7.4.3 Functional groups

A topic which has been debated extensively recently, the relevance of which can be seen in the above discussion of nutrient cycling in chaparral, is that of functional groups (Walker 1992). The concept of a functional group rests on the notion that organisms which arc taxonomically distinct can be functionally similar. An obviously coherent functional group is the set of plants that forms symbiotic relationships with nitrogen-fixing bacteria -these are most notably legumes (Rundel 1989), but it includes many other taxa. In MTEs. nitrogen-fixers are epitomized by the genera Ceanothus, Lotus and Cercocarpus in California (Hanes 1977), Apalathus and Psoralea in South Africa (Lamont 1983), Trevoa trinervis in Chile (Rundel 1983) and many species of Acacia and Casurina in Western Australia (Lamont 1983). Blondel and Aronson (¡995) report that nitrogen fixers are not common in the Mediterranean basin, but C'oriaria myrtifolia, which forms dense stands under evergreen oaks in southern France and Spain, and Myrica species may be important fixers of nitrogen (Rundel 1983). Leguminous plants usually form root nodules in association with the bacterium Rhizobium, while other taxa such as Casurina and Myrica will nodulate in response to infection by actinomycetes, and Macrozamia, the Australian cycad, forms corraloid nitrogen-fixing roots in association with blue-green algae (Lamont 1983).

In eucaJpyt woodlands in southwestern Australia, a suite of six to ten Acacia species make up what can be construed as a functional group of nitrogen fixers. There is apparent functional equivalence within the group since all are morphologically similar, and all fix nitrogen. However, this similarity disappears when their response to disturbance is considered. Studies of a subset of these species have indicated that seeds of different species have markedly dilferent responses to elevated temperatures similar to those expected during fire (Figure 7.4). It is well known that seeds of some species require a high temperature treatment to stimulate germination (Bell et at. 1993), while seeds of other species arc intolerant of high temperatures. Some of the Acacia species were inhibited and others stimulated by high temperatures, and subtle differences in temperature response were evident. Thus, different species will be stimulated to germinate depending on the severity of the fire. The diversity within the nitrogen-fixing group thus provides insurance against the complete loss of that group in the face of variations in disturbance intensity. In this example we have been able to show interlinkage between the composition of the system (Eucalyptus and Acacia spp.), its structure (a woody plant community with cycles of accumulating fuel) and some of the ecosystem processes (fire and the flux of nutrients, and the flow of genetic information from one generation to the next). An emergent property of the mechanistic portion of the system, embodied in an otherwise tightly constituted functional group, is the resilience that the whole system is able to manifest over time in response to unpredictability in the pattern of disturbance by fire. There arc probably other equally important emergent properties which have not yet been quantitatively described, such as stability/mestastability, elasticity, plasticity and predictability.

Functional grouping, it is evident, must be regarded as relative (Davis et al. 1994). Clearly a drought-adapted nitrogen fixer will be in a different water-use group to one suited to mesic conditions, and one could only substitute for the other under a limited set of conditions (Hobbs et al. 1995b). This relativity of functional grouping is well demonstrated by the observed dynamics of predator-prey relations in an arid Chilean MTE, described by Jaksic et al. (1993) and Fuentes et al. (1995). In that study a set of 10 predators (four falconiform hawks, four owls and two foxes) were monitored together with their prey, which comprised eight small mammal species (seven rodents and one marsupial). Within each of these sets, animals were recognized as belonging to one of a few trophic guilds; prey species were either granivores, insectivores, folivores or omnivores (with possible preferences), while the predators were classed as either omnivore or exclusive carnivore. Over the 5-year period of the study, the annual rainfall varied from 58 mm to 513 mm (with a long-term average of 206 mm). Associated with these wet and dry years were troughs and peaks in primary production and small mammal population size. Contrary to expectation, small mammal populations within supposed trophic guilds did not irrupt synchronously when high precipitation promoted plant growth, instead, only populations of Phyllotis darwinii (a granivore), Akodon olivaceus (a granivore/omnivore) and Marmosa elegans (a marsupial insectivore) irrupted, while those of Oryzomys longicaudaius (a granivore) and Akodon longipilis (an insectivore/omnivore) did not. In addition, during the relative drought years, five of the eight small mammal species disappeared from the study site. These were O. longicaudatus, A. longipilis and the three folivores Abrocoma bennettii, Octodon dregus and Chinchilla laigera. Fuentes et al. (1995) interpret these observations as evidence for little redundancy in the supposed guilds, and claim that these groups could not be functionally equivalent. The results therefore suggest that functional groups, should they exist for the prey species, cannot be defined in trophic terms alone.

Within the trophic guilds of predators, on the other hand, indications of functional equivalence were much stronger. The two carnivorous owls, Bubo virginianus and Tyto alba, preyed on the same species at approximately the same frequencies, while the omnivore guild comprising the two Pseudalopex foxes (P. culpaeus and P. gr¡sens), the owls Athene cunicularia and Glauci-dium nanum, and the falcon Falco sparverius, also displayed consistently similar feeding habits. The three large falcons (Buteo polyosoma, Geranoaetus melanoleucus and Parabuteo unieinctus) were also consistently carnivorous. During the lean years, predator species started to disappear when small mammal numbers dropped to below 100 individuals ha~\ Although there was a reasonable match between the carnivorous diets of the three larger falcons and three of the owls, it was the former group that disappeared from the study site first. Even the omnivorous falcon F. sparverius disappeared before its owl counterpart A. cunicularia. Of the four predator species that remained at the study site when prey numbers were low, three (the two foxes and the owl G. nanum) were omnivores, and only one species (the owl T. alba) was a carnivore. Of the six species which migrated away from the site during the lean years, five were carnivores. Fuentes et al. (1995) believe that this pattern suggests a high degree of functional equivalence amongst the recognized guilds, but concede that the "acid test" of density compensation within guilds has yet to be performed on this system.

In a review of this work on predator-prey relationships, Wiens (1993) made a further interpretation regarding the relative nature of functional equivalence. He interpreted the early disappearance of the falconiform species during the lean years as a fine-tuning of the functional grouping, and a teasing apart of the broad trophic niche occupied during the good years by both owl and falcon species into separate and narrower niches (Figure 7.5). Taken to its logical conclusion, separation of functional attributes between species along different axes reduces ultimately to an argument about niche

Figure 7.5 The relationship between resource availability and the overlap of niche spaces, based on observations in a Chilean predator prey system, and demonstrating the relative nature of niche breadth. Resource levels. A, B and C reflect conditions of starvation, specialization and opportunism, respectively, in a prey species. Redrawn from Wiens (1993)

Figure 7.5 The relationship between resource availability and the overlap of niche spaces, based on observations in a Chilean predator prey system, and demonstrating the relative nature of niche breadth. Resource levels. A, B and C reflect conditions of starvation, specialization and opportunism, respectively, in a prey species. Redrawn from Wiens (1993)

differentiation (sensu Hutchinson 1958). The existence of such niches in Californian MTEs has been elegantly demonstrated by Cody (1986). Along one set of transects which intersected the main axis of the coastal mountains (representing steep environmental gradients) and another running parallel to the coast (representing shallow gradients over latitudinal environmental changes), he determined the turnover of species in the diverse genera of Ceanothus and Arctostaphylos. The replacement sequences of species along these two sets of gradients were correspondingly steep for the steep gradients, and shallow for the shallower ones, which Cody (1986) interprets as evidence for niche separation in the two speciose genera investigated.

7.4.4 Keystones

The epithet keystone can be applied to a component, or sometimes a process of a system, the removal of which would cause disproportionate changcs to the system (Lamont 1992; Bond 1993). The keystone quality is therefore a concept which links diversity to system function in a particular way, suggesting that maintenance of species richness per se is not always a reliable measure of system stability, and that other species, or even suites of species, may rely on the presence of the putative keystone. In the Australian review of biodiversity in MTEs (Hobbs 1992), Lamont (1992) presented a multilevel interpretation of keystones, drawing on interactions in natural systems to illustrate his model. In that model, depicted in a sketch as a mediaeval-style building, he pointed to three levels of keystones in a Jarrah forest: first-order keystones have only one species dependent on them; second-order keystones support whole suites of species (the example given is N2-fixmg bacteria which support all nodulating legumes); a third-order keystone is one without which the whole system would collapse. For this third-order keystone, Lamont (1992) cites the case of Gastrolobium bilobum, a major nitrogen-fixing plant which also stabilizes soil and provides food and shelter for small vertebrates, such as the small marsupial Bettongia penicillata. Germination of G. bilobums seeds depends on the action of a suit of ectomycorrhizae, which in turn rely on Bettongia for dispersal, as well as for facilitation of spore germination in passing through the animal's gut. Without G. bibobum, it is reasoned, the Jarrah forest system would collapse.

An example of a keystone component in a MTE of the Cape, South Africa, is provided by the recent work on myrmecochory (seed dispersal by ants) (Bond and Slingsby 1984; Bond and Stock 1989; Bond el al. 1992). Over 1300 fynbos plants (20% of the flora) produce seeds which have a protein-rich elaiosome that attracts indigenous ants. The indigenous ants habitually forage for the seeds, which drop close to the parent plant, and haul them away to their nests where the elaiosomes are eaten. The seeds are then abandoned in the nest, affording them protection against granivores and the heat of intense fires. The ants function as dispersers and protectors of seed, and myrmecochory can therefore be regarded as a keystone process which is necessary for the continued survival of the many fynbos plants. The vulnerability of this keystone process was recently shown when the alien Argentine ant, Jridomyrmex humilis, invaded parts of the fynbos. This ant is smaller but more aggressive than the indigenous seed-gathering ants, and regularly displaces the latter. The Argentine ants eat the elaiosomes on the soil surface and do not bury the seeds. In fynbos invaded by this species, seedling regeneration of ant-dispersed plants after fire is much less successful than in uninvaded fynbos (Bond and Slingsby 1984). Besides threatening many fynbos plants with extinction, the collapse of this ant plant mutualism could have ecosystem-level effects since ant-dispersed plants are often dominant components of fynbos shrublands. Proteaccous species are generally deeper rooted than other members of fynbos communities (Higgins et al. 1987), and local extinction would probably therefore also induce a marked change in the hydrology of the host systems. Thus I. humilis may be considered a keystone invader with negative system impact.

7.4.5 Biodiversity and its support of human utility

There are many illustrations of the role that biodiversity can play in the functioning of ecosystems. In some cases the diversity of components themselves, or the structure they provide, may be absolutely essential for the stability and resilience required for the impacts of natural or human perturbation. In some instances, however, altered diversity of ecosystems may act to enhance their human utility. In Chile, for instance, the production side of the honey industry comprises the honeybee. Apis me I lifer a, and a flora from which the raw materials of honey production are obtained. This system has been investigated by Varela el at. (1991) and reported in Fuentes el al. (1995), and has provided considerable insight into the roles that biodiversity can play. Firstly, in a survey of the pollen collected by bees throughout the year, it was shown that regardless of the number of plant species in flower, which were up to 90 at any one time, bees only used up to 15 of them. This suggests a functional saturation of diversity along the lines of the Vitousek and Hooper (1993) model referred to in Section 7.4.2. A second lesson was derived from the fact that the bulk of pollen collected by bees was contributed by a limited number of plant species (Varela et al. 199 i). Galega officinalis (Fabaceae), Lithraea caustica (Anacardiaceae) and three members of the Brassicaceae (Hirschfeldia incana, Raphanus sativus and Rapistrum rugosum) were in this category, for which a seasonal replacement series of pollen supply was observed during the early southern hemisphere summer, the bulk of pollen was supplied by the Brassicaccae species in Octobcr, by

L. caustica in November/December, and by G. officinalis during December/ January (Fuentes et ai. 1995). Of the significant pollen contributors, more than 50% were introduced species (including the Brassicaceae and G. officinalis mentioned above). This indicates that altered diversity plays a major role in the functioning of the ecosystems in central Chile which produce honey for human consumption - remembering too that the main protagonist, Apis mellifera, is also an introduced species.

A similar scenario can be presented for the fynbos region of South Africa, in that case the Cape honeybee (A. mellifera capensis), a fynbos race of the European honeybee (Hepburn and Jacot-Guiilarmod 1991), is indigenous, while many of the plant species that provide it with pollen and nectar are aliens. Introduced plant species used by the Cape honeybee include Eucalyptus, Acacia, citrus fruits and deciduous fruits, as well as many herbaceous species (Anderson et al. 1983), resulting in a far more productive honey industry than would be the case without them.

7.4.6 Intraspecific variation and system function in the Mediterranean basin

In the Mediterranean basin, taxonomically well-known groups of species have been shown to have large amounts of intraspecific variation. Linking this variation to function, however, has been difficult. The common pasture grass, Dactylis glomerata, for instance, comprises a complex which includes as many as J 5 diploid types, three tetraploids and one hexaploid, the latter being confined to North Africa (Lumaret 1988). Stebbins and Zohary {1959) interpreted the differentiation of tctraploid forms of D. glomerata to be the result of autopolyploidy in diploids from both temperate and Mediterranean groups, and an ecological adaptation to the different climatic regions. High environmental heterogeneity in the Mediterranean basin is seen as the selective force behind higher MTE ecotypic variation of this species than in the topographically more uniform part of its range (Lumaret 1988). Adaptations to Mediterranean conditions include morphological traits that support water-saving mechanisms, as well as seed retention throughout the summer drought. Variations in water relations and other physiological characteristics of this species along gradients of water stress are more or less correlated with trends in four different enzyme systems (Roy and Lamaret 1987). Intraspecific variation can therefore be seen as the raw materia! for evolution, which in turn provides functional plasticity at the system level and ensures persistence of ecosystems.

Another important feature of many Mediterranean plants is the presence of volatile essential oils in their tissues. Well-known examples are thyme (Thymus), mint, basil, parsley, fennel, sage, rosemary, lavender, coriandcr, oregano, rue, bay leaves (Lauras nobilis), wormwood (Artemisia spp.), fenugreek, sesame, saffron, licorice, onions, shallots, chives and garlic. The majority of these are grouped in the Lamiaceae, Apiaceae and Asteraceae. While these species are clearly of eulinarily functional value to humans, the role of plants containing volatile aromatic compounds in ecosystems is complex and not fully understood. On the one hand it is thought that these highly flammable oils are linked to the fire ecology of Mediterranean basin systems, but they may also: (i) be a defence against herbivores, bacteria and fungi; (ii) inhibit competitor establishment through allelopathy; (iii) mimic insect pheromones to attract pollinators; (iv) reduce water stress by providing antitranspirant action (Margaris and Vokou 1982). In Thymus, one of the best-studied of aromatic genera, there is significant variation in oil content between species, as well as genetically controlled variation within species. Gouyon et al. (1986) found that the distribution of intraspecific variability in oil content (chemotypes) in Thymus vulgaris is probably strictly determined by the environment. However, they also found that there is a high turnover of chemical polymorphism across very short distances, which makes up a mosaic persistent in time. However poorly it is understood, diversity at the intraspecific level is clearly implicated with the maintenance of system function.

7.4.7 Pleistocene herbivores in the Californian palaeolandscape

In California, near the end of the Pleistocene, there was a massive extinction event in the large mammalian herbivore fauna; over 70% of the genera were lost during a brief period of a few hundred years. This was apparently precipitated by the depletion of large megaherbivore populations through human impacts (Martin 1984). This resulted in a cascade of extinctions involving other mammals in the trophic chain (Owen-Smith 1989). It was proposed that an important role of the megaherbivores was the maintenance of landscape diversity; the loss of this faunal component resulted in loss of habitats required by other herbivores, and hence the loss of food resources for carnivores, which ultimately greatly altered the functioning of these ecosystems (Keeley and Swift 1995).

7.4.8 Formation shifts in South African MTEs

Until recently the prevailing view was that the boundaries bet^ben major vegetation formations in the southern and southwestern Cape of South Africa were controlled by edaphic factors and moisture availability (see review in Cowling and Holmes 1992). However, recent studies revealed the dynamic nature of boundaries between forest and fynbos, fynbos and grassland, fynbos and renosterveld, and renosterveld and grassland, and the boundary between stands of natural vegetation and thickets of alien trees and shrubs. In all cases, changed disturbance regimes (notably fire and

Figure 7.6 A model of the feedback loop which connects system processes with vegetation type in MTEs. This figure refers to the observed dynamics of natural vegetation and that dominated by invasive alien plants in MTEs of the fynbos region of South Africa, as described in Table 7.4

grazing intensity) have caused shifts in the boundaries between formations (Richardson et al. 1995).

Ecosystem functions such as net primary productivity and standing phytomass, water production, fire behavior, fire-induced soil water-repel-lency, and sediment yield and nutrient cycling are very different in the major natural and alien vegetation formations of the Cape floristic region (CFR). These differences are attributable to the structural and functional characteristics of the assemblages, rather than to the biodiversity of these formations per se (Richardson et al. 1995). Changing patterns of land use can and do influence the boundaries between the major formations in the CFR. The replacement of species-rich fynbos by species-poor indigenous forest or stands of a few alien tree species has major effects on ecosystem function. Such vegetation changes are often rapid and irreversible (Bond and Richardson 1990), and their effects are pervasive (Figure 7.6). For example, the recent invasion of the region by many bird species from adjacent biomes is an indicator of profound alterations to many ecosystem features caused by man-induced vegetation change. Intentional or naturally occurring formation shifts offer excellent opportunities for studying the effects of changes in biodiversity on ecosystem function (Table 7.3).

Table 7.3 Dynamics of natural and transformed MTEs in the fynbos region of South Africa, showing feedback influences of functionally different vegetation types (after Richardson et al. 1995)
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