Assessing the true status of the fish species Labeo cylindricus (Peters 1868) (Teleostei: Cyprinidae) in Lake Baringo,Kenya

Eutrophication contributes to the proliferation of alien invasive weed species such as water hyacinth Eichhornia crassipes. Although the South American moth Niphograpta albiguttalis was released in South Africa in 1990 as a biological control agent against water hyacinth, no post-release evaluations have yet been conducted here. The impact of N. albiguttalis on water hyacinth growth was quantified under low-, medium- and high-nutrient concentrations in a greenhouse experiment. Niphograpta albiguttalis was damaging to water hyacinth in all three nutrient treatments, but significant damage in most plant parameters was found only under high-nutrient treatments. However, E. crassipes plants grown in high-nutrient water were healthier, and presumably had higher fitness, than plants not exposed to herbivory at lower-nutrient levels. Niphograpta albiguttalis is likely to be most damaging to water hyacinth in eutrophic water systems, but the damage will not result in acceptable levels of control because of the plant's high productivity under these conditions. Niphograpta albiguttalis is a suitable agent for controlling water hyacinth infestations in eutrophic water systems, but should be used in combination with other biological control agents and included in an integrated management plan also involving herbicidal control and water quality management.     

Impact of nutrients and herbivory by Eccritotarsus catarinensis on the biological control of water hyacinth,Eichhornia crassipes

Many water hyacinth infestations in South Africa are the symptom of eutrophication, and as a result, biological control of this weed is variable. This study examined the effects of herbivory by the mirid, Eccritotarsus catarinensis, on water hyacinth grown at high, medium and low nitrogen (N) and phosphorus (P) nutrient concentrations. Water nutrient concentration appears to be the overriding factor affecting plant growth parameters of water hyacinth plants—at high nutrient concentrations, leaf and daughter plant production were more than double than at low nutrient concentrations, while stem length was twice as great at high nutrient concentrations compared to low concentrations. Chlorophyll content was also twice as high at high nutrient concentrations than low concentrations. Conversely, flower production at high nutrient concentrations was less than half that at low concentrations. Herbivory by E. catarinensis did not have as great an effect on water hyacinth vigour as nutrient concentration did, although it significantly reduced the production of daughter plants by 23 ± 9%, the length of the second petiole by 13 ± 5%, and chlorophyll content of water hyacinth leaves by 15 ± 6%. In terms of insect numbers, mirids performed better on plants grown under medium nutrient conditions (99 ± 28 S.E.), compared to high nutrient concentrations (52 ± 27 S.E.), and low nutrient concentrations (25 ± 30 S.E.). Thus, these results suggest that the fastest and most significant reduction in water hyacinth proliferation would be reached by lowering the water nutrient concentrations, and herbivory by E. catarinensis alone is not sufficient to reduce all aspects of water hyacinth vigour, especially at very high nutrient concentrations.     

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L⁻¹) and phosphorus (0.2–0.01 mg L⁻¹) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L⁻¹ N and 20 mg L⁻¹ P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.     

Fungi associated with Eichhornia crassipes in South Africa and their pathogenicity under controlled conditions

Eichhornia crassipes Mart. Solms-Laubach (Pontederiaceae), water hyacinth, continues to be the world's worst aquatic weed. In South Africa, considerable research has been conducted on biological control agents associated with water hyacinth, with the release of six arthropods and one fungus, but little is known about the occurrence and impacts of native phytopathogenic fungi. Nation-wide surveys were conducted in 2010 and 2011 on various aquatic bodies of South Africa to identify the fungal pathogens associated with water hyacinth. Diseased plant parts were collected and fungi were isolated and identified. Some 250 isolates belonging to more than 25 genera were collected. Some of these represent new host records, as well as undescribed taxa. Isolates of Acremonium zonatum (Sawada) Gams, Alternaria eichhorniae Nag Raj and Ponnappa, Bipolaris hawaiiensis (M.B. Ellis) Uchida and Aragaki, Fusarium Link, Myrothecium roridum Tode ex Fr. and Ulocladium sp., showed the highest pathogenicity and have the potential to be useful in complementing the ongoing biocontrol programme on water hyacinth in South Africa.     

Plant community dynamics relative to the changing distribution of a highly invasive species, <Emphasis Type="Italic">Eichhornia crassipes</Emphasis>: a remote sensing perspective

Eradicating or controlling invasive alien species has frequently had unintended consequences, such as proliferation of other invasive species or loss of ecosystem function. We explore this problem using a case study of a highly invasive floating aquatic macrophyte, water hyacinth (Eichhornia crassipes), in the Sacramento-San Joaquin Delta of California. We used 5 years of remote sensing data to perform change detection analysis to study plant community dynamics contemporaneous with changes in water hyacinth cover. Our results show that as water hyacinth cover decreased, submerged aquatic plant (SAP) cover increased and vice versa. This effect was strongest in large patches of water hyacinth. We found no evidence that the native floating aquatic species, pennywort (Hydrocotyle umbellata), benefitted from reducing cover of water hyacinth. In most years, pennywort cover either showed no trend or followed the same trajectory as water hyacinth cover. In this study a decrease in cover of water hyacinth most often resulted in colonization by SAP species with some habitat returning to open water.     

Predicting the distribution of Eccritotarsus catarinensis, a natural enemy released on water hyacinth in South Africa

Water hyacinth [Eichhornia crassipes (Mart.) Solms (Pontederiaceae)] is the most damaging aquatic weed in South Africa, where five arthropod biological control agents have been released against it. The most recent introduction of Eccritotarsus catarinensis (Carvalho) (Heteroptera: Miridae) has failed to establish permanent populations at a number of sites in South Africa where water hyacinth is a problem. Cold winter temperatures at these sites are assumed to be the reason for these establishment failures. This assumption was tested by investigating the thermal physiology of the mirid, then incorporating these data into various predictive distribution models. Degree‐day models predict 3–14 generations per year at different localities in South Africa, and five generations at a Johannesburg site where the mirid failed to overwinter. The inability to develop sufficiently rapidly during winter months may hinder overwintering of this insect, which was predicted to develop through only one generation during the winter months of April to August in Johannesburg. A CLIMEX model also showed that cold stress limits the mirid's ability to overwinter in the interior of the country, while determination of the lower lethal limit (–3.5 °C) and critical thermal minimum (1.2 ± 1.17 °C) also indicated that extreme temperatures will limit establishment at certain sites. It is concluded that E. catarinensis is limited in its distribution in South Africa by low winter temperatures.     

Megamelus scutellaris (Hemiptera: Delphacidae), a biocontrol agent of water hyacinth,is not sufficiently specific for release in Australia

Water hyacinth Eichhornia crassipes (Pontederiaceae) is one of the world's worst invasive species, responsible for damaging aquatic systems in many warmer parts of the globe including north America, Africa, Asia and Australia. The planthopper Megamelus scutellaris Berg (Delphacidae) has been released in USA and approved for release in South Africa for biocontrol of water hyacinth. We assessed this agent for suitability for release in Australia and found that a related native aquatic plant, Monochoria cyanea (Pontederiaceae) is within the fundamental host range of this insect. Adult survival, oviposition and development of nymphs to adult was equally high on M. cyanea as on the target species, although the quality of these next generation adults was lower than those reared on the target species. This demonstrates that M. scutellaris is not sufficiently specific for release in Australia. Nymphal development to adults occurred only in very low numbers on the three other Australian species of Monochoria. M. cyanea only occurs in Australia so M. scutellaris is still a possible water hyacinth biocontrol candidate for other regions depending on the results of assessment of the risk to local species of Monochoria. This study demonstrates the effectiveness of modern biocontrol agent assessment and reinforces the importance of testing of local non-target species.     

Assessing density–damage relationships between water hyacinth and its grasshopper herbivore

Plants are variable in their responses to insect herbivory. Experimental increases in densities of phytophagous insects can reveal the type of plant response to herbivory in terms of impact and compensatory ability. The relationship between insect density and plant damage of a grasshopper, Cornops aquaticum Brüner (Orthoptera: Acrididae: Tetrataeniini), a candidate biological control agent, and an invasive aquatic plant, water hyacinth, Eichhornia crassipes Mart. Solms‐Laubach (Pontederiaceae), was investigated to assess potential damage to the weed. The impact of different densities of male and female grasshoppers on E. crassipes growth parameters was determined in a quarantine glasshouse experiment. Damage curves indicated that the relationship between plant biomass reduction and insect density was curvilinear whereas leaf production was linear. Female C. aquaticum were more damaging than males, causing high rates of plant mortality before the end of the trial at densities of three and four per plant. Feeding by C. aquaticum significantly reduced the total plant biomass and the number of leaves produced, and female grasshoppers caused a greater reduction in the number of leaves produced by water hyacinth plants than males. Grasshopper herbivory suppressed vegetative reproduction in E. crassipes, suggesting C. aquaticum could contribute to a reduction in the density and spread of E. crassipes infestations. The results showed that E. crassipes vigour and productivity decreases with an increase in feeding intensity by the grasshopper. Cornops aquaticum should therefore be considered for release in South Africa based on its host specificity and potential impact on E. crassipes.     

The effect of two biological control agents, the weevil Neochetina eichhorniae and the mirid Eccritotarsus catarinensis on water hyacinth, Eichhornia crassipes , grown in culture with water lettuce, Pistia stratiotes

We assessed the effect of two biological control agents, the mirid Eccritotarsus catarinensis (Carvalho) and the weevil Neochetina eichhorniae (Warner), singly or in combination, on the competitive ability of their host plant, water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub., grown in a screen house, in competition with another aquatic plant (Pistia stratiotes L.). Water hyacinth plant growth characteristics measured included fresh weight, leaf and petiole lengths, number of inflorescences produced, and new shoots. Without herbivory, water hyacinth was 18 times more competitive than water lettuce (across all experimental combinations of initial plant densities), as estimated from fresh weights. Both insect species, singly or in combination, reduced water hyacinth plant growth characteristics. E. catarinensis alone was less damaging than the weevil and under normal conditions, i.e., floating water hyacinth, is not expected to increase control of water hyacinth beyond that of the weevil. When combined with the weevil, half the inoculum of weevils and half the inoculum of mirids produced the same growth reduction as the full inoculum of the weevil. Under conditions where the weevils are not effective because water hyacinths are seasonally rooted in mud, the mirid, which lives entirely on leaves, should become a useful additional biological control agent. Handling Editor: John Scott.     

Decomposition of macrophyte litter in a subtropical constructed wetland in south Florida (USA)

The South Florida Water Management District has constructed large treatment wetlands (stormwater treatment areas (STAs)) to reduce total phosphorus concentrations in agricultural runoff before this water enters the Everglades. An important component of nutrient removal and storage in these systems is incorporation of nutrients into aquatic macrophytes and burial of this biomass in the sediments. However, decomposition of plant biomass before burial returns nutrients to the water column and may reduce STA treatment efficiency. As part of research on biogeochemical control of STA performance, we conducted a summer (July–September) and a long-term (12-month) experiment (February–February) that measured decomposition rates and release of chemical constituents from dominant aquatic macrophytes in a constructed wetland located in south Florida. The rank order of mean decomposition rates was Najas/Ceratophyllum (0.0568 d⁻¹) > Pistia (0.0508 d⁻¹) > Eichhornia (0.0191 d⁻¹) > submerged Typha (0.0059 d⁻¹) > aerial Typha (0.0008 d⁻¹). Summer decomposition rates were generally higher than rates from the long-term experiment, which suggested a temperature effect. Decomposition rates were negatively correlated with litter C:N and C:P molar ratios and cellulose and lignin content and positively correlated with N and P content. There was no significant difference in decomposition rates among sampling stations despite the fact that there was a decreasing gradient in water column inorganic phosphorus and nitrogen concentrations at these sites. Relatively little of the initial P mass remained in the litter of all species, except Typha, by the end of both experiments. First-order decomposition models derived using nonlinear regression generally had explanatory power, i.e. accounted for variance, comparable to more complex decreasing-coefficient models. Decomposition rates for the species examined in this study were within the range of published values when comparisons were made either by species or by plant group.     

Catchment context and the bottom‐up regulation of the abundance of specialist semi‐aquatic weevils on water hyacinth

1. Bottom‐up regulation is prevalent in plant–herbivore interactions and is thought to be particularly important in the case of aquatic plants and their specialist insect herbivores. 2. Recently published mesocosm studies have shown that the abundance of specialist Neochetina weevils, N. bruchi and N. eichhorniae, on water hyacinth (Eichhornia crassipes) are principally under the influence of nutrients in plant tissues. 3. We examined historical patterns of the abundance of these species of semi‐aquatic weevils in two water bodies from catchments with significantly different nutrient loads in subtropical Australia to test the validity of the published conceptual model of bottom‐up regulation. 4. Our results revealed that these weevils are indeed under bottom‐up regulation under field conditions. However, the nature of this regulation appears to be influenced by the broader catchment context of the water hyacinth‐infested water body, with plant tissue nutrients influencing weevil abundance more in the catchment with lower nutrient run‐offs. 5. Our findings reaffirm the importance of bottom‐up regulation in plant–insect interactions, add to the growing evidence of indirect effects spanning terrestrial and aquatic ecosystems, and inform management of water hyacinth using these weevils as biocontrol agents.     

Herbivory on aquatic vascular plants by the introduced golden apple snail (<Emphasis Type="Italic">Pomacea canaliculata</Emphasis>) in Lao PDR

The effect of naturally found densities of the exotic and herbivorous golden apple snail (Pomacea canaliculata) on three dominant aquatic plants – duckweed (Lemna minor), water hyacinth (Eichhornia crassipes) and morning glory (Ipomoea aquatica) – was assessed in a wetland survey and quantified in a field experiment in Laos in southeast Asia. Snail grazing reduced plant biomass, but plant species were differently affected by grazing. Duckweed had almost disappeared after 6 and water hyacinth after 21 days, whereas morning glory remained at 80% of initial biomass after 32 days. Snail growth was lowest on morning glory and, when all plant species were simultaneously presented to snails, this plant was not preferred. We suggest that the negative effect the golden apple snail had on the growth of these plant species in field enclosures is present in the natural environment as well. This new and intense herbivory could have serious negative effects on invaded freshwater ecosystems in this region.     

Water hyacinth,Eichhornia crassipes (Pontederiaceae), reduces benthic macroinvertebrate diversity in a protected subtropical lake in South Africa

The socio-economic impacts of the free-floating aquatic plant water hyacinth, Eichhornia crassipes (Pontederiaceae), on aquatic systems are well documented, yet the impacts on aquatic biodiversity, particularly invertebrate biodiversity, are less well understood. This study aimed to determine whether the presence of water hyacinth altered the diversity and assemblage structure of benthic macroinvertebrates in a conservation area. The benthic macroinvertebrate assemblage was sampled over 1 year at five sites under water hyacinth mats and at five sites without water hyacinth at Lake Nsezi—Nseleni River in the vicinity of Richards Bay, KwaZulu-Natal, South Africa. Artificial substrates were placed beneath water hyacinth mats or in the open water to allow for colonization by freshwater macroinvertebrates, and left for a period of 6 weeks, repeated on seven occasions. Twenty nine families comprising 18,797 individuals were collected, 817 (13 families) individuals were from under water hyacinth mat sites compared to 17,980 (27 families) individuals from open water sites. Ninety-eight percent of individuals collected were, however, the invasive snail, Tarebia granifera. Open water samples were separated from samples beneath the water hyacinth mat by non-metric Multidimensional Scaling, indicating reduced biodiversity associated with the presence of water hyacinth. Exclusion of the dominant Thiaridae from the analysis did not alter the groupings. Family richness(s) and abundance (N) were significantly higher in open water communities(S: H₃ = 21.09; P = 0.0001; N: H₃ = 22.58; P = 0.00001), while evenness (J’) was higher under water hyacinth mats (H₃ = 20.13; P = 0.0002). The presence of water hyacinth had a significantly negative impact on aquatic macroinvertebrate biodiversity in a conservation area, and therefore the control of this invasive aquatic plant must play a major role in catchment management.     

Field assessment of a frond-feeding weevil, a successful biological control agent of red waterfern, Azolla filiculoides, in southern Africa

Azolla filiculoides (red waterfern) is a small, floating fern native to South America, that has invaded aquatic habitats, predominantly water resevoirs in southern Africa. A frond-feeding weevil, Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae), was imported from Florida, USA, and released as a biological control agent against this weed in South Africa at the end of 1997. To date, 24,700 weevils have been released, which has resulted in local extinction of red waterfern at 81% of the 112 release sites. The weevil has not failed to control a single site. Several sites were, however, lost due to flooding or drainage of dams. The surface area of weed controlled totalled 203.5 ha. On average, A. filiculoides was controlled in infested sites in 6.9 (±4.3) months. The weed recolonized at 22 of the sites (through either spore germination or dispersal by waterfowl), but the weevils subsequently spread to all of these sites and successfully caused local extinction of the weed at 18 of the sites. Five years after the release of the weevil, the weed no longer poses a threat to aquatic systems in southern Africa. In comparison to other biological control programs of aquatic weeds, the program against A. filiculoides in southern Africa ranks among the most successful cases anywhere in the world.     

Economic evaluation of water loss saving due to the biological control of water hyacinth at New Year’s Dam,Eastern Cape province,South Africa

Water hyacinth Eichhornia crassipes is considered the most damaging aquatic weed in the world. However, few studies have quantified the impact of this weed economically and ecologically, and even fewer studies have quantified the benefits of its control. This paper focuses on water loss saving as the benefit derived from biological control of this plant between 1990 and 2013 at New Year’s Dam, Alicedale, Eastern Cape, South Africa. Estimates of water loss due to evapotranspiration from water hyacinth vary significantly; therefore, the study used three different rates, high, medium and low. A conservative raw agriculture value of R 0.26 per m³ was used to calculate the benefits derived by the water saved. The present benefit and cost values were determined using 10% and 5% discount rates. The benefit/cost ratio at the low evapotranspiration rate was less than one, implying that biological control was not economically viable but, at the higher evapotranspiration rates, the return justified the costs of biological control. However, at the marginal value product of water, the inclusion of the costs of damage to infrastructure, or the adverse effects of water hyacinth on biodiversity, would justify the use of biological control, even at the low transpiration rate.     

Biomass and nutrient stock of submersed and floating macrophytes in shallow lakes formed by rewetting of degraded fens

Ecosystem restoration by rewetting of degraded fens led to the new formation of large-scale shallow lakes in the catchment of the River Peene in NE Germany. We analyzed the biomass and the nutrient stock of the submersed (Ceratophyllum demersum) and the floating macrophytes (Lemna minor and Spirodela polyrhiza) in order to assess their influence on temporal nutrient storage in water bodies compared to other freshwater systems. Ceratophyllum demersum displayed a significantly higher biomass production (0.86–1.19 t DM = dry matter ha⁻¹) than the Lemnaceae (0.64–0.71 t DM ha⁻¹). The nutrient stock of submersed macrophytes ranged between 28–44 kg N ha⁻¹ and 8–12 kg P ha⁻¹ and that of floating macrophytes between 14–19 kg N ha⁻¹ and 4–5 kg P ha⁻¹ which is in the range of waste water treatment plants. We found the N and P stock in the biomass of aquatic macrophytes being 20–900 times and up to eight times higher compared to the nutrient amount of the open water body in the shallow lakes of rewetted fens (average depth: 0.5 m). Thereafter, submersed and floating macrophytes accumulate substantial amounts of dissolved nutrients released from highly decomposed surface peat layers, moderating the nutrient load of the shallow lakes during the growing season from April to October. In addition, the risk of nutrient loss to adjacent surface waters becomes reduced during this period. The removal of submersed macrophytes in rewetted fens to accelerate the restoration of the low nutrient status is discussed.     

A first report of water hyacinth (Eichhornia crassipes) soil seed banks in South Africa

We investigated water hyacinth seed banks in several aquatic systems in South Africa. Fifteen sites, mainly in the Eastern and Western Cape provinces, were surveyed from August to October 2009. Soil seed density varied between 0 and 2534 seeds/m² but did not differ significantly between the type of waterbody (impoundment vs. river) or the history of control carried out at a site. Average germination was 54.17% with very fast velocities (Vigour Index = 36.66) and maximum germination around three days. Although we demonstrated the existence of an important reservoir of seeds, results from this study indicated that a combination of factors such as water fluctuation, eutrophication and seed decomposition might have had a great influence on dispersal and persistence of seeds.     

Biomass production and nutrient accumulation in <Emphasis Type="Italic">Sparganium emersum</Emphasis> Rehm. after sediment treatment with mineral and organic fertilisers in three mesocosm experiments

The response of the aquatic plant Sparganium emersum to different sediment nutrient levels was studied in three mesocosm experiments. The aim was to assess plant growth parameters and nutrient accumulation in the plant tissue under conditions relevant for habitats with sediments affected by anthropogenic nutrient enrichment. The experimental treatments were produced by fertilisation of the rooting medium (washed river sand) with differing doses of either NPK mineral fertiliser or digested sludge from solid pig slurry waste. Growth inhibition by high nutrient levels was not observed in any treatment (highest nutrient concentrations in the sediment with mineral fertiliser: N 250 mg kg⁻¹, P 50 mg kg⁻¹; organic fertiliser: N 6300 mg kg⁻¹, P 1800 mg kg⁻¹), which confirms the tolerance of S. emersum to high nutrient loads. The sediment nutrient concentration was best reflected in shoot dry mass. Nutrient contents in plant tissues were similar for most nutrient concentrations in the rooting media; only N increased significantly with N levels in the sediment in belowground parts. Nutrient standing stocks in plants, however, generally corresponded to the nutrient supply, and reached highest values (max. N 3.7 g m⁻², P 1.2 g m⁻²) in the richest treatments with organic fertiliser. The capability of S. emersum to use nutrients from high sediment concentrations and in organically polluted environments recommends this species for use in water quality management including tertiary wastewater treatment.     

Toxic effect of herbicides used for water hyacinth control on two insects released for its biological control in South Africa

The integrated control of water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) has become necessary in South Africa, as biological control alone is perceived to be too slow in controlling the weed. In total, seven insect biological control agents have been released on water hyacinth in South Africa. At the same time, herbicides are applied by the water authorities in areas where the weed continues to be troublesome. This study investigated the assumption that the two control methods are compatible by testing the direct toxicity of a range of herbicide formulations and surfactants on two of the biological control agents released against water hyacinth, the weevil, Neochetina eichhorniae Warner (Coleoptera: Curculionidae) and the water hyacinth mirid, Eccritotarsus catarinensis (Carvalho) (Hemiptera: Miridae). A number of the formulations used resulted in significant mortality of the mirid and the weevil. Products containing 2,4-D amine and diquat as active ingredients caused higher mortality of both agents (up to 80% for the mirid) than formulations containing glyphosate. Furthermore, when surfactants were added to enhance herbicide efficiency, it resulted in increased toxicity to the insects. We recommend that glyphosate formulations should be used in integrated control programmes, and that surfactants be avoided in order to reduce the toxic nature of spray formulations to the insect biological control agents released against water hyacinth.     

Contrasting species responses to continued nitrogen and phosphorus addition in tropical montane forest tree seedlings

Global changes in nutrient deposition rates are likely to have profound effects on plant communities, particularly in the nutrient‐limited systems of the tropics. We studied the effects of increased nutrient availability on the seedlings of six tree species in montane forests of southern Ecuador in situ. After five years of continued N, P, or N+P addition, naturally grown seedlings of each of the two most common species at each elevation (1000, 2000, and 3000 m asl) were harvested for analyses of leaf morphology, nutrient content, herbivory, and tissue biomass allocation. Most species showed increased foliar N and P concentrations after addition of each respective element. Leaf tissue N:P ratios of >20 in the control plants of all species suggest that P is more growth‐limiting in these forests than N. Leaf morphological responses to nutrient addition were species and nutrient specific, with some species (Hedyosmum purparescens, Graffenrieda emarginata) exhibiting increased specific leaf area (SLA), and others (Graffenrieda harlingii) increased leaf area ratios (LAR). Pouteria torta (1000 m) had lower SLA and LAR after P addition. Increased herbivory was only evident in G. emarginata (after N and N+P addition). Only the species from 3000 m asl modified biomass allocation after nutrient addition. In general, N and N+P addition more strongly affected the species studied at the upper elevations, whereas P addition had a similar range of effects on the species at all elevations. We conclude that the responses of the studied tropical montane forest tree seedlings to chronic N and P addition are highly species‐specific and that successful adaptation to increased nutrient availability will depend on species‐specific morphological and physiological plasticity.