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How habitat size affects the abundance of all species living in a community. Digging deep into this issue can bring ecological insights and is also valuable for formulating strategies to promote biodiversity. Chase et al. reported the results of a study in the journal Nature, which may help to settle the long-standing debate about the relationship between habitat area and the species diversity that the habitat can accommodate.
Land change caused by human activities is a major component of global change. The loss of natural habitat reduced the diversity and abundance of local species, and it was related to the extinction of more than one third of the world’s animals from 1600 to 1992. A report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services estimates that there are currently more than 500,000 species—about 9% of all terrestrial species— May lack the number of habitats needed for long-term survival. If these species disappear, many key ecosystem services will be damaged, such as pollination or control of pests or pathogenic factors. The impact of habitat loss on biodiversity is usually estimated based on the relationship between area and species richness-first described more than 150 years ago. This seemingly universal relationship is simple: the larger the area of a particular habitat, the more species it contains, but the number of species increases with the area is nonlinear. Due to the limited resources in an area, the number of individuals of ecologically similar species is also limited. Therefore, when a habitat loses part of its area, for many species, it also loses the ability to support a large enough population. As land use intensifies and habitat area decreases, these species will become extinct [8]. Chase and colleagues proposed a concise and neat method to illustrate the dynamics of communities occupying habitat patches of different sizes. The author did not only consider the total number of species in each habitat fragment, but focused on the number and relative abundance of different species in the samples obtained from these fragments. In this way, the structure of ecological communities can be directly compared, while avoiding problems that may arise when taking into account the differences in the work required for sampling large and small areas. The author's method also allows comparison of changes in the relative abundance of individuals of all species, which is a measure of community structure related to ecosystem dynamics. Thanks to this method, Chase et al. can distinguish three possible modes of change due to habitat loss (Figure 1). In the model described by the "passive sampling" model, the structure of the community remains unchanged in large and small habitat fragments. Therefore, regardless of the total area of the habitat, each sample exhibits similar species abundance (richness, number of species), abundance (number of individuals) and evenness (distribution of individuals of different species). In this case, species reduction will reflect the loss of habitat area under the classical species-area theory, and the total number of species in the entire habitat fragment will depend entirely on its size. The other two models are described as the type of ecosystem decline. This hypothesis assumes that in the process of habitat reduction, there will be disproportionately high biological losses compared with the loss of habitat area. There is a kind of ecosystem decline that occurs due to excessive individual losses. Compared with the larger habitat fragments, the smaller habitat fragments contain fewer individuals per sample, and all species are affected the same. This resulted in a community where there were fewer species in the small fragments, but the relative abundance of species in a single sample between the small fragments and the large fragments remained unchanged. Another type of ecosystem decline is caused by uneven changes in the relative abundance of species plus species loss. In this case, the existing species have different responses to habitat loss, so the species abundance in small fragments increases or decreases relative to the species abundance in large fragments. In the samples from small habitat fragments, the relative abundance of species has become more uneven, because some species have an increased quantitative advantage, making communities poor and causing species poverty. Chase et al. used data from about 120 human-reformed landscapes around the world to show that, in general, samples from small natural habitat fragments contain fewer individuals and fewer species than samples from large fragments. The species abundance is more uneven. This result is consistent with the generalized model of ecosystem aging—mainly because of the decrease in uniformity (see Figure 1). This result is valid regardless of the habitat or organism being studied. This means that changes in natural habitats will cause major functional changes in ecosystem dynamics, not just the loss of populations and species. Therefore, the current estimation of species extinctions related to habitat loss based on passive sampling models may not only underestimate the number of threatened or disappeared species, but also underestimate the impact of loss of these species on ecological functions and ecosystem service provision . Changes in biodiversity caused by habitat loss will change many ecological processes, eventually causing catastrophic effects and accelerating the process of extinction. However, local extinctions often do not happen immediately. Some species will continue to exist, only with a decline in abundance and population dynamics-known as "extinction debt"-until the final individual perishes. This can lead to uneven distribution of species abundance, which is vividly demonstrated by the method of Chase and colleagues. Their analysis revealed a small number of "2021 hermes bags" species and a large number of rare species. The former dominate communities in small habitats, and many of the latter may be going extinct. The declining species may be replaced by other species from the adjacent human modified landscape, especially at the edge of the habitat, resulting in the so-called "edge effect", which is relatively more important in small habitat fragments. In fact, in the early stages of land reconstruction, the communities in small fragments are more different from the original communities than the communities in large fragments. As time goes by, the communities in small fragments and the communities in large fragments grow more and more. The more similar, because they gradually recovered from the impact of land reform. According to research by Chase and colleagues, the diversity and species abundance found between large fragments and small fragments is attenuated in the older or "softer" European landscape than in the newer and more dramatic The transformed North American landscape is smaller. This suggests that over time, species migrating from the margins of human-altered habitats may at least partially compensate for the ecological functions performed by native species in larger habitats, thereby allowing small habitat fragments to reach new - but not the same The-ecological balance. Although this work emphasizes the key role of habitat area in maintaining ecosystems, it rarely discusses how these processes change due to habitat loss. Species from higher trophic levels (upper food chain) (such as predators) require larger habitats to maintain their populations than species from lower trophic levels, so the number of individuals supported by smaller habitat fragments may be It is not enough to maintain the population of top predators or consumers, so it will produce a shorter food chain and change the structure of the ecosystem. The difference in extinction rates between trophic levels will cause significant changes in ecosystem functions at the edge of habitat. As the area of natural habitats shrinks, this will endanger ecosystem functions and the provision of ecosystem services. Chase and colleagues’ research results require reconsideration of the controversy: whether a single large conservation area will protect more species than several small conservation areas (replica hermes). Some current evidence suggests that a continuous habitat may contain fewer species than many small habitats with the same total area. However, compared with a single large-scale protected area, these small habitats may undergo huge ecological changes, which may eventually lead to a large-scale reduction of ecosystem functions, and in the long run, increase the extinction rate of native species. The method of Chase and colleagues provides a good overview of the extent of these effects, but more details are needed to understand exactly how local ecological processes change. This requires going beyond the study of the nutrient chain, evaluating more complex food webs, and collecting information on changes in species functional responses and trait diversity in ever-shrinking habitats. Ultimately, this information will reveal which ecological processes are declining, and how this decline in ecosystems affects the maintenance of normal biodiversity.
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