Prof. Dr. Martin Schnittler

introduction	Myxomycete Research			Vascular Plant Research		Scientific papers
	The life of a Slime Mould	Main Results	Myxomycete Collection	Red Lists and Conservation work	Vascular Plant Collection

 

Research on myxomycete ecology and distribution

From obvious reasons, ecological field research is almost exclusively limited to macroscopically visible organisms. However, myxomycetes have the advantage that one can apply the successfully proven methods of field ecology to organisms spending most of their life as true microorganisms. The fructifications of myxomycetes, although only the ‘tip of the iceberg’ in terms of their life cycle, give the handle to this kind of research.

Since myxomycete prey on other microorganisms, more insights into their ecology will ultimately elucidate the ecology of the microbial world as a whole. The results of this research will hopefully bring our knowledge about myxomycetes closer to ecologically better understood groups of organisms, like macroscopic fungi, mosses, or vascular plants.

Myxomycete taxonomy

The most recent checklist [LINK Nomenmyx Lado] of the group lists more than 1000 taxa, but only about 40% of these taxa are better known, i.e. recorded from more than 20 localities. From 1012 subgeneric taxa (Oct. 2000) 305 (31%) are known only from the type locality, and further 258 (26%) are estimated to be very rare (observed from less than 20 localities world-wide). The high proportion of “singleton species” indicates the still insufficient level of knowledge.

image number of described species from Stapfia paper

Due to a mixed mode of reproduction, probably including both apomixis and sexual reproduction, the current morphological species concept does not appropriately reflect the biology of myxomycetes. Most of the ca. 100 species investigated so far consist of homothallic (presumably apomictic) and heterothallic (sexual) strains. [LINK to Jim Clark?] Thus, like in apomictic genera of vascular plants (such as Rubus or Hieracium) the existence of an indeterminate number slightly different local clones is very likely for most myxomycete morphospecies.

As a consequence of these considerations, it is proposed that a taxon described as new for science should be known by several specimens from more than one locality, and should differ from all others in more than only one significant character. Intraspecific variability as well as its microhabitat should be described. These requirements will minimize the risk of describing local clones represented by single specimens or aberrations during sporocarp development as new species.

Field surveys

Quantitative local species inventories, with both records and number of sporocarps per colony counted, allow to generate abundance data and to estimate the total number of species to be expected.

image rank-abundance plot Kazakhstan

Estimations show these surveys to be complete to 70–90%. From rank-abundance plots, as well as from species-sample curves, the total number of species to be expected for a survey can be estimated. Such fairly complete surveys can function as “calibration tools” to judge pure species lists for the inclusion into biodiversity studies.

Global patterns of myxomycete diversity

Arctic regions have a relatively stable but species-poor myxomycete assemblage, and distribution of myxomycetes is limited more by microhabitat availability than by macroclimatic conditions. About 35 species occur regularly in the Arctic, and a considerable number of wood-inhabiting myxomycetes occur as far north as woody debris is present. However, almost all taxa seem to be commoner in boreal and/or temperate zones.

In boreal regions, species numbers are three times higher than in Arctic regions. Besides the significantly higher number of wood-inhabiting species, species of algae-covered rocks and nivicolous myxomycetes were observed in boreal zones.

As to expect, temperate zones are even richer in species. A study of montane myxomycetes provides evidence for a group of species specialized on feeding upon algae. Southern temperate zones with a climate characterized by a summer rainfall peak seem to provide the best conditions for corticolous myxomycetes. A preliminary checklist from the Great Smoky Mountains, eastern North America, records 168 species, with 47 (28%) inhabiting bark.

Deserts possess the most distinctive myxomycete assemblage of any major vegetation zone. A winter-cold desert in Kazakhstan was found to support 28 species of myxomycetes, with 18 of them more common. A few species were exceedingly abundant.

Tropical forests are dominated by litter-inhabiting, robust forms of myxomycetes. Typical features of tropical myxomycetes are: phaneroplasmodia, preferring litter substrata, stalked and relatively large fructifications. Corticolous species are almost completely absent in wet tropical forests.

Tropical rainforests exhibit an ‘reverse’ pattern of myxomycete diversity which decreases with increasing elevation and annual rainfall. Investigations in Costa Rica, Ecuador and Puerto Rico show that both abundance and species richness decrease significantly along a gradient from tropical dry or moist to tropical wet forest.

The reason for this pattern deviating from most other groups of organism is likely to be the continuously high moisture in tropical wet forests. As derived from a comparison between Ecuador and eastern North America, the same species grow longer stalks in the Tropics, which increases the proportion of resources which could otherwise be used for spore development. High moisture hinders the drying of spores which enables them to become airborne, and promotes the development of myxomyceticolous fungi, which in turn reduces the number of viable spores.

Myxomycete diversity on Earth increases from the Arctic to the southern temperate zone, but decreases again in the humid Tropics. All three approaches indicate that eastern North America, and perhaps eastern Asia, are the global “hot spots” for myxomycetes. As such, patterns of species diversity differ from those known for vascular plants.

image myxo biodiv on earth (from PP file)

Species distribution

Distribution patterns of myxomycetes can be explained by a combination of both microhabitat and macroclimate requirements. An example is a first map for the highly disjunct range of Barbeyella minutissima, a minute species following the distribution of montane spruce-fir forests on Earth. [Link pdf]

image PP map BARmin

Most myxomycete species are not cosmopolitan, especially when abundance values are regarded. Preliminary dot maps for two species show discrete patterns, which would be even more pronounced if abundance values would be available troughout.

image dist LEOfra habil

image dist CERmor habil

New Tropical Microhabitats

In tropical forests, aerial microhabitats display a higher myxomycete diversity than those at the forest floor. Aerial substrata such as decaying leaves, stems, fruits and flower remnants have a higher probability of drying out, thus providing the spores with a better chance to become airborne for long-distance dispersal.

In wet tropical forests myxomycetes occur on foliicolous liverworts that inhabit living leaves. This newly discovered microhabitat supports small populations of myxamoebae, which can develop fructifications in culture. At least three species occur regularly and with a much higher frequency than on litter from the forest floor.

image epi myxo

Inflorescences of giant Zingiberales forbs harbour a specialized community of myxobacteria and myxomycetes. By means of sporocarp counts and multivariate statistics, this community was characterized ecologically. Three species of myxobacteria and six myxomycetes have a clear preference for this microhabitat, formed by decaying corolla parts which possess a basic pH and which remain in the still living inflorescence bracts. Evidence for birds functioning as dispersal factors of these myxomycetes is presented.

image inflo myxo (C:\plates\slides\HELmar2.jpg)

Physarum didermoides fruiting in 3 m height on the inflorescence of Heliconia mariae, Costa Rica.

Myxomycete ecology

Interspecific competition seems to be rare, but may occur in myxomycetes. Based upon substratum cultures, sporocarp numbers were counted or estimated for the community from a Kazakh desert. These counts were used to compute niche breadths and niche overlap for the commoner species. Evidence of competition was found among species with large phaneroplasmodia.

Myxomycetes are best adapted to fluctuating moisture in the environment. Dry conditions facilitate dispersal. Together with durable dormant stages, this enables myxomycetes to respond rapidly to temporally and spatially changing microhabitats. As a general pattern for all surveys, myxomycetes need very high moisture for their amoebal stages, high moisture for the plasmodial phase and dry conditions to ensure effective spore dispersal by fructifications. This explains the low abundance of myxomycete fructifications in tropical wet forests versus the high abundance in arid regions with fluctuating rainfall.

The evolution of the myxomycete fructification

The stalked fructification is typical for myxomycetes, and represents the major evolutionary advantage compared with other groups of micro-organisms. Almost two thirds of all species with solitary fructifications develop a stalk. Its main function is to elevate the sporotheca above the substratum surface, to allow the spores to dry out and become airborne. Perhaps as a case of parallel evolution, also Myxobacteriales, Protosteliales, Dictyosteliales and Acrasiales “invented” this kind of fructification.

A resource allocation model shows that stalked fructifications face a size limit, which is the reason that large plasmodia usually divert their resources into many small sporocarps. As shown by a database of the better-known myxomycete species, stalk length increases with sporotheca volume. The limit is reached with a stalk length of about 1 mm and a volume between 0.1 and 0.5 mm³. For larger volumes, stalk length decreases again.

image PP relationship stalk length and spc volume