Terrestrial ecosystems

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Introduction

Although terrestrial environments cover only a third of the earth’s surface, their productivity is higher than marine ecosystems (Figure 353). The reason is a better efficiency of photosynthesis linked to the higher availability of carbon dioxide and the greater light intensity. Terrestrial ecosystems are more prone to variations in environmental conditions. Seasonal effects are often significant here. Altitude, latitude and water availability are major regulators. Large regions are hot deserts, too cold or high in altitude. However, in these inhospitable ecosystems specific flora and fauna of eukaryotic protists are present. In particular, lichens (Figure 46 and Box 27) can be the most abundant primary producers there. For other biotopes, plants provide the majority of carbon fixation, except in freshwater environments where Viridiplantae algae generally perform this role.


Three terrestrial biomes

Plants live associated with numerous Eumycota and bacteria which will facilitate their photosynthetic activity (Box 28). They have the ability to make lignocellulose, a very resistant mixture that only Dikarya fungi can effectively break down (Box 29). The result of this degradation is the production of humic acids which play crucial roles in the retention of water and mineral salts. Wood-degrading fungi therefore have a major role in soil health and their effect on carbon fixation is positive: they allow more carbon to be fixed than they release through their degradation activity! Much more than protozoa or animals, Eumycota play fundamental roles in recycling organic matter in terrestrial ecosystems. The species involved obviously depend on the biotopes. There are three main types of biotope or biomes that have significant impacts on the carbon cycle: boreal forests, tropical and equatorial forests, and savannas and grasslands (Figure 355). In addition to these three biotopes, there are freshwater ecosystems which host species that are very different from terrestrial ecosystems (Figure 355). As in marine environments, terrestrial ecosystems are subject to pressure from parasites. In the case of plants, parasites consist mainly of the Eumycota fungi (see pages 441-445) and Oomycota, together accounting for about 85% of plant diseases. It is estimated that their damage collectively causes a decrease in production equivalent to that of grazing by vertebrate and invertebrate herbivores. For animals, the parasites are mainly of bacterial and viral origin, but some are also fungi and protozoa belonging to various lineages (Figure 349). The action of humans that disrupts ecosystems allows invasions of exogenous species at the expense of endemic species. It seems that it particularly favors Eumycota which are often the cause of epidemics in nature, attacking both plants and animals. For the latter, there are currently two major worrying epidemics. One is caused by Batrachochytrium dendrobatidis, or “Bd”, a Chytridiomycota that decimates frogs, especially in Central America; the other, which affects North American bats, is caused by Geomyces destructans, an Ascomycota of the Leotiomycetes class.

In the three large terrestrial biomes, the trophic chains start mainly with plants and the first consumers will be more or less large animals: insects, nematodes, vertebrates… Note for example that it is estimated that in a typical African savannah, the plant biomass which passes through the intestines of ants is very close to that which passes through the intestines of large herbivores. In temperate ecosystems, many food chains start at soil level by fungal recyclers living on dead plant matter.


Temperate forests

In temperate forests, carbon fixation occurs mainly by ectomycorrhizal trees. They only constitute 3% of plant species, but these trees, which make up the majority of forests in boreal and temperate zones, are therefore major partners in the global carbon cycle. The fungi involved are diverse: Endogonal, Ascomycota and especially Basidiomycota, with more than 5,000 known ectomycorrhizal species. The estimated plant productivity is 5 tonnes of plant material per hectare. Part of the plant biomass is consumed in food chains, the first link of which is generally small animals. Another part of the biomass is exploited by microorganisms in the soil. The supply of organic matter to the soil is seasonal with leaf fall in autumn. The leaves undergo a complex cycle of chemical modifications mainly by Eumycota fungi and to a lesser extent by Streptomycetes eubacteria. Leaves are also physically modified by small animals. Indeed, the work of microorganisms is accompanied by that of burrowing animals such as springtails, earthworms or nematodes, which participate in the fragmentation of the biomass and thus facilitate its degradation. The faeces of these animals are the preferred substrate for many microorganisms. The relationships between the different groups of organisms involved in soil formation are complex because animals participate not only in the fragmentation and degradation of matter, but also in the oxygenation of the soil. This role is important because fungi and some bacteria need oxygen to live and break down lignocellulose (Box 29). These animals consume the fungi in return, and it has been experimentally shown that burrowing organisms prefer to eat leaves colonized by fungi to non-colonized leaves. Protists also participate in this feast. For example, in decaying woods, Mycetozoa are major consumers as evidenced by their ubiquity.

The action of different microorganisms to degrade leaves and pieces of wood that have fallen to the ground is often sequential and can take place over several years, especially when it comes to degrading tree trunks. This leads to stratification of the soils with the surface cover or litter containing a lot of organic matter and the deeper layers containing less and less (Figure 356). On the whole, the Eumycota fungi play a preponderant role in the upper layers by their biomass and their capacity to degrade lignin; in temperate forests, they can constitute up to 80% of the living biomass excluding roots. Typically, a liter of soil contains about 600 kilometers of mycelial hyphae, which is the mass equivalent of about 20 cows weighing 600 kg in the soil of an area the size of a football field! Bacteria, especially the anaerobic ones, are more active in the lower layers. Residual organic matter is gradually buried and is transformed in the deeper layers via chemical reactions when temperatures become incompatible with life.


Tropical and equatorial forests

The dynamics in tropical and equatorial forests are different from those in temperate and boreal forests. Seasonal effects are reduced here. Ectomycorrhizal trees appear to be less important than endomycorrhizal ones. Primary production is greater in tropical forests than boreal ones, on average being 60 tons per hectare and per year in the tropical forests, mainly due to increased availability of light. Yet the soil of these forests is often poorer in plant debris than their high productivity would suggest. The main reason is that recycling mainly goes through termites and possibly fungal ants which harvest and digest the biomass. The enzymatic digestion of plant biomass, however, once again involves eukaryotic protists (see the function of the termite gut), including fungi that are cultivated by insects.


Savannas

Production in savanna ecosystems is dominated by herbaceous plants, although many trees are also present there. Almost all herbaceous plants, including grasses which form the majority group, are endomycorrhizal. In these ecosystems, grazing is easy and a large fraction of the plant material circulates through the digestive tract of herbivores. Once again, fungi play an important role in the recirculation of the carbon present in these excrements (see Figure 68 for the succession of species in this biotope).

Another effect of the action of fungi on lignocellulose is the production of humic acids which, through their properties of water retention and trace elements, will ensure the sustainability of terrestrial ecosystems. The secretion of glomalin by endomycorrhizal fungi also clumps soils, preventing their leaching. Intensive agriculture modifies the natural cycle of degradation of plant material in soils by fungi, both in temperate and tropical regions. It greatly disrupts soil health by depletion of humic acids and glomalin, as well as by unbalancing the ratios of microorganisms towards an excess of bacteria. It is not uncommon for bacterial biomass to be greater than fungal biomass in soils damaged by overly intensive practices. Adding fertilizers, antifungals and other pesticides only accelerates the damage. The process ends with soil erosion and desertification. Currently, new agricultural techniques are being developed to take better account of the natural soil cycle. Likewise, remedial techniques have been developed to restore soil health. They are based on a good knowledge of the functioning of terrestrial ecosystems. A simple practice for restoring soil is to cover it with twigs a few centimeters in diameter crushed into pieces. This technique known as “fragmented rameal wood” initiates a new virtuous cycle of production of humic acids by fungi. It also promotes oxygenation of the soil and discourages the presence of anaerobic bacteria.


Freshwater ecosystems

In freshwater ecosystems, the main primary producers appear to be mainly Viridiplantae algae, although many plants have adapted to aquatic life. The trophic chains resemble those found in the oceans, and various phagotrophic protozoa often form the first link. They serve as prey for larger protists or small animals, etc. These ecosystems also collect a lot of organic matter from the surrounding soils in the form of leaves, trunks, animal corpses… As in terrestrial ecosystems, these materials are mainly recycled by Eumycota fungi and to a lesser extent by Oomycota. They will thus start trophic chains similar to those present in soils.


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