Logging cuts the functional importance of invertebrates in tropical rainforest

Abstract

Invertebrates are dominant species in primary tropical rainforests, where their abundance and diversity contributes to the functioning and resilience of these globally important ecosystems. However, more than one-third of tropical forests have been logged, with dramatic impacts on rainforest biodiversity that may disrupt key ecosystem processes. We find that the contribution of invertebrates to three ecosystem processes operating at three trophic levels (litter decomposition, seed predation and removal, and invertebrate predation) is reduced by up to one-half following logging. These changes are associated with decreased abundance of key functional groups of termites, ants, beetles and earthworms, and an increase in the abundance of small mammals, amphibians and insectivorous birds in logged relative to primary forest. Our results suggest that ecosystem processes themselves have considerable resilience to logging, but the consistent decline of invertebrate functional importance is indicative of a human-induced shift in how these ecological processes operate in tropical rainforests.

ARTICLE

Received 29 Sep 2014 | Accepted 4 Mar 2015 | Published 13 Apr 2015

DOI: 10.1038/ncomms7836 OPEN

Logging cuts the functional importance of invertebrates in tropical rainforest

Robert M. Ewers1, Michael J.W. Boyle1, Rosalind A. Gleave1, Nichola S. Plowman1,2,3, Suzan Benedick4,Henry Bernard5, Tom R. Bishop1,6,7, Effendi Y. Bakhtiar5, Vun Khen Chey8, Arthur Y.C. Chung8,Richard G. Davies9, David P. Edwards10, Paul Eggleton11, Tom M. Fayle1,2,3,5, Stephen R. Hardwick1,Rahman Homathevi5, Roger L. Kitching12, Min Sheng Khoo1, Sarah H. Luke9,13, Joshua J. March11, Reuben Nilus8, Marion Pfeifer1, Sri V. Rao14, Adam C. Sharp1, Jake L. Snaddon15, Nigel E. Stork12, Matthew J. Struebig16, Oliver R. Wearn1,17, Kalsum M. Yusah5 & Edgar C. Turner1,13

Invertebrates are dominant species in primary tropical rainforests, where their abundance and diversity contributes to the functioning and resilience of these globally important ecosystems. However, more than one-third of tropical forests have been logged, with dramatic impacts on rainforest biodiversity that may disrupt key ecosystem processes. We nd that the contribution of invertebrates to three ecosystem processes operating at three trophic levels (litter decomposition, seed predation and removal, and invertebrate predation) is reduced by up to one-half following logging. These changes are associated with decreased abundance of key functional groups of termites, ants, beetles and earthworms, and an increase in the abundance of small mammals, amphibians and insectivorous birds in logged relative to primary forest. Our results suggest that ecosystem processes themselves have considerable resilience to logging, but the consistent decline of invertebrate functional importance is indicative of a human-induced shift in how these ecological processes operate in tropical rainforests.

1 Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, UK. 2 Faculty of Science, University of South Bohemia, Branisovska 31, Ceske Budejovice CZ-370 05, Czech Republic. 3 Institute of Entomology, Biology Centre of Czech Academy of Sciences, Branisovska 31, Ceske Budejovice CZ-370 05, Czech Republic. 4 Faculty of Sustainable Agriculture, Universiti Malaysia Sabah, Locked Bag No. 3, Sandakan, Sabah 90509, Malaysia. 5 Institute for Tropical Biology and Conservation, Universiti Malaysia Sabah, Jln UMS, Kota Kinabalu, Sabah 88400, Malaysia. 6 School of Environmental Sciences, University of Liverpool, Liverpool L69 3GP, UK. 7 Centre for Invasion Biology, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa. 8 Forest Research Centre (Sepilok), Sabah Forestry Department, PO Box 1407, Sandakan, Sabah 90715, Malaysia.

9 School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK. 10 Department of Animal and Plant Sciences, University of Shefeld, Western Bank, Shefeld S10 2TN, UK. 11 Entomology Department, Natural History Museum, Cromwell Road, London SW7 5BD, UK. 12 Environmental Futures Research Institute and Grifth School of the Environment, Grifth University, Nathan, Queensland 4111, Australia. 13 Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. 14 School of Rural, Animal and Environmental Sciences, Nottingham Trent University, Brackenhurst Campus, Southwell, Nottinghamshire NG25 0QF, UK. 15 Centre for Biological Sciences, University of Southampton, Southampton SO17 1BJ, UK. 16 Durrell Institute of Conservation and Ecology, School of Anthropology and Conservation, University of Kent, Canterbury CT2 7NR, UK. 17 Institute of Zoology, Zoological Society of London, Regents Park, London NW1 4RY, UK. Correspondence and requests for materials should be addressed to R.E. (email: mailto:[email protected]

Web End [email protected] ).

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Invertebrates are the little things that run the world1, and nowhere is this more evident than in tropical rainforests. Invertebrates are dominant prey2, predators of seeds3 and

other invertebrates2,4, herbivores5 and pollinators6,7 in rainforest ecosystems, and are among the most important organisms for breaking down dead organic matter8. Tropical rainforests are estimated to host around six million invertebrate species9,10, and over 18,000 species can be present in a single hectare11. This diversity is expected to confer substantial redundancy to ecosystem processes. Differences in environmental sensitivity among functionally similar species give stability to ecosystem processes in the face of environmental change, as the loss of environmentally sensitive species will be compensated for by more robust species12. Such functional resistance to disturbance would indicate a resilient ecosystem13, and understanding the resilience of natural habitats represents one of the great challenges for predicting the future impacts of human-caused global change on biodiversity14.

We conducted a set of experiments to quantify the resistance to intensive logging of three ecosystem processes that operate at three different trophic levels in the tropical rainforests of Borneo. The logged forest had been logged twice15, the second time a salvage logging rotation that ended o10 years before data collection, and was conducted under a regime in which any sized tree could be cut. In total, a cumulative amount of 179 m3 ha 1 timber was removed16, placing our sites among the most heavily logged forests in the tropics17. Overall, we nd that ecosystem processes are remarkably resilient to very heavy logging, but the taxa performing those processes change. Ecosystem processes in primary forest are dominated by the actions of invertebrates such as ants, termites, beetles and earthworms. In logged forest, however, invertebrates are much less dominant and we nd the actions of vertebrate taxa, such as birds, amphibians and small mammals, increase in importance. Our results are consistent with those expected in a resilient ecosystem, with that resilience conferred through functional similarity between invertebrates and vertebrates.

ResultsResilience of ecosystem processes. We found a 15% decrease in the rate of leaf litter decomposition (likelihood ratio test, w2(1) 5.90, P 0.015, N 25) in logged compared with primary

forest, a 13% increase in the rate at which seeds were removed and/or predated (likelihood ratio test, w2(1) 3.98, P 0.047,

N 194), and no difference in invertebrate predation rate

(Fig. 1; likelihood ratio test, w2(1) 1.03, P 0.310, N 51;

Supplementary Table 1). In combination, this indicates considerable resilience of ecosystem processes to the biodiversity changes caused by logging. However, experimental manipulations demonstrated that invertebrates contributed signicantly less to delivering all three ecosystem processes in logged relative to primary forest (Fig. 1).

Invertebrate contributions to ecosystem processes. We quantied the rate of decomposition of Macaranga sp. leaves, a common genus of early successional trees in southeast Asian rainforests found in both primary and logged forest. When invertebrates were excluded from leaf litter, decomposition rates were reduced by 39% in primary forest but by just 16% in logged forest (Fig. 1a,b), meaning the invertebrate contribution to decomposition processes in logged forest is less than one-half of that in primary forest (habitat treatment interaction effect:

likelihood ratio test, w2(2) 41.48, Po0.001, N 25;

Supplementary Table 2). This experiment does not, however, account for potential bias that may arise from differential

Absolute rate

1.2

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Logged

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Figure 1 | Ecosystem process rates in primary (green) and logged (orange) forest in response to experimental removal of invertebrates, fungi or vertebrates. Ecosystem processes were quantied at three trophic levels represented by (a,b) leaf litter decomposition rate, (c,d) seed disturbance, dened as the combined removal and/or predation rate; and (e,f) invertebrate predation rate. Symbols indicate the taxa that contributed to the rates displayed. Absolute values (mean95% CI) of the ecosystem process rates are presented in the left-hand column and are measured as (a) the slope of a linear regression model relating loge-transformed litter mass (g) as a function of loge-transformed time (number of days), (c) the proportion of experimental seeds removed or predated per day, and (e) the proportion of experimental mealworm larvae predated per day. Letters indicate habitat treatment categories that did not signicantly differ from

each other (Po0.05). There was a signicant treatment habitat

interaction for all three ecosystem processes, demonstrating that the role of invertebrates was stronger in primary than logged forests. In the right-hand column (panels b,d,f), values represent the proportional change in ecosystem process rates relative to control sites (calculated from data presented in the left-hand column). Values o1 (dashed line) indicate functions whose rate is reduced following the exclusion of a taxon; smaller values indicate larger reductions in the rate and hence a stronger contribution of that taxon to delivering the ecosystem process. Posthoc signicance tests were used to examine the treatment habitat interaction

effects. For example, in panel b, the asterisk indicates that the effect on decomposition of excluding invertebrates was signicantly (Po0.05) larger in primary forest than in logged forest. NS indicates nonsignicant interactions.

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invertebrate preferences for different litter types in the primary and logged forest habitats. Treating litter with a broad-spectrum fungicide had little effect, reducing decomposition rates by o2%

in both habitats, suggesting that bacteria rather than fungi are important in the early phases of leaf decomposition in these forests18. Soil bacteria in Borneo are known to be resistant to logging19 and have been shown to be more resistant to the logging associated compaction of soil than fungi20. At our site there was a reduction, albeit nonsignicant, in the abundance of above-ground litter-trapping marasmioid fungal networks21 in logged relative to primary forest (Fig. 2; likelihood ratio test, w2(1) 3.76, P 0.052, N 114).

We placed seeds on the forest oor to quantify the rate at which seeds were disturbed (either predated and/or removed) over a 24-h period. Excluding invertebrates from seeds in primary forest reduced the seed disturbance rate by almost three quarters (72%), but their role was smaller in logged forest, where excluding invertebrates reduced the rate of seed disturbance by only

one-half (52%; Fig. 1c,d; habitat treatment interaction effect:

likelihood ratio test, w2(2) 46.19, Po0.001, N 194;

Supplementary Table 2). By contrast, vertebrates, primarily small mammals, accounted for just 3% of seed disturbance in primary forest, but their importance increased signicantly in logged forest where they accounted for 10% of all seed disturbance.

We found similar results in predation experiments quantifying the rate at which live mealworms tethered to articial leaves were predated. Predation rates decreased by 96% in primary forest when invertebrates were excluded (Fig. 1f). In logged forest, however, invertebrates contributed just 62% of all predation (Fig. 1e,f; habitat treatment interaction effect: likelihood ratio

test, w2(2) 10.77, P 0.005, N 51; Supplementary Table 2).

Excluding vertebrates, such as insectivorous birds and bats and small mammals such as treeshrews that forage in the understorey, also reduced predation of invertebrates in primary forest (48%), but their contribution to predation was signicantly higher in logged forest (69%).

Primary Logged

Environment Invertebrate Vertebrate

+

+

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Leaf area index (Morph)

Maximum air temperature (Clim)

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Earthworms (Abun)

Earthworm biomass (Mass)

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Termites (Occ)

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Predatory beetles (Abun)

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Small mammals (Abun)

Frogs (Occ)

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Granivorous birds (Occ)

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3 2 1 0 1 2 3

Standardized value

Figure 2 | Differences in the physical environment and the functional composition of invertebrate and vertebrate communities between primary and logged tropical rainforest. Response variables fall into the categories of microclimate (Clim), morphology and structure (Morph), biomass (Mass), diversity (Div), occurrence (Occ) and abundance (Abun). Invertebrate groups tend to be more abundant in primary than in logged forest, whereas the reverse is generally true for vertebrate groups. For comparison, all values were standardized to represent standard deviations from the mean. Dark lines represents the median, boxes the rst and third quartiles, and whiskers the range. Signicant differences (Po0.05) from mixed effect models on untransformed data are indicated with a green minus ( ) or orange plus ( ) sign if the effect of logging was negative or positive.

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Altered functional compositional of the rainforest fauna. To understand why the relative importance of invertebrates versus vertebrates changes after logging, we conducted a comprehensive assessment of the functional diversity of the rainforest fauna in primary and logged forest. We dug soil pits to obtain data on the occurrence patterns of 27 genera of termite (N 429 individuals)

and 416 earthworms, and made eld observations on the abundance of 192 species of foraging ant (N 4,173). We used canopy

fogging to collect invertebrate herbivores (N 1,492), and mod

ied ight intercept/pitfall traps to quantify invertebrate biomass, the abundance of leaf litter beetles (N 1,820) and occupancy of

leaf litter frogs (collected as by-catch; N 33). Harp traps were

used to capture 26 species of bat (N 687), point counts to

identify 115 species of understorey bird (N 1,972) and live-

capture traps to collect 26 species of small mammals (N 1,897).

The reduced role of invertebrates in decomposition processes is probably due to declines in the abundance of key invertebrate decomposers in logged forest (Supplementary Table 3). The abundance of leaf litter beetles (Fig. 2; likelihood ratio test, w2(1) 10.87, Po0.001, N 198), the occurrence of termites

(Fig. 2; likelihood ratio test, w2(1) 4.14, P 0.042, N 41),

including those belonging to the functional group associated with eating dead wood, grass or leaf litter (Fig. 2; likelihood ratio test, w2(1) 5.87, P 0.015, N 41), and the genus-level diversity of

termites (Fig. 2; likelihood ratio test, w2(1) 4.51, P 0.034,

N 41) were all reduced by two-thirds in logged relative to

primary forest. In addition, the biomass of earthworms in logged forest was almost half of that in primary forest (Fig. 2; likelihood ratio test, w2(1) 5.41, P 0.020, N 27), although their

abundance remained constant (Fig. 2; likelihood ratio test, w2(1) 0.44, P 0.506, N 27), probably because individual

worms in primary forest have larger body sizes. The most likely explanation for reductions in the abundance or biomass of these key functional groups is altered microclimate (Supplementary Table 4). Logging opens gaps in the rainforest canopy and logged forests therefore have a lower leaf area index (Fig. 2; likelihood ratio test, w2(1) 5.51, P 0.019, N 98) resulting in higher daily

maximum air temperature22 (Fig. 2; likelihood ratio test, w2(1) 5.82, P 0.016, N 161) and lower minimum humidity

(Fig. 2; likelihood ratio test, w2(1) 6.79, P 0.009, N 161) than

primary forests. Soft-bodied invertebrates such as earthworms and termites are particularly sensitive to desiccation23, and their decline in logged forests is consistent with the altered microclimate patterns we observed.

Invertebrates, including leaf litter beetles and ants, are assumed to be dominant predator and seed disturbance agents in tropical rainforests4, and we found substantial changes in the abundance and functional composition of these communities in logged forest (Supplementary Table 5). There was no difference in the total abundance of ants foraging in the leaf litter in logged relative to primary forest (Fig. 2; likelihood ratio test, w2(1) 0.01, P 0.912,

N 210), but there were 25% fewer ant species (Fig. 2; likelihood

ratio test, w2(1) 6.74, P 0.009, N 210), the average body size

of foraging ant species was one-third smaller (Fig. 2; likelihood ratio test, w2(1) 4.73, P 0.030, N 210), and the abundance of

predatory beetles (Fig. 2; likelihood ratio test, w2(1) 5.04,

P 0.025, N 198) and predatory ants (Fig. 2; likelihood ratio

test, w2(1) 3.41, P 0.065, N 210) in the leaf litter were halved,

although this latter difference was not statistically signicant. Large-bodied, carnivorous ants are known to be particularly sensitive to land use change in Borneo24, a pattern that may in part arise because of the altered structure of the forest canopy and an increase in generalist, visual vertebrate predators.

Concurrent with the changes to the functional composition of invertebrate communities, the abundance of key vertebrate functional groups tended to increase in logged relative to primary

forest (Supplementary Table 6). Capture rates of small mammals at ground level were more than three times higher in logged forest than in primary forest (Fig. 2; likelihood ratio test, w2(1) 6.47,

P 0.011, N 1,248). Frogs were much more likely to be present

on the ground (Fig. 2; likelihood ratio test, w2(1) 7.62,

P 0.006), and there was a signicantly higher abundance of

insectivorous birds (Fig. 2; likelihood ratio test, w2(1) 10.07,

P 0.002, N 114), in logged than in primary forest. It is

unlikely that trophic release explains the increased abundance of small mammals because there is little evidence of trophic release in the region25 and the logged forest we worked in has a high abundance of top carnivores26. Rather, these vertebrate groups probably increase in abundance because of increased resource availability: vegetation plots were twice as likely to contain trees that were fruiting or owering (Fig. 2; likelihood ratio test, w2(1) 4.48, P 0.034, N 112), and the total invertebrate

biomass was doubled (Fig. 2; likelihood ratio test, w2(1) 11.59,

Po0.001, N 198), in logged relative to primary forest. The

increased invertebrate biomass is predominantly comprised of large herbivorous invertebrates (Fig. 2; Supplementary Table 5; likelihood ratio test, w2(1) 3.94, P 0.047, N 107). These

herbivores do not contribute directly to the ecosystem functions we examined, suggesting that herbivory may be one function that runs counter to the general trend for invertebrates to decline in importance in logged forest. We did not nd a denitive effect of logging on the occurrence of granivorous birds, which were rarely recorded at this study site (Fig. 2; likelihood ratio test, w2(1) 1.53, P 0.306, N 114), or on the abundance of

insectivorous bats16 (Fig. 2; likelihood ratio test, w2(1) 0.53,

P 0.469). Together, the reduced abundance of large and

predatory ants and beetles, combined with the increased abundance of a suite of generalist vertebrates, help explain why the contribution of invertebrates to seed disturbance and predation rates in logged forests decreased while that of vertebrates increased.

DiscussionOur data suggest that the ecosystem processes operating in tropical forests remain considerably resistant to the human disturbance of high-intensity logging. This occurs despite large changes in the abundance, diversity and functional composition of termites, ants, beetles and earthworms, the key invertebrate groups primarily responsible for those functions in primary forest. The fact that taxa other than invertebrates are able to perform the same ecosystem processes probably confers the resilience to human disturbance. For the decomposition processes, this resilience may potentially arise because of increased physical fracturing of litter under the harsher microclimatic conditions in logged forest or possibly through a density compensation effect12 of bacteria or other decomposer micro-invertebrates. For the seed disturbance and invertebrate predation processes, our data indicate the resilience is partially conferred through a density compensation effect of vertebrates. However, the susceptibility of vertebrates to a range of anthropogenic pressures has seen their threat status steadily rise over the last two decades27, meaning an increased reliance on vertebrates to deliver ecosystem processes in logged forests may leave these systems vulnerable to future change.

Primary forests remain irreplaceable for conserving tropical biodiversity28, but the conservation value of logged forests should not be ignored. Depending on the intensity of selective logging17 and the timber-harvesting method used29, logged forests can retain high biodiversity value in southeast Asia and elsewhere28, supporting many of the tree30, bird31, bat16, cat26 and invertebrate31 species found in primary forests and justifying

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decisions to protect large areas of logged forest32. But forests are more than the sum of their component species. They are complicated networks of species that interact with each other and with the environment to perform ecological functions such as nutrient cycling, the regulation of water regimes and, ultimately, the modulation of local and global climates. Our data show that ecological processes associated with primary forest are retained in logged forest, but that the role of the worlds most diverse taxon in performing those processes is greatly diminished. Invertebrates remain an important actor in logged forests, but human actions are deposing them from their dominant role in rainforest ecosystems.

Methods

Study site and design. Data were collected at the Stability of Altered Forest Ecosystems Project in Sabah, Malaysia15. Logged forests had been through one round of selective logging (removing 113 m3 ha 1) in the 1970s and a second round of salvage logging16 (removing 66 m3 ha 1) that occurred between 2000 and 2008. Data collection took place between 2010 and 2012, B510 years after logging ended.

All data collection had a nested structure with sample sites clustered at up to four spatial scales based on a fractal sampling pattern15,33 (Supplementary Fig. 1). At the nest scale, sites were separated by 101.75 m (rst order), with those clusters of sites separated by 102.25 m (second order) and again at 102.75 m (third order). All third-order sites were nested within blocks separated by 41 km and the average area of a block was 71 ha. There are 17 sampling blocks at the Stability of Altered Forest Ecosystems Project, which vary in the level of historical disturbance15. For these analyses, we used data collected from eight blocks: two blocks located in unmodied primary forest (blocks OG1 and OG2) and six blocks located in salvage logged forest (blocks A, B, C, D, E and F). We used larger sample sizes in the logged forest because data on leaf area index, an index of forest structure, demonstrated the habitat there was more variable than in the primary forest, with higher variance(0.51 versus 0.39) and a larger range (0.835.56 versus 3.125.48) of values. Not all datasets were collected at all points, so sample sizes differ among the datasets used in analyses.

Ecosystem process experiments. Litter decomposition: naturally senesced whole leaves of Macaranga sp. were collected within 24 h of falling. We used a morphospecies (M. cf. pearsonii) that is a common early successional tree species, is highly abundant within logged forest and also common along riverine margins and in treefall gaps within primary forest. This standardization of the litter does, however, mean our experiment was unable to control for any potential biases in invertebrate preference behaviour among the two habitats. Leaves were cut into roughly 2 cm2 pieces, discarding large veins and petioles, and dried to constant weight. Litter pieces were separated into 4-g units and placed in nylon bags with a 1-mm mesh. To allow invertebrates access to the litter, ve 1-cm2 perforations were made on each side of the bag. Fungi were excluded by treating the lled litter bags with broad-spectrum fungicide containing 40% chlorothalonil34. Data were collected at second-order sites with nine primary forest (block OG2) and 16 logged forest (block E) sites. At each site we placed nine litter bags; three each for the control, invertebrate exclosure and fungal exclosure treatments. One bag from each treatment was collected after 14, 27 and 40 days (2 days), respectively, and on each day, ve or six additional bags were carried to and from the eld to calculate the amount of litter mass lost through handling. Although a short period, this was long enough to detect invertebrate impacts on decomposition that are consistent with results reported elsewhere in the tropics3436. Collected bags were dried to constant weight. Decomposition rate for each site treatment combination was

quantied as the slope of linear regression modelling litter mass as a function of loge-

transformed number of days in the eld, using handling loss as an offset in the model.

Seed disturbance: to compare seed disturbance rates between vertebrates and invertebrates, we required an intermediate seed size and type that was attractive to both taxa. We conducted pilot trials that indicated large seeds such as peanuts were not disturbed by invertebrates, and small seeds such as sesame were ignored by vertebrates. We found that pumpkin seeds were disturbed by both vertebrates and invertebrates, and thus were chosen for the experiments.

Data were collected at rst-order sites with 48 primary forest (block OG2) and 146 logged forest (blocks D and F) sites. At each site we placed 20 pumpkin seeds on each of three standardized brown plastic leaves (80 120 mm) left sitting on

the forest oor. One of the three leaves had ground-moving invertebrates excluded by surrounding the leaf with a wooden frame coated with insect-trapping glue, a second had vertebrate predators excluded by placing the articial leaf inside a30 30 30 cm wire cage with a 1-cm mesh, and the third was left as a control.

We dened the seed disturbance rate as the proportion of seeds that were either removed from the leaf, or partially eaten but left in place on the leaf, over a 24-h period.

Invertebrate predation: data were collected at rst-order sites with 21 primary forest (block OG2) and 30 logged forest (block E) sites. At each site we attached

one live larval mealworm (Tenebrio sp.) to each of three standardized green plastic leaves (80 120 mm) using ne cotton thread and clear tape. Enough cotton

thread was provided to allow the mealworm to crawl around the leaf, and articial leaves were attached to saplings 1.5 m above ground. One of the three leaves had ground-moving invertebrate predators excluded by applying insect-trapping glue to the sapling stem 20 cm above and below the articial leaf, a second had vertebrate predators excluded by placing the articial leaf inside a 30 30 30 cm

wire cage with a 1-cm mesh, and the third was left as a control. Predation of mealworms was recorded over a 24-h period.

Functional composition of the rainforest fauna. Invertebrate collections: leaf litter beetles were collected at 54 rst-order sites in primary forest (blocks OG1 and OG2) and 144 rst-order sites in logged forest (blocks A, C and E) over 3 days during the wet season. We used modied ight intercept traps dug into the ground to simultaneously act as a pitfall trap (23 cm diameter 60 cm high). Traps were

part-lled with 70% ethanol to act as a killing agent. Traps collected frogs as by-catch that allowed us to analyse the presence or absence of frogs in relation to habitat. The total invertebrate biomass of samples was estimated by placing all invertebrates with body length 45 mm on blotting paper and weighing the blotted invertebrates. We excluded small invertebrates despite their high abundance because their small body size means they are likely to contribute less to total community biomass37,38, and also to community-level energy uxes37,39, than the less abundant, but larger, organisms. We calibrated sample-level wet weight biomass of invertebrates with sample-level dry weight biomass using 42 samples that were rst wet weighed and then oven dried to constant weight. Wet weight was strongly and linearly correlated with dry weight (linear regression through the origin, F1,41 1,578, Po0.001, R2 0.97, slope 0.2170.005 s.e.). Canopy

invertebrates were collected by fogging at 12 second-order sites in primary forest (block OG2) and 95 second-order sites in logged forest (blocks A, B, C, D, E and F). At each site, four trays of 1-m diameter were laid out with collecting pots lled with 95% ethanol attached. Fog formulation was synthetic pyrethrum insecticide (active compound: alphacypermethrin with synergist 2.27 %) and diluted in diesel by a ratio of 15:1. Fogging activity started at 07:00 and lasted for 4 min at each site. Arthropods were collected after a 2-h period and identied to order.

Termites: we hand-collected termites from 16 soil pits (12 cm diameter 10 cm

deep) at each of nine second-order primary sites (block OG2) and 32 second-order logged forest sites (blocks C and F), and were identied to genus40. Earthworms were collected from four soil monoliths per site (three monoliths of 50 50 cm

wide 30 cm deep and a fourth, smaller monolith of 25 25 cm wide 30 cm

deep) at each of 9 second-order primary sites (block OG2) and 18 second-order logged forest sites (blocks B and F). Foraging ant abundance and species richness was quantied at 18 rst-order primary forest and 192 rst-order logged forest sites by counting and identifying the number of ants entering a 12 14 cm plastic card,

laid at in the leaf litter and baited with 30 compressed dried earthworm pellets, over a 40-min period. All observations were conducted between 10:00 and 15:00.

Invertebrate functional traits: beetles were identied to family and families were classied according to whether they contained predominantly predatory species or not41. All foraging ants were identied to genus, split into morphospecies and assigned species names where possible. Termites were also identied to genus and grouped according to feeding position along a four-step humication gradient, with the second group representing those that feed on grass, dead wood and leaf litter42. We measured Webers length, a common proxy for body size in ants, for each of the 192 species of foraging ant, averaging measurements from between one and ve minor workers per species. Ant genera were also classied according to whether they belonged to the specialist predator functional group or not43. We used the abundance-weighted mean body size (log10-transformed) of all ants, and the total abundance of specialist predators, visiting each site as the response variable in analyses. Arthropods from canopy fogging samples belonging to the Orthoptera, Phasmida, Homoptera and Heteroptera were classied as herbivores.

Small mammals: we trapped small mammals within 1.75 ha grids overlaying the nested sampling design used in other data collection. Each grid consisted of a4 12 rectangular arrangement of points separated by 23 m. Two locally made

small mammal traps (280 140 140 mm), baited with oil palm fruit, were placed

at or near ground level (r1,500 mm) within 10 m of each grid point, making 96 traps in total per grid. For this analysis, we included captures only from those traps that lay within 30 m of a rst-order sampling point, including 265 traps in primary forest (at 27 rst-order primary sites in blocks OG1 and OG2) and 983 traps in logged forest (at 103 rst-order sites in blocks D, E and F). Trapping sessions ran for seven consecutive days and we used the capture rate of all species combined (captures per seven days) in the analyses.

Bats: four-bank harp traps were set across trails and logging skids at nine second-order sites in primary forest (block OG2) and 81 second-order sites in logged forest (blocks A, B, C, D, E and F) to target insectivorous bats foraging in the forest understory22. Up to seven traps were set each night, 50 to 150-m apart, and moved to a new position the following day. All bats were marked and released at the capture point.

Birds: we sampled birds using 15-minute point counts of 50 m radius at 18 second-order primary forest sites (blocks OG1 and OG2) and 96 second-order logged forest sites (blocks A, B, C, D, E and F). Birds were identied by a single experienced observer (David P. Edwards) and any unknown vocalizations were

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recorded using a Sennheiser ME-66 directional microphone and Edirol R09-HR digital recorder, and subsequently identied against reference collections available from http://www.xeno-canto.org/

Web End =http://www.xeno-canto.org/ . Swifts (Apodidae) and swallows (Hiurundae) were not recorded because they are difcult to observe in closed-canopy forest. We recorded the combined abundance of all bird species classied as belonging to insectivore (including both obligate and generalists) and granivore guilds44.

Environmental variables. Leaf area index was calculated from 13 hemispherical photographs per plot at 18 second-order primary forest sites (blocks OG1 and OG2) and 80 second-order logged forest sites (blocks A, B, C, D, E and F), and processed following the methods of Pfeifer et al.45. The presence of owers or fruits in 25 25 m vegetation plots was recorded at 18 second-order primary forest sites

(blocks OG1 and OG2) and 96 second-order logged forest sites (blocks A, B, C, D, E and F). Within each vegetation plot we counted the number of clumps of the litter-trapping fungi belonging to the genus Marasmius spp., which forms abundant and easily recognizable networks of brown rhizomes that trap leaf litter above the ground21. We placed iButton DS1923-F5 dataloggers (Dallas Semiconductor) to record air temperature and relative humidity 1-m above ground every 3 h, from which we determined the average maximum daily air temperature and minimum daily humidity in the dry season (March and April)46, when the forest experiences the most extreme microclimatic conditions, at 34 rst-order primary forest sites (blocks OG1 and OG2) and 127 rst-order logged forest sites (blocks A, B, C, D, E and F).

Statistical analyses. We tted generalized linear mixed effect models to the data with random effects reecting the nested structure of data collection (multiple observations within rst-order sites within second-order sites within third-order sites within blocks). Error distributions for the models were selected according to the nature of the response variables and are recorded in Supplementary Tables 16. Count data, which was used for all estimates of animal abundance and species richness, was modelled with a Poisson error distribution. Presenceabsence and occurrence data, along with binary response variables such as invertebrate predation and seed disturbance, were modelled with a binomial error distribution. All other variables were modelled using Gaussian errors, with response variables log10-

transformed where this improved normality. All models were tted using the lme4 package47 of the R statistical computing environment48. We used likelihood ratio tests to determine parameter signicance by comparing models with habitat (primary versus logged forest) to a null model with no predictor. The proportion of variance explained by xed effects R2GLMM m

was calculated and is reported in

Supplementary Tables 16.

Posthoc signicance tests using the glht function in the multcomp package49 were used to compare treatment effects and their interactions. We determined the proportion of variance explained by xed effects and by the model as a whole including variance explained by the random effects50. Residuals of all models were tested for spatial autocorrelation to estimate the spatial dependence of model residuals as a continuous function of distance51. In no case did we detect signicant spatial autocorrelation (Supplementary Tables 16) so this is not discussed further. Full results of the partitioning of variance explained and tests of spatial autocorrelation are presented in Supplementary Tables 16.

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Acknowledgements

We thank the Royal Societys South East Asia Rainforest Research Programme for logistical support, and Yayasan Sabah, Maliau Basin Management Committee, the State Secretary, Sabah Chief Ministers Departments, the Malaysian Economic Planning Unit and the Sabah Biodiversity Council for permission to conduct research. This work was funded by the Sime Darby Foundation. RME and MP were supported by European Research Council Project number 281986; TMF by the Czech Science Foundation (1432302S) and project CZ.1.07/2.3.00/20.0064 co-nanced by the European Social Fund and the state budget of the Czech Republic; RLK, NES, TMF and KMY by the Australian Research Council (DP140101541); SRH by the Grantham Institute for Climate Change; RH by Fundamental Research Grant Scheme (FRG0302-STWN-1/ 2011) from the Ministry of Higher Education, Malaysia; SHL by the UK Natural

Environment Research Council (NERC), The University of East Anglia and The Sir Philip Reckitt Educational Trust; and MJS by a Leverhulme Trust Early Career Fellowship. Jeremy Cusack, Leah Findlay, Jessica Haysom, Tim Harvey-Samuel, Matthew Holmes, Robin Loveridge, Jack Thorley and Faye Thompson assisted in the eld. RaphaelK. Didham and Jason M. Tylianakis provided comments. This paper is a contribution to Imperial Colleges Grand Challenges in Ecosystems and the Environment initiative.

Author contributions

R.M.E. designed the study, conducted all analyses and wrote the rst draft of the manuscript. R.M.E., M.J.W.B., R.A.G., N.S.P., T.R.B., R.G.D., D.P.E., P.E., T.M.F., S.R.H., R.L.K., K.M.S., S.H.L., J.J.M., M.P., S.V.R., A.C.S., N.E.S., M.J.S., O.R.W., K.M.Y. and E.C.T. contributed the data. All authors commented on manuscript drafts.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

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How to cite this article: Ewers, R. M. et al. Logging cuts the functional importance of invertebrates in tropical rainforest. Nat. Commun. 6:6836 doi: 10.1038/ncomms7836 (2015).

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