Dominant species can produce a negative relationship between species diversity and ecosystem function

Several studies have reported a positive relationship between species richness and ecosystem functioning. However, if much of a particular ecosystem function is performed by one species (i.e. a functionally dominant species) and this species is also a competitive dominant that excludes other taxa from a habitat, then it is possible to obtain a negative relationship between richness and ecosystem functioning. Results of a leaf pack breakdown experiment in a small stream suggested that the caddisfly Pycnopsyche gentilis , a common detritivorous insect in North American headwater streams, was both a functional and competitive dominant. In a second experiment we compared the effect of Pycnopsyche on leaf breakdown to that of other detritivore taxa by enclosing them with leaf packs in a section of headwater stream in which they were uncommon ( Pycnopsyche transplant experiment). Final leaf pack mass was significantly lower in the Pycnopsyche enclosure treatment; leaves exposed to a greater diversity of detritivores displayed little reduction in leaf mass. These results demonstrated that Pycnopsyche was a functionally dominant detritivore. In a third experiment ( Pycnopsyche density experiment) we found that Pycnopsyche was also a competitively dominant species. Leaf packs and large Pycnopsyche were placed in enclosures that were permeable to the majority of other detritivores but not Pycnopsyche . Leaf mass lost increased with increasing Pycnopsyche density. Leaf packs exposed to Pycnopsyche , however, contained fewer detritivore taxa which suggested that Pycnopsyche was also a competitive dominant. There was a negative relationship between three measures of diversity and leaf litter breakdown in the Pycnopsyche density experiment. Experiments conducted in natural communities that incorporate important species interactions may produce diversity-ecosystem function relationships other than the positive ones that are commonly reported.     

Phenological complementarity, species diversity, and ecosystem function

Increasing species diversity frequently enhances ecosystem function. Phenological complementarity, the asynchrony of species resource use and growth, may explain how species diversity influences ecosystem function but remains largely untested. We used an early successional plant community containing species with a variety of phenologies to test whether increasing species diversity enhances ecosystem function by increasing phenological complementarity. Over a two-year period, we increased environmental heterogeneity within an abandoned field with variation in disturbance, soil nutrients, water, light availability, and disturbance in 160 permanent plots, and measured percent cover of each plant species three times in each growing season. We did not manipulate species composition directly, and thus diversity and complementarity in each plot were the result of pre-existing conditions and responses of individuals to experimental treatments. Species diversity was measured in two ways, as the total number of species per plot and as the evenness of species abundances. Phenological complementarity was measured as the negative logarithm of the variance ratio. We tested whether the number of plant species per plot, species evenness, and their phenological complementarity in the first year predicted total annual cover in the second year. Total annual cover increased only moderately with number of species and evenness, consistent with studies that randomize species composition among replicate plots. Any effect that species number or evenness had on total annual cover, however, was not due to phenological complementarity. Rather, diversity was unrelated to phenological complementarity. These results indicate that naturally occurring variation in species diversity had little effect on whether phenological complementarity can enhance ecosystem function.     

物种多样性与生态系统功能时间变异性的关系研究进展

近年来物种多样性的急剧丧失使得物种多样性与生态系统功能的时间变异性的关系及其机制问题的研究成为了生态学研究的一个热点。综述了物种多样性与群落集合性质变异性以及种群性质变异性的关系及其机制的最新研究成果：1、理论上探讨造成物种多样性与群落集合性质变异性负相关关系的机制包括：抽样效应、资源利用分化假说、统计平均效应、保险假说、种群变异性的均匀度效应等;但实验研究对理论预期的支持并不是普遍的;2．多样性与种群变异性之间的关系主要依赖于均值-方差尺度系数Z;理论上大部分自然群落是种群变异性应该随着多样性的增加而增加;但有研究表明：在变动环境中多样性对单个组分物种的种群水平有稳定性作用;而经验研究并不能得出多样性对种群变异性效应的清晰模式。讨论了目前的理论和实验研究中存在的和今后研究中需要认真思考的问题。     

Ecosystem function and species loss – a microcosm study

There are too many kinds of organisms to be able to study and manage each, yet the loss of a single species can sometimes unravel an ecosystem. Such `fusewire species'– critical in the same sense that an electrical fuse can cut out a whole circuit – would be a rewarding focus for research and management effort. However, this approach can only be effective if these `fusewires' represent but a small proportion of the number of species in the system.  

Aim

To demonstrate methods for measuring what proportion of the species in a system are critical to ecosystem function.  

Methods

The prevalence of fusewire species was measured in manipulative experiments on an aquatic microcosm.  

Results

No single genus deletion caused changes in key characteristics of the system.  

Main conclusions

Comparison of these results with other published studies shows that the proportion of critical fusewire species varies amongst different ecosystems. The oxidation pond microcosms were shown to contain no single species indispensable to system function. They appear to be ill-suited to a management strategy which focuses on priority eukaryote species. However, a single study provides no evidence that this result is general or even typical of other kinds of ecosystems; it is presented here as an empirical model. Other methods of investigation are available; they are less experimentally rigorous but more practical. These could provide important guidance in planning an approach to management in a particular ecosystem.     

Effects of plant species richness and evenness on soil microbial community diversity and function

Understanding the links between plant diversity and soil communities is critical to disentangling the mechanisms by which plant communities modulate ecosystem function. Experimental plant communities varying in species richness, evenness, and density were established using a response surface design and soil community properties including bacterial and archaeal abundance, richness, and evenness were measured. The potential to perform a representative soil ecosystem function, oxidation of ammonium to nitrite, was measured via archaeal and bacterial amoA genes. Structural equation modeling was used to explore the direct and indirect effects of the plant community on soil diversity and potential function. Plant communities influenced archaea and bacteria via different pathways. Species richness and evenness had significant direct effects on soil microbial community structure, but the mechanisms driving these effects did not include either root biomass or the pools of carbon and nitrogen available to the soil microbial community. Species richness had direct positive effects on archaeal amoA prevalence, but only indirect impacts on bacterial communities through modulation of plant evenness. Increased plant evenness increased bacterial abundance which in turn increased bacterial amoA abundance. These results suggest that plant community evenness may have a strong impact on some aspects of soil ecosystem function. We show that a more even plant community increased bacterial abundance, which then increased the potential for bacterial nitrification. A more even plant community also increased total dissolved nitrogen in the soil, which decreased the potential for archaeal nitrification. The role of plant evenness in structuring the soil community suggests mechanisms including complementarity in root exudate profiles or root foraging patterns.     

Productivity and species richness in an arid ecosystem: a long-term perspective

To examine the different effects of rain pulse size on uptake of summer rains by three dominant desert plants in field conditions of desertified grasslands on the Ordos Plateau of northwestern China, we studied relationships between precipitation event size and rainwater uptake using stable isotopes of hydrogen in plant and soil water. Four natural precipitation events that represented precipitation sizes of 5.3, 8.3, 13.3, and 65.3 mm in the summer were chosen for the experiment. The perennial grass Stipa bungeana, the shrub Artemisia ordosia, and the herb Cynanchum komarovii – the dominant species in the communities – were compared for their use of summer rains with different pulse sizes based on the changes in the hydrogen isotope ratios (δD) of their stem water 7 days following each natural rain event. We found that S. bungeana and C. komarovii took advantage of shallow water sources derived from small (< 10 mm) rain events, A. ordosia took advantage of deeper soil water recharged by large (> 65 mm) rain events, and C. komarovii relied primarily on rain events of intermediate (10–20 mm) size. These different responses to rain pulses among species suggested that more frequent small rain events will promote the dominance of S. bungeana and C. komarovii, medium-sized events will facilitate development of C. komarovii, and large events will advance A. ordosia in this community. The rainwater utilization patterns of the three species would allow the coexistence of S. bungeana and A. ordosia or the coexistence of A. ordosia and C. komorovii in various successional serals following the disturbances. With an increase in variability of summer rain pulse size as predicted by climate change models, we expect that the structure of this community will undergo significant change in the future. Altered precipitation regimes, especially in combination with anthropogenic-related disturbances such as over-grazing, are likely to accelerate rates of degradation in northwestern China.     

Helminth species diversity of mammals: parasite species richness is a host species attribute

Studies investigating parasite diversity have shown substantial geographical variation in parasite species richness. Most of these studies have, however, adopted a local scale approach, which may have masked more general patterns. Recent studies have shown that ectoparasite species richness in mammals seems highly repeatable among populations of the same mammal host species at a regional scale. In light of these new studies we have reinvestigated the case of parasitic helminths by using a large data set of parasites from mammal populations in 3 continents. We collected homogeneous data and demonstrated that helminth species richness is highly repeatable in mammals at a regional scale. Our results highlight the strong influence of host identity in parasite species richness and call for future research linking helminth species found in a given host to its ecology, immune defences and potential energetic trade-offs.     

Successional diversity and forest ecosystem function

Forest inventory data was used to examine the relationship between successional diversity and forest ecosytem function. The inventory data show that stands composed of early successional species are more productive than stands composed of late successional species, whereas stands composed of late successsional species have lower turnover than stands composed of early successional species. Taken alone, these results would suggest that forests should be managed in a way that favors the most productive early successional species or longest-lived late successional species, depending on whether the goal is to maximize productivity or maximize carbon storage. However, the inventory data also show that stands with low successional diversity fix and store less carbon than stands with high successional diversity. This result suggests that forests should be managed in such a way as to retain species diversity while also favoring species that maximize the ecosystem function of interest.     

Gopher tortoise herbivory increases plant species richness and diversity

Plant Ecology - Mammalian herbivores often alter plant species richness and diversity, but such impacts have not been much investigated in reptiles. This study examined the effects of gopher...     

Regional species richness in an obligate subterranean dwelling fauna – epikarst copepods

Aim  The diversity of the obligate cave-dwelling fauna has proved difficult to measure because of the highly localized distributions of most species. We investigated: (1) the local and regional diversity patterns of a major component of the obligate cave-dwelling fauna living in the epikarst zone, the karst layer closest to the surface; (2) variations in local and regional patterns of species richness; and (3) sampling sufficiency at multiple scales.
Location  Caves in the Dinaric Mountains of Slovenia.
Methods  We sampled continuously the abundance of 37 species of copepods dislodged from the epikarst from 35 ceiling drips in six caves for a period of one year. Copepods were collected in a specially designed net that allowed continuous collection.
Results  Based on species accumulation curves and Chao estimates of total diversity, we determined that 3–4 months of continuous sampling were sufficient to find 90% of the species in a drip, that five drips were sufficient to find 90% of the species in a cave, and that five caves were sufficient to find 90% of the species in a region.
Main conclusions  The epikarst copepod fauna is a significant part of the aquatic cave fauna, contributing about 20% at the regional level. Because of the scale of variation, much of which occurs within a cave, and because of the availability of continuous sampling devices, the epikarst component of subterranean diversity seems to be more thoroughly and accurately measured than do other components.     

Interactive effects of species richness and species traits on functional diversity and redundancy

The importance of species diversity for ecosystem function has emerged as a key question for conservation biology. Recently, there has been a shift from examining the role of species richness in isolation towards understanding how species interact to effect ecosystem function. Here, we briefly review theoretical predictions regarding species contributions to functional diversity and redundancy and further use simulated data to test combined effects of species richness, number of functional traits, and species differences within these traits on unique species contributions to functional diversity and redundancy, as well as on the overall functional diversity and redundancy within species assemblages. Our results highlighted that species richness and species functional attributes interact in their effects on functional diversity. Moreover, our simulations suggested that functional differences among species have limited effects on the proportion of redundancy of species contributions as well as on the overall redundancy within species assemblages, but that redundancy rather was determined by number of traits and species richness. Our simulations finally indicated scale dependence in the relative effects of species richness and functional attributes, which suggest that the relative influence of these factors may affect individual contributions differently compared to the overall ecosystem function of species assemblages. We suggest that studies on the relationship between biological diversity and ecosystem function will benefit from focusing on multiple processes and ecological interactions, and that the relative functional attributes of species will have pivotal roles for the ecosystem function of a given species assembly.     

Australian Thysanoptera – biological diversity and a diversity of studies

Abstract   Studies in Australia on thrips have had extensive impacts worldwide. In behaviour, the latest definition of eusociality is derived from work on the radiation of thrips on Acacia species in central Australia, and these Acacia thrips also having been used to develop the concept of 'model clades' for analysing the evolution of behavioural and ecological diversity. In ecology, the concept of the lack of density dependent factors in population dynamics was elaborated through studies on the plague thrips of southern Australia. In virology, thrips were first shown in Australia to be the vectors of tospoviruses, although these viruses, their vectors and the plants attacked are all non-native to this continent. Work in Australia has included the development of electronic methods of illustration, identification and information transfer about thrips, including the use of molecular methods for pest species recognition, and considerable advances have been made in Australia in our knowledge of the relationships between thrips and plants, from polyphagy to pollination.     

Performance of nonparametric species richness estimators in a high diversity plant community

Abstract. The efficiency of four nonparametric species richness estimators — first‐order Jackknife, second‐order Jackknife, Chao2 and Bootstrap — was tested using simulated quadrat sampling of two field data sets (a sandy ‘Dune’ and adjacent ‘Swale’) in high diversity shrublands (kwongan) in south‐western Australia. The data sets each comprised > 100 perennial plant species and > 10 000 individuals, and the explicit (x‐y co‐ordinate) location of every individual. We applied two simulated sampling strategies to these data sets based on sampling quadrats of unit sizes 1/400th and 1/100th of total plot area. For each site and sampling strategy we obtained 250 independent sample curves, of 250 quadrats each, and compared the estimators’ performances by using three indices of bias and precision: MRE (mean relative error), MSRE (mean squared relative error) and OVER (percentage overestimation). The analysis presented here is unique in providing sample estimates derived from a complete, field‐based population census for a high diversity plant community. In general the true reference value was approached faster for a comparable area sampled for the smaller quadrat size and for the swale field data set, which was characterized by smaller plant size and higher plant density. Nevertheless, at least 15–30% of the total area needed to be sampled before reasonable estimates of S_t (total species richness) were obtained. In most field surveys, typically less than 1% of the total study domain is likely to be sampled, and at this sampling intensity underestimation is a problem. Results showed that the second‐order Jackknife approached the actual value of S_t more quickly than the other estimators. All four estimators were better than S_obs (observed number of species). However, the behaviour of the tested estimators was not as good as expected, and even with large sample size (number of quadrats sampled) all of them failed to provide reliable estimates. First‐ and second‐order Jackknives were positively biased whereas Chao2 and Bootstrap were negatively biased. The observed limitations in the estimators’ performance suggests that there is still scope for new tools to be developed by statisticians to assist in the estimation of species richness from sample data, especially in communities with high species richness.     

Soil microbes regulate ecosystem productivity and maintain species diversity

One of the major goals in ecology is to determine the mechanisms that drive the asymptotic increase in ecosystem productivity with plant species diversity. Niche complementarity, the current paradigm for the asymptotic diversity-productivity pattern, posits that the addition of species to a community increases productivity because each species specializes on different resources and thus can more thoroughly utilize the available resources. At higher diversity the increase in productivity decreases because resources become limiting, resulting in the classic asymptotic diversity-productivity pattern. An alternative but less tested explanation is that density-dependent disease from species-specific soil microbes drive the diversity-productivity relationship by increasing disease and thus decreasing productivity at low diversity. At higher diversity, productivity asymptotes because disease decreases with increasing diversity until it reaches a uniformly low level. Using a series of field experiments, we found that the classic asymptotic diversity-productivity pattern existed only when soil microbes were present. Soil microbes created the well-known pattern by depressing plant growth at low productivity though negative density dependent disease. In contrast, niche complementarity played only a weak role in explaining the diversity-productivity relationship because productivity remained high at low abundance in the absence of soil microbes. Based on our findings, the ongoing loss of species in natural ecosystems will likely increase per capita plant disease and lower ecosystem productivity. Furthermore, recent evidence suggests that negative density dependent disease maintains plant species diversity, and thus this single mechanism appears to link diversity maintenance to the diversity-productivity curve—two important ecological processes.Key words: density dependence, diversity-productivity, negative feedback, pathogens, species richness, soil microbesThe asymptotically saturating increase in ecosystem productivity with increasing diversity is a well know pattern in nature^1–4 (Fig. 1). The pattern has been used as an argument for the importance of species diversity,⁵ and understanding the mechanisms that drive the pattern is critical to determine the potential loss in productivity with ongoing and accelerating species loss in many ecosystems. The cause of the diversity-productivity pattern can be explained by either bottom-up control, such as plant resource competition, or top-down control from plant herbivores or pathogens. Most contemporary explanations for the pattern are centered on the bottom-up concept of niche-based resource competition, in which different species utilize different resources. The commonly accepted explanation, the niche complementarity hypothesis, states that the increase in species diversity increases productivity because each additional species uses a differ set of resources (e.g., nutrients) and thus more thoroughly utilizes whole-ecosystem resources.^3,4,6 At high diversity, however, the resource requirements of additional species overlap with existing ones and thus productivity no longer increases with diversity, resulting in the asymptotic diversity-productivity pattern (Fig. 1).Open in a separate windowFigure 1Theoretical relationship between species number and biomass. As diversity increases, total biomass increases asymptotically.Top-down control from plant enemies may also produce the asymptotic diversity-productivity pattern if the enemies are species-specific and have a strong negative density-dependent effect at low diversity. One general group of enemies is plant pathogens and parasites (bacterial, fungal, viral) that live in the soil and infect plant roots (hereafter referred to as soil pathogens). The specificity of soil pathogens has been shown in various studies and is now generally accepted.^1,7,8 The negative density dependent effect of plant pathogens at low diversity is likely because when diversity is low the relative abundance of each remaining species is high,^9–11 which leads to most individuals growing in close proximity of conspecifics and thus a greater probability of species-specific disease transmission. Unlike other plant enemies, such as foliar pathogens or insect and mammalian herbivores, which can be broadly dispersed, soil-borne pathogens may be a particularly effective driver of negative density dependent effects because they have low mobility and thus are more likely to infect nearby conspecifics, which causes increased disease at low diversity.^9–11 As diversity increases, the effect of soil-borne pathogens decreases because there is a lower likelihood of growing near a conspecific and there are lower concentrations of host-specific soil enemies.¹⁰ Consequently, soil-borne, species-specific disease may limit ecosystem productivity through top-down density-dependent regulation, even in the absence of niche-based explanations. Few studies, however, have considered the role of plant soil pathogens in driving the classic diversity-productivity relationship¹ (see also ref. ²) and, until now, no study has compared the two potential drivers simultaneously.¹We used a modeling approach to first demonstrate that both niche complementarity and species-specific soil pathogens can both theoretically drive the well-known diversity-productivity pattern.¹ We then used a series of complementary field experiments in grasslands in North America (Ontario, Canada and Minnesota, USA) to determine how plant disease and productivity change over a gradient of plant species richness in the presence and absence of soil microbes, and whether feedback between plants and their species-specific soil biota influenced the diversity-productivity pattern.¹ We first tested whether the asymptotic diversity-ecosystem productivity relationship arose in the presence of soil pathogens (a test of the negative density dependence hypothesis) or in the absence of soil pathogens (a test of the niche complementarity hypothesis). We then confirmed that soil biota were species specific and examined the decrease in plant disease and increase in productivity with increasing plant diversity.     

Tree species richness and density affect parasitoid diversity in cacao agroforestry

In Brazil, cacao is mostly planted beneath shade trees. The diversity of shade trees varies from monospecific to highly diverse canopies, characteristic of pristine Atlantic Forest. This study evaluates the relationships between family richness of Hymenoptera-Parasitica and Chrysidoidea, and tree species richness and density, the species richness of herbaceous understorey, and the area and age of the cacao agroforestry system. We sampled 16 cacao agroforestry systems, with canopy diversity ranging from one to 22 tree species per hectare, in three seasons: summer (March), winter (August) and spring (November). Parasitoids were sampled using eight Malaise-Townes traps per site. Tree species richness and density were enumerated within 1 ha at each site, and herbaceous plant species richness was calculated in eight 1 m² plots, within the hectare. The number of parasitoid families increased with tree species richness and density in spring and summer, but decreased in winter. Neither species richness of herbaceous plants nor area and age of the system affected parasitoid family richness. We suggest that the increase of parasitoid diversity with tree species richness and density in warmer seasons reflects increasing heterogeneity and availability of resources. The decrease in parasitoid family number with tree density in winter may be due to local impoverishment of resources, leading to parasitoid emigration to neighbouring forest remnants. This result implies that a higher diversity of shade trees will help to maintain high parasitoid levels and, in consequence, higher levels of natural enemies of cacao pests, particularly in the warmer seasons. This prediction is borne out in the experience of cacao producers. The proper management of shade tree diversity will play a vital role in maintaining the sustainability of cacao agroforestry production systems in the tropics and, concurrently, will maintain high biodiversity values in these locations.     

Regional patterns of diversity and estimates of global insect species richness

The total number of insect species in the world is an important if elusive figure. We use a fresh approach to estimate global insect species richness, based on biogeographic patterns of diversity of well or better documented taxa. Estimates generated by various calculations, all variations on a theme, largely serve to substantiate suggestions that insect species are likely to number around 10 million or less.     

Soft bottom species richness and diversity as a function of depth and iceberg scour in Arctic glacial Kongsfjorden (Svalbard)

Macrozoobenthic soft-sediment communities of central Arctic Kongsfjorden inhabiting six depth zones between 5 and 30 m were sampled using SCUBA-diving during June–August 2003 and analysed comparatively. About 63 taxa were found, nine of which had not been reported for Kongsfjorden and four for Svalbard. Suspension feeding or surface and sub-surface detritivorous polychaetes and deposit-feeding amphipods were dominant. Only 11 of the 63 taxa (45 species and additional 18 families not further identified) inhabited the complete depth range. Biomass ranged from 3.5 to 25.0 g ash free dry mass m⁻² and mean Shannon diversity (Log e) was 2.06. Similarity clustering from abundance and biomass data showed a significant difference between the shallow station (5 m) and the rest. The latter formed two sub-groups (10–20 and 25–30 m). Depth is irrevocably correlated with ice-scouring. Thus the differences in diversity together with the predicted iceberg scour intensity support the ‘intermediate disturbance hypothesis’ indicating that habitats impacted by moderate iceberg scouring enable higher diversity. In contrast, biotopes frequently affected only host pioneer communities, while mature, less diverse assemblages dominate depths of low impact.     

Mussels as ecosystem engineers: Their contribution to species richness in a rocky littoral community

Mussels are important ecosystem engineers in marine benthic systems because they aggregate into beds, thus modifying the nature and complexity of the substrate. In this study, we evaluated the contribution of mussels (Brachidontes rodriguezii, Mytilus edulis platensis, and Perna perna) to the benthic species richness of intertidal and shallow subtidal communities at Cerro Verde (Uruguay). We compared the richness of macro-benthic species between mussel-engineered patches and patches without mussels but dominated by algae or barnacles at a landscape scale (all samples), between tidal levels, and between sites distributed along a wave exposition gradient. Overall, we found a net increase in species richness in samples with mussels (35 species), in contrast to samples where mussels were naturally absent or scarce (27 species). The positive trend of the effect did not depend upon tidal level or wave exposition, but its magnitude varied between sites. Within sites, a significant positive effect was detected only at the protected site. Within the mussel-engineered patches, the richness of all macro-faunal groups (total, sessile and mobile) was positively correlated with mussel abundance. This evidence indicates that the mussel beds studied here were important in maintaining species richness at the landscape-level, and highlights that beds of shelled bivalves should not be neglected as conservation targets in marine benthic environments.     

Effects of trophic skewing of species richness on ecosystem functioning in a diverse marine community

Widespread overharvesting of top consumers of the world's ecosystems has "skewed" food webs, in terms of biomass and species richness, towards a generally greater domination at lower trophic levels. This skewing is exacerbated in locations where exotic species are predominantly low-trophic level consumers such as benthic macrophytes, detritivores, and filter feeders. However, in some systems where numerous exotic predators have been added, sometimes purposefully as in many freshwater systems, food webs are skewed in the opposite direction toward consumer dominance. Little is known about how such modifications to food web topology, e.g., changes in the ratio of predator to prey species richness, affect ecosystem functioning. We experimentally measured the effects of trophic skew on production in an estuarine food web by manipulating ratios of species richness across three trophic levels in experimental mesocosms. After 24 days, increasing macroalgal richness promoted both plant biomass and grazer abundance, although the positive effect on plant biomass disappeared in the presence of grazers. The strongest trophic cascade on the experimentally stocked macroalgae emerged in communities with a greater ratio of prey to predator richness (bottom-rich food webs), while stronger cascades on the accumulation of naturally colonizing algae (primarily microalgae with some early successional macroalgae that recruited and grew in the mesocosms) generally emerged in communities with greater predator to prey richness (the more top-rich food webs). These results suggest that trophic skewing of species richness and overall changes in food web topology can influence marine community structure and food web dynamics in complex ways, emphasizing the need for multitrophic approaches to understand the consequences of marine extinctions and invasions.