1,721,103 research outputs found
Parasitic worms: how many really?
No full textAccumulation curves are useful tools to estimate species diversity. Here
we argue that they can also be used in the study of global parasite
species richness. Although this basic idea is not completely new, our
approach differs from the previous ones as it treats each host species
as an independent sample. We show that randomly resampling host-parasite
records from the existing databases makes it possible to empirically
model the relationship between the number of investigated host species,
and the corresponding number of parasite species retrieved from those
hosts. This method was tested on 21 inclusive lists of parasitic worms
occurring on vertebrate hosts. All of the obtained models conform well
to a power law curve. These curves were then used to estimate global
parasite species richness. Results obtained with the new method suggest
that current predictions are likely to severely overestimate parasite
diversity. (C) 2014 Australian Society for Parasitology Inc. Published
by Elsevier Ltd. All rights reserved
On the methods to assess significance in nestedness analyses
No full textUse of Z values to evaluate nestedness significance is a common
procedure. An appealing alternative to the use of Z values is that of
using a value of relative nestedness (RN). However, there is no
agreement on the preferable procedures to generate the null matrices
needed to compute both Z and RN. In general, it is recommended to use
restrictive null models that take into account row and column totals.
The two most widely used null models of this kind, namely, FF and CE
[that generate matrices with row and column sums equal (FF) or
proportional (CE) to the row and column totals of the original matrix,
respectively], are very different in terms of restrictiveness. We
performed a set of comparative analyses on both theoretical and real
matrices to investigate the differences between the use of Z and RN
values, and between the use of FF and CE null models, when NODF
(Nestedness metric based on overlap and decreasing fill) or rho(A)
(i.e., the largest eigenvalue of the adjacency matrix) are used to
measure nestedness. We found no difference in the use of Z or RN values.
On the other hand, we found that different combinations of nestedness
measures and null models may lead to inconsistent outcomes. Our results
offer some clarity on a few issues that, despite playing a central role
in the practical application of nestedness analysis, have been little
explored, and highlight the need for the definition of some commonly
accepted standards
A Few Good Reasons Why Species-Area Relationships Do Not Work for Parasites
Full text linkSeveral studies failed to find strong relationships between the
biological and ecological features of a host and the number of parasite
species it harbours. In particular, host body size and geographical
range are generally only weak predictors of parasite species richness,
especially when host phylogeny and sampling effort are taken into
account. These results, however, have been recently challenged by a
meta-analytic study that suggested a prominent role of host body size
and range extent in determining parasite species richness (species-area
relationships). Here we argue that, in general, results from
meta-analyses should not discourage researchers from investigating the
reasons for the lack of clear patterns, thus proposing a few tentative
explanations to the fact that species-area relationships are infrequent
or at least difficult to be detected in most host-parasite systems. The
peculiar structure of host-parasite networks, the enemy release
hypothesis, the possible discrepancy between host and parasite ranges,
and the evolutionary tendency of parasites towards specialization may
explain why the observed patterns often do not fit those predicted by
species-area relationships
Fish parasites resolve the paradox of missing coextinctions
No full textModels of coextinction identify parasites as one of the most menaced
ecological groups. The number of host species a parasite uses should
strongly affect its risk of coextinction. The naive expectation is that
the lower the number, the higher is the parasite's risk of being left
with no hosts. Here we analyse the coextinction risk of 12,141 fish
parasite species and find that highly specific parasites are not the
most endangered, because they tend to use hosts with low vulnerability
to extinction. This unexpected result may explain why the number of
documented host-parasite coextinctions is much lower than predicted by
theoretical studies
Human population density and tenebrionid richness covary in Mediterranean islands
Full text linkHuman population growth is expected to drive several species to local
extinction. Yet, an unexpected high biodiversity can be found even in
densely populated areas. Although a positive correlation between human
density and biodiversity can be explained by the intermediate disturbance
hypothesis, an alternative possible explanation may come from the tendency
of human settlements to be located in sites whose environmental conditions
are particularly favourable also for many other animal species. To
investigate this hypothesis, we studied the relationships between human
population density and species richness of native tenebrionid beetles in
small Italian islands. We used partial regression analysis to assess the
individual contribution of island area (to account for the species–area
relationship), elevation (used as a proxy of environmental diversity), and
human density to species richness. We found that tenebrionid diversity
increased with human population density even after controlling for area and
elevation. This may suggest that islands that were (and are) more hospitable
to humans are also those which can be more favourable for tenebrionids
Calling for a new strategy to measure environmental (habitat) diversity in Island Biogeography: a case study of Mediterranean tenebrionids (Coleoptera, Tenebrionidae)
Full text linkMany recent researches in island biogeography attempted to disentangle the effects of area per se and “habitat diversity” on species richness. However, the expression “habitat diversity” in this context should be avoided, because habitats can be only recognized by referring to the resources needed by a particular species. What is really measured in such researches is some form of “environmental heterogeneity”. Although habitat heterogeneity can be measured in various ways, most researches in island biogeography simply used the number of biotopes (typically classified as land cover categories). However, not all biotopes have the same surface.On the basis of the area occupied by each land cover categories, it is possible to calculate indices of environmental diversity, evenness and dominance, as commonly done in community ecology research. These indices may be useful to investigate the role of environmental diversity in determining species richness. We used the tenebrionid beetles inhabiting twenty-five small islands around Sicily (Central Mediterranean) to illustrate these concepts. We found that both area per se and environmental heterogeneity contributed to determine species richness. Moreover, we found that the relationship between species richness and environmental homogeneity followed a power function model. This indicates that environmental homogenization may determine a rapid, non linear decline in species richness.JRC.H.3 - Forest Resources and Climat
Identification of Monogenea made easier: a new statistical procedure for an automatic selection of diagnostic linear measurements in closely related species
No full textWe introduce a new statistical method to select which morphological
characters are most useful to identify monogenean species. The method
estimates the average size overlap of candidate diagnostic structures
among a set of species to individuate those that mostly differ between
the species. To demonstrate our approach, we report a comprehensive
analysis conducted on two of the most species-rich monogenean genera:
Dactylogyrus Diesing, 1850 and Gyrodactylus von Nordmann, 1832. We
demonstrate that, in contrast to common taxonomic practice, very few but
highly diagnostic measurements are necessary to correctly identify a
specimen. In particular, we found that most of Dactylogyrus and
Gyrodactylus species can be classified on the basis of the width of the
supplementary connecting bar and of the length of the hook sickle,
respectively
When human needs meet beetle preferences: tenebrionid beetle richness covaries with human population on the Mediterranean islands
Full text link1. Human presence can affect biodiversity in many ways. If anthropization is one of the major drivers of species extinctions, at the same time, human induced increase in environmental heterogeneity may also increase species richness. 2. In many cases, however, heterogeneity is not enough to explain the unexpectedly high biodiversity found in some densely populated areas. 3. A possible explanation to such situations is the partial overlap in resource requirements between man and other species, which promotes a tendency for humans to settle in sites characterised by environmental conditions that are particularly favourable also for many other organisms. 4. To test this hypothesis, we investigated the relationships between human population and species richness of native (non-synanthropic) tenebrionid beetles in the Mediterranean islands, many of which have been inhabited by humans for millennia. 5. Using partial correlation analyses, we found that tenebrionid diversity increased not only with island area and maximum elevation (used herein as a measure of environmental heterogeneity), but also with human population. 6. This may suggest that the islands that were (and are) more accessible and hospitable to humans are also those which can be more easily colonised by tenebrionids, owing to their larger areas and higher environmental heterogeneity
A protocol to compare nestedness among submatrices
No full textSearching for nestedness has become a popular exercise in community
ecology. Significance of a nestedness index is usually evaluated using z
values, and finding that a matrix is nested is typically a common
result. However, nestedness is not likely to be spread uniformly within
a matrix of species presence/absence per site. Selected parts of the
matrix may show a degree of nestedness significantly higher (or lower)
than expected from the overall pattern. Here we describe a procedure to
assess if a particular submatrix (i.e., a peculiar combination of rows
and columns extracted from the complete matrix) is more or less nested
than expected for an assortment of sites and species taken at random
from the same overall matrix. The idea is to obtain several submatrices
of different sizes from the same overall matrix and to calculate their z
values. A regression is then performed between z values of submatrices
and their sizes. A nestedness index independent of matrix size is
suggested as the deviation of the z value of a particular submatrix from
that expected according to the regression line. We applied our protocol
to 55 matrices with different nestedness indices under various
null-models and, for purpose of demonstration, we discussed in detail a
single case study regarding various animal groups of the Aegean Islands
(Greece). The obtained results strongly encourage further research to
focus not only on the question whether a matrix is nested or not, but
also on where and why nestedness is confined
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