Insect Ecology - An Ecosystem Approach 2nd ed - Chapter 15 pptx

15 Insects as Regulators of Ecosystem Processes I. Development of the Concept II. Ecosystems as Cybernetic Systems A. Properties of Cybernetic Systems B. Ecosystem Homeostasis C. Definition of Stability D. Regulation of Net Primary Productivity by Biodiversity E. Regulation of Net Primary Productivity by Insects III. Summary INSECTS, AND OTHER ORGANISMS, INEVITABLY AFFECT THEIR ENVIRONMENT through spatial and temporal patterns of resource acquisition and redistribution. Insects respond to environmental changes in ways that dramatically alter ecosystem conditions, as discussed in Chapters 12–14. These effects of organisms do not necessarily provide cybernetic (stabilizing) regulation. However, the hypothesis that insects stabilize ecosystem properties through feedback regulation is one of the most important and revolutionary concepts to emerge from research on insect ecology and should be considered in making pest management decisions in natural ecosystems. The concept of self-regulation is a key aspect of ecosystem ecology. Vegeta- tion has a documented role in ameliorating variation in climate and biogeochemical cycling (Chapter 11), and vegetative succession facilitates recovery of ecosystem functions following disturbances. However, the concept of self- regulating ecosystems has seemed to be inconsistent with evolutionary theory (emphasizing selection of “selfish” attributes) (e.g., Pianka 1974), with variable successional trends following disturbance (e.g., H. Horn 1981) and with the lack of obvious mechanisms for maintaining homeostasis (e.g., Engelberg and Boyarsky 1979). The debate over the self-regulating capacity of ecosystems, and especially the role of insects, is somewhat reminiscent of debate on the now-recognized importance of density-dependent feedback regulation of population size (Chapter 16) and is a useful example of how science develops. The outcome of this debate has significant consequences for how we manage ecosystems and their biotic 437 015-P088772.qxd 1/24/06 11:05 AM Page 437 resources. Although controversial, this concept is an important aspect of insect ecology, and its major issues are the subject of this chapter. I. DEVELOPMENT OF THE CONCEPT The intellectual roots of ecosystem self-regulation lie in Darwin’s (1859) recognition that some adaptations apparently benefit a group of organisms more than the individual, leading to selection for population stability. The concept of altruism and selection for homeostasis at supraorganismal levels has remained an important issue, despite recurring challenges and alternative models (e.g., Axelrod and Hamilton 1981, Schowalter 1981, E. Wilson 1973, 1997). Behavioral ecologists have been challenged to explain the evolution of altruistic behaviors that are fundamental to social organization. Even sexual reproduction could be considered a form of self-restraint because individuals contribute only half the genotype of their progeny through sexual reproduction, compared to the entire genotype of their progeny through asexual reproduction (Pianka 1974). Cooperative interactions, such as mutualism, and self-sacrificing behavior, such as suppression of reproduction and suicidal defense by workers of social insects, have been more difficult to explain in terms of individual selection. Haldane (1932) proposed a model in which altruism would have a selective advantage if the starting gene frequency were high enough and the benefits to the group outweighed individual disadvantage. This model raised obvious questions about the origin of altruist genes and the relative advantages and disad- vantages that would be necessary for increased frequency of altruist genes. Group selection theory was advanced during the early 1960s by Wynne- Edwards (1963, 1965), who proposed that social behavior arose as individuals evolved to curtail their own individual fitnesses to enhance survival of the group. Populations that do not restrain combat among their members or that overex- ploit their resources have a higher probability of extinction than do populations that regulate combat or resource use. Selection thus should favor demes with traits to regulate their densities (i.e., maintain homeostasis in group size). Behav- iors such as territoriality, restraint in conflict, and suppressed reproduction by subordinate individuals (including workers in social insect colonies) thereby reflect selection (feedback) for traits that prevent destructive interactions or oscillations in group size. This hypothesis was challenged for lack of explicit evolutionary models or experimental tests that could explain the progressive evolution of homeostasis at the group level (i.e., demonstration of an individual advantage to altruistic individuals over selfish individuals). Furthermore, Wynne-Edwards’ proposed devices by which individuals curtail their individual fitnesses, and communicate their density and the degree to which each individual should decrease its individual fitness, were inconsistent with available evidence or could be explained better by models of individual fitness (E.Wilson 1973). Nevertheless, the concept of group selection was recognized as an important aspect of social evolution (E. Wilson 1973). Hamilton (1964) and J. M. Smith (1964) developed an evolutionary model, based on kin selection, whereby individual fitness is increased by 438 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES 015-P088772.qxd 1/24/06 11:05 AM Page 438 behaviors that favor survival of relatives with similar genotypes.They introduced a new term, inclusive fitness, to describe the contributions of both personal reproduction and reproduction by near kin to individual fitness. For example,care for offspring of one’s siblings increases an individual’s fitness to the extent that it contributes to the survival of related genotypes. Failure to provide sufficient care for offspring of siblings reduces survival of family members. This concept explained evolution of altruistic behaviors, such as maternal care; shared rearing of offspring among related individuals; alarm calls (that may draw attention of predators to the caller); and voluntary suppression of reproduction and suicidal defense by workers in colonies of social insects, which usually benefit close relatives. For social Hymenoptera, Hamilton (1964) noted that males are produced from unfertilized eggs and have unpaired chromosomes. Accordingly, all the daughters in the colony inherit only one type of gamete from their father and thereby share 50% of their genes through this source. In addition, they share another 25%, on average, of their genes in common from their mother. Overall, the daughters share 75% of their genes with each other compared to only 50% of their genes with their mother. Therefore, workers maximize their fitness by helping to rear siblings, rather than by having their own offspring. This model does not apply to termites. Husseneder et al. (1999) and Thorne (1997) suggested that developmental and ecological factors, such as slow development, iteroparity, overlap of generations, food-rich environment, high risk of dispersal, and group defense, may be more important than genetics in the maintenance of termite eusociality, whatever factors may have favored its original development. Levins (1970) and Boorman and Levitt (1972) proposed interdemic selection models to account for differential extinction rates among demes of metapopula- tions that differ in altruistic traits. In the Levins model, colonists from small populations found other small populations in habitable sites. Increasing frequency of altruist genes decreases the probability of extinction of these small populations (i.e., cooperation elevates and maintains each deme above the extinction threshold; see Chapters 6 and 7). In the Boorman–Levitt model, colonists from a large, stable population found small, marginal populations in satellite habitats. Altruist genes do not influence extinction rates until marginal populations reach demo- graphic carrying capacity (i.e., altruism prevents destructive population increase above carrying capacity; see Chapters 6 and 7). Both models require restrictive conditions for evolution of altruist genes. Matthews and Matthews (1978) noted that group selection requires that an allele become established by selection at the individual level. Thereafter, selection could favor demes with altruist genes that reduce extinction rates, relative to demes without these genes. Interdemic selection has become a central theme in developing concepts of metapopulation dynamics (Chapter 7). Meanwhile, the concept of group selection was implicit in early models of ecological succession and community development. The facilitation model of succession proposed by Clements (1916) and elaborated by E. Odum (1953, 1969) emphasized the apparently progressive development of a stable,“climax,” ecosystem through succession. Each successional stage altered conditions in ways that I. DEVELOPMENT OF THE CONCEPT 439 015-P088772.qxd 1/24/06 11:05 AM Page 439 benefited the replacing species more than itself. However, such facilitation con- tradicted individual self-interest that was fundamental to the theory of natural selection. Furthermore, identification of alternative models of succession, including the inhibition model (Chapter 10), made succession appear to be more consistent with evolutionary theory. D. S. Wilson (1976, 1997) developed a model that specifically applied the concept of group selection to the community level. Wilson recognized that individuals and species affect their own fitness through effects on their environment, including the fitness of other individuals. For example, earthworm effects on soil development stimulate plant growth, herbivory, and litter production (see Chapter 14) and thereby increase the detrital resources exploited by the worms, a positive feedback. Furthermore, spatial heterogeneity, from large geographic to microsite scales, in population distribution results in intrademic variation in effects of organisms on their community. Given sufficient iterations of Wilson’s model, every effect of a species on its community eventually affects that species, positively or negatively, through all possible feedback pathways. Intrademic variation in effects on the environment is subject to selection for adaptive traits of individuals. The models described earlier in this section help explain the increased frequency of altruist genes, but what selective factors can maintain altruist genes in the face of evolutionary pressure to “cheat” among nonrelated individuals? Trivers (1971) and Axelrod and Hamilton (1981) developed a model of reciprocal altruism based on the Prisoner’s Dilemma (Fig. 15.1), in which each of two players can cooperate or defect. Each player can choose to cooperate or defect if the other player chooses to cooperate or defect. If the first player acts coop- eratively, the benefit/cost for cooperation by the second player (reward for mutual cooperation) is less than that for defection (temptation for the first player 440 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES FIG. 15.1 Prisoner’s Dilemma, defined by T > R > P > S and R > (S + T)/2, with payoff to player A shown using illustrative values. From Axelrod and Hamilton (1981) with permission from the American Association for the Advancement of Science. Please see extended permission list pg 573. 015-P088772.qxd 1/24/06 11:05 AM Page 440 to defect in the future); if the first player defects, the benefit/cost for cooperation by the second player (sucker’s payoff) is less than that for defection (punishment for mutual defection). Therefore, if the interaction occurs only once, defection (noncooperation) is always the optimal strategy, despite both individuals doing worse than they would if they both cooperate. However, Axelrod and Hamilton (1981) recognized the probability of repeated interaction between pairs of unrelated individuals and addressed the initial viability (as well as final stability) of cooperative strategies in environments dominated by noncooperat- ing individuals or more heterogeneous environments composed of other individuals using a variety of strategies. After numerous computer simulations with a variety of strategies, they concluded that the most robust strategy in an environment of multiple strategies also was the simplest, Tit-for-Tat. This strategy involves cooperation based on reciprocity and a memory extending only one move back (i.e., never being the first to defect but retaliating after a defection by the other and forgiving after just one act of retaliation). They also found that once Tit-for-Tat was established, it resisted invasion by possible mutant strategies as long as the interacting individuals had a sufficiently large probability of meeting again. Axelrod and Hamilton emphasized that Tit-for-Tat is not the only strategy that can be evolutionarily stable. The Always Defect Strategy also is evolutionarily stable, no matter what the probability of future interaction.They postulated that altruism could appear between close relatives, when each individual has part interest in the partner’s gain (i.e., rewards in terms of inclusive fitness), whether or not the partner cooperated. Once the altruist gene exists, selection would favor strategies that base cooperative behavior on recognition of cues, such as relat- edness or previous reciprocal cooperation. Therefore, individuals in relatively stable environments are more likely to experience repeated interaction and selection for reciprocal cooperation than are individuals in unstable environments that provide low probabilities of future interaction. These models demonstrate that selection at supraorganismal levels must be viewed as contributing to the inclusive fitness of individuals. Cooperating individuals have demonstrated greater ability in finding or exploiting uncommon or aggregated resources, defending shared resources, and mutual protection (Hamil- ton 1964). Cooperating predators (e.g., wolves and ants) have higher capture effi- ciency and can acquire larger prey compared to solitary predators. The mass attack behavior of bark beetles is critical to successful colonization of living trees. Co-existing caddisfly larvae can modify substrate conditions and near-surface water velocity, thereby enhancing food delivery (Cardinale et al. 2002). Animals in groups are more difficult for predators to attack. Reciprocal cooperation reflects selection via feedback from individual effects on their environment. The strength of individual effects on the environment is greatest among directly interacting individuals and declines from the population to community levels (Fig. 1.2) (e.g., Lewinsohn and Price 1996). Reciprocal cooperation can explain the evolution of sexual reproduction and social behavior as the net result of tradeoffs between maximizing the contribution of an individual’s own genes to its progeny and maximizing the contribution I. DEVELOPMENT OF THE CONCEPT 441 015-P088772.qxd 1/24/06 11:05 AM Page 441 of genes represented in the individual to progeny of its relatives. Similarly, species interactions represent tradeoffs among positive and negative effects (see Chapter 8). Population distribution in time and space (i.e., metapopulation dynamics; see Chapter 7) is a major factor affecting interaction strengths. Individuals dispersed in a regular pattern (Chapter 5) over an area will affect a large proportion of the total habitat and interact widely with co-occurring populations, whereas the same total number of individuals dispersed in an aggregated pattern will affect a smaller proportion of the total habitat but may have a higher frequency of interactions with co-occurring populations in areas of local abundance. Consistency of population dispersion through time affects the long-term frequency of interactions and reinforcement of selection from generation to generation. Metapopulation dynamics interacting with disturbance dynamics provide the template for selection of species assemblages best adapted to local environmental variation. II. ECOSYSTEMS AS CYBERNETIC SYSTEMS The cybernetic nature of ecosystems, from patch to global scales, has been a central theme of ecosystem ecology. J.Lovelock (1988) suggested that autotroph– heterotroph interactions have been responsible for the development and regulation of atmospheric composition and climate that are suitable for the persist- ence of life.The ability of ecosystems to minimize variability in climate and rates of energy and nutrient fluxes would affect responses to anthropogenic changes in global conditions. A. Properties of Cybernetic Systems Cybernetic systems generally are characterized by (1) information systems that integrate system components, (2) low-energy feedback regulators that have high- energy effects, and (3) goal-directed stabilization of high-energy processes. Mech- anisms that sense deviation (perturbation) in system condition communicate with mechanisms that function to reduce the amplitude and period of deviation. Neg- ative feedback is the most commonly recognized method for stabilizing outputs. A thermostat represents a simple example of a negative feedback mechanism. The thermostat senses a departure in room temperature from a set level and communicates with a temperature control system that interacts with the thermostat to readjust temperature to the set level. The room system is maintained at tem- peratures within a narrow equilibrial range. Organisms are recognized as cybernetic systems with neurological networks for communicating physiological conditions and various feedback loops for maintaining homeostasis of biological functions. Cybernetic function is perhaps best developed among homeotherms. These organisms are capable of self-regulating internal temperature through physiological mechanisms that sense change in body temperature and trigger changes in metabolic rate, blood flow, and sweat 442 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES 015-P088772.qxd 1/24/06 11:05 AM Page 442 that increase or decrease temperature as necessary. However, energy demand is high for such regulation. Heterotherms also have physiological and behavioral mechanisms for adjusting body temperature within a somewhat wider range but with lower energy demand (see Chapters 2 and 4). Regardless of mechanism, the result is sufficient stability of metabolic processes for survival. Although self-adjusting mechanical systems and organisms are the best- recognized examples of cybernetic systems, the properties of self-regulating systems have analogs at supraorganismal levels (B. Patten and Odum 1981, Schowalter 1985, 2000). Human families and societies express goals in terms of survival, economic growth, improved living conditions, and so on and accomplish these goals culturally through governing bodies, communication networks, and balances between reciprocal cooperation (e.g., trade agreements, treaties) and negative feedback (e.g., economic regulations, warfare). B. Ecosystem Homeostasis E. Odum (1969) presented a number of testable hypotheses concerning ecosystem capacity to develop and maintain homeostasis, in terms of energy flow and biogeochemical cycling, during succession. Although subsequent research has shown that many of the predicted trends are not observed, at least in some ecosystems, Odum’s hypotheses focused debate on ecosystems as cybernetic systems. Engelberg and Boyarsky (1979) argued that ecosystems do not possess the critical goal-directed communication and low-cost/large-effect feedback systems required of cybernetic systems. Although ecosystems can be shown to possess these properties of cybernetic ecosystems, as described later in this section, this debate cannot be resolved until ecosystem ecologists reach consen- sus on a definition and measurable criteria of stability and demonstrate that potential homeostatic mechanisms, such as biodiversity and insects (see later in this chapter), function to reduce variability in ecosystem conditions. Although discussion of ecosystem goals appears to be teleological, nonteleo- logical goals can be identified (e.g., maximizing distance from thermodynamic ground; see B. Patten 1995, a requisite for all life). Stabilizing ecosystem conditions obviously would reduce exposure of individuals and populations to extreme, and potentially lethal, departures from normal conditions. Furthermore, stable population sizes would prevent extreme fluctuations in abundances that would jeopardize stability of other variables. Hence, environmental heterogeneity might select for individual traits that contribute to stability of the ecosystem. The argument that ecosystems do not possess centralized mechanisms for communicating departure in system condition and initiating responses (e.g., Engelberg and Boyarsky 1979) ignores the pervasive communication network in ecosystems (see Chapters 2, 3, and 8). However, the importance of volatile chemicals for communicating resource conditions among species has been recognized relatively recently (Baldwin and Schultz 1983, Rhoades 1983, Sticher et al. 1997, Turlings et al. 1990, Zeringue 1987). The airstream carries a blend of volatile II. ECOSYSTEMS AS CYBERNETIC SYSTEMS 443 015-P088772.qxd 1/24/06 11:05 AM Page 443 chemicals, produced by the various members of the community, that advertises the abundance, distribution, and condition of various organisms within the community. Changes in the chemical composition of the local atmosphere indicate changes in the relative abundance and suitability of hosts or the presence and proximity of competitors and predators. Sensitivity among organisms to the chemical composition of the atmosphere or water column may provide a global information network that communicates conditions for a variety of populations and initiates feedback responses. Feedback loops are the primary mechanisms for maintaining ecosystem stability,regulating abundances and interaction strengths (W. Carson and Root 2000, de Ruiter et al. 1995, B. Patten and Odum 1981, Polis et al. 1997a, b, 1998). The combination of bottom-up (resource availability), top-down (predation), and lateral (competitive) interactions generally represent negative feedback, stabilizing food webs by reducing the probability that populations increase to levels that threaten their resources (and, thereby, other species supported by those resources). Mutualistic interactions and other positive feedbacks reduce the probability of population decline to extinction thresholds. Although positive feedback often is viewed as destabilizing, such feedback may be most important when populations are small and likely is limited by negative feedbacks as populations grow beyond threshold sizes (Ulanowicz 1995). Such compensatory interactions may maintain ecosystem properties within relatively narrow ranges, despite spatial and temporal variation in abiotic conditions (Kratz et al. 1995, Ulanowicz 1995). Omnivory increases ecosystem stability, perhaps by increasing the number of linkages subject to feedback (Fagan 1997). Ecological succession represents one mechanism for recovery of ecosystem properties following disturbance-induced departures from nominal conditions. The concept of self-regulation does not require efficient feedback by all ecosystems or ecosystem components. Just as some organisms (recognized as cybernetic systems) have greater homeostatic ability than do others (e.g., homeotherms vs. heterotherms), some ecosystems demonstrate greater homeostatic ability than do others (J.Webster et al. 1975). Frequently disturbed ecosystems may be reestablished by relatively random assemblages of opportunistic colonists and select genes for rapid exploitation and dispersal. Their short dura- tion provides little opportunity for repeated interaction that could lead to stabilizing cooperation (cf. Axelrod and Hamilton 1981). Some species increase variability or promote disturbance (e.g., brittle or flammable species; e.g., easily toppled Cecropia and flammable Eucalyptus). Insect outbreaks increase variation in some ecosystem parameters (Romme et al. 1986), often in ways that promote regeneration of resources (e.g., Schowalter et al. 1981a). Despite this, relatively stable environments, such as tropical rainforests, might not select for stabilizing interactions. However, stable environmental conditions should favor consistent species interactions and the evolution of reciprocal cooperation, such as demonstrated by a diversity of mutualistic interactions in tropical forests. Selection for stabilizing interactions should be greatest in ecosystems characterized by intermediate levels of environmental variation. Interactions that reduce such variation would contribute to individual fitnesses. 444 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES 015-P088772.qxd 1/24/06 11:05 AM Page 444 C. Definition of Stability B. Patten and Odum (1981) proposed that a number of time-invariant or regularly oscillating ecosystem parameters represent potential goals for stabilization. These included total system production (P) and respiration (R), P : R ratio, total chlorophyll, total biomass, nutrient pool sizes, species diversity, population sizes, etc. However, the degree of spatial and temporal variability of these parameters remains poorly known for most, even intensively studied, ecosystems (Kratz et al. 1995). Kratz et al. (1995) compiled data on the variability of climatic, edaphic, plant, and animal variables from 12 Long Term Ecological Research (LTER) sites, rep- resenting forest, grassland, desert, lotic, and lacustrine ecosystems in the United States. Unfortunately, given the common long-term goals of these projects, comparison was limited because different variables and measurement techniques were represented among these sites. Nevertheless, Kratz et al. offered several important conclusions concerning variability. First, the level of species combination (e.g., species, family, guild, total plants or animals) had a greater effect on observed variability in community structure than did spatial or temporal extent of data. For plant parameters, species- and guild-level data were more variable than were data for total plants; for animal parameters, species-level data were more variable than were guild-level data, and both were more variable than were total animal data. As discussed for food-web properties in Chapter 9, the tendency to ignore diversity, especially of insects (albeit for logistic reasons), clearly affects our perception of variability. Detec- tion of long-term trends or spatial patterns depends on data collection for parameters sufficiently sensitive to show significant differences but not so sensitive that their variability hinders detection of differences. Second, spatial variability exceeded temporal variability. This result indicates that individual sites are inadequate to describe the range of variation among ecosystems within a landscape. Variability must be examined over larger spatial scales. Edaphic data were more variable than were climatic data, indicating high spatial variation in substrate properties, whereas common weather across landscapes homogenizes microclimatic conditions.This result also could be explained as the result of greater biotic modification of climatic variables compared to substrate variables (see the following text). Third, biotic data were more variable than were climatic or edaphic data. Organisms can exhibit exponential responses to incremental changes in abiotic conditions (see Chapter 6). The ability of animals to move and alter their spatial distribution quickly in response to environmental changes is reflected in greater variation in animal data compared to plant data. However, animals also have greater ability to hide or escape sampling devices. Finally, two sites, a desert and a lake, provided a sufficiently complete array of biotic and abiotic variables to permit comparison. These two ecosystem types represent contrasting properties. Deserts are exposed to highly variable and harsh abiotic conditions but are interconnected within landscapes, whereas lakes exhibit relatively constant abiotic conditions (buffered from thermal change by II. ECOSYSTEMS AS CYBERNETIC SYSTEMS 445 015-P088772.qxd 1/24/06 11:05 AM Page 445 mass and latent heat capacity of water, from pH change by bicarbonates, and from biological invasions by their isolation) but are isolated by land barriers. Comparison of variability between these contrasting ecosystems supported the hypothesis that deserts are more variable than lakes among years, but lakes are more variable than deserts among sites. Kratz et al. (1995) provided important data on variation in a number of ecosystem parameters among ecosystem types. However, important questions remain. Which parameters are most important for stability? How much deviation can be tolerated? What temporal and spatial scales are relevant to ecosystem stability? Among the parameters that could be stabilized as a result of species interactions, net primary production (NPP) and biomass structure (living and dead) may be particularly important. Many other parameters, including energy, water and nutrient fluxes, trophic interactions, species diversity, population sizes, climate, and soil development, are directly or indirectly determined by NPP or biomass structure (Boulton et al. 1992; see Chapter 11). In particular, the ability of ecosystems to modify internal microclimate,protect and modify soils,and provide stable resource bases for primary and secondary producers depends on NPP and biomass structure. Therefore, natural selection over long periods of co-evolution should favor individuals whose interactions stabilize these ecosystem parameters. NPP may be stabilized over long time periods as a result of compensatory community dynamics and biological interactions, such as those resulting from biodiversity and herbivory (see later in this chapter). No studies have addressed the limits of deviation, for any parameter, within which ecosystems can be regarded as qualitatively stable. Traditional views of stability have emphasized consistent species composition, at the local scale, but shifts in species composition may be a mechanism for maintaining stability in other ecosystem parameters, at the landscape or watershed scale. This obviously is an important issue for evaluating stability and predicting effects of global environmental changes. However, given the variety of ecosystem parameters and their integration at the global scale, this issue will be difficult to resolve. The range of parameter values within which ecosystems are conditionally stable may be related to characteristic fluctuations in environmental conditions or nutrient fluxes. For example, biomass accumulation increases ecosystem storage capacity and ability to resist variation in resource availability (J.Webster et al. 1975) but also increases ecosystem vulnerability to some disturbances, including fire and storms. Complex ecosystems with high storage capacity (i.e., forests) are the most buffered ecosystems, in terms of regulation of internal climate, soil conditions, and resource supply, but also fuel the most catastrophic fires under drought conditions and suffer the greatest damage during cyclonic storms. Hence, ecosystems with lower biomass, but rapid turnover of matter or nutrients, may be more stable under some environmental conditions. Species interactions that periodically increase rates of nutrient fluxes and reduce biomass (e.g., herbivore outbreaks) traditionally have been viewed as evidence of instability but may contribute to stability of ecosystems in which biomass 446 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES 015-P088772.qxd 1/24/06 11:05 AM Page 446 [...]... arid conditions and frequent droughts that historically maintained a sparse woodland dominated by drought- and fire-tolerant (but shade-intolerant) pine trees and a ground cover of grasses and shrubs, with little understory (Fig 15. 7) Low-intensity ground fires occurred frequently, at intervals of 15 25 years, and covered large areas (Agee 1993), minimizing droughtintolerant vegetation and litter accumulation... later successional fir FIG 15. 7 The relatively arid interior forest region of North America was characterized by open-canopied forests dominated by drought- and fire-tolerant pines, and by sparse understories, prior to fire suppression beginning in the late 1800s (A) Fire suppression has transformed forests into dense, multistoried ecosystems stressed by competition for water and nutrients (B) From Goyer... stability and “health” of various ecosystems? Until recently, insect outbreaks and disturbances have been viewed as destructive forces The increased productivity of ecosystems in the absence of fire and insect outbreaks supported a view that resource production could be freed from limitations imposed by these regulators However, fire now is recognized as an important tool for restoring sustainable (stable) ecosystem. .. processes Dominant organisms in any ecosystem are adapted to survive environmental changes or disturbances that recur regularly with respect to generation time Therefore, adaptation to prevailing conditions (evolution) constitutes a feedback that reduces ecosystem deviation from nominal conditions For example, many grassland and pine forest species are adapted to survive low-intensity fires and drought... accumulation The relatively isolated higher elevation and riparian zones were more mesic and supported shade-tolerant (but fire- and drought-intolerant) fir and spruce forests Fire was less frequent (every 150 –1000 years) but more catastrophic at higher elevation as a result of the greater tree densities and understory development that facilitated fire access to tree canopies (Agee 1993, Veblen et al 1994)... virtually all ecosystems (e.g., Table 9.1) and are capable of controlling a 451 452 15 INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES FIG 15. 4 Relationship between plant species diversity prior to drought and drought resistance in experimental grassland plots planted with different species diversities Mean, standard error, and number of plots with given species richness are shown 1 dB/Bdt (yr-1) = 0.5 ln... grasshoppers depressed plant growth and survival more than could be offset by increased nitrogen cycling and plant productivity Despite the obvious influence of animals on key ecosystem processes, their regulatory role has remained controversial and largely untested Herbivorous insects possess the characteristics of cybernetic regulators (i.e., low maintenance cost and rapidly ampli ed effects, sensitivity... similar situation has been inferred from insect demography in pine-hardwood forests of the southern United States (see Fig 10.5) Van Langevelde et al (2003) also suggested a cycle of alternating vegetation states maintained by interaction of fire and herbivores in African savanna III SUMMARY FIG 15. 8 Phytophage modification of succession in central Sierran mixed conifer ecosystems during 1998 Understory... generally reduce biomass and improve water or nutrient balance or, in extreme cases, reduce biomass of the most stressed plants, regardless of their abundance, and promote replacement by better adapted plants (e.g., Ritchie et al 1998, Schowalter and Lowman 1999) Second, high densities of particular plant species, as a result of artificial planting or of inhibitive successional stages, enhance host availability... Our management of ecosystem resources, and in particular our approach to managing phytophagous insects, requires that we understand the extent to which phytophages contribute to ecosystem stability III SUMMARY The hypothesis that phytophagous insects regulate ecosystem processes is one of the most important and controversial concepts to emerge from research on insect ecology The extent to which ecosystems . (1964) and J. M. Smith (1964) developed an evolutionary model, based on kin selection, whereby individual fitness is increased by 438 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES 01 5- P088772.qxd. shared resources, and mutual protection (Hamil- ton 1964). Cooperating predators (e.g., wolves and ants) have higher capture ef - ciency and can acquire larger prey compared to solitary predators compartmental- ized biodiversity and ecosystem stability. They concluded that biodiversity loss 448 15. INSECTS AS REGULATORS OF ECOSYSTEM PROCESSES 01 5- P088772.qxd 1/24/06 11:05 AM Page 448 reduces

Định dạng
Số trang	25
Dung lượng	825,16 KB