The functional trophic role of lotic primary consumers: generalist versus specialist strategies

1. The relative extent of generalist or specialist resource use strategies is an important question in ecology. A community dominated by specialist strategies suggests a high level of interspecific competition for resources, resulting in the evolutionary development of isolating mechanisms between species (e.g. resource specialization to avoid and/or outcompete other species). A community dominated by generalist strategies suggests less interspecific competition for resources, allowing many taxa to utilize the same resources. In stream systems, generalist food habits are a common strategy among primary consumers, but little is known about resource assimilation strategies (resources incorporated into tissue growth). Published data indicate that generalist resource assimilation strategies may prevail in lotic systems as well.
2. Functional feeding groups (FFGs) are often used to infer resource assimilation among lotic macroinvertebrates (e.g. shredder-detritivore, scraper-herbivore). While these groupings are aptly used to describe invertebrate feeding modes and community structure, the use of FFGs to describe resource assimilation among lotic consumers is not appropriate. Sufficient data now exist to seriously question how accurately FFG assignments describe the processes of energy flow and material transfer between trophic levels in stream ecosystems.
3. Because FFGs may not accurately describe functional attributes in lotic systems, an alternative approach is needed. One approach is to determine the amount of secondary production that is derived from autochthonous (e.g. periphyton and algae) and allochthonous (e.g. detritus) resources directly. A simple model of community function based on this approach is presented. The model incorporates trophic generalists into measurement of consumer–resource energetics in lotic systems.     

The functional trophic role of lotic primary consumers: generalist versus specialist strategies

1. The relative extent of generalist or specialist resource use strategies is an important question in ecology. A community dominated by specialist strategies suggests a high level of interspecific competition for resources, resulting in the evolutionary development of isolating mechanisms between species (e.g. resource specialization to avoid and/or outcompete other species). A community dominated by generalist strategies suggests less interspecific competition for resources, allowing many taxa to utilize the same resources. In stream systems, generalist food habits are a common strategy among primary consumers, but little is known about resource assimilation strategies (resources incorporated into tissue growth). Published data indicate that generalist resource assimilation strategies may prevail in lotic systems as well.
2. Functional feeding groups (FFGs) are often used to infer resource assimilation among lotic macroinvertebrates (e.g. shredder-detritivore, scraper-herbivore). While these groupings are aptly used to describe invertebrate feeding modes and community structure, the use of FFGs to describe resource assimilation among lotic consumers is not appropriate. Sufficient data now exist to seriously question how accurately FFG assignments describe the processes of energy flow and material transfer between trophic levels in stream ecosystems.
3. Because FFGs may not accurately describe functional attributes in lotic systems, an alternative approach is needed. One approach is to determine the amount of secondary production that is derived from autochthonous (e.g. periphyton and algae) and allochthonous (e.g. detritus) resources directly. A simple model of community function based on this approach is presented. The model incorporates trophic generalists into measurement of consumer–resource energetics in lotic systems.     

Structure of the Photosynthetic Apparatus in Leaves of Freshwater Hydrophytes: 1. General Characteristics of the Leaf Mesophyll and a Comparison with Terrestrial Plants

The structure of photosynthetic elements was investigated in leaves of 42 boreal plant species featuring different degrees of submergence (helophytes, neustophytes, and hydatophytes). The mesophyll structure types were identified for all these species. Chlorenchyma tissues and phototrophic cells were quantitatively described by such characteristics as the sizes of cells and chloroplasts in the mesophyll and epidermis, the abundance of cells and chloroplasts in these tissues, the total surface area of cells and chloroplasts per unit leaf area, the number of plastids per cell, etc. The hydrophytes typically had thick leaves (200–350 m) with a well-developed aerenchyma; their specific density per unit area (100–200 mg/dm²) was lower than in terrestrial plants. Mesophyll cells in aquatic plants occupied a larger volume (5–20 × 10³m³) than epidermal cells (1–15 × 10³m³). The number of mesophyll cells per unit leaf area was nearly 1.5 times higher than that of epidermal cells. Chloroplasts were present in the epidermis of almost all species, including emergent leaves, but the ratio of the chloroplast total number to the number of all plastids varied depending on the degree of leaf submergence. The total number of plastids per unit leaf area (2–6 × 10⁶/cm²) and the surface of chloroplasts per unit leaf area (2–6 cm²/cm²) were lower in hydrophytes than in terrestrial plants from climatically similar habitats. The functional relations between mesophyll parameters were similar for hydrophytes and terrestrial plants (a positive correlation between the leaf weight per unit area, leaf thickness, and the number of mesophyll cells per unit leaf area), although no correlation was found in hydrophytes between the volume of mesophyll cells and the leaf thickness. Phototrophic tissues in aquatic plants contributed a larger fraction to the leaf weight than in terrestrial plants, because the mechanical tissues were less developed in hydrophytes. The CO₂assimilation rates by leaves were lower in hydrophytes than in terrestrial plants, because the total surface area of chloroplasts per unit leaf area is comparatively small in hydrophytes, which reduces the conductivity for carbon dioxide diffusion towards the carboxylation sites.     

Construction costs and mesostructure of leaves in hydrophytes

Construction costs (CC) and parameters of leaf structure (specific leaf weight, dry matter content, volume of photosynthesizing cells, and the number of cells per leaf area unit) were determined for 19 species of aquatic higher plants. The CC of 1 g dry matter varied from 0.98 g glucose in Lemna gibba L. to 1.48 g glucose in Nuphar pumila (Timm) DC. and Potamogeton natans L. The CC of leaf area unit varied to a greater extent than the CC of 1 g dry wt (from 10 to 97 g glucose/m²) and depended on the type of mesophyll structure. In leaves of hydrophytes with dorsoventral mesophyll structure, the CC of 1 m² leaf area was 3–9 times larger than in leaves with homogeneous structure. Variations in CC of 1 m² leaf area in hydrophytes were affected insignificantly (by 2% only) by variations of CC per 1 g dry wt and were mainly determined (by 82%) by changes in specific leaf weight. Two-factor analysis of variance has shown that the CC of 1 g dry wt in hydrophytes depended on the attachment of plants to the sediment: the CC was 1.2 times larger in rooted hydrophytes than in free floating plants. The second factor (the extent of submergence) potentiated the effect of rooting on CC. Reliable differences were found between the leaf CC for hydrophytes belonging to four groups distinguished by the extent of their contact with water and sediment. In a group series: rooted hydrophytes with floating leaves → submerged rooted hydrophytes → free floating submerged hydrophytes → free floating surface inhabiting hydrophytes, the CC of 1 g dry wt decreased by 1.3 times. Path analysis has shown that this trend was due to the increase in photosynthesizing cell volume and to reduction in number of cells per leaf area unit, which caused the decrease in dry matter content. The decrease in the content of leaf dry matter was accompanied by changes in its chemical composition: the content of carbon and nitrogen decreased. This led to a consistent decrease in leaf CC expressed per 1 g dry wt upon the increase in extent of plant hydrophilicity.     

Littoral macrophyte–periphyton complexes in two Hungarian shallow waters

Periphyton developing on the surfaces of emergent and submerged aquatic plants has a significant influence on water quality. The periphyton types that form on various plant species can be characterized by their mass values, the proportion of the present organic and inorganic fractions, as well as their chlorophyll-a contents. Studies on periphyton complexes constituting integrated biomonitoring systems are useful to gain essential long-term information about the performance of shallow water bodies. The filtering and settling effect of Phragmites and other aquatic plants, as well as their periphyton was examined and clearly observable in the water areas and non-flooded aquatic habitats belonging to the second phase of Kis-Balaton Protection System, as it was indicated by the mass values and ash contents. The periphyton forming on the aquatic vegetation that annually develops in Kisköre Reservoir and yields a considerable biomass has a critical part in influencing water quality. The only difference (p<0.05) was found in the ash content of the periphyton, being lower in Kis-Balaton (48.64 ± 2.29 S.E., %) and higher in Kisköre Reservoir (57.42 ± 2.54 S.E., %). This paper presents the dry mass of the periphyton, as well as its ash and chlorophyll-a content, and the results obtained on the composition of the alga species of the periphyton.

     

Do submerged aquatic plants influence periphyton community composition for the benefit of invertebrate mutualists?

• 1 It has been suggested that submerged aquatic plants can influence the periphyton which grows on their surfaces, making it nutritionally beneficial to snails. In return, preferential feeding by snails clears the plants from a potential competitor, with both plants and grazers gaining from this mutualistic relationship.
• 2 A highly replicated experiment was conducted, in which the nature of the plant (isoetid and elodeid types compared with similar shaped inert substrata), the nutrient availability (10–200 µg L^‐1 P, 0.2–4 mg L^‐1 N) and the influence of periphyton grazers, Physa fontinalis, were controlled. The plants were cleaned of periphyton before use and an algal inoculum added to all treatments. At the end of the growth period, quantitative measures of the periphyton community composition were made and related to the treatments using both ordination and analysis of variance.
• 3 Grazing had the largest influence on community composition and algal numbers. A community of unicellular and adpressed filamentous forms developed in the presence of snails, and of erect filamentous forms in their absence. Three algal species, Cocconeis placentula, Chamaesiphon incrustans and Aphanochaete repens, increased in real numbers in the presence of snails, probably as a result of reduced competition whilst being able to withstand grazing.
• 4 The second largest effect was the influence of host plant. However, differences between the two artificial plants were as great as between the real plants and their artificial counterparts, indicating that physical structure was as important as any active contribution by the plants. Nutrients had a small but significant effect on community composition, but not all species responded in the same way to nutrient enrichment.
• 5 Although submerged aquatic plants exert an influence over the community composition of the periphyton which develops on their surfaces, it is unlikely that they manipulate it to make it more attractive to grazers such as snails.

     

Energy content of water- and bog-plant associations in the region of Valdivia (Chile)

Summary Mapping the vegetation of the littoral zone of different types of aquatic habitats in the region of Valdivia (Chile) submerged Egerietum densum, the emergent Sagittario-Alismetum and the Scirpetum californiae were found. In the littoral zone of the banados (i.e. lakes) formed by inundation after subsidence the submerged Egerietum densum, the floating-leaved Polygono-Jussiaetum and the emergent Juncetum procerii; the analyzed pond of Mehuin contained the Callitrichetum stagnalis chilensis and the artificial ponds close to La Union the Lemno-Azolletum. The caloric values of 18 hydrophytes showed a decline from the large emergent hydrophytes (3 818 cal/g) to the submerged (2 907 cal/g) and there was also a decrease from the free-floating (3 652 cal/g) to the floating-leaved (3 364 cal/g) and to the submerged plants. In the most eases high caloric values correspond to a high content of lignin (large emergent hydrophytes) or lipids (floating-leaved plants).We thank Mrs. Bölke and Mr. Delgado for technical help and the Universidad Austral de Chile, Valdivia for fluancial support.     

The Behavior of Organic Phosphorus under Non-Point Source Wastewater in the Presence of Phototrophic Periphyton

To understand the role of ubiquitous phototrophic periphyton in aquatic ecosystem on the biogeochemical cycling of organic phosphorus, the conversion and removal kinetic characteristics of organic phosphorus (Porg) such as adenosine triphosphate (ATP) were investigated in the presence of the periphyton cultured in artificial non-point source wastewater. The preliminary results showed that the periphyton was very powerful in converting P_org evidenced by the fact that inorganic phosphorus (P_inorg) content in solution increased from about 0.7 to 14.3 mg P L⁻¹ in 48 hours in the presence of 0.6 g L⁻¹ periphyton. This was because the periphyton could produce abundant phosphatases that benefited the conversion of P_org to P_inrog. Moreover, this conversion process was described more suitable by the pseudo-first-order kinetic model. The periphyton was also effective in removing P_org, which showed that the P_org can be completely removed even when the initial P_org concentration was as high as 13 mg P L⁻¹ in 48 hours in the presence of 1.6 g L⁻¹ periphyton. Furthermore, it was found that biosorption dominated the P_org removal process and exhibited the characteristics of physical adsorption. However, this biosorption process by the periphyton was significantly influenced by biomass (absorbent dosage) and temperature. This work provides insights into P_org biogeochemical circulation of aquatic ecosystem that contained the periphyton or similar microbial aggregates.     

Metal content in higher aquatic plants in a small siberian water reservoir

The dynamics of metal content in higher aquatic plants (macrophytes) in a small Bugach water reservoir in 1998–2006 was studied. A comparative estimation of the metal content in six macrophyte species (Typha latifolia L., Typha angustifolia L., Polygonium amphibium L., Potamogeton perfoliatus L., Potamogeton pectinatus L., Phragmites australis (Cav) Trin. Ex Steud.) showed that their metal concentrations do not generally exceed those known from the literature. Cluster analysis showed that the macrophyte species under study form two ecological groups with respect to the metal content, i.e., submerged plants (hydrophytes) and emergent aquatic plants (heliophytes).     

Effects of nutrient (N,P, C) enrichment,grazing and depth upon littoral periphyton of a softwater lake

Three field experiments were performed in Lake Lacawac, PA to determine the importance of potentially limiting nutrients relative to other factors (grazing, depth) in structuring shallow water algal periphyton communities. All three experiments measured periphyton growth (as chlorophyll-a, AFDM or biovolumes of the algal taxa) on artificial clay flower pot substrates which released specified nutrients to their outer surfaces.Control of standing crop by nutrient supply rate vs. grazing was examined in Expt. I. Substrates releasing excess N and P, together with one of 4 levels of C (as bicarbonate) were placed either inside or outside exclosures designed to reduce grazer densities. Chlorophyll-a rose from 1.1–25.6 µg.cm^–2, and some dominant taxa (e.g., Oedogonium, Nostoc, Anacystis) were replaced by others (e.g., Scenedesmus, Cryptomonas) as bicarbonate supply increased. Reductions in invertebrate density did not significantly affect chlorophyll-a at any of the nutrient levels.Reasons for the species shift were further evaluated in Expt. II, using a minielectrode to measure the elevation of pH within the periphyton mat through photosynthetic utilization of bicarbonate. The pH adjacent to pots diffusing N, P and large quantities of bicarbonate, and supporting high chlorophyll-a densities of 32 µg cm^–2, averaged 10.0 compared to 6.3 in the water column. Pots diffusing only N and P supported 0.7 µg chlorophyll-a cm^–2 and elevated pH to 8.2. We suspect that bicarbonate addition favored efficient bicarbonate users (e.g., Scenedesmus), while inhibiting other taxa (e.g., Oedogonium) because of the attendant high pH.Expt. III was designed to test effects of depth (0.1 m vs. 0.5 m) and N (NH₄ ⁺ vs. NO₃ ^–) upon the growth response to bicarbonate observed in Expts. I and II. Similar standing crop and species composition were noted on pots at 0.1 m vs. 0.5 m. Enrichment with NH₄ ⁺ vs. NO₃ ^– also appeared to have little effect upon the periphyton community.Shallow water periphyton communities in Lake Lacawac, when supplied with sufficient N and P, appear to show a distinctive response to increasing bicarbonate concentration and pH which is robust to moderate variation in grazer densities, distance from the water surface, and the form of N enrichment.     

Superoxide radical production and performance index of Photosystem II in leaves from magnetoprimed soybean seeds

Priming of soybean seeds with static magnetic field exposure of 200 mT (1 h) and 150 mT (1 h) resulted in plants with enhanced performance index (PI). The three components of PI i.e the density of reaction centers in the chlorophyll bed (RC/ABS), exciton trapped per photon absorbed (φpo) and efficiency with which a trapped exciton can move in electron transport chain (Ψo) were found to be 17%, 27%  and 16% higher, respectively in leaves from 200 mT (1h) treated compared to untreated seeds.  EPR spectrum of  O₂^{. –} - PBN  adduct revealed that the O₂^{. –} radical level was lower by 16% in the leaves of plants that emerged from magnetic field treatment. Our study revealed that magnetoprimed seeds have a long lasting stimulatory effect on plants as reduced superoxide production and higher performance index contributed to higher efficiency of light harvesting that consequently increased biomass in plants from treated plants.     

Use of carbon-13 and carbon-14 natural abundances for stream food web studies

We review the use of stable carbon isotope ratios (δ¹³C) and radiocarbon natural abundances (Δ¹⁴C) for stream food web studies. The δ¹³C value of primary producers (e.g., periphytic algae, hereafter periphyton) in streams is controlled by isotopic fractionation during photosynthesis and variable δ¹³C of dissolved CO₂. When periphyton δ¹³C differs from that of terrestrial primary producers, the relative contribution of autochthony and allochthony to stream food webs can be calculated. Moreover, the variation in periphyton δ¹³C can reveal how much stream consumers rely on local resources because each stream habitat (e.g., riffle vs. pool, open vs. shaded) usually has a distinctive δ¹³C. However, periphyton δ¹³C often overlaps with that of terrestrial organic matter. On the other hand, periphyton Δ¹⁴C is less variable than δ¹³C among habitats, and reflects the Δ¹⁴C of dissolved CO₂, which could be a mixture of “aged” (Δ¹⁴C < 0 ‰) and “modern” (Δ¹⁴C > 0 ‰) carbon. This is because the Δ¹⁴C is corrected by its δ¹³C value for the isotopic fractionation during photosynthesis. Recent studies and our data indicate that many stream food webs are supported by “aged” carbon derived from the watershed via autochthonous production. The combined use of δ¹³C and Δ¹⁴C allows robust estimation of the carbon transfer pathway in a stream food web at multiple spatial scales ranging from the stream habitat level (e.g., riffle and pool) to watershed level (autochthony and allochthony). Furthermore, the Δ¹⁴C of stream food webs will expand our understanding about the time frame of carbon cycles in the watersheds.     

Periphyton density is similar on native and non‐native plant species

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Artificial Regulation of Water Level and Its Effect on Aquatic Macrophyte Distribution in Taihu Lake

Management of water levels for flood control, water quality, and water safety purposes has become a priority for many lakes worldwide. However, the effects of water level management on the distribution and composition of aquatic vegetation has received little attention. Relevant studies have used either limited short-term or discrete long-term data and thus are either narrowly applicable or easily confounded by the effects of other environmental factors. We developed classification tree models using ground surveys combined with 52 remotely sensed images (15–30 m resolution) to map the distributions of two groups of aquatic vegetation in Taihu Lake, China from 1989–2010. Type 1 vegetation included emergent, floating, and floating-leaf plants, whereas Type 2 consisted of submerged vegetation. We sought to identify both inter- and intra-annual dynamics of water level and corresponding dynamics in the aquatic vegetation. Water levels in the ten-year period from 2000–2010 were 0.06–0.21 m lower from July to September (wet season) and 0.22–0.27 m higher from December to March (dry season) than in the 1989–1999 period. Average intra-annual variation (CV_a) decreased from 10.21% in 1989–1999 to 5.41% in 2000–2010. The areas of both Type 1 and Type 2 vegetation increased substantially in 2000–2010 relative to 1989–1999. Neither annual average water level nor CV_a influenced aquatic vegetation area, but water level from January to March had significant positive and negative correlations, respectively, with areas of Type 1 and Type 2 vegetation. Our findings revealed problems with the current management of water levels in Taihu Lake. To restore Taihu Lake to its original state of submerged vegetation dominance, water levels in the dry season should be lowered to better approximate natural conditions and reinstate the high variability (i.e., greater extremes) that was present historically.     

Primary production in Rattlesnake Springs,a cold desert spring-stream

Primary productivity and respiration were measured in Rattlesnake Springs, Washington, using the upstream-downstream diel pH-CO₂ curve and harvest methods.Daily P_g and P_n rates averaged 8.7 and 0.9, 0.6 and 0.3, and 9.3 and 1.2 g C m^–2 d^–1 for periphyton, watercress, and total community, respectively. Average photosynthetic efficiencies (%, P _n Lt^–1) were approximately 0.22 and 0.07 for periphyton and watercress, respectively. Annual community P_g was 2 700 g C m^–2 a^–1 and was highest for periphyton (2 526 g C m^–2 a^–1). Periphyton P_n (356 g C m^–2 a^–1) exceeded that of watercress (87 g C m^–2 a^–1). Community R was 2 257 g C m^–2 a^–1, and was highest for periphyton (2 170 g C m² a^–1).Desert streams appear to be enigmas in terms of their relationship between autotrophy and heterotrophy and their ability to be net importers or exporters of organic matter. The fact that they can be autotrophic and net importers of organic matter is probably related to the characteristic flash-flooding of desert streams, and emphasizes the necessity of examining these systems over more than a single annual cycle.     

Comparative Characterization of the Pigment Complex in Emergent, Floating, and Submerged Leaves of Hydrophytes

The content of chlorophylls (Chls) and carotenoids was studied in the leaves of 42 species of boreal aquatic plants with different degree of submergence (emergent, floating, and submerged) and isopalisade, dorsoventral, and homogenous types of mesophyll structure. Hydrophytes were shown to have a low Chl content (1–2 mg/g fr wt) and low Chls/carotenoids ratio (2.3–3.5) as compared to terrestrial plants. The pigment content per dry wt unit and unit leaf area was dependent on the type of mesophyll structure. It was a consequence of the changes in the parameters of leaf mesophyll structure characterizing the density of photosynthetic elements. In a sequence emergent floating submerged forms, the content of Chls and carotenoids decreased, and the photosynthetic capacity decreased due to a reduction in the chloroplast number per unit leaf area. Adaptation of submerged leaves to low illumination and slow CO₂ diffusion changed the functional properties of chloroplasts. An increase in the pigment content in the chloroplasts of submerged leaves (7 × 10^–9 mg Chl, 2 × 10^–9 mg carotenoids) as compared to emergent and floating leaves was accompanied by a decline in the photosynthetic capacity per Chl comprising 1.6 mg CO₂/(mg Chl h) versus 3.9 and 3.8 mg CO₂/(mg Chl h) in emergent and floating leaves, respectively.     

Arbuscular Mycorrhizae in association with aquatic and marshy plant species in Goa,India

Of the 20 plant species of hydrophytes screened for Arbuscular Mycorrhizal (AM) fungal root colonization, 10 aquatic plants (out of 14 species) and five marshy plants (out of six species) were found to be mycorrhizal, while the remaining species were non mycorrhizal. Vesicular colonization occurred in 12 plant species while arbuscular colonization was restricted to only three plant species. A rooted submerged pteridophyte viz., Isoetes coromandelina L. was found to be mycorrhizal exhibiting vesicular colonization. In all, two genera viz., Glomus and Scutellospora, the former being dominant, were recorded. The most common AM fungal species Glomus claroideum was recovered from 14 plant species.     

Heterotrophic utilization and decomposition of extracellular carbon released by the aquatic angiosperm Littorella uniflora (L.) aschers

The heterotrophic assimilation of extracellular organic carbon (EOC) released by Littorella uniflora (L.) Aschers. was assayed. Utilization and decompostion of EOC by heterotrophs in the periphyton community were measured in a two-chamber open flow-system and, by the bacterioplankton, in bottle incubations. Under the experimental conditions 12–30% of the released EOC was metabolized in the periphyton community. This corresponded to a theoretical removal rate of 72–100% h^?1. The bacterioplankton assimilated EOC more slowly (1% h^?1). These results show that the EOC (mainly small molecules < 700 Daltons) was highly labile and accessible to heterotrophic microorganisms. The EOC released by submerged macrophytes in Lake Kalgaard is distributed approximately 20% to the periphyton community, 20% to the bacterioplankton, and 60% to the sediment.     

Geographic patterns and environmental correlates of taxonomic and phylogenetic diversity of aquatic plants in China

Studies on large-scale geographic patterns of aquatic plant diversity can promote research on the generality of macroecological patterns in different ecosystems. Here, we compiled a checklist of 889 aquatic angiosperms in China, including 738 helophytes (emergent and marshy plants) and 151 hydrophytes (submerged, free-floating, and floating-leaved plants). We explore the geographic patterns and environmental correlates of aquatic plant diversity based on six metrics including species richness (SR), weighted endemism (WE), phylogenetic diversity (PD), phylogenetic endemism (PE), the standardized effect size of phylogenetic diversity (PDses), and the standardized effect size of mean phylogenetic distance (MPDses). Our results show that the diversity of aquatic plants in China is extremely uneven, with high diversity in southeastern China and low diversity in northwestern China, and the geographic patterns of taxonomic and PD are generally consistent. The pattern of helophytes differs from that of hydrophytes. Notably, the wavy-shaped pattern of aquatic plant diversity (especially SR and PD for hydrophytes) across the latitude observed in this study is not consistent with those previously observed for aquatic plants in other continents. Climatic variables and water environmental variables are the main drivers of aquatic plant diversity in China; however, the effects of individual variables differ between helophytes and hydrophytes. Water environmental variables have a greater impact on PDses and MPDses of hydrophytes than those of helophytes. Overall, our work provides insight into understanding the large-scale patterns of aquatic plant diversity and is a critical addition to previous studies on the macroecological pattern of terrestrial organisms.