Today's lecture will be the last dealing with the physical factors controlling ecosystem function - Next week we will get into the biological side of ecosystems, starting with chapters 10 and 11.
But today, it is ....
Nutrient regeneration is a fundamental process that profoundly influences the productivity of an ecosystem. We will examine nutrient regeneration in terrestrial and aquatic systems separately since they differ quite a bit.
In terrestrial systems the major source of new nutrients comes from the soil, which is formed by weather of bedrock. Essentially, the rock underneath the soil is broken down by various physical and biological processes and mineral nutrients are released.
Although weathering is an important source of new nutrients, nutrient recycling of dead organic matter is a far more important source of nutrients in most ecosystems. This dead organic matter that sits on the top layer of the soil is referred to as detritus and is composed primarily of dead plants - e.g. after Hugo passed through SC, many trees were killed and knocked to the ground - these dead trees are now the source of much nutrient recycling. This reservoir of nutrients is recycled by the action of worms, snails insects mites bacteria and fungi - all of which consume this detritus.
Leaf litter is another major source of detritus - fall every year - The breakdown of leaf-litter occurs in three ways -
1) leaching of soluble minerals and small organic compounds by water - 10 - 30% of substance in leaves dissolve
-Fig. 4.3 losses of water soluble organic and inorganic material from deciduous vs. coniferous trees -
2) consumption by detritus feeding organisms e.g. millipedes, earthworms, woodlice, and other inverts - 15 - 45% assimilation, but also breakdown detritus into small particles
- Table 3.1 list of decomposing animals
3) attack by fungi, and further breakdown by bacteria
- table 3.2 list of decomposer microorganisms
A basic model of soil decomposition process is shown in fig 2.5.
Breakdown of detritus varies a lot primarily depending upon cellulose and lignin content. In fact it is primarily fungi that are capable of digesting lignin - when you look on your lawn and see a mushroom - this usually means that there is a dead root or branch near the soil surface.
The rate at which decomposition takes place differs greatly in different ecosystems, as well as among different tissues.
- Table 4.1 - turnover of plant tissues - total decomposition
Physical factors that contribute to variation in decomposition rate include 1) surface properties - waxy surfaces are hydrophobic and restrict the development of water films, which inhibits the germination of fungal spores, the growth of fungal mycelium, and activity of exoenzymes - secrete by fungi to digest plant tissue - e.g. magnolia leaves last forever while water oak and grass break down quick.
- conifer needles often have a mat of tubules extending over the stomata thus blocking this as a point of entry for fungal mycelium
- herbivorous insects are often deterred by leaf thickness and pubescence (hairs - trichomes) - physical and chemical defenses
2) Toughness -
resistance to fungal penetration has been shown to be directly related to toughness - applies especially to living plants, but also to dead material -- turkey oak / Brachys tessellatus system and mine initiation
3) Particle size - fig 4.14 - detritus is broken down into small pieces by the action of invertebrates in the soil - the smaller the particles the quicker the decomposition by microorganisms like fungus and bacteria - this stems from the greater surface area -
Many plants have actually evolved a symbiotic relationship with many species of fungi know as mycorrhizae that live on or even in the roots of many plants. Two kinds - endomycorrhizae where the fungus actually penetrates the root structure - ectomycorrhizae which forms a sheath over the root -
Many plants are completely dependent on these fungi - fig 8.5
The decomposer community is actually very complex. The are a wide variety of species that have evolved specialized roles in the detritus community.
There are carnivores - feed on animals
microbivores - feed on microorganisms
- saprovores - feed on dead plant and animal remains
As for the fungi, they can be divided into distinct groups as well:
necrotrophs - short term exploitation of living organisms which results in the rapid death of the food resource - these include species that are herbivores, plant parasites that feed and kill plant tissues, predators - trap and kill animals and microorganisms like nematodes,
Biotrophs - are fungi with long term associations with plants - usually not killing them - e.g. mycorrhiza and rhizobium
Saprotrophs - utilizing dead material - most fungi fall into this category.
Fungi have evolved several growth forms that enable them to exploit there resources - there is colonial growth that arises by division of unicells which is seen in the yeasts, and mycelial growth, which typifies most fungi -
- unicellular microbes are well adapted for surface habitats, rapid dispersal, and entry into small spaces-
- mycelial form is very important to the decomposer life style because it confers the ability to rapids penetrate and invade detrital materials - can spread very quickly -
The invertebrate community in the detritus is even more diverse and many organisms have evolved very specialized behavior, physiology and morphology in order to exploit detrital food resources.
Looking first at saprotrophic invertebrates - there are two main challenges to feeding on detritus - the first has to do with the physical act of ingesting the material - getting it into your mouth so to speak. Obviously, very few of us are capable of eating a big mac in a single bite and much or our jaw morphology has evolved to permit the mastication and dissection of our food items into a size small enough for transport to the gut and efficient digestion once there. The same is true of most detrital feeders.
You might expect this problem to be largest for the smallest organisms, the protozoa which are small single celled animals. Many protozoa have got around the particle size problem by evolving symbiotic relationships with larger animals. For example these three species of protozoa inhabit the guts of detritus feeding insects. - Fig 3.6 - these guys take advantage of the insect host's ability to chow down and fragment the plant material. In exchange, the insect benefits from the protozoans ability to decomposed cellulose -
Nematodes make up a sizable portion of the detrital community and have evolved a very wide range of pharyngeal structures that are related to their mode of food consumption - Fig 3.7 - fungivore, bacterivore, predator, and omnivore
Earthworms have very characteristic feeding structures - they are capable of pulling apart soft plant tissue by grasping the material with their mouth then contracting their pharynx - it is hard to think of an analogy for this but it might be a bit like eating an extra thick milkshake -- you suck on the straw and get a mouthful, but then it stops because the milkshake is so thick that you create an air hole and so you have to move the straw? The earthworm is just like that - it can suck and break apart soft plant tissues, but it can't handle a tough steak. - and you can see the result of this in many deciduous forests where earthworms are a major player in the detrital community - if you look down at the leaf litter most of the leaves will be stripped of the interior material but the major veins, petioles and small twigs will be untouched - this is mainly because the earthworms can't handle this tough material.
Slugs and snails feed on plant material by rasping the tissues away with a grater like radula -
Many insect larvae have sclerotised mouthparts that allow them to chomp up leaf litter - fig 3.9
- among dipteran flies, there is a high degree of specialization in mouth parts.
Picture of bibionid - a critter we work on in my lab is the bibionid fly, Plecia nearctica, otherwise known as the luv-bug - occurs in great swarms twice a year down on the coast - spends most of its 6 month life cycle in the leaf littler layer as a larvae - has a very well developed head and mouthparts and are very important player in the production of small particle sizes for consumption by smaller organisms and fungi.
Terrestrial Chironomids are frugivorous eating fungi and small plant detritus - these guys have very small mouth parts
Tabanid fly larvae have even smaller mouthparts and eat algal cells, pollen, spores, amoebae and can scrape the surface of leaves.
Muscid larvae - have extremely reduced mouthparts that have been reduced to a pair of chitinous hooks, and are not capable of feeding on solid material - these guys do however feed in compost and dung, eating primarily the bacteria that abound in these microhabitats -
Collembola are very primitive insects that are very common in the leaf litter - common name = springtails -
- fig 3.10
- the majority of colembola chew their food and have strongly developed mandibles and maxillae - mandibles have a roughened plate akin to molars in mammals for grinding up hard plant litter (A)
- species that feed on soft plant tissue have much reduced plates (B)
- species that suck out materials have pointy mandibles adapted to piercing
- mites also vary tremendously in the feeding structures - fig 3.11
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Many other litter dwelling arthropods have generalized mouthparts with strong mandibles for biting and tearing, and strong pharynx muscles adapted for swallowing large chunks of food - these include Isopods, amphipods, orthops (e.g. grasshoppers, crickets, roaches).
Cellulose and lignin are two very important components of a detrital diet.
- the various pathways for cellulose digestion are shown in this figure fig 3.12
- Some animals appear to have evolved specialized cellulose specific digestive enzymes known as cellulases - in some mollusks it has been conclusively demonstrated that cellulases are secreted by the digestive gland -
- However in every other organism cellulases are secreted by microbial organisms that inhabit the gut. This symbiotic relationship ranges from obligate as is found in termites where symbiotic protozoa can make up 2/3 rds of total body weight to facultative where the animals rely on the activity of microbes that are ingested along with the detritus.
The strength of the symbiotic relationship is correlated to the efficiency of cellulose assimilation - for example in one study of termites, the original wood contained 55% cellulose and 27% lignin - the feces produced by these termites contained only 18% cellulose and 76% lignin - very efficient at digesting cellulose, not so good at lignin
- termites can not live without the symbiotic protozoans,
That's about all I am going to say about nutrient cycling in terrestrial systems - you text has bunch on recycling in aquatic systems - please read this.