Why is species abundance important

Table 2. Indicative response of conservation and conventional tillage practices on weed abundance and diversity. Fertilization affects soil fertility and nutrient uptake, thus resulting inincreasing agricultural yields, as well as in modifications of weed communities Allan et al.

Weed communities tend to be more diversified in low than in high input systems Gough et al. Finally, it has been proposed that at higher nitrogen level of soils, invasive annual grasses can be reproduced better compared to native species Vasquez et al.

Blackshaw et al. In addition, Sweeney et al. At the same time, soil N fertility was considered to have effects on weed seed mortality for some specific weed species, such as Abutilon theophrasti, Ambrosia trifida , and Eriochloa villosa Davis, On the other hand, Kakabouki et al.

Cover crops and intercropping are considered to be practices that improve soil fertility with adverse effects on weed communities Bilalis et al. As mentioned by Bilalis et al. Similarly, the lowest value of Shannon's index was also found in the same plots. It is reported that increased nutrients level increased the dry matter of weeds Mohammaddoust-e-Chamanadad et al. Blackshaw and Brandt reported that the competitiveness of Lolium persicum low N-responsive was not influenced by nitrogen added.

On the contrary, the competitive ability of Amaranthus retroflexus high N responsive increased with higher levels of nitrogen. In the case of Avena ludoviciana the added N increased the negative effect on wheat grain yield Lack et al. Previous studies had shown that soil enrichment significantly reduced the richness of native species in grassy woodland ecosystems McIntyre and Martin, In addition, species richness was negatively correlated with phosphorus, and species evenness was negatively correlated with the ratio of organic carbon to total nitrogen in soil according to the study of Ma Blackshaw and Brandt also mentioned that P-responsive species were more competitive as added P increased.

On the contrary, Freyman et al. In another study, Digitaria ischaemum was found to be the dominant species under NK and non-fertilized treatments, Cyperus rotundus dominated under phosphorus PK treatment, while more weed species and higher Shannon' s diversity values were detected in the balanced fertilization treatment Yin et al.

Wan et al. Than et al. The results showed that the N and P fertilizer application had a more significant impact on weed community compared to the K application. In another study, the growth responses of common crops and weeds with addition of composted poultry manure CPM were compared Little et al.

The results indicated that weed growth response to CPM was not explained by K or N added with the exception of velvetleaf. In another study, Ugen et al.

The results revealed that the weed nutrient uptake and growth was increased with N and P application, whereas the relative competitiveness of bean was increased further to K application. Tang et al. The same authors reported that the PK treatment favored weed density, shoot biomass and diversity compared to N plus P fertilizer treatments.

Weed community composition and structure can be greatly influenced by different management practices, such as tillage, and fertilization.

Weed species abundance and diversity can be incorporated into numerous population indices. This article cites only a few of the most commonly used indices. Choosing the accurate index with regard to the aims and context of each study should be a priority for species' abundance and diversity estimations.

In general, conservation of tillage systems seems to be associated with higher weed richness and diversity, as the elimination of tillage creates more enhancing conditions for some weed species. However, there are cases where reduced tillage systems led to less diverse weed communities compared to more intensive tillage systems.

Similarly, various results about the influence of different fertilization patterns were shown in the carried out experiments. There is a common trend that weed abundance and richness are positively affected by organic farming. Furthermore, diversity of weed species seems to be enhanced under low-input conditions, while low N fertilization level enhanced the effective control of weeds.

On the other hand, demands of weeds on nutrients are quite often proved to be species dependant. It is noteworthy that, at field level, predicting weed flora species responses to management filters, such as tillage or fertilization, remains a difficult task due to the environmental conditions which vary in time and space. This overview of the numerous experiments that determine the effect of tillage systems and fertilization on the composition and abundance of weed species in crop fields can be helpful in understanding how particular weed species increase or decrease, in terms of numbers and diversity, and how crop management can contribute to the suppression of weeds.

Another aim of this review is to raise awareness on the importance of conserving weeds biodiversity as an integral part of balanced agroecosystems.

Further research is essential in order to understand the complex relationships of weed species and how they are affected by different tillage amendments and fertilization patterns. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Allan, E. Land use intensification alters ecosystem multifunctionality via loss of biodiversity and changes to functional composition. Amuri, N. Weed populations as affected by residue management practices in a wheat—soybean double-crop production system.

Weed Sci. Armengot, L. Long-term feasibility of reduced tillage in organic farming. Tillage as a driver of change in weed communities: a functional perspective. Banks, P. Influence of long-term soil fertility treatments on weed species in winter wheat. Barkman, J. Acta Botanica Neerlandica 13, — Bengtsson, J. The effects of organic agriculture on biodiversity and abundance: a meta-analysis.

Berner, A. Crop yield and soil fertility response to reduced tillage under organic management. Soil Tillage Res. Bilalis, D. Effect of three tillage Systems on weed flora in a 3-year rotation with four crops.

Crop Sci. Effects of two legume crops, for organic green manure, on weed flora, under Mediterranean conditions: competitive ability of five winter season weed species. Google Scholar. Weed-suppressive effects of maize—legume intercropping in organic farming. Pest Manag. Effect of different levels of wheat straw soil surface coverage on weed flora in Vicia faba crops.

The effect of tillage system and rimsulfuron application on weed flora, arbuscular mycorrhizal AM root colonization and yield of maize Zea mays L. Blackshaw, R. Nitrogen fertilizer rate effects on weed competitiveness is species dependent. Phosphorus fertilizer effects on the competition between wheat and several weed species. Weed Biol. Weed species response to phosphorus fertilization.

Fertilizer, manure and compost effects on weed growth and competition with winter wheat in western Canada. Crop Protect. Bonham, C. Measurements for Terrestrial Vegetation. Booth, B. Weed Ecology in Natural and Agricultural Systems. Braun-Blanquet, J. Berlin; Heidelberg: Springer-Verlag. Bray, J. An ordination of the upland forest communities of southern Wisconsin. Brix, A. The relation between densities and frequencies of weeds in arable fields.

Buhler, D. Population dynamics and control of annual weeds in corn Zea mays as influenced by tillage systems. Influence of tillage systems on weed population dynamics and management in corn and soybean in the central USA. Perennial weed populations after 14 years of variable tillage and cropping practices. Cardina, J. Crop rotation and tillage system effects on weed seedbanks. Carter, M. Hillel Oxford: Elsevier , — Cheetham, A. Binary presence absence similarity coefficients.

Cheimona, N. Effect of different types of fertilization on weed flora in processed tomato crop. Procedia 10, 26— Clifford, H. An Introduction to Numerical Classification. Coffman, C. Corn-weed Interactions with long-term conservation tillage management. Cousens, R. Dynamics of Weed Populations.

Daubenmire, R. A canopy-coverage method of vegetational analysis. Northwest Sci. Davis, A. Nitrogen fertilizer and crop residue effects on seed mortality and germination of eight annual weed species. Effects of tillage systems and crop rotation on weed density, weed species composition and weed biomass in maize. Dorado, J. The effect of tillage system and use of a paraplow on weed flora in a semiarid soil from central Spain.

Weed Res. Evans, F. The vegetational structure of an abandoned field in southeastern Michigan and its relation to environmental factors. Ecology 36, — Fennimore, S. Organic amendment and tillage effects on vegetable field weed emergence and seedbanks. Weed Technol. Floyd, D. A comparison of three methods for estimating plant cover. Freyman, S. Effect of nitrogen, phosphorus and potassium on weed emergence and subsequent weed communities in south coastal British Columbia.

Canadian J. Plant Sci. Fried, G. Environmental and management factors determining weed species composition and diversity in France. Gamito, S. Caution is needed when applying Margalef diversity index. Gibson, R. Pollinator webs, plant communities and the conservation of rare plants: arable weeds as a case study. Gill, K.

Weed flora in the early growth period of spring crops under conventional, reduced, and zero tillage systems on a clay soil in northern Alberta, Canada.

Soil and Tillage Res. Gough, L. Fertilization effects on species density and primary productivity in herbaceous plant communities. Oikos 89, — Grey, T. Although it may be relatively easy to explain patterns when only a handful of species are involved, it has proven difficult to provide general rules about entire communities where many species co-occur and interact.

Such general rules would be very useful as they increase our understanding of how communities function and allow predictions as to how they would respond to environmental changes, for example global warming or restoration efforts. Although there are few rules that are universally true in community ecology Lawton , there are regular, predictable patterns.

Within most natural assemblages a few species comprise the majority of the individuals Figure 1; Preston This pattern in distribution of species abundances is accompanied on a larger scale level, by the tendency of widespread species to also occur in higher densities compared to species restricted in their geographic distribution. This relationship is termed the distribution-abundance relationship. Figure 1: The commonness and rarity of species in a community In nearly every community in which species have been identified and counted, distributions are highly skewed such that a few species are present in the greatest numbers.

In a mesotrophic ditch more than half of all the individuals belonged to one species Hygrotus decoratus A1—A3. Verberk All rights reserved. Both patterns are among the most robust patterns in community ecology.

Their robustness and consistency across species groups and ecosystems suggests that there are general macroecological rules underlying the abundance and distribution of species.

In the remainder of this article, I discuss each one in more detail. Knowing the abundance of different species can provide insight into how a community functions. Data on species abundances are relatively easy to obtain, and may give insight into less visible aspects of a community, such as competition and predation. For example, observations that two species occur together in many places, yet never co-occur at high densities i.

Comparing species abundance among communities can be difficult because communities often comprise many different species whose abundance profiles differ widely among the communities.

Species abundance curves, in which the number of species is plotted against their abundance, are used to deal with this complexity. By condensing the information on species abundances, they allow for comparisons of how various communities differ in the way they are organized. Contrasting patterns in species abundance may similarly indicate differences in the way that communities are organized.

For example, the periodic intrusion of seawater into salt marshes Figure 2A may constitute a disturbance that prevents most species from becoming abundant other than a few species that are able to cope with the periodic exposure to salt water. As a result, there is a large disparity in the abundance of species from the species-poor communities found in salt marshes Figure 2C. The rich structural complexity in fen wetlands Figure 2B may lead to a fine partitioning of available habitat space.

As a result, the pattern in species abundance may be more uniform; for the many species that co-occur in fen wetlands, differences in their abundance are much more gradual, sometimes also referred to as being more evenly distributed Figure 2D. Figure 2: Differences between communities in equitability of abundances A Communities in salt marshes are species poor and characterized by a very skewed pattern in species abundance, possibly owing to periodic disturbance by seawater.

B In contrast, structurally complex fen systems are species rich and have a more even community abundance pattern, possibly owing to a fine partitioning of available niches.

Hypothetical species abundance distributions illustrate the differences in species abundance distribution between C the salt marsh where 15 species show quite unequal abundances and D the fen system where the same number of individuals is distributed more equally over twice as many species.

In nearly every community in which species have been identified and counted, distributions are highly skewed. That is, there are many rare species and only a few common species. Although a skewed pattern in species abundance seems to be a universal feature of all communities, it may be more pronounced in some ecosystems, such as salt marshes Figure 2A, C , and less pronounced in other ecosystems, such as fens Figure 2B, D.

To determine what "more" and "less" actually mean, we need to quantify the degree to which such patterns are skewed. One way to deal with skewed abundances is to transform them using logarithms. This is visualized by Preston's log-normal distribution of species abundance Preston He constructed abundance intervals, each interval being twice the preceding one and graphed the number of species within each interval Figure 3.

When plotted on this log 2 scale, patterns in abundance were found to approach a normal distribution hence the name log-normal distribution. For many communities a log-normal distribution provides an accurate description of the abundances of species in a community, and this distribution is therefore frequently used to model the relative abundance of species in communities. Besides the log-normal distribution, there are other mathematical models e.

Using logarithms to transform data is simply another way of plotting abundance data that facilitates comparisons of the data across communities. However, assuming that abundances in natural communities are indeed log-normally distributed has some practical consequences.

According to a log-normal distribution, not only are there few common species with a high local abundance, but also few species seem to be truly rare.

This makes sense intuitively, as species with small populations are more likely to go extinct. Hence the number of species with small populations should be limited. Yet this prediction is not always borne out by field data; indeed, most species in a sample are rare. One reason may be that it requires extensive sampling before the log-normal distribution becomes evident. Incomplete sampling will often miss some species.

Preston noted that more intensive sampling will "unveil" these species i. Deviations from a log-normal distribution, therefore, can be used to estimate the completeness of a species inventory and estimate how many more species might be found if sampling is more intensive. As community sampling is always incomplete, this information has significant practical value.

Being able to extrapolate information from a limited number of samples improves the effectiveness of species inventories, including monitoring changes in species abundances due to global warming or restoration efforts, for example. Figure 3: Species abundance distributions on a log-normal scale The number of species plotted for different abundance intervals, each interval being twice the preceding one.

The portion of the graph red left of Preston's veil line is theoretical, depicting those species that are expected to be present but their low abundance prevents them from being represented in the sample. Species that are restricted in their geographic distribution tend to be scarce whereas widespread species are likely to occur at high densities.

This positive interspecific distribution-abundance relationship Figure 4A is intimately related to the patterns in species abundance discussed earlier.

This relationship may seem self-evident: Surely there is a positive link between measures of a species' success on a local scale its density and on a regional scale its geographic distribution.

Yet although a larger area is more likely to be able to sustain a higher total number of individuals of a species, it is not clear why the density number of individuals in a given area should also increase. Figure 4: The interspecific distribution-abundance relationship A Generally, a positive relationship results when plotting measures of abundance against measures of distribution for different species from a species group.

B The same data, with species subdivided into habitat specialists red and habitat generalists green , showing that habitat specialists may be more abundant relatively i. It is vital to understand how the processes linking the local abundance of a species and its regional distribution because this has some far reaching consequences.

For example, suppose that restricted ranges inevitably lead to low densities. Then efforts to conserve a particular species by protecting a small part of high quality habitat will not be effective. Similarly, control of invasive species can be most fruitfully directed to prevent range expansion. If on the other hand, high local densities inevitably lead to range expansion, then the conservation efforts outlined above may be very effective, while control of invasive species is best achieved by local eradication efforts.

There are two broad classes of ecologically based explanations for interspecific distribution-abundance relationships. The first class postulates the existence of a positive feedback between local abundance and the regional distribution of a species Figure 5A.

Species that occur in large numbers across many localities will be more likely to maintain their wide distributions and high abundance. Larger populations produce more offspring, which increases the chances that the species will reach other localities higher colonization and expand its geographic range.

Similarly, being widespread will ensure the continuous arrival of individuals to all places and thus a species will be less likely to disappear from a particular locality lower local extinction. A consequence of this positive feedback is that there is a dichotomy: Species will either be widespread and abundant so called core species or they will be restricted and scarce so called satellite species.

Figure 5: Explanations for a positive distribution-abundance relationship A A positive feedback between local population size and regional distribution may generate a positive distribution-abundance relationship. B Spatially autocorrelated differences in habitat quality and the way they are experienced by different species generates a positive relationship between the abundance and distribution of species. The second class focuses on niche differences among species. Both physical temperature, rainfall and biotic predators, competitors factors may limit the survival and reproduction of a species, and hence its local density and geographic distribution.

The combination of these factors, and the point at which they limit a species' survival and reproduction, constitutes the multidimensional niche of a species Hutchinson The density that a given species can attain at a certain locality is assumed to also reflect its tolerance to the environmental conditions prevailing at that locality.

Localities are likely to be more similar if they are in close proximity because of spatial autocorrelation i. Hence locally abundant species will also be able to occupy localities across a larger geographic range Figure 5B.

A distinction is made between species with a broad niche generalists and species with a narrow niche specialists. Generalists are able to tolerate a broad set of environmental conditions and use a wide range of resources, enabling them to become both widespread and locally abundant; in other words, the jack-of-all-trades is the master of all Brown A relevant question is whether geographical distribution and local abundance both contribute to extinction risk.

More specifically, if specialist and satellite species face a double jeopardy of extinction i. Although measuring a species' niche breadth is not a simple task given the large number of potentially relevant factors that may limit the survival and reproduction of a species, there is currently little evidence that generalists are more abundant than specialists.

Indeed, specialists may even be more abundant than generalists after accounting for their narrow distribution Figure 4B. Thus specialists may persist through numerically larger local populations. This suggests that the jack-of-all-trades i. The patterns in species abundance and geographic distribution discussed above have been explained in various ways. The various explanations differ in the assumptions and operational mechanisms, yet they all provide an explanation for the same, near universal patterns of a skewed pattern in species abundances or a positive distribution-abundance relationship.

As the same pattern can arise in different ways, a more detailed description of the shape and form of these patterns in species abundance and geographic distribution is unlikely to tell us which explanation is most valid in a given situation. Species abundance is the number of individuals of each species in an area. It is also a numerical characteristic. Species diversity is a term used to define the different number of species in an area Species richness and its abundance and the distribution of these species in that ecosystem.

You could have high species richness but low abundance and therefore, low species diversity in an ecosystem. An aquarium with many different species of animals, but very few individuals of each species confined to a small space is an example.

Likewise, you could have an ecosystem with high abundance, low species richness and therefore, low species diversity.

An Oak forest is an example of this. Most of the trees in the forest are Oak; other tree species are limited in numbers and have poor distribution throughout the forest.

Large scale interactions are important, because it ensures that the ecosystem will not collapse after a short while.

For example-. This is an example of a food web which clearly shows high species richness so many different organisms. Now imagine an ecosystem where only a maximum of 5 individuals of each organism is present low abundance : low species diversity. All conservation efforts that focus on species diversity need to improve species richness and abundance and their spatial distribution. Usually found sitting with a good book, nibbling on a piece of dark chocolate.

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