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Trait and Density Mediated Indirect Interactions
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Indirect Effects of an Exotic Engineer 395, autogenic engineering may be a particularly important outcomes Werner and Peacor 2003 Griffin and Thaler. mechanism by which architecturally unique exotic plants 2006 Additionally TMIIs commonly result in less pre. impact invaded communities through both direct and in dictable outcomes and can produce highly nonlinear effects. direct effects Crooks 2002 Importantly many plant spe that magnify their strength relative to DMIIs Wootton. cies may commonly participate in multiple such indirect 1994 Krivan and Schmitz 2004 Thus decomposing these. interactions influencing one or many species of organisms components and evaluating their individual and joint con. simultaneously or sequentially Ohgushi 2005 Ohgushi et tributions is necessary to fully understand indirect effects. al 2007 Hence plants commonly serve as initiator and Consistent with the general top down emphasis in. transmitter species in indirect interactions fig 1 and rel indirect interaction studies current research on TMIIs and. egating them to only the role of receiver species greatly limits DMIIs has focused on interactions where predators are the. understandings of indirect effects initiators their consumer prey are the transmitters and. A primary focus of current indirect effects research has lower trophic levels usually primary producers are the re. been to quantify the relative influence of the two function ceiver species that is predator r consumer r producer. ally distinct types of indirect interactions Werner and Pea pathways e g Werner and Peacor 2003 Schmitz et al. cor 2003 Schmitz et al 2004 Preisser et al 2005 density 2004 However the theoretical framework applied is not. mediated indirect interactions DMIIs and behavioral or limited to top down trophic interactions Theoretically any. trait mediated indirect interactions TMIIs DMIIs arise organism at any taxonomic level can act as an initiator a. when an initiator species indirectly affects the abundance transmitter or a receiver species Wootton 1994 Abrams. of a receiver species through its direct effects on the abun et al 1996 Moreover both density and trait mediated. dance or density of a transmitter species Abrams et al interactions are likely to occur concurrently in many in. 1996 In contrast TMIIs arise when the initiator species teraction pathways All species are presumably capable of. indirectly affects the abundance of the receiver species by density responses and most organisms are capable of ex. altering the behavior or traits e g morphological physi hibiting some degree of plasticity that can result in trait. ological chemical etc of the transmitter species Both mediated interactions Miner et al 2005 Thus DMIIs and. density and trait mediated interactions commonly transmit TMIIs are not restricted to predator r consumer r pro. simultaneously through the same pathway and can have ducer pathways Moreover understanding the relative con. additive synergistic or opposing effects on net interaction tributions of TMIIs and DMIIs to net interaction outcomes. Figure 1 Indirect interaction pathways depicting initiator transmitter and receiver species in different trophic positions Research to date examining. density mediated indirect interactions DMIIs and trait mediated indirect interactions TMIIs has focused on top down trophic pathways where. the initiator I is a predator the transmitter T is a consumer and the receiver R is a primary producer a However initiator transmitter and. receiver species can theoretically occur at all trophic levels The scenario presented in b represents a classic exploitation competition pathway where. the initiator and receiver species are both herbivores and the transmitter is a plant In this case feedbacks are likely The scenario presented in c. represents a poorly recognized case involving an autogenic engineer as the initiator that directly affects a predator through a nontrophic linkage. the primary receiver is a consumer and the secondary receiver is a plant In this particular case the secondary receiver is also the initiator but. more commonly it would be a different plant species These pathways are not exhaustive they focus on illustrating the fact that transmitter species. which are key to understanding DMIIs and TMIIs can occur at all trophic levels The labels predator consumer and producer indicate the. trophic levels across scenarios Arrows indicate direction of the interactions. 396 The American Naturalist, is as important in other types of interaction pathways as it different architectures and occur at very different stem den. is in predator r consumer r producer pathways Wootton sities D E Pearson Y K Ortega and S J Sears unpub. and Emmerson 2005 Yet studies have not attempted to lished data Variation in plant architecture influences web. partition and quantify DMIIs and TMIIs in other interaction size an important component of web spider foraging be. pathways as is currently done in predator r consumer r havior the trait effect while variation in substrate density. producer pathways influences spider density the density effect Pearson 2009. Here I examine an exotic plant invader that serves as an In my experiments I varied substrate size to influence TMIIs. autogenic ecosystem engineer initiating indirect interactions and manipulated substrate abundance to influence DMIIs. via both DMIIs and TMIIs that are transmitted by a native in a two by two full factorial design see below I used the. predator to both primary and secondary receiver species gallfly biological control agent Urophora affinis Tephriti. Spotted knapweed Centaurea maculosa is an exotic pe dae and its live host plant C maculosa for the primary. rennial forb that has invaded millions of hectares of grass and secondary receiver species respectively Hence the ini. land in the western United States Sheley et al 1998 Within tiator and secondary receiver species were the same How. grasslands in western Montana C maculosa differs archi ever I selected these species because the biology of this. tecturally from native forbs in that it generates larger more system is well studied and readily manipulated and because. expansive and more persistent stems and establishes sub U affinis is commonly consumed by Dictyna in the field. stantially higher overall stem densities Pearson 2009 D E Story et al 1995 Pearson 2009 My primary objective was. Pearson Y K Ortega and S J Sears unpublished data As to quantify the strength of autogenically engineered indirect. a result C maculosa invasion has increased densities of interactions Inferences about feedbacks in this particular. native Dictyna coloradensis and Dictyna major web spiders biocontrol system were secondary. by 46 fold density effect relative to uninvaded sites by Centaurea maculosa is a widespread perennial forb As. increasing availability of a limited substrate Pearson 2009 teraceae that produces one to many persistent panicle. The novel substrates have also altered Dictyna behavior by shaped flowering stems that are approximately 4 15 dm. producing more expansive architectures that enable spiders tall Individual C maculosa stems from the previous growing. to build larger webs that double their per capita prey capture season were collected near Missoula to serve as experimental. rates trait effect Thus in this system the initiator species substrates All primary stems were cut to a height of 50 cm. is a plant C maculosa that engineers vegetation architec and all lateral stems were trimmed back to emulate either. ture the transmitter species is a predator Dictyna spp that small substrates or large substrates In constructing arenas. exhibits density and trait responses to the initiator and the I simulated natural conditions by setting treatment effects. receiver species are Dictyna prey primarily Diptera and Hy for the density and trait treatments on the basis of field. menoptera plant r predator r consumer pathway Field estimates for these parameters Pilot work indicated that a. studies suggest that ecosystem engineering by the plant has substrate width of approximately 4 cm resulted in web sizes. had substantial indirect effects in this system elevating spi that were comparable to Dictyna s primary native substrate. der predation rates on their prey by 90 fold via combined Achillea millefolium Thus small substrates were designed. density and trait mediated pathways Pearson 2009 In to emulate natural conditions experienced by spiders before. this study I developed microcosms to fully partition and invasion Larger substrates were trimmed to widths of ap. quantify the independent and interactive effects of the trait proximately 20 cm to emulate typical C maculosa plants in. and density interactions in this plant r predator r con the field Trait effects approximated field parameter esti. sumer pathway parallel to approaches commonly used in mates well see Results Substrate densities were set at. predator r consumer r producer pathways I also examined one stem for low density the biological minimum and 16. how DMIIs and TMIIs affected the fecundity of the prey s stems for high density the highest density that could be. host plant a secondary receiver species fig 1c achieved while keeping stems separated The difference be. tween low and high density treatments was conservative. 16 fold relative to natural conditions where stem densities. Material and Methods, in C maculosa invaded areas were on average 47 times. Experiments were conducted in microcosms within a fenced higher than those in native habitat D E Pearson Y K. outdoor space at Diettert Gardens on the University of Ortega and S J Sears unpublished data. Montana campus in Missoula Microcosms comprised four Dictyna coloradensis Dictynidae are small carapace. components 1 models representing plant substrates the length 1 mm univoltine spiders that construct irregular. initiator species 2 Dictyna coloradensis spiders the trans cribellate webs and are active from about April to October. mitter species 3 Dictyna prey the receiver species and These spiders build up their webs over several days but do. 4 a plant host for the prey species a secondary receiver not reconstruct webs daily like some orb weavers Jackson. species Native and exotic forbs in this system have very 1978 Webs are constructed within not among substrates. Indirect Effects of an Exotic Engineer 397, Dictyna spp capture and feed on a variety of taxa ranging. from about 1 to 20 mm in length but most prey are Diptera. and Hymenoptera Pearson 2009 They retain prey items. in their webs so captures can be readily quantified Adult. and late instar female D coloradensis individuals adult. males do not construct webs were collected in the Missoula. valley and randomly introduced onto model substrates in. a greenhouse Dictyna coloradensis cannot be distinguished. from Dictyna major when alive J Slowik Denver Museum. of Nature and Science Denver CO so it is possible that. a few experimental individuals were D major However. prior identification of spiders from the collection areas pro. duced only D coloradensis and the spiders are ecologically. similar Jackson 1978 and exhibit similar responses to plant. architecture Pearson 2009 Thus the study species is re. ferred to as D coloradensis, Urophora affinis larvae overwinter within galls in C ma.
culosa seed heads Story et al 1992 They pupate in spring. and emerge as adults from May to July Adults display and. copulate on C maculosa plants or nearby substrates and. females oviposit within immature C maculosa flowerheads. The eggs hatch into larvae and induce gall formation in the. plants which reduces seed production via an energy sink. Harris 1980 Thus U affinis impact their host by reducing. fecundity Urophora affinis individuals were reared for the. study by collecting and caging C maculosa seedheeds before. the emergence of U affinis Urophora affinis indiviudals were. randomly collected from the rearing cage without regard to. sex when introduced into experimental arenas, I gathered live C maculosa plants from a wild population. in the Missoula valley in May to serve as host plants Sim. ilarly sized plants that had not initiated bolting were selected Figure 2 Experimental arenas The upper panel shows an arena being. and placed in 1 gal pots with a 50 50 sand and potting prepared for the high density small substrate treatment The lower panel. soil mix in the bottom third of the pot to facilitate drainage shows a low density large substrate arena out in the field toward the. The remainder of each pot was filled with soil from the root end of the experiment The arrow in the lower panel points to the spider. zone of the collected plants Plants were reared in a green. house until they bolted and initiated flowering When flow spiders were introduced with plants standardized by size. ers began to reach a stage suitable for U affinis oviposition and flower development within each block I placed D co. plants were transferred to experimental arenas loradensis on substrate models one spider per substrate. Each experimental arena fig 2 consisted of an oversized and allowed them to establish and begin constructing webs. 1 9 cm thick plywood base covered with a 1 m wide 1 overnight before the arenas were covered and placed out. m wide 0 75 m tall frame of 1 3 cm diameter PVC pipe doors Twenty U affinis were added to each cage when. overlaid with insect netting Wondermesh Garvock Lau microcosms were first set out and 20 more were added. rencekirk Scotland The cloth mesh size was 0 8 mm and every 2 days to offset natural mortality non spider inflicted. it allowed wind precipitation and 85 natural light to pass mortality and to maintain populations at around 20 30. through but prevented the passage of all but extremely small flies per arena over the course of the study I ran experiments. invertebrates In the high density treatment the stems were for 8 days with each arena receiving 60 total U affinis over. placed in a 4 4 array with 20 cm spacing with a little the course of the experiment Field studies indicated that. extra space in the center for the live host plant In the low this period was long enough for web size and prey captures. density treatment the single stem was randomly placed in to equilibrate Pearson 2009 but short enough to avoid. one of the four central points on the array Stems were confounding effects Preisser et al 2005 Abrams 2008 I. plastered into holes drilled in the wooden bases One live initiated arenas over a 2 week period in June to allow all. host plant was placed in the center of each arena before the host plants to reach the proper developmental stage for U. 398 The American Naturalist, affinis oviposition Arenas were set out in blocks of four from web length and width on the basis of the geometry. cages representing the full combination of treatments of a triangle Jackson 1978 Pearson 2009. n p 18 blocks Each day I examined all webs noting the. presence or absence of spiders and counting new prey cap. tures Spiders that abandoned substrate models often as a Results. result of wind or rain events occurring before they were Treatments were effective and represented natural condi. fully established were immediately removed from the are tions reasonably well fig 3 Mean web area on small sub. nas to prevent them from moving to the covers and killing strates was nearly identical to field measurements of webs. prey independent of experimental substrates built on Achillea millefolium fig 3A Mean web area on. At the end of the experiment I measured webs following large substrates was comparable to that of webs built on. Pearson 2009 tallied final prey captures and maintained Centaurea maculosa in the field 95 confidence interval. live plants in the greenhouse so flowers could mature and of experimental mean included mean field estimate In the. set seed and U affinis larvae could develop Plants remained high density treatment about half x p 7 7 of the Dictyna. covered with insect netting in the greenhouse to preclude coloradensis individuals introduced into experimental arenas. further access by Urophora I pollinated the experimental. plants using live flowers collected from wild plants in the. field because netting excluded pollinators Once all flowers. on a plant set seed seed heads were collected and dissected. to count the seeds and U affinis larvae within, To evaluate the indirect effects of plant architecture on. D coloradensis prey the primary receiver species I used. the total number of adult U affinis captured in all webs in. each arena over the 8 day study as the response variable in. a generalized mixed linear model with density 1 vs 16. stems and trait large vs small stems effects and their. interaction entered as fixed factors and experimental block. treated as a random factor PROC MIXED SAS Institute. 2003 Since the total number of U affinis introduced into. each cage was the same across treatments total captures. provided a direct measure of impact on the prey population. the amount by which the total prey population was re. duced As an added measure of the indirect effects of plant. architecture on the prey of D coloradensis I also examined. the number of U affinis larvae produced per seed head in. each arena This allowed me to assess whether the reduction. in adult U affinis numbers was sufficient to affect U affinis. recruitment It also provided a measurement of U affinis. attack on C maculosa I used the same model as above to. test for treatment effects except that arena was added as a. random factor to account for the multiple seed heads pro. duced in each arena To evaluate the indirect effects of plant. architecture on C maculosa host plants the secondary re. ceiver species I examined the number of seeds produced. per C maculosa seed head in each arena employing the. same model used to assess U affinis larvae All dependent. variables were positively skewed To meet assumptions of. normality and homoscedasticity the adult U affinis data. were square root transformed and the U affinis larvae and. seed data were inverse transformed Tabachnick and Fidell Figure 3 Treatment effect size mean SE for A trait effects web. size on large vs small substrates and B density effects number of. 1989 Data presented in figures are back transformed least spiders or active webs in arenas with high vs low substrate density. squares means and the variances are back transformed up Arrows indicate mean estimates for equivalent parameters obtained from. per or lower limits Haase et al 2008 I calculated web area field data for comparison based on Pearson 2009. Indirect Effects of an Exotic Engineer 399, remained on the substrates For low density treatments spi density interaction F1 51 p 1 39 P p 24 indicating that. ders that abandoned were replaced with established spiders the combined effect would be additive The density. to ensure that the minimum density was met Thus the mediated indirect effects of plant architecture on adult U. effective difference between low and high density treat affinis were strong enough to reduce the number of U affinis. ments a ratio of 1 8 was quite conservative relative to larvae produced per seed head by nearly 50 F1 43 p. natural conditions a ratio of 1 47 fig 3B 4 58 P p 04 fig 4B but trait mediated indirect effects. Both density mediated F1 51 p 110 75 P 01 fig 4A alone did not significantly reduce U affinis larval densities. and trait mediated F1 51 p 6 23 P p 02 indirect effects F1 43 p 0 52 P p 47 nor was there a significant inter. of plant architecture significantly reduced adult Urophora action between density and trait mediated effects on U. affinis abundance and there was no significant trait by affinis larval densities F1 43 p 0 27 P p 60 For the sec. ondary receiver species C maculosa density mediated in. direct effects of plant architecture significantly reduced. seed production F1 34 p 8 36 P p 01 fig 5 but trait. mediated indirect effects did not F1 34 p 0 21 P p 65. and there was no significant density by trait interaction. F1 34 p 2 32 P p 14 The presence of U affinis larvae. negatively correlated with number of seeds in C maculosa. seed heads r p 0 37 P 01,Discussion, Prior field studies in western Montana have shown that.
changes in vegetation architecture due to autogenic eco. system engineering by the exotic invasive plant Centaurea. maculosa have increased rates of predation on insects 90. fold by increasing the abundance and intensifying predatory. behavior of native Dictyna spiders Pearson 2009 These. field studies suggest that plants as autogenic engineers may. contribute substantively to community assembly via direct. nontrophic influences on predators that can transmit. through both DMIIs and TMIIs to other species Using. microcosms to isolate and quantify density and trait effects. I show here that the direct effects of autogenic engineering. by C maculosa which increased the abundance and per. capita capture rates of Dictyna coloradensis spiders signif. icantly reduced populations of their prey Urophora affinis. via both DMIIs and TMIIs with no significant density by. trait interactions TMIIs were strong enough to reduce adult. U affinis populations but not strong enough to reduce U. affinis recruitment rates and so they did not affect recruit. ment of its host plant the secondary receiver species In. contrast the stronger DMIIs impacted U affinis populations. sufficiently to reduce their recruitment which translated to. reduced seed production in their host plant The observed. reductions in seed production were negatively correlated. Figure 4 Density and trait mediated indirect effects of plant architecture. on A adult Urophora affinis fly abundance and B U affinis larvae with number of U affinis larvae in the C maculosa seed. production as transmitted by Dictyna coloradensis spider predation Re heads consistent with the well documented ability of U. sults are based on 8 day trials in experimental arenas where plant ar affinis to suppress C maculosa seed production Maddox. chitecture was represented by large or small substrates inducing trait 1982 Story et al 1989 Overall the effects of the DMIIs. mediated responses in the transmitter species and high or low substrate. were 4 3 times stronger than those of the TMIIs on the. density inducing density mediated responses in the transmitter species. in a full factorial design Different letters over bars indicate significant primary receiver species U affinis This difference in the. differences at the P p 05 level Means and variance are back transformed strength of DMIIs versus TMIIs is consistent with though. see Material and Methods substantially weaker in magnitude than the difference es. 400 The American Naturalist, initiator species overrode its weak per capita density effects. Per capita effects of C maculosa on Dictyna density were. low relative to trait effects per capita density effect p 0 5. trait effect p 2 8 However natural stem densities resulting. from C maculosa invasion can average 47 times higher than. those in the native system D E Pearson Y K Ortega and. S Sears unpublished data Emulating this effect in the. experiment even conservatively only an eightfold treat. ment effect resulted in very strong DMIIs because the. initiator s density drove the overall indirect effect In other. words per capita density effects were relatively weak but. cumulative density effects the overall DMII were strong. relative to the overall TMII because the invasive autogenic. engineer achieves unnaturally high densities relative to. that of the natives This result highlights the fact that au. togenic architectural engineering will tend to cause DMIIs. that are driven by initiator density or size This requires. manipulation of initiator density to examine receiver re. Figure 5 Density and trait mediated indirect effects of plant architecture. on a secondary receiver species Centaurea maculosa as measured by sponses something that is rarely done in experiments of. seeds produced per seed head while under attack by the primary receiver predator initiated pathways This result also highlights an. species Urophora affinis Different letters over bars indicate significant important concept applicable to all indirect interaction. differences at the P p 05 level Means and variance are back transformed pathways In nature weak per capita effects can be overcome. see Material and Methods, via naturally high organism densities to generate strong net. indirect effects sensu Jones et al 1994 Net indirect. timated in field studies which suggested that the density interaction outcomes likely commonly involve the integra. mediated pathway was 23 times stronger than the trait tion of per capita effects across varying initiator densities. mediated pathway and also indicated no significant even in predator and consumer initiated systems For ex. density by trait interactions Pearson 2009 The difference ample predators commonly aggregate in response to re. in the relative strength of DMIIs and TMIIs between the source pulses with important community level ramifica. microcosm and the field studies is attributable to the fact tions Holling 1959 Such responses may strongly influence. that density effects in microcosms were conservative relative net indirect effect outcomes in nature but they have not. to those of field densities due to space constraints in arenas been incorporated into TMII DMII studies. Actual differences between DMIIs and TMIIs are likely One of the biggest challenges in evaluating TMIIs and. much stronger in nature Regardless the conclusions that DMIIs in predator driven systems has been fully partition. DMIIs were substantially stronger than TMIIs and there ing density and trait effects to quantify their individual and. were no significant interactions are consistent between field synergistic contributions Werner and Peacor 2003 Griffin. studies and controlled experiments and Thaler 2006 Abrams 2008 This is extremely chal. This study demonstrates that the TMII DMII concept can lenging because predator cues can be so closely linked to. apply beyond predator r consumer r plant linkages to a predation that it requires creative experimental approaches. broader range of interaction pathways with two important like gluing the predator s mouth parts Schmitz 1998 or. general conclusions First expanding the concept requires complicated study designs to fully partition trait and density. certain caveats regarding its new application and interpre effects Trussell et al 2004 Griffin and Thaler 2006 Abrams. tation Second expanding an established concept can gen 2008 In this system TMIIs and DMIIs were fully parti. erate new insights even for prior applications by providing tioned by a standard full factorial experiment manipulating. novel perspectives For example indirect effects studies have initiator morphology and density because DMIIs were. focused on quantifying the relative importance of TMIIs linked solely to the initiator s density and TMIIs were linked. and DMIIs in contributing to net interaction outcomes solely to its morphology However in other cases of auto. Studies examining predator r consumer r producer path genic engineering trait and density effects may be closely. ways have concluded that TMIIs are generally as strong as linked For example the orb weaver Aculepeira packardi uses. or stronger than DMIIs Werner and Peacor 2003 Schmitz multiple C maculosa stems to construct its webs in this. et al 2004 Preisser et al 2005 In this system I found that system Dictyna build webs within stems Thus density. DMIIs were substantially stronger than TMIIs This oc and morphology of the engineer likely simultaneously in. curred because the high natural densities achieved by the fluence the abundance and behavior of A packardi and. Indirect Effects of an Exotic Engineer 401, teasing apart trait and density effects would be difficult vasions Pearson 2009 In general if the invader differs. Given the variety of ways that plants can affect other species functionally from the natives a strong response may be. Ohgushi et al 2007 Karban 2008 density and trait effects expected from those natives sensitive to its unique attributes. may commonly be closely linked in plant initiated path positively or negatively However it is important to note. ways creating challenges for partitioning effects closely par that exotic ecosystem engineers can also have extensive non. alleling those in predator r consumer r producer pathways engineering effects For example C maculosa dramatically. This study also addresses several information gaps re reduces native plant abundance via competition which in. garding ecosystem engineering related to engineering feed directly affects herbivores and predators through trophic. backs engineers in biological invasions and efforts to merge interactions Ortega and Pearson 2005 Ortega et al 2006. engineering with trophic ecology Jones et al 1994 Crooks Accounting for nonengineering effects of invasive engineers. 2002 Wright and Jones 2006 Since the inception of the greatly complicates the prediction of their impacts. ecosystem engineering concept it has been recognized that Ecosystem engineers can produce such powerful and. any engineering effect that feeds back on the engineer could widespread indirect interactions that they have been com. have broad system level ramifications Jones et al 1994 pared with keystone species e g Jones et al 1994 Al. The particular species combination used here illustrates how though most autogenic ecosystem engineers as in this case. feedbacks could be important for autogenic engineering would not strictly qualify as keystone species Power et al. Centaurea maculosa s engineering effects could potentially 1996 these powerful effects emphasize the need to better. result in increasing its own populations by reducing natural incorporate ecosystem engineering into indirect interaction. enemy attacks My results suggest that Dictyna spp can frameworks Additionally there are many other types of. reduce Urophora enough via indirect interactions to actually nontrophic interactions cataloged as competition facilita. diminish their impacts on C maculosa seed production and tion and mutualisms for example that are also extremely. potentially recruitment Whether autogenic engineering in important and need to be more fully incorporated into. this particular system actually results in C maculosa s release studies of community interactions The current study sug. from natural enemies through these indirect effects is un gests that nontrophic interactions can be better incorporated. clear because while some studies suggest that C maculosa into community ecology by simply expanding current eco. is not seed limited and is unaffected by Urophora Harris logical concepts and frameworks It also illustrates the un. 1980 Maddox 1982 Myers et al 1988 more recent work derutilized potential of biological invasions for advancing. suggests that Urophora suppresses C maculosa populations community ecology and integrating ecology and evolution. Story et al 2008 Nonetheless this system illustrates how Hairston Smith and Slobodkin s 1960 green world hy. such interactions could play out in nature and that auto pothesis HSS stimulated tremendous advances in ecology. genic engineers may exert feedbacks that are strong enough However it also created a predator centric perspective that. to affect their own populations despite the fact that as noted currently constrains ecological thinking Pringle et al 2007. by Jones et al 1994 engineering feedbacks are likely to Ecological concepts such as trait and density mediated in. be complex teractions have been underutilized because of such restricted. Exotic invasive species may commonly impact native sys perspectives Thus nontrophic interactions could be more. tems through ecosystem engineering Crooks 2002 Crooks fully incorporated into ecology by simply loosening the. 2002 explored the potential for predicting invasive engi predator centric veil of HSS Expanding on this the more. neering impacts by examining how vegetation complexity broadly an ecological concept is applied the more it will. affects species richness but with mixed results In this sys contribute All ecological concepts should be stretched to. tem autogenic engineering by C maculosa may increase their limits for two reasons First applying concepts in new. Dictyna predation sufficiently to reduce some natural prey contexts can provide unique insights and perspectives as is. populations and possibly increase certain prey resources noted by this study Second finding the boundaries of con. These results illustrate the strength and complexity of in ceptual applications can be equally insightful For example. vasive autogenic engineering effects They also emphasize the conclusion that trophic cascades were more prominent. the importance of functional roles in predicting outcomes in aquatic than terrestrial environments stimulated extensive. Duffy 2002 The community response to invasive engi discussions about the attenuation of trophic cascades that. neers will depend on the degree to which the invader s increased knowledge of indirect interactions in both aquatic. functional roles differ from those of the natives Centaurea and terrestrial systems Strong 1992 Stretching concepts. maculosa and other exotic forbs invading western Montana with discrete origins across ecological e g aquatic vs ter. differ architecturally and in other attributes from the na restrial taxonomic e g plant vs animal or vocational. tives D E Pearson Y K Ortega and S Sears unpublished ecology vs evolution boundaries will generate new in. data Thus the populations of Dictyna spp and other web sights by offering new perspectives and defining important. spiders have predictably increased in response to these in attributes that truly delineate conceptual boundaries. 402 The American Naturalist, The food web framework also has inhibited integration G A Polis and K O Winemiller eds Food webs integration of. of nontrophic interactions Food webs provide the concep patterns and dynamics Chapman Hall New York. tual core for much ecological theory However food webs Abrams P A 2008 Measuring the impact of dynamic antipredator. traits on predator prey resource interactions Ecology 89 1640 1649. exclude the many nontrophic interactions known to pro. Agrawal A A C Kobayashi and J S Thaler 1999 Influence of. foundly affect community structure and function Com prey availability and induced host plant resistance on omnivory. munity interaction webs were developed to incorporate by western flower thrips Ecology 80 518 523. nontrophic interactions into community assembly theory Bailey J K and T Whitham 2006 Interactions between cottonwood. Paine 1980 but they remain underutilized Ohgushi and beavers positively affect sawfly abundance Ecological Ento. 2008 Food web complexity has been addressed by parti mology 31 294 297. tioning webs into subcompartments or motifs to examine Callaway R M 2007 Positive interactions and interdependence in. the influence of different motifs on community outcomes plant communities Springer Dordrecht. Rip et al 2010 Conceptually food webs could be ex Carpenter S R J F Kitchell and J R Hodgson 1985 Cascading. trophic interactions and lake productivity BioScience 35 634 639. panded into interaction webs by incorporating hybrid mo. Cox G W 2004 Alien species and evolution the evolutionary ecol. tifs that include nontrophic interactions Broader integra ogy of exotic plants animals microbes and interacting native. tion will be achieved once it is recognized that food webs species Island Washington DC. are merely a class of motifs that provide the scaffolding for Crooks J A 2002 Characterizing ecosystem level consequences of. more complex interaction webs biological invasions the role of ecosystem engineers Oikos 97. Finally biological invasions present unique natural ex 153 166. periments offering novel insights for ecology and evolution Duffy J E 2002 Biodiversity and ecosystem function the consumer. This fact has been noted Sax et al 2005 but is rarely fully connection Oikos 99 201 219. Estes J A and J F Palmisano 1974 Sea otters their role in struc. capitalized on A massive literature exists on biological in. turing nearshore communities Science 185 1058 1060. vasions Richardson and Pysek 2008 but most studies fo. Griffin C A M and J S Thaler 2006 Insect predators affect plant. cus on the causes of invasion or the impacts of invaders resistance via density and trait mediated interactions Ecology. Very few studies actually examine invasions as natural ex Letters 9 333 346. periments in community assembly When a strong invader Haase J R Brandl S Scheu and M Schadler 2008 Above and. sensu Ortega and Pearson 2005 enters a new system the below ground interactions are mediated by nutrient availability. native community is forced to rapidly reassemble in re Ecology 89 3072 3081. sponse to the new conditions Thus the roles that com Hairston N G F E Smith and L B Slobodkin 1960 Community. petition herbivory predation and facilitation for example structure population control and competition American Natu. ralist 94 421 425, play in structuring the community as well as the importance.
Harris P 1980 Effects of Urophora affinis Frfld and U quadri. of rapid evolutionary responses to these ecological changes fasciata Meig Diptera Tephritidae on Centaurea diffusa Lam. Cox 2004 can be observed in real time Biological inva and C maculosa Lam Compositae Zeitschrift fu r Angewanate. sions represent a lucrative frontier for exploring concepts Entomologie 90 190 210. in ecology and evolution and the interface between the two Holling C S 1959 The components of predation as revealed by a. that demands greater attention study of small mammal predation of the European pine sawfly. Canadian Entomologist 91 293 320, Huntzinger M R Karban and J H Cushman 2008 Negative effects. Acknowledgments of vertebrate herbivores on invertebrates in a coastal dune com. Discussions with J Maron and M Rout stimulated ideas munity Ecology 89 1972 1980. for the study design J Maron provided the use of his Jackson R R 1978 Comparative studies of Dictyna and Mallos. 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