Whole-ecosystem experimental manipulations of tropical forests
Authors: Tom M. Fayle, Edgar C. Turner, Yves Basset, Robert M. Ewers, Glen Reynolds, Vojtech Novotny
Journal: Trends in Ecology & Evolution (2015)

•Tropical forests are highly complex systems, threatened by multiple global changes.•Understanding these habitats requires manipulating drivers at whole-ecosystem scales.•These include climate, nutrients, biotic interactions, human impacts, and restoration.•We describe the current range of these manipulations and suggest future experiments. Tropical forests are highly diverse systems involving extraordinary numbers of interactions between species, with each species responding in a different way to the abiotic environment. Understanding how these systems function and predicting how they respond to anthropogenic global change is extremely challenging. We argue for the necessity of 'whole-ecosystem' experimental manipulations, in which the entire ecosystem is targeted, either to reveal the functioning of the system in its natural state or to understand responses to anthropogenic impacts. We survey the current range of whole-ecosystem manipulations, which include those targeting weather and climate, nutrients, biotic interactions, human impacts, and habitat restoration. Finally we describe the unique challenges and opportunities presented by such projects and suggest directions for future experiments. Tropical forests are highly diverse systems involving extraordinary numbers of interactions between species, with each species responding in a different way to the abiotic environment. Understanding how these systems function and predicting how they respond to anthropogenic global change is extremely challenging. We argue for the necessity of 'whole-ecosystem' experimental manipulations, in which the entire ecosystem is targeted, either to reveal the functioning of the system in its natural state or to understand responses to anthropogenic impacts. We survey the current range of whole-ecosystem manipulations, which include those targeting weather and climate, nutrients, biotic interactions, human impacts, and habitat restoration. Finally we describe the unique challenges and opportunities presented by such projects and suggest directions for future experiments. Tropical forest ecosystems are highly complex and heterogeneous, comprising diverse networks of interactions between species within communities, and between those communities and the environment [1Novotny V. et al.Guild-specific patterns of species richness and host specialization in plant–herbivore food webs from a tropical forest.J. Anim. Ecol. 2010; 79: 1193-1203Crossref PubMed Scopus (255) Google Scholar, 2Basset Y. et al.Arthropod diversity in a tropical forest.Science. 2012; 338: 1481-1484Crossref PubMed Scopus (406) Google Scholar]. Understanding how these systems function is important because they provide globally significant ecosystem services, including carbon sequestration, climate regulation, supply of water, and conservation of biodiversity [3Edwards D.P. et al.Maintaining ecosystem function and services in logged tropical forests.Trends Ecol. Evol. 2014; 29: 511-520Abstract Full Text Full Text PDF PubMed Scopus (278) Google Scholar]. Understanding these habitats can require manipulations that target entire ecosystems. We define whole-ecosystem experiments as manipulations aiming to replicate drivers at spatial and temporal scales similar to those occurring in either natural or human-modified ecosystems. We include only studies where either the entire ecosystem is manipulated (e.g., during logging [4van der Hout P. Testing the applicability of reduced impact logging in Greenheart forest in Guyana.Int. Forest. Rev. 2000; 2: 24-32Google Scholar]) or where the manipulation is expected to impact on the entire ecosystem (e.g., removal of ecosystem engineers [5Klimeš P. et al.Experimental suppression of ants foraging on rainforest vegetation in New Guinea: testing methods for a whole-forest manipulation of insect communities.Ecol. Entomol. 2011; 36: 94-103Crossref Scopus (35) Google Scholar]; see Glossary). Our definition is not based explicitly on any spatial or temporal scale because this would be arbitrary, but instead on the scope of the expected impacts. Hence, we include manipulations smaller in scope if impacts are expected to ramify through the ecosystem, or if results can be used to predict ecosystem-wide impacts. For example, excluding ants from a 1 ha area is small in taxonomic scope [5Klimeš P. et al.Experimental suppression of ants foraging on rainforest vegetation in New Guinea: testing methods for a whole-forest manipulation of insect communities.Ecol. Entomol. 2011; 36: 94-103Crossref Scopus (35) Google Scholar], but the importance of ants in tropical ecosystems means that impacts are expected across all trophic levels. Furthermore, we restrict our definition to purposefully designed experiments with at least some sites in forest, and do not include natural experiments [6Sagarin R. Pauchard A. Observational approaches in ecology open new ground in a changing world.Front. Ecol. Environ. 2009; 8: 379-386Crossref Scopus (108) Google Scholar] such as monitoring following colonization by invasive species (e.g., [7Sanders N.J. et al.Community disassembly by an invasive species.Proc. Natl. Acad. Sci U.S.A. 2003; 100: 2474-2477Crossref PubMed Scopus (369) Google Scholar]). We emphasize that experiments and observations falling outside our definition are still worthwhile but that whole-ecosystem experiments are needed for understanding whole-ecosystem responses. Disentangling the drivers of functioning in tropical forests usually relies on observed changes following environmental perturbations [6Sagarin R. Pauchard A. Observational approaches in ecology open new ground in a changing world.Front. Ecol. Environ. 2009; 8: 379-386Crossref Scopus (108) Google Scholar]. These perturbations are often correlated and synergistic, necessitating complementary experimental manipulations if their effects are to be properly understood [8Rogers H. et al.'Natural experiment' demonstrates top-down control of spiders by birds on a landscape level.PLoS ONE. 2012; 7: e43446Crossref PubMed Scopus (65) Google Scholar]. However, constraints of cost and logistics mean that manipulations of entire ecosystems are rare. Most experiments are limited in scale and take a reductionist approach, focusing on a tractable subcomponent of the ecosystem (for example the relationship between a particular plant and its herbivores). Owing to the subtleties of interactions at larger scales and across multiple trophic levels in tropical forests [9Morris R.J. et al.Experimental evidence for apparent competition in a tropical forest food web.Nature. 2004; 428: 310-313Crossref PubMed Scopus (231) Google Scholar], manipulations focusing on a specific target taxon, or those conducted at small spatial or temporal scales, are often unable to determine the drivers of ecological processes. For example, environmental variables are often measured at spatial scales too small for optimal prediction of species distributions [10Jackson H.B. Fahrig L. Are ecologists conducting research at the optimal scale?.Global Ecol. Biogeogr. 2015; 24: 52-63Crossref Scopus (377) Google Scholar]. A related issue is that of spill-over effects from the surrounding unmanipulated area, which will be greater at smaller scales, and for plots without buffer zones. Furthermore, during manipulations employing a barrier, populations that are isolated (but otherwise unmanipulated) can behave differently to those in continuous areas [11Ostfeld R.S. The fence effect reconsidered.Oikos. 1994; 70: 340-348Crossref Scopus (57) Google Scholar]. Thus a more holistic approach is needed that borders on a reinvention of systems ecology [12Evans M.R. et al.Predictive ecology: systems approaches.Philos. Trans. R. Soc. Lond. B: Biol. Sci. 2012; 367: 163-169Crossref PubMed Scopus (90) Google Scholar]. Here we advocate such a whole-ecosystem experimental approach (Figure 1) to unravel the complex interactions and processes within tropical forests. The diverse, heterogeneous nature of tropical forests, in which many species are rare, means that whole-ecosystem manipulations as defined here are challenging to implement but are vital for understanding these habitats. This approach allows prediction of responses of communities comprising multiple trophic levels and associated ecosystem processes. For example, exclusions of mammalian herbivores from tropical forests suggest that this group is relatively unimportant in controlling vegetation structure [13Royo A. Carson W.P. The herb community of a tropical forest in central Panamá: dynamics and impact of mammalian herbivores.Oecologia. 2005; 145: 66-75Crossref PubMed Scopus (30) Google Scholar]. However, manipulations at larger spatial scales in tropical grasslands reveal strong whole-ecosystem responses, with impacts on vegetation structure and cascading effects on other taxa [14Pringle R.M. et al.Herbivore-initiated interaction cascades and their modulation by productivity in an African savanna.Proc. Natl. Acad. Sci U.S.A. 2007; 104: 193-197Crossref PubMed Scopus (183) Google Scholar]. Understanding whether this reflects a difference in the importance of mammalian herbivores between habitats requires exclusions at comparable (whole-ecosystem) spatial scales in tropical forests (recent extinction of large mammals in tropical forests is one explanation for these differences [15Harrison R.D. et al.Consequences of defaunation for a tropical tree community.Ecol. Lett. 2013; 16: 687-694Crossref PubMed Scopus (195) Google Scholar]). Manipulative experiments can also be useful to complement natural experiments in situations where potential confounding factors are present. For example, forest fragments in the tropics are usually left on steep slopes and high areas [16Liu J-J. Slik J.W.F. Forest fragment spatial distribution matters for tropical tree conservation.Biol. Conserv. 2014; 171: 99-106Crossref Scopus (66) Google Scholar]. Hence, fragmentation is confounded with topographic variables, and only large-scale manipulations are able to disentangle these effects. Furthermore, manipulations allow collection of pre-treatment data, which is not always possible with natural experiments relying on unpredictable events such as fires [17Balch J.K. et al.Negative fire feedback in a transitional forest of southeastern Amazonia.Global Change Biol. 2008; 14: 2276-2287Crossref Scopus (149) Google Scholar] or hurricanes [18Shiels A.B. et al.Plant responses to simulated hurricane impacts in a subtropical wet forest, Puerto Rico.J. Ecol. 2010; 98: 659-673Crossref Scopus (65) Google Scholar]. Such data allow the impacts of events to be accurately assessed, rather than inferring effects through comparisons between different areas, which can differ for other reasons. The insights provided by manipulations conducted at whole-ecosystem scales have motivated an ongoing increase in such experiments in tropical forests (Figure 2), with emerging opportunities for collaborative studies. Here we describe the findings of whole-ecosystem manipulations in tropical forests to date, assess the logistic, analytical, and ethical challenges that these projects present, and suggest future research directions. Although this review focuses on whole-ecosystem manipulations in tropical forests, such experiments are useful in other habitats, and many of the points made here are more widely relevant. Whole-ecosystem manipulations (Table 1) comprise those assessing (i) effects of weather and climate, (ii) role of key ecological groups, (iii) importance of nutrients, (iv) effects of direct anthropogenic activities, such as logging, and (v) efficacy of different forest restoration strategies.Table 1Examples of the range of whole-ecosystem manipulations conducted to dateProcess (manipulation)Example project name and regionMethods usedResponses measuredReplicate area (ha)aAreas refer to the manipulated area, not to the size of sampled plots, which is smaller when an unsampled buffer is employed.NStartKey RefsOther comparable experimentsbWhere multiple experiments have been conducted on the same broad research question, a single example is described in detail, with the final column giving details of representative other experiments on the same topic.Weather and climateFireMato Grosso, BrazilImplemented different fire regimes in three treatment areas (control – no burning, once burned, and annual burning). See also Box 1Tree and liana mortality; above ground biomass; invertebrates503200417Balch J.K. et al.Negative fire feedback in a transitional forest of southeastern Amazonia.Global Change Biol. 2008; 14: 2276-2287Crossref Scopus (149) Google Scholar, 22Balch J.K. et al.Size, species, and fire behavior predict tree and liana mortality from experimental burns in the Brazilian Amazon.Forest Ecol. Manag. 2011; 261: 68-77Crossref Scopus (84) Google Scholar, 74Carvalho K.S. et al.Leafcutter ant nests inhibit low-intensity fire spread in the understory of transitional forests at the Amazon's forest–savanna boundary.Psyche. 2012; (780713)https://doi.org/10.1155/2012/780713Crossref Scopus (15) Google ScholarExperimentally cleared and burned forest in Amazonas and Pará, Brazil 75Carvalho Jr, J.A. et al.A tropical rainforest clearing experiment by biomass burning in the Manaus region.Atmospheric Environ. 1995; 29: 2301-2309Crossref Scopus (44) Google ScholarRainfallDroughtTapajós National Forest, Brazil. Part of Large-Scale Biosphere Atmosphere-Experiment in AmazoniaDroughting of treatment plot (reduction of ∼40% of rainfall) using plastic panels and guttering, compared to control without treatment in BACI designSoil and gas flux; tree and liana mortality; net primary productivity; above ground biomass; soil water movement; water uptake12199820Nepstad D.C. et al.The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest.J. Geophys. Res. Atmospheres. 2002; 107: 8085Crossref Google ScholarDirectly comparable with experiments in Caxiuana National Forest, Brazil 76Fisher R.A. et al.The response of an Eastern Amazonian rain forest to drought stress: results and modelling analyses from a throughfall exclusion experiment.Global Change Biol. 2007; 13: 2361-2378Crossref Scopus (219) Google Scholar and inSulawesi, Indonesia 77van Straaten O. et al.Simulated drought reduces soil CO2 efflux and production in a tropical forest in Sulawesi, Indonesia.Ecosphere. 2011; 2: 1-22Crossref Google ScholarIrrigation during dry seasonBarro Colorado Island (BCI), PanamaDry season irrigation in treatment plots compared to control plotsLitterfall2.254198621Wright S.J. Cornejo F.H. Seasonal drought and leaf fall in a tropical forest.Ecology. 1990; 71: 1165-1175Crossref Scopus (181) Google ScholarIrrigation and leaf removal experiment in Brazil 78Vasconcelos S.S. et al.Effects of seasonality, litter removal and dry-season irrigation on litterfall quantity and quality in eastern Amazonian forest regrowth, Brazil.J. Trop. Ecol. 2008; 24: 27-38Crossref Scopus (24) Google ScholarHurricane damageSimulated hurricane damageCanopy Trimming Experiment, Luquillo Forest, Puerto RicoCanopy thinning and addition of material to forest floor compared to control in factorial BACI designSeedling mortality and growth; forest floor and canopy arthropods0.0912200318Shiels A.B. et al.Plant responses to simulated hurricane impacts in a subtropical wet forest, Puerto Rico.J. 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Springer, 2013: 209-214Crossref Scopus (22) Google ScholarNoneRemoval of arthropodsFlorida BayIslands covered with tents and fumigated with methyl bromideField monitoring (non-lethal) of all arthropod species' populations0.01–0.05cOnly island diameter is given here, and areas therefore assume that islands are circular.9196684Simberloff D.S. Wilson E.O. Experimental zoogeography of islands. A two-year record of colonisation.Ecology. 1970; 51: 934-937Crossref Google ScholarNoneAnt exclusion and suppressionPapua New GuineaAnts excluded and poisoned in treatment plots compared to control plotsAnt abundance0.06420075Klimeš P. et al.Experimental suppression of ants foraging on rainforest vegetation in New Guinea: testing methods for a whole-forest manipulation of insect communities.Ecol. Entomol. 2011; 36: 94-103Crossref Scopus (35) Google ScholarNoneSuppression of invasive antsChristmas IslandApplication of insecticide bait to three supercolonies of A. gracilipesAnt activity and abundance; scale insect abundance9.1–35.66200024Abbott K.L. Green P.T. Collapse of an ant–scale mutualism in a rainforest on Christmas Island.Oikos. 2007; 116: 1238-1246Crossref Scopus (35) Google ScholarNoneHerbivore exclusionGigante Peninsula and BCI, PanamaPaired fenced enclosure and control plots in BACI designHerb diversity, density and cover0.1416199313Royo A. Carson W.P. The herb community of a tropical forest in central Panamá: dynamics and impact of mammalian herbivores.Oecologia. 2005; 145: 66-75Crossref PubMed Scopus (30) Google ScholarNoneMammalian seed predator exclusionBladen Nature Reserve, BelizeLive trapping and removal of dominant granivorous small mammalFruit removal rate and fate of removed fruits0.56200485Klinger R. Rejmánek M. Experimental seed predator removal reveals shifting importance of predation and dispersal limitation in early life history stages of tropical forest trees.Folia Geobot. 2013; 48: 415-435Crossref Scopus (8) Google ScholarNoneAnthropogenic impactsDeforestationSelective loggingGunung Rara Forest Reserve, Sabah, MalaysiaTwo modes of selective harvesting compared as well as pre-logging vine cutting in BACI designTree diameter breast height, species identity, damage and mortality; skid trail cover5.7620199234Forshed O. et al.Initial impact of supervised logging and pre-logging climber cutting compared with conventional logging in a dipterocarp rainforest in Sabah, Malaysia.Forest Ecol. Manag. 2006; 221: 233-240Crossref Scopus (8) Google ScholarExperiment investigating the impact of logging intensity and type in Guyana 4van der Hout P. Testing the applicability of reduced impact logging in Greenheart forest in Guyana.Int. Forest. Rev. 2000; 2: 24-32Google ScholarClearance and plantation establishment in river catchmentsMendolong Research Area, Sabah, MalaysiaClearance of forest and establishment of Acacia mangium plantation compared across three extraction methods and two control habitats in BACI designPrecipitation; stream flow and quality; surface runoff; soil moisture; soil physical properties; dissolved nutrients; Acacia growth3.4–18.2 catchments5198586Malmer A. Hydrological effects and nutrient losses of forest plantation establishment on tropical rainforest land in Sabah, Malaysia.J. Hydrol. 1996; 174: 129-148Crossref Scopus (91) Google ScholarExperiment investigating the impact of forest clearance and burning in Costa Rica 33Ewel J. et al.Slash and burn impacts on a Costa Rican wet forest site.Ecology. 1981; 62: 816-829Crossref Google ScholarForest FragmentationExperimental fragmentationThe Biological Dynamics of Forest Fragments Project (BDFFP), Manaus, BrazilFragmentation of continuous primary forest into fragments. Includes comparisons with surrounding continuous forest, cattle ranches and regrowth areasWide range of studies from biodiversity to ecosystem functions and services1–10011197935Laurance W.F. et al.The fate of Amazonian forest fragments: a 32-year investigation.Biol. Conserv. 2011; 144: 56-67Crossref Scopus (649) Google ScholarForest fragmentation experiment in Sabah, Malaysia 66Ewers R.M. et al.A large-scale forest fragmentation experiment: the Stability of Altered Forest Ecosystems Project.Philos. Trans. R. Soc. Lond. B: Biol. 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R. Soc. B: Biol. Sci. 2011; 366: 3303-3315Crossref PubMed Scopus (82) Google ScholarVine manipulation comparable in Oquiriquia, Bolivia, recorded the regrowth of vines and cost-effectiveness of cutting 43Perez-Salicrup D.R. et al.Cost and efficiency of cutting lianas in a lowland liana forest of Bolivia.Biotropica. 2001; 33: 324-329Crossref Scopus (35) Google ScholarReplanting of former plantation with different combinations of seedlingsReserva Natural Vale, Espírito Santo, BrazilFactorial design with high and low seedling diversity; low, medium, and high density of seedlings; and the inclusion of pioneer species or notSeedling growth; herbivory; plant defenses0.2536200487Massad T.J. et al.Restoration of pasture to forest in Brazil's Mata Atlântica: the roles of herbivory, seedling defenses, and plot design in reforestation.Restoration Ecol. 2011; 19: 257-267Crossref Scopus (28) Google ScholarBiodiversity, Panama 88Potvin C. Gotelli N.J. 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Manag. 1999; 124: 123-135Crossref Scopus (87) Google ScholarExperiment investigating silviculture treatments, Heredia Province, Costa Rica 89Rincón M. et al.Understory bees and floral resources in logged and silviculturally treated Costa Rican rainforest plots.J. Kansas Entomol. Soc. 1999; 72: 379-393Google Scholara Areas refer to the manipulated area, not to the size of sampled plots, which is smaller when an unsampled buffer is employed.b Where multiple experiments have been conducted on the same broad research question, a single example is described in detail, with the final column giving details of representative other experiments on the same topic.c Only island diameter is given here, and areas therefore assume that islands are circular. Open table in a new tab Whole-ecosystem experiments can provide particularly useful insights into climate and weather impacts because these events usually involve a suite of correlated changes, and tropical forests are predicted to be particularly vulnerable to climate change [19Brando P.M. et al.Abrupt increases in Amazonian tree mortality due to drought–fire interactions.Proc. Natl. Acad. Sci U.S.A. 2014; 111: 6347-6352Crossref PubMed Scopus (514) Google Scholar]. Only with manipulations can the relative importance of, and interactions between, drivers be disentangled. Water availability manipulations have demonstrated reduced tree growth rates in response to drought [20Nepstad D.C. et al.The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest.J. Geophys. Res. Atmospheres. 2002; 107: 8085Crossref Google Scholar] and that reduced rainfall is not a proximal cue for leaf fall [21Wright S.J. Cornejo F.H. Seasonal drought and leaf fall in a tropical forest.Ecology. 1990; 71: 1165-1175Crossref Scopus (181) Google Scholar]. Furthermore, separate manipulations of water availability [20Nepstad D.C. et al.The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest.J. Geophys. Res. Atmospheres. 2002; 107: 8085Crossref Google Scholar] and fire regimes [17Balch J.K. et al.Negative fire feedback in a transitional forest of southeastern Amazonia.Global Change Biol. 2008; 14: 2276-2287Crossref Scopus (149) Google Scholar, 22Balch J.K. et al.Size, species, and fire behavior predict tree and liana mortality from experimental burns in the Brazilian Amazon.Forest Ecol. Manag. 2011; 261: 68-77Crossref Scopus (84) Google Scholar] offer the potential to partition the impacts of these correlated drivers. Simulations of hurricane damage also show that detritus deposition kills existing saplings, while increased canopy openness stimulates germination from the seedbank [18Shiels A.B. et al.Plant responses to simulated hurricane impacts in a subtropical wet forest, Puerto Rico.J. Ecol. 2010; 98: 659-673Crossref Scopus (65) Google Scholar]. Direct manipulations of particular taxa have been used to reveal the roles these groups play in structuring tropical forest ecosystems. For example, experiments have demonstrated that mammalian herbivores do not strongly affect vegetation structure in tropical forests [13Royo A. Carson W.P. The herb community of a t