--I doubt that the Aegla are the reason why crayfish did not spread in South America. If you look at the distribution of the crayfish in South America you will notice that most of them are behind the Andes and in temperate zones. So, natural barriers such as mountain ranges and temperature are most likely culprits.
Sorry about my wording – I was more focused on past interspecific competition between crayfish and
Aegla sp., as outlined in the “Introduction” of
http://www.museum.vic.gov.au/memoirs/do ... _Tudge.pdf. Do you accept that these generally temperate freshwater anomurans may have played a role in forcing crayfish (along the "fault line" of historical overlap) out of their traditional aquatic benthic roles?
Even on the basis of gross external morphology, the
Aegla evince distinct covergency (even considering homology) with crayfish, and are often confused with such.
--Again, unlikely. Macrorachium and crayfish have been living happily side by side in the Mississipi, Madagascar, New Guinea and Australia.
The Big River Region includes the Missouri and Mississippi rivers. Many kinds of fish are characteristic of the channels of these streams, but crayfish occur only as small local populations or stray individuals [Incision: there is presumably no outlier of salinity]. Four-inch shrimp (Macrobrachium ohione) was formerly abundant in the Mississippi River, but disappeared about 30 years ago [Incision: this was due to unsustainable harvesting and artificial channelization of the river; they have, since then, been "rediscovered" in stretches of their former range]. Several crayfish species characteristic of the other regions are common in sloughs and marshes on the river floodplains [Incision: atypical habitat for a fluviate prawn]. These include the White River crayfish (P. acutus) and dwarf crayfish (C. shufeldtii) along the Mississippi River, and the papershell crayfish (O. immunis) along the Missouri River. The devil crayfish (C. diogenes) is the common burrowing crayfish on floodplains of both rivers.
(From
http://www.conservation.state.mo.us/nat ... istrib.htm)
Crayfish-prawn "coexistance" as applies to that regional case, is tenuous at best. However, the text is unclear in several regards; while it seems to suggest that crayfish are scarce in streams of the Mississippi/Missouri watersheds of the surveyed region, it gives little indication for or against their presence in the main river body. If competitive exclusion by riverine prawns was a major limiting influence, one might expect that crayfish have today expanded to assume the now-largely vacated stretches (if dam construction and pollution have not impeded such movement).
It must furthermore be noted that geographical adjacency is not interchangeable with "side-by-side" sympatry, per se.
The overlapping fronts between crayfish and Macrobrachium prawns (Southern North America, Irian Jaya, Japan, New Zealand) tend to result in the ecological marginalization of both parties.
--I can't seem to make sense of this sentence. How would *both* groups be marginalized? You would think that one group would outcompete the other if they are occupying the same ecological niche. In any case...where are you getting the information from that they are even competing in the areas where they occur together? As I told you obove they have been living side by side in some areas for a long time and they don't seem to affect each other all that much.
When species meet, competition reduces population densities in the region of overlap, which, in turn, intensifies the asymmetry in gene flow from center to margin. This reduces the ability of each species to adapt to local physical conditions at their range limits. -
http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract
Interspecific competition tends to narrow the use of resources (encourages specialization as a way to reduce amount of competition between species)
A) exploitative – One species indirectly reduces the abundance of another species by reducing the shared limiting resource
B) interference - One species directly or aggressively interferes with another species access to a resource.
http://www.britannica.com/eb/print?tocI ... icle=false
Apparant "coexistance" of prawns and crayfish comes at the margins of both groups' range; overlap rarely occurs in zones of peak diversification (in which established crayfish fauna has firmly occupied benthic crustacean roles). One might put forth the case of Australia, the epicenter of crayfish diversity, and make note of the freshwater
Macrobrachium dwelling there, but, for a variety of reasons (forthcoming), this is not inconsistent with occupation of similar ecological niche.
Evidence/consequences of competition:
1) Adjacent, but non-overlapping ranges (i.e., parapatry)
2) Differences in resource use when two species are allopatric
3) Less than random co-occurrence pattern
4) Change in abundance of natives, with the introduction of nonnatives:
a) Replacement hypothesis – environmental degradation eliminated sensitive native fish; tolerant fish assumed ecological role
b) Displacement hypothesis – indigenous species are displaced by invading nonnative
[Parapatry is a type of speciation in the concept of natural selection. It occurs when the zones of two species with a similar ecological niche abut but do not overlap. The two species may have contact at the common boundary of their zones. The concept of parapatry is necessary to satisfy the postulate of natural selection that two species cannot co-exist in the same location over the longer term with the same ecological niche. It suggests that there may be some factor apart from distance or ecological role which keeps the two species apart.
It contrasts with allopatry where different species are geographically isolated from each other, and sympatry, where different species evolve in the same area by finding different ecological roles.]
See
http://aentry.nrifs.affrc.go.jp/~dbmngr ... _1_001.jpg
An understanding of crayfish-prawn interaction must take into account a broad range of biological conceptions:
- Ecological release. In ecological release, a species occupies a broader niche or geographical area in the absence of a closely related competitor. An example comes from the distribution of two species of planaria in streams. When found alone in a stream (allopatric distribution) each species occupies a wide range of stream temperatures. When both species are found in the same stream (sympatric distribution), however, the distribution of both species is restricted.
P. montenegrina is found from 5 to about 13.5 C, whereas
P. gonocephala occupies the warmer portions of the stream from 13.5 to approximately 23 C (Beauchamp and Ullyott 1932).
- Contiguous allopatry. In this phenomenon, two species occupy distinctly different geographical areas directly adjacent (contiguous) to one another. Another study on chipmunks on the east slope of the Sierra Nevada Mountains in California provides an example of both contiguous allopatry and ecological release. Heller and Gates (1971) found four species of chipmunks living at different altitudes. The least chipmunk (
Eutamias minimus) is found at the lowest elevations in the sagebrush range. When all other chipmunks are absent, the least chipmunk can occupy all altitudes up to the alpine. However, the yellow pine chipmunk (
E. amoenus) through aggressive behavior restricts the least chipmunk to the hot, dry sagebrush areas. If the least chipmunk is absent, the yellow pine chipmunk does not invade the hot, dry habitats. Evidently this dry area is not part of the fundamental niche of the yellow pine chipmunk. The lodgepole pine chipmunk (
E. speciosus) is the most aggressive of the four species, but it is most vulnerable to heat stress and is restricted to shady, cool forests. It apparently limits both the upper distribution of the yellow pine chipmunk and the lower distribution of the alpine chipmunk (
E. alpinus). Therefore, these four species, when all are present, show contiguous allopatry by altitude. When one or another is absent, the least chipmunk, and perhaps others, shows ecological release.
- Niche partitioning. In niche partitioning, two or more species coexist while sharing one or more resources in such a way that the niche overlap apparently violates the competitive exclusion principle. Upon closer investigation, the resources, though shared, are used with different frequencies or are used in different ways so as to allow coexistence. For example, the root systems of coexisting annual plants can be shown to partition the soil by depth, thereby avoiding direct resource competition (Wieland and Bazzaz 1975). In a classic study, MacArthur (1958) showed that five species of Dendroica warblers coexisted by foraging in different portions of trees in a coniferous forest. Although there was overlap, each species spent the majority of its foraging time in a unique portion of the trees. Other examples come from Diamond (1978). In one case he describes the coexistence of four species of fruit pigeons on islands of the Bismarck Archipelago. The pigeons range in size from 91g to 722g. The fruit sizes they consume range from 3 mm in diameter to 50 mm in diameter. Again, while there is a great deal of overlap among the pigeons in terms of fruit sizes consumed, the mean usage rates are clearly different. The smallest pigeon consumes fruits that are on average, 8 mm in size, the 135g pigeon consumes fruits that average 11 mm, the 470g pigeon, 18 mm and the 722g pigeon, 25 mm. In a similar analysis, Diamond (1978) shows that eight species of fruit pigeons in New Guinea coexist not only by specializing on different sizes of fruit, but also by the position of the fruit on the branches.
Niche partitioning can also occur within a species. Male red-eyed vireos forage for insects primarily in the upper canopy of the forest, while females concentrate their foraging in the lower canopy and near the ground (Williamson 1971).
- Character displacement. Character displacement describes a situation in which two species, when living in separate geographical ranges (allopatric distributions) have nearly identical physical characteristics (i.e. beak sizes in birds, overall body sizes in lizards and snails, canine sizes in the cat family). When sympatric, however, these physical or morphological characteristics diverge in one or both species. This divergence minimizes competition for food and allows the two species to coexist. Brown and Wilson (1956) appear to have introduced this idea. When examining the overall size and beak lengths of specimens of the eastern (
Sitta tephronota) and western rock nuthatches (
S. neumayer), they found that the allopatric populations were almost identical in both average size and in the range of sizes. However, these two species become sympatric in Iran. In sympatry, the eastern rock nuthatch increases in size while the western species becomes smaller. In this sympatric zone their beak and body sizes are completely non-overlapping. This allows them to feed on different sized prey items and therefore coexist.
In the above examples only pairs of species were examined. However, size displacements may occur among several coexisting species. Strong et al. (1979) called this “community-wide character displacement.” A pattern of regular differences in some size-ranked sequence has been accepted as evidence that past competition helps to mold niche differences. In fact G.E. Hutchinson (1959) proposed that a ratio of only 1.1 to1.3 between closely related species in this size-ranked sequences was necessary for coexistence. This is known in the literature as Hutchinson’s ratio or rule. There are many who have questioned the meaning or significance of such a rule (Simberloff and Boecklen 1981, Strong and Simberloff 1981). However, an interesting paper by Dayan et al. (1990) has shown that among wild felines in Israel, character displacement of canine diameters is remarkably constant at 1.10-1.14. These size ratios include three species of Felis, but also include male and female dimorphism in canine sizes. A great deal more information is needed on: 1) the size distribution of potential prey; 2) whether prey populations are limiting to predator populations; 3) whether larger canines correlate with the capture of larger prey; and 4) habitat use by the cats. Nevertheless, the data suggest that competition has played an important role in the evolution of the feline community.
Taper and Case (1992) have cautioned that any study purporting to demonstrate character displacement should meet the following criteria:
1. Morphological differences between a pair of sympatric species must be statistically greater than the differences between allopatric populations.
2. The observed differences between sympatric and allopatric populations must have a genetic basis.
3. Differences between sympatric and allopatric populations must have evolved on the site and not be due to different founder populations.
4. Variation in the character must have a known, ecologically important function.
5. Competition must be known to occur for a resource in short supply and the character must play an important role in that competition.
6. Differences in the character cannot be explained by differences in resources available to the sympatric and allopatric populations.
- Historical replacement. Finally, over human history there are documented cases of one species invading and eliminating another species from its original range. For example, Diamond (1978) has described both historical replacement and niche segregation in two species tits (
Parus). The blue tit (
P. caeruleus) is found in Europe west of the Ural Mountains where it occupies a wide variety of habitats from forest to riparian thickets. The azure tit (
P. cyanus) was formerly found in Asia east of the Urals. But in the late 19th Century it spread 1000 miles west across Russia to the Baltic Sea. Over the next 10 years, however, the azure tit retreated several hundred miles eastward from the Baltic. At present, the two species overlap on eastern edge of the geographical range of the blue tit. However, in the overlap zone the two species segregate by habitat. The azure tit is found in riparian thickets, while the blue tit is in upland forest.
There are many known examples where introduced species have outcompeted the native flora or fauna. Starlings and house sparrows are infamous for evicting native cavity nesting birds such as bluebirds and flickers from nest sites. The purple loosestrife (
Lythrum salicaria), a native of Europe, has invaded wetlands in temperate North America is crowding out native wetland plants. Finally, Australia and Hawaii are living laboratories demonstrating the effects of introduced competitors (as well as predators and herbivores) on native flora and fauna.
http://mason.gmu.edu/~lrockwoo/sp02%20I ... 0Niche.htm
--No idea. I am not aware of any studies about this. It's also puzzling why they would introduce crayfish for snail control since Macrobrachium shrimp eat shrimp just fine.
I share your confusion. It may be that the regions of introduction do not possess native
Macrobrachium or that they have suffered from environmental degradation. Even in such a situation, it may have been a more conscientious to favor the introduction (with respect to water systems with no linkage to estuarine or marine bodies) of
Macrobrachium over crayfish, as the former option restricts potential for unwanted population recruitment.
Macrobrachium & Snails:
http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract
Crayfish & Snail Fever:
http://www.unm.edu/~quantum/quantum_sp1 ... fever.html
