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STF1053 BIODIVERSITYLU3: Species DiversityPART I

SPECIESSPECIES are the different kinds of organisms found on the Earth.

A more exact definition of species is a group of interbreeding organisms that do not ordinarily breed with members of other groups.SPECIES (cont.)If a species interbreeds freely with other species, it would no longer be a distinctive kind of organism.

This definition works well with animals.SPECIES (cont.)However, in some plant species fertile crossings can take place among morphologically and physiologically different kinds of vegetation.

In this situation, the definition of species given here is not appropriate.SPECIES (cont.)There are several competing theories, or "species concepts

The most widely accepted are the morphological species concept, the biological species concept, and the phylogenetic species concept.

Although the Morphological Species Concept (MSC) is largely outdated as a theoretical definition, it is still widely used.SPECIES (cont.)According to this concept:

species are the smallest groups that are consistently and persistently distinct, and distinguishable by ordinary means.

Cronquist (1978): states that "a species is a community, or a number of related communities, whose distinctive morphological characters are, sufficiently definite to entitle it, or them, to a specific name"Cronquist, A. (1978). Once again, what is a species? In L.V. Knutson (Ed.),Biosystematics in Agriculture. (pp. 3-20). Montclair, New Jersey, U.S.A: Allenheld Osmin.SPECIES (cont.)The Biological Species Concept (BSC), states that "a species is a group of interbreeding natural populations that is reproductively isolated from other such groups".

According to the Phylogenetic Species Concept (PSC), is defined a species : "is the smallest diagnosable cluster of individual organism (that is, the cluster of organisms are identifiably distinct from other clusters) within which there is a parental pattern of ancestry and descent".SPECIES (cont.)These concepts are not in agreement, and considerable debate exists about the advantages and disadvantages of all existing species concepts.

In practice, systematists usually group specimens together according to shared features (genetic, morphological, physiological).SPECIES (cont.)When two or more groups show different sets of shared characters, and the shared characters for each group allow all the members of that group to be distinguished relatively easily and consistently from the members of another group, then the groups are considered different species.SPECIES (cont.)This approach relies on the objectivity of the phylogenetic species concept (i.e., the use of natural, shared, characters to define or diagnose a species) and applies it to the practicality of the morphological species concept, in terms of sorting specimens into groups.Species DiversitySpecies diversity is the number of different species in a particular area (species richness) weighted by some measure of abundance such as number of individuals or biomass.

However, it is common for conservation biologists to speak of species diversity even when they are actually referring to species richness.Species Diversity (cont.)Another measure of species diversity is the species evenness, which is the relative abundance with which each species is represented in an area.

An ecosystem where all the species are represented by the same number of individuals has high species evenness.Species Diversity (cont.)An ecosystem where some species are represented by many individuals, and other species are represented by very few individuals has a low species evenness.Species Diversity as a Surrogate for Global BiodiversityGlobal biodiversity is frequently expressed as the total number of species currently living on Earth, i.e., its species richness.Between about 1.5 and 1.75 million species have been discovered and scientifically described thus far (Cracraft, 2002).Cracraft, C. (2002). The seven great questions of systematic biology: an essential foundation for conservation and the sustainable use of biodiversity.Annals of the Missouri Botanical Garden,89, 127-144.(cont.)Estimates for the number of scientifically valid species vary partly because of differing opinions on the definition of a species.

For example, the phylogenetic species concept recognizes more species than the biological species concept.(cont.)Also, some scientific descriptions of species appear in old, uncertain, or poorly circulated publications.

In these cases, scientists may accidentally overlook certain species when preparing inventories of biota, causing them to describe and name an already known species.(cont.)More significantly, some species are very difficult to identify. For example, taxonomically "cryptic species look very similar to other species and may be misidentified (and hence overlooked as being a different species).

Thus, several different, but similar-looking species, identified as a single species by one scientist, are identified as completely different species by another scientist(cont.)Scientists expect that the scientifically described species represent only a small fraction of the total number of species on Earth today. Many additional species have yet to be discovered, or are known to scientists but have not been formally described.

Scientists estimate that the total number of species on Earth could range from about 3.6 million up to 7.7 million, with 13 to 20 million being the most frequently cited range (Cracraft 2002).Cracraft, C. (2002). The seven great questions of systematic biology: an essential foundation for conservation and the sustainable use of biodiversity.Annals of the Missouri Botanical Garden,89, 127-144.(cont.)Species are grouped together according to shared characteristics (genetic, anatomical, biochemical, physiological, or behavioral) and this gives us a classification of the species based on their phylogenetic, or apparent evolutionary relationships.

We can then use this information to assess the proportion of related species among the total number of species on Earth.(cont.)Most public attention is focused on the biology and ecology of large, charismatic species such as mammals, birds, and certain species of trees (e.g., mahogany, sequoia).

However, the greater part of Earth's species diversity is found in other, generally overlooked groups, such as mollusks, insects, and groups of flowering plants.Regional patterns of species diversity(i) Species diversity or richnessA basic "statistic" of zoogeography is species diversity or "richness" which describe the numbers of species within a prescribed area, without regard to their numerical abundance or ecological importance in a given community.

There are three basic kinds of richness indices:1. Alpha diversity or the numbers of species within a generally small area (e.g., a hectare, sq. km, or a defined habitat such as a single lake).(cont.)2. Beta diversity

the change in the species richness between communities across an environmental gradient over a relatively small distance (e.g., the change across a salinity gradient or across a mountain slope). Beta diversity is often estimated by calculating a species turnover rate(cont.)

(cont.)3. Gamma diversity

the numbers of species across a very large area such as a biome or a continent. Gamma diversity tends to be a function of both alpha and beta diversity, i.e., it will trend to be high in areas of high alpha diversity and particularly across a diversity of habitats. Gamma diversity is simply the sum of all species across each area (without double counting shared species). In the table below, an "X" means that a species (1-9) is present in one of three areas (Forest, grassland, and scrub ecosystems).(cont.)

(cont.)(ii) The latitudinal gradient in speciesdiversityDescribed by some as the only true "law" of ecology is the observation made even by early zoogeographers that there tend to be more species (higher alpha, beta, and gamma diversities) in the tropics than at higher or lower latitudes.(cont.)(iii) What can explain the latitudinal gradient in species diversity?

The gradient in species diversity is clearly not just an distinctive characteristic of a few species. Its broad observance suggests a general shared process that consistently "produces" more species of any given taxon in the tropics than the poles.

There have been up to 35 different hypotheses proposed to explain the gradient in species diversity, many of which are not mutually exclusive (because many things co-vary with latitude, e.g., temperature, productivity, etc).(cont.)There are, however, several ways in which hypothese can be grouped. The so-called "mid-domain effect of Colwell and Hurtt (1994), for example, states that random distribution of geographic range boundaries between the "hard edges" of the poles of the earth results de facto in the greatest range overlaps mid-way between these edges, i.e., the equatorial regions, and, consequently, greater species diversity at the equator than at either pole.

Hubbell's (2001) "neutral theory of biodiversity and biogeography" is another null model against which predictions can be objectively tested.Colwell, R. K., and Hurtt, G. C. 1994. Nonbiological gradients in species richness and a spurious Rapoport effect.American Naturalist144:570-595.Hubbell, S.P. (2001) The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press, Princeton, NJ.(cont.)Two other general classes of hypotheses: equilibrium and non-equilibrium hypotheses. There have been alternative classification schemes proposed (time-based, habitat heterogeneity-based, evolutionary-based, etc ).

Equilibrium hypotheses suggest that a "steady state has been reached among the processes that influence species diversity and that the forces that increase diversity are exactly balanced by those that reduce diversity such that species diversity remains fixed through time.Willig, M. R., D. M. Kaufmann, and R. D. Stevens. 2003. Latitudinal gradients of biodiversity: pattern, process, scale and synthesis. Annu. Rev. Ecol. Syst. 34:273-309.(cont.)In other words, if we revisited an area in 100,000 years it would have the same species diversity (although the species composition may have changed).

The differences among areas in observed species diversity simply reflect the different relative magnitudes of forces that increase or decrease species diversity in a given area - imagine two containers each with a hole at a different height from the bottom. Water enters each container at the same rate, but the one with the lower hole holds less water (fewer species).(cont.)Non-equilibrium hypotheses involve the idea that a given community has not yet reached a "steady state and species diversity is still in the process of increasing (or decreasing) after some historical disturbance.

In other words, if we came back 100,000 years later, the species diversity may have changed to reach its "true" level. For example, Pleistocene glaciations eliminated most life from North America north of about 46 degrees latitude. After the ice sheets left, animals and plants re-colonized the vast area and perhaps the gradient in species diversity is simply due to the fact that not enough time has elapsed to allow all species to reach or establish populations in the newly-exposed habitat.(cont.)Such habitats are considered as "unsaturated" and given enough time the species diversities will increase to be the same as at more tropical latitudes.Six major sub-hypotheses have beenproposed to account for the latitudinal gradientHistorical shift (non-equilibrium): as explained above

Productivity: tropical areas are more productive and there is more useable energy available to be subdivided amongst more niches and hence more species. Opportunities for resource specialization increase species diversity(cont.)Note that in the productive environment, more species are able to achieve some critical value of population size although each occupies a narrower niches. This way, more productive environments may promote greater species diversity by allowing the persistence of more species that show a greater degree of resource specialization.(cont.)Although productivity is typically correlated positively with increase species diversity, it has its limits.

In fact, the productivity-species diversity relationship is often "dome-shaped", i.e., maximal species diversity is found in areas of intermediate productivity, that is, at intermediate latitudes.

Some of the most productive environments on earth are not the most speciose (rich in species). E.g., estuaries, salt marshes, hydrothermal vents, shallow eutrophic lakes).3. Harshness:small isolated or otherwise "harsh environments (like extreme temperature or moisture) have higher extinction rates, lower colonization potential, and less opportunity for resource specialization than more benign tropical environments.

This may explain why some of the most productive environments are not the most species in many cases, they are too stressful (e.g., salt marches [salinity stress], hot springs [heat stress]).

Stressful environments may simply increase extinction rates or reduce the potential for specialization and place a limit on the numbers of species that exist in such environments because only a few possess the requisite adaptations to extreme conditions.4. Climate stabilitymore variable climates prevent resource specialization and hence are able to support fewer species.

Again, problems with this hypothesis include the many exceptions. for instance, many regions that are quite stable (high mountain tops, deep abyssal regions of the ocean) have low species diversity. By contrast, many variable environments (e.g., the "dry" and "flood" seasons in Amazonia) are incredibly species rich.

Climate stability in terms of seasonality of solar radiation, like the tropics, are more productive5. Habitat heterogeneitydiverse physical environments promote isolation, and resource specialization, and hence speciation, and the coexistence of more species.

Such relationships have been documented for desert rodents and some aquatic habitats. Coral reefs, for instance, are outstanding examples of complex physical environments that support some of the highest diversities of marine species.6. Interspecific interactions(2-6 are equilibrium hypotheses): More species in tropical areas create a positive feedback through increased interspecific interactions such as competition, predation or parasitism.

The intensity of these interactions prevents a few species from dominating the resources, promotes specialization, and, therefore, potential coexistence of more species.Rapoport's RuleRapoport in 1982 formalized the observation that subpopulations of mammals in North America (i.e., within species) tend to show larger average geographic range sizes with increasing latitude.

This trend has been observed in several other taxa and is known as "Rapoport's Rule.Species DominanceAlso noted in community comparisons is another latitudinal trend. Tropical areas tend to have more species, but these species also tend to be rarer, i.e., any individual species accounts, in terms of numerical abundance, for a lower proportion of the total individuals summed across all species in that area, than the average species in more temperate areas.

In other words, there are fewer species in temperate areas, but those fewer species tend to be more abundant (a relatively few species "dominate" the landscape) and they have larger geographic ranges than those in tropical areas (where species are more numerous, but each one is less abundant and has a smaller average range size).STF1053 BIODIVERSITYLU3: Species DiversityPART II

SPECIES CONCEPT &SPECIATION

What is a species?1. Biological species concept

A species is a population or group of populations whose members have the potential to interbreed with one another and produce viable offspring, but who cannot produce viable offspring with other species.(cont.)Speciation is the process by which a new species originates and involves the creation of a population of organisms that are novel enough to be classified in their own group. There are two processes by which this can occur:

Anagenesis is the accumulation of heritable traits in a population, that transforms that population into a new species

Cladogenesis is branching evolution, in which a new species arises as a branch of from the evolutionary tree. The original species still exists. This process is the source of biological diversity

(cont.)For a new species to form, there needs to be isolation of some members of a species as a separate population. Forms of isolation, that interfere with breeding include both..2. Prezygotic and postzygotic barriersPrezygotic barriers prevent mating or egg fertilization if members of different species try to mate. Examples:

a. Habitat isolation- Two species that live in the same area, but occupy different habitats rarely encounter each other.

b. Behavioral isolation - Signals that attract mates are often unique to a species (e.g. different species of fireflies flash different patterns).

(cont.)c. Temporal isolation- Two species breed at different times of the day or during different seasons.

d. Mechanical isolation- Closely related species attempt to mate, but are anatomically incompatable. (Example: flowering plants with pollination barriers; some plants are specific with respect to the insect pollinator, often occurs with butterflies/moths)

e. Gametic isolation- Gametes must recognize each other. (Example: fertilization of fish eggs, chemical signals between sperm and egg allows sperm to recognize the correct egg)Postzygotic barriersprevent a hybrid zygote from developing into a fertile adult. Examples:

a. Reduced hybrid viability- Abort development of hybrid at some embryonic stage.

b. Reduced hybrid fertility- Meiosis doesnt produce fertile gametes in vigorous hybrids.

c. Hybrid breakdown- First-generation hybrids are fertile, but they cannot produce fertile offspring in the next generation (e.g. different species of cotton). Alternative concepts of speciesEcological species concept

Species are defined by their use of environmental resources; their ecological niche (e.g. species that are defined by their food source such as butterflies with certain flowers)

Morphological species concept

- Takes into consideration factors such as body shape, size, etc.

(cont.)Paleontological species conceptSpecies in the fossil record are characterized according to a unique set of structural features.

Phylogenetic species concept- Recognizes species are sets of organisms with unique genetic histories. This idea is based often on molecular analyses such as DNA sequences.Modes of speciation1. Allopatric speciation

- Allopatric speciation describes speciation that takes place in populations with geographically separate ranges. Gene flow is interrupted and new species evolve.

2. Sympatric speciation

Sympatric speciation describes speciation that takes place in geographically overlapping populations. Chromosomal changes and nonrandom mating reduce gene flow.

Remember: Species arise when individuals in a population become isolated one from the other.Modes of speciation (cont.)

Modes of speciation (cont.)

Sympatric speciationSympatric speciation describes speciation that takes place in geographically overlapping populations. This can occur by chromosomal changes and nonrandom mating. Both can reduce gene flow between organisms and cause populations to evolve to new species.