Species pool hypothesis
The species pool hypothesis (Engl. Species pool hypothesis) emerged at the end of the last century as an alternative top-down approach to the previously propagated bottom-up explanations for the biodiversity at Community level ( biological communities ). It generally states that the number of species on a smaller scale is essentially determined by the available species (number of species) on the next higher scale.
Origin of the hypothesis
The hypothesis was developed in the context of a discussion of the CSR model . The starting point is the unimodal curve, which is shown in the relationship between the number of species of communities and the environmental conditions ( site conditions ) under which they occur. This pattern, known as the hump-backed curve , is generally explained by small-scale processes: Under “bad” (less fertile / infertile) conditions, the (environmental) stress is so great that only a few species can be found can settle. If the conditions improve, more and more species can be found. If the conditions continue to improve, competition between species and the exclusion of competition will increasingly occur - the number of species will decrease, as few highly competitive species will prevail.
Alternatively, however, it is also possible that there are simply fewer species that are adapted to fertile locations. On the right-hand side of the curve, species are therefore not displaced by competitors; instead, there are fewer species because the species pool (a group of species that can potentially occur under the conditions under consideration) is smaller. The reason given is that fertile sites are rarer and therefore there was only a small possibility of developing characteristics of adaptation (e.g. high potential biomass ) to these conditions in the course of evolution . Since intermediate locations are more widespread, more species were able to adapt to the conditions - the species pool is therefore larger. Poor locations, on the other hand, were dominant at the beginning of the settlement of the country (or the location conditions prevailing everywhere), but were increasingly converted into intermediate to fertile locations and are also "rare" today. In addition, the resources are still limiting in poor locations .
In addition to the area occupied by a location or habitat type , the age of such an area is also important. With increasing ( geological ) age, the probability of adaptation and thus the size of the species pool (species accumulation) also increase. Examples are the tropical rainforest and the Sonoran desert . Both are very old or relatively stable habitats for a long time and, regardless of their different fertility, are very species-rich. Another example is the species poverty of temperate European deciduous forests in relation to the forests of the same vegetation zone in North America and East Asia . The glaciation in the Pleistocene resulted in greater species loss in Europe than in the other two regions, as mountains such as the Alps and Pyrenees , as well as the Mediterranean, formed barriers to the retreat and re-immigration of species.
Definitions
The species pool concept can be understood as nested pools on different scales. The size of the outer pool determines the size of the subsequent pool, with each level being characterized by certain processes. The highest level is the global species pool. This has not yet been named in the literature, but is to be supplemented here as a pool encompassing all species worldwide (depending on the intention, a certain group of organisms such as plants , animals or vascular plants can be considered here). On this global scale, speciation and large-scale migration are fundamental processes; they determine which species can immigrate to the regional species pool (the level below). The regional species pool is a group of species that occurs in a certain region and which is possible to coexist in the target community under consideration . A region in this context is a physiographically and climatically be reached relatively uniform area from which the target community. There is also a flora for the region . The next level is called the local species pool ; H. a group of species found in the landscape around the target community. It is possible for the species to coexist in the target community and they can immigrate to it relatively quickly (within a few years). The community species pool (also the current species pool) finally includes all species that are currently present in the target community. At the boundary between the local and community species pool, abiotic and biotic conditions act as ecological filters to determine which species migrate into the community under consideration.
criticism
A major point of criticism is that the species pool hypothesis is too restricted to the top-down approach. The influence of the species pool should not be seen as the sole determining factor in relation to local biodiversity. Rather, it should be seen as a supplement to local mechanisms such as niche and competition. It is also pointed out that the direction of the relationship between local and regional biodiversity and the underlying mechanisms are unknown.
The second weak point of the hypothesis is its testability. For statistical reasons, the pools of communities in test studies must not overlap (must be independent). Nevertheless, they must contain similar taxa with similar location requirements in order to be comparable. However, this is practically impossible to implement for most groups of organisms. In some cases, one could work with one test community per continent , but the number of continents and thus samples is then too small to meet statistical requirements. Previous studies (e.g.) are useless in this regard. If not testable, however, the entire hypothesis also becomes unusable.
Individual evidence
- ↑ Zobel (1997): The relative role of species pools in determining plant species richness: an alternative explanation of species coexistance? Trends in Ecology & Evolution 12, pp. 266-269.
- ↑ Taylor, Aarssen, Loechle (1990): On the relationship between r / K selection and environmental carrying capacity: a new habitat template for plant life history strategies. Oikos 58, pp. 239-250.
- ↑ Grace (2001): Difficulties with estimating and interpreting species pools and the implications for understanding patterns of diversity. Folia Geobotanica 36, pp. 71-83.
- ↑ Herben (2001): Correlation between richness per unit area and the species pool cannot be used to demonstrate the species pool effekt. Journal of Vegetation Science 11, pp. 123-126.
- ↑ Lepš (2001): Species-pool hypothesis: limits to its testing. Folia Geobotanica 36, pp. 45-52.
- ^ Wilson, Anderson (2001): Species-pool relations: like a wooden light bulb? Folia Geobotanica 36, pp. 35-44.
- ↑ Pärtel, Zobel M., K. Zobel, van der Maarel (1996): The species pool and is relation to species richness: evidence from the Estonian plant communities. Oikos 75, pp. 111-117.
- ↑ Duncan, Buckley, Urlich, Stewart, Geritzlehner (1998): Small-scale richness in forest canopy gaps: the role of niche imitation versus the size of the species pool. Journal of Vegetation Science 9, pp. 455-460.
- ↑ Pärtel, Zobel (1999): Small-scale plant species richness in calcareous grasslands determined by the species pool, community age and shoot density. Ecography 22, pp. 153-159.
- ↑ Safford, Rejmanek, Hadač (2001). Species pools and the “hump-back” model of plant species diversity: an empirical analysis at a relevant spatial scale. Oikos 95, pp. 282-290.