Browsing by Author "Kelly, Eugene F., committee member"
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- ItemOpen AccessAdvancing understanding of the formation and stability of soil organic matter in a changing environment(Colorado State University. Libraries, 2015) Lavallee, Jocelyn M., author; Conant, Rich T., advisor; Paul, Eldor A., advisor; Cotrufo, M. Francesca, committee member; Borch, Thomas, committee member; Kelly, Eugene F., committee memberSoil is one of our most precious natural resources. It plays a key role in maintaining soil fertility and water quality, and represents a major reservoir in both the global carbon (C) and nitrogen (N) cycles. Soils contain more C and reactive N than the atmosphere and all vegetation combined, the majority of which is found in soil organic matter (SOM). Despite its considerable significance, little is known about the factors that control the formation of SOM, and its stability in the environment. Key questions pertain to whether environmental changes will increase the production of CO₂ during SOM formation and decomposition, forming a large positive feedback to climate change. Answering those questions required a better understanding of how various mechanisms that confer SOM stability are affected by environmental change. My dissertation research aimed to address some of these key questions, and to advance our overall understanding of SOM formation, SOM stability, and the response of stable SOM to changes in the environment. First, I conducted two soil incubation experiments using isotopically labeled (¹³C and ¹⁵N) plant material, which allowed me to track the incorporation of plant-derived C and N into SOM, and efflux of plant-derived C in CO₂. In one soil incubation, I tested the effects of plant litter quality and on the rate and efficiency of SOM formation (a measure of the amount of SOM formed versus the amount of CO₂ lost in the process) by comparing SOM formation from leaves versus roots. I found that plant litter chemistry (C/N ratio) was a reliable predictor of SOM formation after the initial stage of decomposition, with low C/N ratios resulting in more SOM formation and higher formation efficiencies overall. In the second soil incubation, I tested the effect of warming on the rate and efficiency of SOM formation, as well as the rate of destabilization of stable SOM. I found that warming generally led to lower formation efficiencies, causing greater CO₂ production per unit of SOM formed. Warming also led to higher rates of destabilization of stable SOM throughout the experiment. Next, I aimed to investigate the effect of warming on SOM in the field, using soils from two multi-factor climate change experiments. Results from that study suggested that while warming increased the rate of turnover of SOM in some cases, any resulting losses of SOM were offset by increased inputs of SOM, so that total SOM stocks were unchanged. Last, I investigated the persistence of pyrogenic SOM, which is thermally transformed by fire, in the face of land use change at three agricultural sites across the US. I found that pyrogenic SOM was present in all three soils, and had persisted to a greater extent than other SOM with land use change. Many studies of SOM dynamics do not account for pyrogenic SOM, and the results of my work suggest that this lack of accounting can preclude us from fully understanding the mechanisms behind SOM stability. Overall, my work advances our understanding of stable SOM in terms of how it is formed, and whether it will persist in the face of environmental change. Changes in plant litter quality and temperature may lead to changes fluxes of CO₂ to the atmosphere during SOM formation, and while some SOM (pyrogenic SOM) is highly stable in the environment, other SOM is susceptible to loss with warming and land use change. However, in the case of warming, increased plant inputs may offset increased rates of SOM decomposition.
- ItemOpen AccessBiogeochemical response of U.S. Great Plains grasslands to regional and interannual variability in precipitation(Colorado State University. Libraries, 2002) McCulley, Rebecca Lynne, author; Burke, Ingrid C., advisor; Lauenroth, William K., committee member; Kelly, Eugene F., committee memberCurrent climate change scenarios predict increasing variability in both the amount and timing of rainfall for the Great Plains region of North America. In this region, aboveground production is tightly linked to both long-term average and interannual precipitation patterns, suggesting that future changes in climate may have significant consequences for grassland ecosystem function. However, aboveground production accounts for only ~50% of the carbon input into these ecosystems, and little is known about the belowground production response or biogeochemical consequences of interannual variability in precipitation. Biogeochemical processes, such as nitrogen mineralization, determine the amount of resources available for plant growth and have shown sensitivity to alterations in water availability. Thus, interannual variability in precipitation is likely to have direct and indirect effects on plant production by influencing water availability and by altering biogeochemical processes. In this dissertation, I address the influence of regional, seasonal, and interannual variability in precipitation on nitrogen (N) and carbon (C) cycling and microbial biomass and community composition in grassland ecosystems of the Great Plains. At 5 sites spanning a 500 mm mean annual precipitation gradient and encompassing, from west to east, shortgrass steppe, mixed grass prairie, and tallgrass prairie plant community types, I measured monthly in situ net N mineralization and soil respiration rates and annual above- and belowground net primary production and litter decomposition rates during the 1999-2001 time period. To quantify variability in the microbial biomass and community composition I analyzed the phospholipid fatty acid content of soil samples taken in October 2000 and June 2001 from these 5 sites. Carbon cycling rates and microbial biomass increased from semi-arid shortgrass steppe to sub-humid tallgrass prairie. At each site, C cycling rates were responsive to interannual variability in precipitation and this responsiveness varied across grassland community types. There were no significant regional, seasonal, or interannual trends in N cycling rates. Microbial biomass was larger during the growing season than in the fall, and microbial community composition was different for each of the 3 grassland types but was not significantly different across landscapes (uplands or lowlands) or between seasons at any of the sites.
- ItemOpen AccessClimate change consequences of geographic variation in growth and penology of two dominant central US grasses(Colorado State University. Libraries, 2012) Giuliani, Amanda Lease, author; Knapp, Alan K., advisor; Kelly, Eugene F., committee member; Wall, Diana H., committee memberSpecies can exist in a given range of climatic conditions, and these ranges have shifted in response to geologic climate change. Plant species with slower migration rates, however, may not be able to keep up with the current predicted rate of climate change. Thus, populations located peripherally to a biometry may play a more significant role in sourcing future generations than previously thought. As a result of many studies, we know a lot about populations that exist central to their range, or dominant populations (DPs), of two key C4 grass species, Bounteous gracilis and Andropogon gerardii, that account for much of the biomass of the shortgrass steppe and tallgrass prairie, respectively. However, we know little about their corresponding peripheral populations (PPs). This study examines ecophysiological, morphological, and whole plant differences between DPs and PPs of B. gracilis and A. gerardii under well-watered and water-stressed conditions in a reciprocal common garden experiment. Traits that were measured included predawn and midday leaf water potential, total biomass, reproductive biomass percentage, and individual reproductive and vegetative tiller mass (A. gerardii only), specific leaf area, fluorometer, height, and reproductive tiller density. We found that key traits differed between DPs and PPs in both species, but these key traits were unique to each species. For B. gracilis phenological traits of DPs and PPs were primarily different, whereas productivity traits were significantly different between populations of A. gerardii. If, in fact, PPs of these two C4 grasses are the propagules of future generations, the differences observed in this study may have implications as we look ahead to predicted climate patterns. In B. gracilis, an understanding of the differences in phenological traits may be important when we account for future adaptation, whereas knowledge about productivity differences in A. gerardii may help us better predict effects on ecosystem function. In both cases, more research is necessary to further develop our understanding of PPs and the potentially significant role they will play in the future.
- ItemOpen AccessCompetition from neighboring trees in eucalyptus monoculture and in mixed species native forest restoration plantations(Colorado State University. Libraries, 2012) Luu, Trung Canh, author; Binkley, Dan, advisor; Rocca, Monique, committee member; Laituri, Melinda, committee member; Kelly, Eugene F., committee memberCompetition has been recognized as a crucial factor in determining stand structure and productivity. However, competition is not a simple pattern. Its intensity and importance vary with structures of neighboring tree size and composition, and nutrient gradients. Our studies examined the influence of neighborhood uniformity on growth of individual trees in Eucalyptus monoculture, and competition between pioneer and non-pioneer species in mixed native species restoration plantations by developing a number of alternative neighborhood growth models. Our analyses showed that neighborhood uniformity of tree sizes had significant effects on growth of individual clonal Eucalyptus trees and these effects increased with increasing age of stand because stand and neighborhood tree size became less uniform with age. For competition from pioneer trees to non-pioneer trees, competition from neighboring trees had strong effects on the growth of individual non-pioneer trees, and the intensity of competition from neighboring trees varied with focal tree species guild and degrees of silviculture interventions. For example, non-pioneer legumes experienced competition as a function of neighboring tree sizes and distances only. Non-pioneer non-legumes experienced competition as a function of neighboring tree sizes and distances, and also by the identity of neighboring species guilds. The non-pioneer non-legumes experienced stronger competition in the intensive silviculture treatment, probably resulting from the neighboring species guild of pioneer non-legumes, unlike the non-pioneer legumes. Although intensive silviculture initially enhanced forest stand productivity (both density and tree size), strong competition from fast-growing lowered the later growth of individual non-pioneer trees. Our analyses suggested implications to: (i) increase and maintain stand uniformity to increase stand stem productivity and quality; and (ii) control strong, even exclusive completion in some cases from pioneer trees to non-pioneer trees through matching species to be mixed and managing their abundance.
- ItemOpen AccessEffects of irrigated and dryland cultivation on soil carbon, nitrogen and phosphorus in northeastern Colorado(Colorado State University. Libraries, 2001) Sinton, Penelope J., author; Burke, Ingrid C., advisor; Kelly, Eugene F., committee member; Peterson, Gary A., 1940-, committee member; Lauenroth, William K., committee memberI investigated the effects of irrigated and fertilized com agriculture on soil C, N and P in northeastern Colorado as they compare to dryland wheat-fallow fields and native rangelands in the semiarid shortgrass steppe of northeastern Colorado. Three replicates each of native rangeland, dry land wheat-fallow, and irrigated corn fields located in or adjacent to the Pawnee National Grasslands were selected for this study. I measured potentially mineralizable C and N from 0-15cm in the soil profile, particulate organic matter (POM) C and Nin the upper 30cm, total and NaHC03-P to a depth of 105cm, and total soil C and N to a depth of 195cm in the soil profile. Irrigated corn fields contained significantly lower mineralizable, POM, and total C and N than rangelands in the upper 5cm of soil. Com fields also had significantly greater NaHCOrP content than rangelands or wheat-fallow fields to a 1-meter depth in the soil. Wheat-fallow fields had significantly less potentially mineralizable and POM C and N than rangelands or corn fields in the upper 5cm of soil. Cumulative losses of total C and N in wheat-fallow fields extended to depths of 75cm or more. There were no significant differences in total P among land use types. Differences in C and N between corn and wheat-fallow fields are likely due to differences in the quantity of plant residue inputs. The distribution of C, N and NaHC03-P through the soil profile in corn fields also differed from rangelands. Soil C, N and NaHC03-P in the soil profile of rangelands decreased from the surf ace down, whereas in com fields C, N and NaHC03-P increased from the surf ace to 30cm and then decreased. Distribution of C, N and P in corn fields may be due to leaching of C or N or decomposition changes in the soil profile. In wheat-fallow fields, C, N and NaHC03-P showed a more uniform distribution in the upper 30cm of soil than rangelands, likely due to tillage practices that mix the upper soil layers in wheat-fallow fields. These results indicate that irrigated and fertilized corn crops in this region of the semiarid shortgrass steppe depletes pools of C and N at the soil surf ace but does not cause a change in C or N below the 5cm layer of soil. The differences in amount and distribution of C and N observed in this study among dryland wheat-fallow and irrigated corn fields indicate that the type of crop grown in this region should be an important consideration for regional studies that evaluate C and N changes due to cultivation.
- ItemOpen AccessFire management effects on carbon flow from root litter to the soil community in a tallgrass prairie(Colorado State University. Libraries, 2013) Shaw, Elizabeth Ashley, author; Wall, Diana H., advisor; Cotrufo, M. Francesca, committee member; Kelly, Eugene F., committee memberBelowground litter decomposition is a major component of carbon cycling in grasslands, where it provides energy and nutrients for soil microbes and fauna. Fire, a historically frequent disturbance and a common management tool, removes above ground biomass and litter accumulation making belowground root litter of greater importance to decomposer food webs. While many studies use biomass measures of soil faunal groups to estimate changes in soil food web structure and energy flow, little is known about the flow of C from root litter to soil microbial and nematode communities in grasslands and if biomass measures can indicate this flow of C at a fine scale. Our greenhouse experiment first investigated how C from Andropogon gerardii (big bluestem) root litter was allocated into different soil microbial and nematode groups in frequently burned (FB) and infrequently burned (IB) tallgrass prairie soil. Incorporation of 13C into microbial fatty acids and nematode communities was determined on six occasions during decomposition in order to examine whether different groups of microorganisms and fauna were specialized on the root-litter derived C. Results showed that FB and IB soils supported microbial communities of differing community composition and abundance. IB had, generally, higher microbial abundance, more strongly dominated by bacteria than FB soil. Compound-specific stable isotope ratio analysis showed that root litter-C was more quickly incorporated into FB soil microbes. By the end of the experiment, all microbial groups were more highly 13C enriched in FB soils than in IB soils, with the exception of gram-negative bacteria for which there was no significant difference between the two soils. For nematodes, there was no significant difference in abundances; however, fungivore nematodes only incorporated root litter-C in FB soil while bacterivores, omnivores and predators derived at least some C from root litter in both treatments. Despite lower abundance of microbes in FB soil, total root litter mass loss did not differ between FB and IB soil, indicating higher microbial activity in FB soil. Our results reveal that FB prairie soil food webs are more closely coupled to root litter decomposition, where root litter is of increased importance as a C and nutrient source due to the frequent removal of standing biomass and shoot litter by fire. In the second part of our greenhouse experiment, we compared soil energy channel biomass measures with C flow into the soil food web. By coupling the energy channel biomass measurement approach with our decomposition study (using stable isotope enrichment to trace the flow of C into nematode trophic groups), we compared the quantified C flow to nematode energy channel biomass measures during decomposition of 13C-labeled big bluestem root litter. We hypothesized that biomass measures for nematode bacterial and fungal energy channels would indicate the proportion of root litter derived C incorporated into each nematode energy channel. Nematode biomasses and δ13C values were assessed initially (day 0) and after 180 days of incubation. Results showed the nematode bacterial energy channel dominated over the nematode fungal energy channel in both FB and IB grasslands. Yet, FB grassland soil had significantly higher nematode bacterial energy channel biomass than IB at time 0. In both soils, the nematode bacterial energy channel biomass increased significantly after the addition of root litter and there were no differences in the nematode bacterial channel biomass between the two soils at the final harvest (180 days). There were no differences between FB and IB soil's nematode fungal energy channel biomass at either day 0 or 180 days. 13C analysis of nematodes confirmed our hypothesis, as more root litter-C was concentrated in the dominant nematode bacterial energy channel in both FB and IB grassland soils. However, the IB soil's nematode bacterial energy channel had incorporated significantly more root litter derived C than the FB soil, despite no differences in these energy channel biomasses at the final harvest. The FB soil food web showed the opposite effect for the nematode fungal energy channel. These results indicate that while energy channel biomass measurements of nematodes give a broad overview of C flow, 13C decomposition tracer studies are more precise, and provide exact measures of C flow through soil food webs for ecosystem research. Overall, our results highlight the general view that plant litter is an important C-source in grasslands and further show that root litter-C is incorporated differently in frequently and infrequently burned soil food webs. We show that frequently burned soil food webs may be more specialized to decompose grass root litter. Our results indicate the C flow within soil food webs in differing burn management areas, and show differences between the frequently and infrequently burned tallgrass prairie.
- ItemOpen AccessImpact of iron and redox chemistry on the environmental fate and transport of metalloids and radionuclides(Colorado State University. Libraries, 2014) Troyer, Lyndsay D., author; Borch, Thomas, advisor; Ladanyi, Branka M., committee member; Levinger, Nancy E., committee member; Henry, Charles S., committee member; Kelly, Eugene F., committee memberMillions of cubic meters of uranium (U) mine tailings worldwide and millions of gallons of contaminated groundwater are the result of U mining and milling activity. Arsenic can occur at up to 10 weight percent in U ore, so both U and As can be released during U mining. Although these elements commonly occur together, little research into their redox behavior when present in the same environmental system has been performed. The goal of this research is to gain an improved understanding of how redox chemistry affects U and As speciation and complexation when the two elements are present together as co-contaminants. The North Cave Hills in Harding County, South Dakota is an abandoned U mine where overburden has been left open to weathering and transport since mining began in 1955. The exposed overburden has resulted in above-background level concentrations of U and As in sediments and groundwater in the surrounding wetlands. We conducted a field-scale study to investigate U and As redox chemistry at the North Cave Hills by taking sediment samples from the tailings pile and the down gradient watershed in order to assess U and As fate and transport. As sediments pass through anoxic zones at the field site, U is immobilized as reduction takes place but As can simultaneously be released into surface waters as reductive dissolution of Fe minerals also occurs. A laboratory-based study was conducted in order to examine the redox chemistry of U and As in North Cave Hills sediments under controlled conditions. Upon microbial reduction of sulfate and formation of mackinawite in batch systems, U(VI) and As(V) were reduced to nano- UO2 and a reduced As-sulfide mineral phase respectively during biostimulation by three different electron donors. When these systems were exposed to air for 24 hours, mackinawite protected U and As from oxidation and little change in their solid-phase speciation was observed. While mackinawite was shown to play a role in reduction, we could not determine if direct microbial reduction of U and As was also taking place in the systems. In order to further explore the reduction of U(VI) and As(V) by mackinawite, an experiment was set up to determine if As(V) prevented U(VI) reduction, especially following the formation of uranyl arsenate precipitates. As(V) only had an impact on the extent of U reduction at concentrations higher than would occur in most environmental systems. When As(V) concentrations were high, U(VI) was shown to be resistant to reduction because of the precipitation of a uranyl arsenate mineral phase. The findings in this dissertation contribute important information that will improve our current understanding of U and As redox behavior that will lead to improved remediation strategies to effectively prevent the mobilization of both elements in environmental systems.
- ItemOpen AccessSeasonal precipitation and soil moisture dynamics of a hyperarid wash in the Sonoran Desert, U.S.A.(Colorado State University. Libraries, 2013) Howe, Susan R., author; Wohl, Ellen E., advisor; Laituri, Melinda J., committee member; Kelly, Eugene F., committee member; Rathburn, Sara L., committee memberPrecipitation and runoff in arid and hyperarid landscapes is infrequent and both spatially and temporally variable, and the relationship between these hydrologic components and vegetation, soils, and geomorphology in these environments is complex and not well understood. In this study, precipitation and soil moisture were monitored beneath three cover types in three locations across two geomorphic surfaces in the Yuma Wash watershed, located in the Lower Colorado River Valley of the Sonoran Desert, on the US Army Yuma Proving Grounds in Yuma, Arizona. Monitoring, sampling, and characterization occurred from July 2006 to February 2010. Six tipping bucket rain gages and sixty time domain reflectometry soil moisture sensors recorded moisture inputs and storage on a middle to late Pleistocene age alluvial terrace, and a younger, Holocene age alluvial wash. Sensors were spatially distributed in the lower, middle and upper locations of the watershed, beneath bare ground at 2.5, 25, 50, and 100 cm, and beneath the dripline radius of Olneya tesota and Parkinsonia microphylla, at 25, 50, and 100 cm depths. These data suggest that precipitation is highly variable in space and time, and is generally greater than the surrounding valley bottoms of Yuma Proving Grounds. Findings also suggest that soils beneath the dripline radius of these plant species on terraces are wetted more frequently and to greater depths in response to smaller magnitude and lower intensity storm events relative to soils beneath the same species on washes, and relative to bare ground soils. Threshold precipitation conditions necessary to generate changes in soil moisture were compared across surfaces, and illustrate that the vesicular structure in the A (Av) horizons beneath desert pavement plays a key role in redistribution of moisture as runon to O. tesota and P. microphylla on terraces, and that soils beneath the dripline radius of both species on washes receive moisture only during rainfall events exceeding 30 mm. There is also some evidence to suggest precipitation and near surface soil moisture may be greater in the upper basin relative to the mid- and lower basin on both surfaces, but at depths of 25-100 cm, soil moisture responses were difficult to interpret due to local soil properties not quantified in this study. The influence of soil temperature on the imaginary permittivity component of soil moisture readings due to high soluble salt content, the presence of enriched clay layers, soil compaction and induration is discussed. Findings highlight the need to quantify these age-dependent soil pedogenic and hydrologic properties when assessing soil moisture response to spatially variable precipitation in these water-limited environments. Implications for management of military lands are discussed.
- ItemOpen AccessThe influence of moisture availability on terrestrial ecosystems: effects on soil animal communities along a regional/global scale climate gradient(Colorado State University. Libraries, 2013) Sylvain, Zachary Adam, author; Wall, Diana H., advisor; Cotrufo, M. Francesca, committee member; Kelly, Eugene F., committee member; Knapp, Alan K., committee member; Seastedt, Timothy R., committee memberEarth's climate is being altered at an alarming rate, and the consequences of these changes on the planet's ecosystems are unclear. In addition to increased warming due to rising CO2 concentrations, alterations to precipitation patterns will influence soil moisture availability in terrestrial ecosystems and this will have important consequences for plant growth and the ability of soil systems to perform functions such as decomposition and nutrient cycling. The effects on soil systems are especially poorly understood, partly due to the many interactions between environmental conditions and the numerous species found within soil ecosystems, ranging from microbial organisms such as bacteria, archaea and fungi to soil animals including mites and nematodes. With chapter 2, I provide an overview of the role of soil biodiversity and the implications climate and land-use changes may have for ecosystems as a consequence of their effects on soil biodiversity. I then examine the current state of understanding for the influence of soil moisture availability on plant and soil communities of temperate ecosystems in chapter 3, and highlight challenges for future research such as the inclusion of diversity metrics and soil animal community responses in climate change experiments as well as studies that operate at scales larger than single sites in order to better capture the dynamics of ecosystem changes. My research focused on one aspect of climate change, the alteration of soil moisture availability within ecosystems due to changes in precipitation regimes, and whether it affected soil organisms, particularly soil mites and nematodes (Chapter 4 of this dissertation). These two groups were selected because of their high abundances and diversity within soil ecosystems, their dependence upon soil water availability as a consequence of life history traits and their contributions to decomposition and nutrient cycling processes. To examine the effects of changing moisture availability on communities of mites and nematodes I analyzed soil samples along a large scale regional/global climate gradient made up of four long-term ecological research (LTER) sites including Konza Prairie LTER (KNZ), Kansas, Shortgrass Steppe LTER (SGS), Colorado, Jornada Basin LTER (JRN), New Mexico and McMurdo Dry Valleys LTER (MCM), Antarctica. I established elevation transects across hill slopes to obtain landscape-scale gradients of soil moisture availability within each of these ecosystems and sampled existing experimental manipulations of moisture availability from 2009-2011. Mites and nematodes were sorted to trophic groups to determine their ecological role and how changes to their abundances may affect ecosystems. Mite and nematode abundances responded strongly to changes in moisture availability. Across the large-scale climate gradient of all four sites, a positive non-linear response was found with particularly large increases in animal abundances corresponding to incremental moisture increases at the lower limits of moisture availability. Within each of the ecosystems, however, the responses of soil animal trophic group abundances to moisture availability were very similar and were largely negative. In chapter 5 of this dissertation, I further explore the effects of soil moisture and top-down or bottom-up community dynamics on mite and nematode abundances. To do this, I constructed a structural equation model examining the direct and indirect effects of soil moisture availability and trophic interactions on soil animal trophic group abundances. Results of this model suggest that soil moisture strongly controls populations of these organisms. Additionally, predatory mite and nematode trophic groups have top-down controls on lower trophic groups, although these interactions do not appear to be due to predation and instead suggest the influence of additional, unmeasured environmental factors acting indirectly on lower-level soil animal trophic groups. With this dissertation, I demonstrate that changes to soil moisture regimes can have important effects on soil animal communities. A review of the literature (Chapter 3 of this dissertation) showed altered soil moisture availability had the clearest effects on plants, with effects on soil organisms being more idiosyncratic, likely as a result of stronger indirect than direct effects. Experimental evidence along a regional/global climate gradient of two desert and two grassland sites (Chapter 4 of this dissertation) show that increases to moisture availability have strong positive effects on mite and nematode communities, especially at low levels of moisture availability across this large, multi-site scale. At smaller scales (within individual ecosystems) this response becomes weaker and results in declines to animal groups at most sites. These results suggest that as precipitation regimes are altered as a consequence of climate change, the resultant alterations to soil moisture availability may have important feedbacks to terrestrial ecosystems. Observed changes to trophic group structuring in response to changes in moisture availability (Chapters 4 and 5 of this dissertation) show that food webs may be restructured due to future changes in moisture availability, leading to increases to root herbivory and increasing the amount of energy flowing through bacterial rather than fungal decomposition pathways. These changes to food webs can result in alterations to nutrient cycling pathways and shifts in carbon allocation within plant communities, which will further influence ecosystem dynamics.