Nutrient Patches Research Articles

Influence of orographically enhanced SW monsoon flux on coastal processes along the SE Arabian Sea

[1] The Arabian Sea has an excess evaporation over precipitation regime. The southeast Arabian Sea is, however, anomalous because it has ∼2800–4800 mm rainfall during the southwest monsoon (SWM). Despite a high rainfall, the fluvial influence on supply of total suspended matter (TSM) and marine productivity is deemed insignificant and remains unevaluated. We evaluated the poorly resolved influence of fluvial influx on shelf processes. We documented low salinity, stratification, high productivity, and turbidity over the entire inner shelf (chlorophyll-a ∼2.4 mg m−3; PO43− >2.5 μM; NO3− 6.8–2.1 μM; TSM 88–182 mg l−1; salinity nearshore region 26; offshore region 33.8–34.6 practical salinity units). The deeper regions (>40 m), however, had greatly reduced TSM and nutrient levels (NO3− and PO4−3 <1.0 μM), TSM (<24 mg l−1), and patches of high chlorophyll-a. Upon cessation of the SWM, nutrient levels and TSM reduced considerably. We identify two processes that contribute to the marine productivity and turbidity during the SWM. Over the deeper regions, there is a poor influence of fluvial supply and upwelling regulates productivity. Over the shallow inner shelf, the sequestering of fluvial influx due to the prevalence of strong winds, upwelling, and equatorward flow is dominant. The later processes induced high marine productivity and eutrophication in the nearshore region and may have implications for siltation of channels. Reduced turbidity, nutrient and chlorophyll-a levels, and higher salinity of the coastal waters during rest of the year imply a substantial role of fluvial fluxes on coastal processes.

Microscale patchiness leads to large and important intraspecific internal nutrient heterogeneity in phytoplankton

Phytoplankton stoichiometry or nutrient content has been shown to vary in a number of dimensions (species, condition, time, space), but the heterogeneity within a species at a given time and location, and the underlying mechanisms and importance have not been explored. There are a number of mechanisms that can create intraspecific heterogeneity, and theory suggests it can affect the population growth rate. We studied heterogeneity in P content of the freshwater diatom Cyclotella meneghiniana in the Charles River in Boston. Single-cell observations using synchrotron-based X-ray fluorescence show that the nutrient status varies from P-starved to P-replete. We simulate individual cells using an agent-based model that accounts for a number of mechanisms that can create heterogeneity, including surface area–based uptake, mortality differentiation, stochastic biological variability in states and behavior, macroscale mixing, and microscale nutrient patch encounter. By performing a number of simulations with various mechanisms turned on/off and comparing to data, we conclude that the heterogeneity is mostly due to microscale patchiness (85%). We explore the importance of accounting for heterogeneity in models by performing a simulation with the growth rate based on the population-average internal nutrient, as is done in conventional population-level models. This shows that ignoring heterogeneity increases the population growth rate by a factor of 1.47. To account for different heterogeneity in the laboratory and field, population-level ecosystem models should reduce maximum growth rates. The magnitude of this correction depends on local conditions, and in our case, it is a factor of 0.72.

Effects of different organic materials and mineral nutrients on arbuscular mycorrhizal fungal growth in a Mediterranean saline dryland

Arbuscular mycorrhizal (AM) fungi are root symbionts that enhance plant growth and improve soil fertility and soil structure in drylands. Even though AM fungi are obligate biotrophs, organic matter (OM) can stimulate their growth, but the mechanisms behind this are still unknown. Here, we compared the effect of nutrient patches of different OM sources to intrinsic components of OM such as inorganic nutrient supply and an improved soil water-holding capacity (WHC; via application of hydrophilic polymers), on AM fungal growth. Fatty acids extracted from in-growth mesh bags incubated in the field were used as biomarkers for AM fungi and other soil microbes. We found an enhancement of AM fungal growth in certain nutrient patches. Two out of three OM types stimulated AM fungal growth strongly, and also the addition of inorganic nutrients enhanced AM fungi, though to a lesser extent than OM. Enhanced soil WHC, on the other hand, did not influence AM fungal growth. AMF were more strongly enhanced by the mineral nutrients relative to other soil organisms. Intrinsic nutrients might be an important factor for AMF growth stimulation in OM additions, but there was no evidence that nutrients alone can explain this phenomenon.

Landscape nutrition: seeing the forest instead of the trees

Recent theories suggest that herbivores forage across many scales and that foraging decisions are driven by the distribution of nitrogen. However, experimental tests of these predictions across large landscapes are rare and difficult. Pretorius et al. (2011) present an elegant experimental design to test how patch size, local nutrient density and total nutrient load are detected by foraging African elephants (Loxodonta africana) in the Colophospermum mopane shrub veld in South Africa. This experiment should serve as a model for investigations of how herbivores detect and respond to high nutrient patches of different size; it also raises questions for further research, such as the fate of high nutrient patches as elephants disperse nutrients from them in urine, faecal material and carcasses deposited elsewhere in the landscape.

EXPLOITATION OF PHOSPHORUS PATCHES WITH DIFFERENT PHOSPHORUS ENRICHMENT BY THREE ARBUSCULAR MYCORRHIZAL FUNGI

The effect of three arbuscular mycorrhizal (AM) fungi on phosphorus (P) nutrient activation and acquisition by maize from spatially heterogeneous sand was investigated using dual-mesh packages enriched with different P concentrations and compared with non-mycorrhizal cotrols. As would be expected the AM fungi significantly enhanced leaf photosynthetic rate and the biomass and P concentrations in shoots and roots. All three fungi (Glomus intraradices, Glomus mosseae and Glomus etunicatum) displayed the capacity to dissolve inorganic P and promoted P nutrient availability in the packages (P patches). G. etunicatum showed the largest effect comparing with Glomus intraradices and Glomus mosseae, particularly in packages with high concentrations of P. Possible mechanisms involved include the acidification of the P patches by the AM fungi, promotion of the dissolution of the P, and more marked effects of the three fungal isolates with increasing enrichment of P in the patches. Inoculation with G. etunicatum resulted in greater acidification compared to the other two fungi. We conclude that AM fungi can promote P availability by acidifying the soil and consequently exploiting the P in nutrient patches and by facilitating the growth and development of the host plants.

Contrasting root behaviour in two grass species: a test of functionality in dynamic heterogeneous conditions

Root systems are highly plastic as they express a range of responses to acquire patchily distributed nutrients. However, the ecological significance of placing roots selectively in nutrient hotspots is still unclear. Here, we investigate under what conditions selective root placement may be a significant functional trait that determines belowground competitive ability. We studied two grasses differing in root foraging behaviour, Festuca rubra and Anthoxanthum odoratum. The plants were grown in stable and more dynamic heterogeneous environments, by switching nutrient patches halfway through the experiment. A. odoratum was a factor of two less selective in placing its roots into nutrient-rich patches than F. rubra. A. odoratum produced overall higher root length densities with higher specific root length than F. rubra and acquired more nutrients. A. odoratum appeared to be the superior competitor, irrespective of the nutrient dynamics. Our results suggest that root behaviour consisting of producing high root length densities at relatively low biomass investments can be a more effective foraging strategy than placing roots selectively in nutrient hotspots. When understanding the functionality of root traits among different species, specific root length may play a key role.

Trophic interactions in an arid ecosystem: From decomposers to top-predators

In this review we compile results on interactions above and below ground and food web functioning in an arid environment at the Baza Basin, in the Iberian southeast. Our data reveal that herbivory is difficult to estimate in our system. Some herbivores (rabbits) and granivores ( Messor ants) create nutrient and detritus-rich patches, with important effects on the diversity and abundance of species both above and below ground. Fluctuations of prey availability, especially those caused by changes in their circadian rhythms, provoke seasonal migrations and omnivory in predators. We also present experimental evidence of the effects of interactions between aboveground and belowground macroarthropods on litter degradation; belowground detritivores are responsible for a high percentage of ground surface litter decomposition. We demonstrate that belowground herbivores and decomposers can dramatically change aboveground multitrophic interactions. Finally, we identify the biotic and abiotic factors controlling aboveground and belowground macrofaunal distribution at a broad scale. We conclude that trophic interactions in this semi-arid area are numerous and complex with many of the interactions involving more than two or three organisms. The interactions between above- and belowground organisms added complexity to this system. These habitats in which organisms deal with extreme abiotic conditions promote odd interactions resulting in an increase of biodiversity. Finally, despite the large research effort devoted to understanding food web structure and dynamics in this area recently, our state of knowledge is still far from providing a complete picture of interactions and their implications in the regulation and functioning of the system.

Responses of nitrogen transformation and microbial community composition to nitrogen enrichment patch

Fertilization produces many nutrient patches that have been confirmed to affect root growth. However, it is not clear how nutrient transformation and microbial community composition are affected in an inorganic nutrient patch. In this experiment, a nitrogen enrichment patch was formed by the diffusion of a urea fertilizer layer in a specially-designed container. Responses of nitrogen transformation and microbial community composition to the nitrogen enrichment patch were investigated at different incubation times. Results showed that nitrogen status and microbial community composition were slightly affected in the control patch (CK patch). In the nitrogen enrichment patch, however, soil pH was significantly increased in most soil layers close to the urea fertilizer layer; NO2−–N was the predominant form of mineral N, and its transformation to NO3−–N was delayed. Microbial community composition shifted significantly, especially before day 28 of incubation. Principal components analysis (PCA) of phospholipid fatty acids (PLFAs) patterns showed that the microbial community presented different sensitivity to high nitrogen concentration. Fungi (18:2ω6,9) showed the least sensitivity to high concentrations of NO2−–N and NO3−–N. Gram-positive bacteria showed the most sensitivity to NO2−–N. Gram-negative bacteria (cy17:0, cy19:0, 18:1ω9, and 18:1ω7) and actinomycetes (10Me17:0 and 10Me18:0) presented similar responses to NO2−–N and NO3−–N. Results of this study indicate that changes in nitrogen transformation and microbial community composition are likely to occur in nitrogen enrichment patches, but the extent of those changes depend on the microbial species and the distance of soil layers from the urea layer.

Responses to mineral nutrient availability and heterogeneity in physiologically integrated sedges from contrasting habitats

Clonal plants from poor habitats benefit less from morphologically plastic responses to heterogeneity than plants from more productive sites. In addition, physiological integration has been suggested to either increase or decrease the foraging efficiency of clonal plants. We tested the capacity for biomass production and morphological response in two closely related, rhizomatous species from habitats that differ in resource availability, Carex arenaria (from poor sand dunes) and C. disticha (from nutrient-richer, moister habitats). We expected lower total biomass production and reduced morphological plasticity in C. arenaria, and that both species would produce more ramets in high nutrient patches, either in response to signals transported through physiological integration, or by locally determined responses to nutrient availability. To investigate mineral nutrient heterogeneity, plants were grown in boxes divided into two compartments with homogeneous or heterogeneous supply of high (H) or low (L) nutrient levels, resulting in four treatments, H-H, H-L, L-H and L-L. Both C. arenaria and C. disticha produced similar biomass in high nutrient treatments. C. disticha responded to high nutrients by increased biomass production and branching of the young parts and by altering root:shoot ratio and rhizome lengths, while C. arenaria showed localised responses to high nutrients in terms of local biomass and branch production in high nutrient patches. The results demonstrated that although it has a conservative morphology, C. arenaria responded to nutrient heterogeneity through morphological plasticity. An analysis of costs and benefits of integration on biomass production showed that young ramets of both species benefited significantly from physiological integration, but no corresponding costs were found. This suggests that plants from resource-poor but dynamic habitats like sand dunes respond morphologically to high nutrient patches. The two species responded to nutrient heterogeneity in different traits, and this is discussed in terms of local and distant signalling of plant status.

The capacity for nitrate regulation of root hydraulic properties correlates with species’ nitrate uptake rates

A mechanism whereby water flow towards root surfaces is stimulated when exposed to nutrient patches may be evolutionarily desirable in environments with heterogeneous soils. Indeed, the presence of nitrate has been shown to increase root hydraulic conductance in a few agricultural species characterized by high nitrate demand. Does a similar stimulation of root conductivity in response to external nitrate addition exist among wild type organisms? To answer this question, we studied and compared the effect of a sudden increase of nitrate concentration on root hydraulic properties in seven species. These included three agricultural species (Cucumis sativus L., Solanum esculentum L., Zea mays L.) and four wild type species (Arabidopsis thaliana L., Festuca arundinacea Schreb., Populus trichocarpa Torr.&Gray, Nephrolepis exaltata L.). The selected species differed in overall nitrate demand and specific nitrate uptake rates. Changes in root hydraulic conductance induced by nitrate varied from non-existent (N. exaltata L.) to more than 50% increases (Z. mays L.). The magnitude of the hydraulic response to nitrate presence was significantly correlated with species nitrate uptake rate. This finding suggests an emergent physiological trait that links together plant nitrate needs, nitrate availability and root hydraulic properties.

Phragmites australis (Common Reed) Invasion in the Rhode River Subestuary of the Chesapeake Bay: Disentangling the Effects of Foliar Nutrients, Genetic Diversity, Patch Size, and Seed Viability

The invasion and expansion of the introduced haplotype of Phragmites australis across North America is of growing concern. Previous studies in the Chesapeake Bay region found that Phragmites was more abundant, had higher foliar nitrogen, and produced more viable seeds in brackish wetland subestuaries with more anthropogenic development of the watershed. Here, we focus on a different scale and address issues related to the invasion of Phragmites within a single subestuary, the Rhode River. We evaluated patterns in seed viability, foliar nutrient concentrations, patch size, and genetic variation in ten Phragmites patches in wetlands that occur in the side of the subestuary that is surrounded by forest and 10 patches in wetlands that are in the side of the subestuary that has extensive anthropogenic development. Seed viability varied from 0–60% among the 20 patches but did not differ significantly between patches on the developed vs. forested sides of the Rhode River. Foliar nutrients also did not differ between patches on the two sides of the Rhode River. Seed viability, however, was negatively related to foliar nutrients. Most Phragmites patches consisted of >1 genotype. Larger patches had multiple genotypes, and patches with more genotypes produced more viable seeds. Our study indicates that the Rhode River subestuary behaves as one system with no differences in the measured Phragmites variables between the forested vs. developed sides of the watershed. Our findings also suggest a cyclical process by which Phragmites can spread: larger patches contain more genetic diversity, which increases the chances for cross-fertilization. The subsequent increased production of viable seeds can increase local levels of genetic diversity, which can further facilitate the spread of Phragmites by seed.

First come, first served: grasses have a head start on forbs with prompt nutrient patch occupation

Graminoids and forbs are important entities in grassland community assembly, differing in their functional properties. In our study, we asked 1. Do graminoids and forbs differ in the speed of root proliferation into soil patches established under field conditions? 2. Is the patch occupation dynamics affected by the nutrient concentration in the patch? 3. What is the temporal dynamics of available macronutrients in an experimental patch and does it provide comparative advantage to any of these two categories in connection with their root proliferation dynamics? We used ingrowth core technique. Proliferated roots were sampled after 1, 2, 4, 8, or 15 weeks, with forb and graminoid categories distinguished on anatomical basis. We measured root length and root dry weight, concentration of NH4+, NO3−, and of exchangeable phosphorus in the soil. Roots of both functional groups proliferated more intensively in enriched soil patches than in the control ones, but graminoids entered experimental patches more rapidly. Soil concetration of available N fell down to the background level in four weeks. The initial head start by graminoids seems to be crucial for the overall patch exploitation, because the concentration of available nutrient forms, namely nitrate and ammonium, decreases rapidly.

Root decisions

Root systems have recognizable developmental plans when grown in solution or agar; however, these plans often must be modified to cope with the prevailing conditions in the soil environment such as the avoidance of obstacles and the exploitation of nutrient-rich patches or water zones. The modular structure of roots enables them to respond to their environment, and roots are very adaptive at modifying growth throughout the root system to concentrate their efforts in the areas that are the most profitable. Roots also form associations with microorganisms as a strategy to enhance resource capture. However, while the responses of roots in nutrient patches are well-recognized, overall 'rules of response' and variation in strategy among plant species that can be applied in a number of different environments are still lacking. Finally, there is increasing evidence that root-root interactions are much more sophisticated than previously thought, and the evidence for roots to identify self from non-self roots will be briefly discussed.

Does productivity drive diversity or vice versa? A test of the multivariate productivity–diversity hypothesis in streams

The idea that productivity regulates species diversity is deeply ingrained in the field of ecology. Yet, over the past few decades, an increasing number of experiments have shown that species diversity controls, rather than simply responds to, biomass production. These contrasting perspectives have led to a seeming paradox: Is diversity the cause or the consequence of biological production? Here we present empirical evidence for the multivariate productivity-diversity (MPD) hypothesis, which argues that differing perspectives on productivity-diversity relationships can be resolved by recognizing that historical research has focused on how resource supply regulates both the productivity and richness of local competitors, whereas more recent studies have focused on how the richness of a colonist pool regulates the efficiency by which resources are captured and converted into new tissue. The MPD hypothesis predicts that three pathways operate concurrently to generate productivity diversity relationships in nature: (1) resource supply directly limits the standing biomass and/or rate of new production by primary producers, (2) producer biomass is directly influenced by the richness of species that locally compete for resources, and (3) resource supply rate indirectly affects producer biomass by influencing the fraction of species from a colonist pool that locally coexist. To examine whether this set of pathways explains covariation between productivity and diversity in natural streams, we used nutrient-diffusing agar "patches" to manipulate resource supply rates in 20 streams throughout the Sierra Nevada mountain range, California, USA. We then measured the fraction of periphyton species from the stream colonist pool co-occurring on each nutrient patch, as well as the standing biomass and rates of primary production. Natural patterns of covariation agreed with predictions of the MPD hypothesis. Algal biomass was an increasing function of nutrient supply, and an increasing function of local richness. The fraction of species from the colonist pool found co-occurring on a patch was a concave-down function of nutrient supply, causing nutrients to indirectly affect biomass via control over local richness. These results suggest that the MPD hypothesis is a viable explanation of patterns of diversity and productivity in natural stream ecosystems, and that it has potential to merge the historical view that productivity drives diversity with a parallel view that diversity drives productivity.

Enhancement of Bacterial Motility due to Speed-Dependent Absorption

Marine bacteria often reach high swimming speeds, either to take advantage of evanescent nutrient patches or to beat Brownian forces. Since this implies that a sizable part of their energetic budget must be allocated to motion, it is reasonable to assume that some bacteria are able to increase their nutrient intake by increasing their speed v. We formulate a model that uses the concept of internal energy depot originally developed by Schweitzer, Ebeling, and Tilch to investigate this hypothesis. We postulate that the nutrient absorption rate is of the form q(v) = q0 +Av, with q0 and A being constants. If the fraction c of energy spent non-mechanically is low, we find that there is a single stable velocity v1*, but if c is large, there is a critical value of A, Ac, below which only the v = 0 solution is stable. Above the bifurcation point Ac a second stable solution appears, whose value v2* increases monotonically with A. The mechanical efficiency of the molecular motors is also shown to increase with A. The description of the motion is further clarified by the use of the Fokker-Planck formalism. Solutions obtained using realistic parameter values indicate that the speed increase due to the enhanced nutrient absorption may be substantial.

Is Storage an Adaptation to Spatial Variation in Resource Availability?

When individuals store resources acquired while moving through a spatially variable habitat, a form of population structure arises. The theoretical consequences of this process for resource competition are studied for phytoplankton species consuming a single nutrient resource, using a Lagrangian modeling approach. Each competitor population is divided into many subpopulations that move through two model habitats with gradients in nutrient availability: an unstirred chemostat and a partially mixed water column. The results provide little indication that resource storage contributes to competitive fitness in the scenarios analyzed. Superior competitors usually reduce the limiting nutrient to low concentrations at steady state or sometimes have high maximal growth rates. Resource storage enhances competitive fitness in temporally variable habitats where encounters with rich nutrient pulses are strongly periodic. However, in purely spatially variable habitats where encounters with rich nutrient patches are random, resource storage does not appear to provide much benefit, at least for passively moving organisms that cannot control their location.

A microfluidic chemotaxis assay to study microbial behavior in diffusing nutrient patches

The nutrient environment experienced by planktonic microorganisms is patchy at spatiotemporal scales commensurate with their motility (µm – cm), and the efficiency with which chemotactic microbes can exploit this heterogeneous seascape influences trophodynamics and nutrient cycling rates in aquatic environments. Yet, methodological limitations have largely prevented direct examinations of microbial behavior within heterogeneous microenvironments. We used soft lithography to fabricate a microfluidic‐based chemotaxis assay to study the foraging response of aquatic microbes to diffusing microscale nutrient patches. The transparency, biocompatibility, and simplicity of microfluidic devices make them ideally suited for microbial ecology studies. A microinjector was used to create a 300 µm‐wide nutrient band, simulating a pulse release of solutes. The chemotactic response of microbes to the diffusing patch was measured at the population and single‐cell level. In contrast to traditional chemotaxis assays, this technique permits the assessment and quantification of chemotaxis toward a potential attractant in real time, enabling rapid screening of multiple chemicals. Furthermore, detailed information on chemotactic behavior can be obtained by tracking individual organisms. Here, we applied this microassay to study the chemotactic behavior of a range of aquatic microorganisms, including three marine bacterial isolates, a species of phagotrophic flagellate, and a species of phytoplankton. Each organism exhibited a rapid chemotactic response to a variety of chemical compounds, suggesting that many marine microbes are adapted to life within patchy microenvironments. The chemotaxis assay described here was found to be a flexible platform for studying both the specific case of microbes foraging within patchy habitats and as a broadly applicable tool for rapidly assessing and quantifying microbial chemotaxis.

Rapid chemotactic response enables marine bacteria to exploit ephemeral microscale nutrient patches

Because ocean water is typically resource-poor, bacteria may gain significant growth advantages if they can exploit the ephemeral nutrient patches originating from numerous, small sources. Although this interaction has been proposed to enhance biogeochemical transformation rates in the ocean, it remains questionable whether bacteria are able to efficiently use patches before physical mechanisms dissipate them. Here we show that the rapid chemotactic response of the marine bacterium Pseudoalteromonas haloplanktis substantially enhances its ability to exploit nutrient patches before they dissipate. We investigated two types of patches important in the ocean: nutrient pulses and nutrient plumes, generated for example from lysed algae and sinking organic particles, respectively. We used microfluidic devices to create patches with environmentally realistic dimensions and dynamics. The accumulation of P. haloplanktis in response to a nutrient pulse led to formation of bacterial hot spots within tens of seconds, resulting in a 10-fold higher nutrient exposure for the fastest 20% of the population compared with nonmotile cells. Moreover, the chemotactic response of P. haloplanktis was >10 times faster than the classic chemotaxis model Escherichia coli, leading to twice the nutrient exposure. We demonstrate that such rapid response allows P. haloplanktis to colonize nutrient plumes for realistic particle sinking speeds, with up to a 4-fold nutrient exposure compared with nonmotile cells. These results suggest that chemotactic swimming strategies of marine bacteria in patchy nutrient seascapes exert strong influence on carbon turnover rates by triggering the formation of microscale hot spots of bacterial productivity.

Evidence that ethylene signalling is not involved in selective root placement by tobacco plants in response to nutrient‐rich soil patches

Ethylene is a strong controller of root development, and it has been suggested that it is involved in the increase of lateral root development in nutrient-rich soil patches (selective root placement). Here, this contention was tested by comparing selective root placement of an ethylene-insensitive transgenic tobacco (Nicotiana tabacum) genotype (Tetr) with that of wild-type (Wt) plants. Wt and Tetr plants were grown in pots with locally increased nitrate or phosphate concentrations, after which the root growth patterns were compared with those of plants grown in pots with homogeneous nutrient distribution. Tetr plants responded to nutrient patches in a similar way to Wt plants, by placing their roots preferentially at locations with higher nutrient content, and other aspects of plant morphology were also not greatly influenced by ethylene insensitivity. The response of both Wt and Tetr plants to high-nitrate patches was considerably stronger than to locally high phosphate, suggesting that the two responses are not linked in any functional or regulatory way. As the response to nutrient patches was similar in ethylene-sensing and ethylene-insensitive plants, it is concluded that selective root placement in response to nitrate or phosphate is, at least in tobacco, not mediated or modified by ethylene action.

Top‐down and bottom‐up effects on the spatiotemporal dynamics of cereal aphids: testing scaling theory for local density

The relationship between density and area depends on local growth rates and the area‐dependence of migration rates. These rates vary among taxa due to dispersal behaviour, plot productivity and natural enemy impact. Previous studies in aphids suggest that aphid densities are highest in patches of intermediate sizes, and lower in small and large patches. The suggested mechanism causing these patterns is that the dispersal behaviour in aphids creates a mixture of area‐ and perimeter‐dependent migration rates. In this paper, we used these predictions to examine the additional consequences of nutrient availability and natural enemies on the density‐area relationship. The derived predictions were compared to data from a system with three aphid species, a set of aphid parasitoids and generalist natural enemies, and at two levels of plant nutrient availability. We find that predictions from the model based only on dispersal and local growth agree with the temporal dynamics of density‐area relationships for aphids in high nutrient patches. In patches with low nutrients, high parasitism rates appeared to cause a negative density‐area relationship for aphids, thereby deviating from predictions driven by the aphids' dispersal behavior. Hence, the dispersal model with scale‐dependent migration rates can provide a useful tool for understanding insect distribution in patch size gradients, but the relative importance of top‐down effects can completely change with plot productivity.