Princip hnízdovitosti (nested subsets or nestedness) druhového složení:

výpočet a ekologické interpretace

Michal HorsákÚstav botaniky a zoologiePřF MU Brno

Nested subset patterns of species composition

• Theory

• Calculations

• Applications

http://aics-research.com/nestedness/tempcalc.html

http://fm1.fieldmuseum.org/aa/staff_page.cgi?staff=patterso&id=438

nestedness - theory

– the nested subset hypothesis states that the species comprising a depauperate insular biota are a proper subset of those in richer biotas, and that an archipelago of such biotas, ranked by species richness, presents a nested series

– a group of species assemblages is said to be perfectly nested when each species is present in all assemblages richer than the most depauperate one in which that species occurs

One of the main goals of community ecology is the identification and explanation of non-random patterns of species composition.

One of the patterns is the nestedness:

Nested

Non-nested

A

C

B

A

CB

nestedness - theory

nestedness - theory

Three conditions are necessary for development of nestedness:

– islands or sites must be ecologically comparable

– species inhabiting these sites must have shared similar biogeographic histories

– species must be hierarchically ordered in terms of their niches

The first two points correspond to homogeneity constrains.

The third condition serves to summarize factors that order

the incidence of species.

nestedness - theory

The nested pattern was first proposed by Darlington (1957): Zoogeography, …colonization of islands only by species with high dispersal capabilities.

Patterson & Atmar (1986): Nested subsets and the structure of insular mammalian faunas and archipelagos.

Now nestedness has proved to be a very common phenomenon - 219 papers since 1980, Web of Science

mammals, births, frogs, fish parasites, plants, dragonflies…

/archipelago = a geographically coherent cluster of islands, also clusters of insular habitats or non-insular sites/

nestedness - theory

Degree of nestedness:

vertebrates >> invertebrates (especially aquatic), plants

- high diversity

- deferent origin and biogeographic histories

- highly variable environment

- scale dependence

Around 10 studies in freshwater systems:

- insects in Sweden (e.g. Malmqvist et al. 1999)

- macroinvertebrates in fragmented Alpine streams (Monaghan et al. 2005)

- Bdelloidea, Ostracoda, Ephemeroptera, Plecoptera, Trichoptera, Dytiscidae, Culicidae, Simuliidae

nestedness - theory

What becomes relevant is what mechanisms promote nestedness in communities with different phylogenetic and ecological attributes.

Several mechanisms were proposed:

– selective extinction

– selective colonization

– historical effects

– speciation

– differential reproductive success

– habitat nestedness

– nested habitat quality

– human disturbances

– sampling artefact

nestedness - calculation

Many different metrics of nestedness have put forward in the last two decades (see Wright et al. 1998).

Two of the most used procedures are:

– “departures“ method of Lomolino (1996)

– “temperature“ method of Atmar & Patterson (1993)

Sfenthourakis et al. (1999) concluded that the later method is more suitable for evaluating the level of nestedness.

nestedness - calculation

The temperature method of Atmar & Patterson (1993)

– their metric measure the heat of disorder

presence-absence matrix

nestedness - calculation

Calculation the temperature of binary matrix involves three steps:

1. to compute an isocline of perfect order – it is a curve that, in a perfectly nested matrix of the same size an fill, would separate cells denoting the presence of a species at a site from cells denoting its absence

2. to find the matrix using permuting rows and columns in the way that maximizes its nestedness

3. to associate with each absence above the isocline and with each presence below it a normalized measure of distance to the isocline.

The temperature of the matrix is the sum of these distances (normalized to range between 0 for a perfectly nested matrix and 100 for a maximally unnested matrix).

nestedness - calculation

The temperature method of Atmar & Patterson (1993)

– their metric measure the heat of disorder

presence-absence matrix

nestedness - calculation

Two forms of noise contribute to the matrix‘s temperature:

1. the random variation of different stochastic events

2. the “coherent“ noise of specific idiosyncratic biogeographic events

– idiosyncratic species

– idiosyncratic sites

nestedness - applications

The phenonenon, termed nested subsets has various biogeographical, ecological, and coevolutionary consequences, as well as important implications for biological conservation. (B.D. Patterson)

What the metric offers:

– a measures of the uncertainty in species extinction order

– a measures of relative populational stabilities

– a means of identifying minimally sustainable population size

– an estimate of the historical coherence of the species assemblages

– a revelation of mechanism promoting the nestedness

– a decision for an appropriate conservation strategy

nestedness - applications

How to reveal a mechanism promoting the nestedness?

– to correlate the ranking order of sites in the final nested matrix with the order of sites after re-arranging the sites along the analysed factor

– a significant correlation suggests that a community is packed in a predictive order owing to the influence of a given factor (Patterson & Atmar 2000)

nestedness - applications

Implications for biological conservation:

– the SLOSS problem has been addressed in a large number of studies (for details see Ovaskainen, 2002), but no coherent conclusion has yet appeared

– an important conclusion from previously published studies is that the result depends on the level of nestedness

– if the biota is highly nested, "single large" is found to be the better strategy. On the contrary, if the biota is not nested the "several small" strategy tends to maximize the number of species

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