology Lab, Rocky Mountain Biological Laboratory, PO Box 519, Crested Butte, Colorado 81224, USA *Email: jdunne@santafe.edu ABSTRACT: Previous studies suggest that food-web theory has yet to account for major differences in food-web properties of marine versus other types of ecosystems. We examined this issue by analyzing the network structure of food webs for the Northeast US Shelf, a Caribbean reef, and Benguela, off South Africa. The values of connectance (links per species2), link density (links per species), mean chain length, and fractions of intermediate, omnivorous, and cannibalistic taxa of these marine webs are somewhat high but still within the ranges observed in other webs. We further compared the marine webs by using the empirically corroborated niche model that accounts for observed variation in diversity (taxon number) and complexity (connectance). Our results substantiate previously reported results for estuarine, fresh-water, and terrestrial datasets, which suggests that food webs from different types of ecosystems with variable diversity and complexity share fundamental structural and ordering characteristics. Analyses of potential secondary extinctions resulting from species loss show that the structural robustness of marine food webs is also consistent with trends from other food webs. As expected, given their relatively high connectance, marine food webs appear fairly robust to loss of most-connected taxa as well as random taxa. Still, the short average path length between marine taxa (1.6 links) suggests that effects from perturbations, such as overfishing, can be transmitted more widely throughout marine ecosystems than previously appreciated. KEY WORDS: Food webs · Network structure · Marine ecosystems · Robustness · Connectance · Niche model · Biodiversity loss Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 273. Online publication date: June 08, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Network structure and biodiversity loss in food webs: robustness increases with connectance

Food‐web structure mediates dramatic effects of biodiversity loss including secondary and `cascading' extinctions. We studied these effects by simulating primary species loss in 16 food webs from terrestrial and aquatic ecosystems and measuring robustness in terms of the secondary extinctions that followed. As observed in other networks, food webs are more robust to random removal of species than to selective removal of species with the most trophic links to other species. More surprisingly, robustness increases with food‐web connectance but appears independent of species richness and omnivory. In particular, food webs experience `rivet‐like' thresholds past which they display extreme sensitivity to removal of highly connected species. Higher connectance delays the onset of this threshold. Removing species with few trophic connections generally has little effect though there are several striking exceptions. These findings emphasize how the number of species removed affects ecosystems differently depending on the trophic functions of species removed.

Vertebrate extinction in Mediterranean islets: an example from Cyprus

B/OGEOGRAPHIA — vol. XVIII — 1995 (Pubblicato il 31 ottobre 1996) Le piccole Isole Circumsarde e il loro significato Biogeografico Vertebrate extinction in Mediterranean islets: an example from Cyprus E. HADJISTERKOTIS” and B. MASALAM‘ qqOfi’zqcer of tlae Game zzml Fauna Service, Mzfiirtry of Interior; Nzrorzkz, Cyprus. ”°qI7zrz‘z'tzIte of General P/Jyrziology mm’ Biological C/Jemzirtry, Faculty of Bz'0logz'czzl Sciences, U72z'verrz'z‘y of Sczsrarz; Szzsrarz; Italy Key words: Vertebrate, extintion, mediterranean islets, Cyprus. SUMMARY Cyprus is the third island in the Mediterranean \vith an area of 9.251 Kmz. Is a 15 million year old island of oceanic origin which has never been connected to the mainland. In this study we attempt to tally all the extinctions that took place on the island of Cyprus and compare the emerging pattern with that of other islands. All the endemic animals recorded from the Pleistocene Fauna, the Pygmy hippopotamus P/Ja/zoz.trz'0s /7zz'IIzIl‘m', the Pygmy elephant Elzzp/Mr cypria/ax, the Cyprus genet Gezzetta /7/er/rtoizler, and the Ancient Cypriot shrew Crociclzzm rzzzzueole/zr [mu/c_vpI'1‘a, are extinct. These animals represent 60% of the endemic mammalian species aver recorded in Cyprus and 20% of the endemic subspecies. From the Archaeological Fauna nine taxa of animals were reported, which (with the exception of the Cyprus mouflon Oz;/: gmelz'm' op/Jz'0Iz) are not present today. From the avifauna two species and seven subspecies (12% of the total number of breeding taxa) were noted as endemic. Two of the latter, the Dipper C/‘uclz/r rinclz/5 oly/up/qcur is extinct and the Raven Coruzzr camx cjvprizzr is in danger of extinction. This pattern of extinction of indigenous spe— cies was also observed on other islands and is a consequence of small isolated island populations. INTRODUCTION A characteristic of modern extinctions of birds and mammals is that they are concentrated on islands (Diamond, 1984). For birds, 171 species and sub- species have become extinct since 1600, and over 90 percent of these extinc- tions have occurred on islands. For mammals out of 115 documented historic extinctions, 36 percent of these have occurred on islands. This pattern is in part due to the nature of island populations. They are small and isolated; thus they cannot recover from local extirpation following environmental perturba- tions or long—term climatic changes by immigration from other areas (MacArthur and \Wilson, 1967; Leigh, 1981). A growing body of evidence for birds and mammals indicates that over the last few thousand years, the most important agent of direct change in the envi- ronments is human disturbance and alteration of habitats (Diamond, 1984). Most extinct species in recorded history are attributed to some aspects of human interference, such as: habitat destruction; human hunting; effects of introduced taxa particularly predators, and trophic cascades (i.e., secondary extinctions caused by previous extinctions; Diamond and Case, 1986). Usually the period immediately following the initial settlement of islands is correlated 691

Biodiversity Conservation in Running Waters

In the concerns about biodiversity conservation, fresh waters have received less attention than tropical forests and oceans. However, running waters harbor a diverse panoply of species, habitats, and ecosystems, including some of the most threatened and many having great value to human society. An overview of the biological diversity of running waters and the state of imperilment is presented. Six major factors that threaten destruction of running water species and ecosystems are discussed: habitat loss and degradation; species invasions; overharvesting; secondary extinctions; chemical and organic pollution; global climate change. General measures for recovery and restoration of running waters conclude the article.

The size and shape of (2) Pallas from the 1983 occultation of 1 Vulpeculae

The analysis and results of an occultation of the spectroscopic binary star 1 Vulpeculae by the asteroid (2) Pallas, observed from 130 locations, are presented. Combination of solutions from this and previous occultations shows the triaxial shape of Pallas to be elongated, but not severely, with the ratio of the largest to the smallest axes being less than 1.15. The data provide a separation and a position angle of the secondary component of 1 Vulpeculae and indicate that the star's actual parallax is probably near 0.008 arcsec, half of the published value. The analysis gives a calibration of the accuracy and the reaction time corrections for visual observations of asteroidal occultations. The absence of any confirmed secondary extinctions shows that any satellites of Pallas must be rare or small. The photometric observations seem to rule out a substantial cloud of dust surrounding Pallas postulated previously.

Thickness Measurement of Epitaxical Thin Films by X-ray Diffraction Method

AbstractA method was described for determining the thickness of epitaxical thin films common to electronic materials. The equations were developed based on the kinematical theory of X-ray diffraction and effects of both primary and secondary extinctions were considered. As an example of the applications of this method, thickness measurement of AlGaAs thin films on GaAs was demonstrated. These films were grown by molecular beam epitaxy. The integrated reflected intensities from the film and the substrate were obtained by the X-ray double crystal diffractometer. An excellent agreement was obtained between the results from X-ray measurements and RHEED oscillation data.

Statistical dynamical theory of crystal diffraction. II. Intensity distribution and integrated intensity in the Laue cases

The theory described in the previous paper [Kato (1980). Acta Cryst. A36, 763-769] has been applied to calculating the intensity distribution in the topography of statistically homogeneous crystalline media. The integrated intensity for parallel-sided crystals is also presented. The formulae cover the whole range of crystal perfection. Primary and secondary extinctions can be treated on a unified theoretical base. A few numerical examples are given to demonstrate the variations of the integrated intensity as a function of the crystal thickness, the extinction distance and two parameters which characterize crystal perfection over short and long ranges compared with the extinction distance.

Simple dynamical theory approach to neutron diffraction by mosaic crystals

A simple model based on dynamical theory of neutron diffraction by perfect crystals was used for the description of neutron diffraction by non-absorbing mosaic crystals. The model predictions were compared with the theory of Bacon and Lowde for mosaic crystals. From the satisfactory agreement it appears that no substantial difference exists between primary and secondary extinctions. In addition this work opens up the possibility to derive, in the near future, theoretical predictions for neutron diffraction by curved mosaic crystals.

Primary and secondary extinctions in the dynamical theory for an imperfect crystal

Primary and secondary extinction are studied using the dynamical theory of X-rays diffracted by imperfect crystals. The transition from dynamical to kinematical scattering is explained in terms of fundamental processes in diffraction. Contrary to existing extinction theories, where the intensities diffracted dynamically by single coherent domains of a mosaic are combined using an ad hoc assumption of mosaic distributions, the present theory permits the dynamical amplitudes to change in response to disturbances of the dynamical interactions by imperfections. Neither the mosaic block model nor the statistical treatment of imperfections is used. The extinction of diffracted intensities is thereby treated as caused solely by inhomogeneous strains in a single coherent domain.

X-ray Diffraction Topographic Studies of Dislocations in Natural Large Ice Single Crystals

The method of X-ray diffraction topography was adopted to reveal the dislocation structure in natural, large ice single crystals which has been hitherto used for the extensive experiments of plastic deformation. The topographs show clear images of curved and straight dislocation lines lying on the basal planes of the crystal. Dislocation density is in the order of 104 cm-2 and Burgers vectors are (a/3)<112̄0> or c[0001]. In many cases, edge dislocations compose the small angle tilt boundaries of which tilt angle is less than 1° of arc. Size of the subgrains bordered with such boundaries is in the order of 1 mm or more and it differs very much from the size of mosaic blocks evaluated from primary and secondary extinctions of X-rays. Another characteristic feature is that screw dislocations of one Burgers vector are much less than those of other two equivalent Burgers vectors of (a/3)<112̄0> system.

Degree of Perfection of Ice Crystals Determined from Primary and Secondary X-Ray Extinctions

The primary and secondary x-ray extinctions have been investigated in ice crystals. This led to an estimate of the sizes and the degree of misalignment of the mosaic blocks.