Species–area relationship - Wikipedia
Species-area relationships (SARs) are fundamental to the study of key and The results revealed a high level of uncertainty in model selection. The species–area relationship (SAR) gives a quantitative description of This model unites the single-species metapopulation theory with the. The species-area relationship (SAR), that is, the increase of species number with shape or if different systems should have different SAR models . .. in meta- analysis will not go away,” Clinical Psychology Review, vol.
The impact of habitat heterogeneity and increased risk of extinction in small areas have been explored most often, but other key ecological processes e. In addition, several studies have examined how aspects of sampling design, such as whether observations are nested or scattered across space or are of regular or irregular shapes and sizes, influence the shape and parameters of the species-area relationship through their different sensitivities to species aggregation, habitat heterogeneity, and biogeographic processes.
Furthermore, species-area relationships are often quantified differently, depending on the goals of a study. Despite the fact that most studies of species-area relationships focus on inferring ecological phenomena from the form of the relationship, small-scale trends often reflect spatial processes that limit the number of individuals that can fit in a small area. In summary, the mathematical functions used to characterize species-area relationships often have different parameters when applied to data from different ranges of area, and these differences in observed species-area functions are often attributed to sampling methodologies and underlying ecological and biogeographical processes.
Looking forward, ecological research is expanding from its past species-centric perspective to a greatly increased focus on traits of organisms and their phylogenetic relationships, which is leading to examination of how these factors also vary with area see Beyond Species-Area Relationships. General Overviews Species-area relationships were first documented and debated among plant ecologists seeking to characterize and compare plant communities.
The subject later gained popularity among animal ecologists with the seminal work of Preston on species abundance distributions and with Robert MacArthur and Edward O. An excellent historical review is provided in McGuinnesswhich connects debates over the form and function of species-area relationships with emerging ecological theory. Connor and McCoy also reviews the evidence linking species-area relationships to biological and ecological explanations, but the authors focus on the statistical validity of attempts to use the form and parameters of species-area curves to discern ecological causality.
Rosenzweig explores in detail several examples of species-area curves and uses them to discuss the many factors that influence the shape of these curves, while Drakare, et al. Because of the variety of research goals inherent in studies of species-area relationships, sampling and analytical methods, as well as definitions of what constitutes a species-area relationship, often vary among studies. Scheiner defines six types of species-area curves that differ in the spatial arrangement of samples, whether larger samples are constructed in a spatially explicit fashion from adjacent smaller samples, and whether means or single values are used for a given spatial scale.
The statistics and biology of the species-area relationship.
Which function describes the species-area relationship best? A review and empirical evaluation.
BioMed Research International
We think that, even if a meta-analysis suggests the existence of general patterns, the value of the original studies that rejected the existence of those patterns should be not diminished. In particular, here we claim that the fact that many studies have found a lack of relationship between host size and parasite diversity deserves explanation.
From this perspective, we can consider the expected positive PHrR and PHsR as the null hypotheses parasite diversity is expected to increase with host size or distribution. In the following paragraphs we will discuss a few tentative explanations for the fact that these null hypotheses have been rejected in several case studies.
Network Structure Host species and their parasites are arranged into antagonistic ecological networks [ 24and references therein]. These networks are characterized by patterns such as nestedness, species cooccurrence and modularity, which are the result of the underlying processes that minimize competition and risk of coextinction [ 25 — 27 ].
For example, in a nested network, the set of parasite species using any host is a subsample of any richer set. This scenario, that seems to be ubiquitous in host-parasite systems [ 26 ], implies, in general, a good degree of variability in parasite species richness per host [ 28 ]. Although species range or body size may contribute to this variability, the number of parasites found on a given host can be regulated by other factors determining the network structure and particularly species interactions.
For example, a recent work demonstrated that specialist parasites minimize their risk of coextinction by using hosts with low vulnerability to extinction [ 27 ].
A Few Good Reasons Why Species-Area Relationships Do Not Work for Parasites
This pattern, which is closely related to the nested structure of antagonistic networks, may emerge from the fact that several features increasing vulnerability of a host may be inversely related to the probability of that host to establish a stable symbiotic relationship with a parasite [ 27 ].
Geographical range plays an important role in determining these relationships, but also several other fundamental aspects of host ecology may be involved, such as host population size which may be negatively correlated with body size [ 29 ] and persistence over evolutionary time [ 27 ]. This last aspect has been also suggested as an important determinant of parasite species richness colonization time hypothesis [ 30 ].
It should be also highlighted that large sized species are typically less abundant and more endangered than small sized ones [ 31 ], which may lead large sized hosts to harbour fewer parasite species than smaller sized ones, in contrast with PHsR expectations.
What we are suggesting here is that the number of parasite species found on a host is determined by how much this host interacts with other hosts and that the degree of interactions can be affected by too many aspects of host ecology, biology, and ethology to be embedded into a general law. Enemy Release One of the reasons for the success of alien species is the absence of parasites in the newly colonized area [ 32 — 34 ].
Plant and animal invaders that escape their native enemies are less parasitized than conspecific populations in the native range [ 323536 ]. Enemy release is due to the fact that most of the parasites that a colonist host might bring with it are either left behind during the colonization process, lost shortly thereafter, or cannot survive in the new area [ 37 ]. This effect may be partly counterbalanced by the fact that alien species tend to acquire local parasites in their new distribution areas [ 32 ].
However, some alien species, despite having acquired new parasites from native hosts, have been observed to be less parasitized than their native competitors in terms of both parasite diversity and abundance [ 38 ].
Despite the enemy release hypothesis has been addressed mainly to introduce species; it is based on principles that are applicable to any species capable of extending its range via jump dispersal. For example, the majority of fish species and particularly reef fish disperse as larvae, which may travel for very long distances [ 39 ] and are, in general, much less parasitized than adult individuals [ 40 ].
One of the main assumptions behind the application of SARs to host-parasite systems is that a host is likely to increase its parasitofauna through range expansion, due to the fact that a wide geographical range raises the odds of encountering new parasite species PHrR. This assumption, however, is clearly in contrast with the fact that hosts may actually loose parasites during range expansion and that the acquisition of new parasite species from native hosts has been observed to be not sufficient to compensate such loss.
Sampling Biases Our knowledge of parasite diversity is far from being complete [ 41 ], with most of the hosts being unsampled or heavily undersampled [ 42 ]. Parasitofaunas of hosts with wide distributions, which are also often more locally abundant [ 11 ], are probably better known than those of less common hosts. This should create artifactual evidence in support of the hypothesis that parasite richness is positively correlated with host geographic range PHrR.
To address this well-known problem, estimates of study efforts are usually introduced in parasitological studies [ 643 ]. However, these corrections cannot solve another fundamental bias due to host spatial sampling. In general, host species have been screened for parasites only from a few localities within their distributional range. This may imply that we know only a small fraction of the parasite diversity associated with a certain host. Moreover, we cannot assume that the geographical distribution of a parasite equals that of its hosts.
The actual ranges of parasite species may be indeed much more restricted than those of their known hosts, due to the fact that, as discussed in the previous paragraph, a host may lose some of its parasites during colonization [ 323536 ].
This problem has no easy solution, and it is further complicated by difficulties in parasite identification. Host taxonomy has been long used to assist the identification of parasite species. Yet, the classification of several parasites has been recently challenged by new molecular evidence that enlightened how broadly distributed generalist parasites are indeed complexes of species [ 44 ], thus reinforcing the idea that parasite ranges are often much smaller than those of their suitable hosts and that SAR studies based on host parasite checklists may be indeed biased, unless detailed information on species distribution is provided along with host-parasite records.
Evolutionary Tendency Towards Host Specificity Most parasites have few chances to find a proper final host, and infecting an unsuitable host would lead a parasite to starve, or to be killed by the host immune response, or to kill the host and hence itself [ 45 ].
Selective pressures would therefore favour two different, opposite evolutionary processes: Although the two strategies seem to draw a clear separation between generalist and host specific parasites, they overlap with various degrees, keeping parasite host ranges from being extremely narrow or extremely wide [ 46 ].
However, it is known that relatively few parasites use many fish hosts, which supports the hypothesis that adaptations enhancing host finding are evolutionarily more stable than those softening parasite niche requirements [ 27 ].
This skewed pattern might be even more pronounced because there are likely more cryptic species among parasites than among hosts.
Host-parasite coevolution is one of the most commonly assumed causes for specialization [ 47 ] and parasites may have a tendency for overspecialization [ 48 ]. The fact that most parasites are species specific would therefore lead to a pronounced skew also in the distribution of the number of parasite species per host, with a few hosts harbouring several specific parasites and many hosts harbouring few generalist parasite species.
Histogram showing the distribution of the number of parasite species per host species obtained using all available data from FishPEST database i.
The most recurrent situation is that of hosts used by only one parasite species.SPECIES -AREA RELATIONSHIP- ECOLOGY NCERT
Concluding Remarks Host-parasite interactions are far from being neutral [ 52 ].