[MUSIC] Hi guys. Welcome to the sixth lecture of the course Biological Diversity, Theories, Measures, and Data sampling techniques. Last time I told you about distribution of biodiversity microscale. Today I will explain you some general patterns in microscale on biodiversity distribution. Let's start to explain the biomass biodiversity relation. Let's have a look at the figure letter a. It's a simple model, is normal, at the bottom model, of niche differentiation mico resistance. The range of condition in which each species can exist is shown with the circle, and the position which is defined is by the center. By randomly choosing locations, for values, numbers, or circles that are species, it is possible to calculate the effect of diversity on the coverage, of such heterogeneous habitat. The amount of such coverage is proportional to community biomass. In the figure, letter b, we can see there is also simulation that are triangles, and the analytical solution that are solid curves to the effect of diversity on community productivity. For the or the bottom mode, so the mode in the letter a. The relation between biomass and biodiversity is a fundamental relation. It is better to use a functional richness instead of a species richness to understand this pattern. We observe an increase of production efficiency of species in producing biomass when more ecological interaction are present. Second, the fact that communities are likely assembled in order succession, and in order successional species from low to high ecological efficiency. The relation between biomass and biodiversity attracts our attention today the ecological system services. Human depends, on large degree, on the goods provided by the natural and ecosystem. These goods and other benefits provided by ecosystem to mankind are qualitatively referred to as ecosystem services. Anthropogenic activities impact the diversity of organisms, found in ecosystem aboveground and belowground and thus influence the provision of ecosystem services. Consequently, there has been an increasing scientific interest in the link between biodiversity and the provision of ecosystem services, but researchers focus mainly on the aboveground systems. Let's have a look at biodiversity distribution in microscale. I will explain what is a primary succession. Primary succession occurs in excess of the lifeless areas, regions in which the soil is incapable of sustaining life, as a result of such factors as lava flows, newly formed sand dunes, or rocks left from the retreating glacier. Secondary succession instead, of course, an area where a community that previously existed has been partially or completely destroyed. Disturbance regimens are very important in this case, for instance, fire, logging, storms, and others. The scientist Clements introduced the idea of climax for the successions. So we need to understand what is a climax, and what's the functional changes during this succession. Clements described the successional development of a ecological community comparable to the autogenic development of an individual organism. Later, ecologists developed this idea that ecological community is a super organism. And even sometimes claimed that communities could be analogous to complex organism, and so to define a single climax type for each area. Species diversity almost necessarily increases during early succession as new species arrive, but may stabilize in later successional stage. Net primary productivity, biomass, and trophic properties all show variable patterns over succession depending on the particular system and size. Early seral stages are marked by rapid growth and biomass accumulation. Grasses, herbs as more shrub, for instance, increase their population and their biomass. Gross primary productivity is low, but net primary productivity tends to be a large proportion of gross primary productivity. As with the little biomass, in the early seral stage, respiration is low. As the community develops toward woodland and biomass increases, so does productivity. But net primary production as a percentage or gross primary production can fall as respiration rates increase with more biomass. One key factor of succession is facilitation. The intermediate disturbance hypothesis has been proposed to explain how species can coexist in the same environment, leading to biodiversity. Diversity of species in the ecosystem is maintained by a level of intermediate disturbance. So this is why you see this kind of hump shaped curves on the graph. If disturbance is infrequent, the subsection may fall to develop behind the pioneer stage. If the disturbance is rare, the climax will be established and diversity reduced according to the competitive exclusion principle. At the intermediate level of disturbance, instead, the arrival of new species will increase the diversity in proportion to the interval between disturbances. But because the competitive exclusion principle is being questioned, this hypothesis is still dubious. At the contrary, three other processes have been suggested to explain the coexistence of species, in other words, biodiversity. The first is the stabilization of a consistent function, the second is the insurance hypothesis, and the third is the portfolio effect. Increasing diversity can stabilize a consistent function. In this video, you'll see that each rectangle represents a planned community containing individuals of either blue or green species and the total number of individuals correspond to the productivity of the ecosystem. Green species increase in abundance in warm years, whereas blue species increase in abundance in cold years. Such that the community containing only blue or green species will fluctuate in biomass when there is an interannual climate variability. In contrast, in the community contain both blue and green individuals, the decrease in one species is compensated for an increase in the other species, thus, creating stability in ecosystem productivity between years. You should note also, that on average, the diverse community exhibits higher productivity than either single species community. This pattern could occur if blue or green species are active at slightly different times, such that competition between the two species is reduced. This difference in when the species are active, leads to the complementary resource utilization and can increase the total productivity of the ecosystem. The second is the insurance hypothesis. Simple communities are represented by a box in this figure. In this case, these communities are too small that can only contain three individuals. For example, this could be the case for a small pocket of soil on a rocky slope. There are three potential species that can colonize these communities, blue, dark green, and light green. And for the sake of this example, let's assume that the blue species have traits that allow it to survive for a longer drought. Looking at all possible combinations, a community containing one, two, or three species, we see that as the number of species goes up, the probability of containing the blue species also goes up. Thus if slopes in this region were to experience a prolonged drought, the more diverse community will be more likely to maintain primary productivity because of the increased probability of having the blue species present. The third hypothesis is the portfolio effect. The portfolio effect compares community with a stock portfolio. This was an idea provided by Tilman and other authors in 1998. Consider two different portfolios, one with two stocks in the figure left-hand side and one with four stocks in the figure right-hand side, each with a total value of $100 in the top panel. A stochastic event causes a 50% reduction in the value of stock A in the bottom panel. In the portfolio with only two stocks, this leads to an overall loss of $25, whereas only $12.50 are lost in the portfolio with four stocks. Similarly, a decrease in the abundance of one species during a disturbance event will have a large impact in the species poor communities than species rich communities. And the function in the diverse communities are therefore likely to all show a lower response. Moreover, the relative abundance of species within communities may also influence our ecosystem function in response to disturbances. Consider two communities with four species each, but one is dominated by one species, low evenness in the left-hand side of the figure, while the four species are evenly represented in the other, high evenness in the right-hand side of the figure. A disturbance event leads to a 50% decrease in the abundance of species one in both communities. In the community dominated by one species, this leads to a 30% reduction in overall abundance, but only a 12.5% reduction in the community with evenness. This is likely to result in greater reduction in functioning in the community with high dominance than in the community with the high evenness. Thus, communities with many species and high evenness are likely to show smaller responses to external stimuli than communities with few species and high dominance by a single species. Now you can understand how biodiversity is very important for the ecosystem. We will continue the discussion during the next lecture.
