Opportunistic top predators partition food resources in a tropical freshwater ecosystem

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The linkage between riparian predators and aquatic insects across a stream-resource spectrum

1. Spatial subsidies, defined as the flow of energy, nutrients, organisms or pollutants from one habitat to another, have been shown to affect the food–web dynamics in a wide range of ecosystems. An important subsidy to riparian communities is the contribution of adult stream insects to terrestrial predators such as birds, bats and lizards, but also invertebrates including ground and web‐building spiders. 2. We surveyed 37 first‐ and second‐order forest streams across differing environmental gradients in the Central South Island, New Zealand, to investigate the relationship between potential aquatic prey subsidies and predatory riparian arachnids. We anticipated that stream‐insect biomass would be positively associated with riparian arachnids, as a result of emergent adult aquatic insect subsidies to the adjacent habitat. 3. We confirmed positive associations between stream‐insect biomass as a predictor variable and riparian arachnid biomass (R² = 0.42, F_1,34 = 25.2, P < 0.001) and web densities (R² = 0.45, F_1,14 = 11.5, P < 0.01) respectively as dependent variables after adjusting for the confounding effects of environmental variables. Hierarchical partitioning confirmed the importance of stream insect biomass as a statistically significant contributor to the total explained variance in analyses calculated for arachnid biomass, abundance and web density. 4. A concurrent survey of spider‐web density along 20‐m transects from the stream edge into the forest indicated a strong decline in web‐building spider density moving away from the stream (R² = 0.41, F_1,158 = 109, P < 0.001), with stream‐insect biomass as a significant covariate (F_1,149 = 17.7, P < 0.001). 5. Our results suggest that productivity gradients present in the donor system affect the magnitude of the interaction between adjacent habitats. Productivity gradients may lead to increased reciprocal subsidies through a positive feedback loop involving the predation of spiders and other predatory terrestrial invertebrates by aquatic predators. However, terrestrial insectivores such as birds, bats and lizards that are not readily used as prey by aquatic predators may circumvent the feedback cycle by consuming a large proportion of emergent aquatic‐insect biomass. This may lead to asymmetry in the strength of food–web linkages between aquatic and terrestrial habitats.     

Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones

1. Streams and their adjacent riparian zones are closely linked by reciprocal flows of invertebrate prey. We review characteristics of these prey subsidies and their strong direct and indirect effects on consumers and recipient food webs. 2. Fluxes of terrestrial invertebrates to streams can provide up to half the annual energy budget for drift‐feeding fishes such as salmonids, despite the fact that input occurs principally in summer. Inputs appear highest from closed‐canopy riparian zones with deciduous vegetation and vary markedly with invertebrate phenology and weather. Two field experiments that manipulated this prey subsidy showed that it affected both foraging and local abundance of stream fishes. 3. Emergence of adult insects from streams can constitute a substantial export of benthic production to riparian consumers such as birds, bats, lizards, and spiders, and contributes 25–100% of the energy or carbon to such species. Emergence typically peaks in early summer in the temperate zone, but also provides a low‐level flux from autumn to spring in ice‐free streams. This flux varies with in‐stream productivity, and declines exponentially with distance from the stream edge. Some predators aggregate near streams and forage on these prey during periods of peak emergence, whereas others rely on the lower subsidy from autumn through spring when terrestrial prey are scarce. Several field experiments that manipulated this subsidy showed that it affected the short‐term behaviour, growth, and abundance of terrestrial consumers. 4. Reciprocal prey subsidies also have important indirect effects on both stream and riparian food webs. Theory predicts that allochthonous prey should increase density of subsidised predators, thereby increasing predation on in situ prey and causing a negative indirect effect via apparent competition. However, short‐term experiments have produced either positive or negative indirect effects. These contrasting results may be due to characteristics of the subsidies and individual consumers, but could also result from differences in experimental designs. 5. New study approaches are needed to better determine the direct and indirect effects of reciprocal prey subsidies. Experiments coupled with comparative research will be required to measure their effects on individual consumer fitness and population demographics. Future work should investigate whether reciprocal prey fluxes stabilise linked stream–riparian ecosystems, explore how landscape context affects the magnitude and importance of subsidies, and determine how impacts of human disturbance can propagate between streams and riparian zones via these trophic linkages. Study of these reciprocal connections is helping to define a more holistic perspective of catchments, and has the potential to shape new directions for ecology in general.     

Experimental reductions in stream flow alter litter processing and consumer subsidies in headwater streams

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Benthic secondary production and biomass of insects emerging from a northern German temperate stream

• 1 Secondary production and emergence of aquatic insects were examined in the outlet of Lake Belau, Northern Germany, by means of benthic samples and emergence traps.
• 2 At three stream sections annual larval secondary production varied between 4.9 and 10.8 gDM (dry mass) m^‐2 year^‐1. Insects contributed with 3.4, 8.9, and 8.7% to the total macroinvertebrate production that varied between 56.5 and 215.1 gDM m^‐2 year^‐1. Emerged biomass was between 1.0 and 2.0 gDM m^‐2 year^‐1. At all three stream sections Diptera dominated with a larval production of 3.0–l6.1 gDM m^‐2 year^‐1, followed by Trichoptera with 1.0–2.1 and Ephemeroptera with up to 0.9 gDM m^‐2 year^‐1.
• 3 Average larval production amounted to 9.0 gDM m^‐2 year^‐1 and emerged biomass to 1.7 gDM m^‐2 year^‐1. Larvae of insects amounted to 7.0% of total macroinvertebrate production.
• 4 The ratio of annual emerged biomass to secondary production (E/P) varied among individual taxa. At the stream sections the ratio ranged from 15.9% to 20.1% with an average of 18.3% for the stream.
• 5 Relative constancy of the E/P ratio suggests that the determination of emerged biomass can be used as a method for estimating the secondary production of aquatic insects. The composition of functional feeding groups clearly differs between emerged biomass and total macroinvertebrate production. Therefore, the method does not allow conclusions on the level of production of the whole benthic community. However, composition of functional feeding groups at emergence roughly reflects composition of these groups in the benthic insect community.

     

Scavenger guild and consumption patterns of an invasive alien fish species in a Mediterranean wetland

Invasive Alien Species (IAS) alter ecosystems, disrupting ecological processes and driving the loss of ecosystem services. The common carp Cyprinus carpio is a hazardous and widespread IAS, becoming the most abundant species in many aquatic ecosystems. This species transforms ecosystems by accumulating biomass to the detriment of other species, thus altering food webs. However, some terrestrial species, such as vertebrate scavengers, may benefit from dead carps, by incorporating part of the carp biomass into the terrestrial environment. This study describes the terrestrial vertebrate scavenger assemblage that benefits from carp carcasses in a Mediterranean wetland. We also evaluate the seasonal differences in the scavenger assemblage composition and carrion consumption patterns. Eighty carp carcasses (20 per season) were placed in El Hondo Natural Park, a seminatural mesohaline wetland in south‐eastern Spain, and we monitored their consumption using camera traps. We recorded 14 scavenger species (10 birds and four mammals) consuming carp carcasses, including globally threatened species. Vertebrates consumed 73% of the carrion biomass and appeared consuming at 82% of the carcasses. Of these carcasses consumed, 75% were completely consumed and the mean consumption time of carcasses completely consumed by vertebrates was 44.4 h (SD = 42.1 h). We recorded differences in species richness, abundance, and assemblage composition among seasons, but we did not find seasonal differences in consumption patterns throughout the year. Our study recorded a rich and efficient terrestrial vertebrate scavenger assemblage benefitting from carp carcasses. We detected a seasonal replacement on the scavenger species, but a maintenance of the ecological function of carrion removal, as the most efficient carrion consumers were present throughout the year. The results highlight the importance of vertebrate scavengers in wetlands, removing possible infectious focus, and moving nutrients between aquatic and terrestrial environments.     

CO2 supersaturation along the aquatic conduit in Swedish watersheds as constrained by terrestrial respiration,aquatic respiration and weathering

We tested the hypothesis that CO₂ supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized‐additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO₂ in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg²⁺ had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg²⁺ for streams compared with TOC. Both DSi and TOC explained 22–46% of the pCO₂ variability in various lake classes (0.01–>100 km²) and Mg²⁺ and TOC explained 11–41% of the pCO₂ variability in the various stream orders. This suggests that aquatic pCO₂ has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO₂ efflux. pCO₂ and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO₂ with the atmosphere and in‐lake mineralization of TOC. For streams, we observed a decreasing trend in pCO₂ with stream orders between 3 and 6. We calculated the total CO₂ efflux from all Swedish lakes and streams to be 2.58 Tg C yr^?1. Our analyses also demonstrated that 0.70 Tg C yr^?1 are exported to the ocean by Swedish watersheds as HCO₃^? and CO₃^2? of which about 0.56 Tg C yr^?1 is also a residual from terrestrial respiration and constitute a long‐term sink for atmospheric CO₂. Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr^?1, which corresponds to 10% of the NEE in Sweden.     

Variation in active and passive resource inputs to experimental pools: mechanisms and possible consequences for food webs

1. Cross‐ecosystem movements of resources, including detritus, nutrients and living prey, can strongly influence food web dynamics in recipient habitats. Variation in resource inputs is thought to be driven by factors external to the recipient habitat (e.g. donor habitat productivity and boundary conditions). However, inputs of or by ‘active’ living resources may be strongly influenced by recipient habitat quality when organisms exhibit behavioural habitat selection when crossing ecosystem boundaries. 2. To examine whether behavioural responses to recipient habitat quality alter the relative inputs of ‘active’ living and ‘passive’ detrital resources to recipient food webs, we manipulated the presence of caged predatory fish and measured biomass, energy and organic content of inputs to outdoor experimental pools of adult aquatic insects, frog eggs, terrestrial plant matter and terrestrial arthropods. 3. Caged fish reduced the biomass, energy and organic matter donated to pools by tree frog eggs by ～70%, but did not alter insect colonisation or passive allochthonous inputs of terrestrial arthropods and plant material. Terrestrial plant matter and adult aquatic insects provided the most energy and organic matter inputs to the pools (40–50%), while terrestrial arthropods provided the least (7%). Inputs of frog egg were relatively small but varied considerably among pools and over time (3%, range = 0–20%). Absolute and proportional amounts varied by input type. 4. Aquatic predators can strongly affect the magnitude of active, but not passive, inputs and that the effect of recipient habitat quality on active inputs is variable. Furthermore, some active inputs (i.e. aquatic insect colonists) can provide similar amounts of energy and organic matter as passive inputs of terrestrial plant matter, which are well known to be important. Because inputs differ in quality and the trophic level they subsidise, proportional changes in input type could have strong effects on recipient food webs. 5. Cross‐ecosystem resource inputs have previously been characterised as donor‐controlled. However, control by the recipient food web could lead to greater feedback between resource flow and consumer dynamics than has been appreciated so far.     

Integrating aquatic and terrestrial components to construct a complete carbon budget for a north temperate lake district

The development of complete regional carbon (C) budgets for different biomes is an integral step in the effort to predict global response and potential feedbacks to a changing climate regime. Wetland and lake contributions to regional C cycling remain relatively uncertain despite recent research highlighting their importance. Using a combination of field surveys and tower‐based carbon dioxide (CO₂) flux measurements, modeling, and published literature, we constructed a complete C budget for the Northern Highlands Lake District in northern Wisconsin/Michigan, a ～6400 km² region rich in lakes and wetlands. This is one of the first regional C budgets to incorporate aquatic and terrestrial C cycling under the same framework. We divided the landscape into three major compartments (forests, wetlands, and surface waters) and quantified all major C fluxes into and out of those compartments, with a particular focus on atmospheric exchange but also including sedimentation in lakes and hydrologic fluxes. Landscape C storage was dominated by peat‐containing wetlands and lake sediments, which make up only 20% and 13% of the landscape area, respectively, but contain >80% of the total fixed C pool (ca. 400 Tg). We estimated a current regional C accumulation of 1.1±0.1 Tg yr^?1, and the largest regional flux was forest net ecosystem exchange (NEE) into aggrading forests for a total of 1.0±0.1 Tg yr^?1. Mean wetland NEE (0.12±0.06 Tg yr^?1 into wetlands), lake CO₂ emissions and riverine efflux (each ca. 0.03±0.01 Tg yr^?1) were smaller but of consequence to the overall budget. Hydrologic transport from uplands/wetlands to surface waters within the region was an important vector of terrestrial C. Regional C fluxes and pools would be misrepresented without inclusion of surface waters and wetlands, and C budgets in heterogeneous landscapes open opportunities to examine the sensitivities of important fluxes to changes in climate and land use/land cover.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Spatiotemporal heterogeneity of soil and sediment respiration in a river‐floodplain mosaic (Tagliamento,NE Italy)

1. In their natural state, river floodplains are composed of a complex mosaic of contrasting aquatic and terrestrial habitats. These habitats are expected to differ widely in their properties and corresponding ecological processes, although empirical data on their capacity to produce, store and transform organic matter and nutrients are limited. 2. The objectives of this study were (i) to quantify the spatiotemporal variation of respiration, a dominant carbon flux in ecosystems, in a complex river floodplain, (ii) to identify the environmental drivers of respiration within and among floodplain habitat types and (iii) to calculate whole‐floodplain respiration and to put these values into a global ecosystem context. 3. We measured soil and sediment respiration (sum of root and heterotrophic respiration; SR) throughout an annual cycle in two aquatic (pond and channel) and four terrestrial (gravel, large wood, vegetated island and riparian forest) floodplain habitat types in the island‐braided section of the near‐natural Tagliamento River (NE Italy). 4. Floodplain habitat types differed greatly in substratum composition (soil to coarse gravel), organic matter content (0.63 to 4.1% ash‐free dry mass) and temperature (seasonal range per habitat type: 8.6 to 33.1 °C). Average annual SR ranged from 0.54 ± 1.56 (exposed gravel) to 3.94 ± 3.72 μmol CO₂ m^?2 s^?1 (vegetated islands) indicating distinct variation in respiration within and among habitat types. Temperature was the most important predictor of SR. However, the Q₁₀ value ranged from 1.62 (channel habitat) to 4.57 (riparian forest), demonstrating major differences in habitat‐specific temperature sensitivity in SR. 5. Total annual SR in individual floodplain habitats ranged from 160 (ponds) to 1205 g C m^?2 (vegetated islands) and spanned almost the entire range of global ecosystem respiration, from deserts to tropical forests.     

Sources of dissolved organic matter (DOM) in a Rocky Mountain stream using chemical fractionation and stable isotopes

Dissolved organic matter (DOM) is an important vehicle for the movement of nutrients from terrestrial to aquatic systems. To investigate how the source and composition of aquatic DOM change in both space and time, we used chemical, spectroscopic, and isotopic analyses to characterize DOM in a headwater catchment in the Colorado Front Range. Streamwater samples for DOM analyses were collected from 2 sites, a lightly vegetated alpine site and a forested, subalpine site, in the North Boulder Creek catchment during the snowmelt runoff season (May–September). Concentrations of dissolved organic carbon (DOC) peaked on the ascending limb of the snowmelt hydrograph at both the alpine (2.6 mg C l⁻¹) and the subalpine sites (7.0 mg C l⁻¹) and decreased sharply on the descending limb of the hydrograph. Fractionation of DOM into operationally defined humic and non-humic components showed that the fulvic acid content of DOC decreased through the season at both sites and that spectroscopic (fluorescence and ultraviolet) properties of the humic DOM fraction shifted in a manner consistent with an increase in the proportion of humic DOM derived from instream sources as compared to terrestrial catchment sources. Humic and non-humic fractions of DOM isolated near peak flow in June and during low flows in September showed a seasonal enrichment in ¹⁵N and ¹³C as well as a seasonal decrease in the ratio of aromatic to aliphatic carbon, both of which were correlated with a decrease in the C:N ratio of the DOM fractions. These results suggest that seasonal shifts in the isotopic and chemical characteristics of DOM are a result of changes in catchment sources of DOM. In particular, it appears that DOM production in alpine lakes is an important contributor to the streamwater DOM load during late season low flows, especially in the alpine reach of the catchment. Our results further suggest that stable isotopes of C and N are useful tools, particularly when combined with ancillary data such as elemental analyses and catchment discharge, for evaluating sources and transformations of DOM at the catchment scale.     

Seasonal contrasts in carbon resources and ecological processes on a tropical floodplain

1. Globally, tropical floodplains are highly productive ecosystems. This is largely because of predictable seasonal rains providing replenishing floodwaters that stimulate nutrient turnover which, in turn, substantially boosts both primary and secondary productivity. This is associated with concomitant shifts in the types of primary producers and associated food webs. 2. The Magela Creek floodplain on Kakadu National Park in northern Australia is one of the most studied tropical freshwater ecosystems in Australia and provides an opportunity to collate and examine information on organic carbon sources and pathways through food webs to gain a fundamental understanding of how these systems may function. 3. We reviewed biophysical information published since the early 1980s to construct an assessment of the carbon resources for the channel and floodplain. 4. We conclude that macrophytes, largely in the form of grasses and aquatic plants, produce the greatest above‐ground biomass on the Magela Creek floodplain. Although macrophytes provide suitable substrata for the attachment of epiphytes, they do not appear to be an important carbon source for aquatic consumers themselves. Nevertheless, macrophytes do provide critical seasonal food and habitat structure for other producers and consumers on the floodplain, such as the abundant magpie geese. 5. We developed a generalised conceptual food web and carbon budget contrasting the ‘wet’ and ‘dry’ seasons for the Magela Creek system, as a representative of tropical seasonal floodplain systems. 6. Our conceptual model of tropical floodplains indicates that knowledge of the seasonal and spatial links and exchanges between the floodplain and the river is critical in understanding ecosystem function.     

Climate change and the future distributions of aquatic macrophytes across boreal catchments

Aim Aquatic–terrestrial ecotones are vulnerable to climate change, and degradation of the emergent aquatic macrophyte zone would have severe ecological consequences for freshwater, wetland and terrestrial ecosystems. Our aim was to uncover future changes in boreal emergent aquatic macrophyte zones by modelling the occurrence and percentage cover of emergent aquatic vegetation under different climate scenarios in Finland by the 2050s. Location Finland, northern Europe. Methods Data derived from different GIS sources were used to estimate future emergent aquatic macrophyte distributions in all catchments in Finland (848 in total). We used generalized additive models (GAM) with a full stepwise selection algorithm and Akaike information criterion to explore the main environmental determinates (climate and geomorphology) of emergent aquatic macrophyte distributions, which were derived from the national subclass of CORINE land‐cover classification. The accuracy of the distribution models (GAMs) was cross‐validated, using percentage of explained deviance and the area under the curve derived from the receiver‐operating characteristic plots. Results Our results indicated that emergent aquatic macrophytes will expand their distributions northwards from the current catchments and percentage cover will increase in all of the catchments in all climate scenarios. Growing degree‐days was the primary determinant affecting distributions of emergent aquatic macrophytes. Inclusion of geomorphological variables clearly improved model performance in both model exercises compared with pure climate variables. Main conclusions Emergent aquatic macrophyte distributions will expand due to climate change. Many emergent aquatic plant species have already expanded their distributions during the past decades, and this process will continue in the years 2051–80. Emergent aquatic macrophytes pose an increasing overgrowth risk for sensitive macrophyte species in boreal freshwater ecosystems, which should be acknowledged in management and conservation actions. We conclude that predictions based on GIS data can provide useful ‘first‐filter’ estimates of changes in aquatic–terrestrial ecotones.     

Trait‐based community assembly of aquatic macrophytes along a water depth gradient in a freshwater lake

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Contribution of terrestrial invertebrates to the annual resource budget for salmonids in forest and grassland reaches of a headwater stream

1. The annual input, contribution to the diet of salmonids, and quantitative input of terrestrial invertebrates to four reaches with contrasting forest (n=2) and grassland riparian vegetation (n=2) were compared in a Japanese headwater stream.
2. The annual input of terrestrial invertebrates falling into the forest reaches (mean±1 SE=8.7×10³±0.3×10³ mg m^?2 year^?1) was 1.7 times greater than that in the grassland reaches (5.1×10³±0.8×10³ mg m^?2 year^?1), with clear seasonality in the daily input of invertebrates in both vegetation types. The daily input, however, differed between the vegetation types only in summer, when it rose to a maximum in both vegetation types.
3. Fish biomass also differed among the seasons in both vegetation types, being less in the grassland reaches. The contribution of terrestrial invertebrates to the salmonid diet in the forest and grassland reaches was 11 and 7% in spring, 68 and 77% in summer, 48 and 33% in autumn, and 1 and 1% in winter, respectively. The prey consumption rate of fish, which was similar between the vegetation types, increased with stream temperature and was highest in summer. Terrestrial invertebrates supported 49% (mean±1 SE=5.3×10³±0.4×10³ mg m^?2 year^?1) of the annual, total prey consumption (10.9×10³±1.7×10³ mg m^?2 year^?1) by salmonids in the forest and 53% (2.0×10³±0.3×10³ mg m^?2 year^?1) (3.8×10³±0.6×10³ mg m^?2 year^?1) in the grassland reaches.
4. Salmonids were estimated to consume 51 and 35% of the annual total (falling plus drift) input of terrestrial invertebrates in the forest and grassland reaches, respectively. The input of terrestrial invertebrates by drift, however, was almost equal to the output in both vegetation types, suggesting that the reach‐based, in‐stream retention of terrestrial invertebrates almost balanced these falling in.
5. Difference in the riparian vegetation, which caused spatial heterogeneity in the input of terrestrial invertebrates, could play an important role in determining the local distribution of salmonids.     

Flood pulses drive the temporal dynamics of assemblages of aquatic insects (Heteroptera and Coleoptera) in a temperate floodplain

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Species richness of gall-forming insects in a tropical rain forest: correlations with plant diversity and soil fertility

We tested two hypotheses to explain changes in species richness ofgall-forming insects. The first hypothesis proposes that gall-forming insectspecies richness increases as more potential host–plant species areavailable. The second hypothesis implies that soil fertility affects plantcolonization by gall-forming insects. Seven sites, representing strongdifferences in vegetation and soil were chosen at the Lacandona tropical rainforest region, Chiapas, Mexico. Overall, we found 1522 individual plantsbelonging to 340 different plant species. From this, we found gall-forminginsects on 737 (43.9%) plants and on 74 (22%) of total plant species. We found asignificant negative correlation between gall-forming insect species richnessand species richness of plants, which does not support the hypothesis that plantspecies richness is an important factor in generating the radiation ofgall-forming insects. Using phosphorus as an indicator of soil fertility, wefound the lowest number of plants with gall-forming insects and the smallestgall-forming insect load per individual plant in the more fertile soil(alluvial). In contrast, the highest number of plants with galls and the highestgall-forming insect load per plant were found at a savanna-like vegetationsite, where the poorest soil was recorded. These results did not support thesoil fertility hypothesis in terms of species richness, but did with respect toabundance of plants with galls.