CONSTR ECOL & METABOL
CONSTR ECOL & METABOL BCN 6586
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Life Cycles Richard A Fleming1 Robert L Fleming1 and Ian A Fleming2 lGreat Lakes Forest Research Centre Sault Ste Marie Ontario Canada 2 Norwegian Institute for Nature Research Trondheim Norway and Coastal Oregon Marine Experiment Station Oregon State University Newport OR USA Although no generally agreed de nition oflife exists various pro cesses associated with living organism often help distinguish them from inert matter These processes include growth or its potential reaction to stimuli in the immediate environment passive or active movement metabolism through which matter and energy are trans formed into the organization of organic compounds unique to each individual and reproduction As organisms grow and age most go through various develop mental stages in which some of these processes or certain aspects of them are particularly emphasized For instance a butterfly began life as an egg and later emerged as a larva the eating for growth stage The larva eventually pupated and underwent metamorphosis before emerging as a butterfly Only as a butterfly can the insect mate reproduce and thus complete its life cycle A species life cycle is thus the sequence ofdevelopmental stages that individuals pass through goingfrom the start ofone stage to the start of that same stage in the next generation Animals and many simple organisms complete their life cycle in a single generation For instance individuals of various species ofprotists microorganisms which have both plant and animal characteristics and bacteria originate from the ssion of a sin gle existing individual grow to maturity and then complete their life cycle by dividing into two new individuals In more advanced animal species the individual originatesfrom thefusion of male and female sex cells gametes grows to reproductive maturity produces gametes and then completes the life cycle through suc cessful mating Most plants and some fungi and protists have life cycles with alternating generations Individuals originate from germinating spores which grow into gameteproducing organisms Fertilized gametes can grow into sporeproducing organisms Spore germi nation completes the life cycle Life cycles provide frameworks for exploring how di erent pec39e u39 39 cope quot 39L quot 39L quot isoccurring because the scale ofhuman activities now exceeds that ofall other species and c ects biospheric uxes of matter and energy Global change encompasses the direct and indirect e ects of changes in atmospheric composition biological diversity land use and cli mate Since di erent stages of a life cycle often have di erent functions e g growth reproduction and requirements e g tem perature habitat food to ful l those functions di erent aspects ofglobal change will likely have dijferent possibly even com pensatory impacts over a complete life cycle Thus one needs to consider how each component of global change might c ect each life stage in forecasting how a species might respond to global change as a whole This approach is illustrated with three very di erent life forms the spruce budworm a naturally out breaking forest insect wild salmon a group of shes which spend parts of their life cycle in freshwater and parts in saltwater and black spruce a ubiquitous tree species ofNorthAmerica s boreal forest RESPONSES OF THE SPRUCE BUDWORM TO GLOBAL CHANGE The spruce budworm Choristoneura fumiferana Lepi doptera Tortricidae the dominant insect defoliator of North America s coniferous forests has been outbreaking on a roughly 35 year cycle since before the rst Europeans arrived on the continent Its outbreaks last 10715 years and during outbreaks spruce Picea species and balsam r Abies balsamea are often killed over vast areas In its life cycle Figure l the spruce budworm has one generation a year In midAugust 273 weeks after the eggs are laid and most adult moths have died the rst instar stage immedi ately prior to shedding the rst skin larvae hatch and move to overwintering sites on the branches They overwinter as small second instar larva and become active again in early May Then they pass through four heavily feeding instars before pupating in late June In July emerged moths mate disperse and lay eggs The spruce budworm s potential for causing tree mortality stems from its rapid growth and high fecundity realized fertility of 1707200 eggs per female A mature sixth instar larva weighs approximately 100 mg fresh weight and is roughly 20730 mm long This is about 1500 times larger than it was as a second instar larva just 679 weeks earlier Global change particularly climate change may affect insect populations directly through their per capital growth rates or indirectly through interactions and feedbacks with other species and abiotic components of the environment These interactions and feedbacks often depend on delicately tuned phenological relationships For instance the spruce budworm like many other herbivores has synchronized its greatest nutritional demand the heavily feeding larval stages with the time when developing rather then mature host plant foliage is most available Developing is often better for herbivores than mature plant tissue because it is lower in bre which can limit digestibility higher in nitrogen nutritional value and lower in secondary metabo lites defensive chemicals In a warmer climate foliage is generally expected to develop more quickly thus dimin ishing the time interval when herbivores can nd the best foliage Since coldblooded animals such as insects also This article is a sample from the forthcoming Encyclopedia of Global Environmental Change Copyright 2002 John Wiley amp Sons Ltd 2 THE EARTH SYSTEM BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE Emergence survival Fecunditysize CC Ace Dispersal LU Mating Ace l Emergence Dispersal survival l survival CC Survival CC BD Feedinggrowth CC Ace Figure1 Generalized life cycle of the spruce budworm Boxes enclose the major stages The principal functions and processes needed to progress from stage to stage are designated by phrases interrupting the directing arrows Letters in parentheses indicate components of global change AC atmospheric chemistry BD biodiversity CC climate change or LU land use which the text describes as potentially influencing the function or process A plus or minus indicates whether the influence is expected to be largely positive or negative for the insect develop faster and feed more actively at higher temper atures however the net effect is uncertain Many insect herbivores may remain well synchronized in phenological development with that of their host plants foliage even in changing climates On the other hand increased atmo spheric C02 concentrations may allow host plant foliage to accumulate secondary metabolites fast enough that her bivory becomes inhibited relatively earlier in development and this may work against some insects Under climate change the southwestern part of the spruce budworm s range can expect drier conditions with the increasing likelihood of drought Warmer drier condi tions directly increase spruce budworm reproductivity and larval survival Indirect effects are also important The vul nerable larval stages can escape many natural enemies eg the fungal pathogen Erym39a Zaaphthara radicans because larvae develop faster at high temperatures than at normal temperatures and this reduces the length of their exposure Furthermore larvae develop more quickly at high temper atures than many of their natural enemies As many of these natural enemies have alternate hosts they are less closely synchronized phenologically with the spruce bud worm than is the budworm with its host trees Hence some phenological desynchronization of the spruce budworm with its natural enemies is expected as climate change progresses In summary warmer drier conditions tend to weaken the regulating effects of natural enemies and simul taneously increase reproductivity and larval survival Thus the expectation for longer or more frequent outbreaks in warmer drier climates Protracted trends toward warmer climates will allow many successive generations of directed natural selection particularly for organisms like some insects eg aphids with many generations per year Increased climatic vari ability may disrupt this process frequently for short periods but seems unlikely to forestall it over the long term Con sequently genotypes best suited to warmer environments are expected to become increasingly common resulting in genetically adapted populations Experimental and obser vational evidence suggests that such adaptations may pro duce increases in the rates of phenological development increases in the number of generations per year39 shifts in geographical and also possibly host range39 changes in morphology physiology and reproductive strategy39 and possibly even speciation or extinction Other aspects of global change could affect spruce bud worm outbreak patterns but these are more speculative For instance biodiversity loss manifested as a lack of suit able plant habitat or lack of alternate hosts could have serious consequences for certain natural enemies Up to a point this loss may be compensated by other natural enemies so that there is little overall effect on the spruce budworm s outbreak cycle In central parts of the range land use changes manifested as increased wild re con trol and harvesting allow the spruce budworm s host tree species to capture ever increasing proportions of the for est The resulting increase in feeding and egglaying sites lends itself toward larger scale and possibly more intense outbreaks In contrast experiments suggest that elevated atmospheric C02 concentrations can impede the chemical communication systems that many insects use for nding mates and food sources The prevalence and importance of this impedance in the wild is unknown Figure 1 summarizes this discussion climate change has a more pervasive in uence throughout the life cycle of the spruce budworm than other components of global change and will likely bene t spruce budworm populations bio diversity loss and land use change are also represented as having net positive in uences for the spruce budworm The only negative aspects of global change diagrammed in Figure l are associated with elevated atmospheric C02 con centrations The large scale consequences of other changes in atmospheric composition are even less certain LIFE CYCLES 3 RESPONSES OF SALMON TO GLOBAL CHANGE Wild salmon populations Oncarhynchus and Salma species are declining globally populations of Atlantic salmon are declining or have been extirpated in twothirds of their native range hundreds of Paci c salmon populations have gone extinct during the past century and hundreds more are at risk The cause of these declines is multifaceted and the relative contributions of hum aninduced changes natural climatic events and other abiotic and biotic factors have not been rmly established Unquestionably however human impact has been substantial The spatial scale at which humaninduced changes affect wild salmon varies through the course of their life his tory with localized impacts predominating in fresh water and global impacts predominating in oceanic environments Figure 2 In fresh water habitat degradation pollution and introductions of exotic species represent the main human induced impacts Logging agricultural urban and resource extraction activities may all degrade habitat eg sedimen tation channelization ow conditions removal of bank vegetation and fragmentation and water quality eg tem perature dissolved oxygen nutrients and pollution The effects of pollution also occur at large scales acid rain is blamed for losses of Atlantic salmon populations in Europe Fresh water Habitat degradation Introductions Pollu ion Climate change Transition l Barriers Habitat degradation Pollu ion Fisheries Climate change Ocean Exploitation Climate change Figure 2 Principal impacts of global change during the life cycle of anadromous salmon from egg incubation through juvenile freshwater and subadult oceanic rearing to adult freshwater breeding Three critical stages are depicted 1 fresh water 2 the transition between fresh and saltwater and 3 ocean and North America Introduction of cultured salmon exotic shes and other aquatic organisms generally have localized impacts in fresh water Escape of aquacultured salmon and stocking of hatchery salmon pose ecological eg resource competition genetic eg loss of diversity and disease threats to wild populations In Norway such practices have introduced and spread the parasite Gyradaclulus salaris which has almost eradicated 40 salmon populations The transition stage is also affected at predominately local scales The major cause of salmon extirpations is blockage of their upstream migration routes particularly by hydropower dams Fortunately the importance of this is declining Effective sh passage mechanisms have been developed inef cient dams removed and with few hydro power opportunities remaining dam construction is falling Habitat degradation and pollution affect the energetic costs susceptibility to predation physiological state and ulti mately survival of salmon during migration Over shing can also severely depress target populations and contribute to their risk of extirpation Oceanic impacts on wild salmon occur mainly at large scales re ecting the vastness of this domain Particularly important are climatic changes see Fisheries Effects of Climate Change 0n the Life Cycles of Salmon Volume 3 and exploitation It has been predicted that modest rises in sea surface temperatures due to climate change could result in salmon disappearing from large areas because of their narrow thermal limits during winter and early spring Warmer sea surface temperatures may also interact with currents to alter the location and availability of food for salmon which current evidence suggests will cause a reduction in salmon body size Exploitation effects occur both directly from salmon sheries discussed above and incidental catches during other sheries and indirectly eg industrial sheries competition of salmon with forage sh such as capelin and sand lance This analysis shows that most human impacts on Atlantic salmon are intensifying throughout the lifecycle that they are predominantly negative and that they are interacting in complex ways Sustainability of wild salmon thus depends on forecasting and minimizing these impacts The latter depends on maintaining a diversity of gene pools and population structures to provide evolutionary exibility for salmon in response to an uncertain future RESPONSES OF BLACK SPRUCE TO GLOBAL CHANGE Trees are amongst the longestlived organisms on earth39 several decades are usually required to reach sexual matu rity or full fecundity and generation times are often 507200 years Adaptations and changes in forest tree composition in response to global change will largely re ect the compet itive abilities and ecological tolerances of existing ecotypes 4 THE EARTH SYSTEM BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE rather than the rapid evolution of new ecotypes As the main structural component of forested landscapes trees are the primary providers of habitat for many terrestrial organisms and changes in the overstory resulting from global change will have wideranging repercussions Black spruce Picea mariana is an abundant evergreen conifer found throughout the North American boreal for est It has the most northerly range of any tree species on the continent extending to the tree line and is commonly found as far south as the temperate forests of the Great Lakes region This slowgrowing tree reaches heights of 15725 In and ages exceeding 200 years on productive sites but assumes a more shrublike stature on very poor sites or in extreme climates Over much of its southern and cen tral range it forms dense stands of considerable standing biomass either by itself on wetter sites or in combina tion with other species on welldrained sites In northern locations stands are more open and growth is reduced39 at its southern limits this species is con ned to cold wet or shallowsoiled sites Black spruce seed and pollen cones emerge in the spring and fertilized seeds mature by fall of the same year Cones continue to disperse seed for several years after ripening Seed production occurs regularly in stands over 25 years old and peaks in stands between 50 and 150 years of age Seed fall is accelerated for 273 years after wild re Seeds are commonly dispersed 1007200m from stand edges but individual seeds can achieve much greater dispersal distances particularly during high winds or on crusted snow Black spruce also reproduces vegetatively from lower living branches which become partially buried by organic debris This form of reproduction is common in open grown stands particularly in the far north where successful pollination and subsequent seed germination are restricted Climate change could have large direct impacts on the range abundance and vigour of virtually all forest trees including black spruce Warmer temperatures should improve seed production and germination at higher lati tudes and may allow considerable extension of black spruce beyond the current tree line In contrast the forecasted northerly extension of the western Canadian prairies would greatly reduce the southern extent of the species range in western Canada The direct effects of changes in precip itation patterns are less predictable Black spruce is well adapted to drier upland sites as well as wetter lowland sites both in the colder continental climates of northwest ern North America and in the cool more humid climates to the east However drier conditions combined with warmer temperatures may restrict germination limit seedling estab lishment and increase drought stress in established stands Increased temperatures and C02 levels will improve tree growth but may put black spruce at a competitive disad vantage on sites which provide suitable habitat for competi tors such as white pine Pinus slrabus jack pine Pinus banksiana and oak Quercus species which have greater temperature or COz growth responses In the southeastern portion of its range climate change effects on black spruce populations may be manifested largely through alterations in competitive relationships with other species Longevity conservative resource require ments and broad ecological tolerances suggest that estab lished black spruce trees and stands may not be greatly affected by the forecasted changes in climate Increased com petition from faster growing andor more shade tolerant competitors however is likely to reduce seedling establish ment and sapling vigour Climate change in uences on the frequency and intensity of rarely occurring catastrophic events eg wild re insect outbreaks storms severe frosts are equally as important to forest trees as the direct effects of incremental changes in climate averages Like most boreal species black spruce is adapted to and dependent upon infrequent wild res which destroy existing stands but produce dense vigorous new stands of the same species Increased re frequencies ie within every 30740 years could reduce black spruce abundance by burning many stands before most trees reach full fecundity In contrast substantial reductions in re frequency could permit the incursion of fastergrowing or more shadetolerant competitors which are less readapted but better resource competitors than black spruce Feedback between changes in stand composition and re frequency are also likely Large increases in broadleaved species will reduce fuel availability and perhaps re frequency Insect epidemics affect individual tree species directly through herbivory and indirectly by altering competitive relationships with other plants Outbreaks of the major insect pest of black spruce the spruce budworm reduce stand vigour and destroy current seed production but cause little longterm damage to black spruce This re ects the phenological development of feeding larvae which is syn chronized with vegetative bud ush and foliar development in balsam r and white spruce the spruce budworm s prin cipal host species Vegetative bud ush in black spruce usu ally occurs 10714 days later Differential effects of climate change on herbivorehost phenology could either reduce or greatly increase defoliation of black spruce by altering the synchrony of bud ush and budworm larval feeding Climateinduced changes in the frequency and severity of insect outbreaks on associated species eg forest tent caterpillar on trembling aspen and spruce budworm on bal sam r could alter the competitive position of black spruce Other facets of global change which have widespread implications for forest trees include logging deforestation and afforestation Most of the land base currently occupied by black spruce has little agricultural value because of cli mate and soil limitations and more land suitable for black spruce is currently reverting from agriculture to forests than vice versa In contrast logging activities have had large and LIFE CYCLES 5 sustained negative impacts on the representation of black spruce on the landscape Black spruce is highly valued for pulpwood and saw timber and has been logged extensively throughout the southern boreal Typically on peatland sites black spruce remains the predominant species in second growth stands although there may be regeneration delays and reductions in stand densities Without planting and stand tending many areas on upland sites revert from black sprucedominated to mixedwood forests with black spruce a minor component The widespread establishment of black spruce plantations has maintained this species on other parts of the landscape but often at reduced densities and with dif ferent stand compositions and structures than are found in stands of wild re origin CONCLUSION The life cycles of three very different species have provided analytic frameworks for developing a qualitative under standing of how each species is likely to react to global change Global change will affect some stages of a life cycle more than others and certain aspects of global change either individually or interactively are more important than others at these particularly vulnerable stages To establish in a new area or persist in its current habitat a species must be able to complete its life cycle By identifying the most vulnerable stages life cycle analyses can provide a focus for management efforts ranging from repelling invad ing species to protecting endangered ones see Integrated Pest Management in an Era of Global Environmental Change Volume 4 FURTHER READING Ayres M P and Lombardero M J 2000 Assessing the Conse quences of Global Change for Forest Disturbance from Herbi vores and Pathogens Sci Total Environ 262 2637286 Black R A and Bliss L C 1980 Reproductive Ecology ofPicea mariana Mill BSP at Tree Line Near Inuvik Northwest Territories Canada Ecol Monogr 50 3317354 Farrar J L 1995 Trees in Canada Fitzhenry and Whiteside Markham Ontario 17502 Fleming RA 2000 Climate Change and Insect Disturbance Regimes in Canada s Boreal Forests World Resour Rev 123 5207555 Johnson E A 1992 Fire and Vegetation Dynamics Studiesfrom the North American Boreal Forest Cambridge University Press Cambri e Loehle C and LeBlanc D 1996 Modelbased Assessments of Climate Change Effects on Forests a Critical Review Ecol Model 90 1731 Mather M E Parrish D L and Flot C L eds 1998 Integrat ing Across Scales Predicting Patterns of Change in Atlantic Salmon Can J Fish Aquat Sci 551 17323 Nelhsen W William J E and Lichatowich J A 1991 Paci c Salmon at the Crossroads Stocks at Risk from California Oregon Idaho and Washington Fisheries 162 4721 Slaney T L Hyatt K D Northcote T G and Fielden R J 1996 Status of Anadromous Salmon and Trout in British Columbia and Yukon Fisheries 2110 20735 United Nations Environment Program 1999 Global Environment Outlook 2000 Earthscan London 17398 Viereck L A and Johnston W F 1990 Picea Mariana Mill BSP in Silvics of North America eds RM Burns and B H Honkala Handbook 654 US Department of Agriculture Forest Service Washington DC 2277237 Vol 1