7000 years of turnover: historical contingency and human niche construction shape the Caribbean's Anthropocene biota.

Understanding plant community assembly and succession has long preoccupied ecologists, with a plethora of different theories (stochastic, deterministic and an intermediate situation of both) put forward over time. Currently, the role of historical contingency in forming alternative vegetation states is attracting increasing attention, with priority effects caused by order of arrival of different species producing long-term and significant effects on ecosystem functioning and diversity. The role of nutrient availability in modulating the strength of priority effects is an important consideration, since significant effects of order of arrival on communities may depend strongly on how many nutrients are available in the soil. The range and the effect size of these key drivers of assembly (historical contingency, nutrients) along temporal, spatial and resource related gradients are rarely addressed. The main underlying goal is to understand community assembly better to gain knowledge that can be applied in the restoration of species-rich grasslands, so that specific desired goals of an ecosystem can be met. The topic of this dissertation is the investigation of priority effects in relation to soil nutrient availability over time during assembly of semi-natural European grasslands. The focus is on both community traits and species-specific traits (including intraspecific responses of traits). The main grassland field experiment (Manuscript 2) showed that priority effects do play a role in community assembly of dry acidic grasslands, but the stability over time depended on the variables measured, with stable priority effects being found in relation to community aboveground biomass and plant functional group composition but not for community composition or species richness. The low nutrient availability negatively influenced the establishment of target species and the breadth of the priority effect was not as widely found in mesic grasslands, which suggested the important role of abiotic factors on community assembly. These results supported the findings of Sutherland (1974), that the vegetation in some ways reached a stable state in terms of the plant functional group relative composition but not in terms of species richness of species composition. Furthermore it could be shown that not the species richness of the initial seed mixture was the determining factor, but functional composition especially the plant functional group of legumes have an abiding positive effect on community traits (aboveground productivity and cover) in these grasslands, which could be also demonstrated using a new automated field measurement system (Manuscript 1). In Manuscript 3 the positive priority effect of the plant functional group of legumes on productivity found in the field, could be confirmed in a pot experiment using more nutrient-rich potting soil. In terms of productivity and functional group composition the simultaneously sown controls and the legume first treatment developed similarly. Which also underlines the importance of…

Pssst…pass the algae: succession in lichen soil crusts

Revealing the role of biodiversity in ecosystem functioning (BEF) has been a major focus of ecological research over recent decades. In general, results from artificially assembled communities point to the important role of biodiversity showing that loss of species has a negative effect on various ecosystem functions (mostly assessed by above‐ground peak biomass). However, the evidence from manipulations of natural communities is scarce, and results are often contradictory between these two approaches. In particular, the importance of species dominance for ecosystem functioning remains poorly understood. We created a gradient of plant species richness in a meadow community following a realistic species loss scenario (removal of less abundant species) to test the effect of diversity on community biomass and assess the importance of subordinate species compared with dominants in a 5‐year experiment. Contrasting with results of BEF experiments with artificial assembly, we did not find any relationship between plant species diversity and above‐ground biomass across the timeframe of the experiment. We provide evidence that dominant species' identity and traits are the main drivers of community biomass because dominant species were able to maintain biomass production after substantial species loss. Furthermore, dominants prevented community biomass from declining and biomass was indirectly influenced not by species richness but through differences in functional diversity. Our results support the mass ratio hypothesis, showing much bigger effect of dominant species on community biomass production and hints to the rather minor importance of the complementarity effect between species. We emphasize that BEF research should more focus on the role of dominant species in maintaining various ecosystem functions. Synthesis . Species diversity is a poor predictor of community above‐ground biomass production and dominant species can effectively compensate the total production after substantial loss of other species in a grassland community.

Biodiversity and ecological functioning of mudflat macrofauna in the Anthropocene

Global biodiversity is rapidly declining, resulting in far-reaching impacts on the functioning of ecosystems and human wellbeing. In recent decades, anthropogenic land use has been identified as a major driver of biodiversity loss, especially through the expansion and intensification of agricultural systems. While the drivers of biodiversity loss have been relatively clearly established, variability in the way that whole ecosystems respond to these drivers is still poorly understood. This is, in part, because we still lack a clear understanding of how species interactions govern the way that complex communities respond to environmental stressors, as well as their role in mediating ecosystem functioning. 
\nSpecies interactions can moderate community responses to land-use change via trophic cascades, whereby extinctions at the top or bottom of a food chain produce cascading effects through the rest of the food web due to the disruption of resource availability or predatory control of consumers. Additionally, species interactions are fundamental for ecosystem functioning as they are almost always directly linked to processes such as decomposition, herbivory, predation, pollination, and seed dispersal. Therefore, an approach to studying biodiversity and ecosystem functioning of naturally complex communities that incorporates multiple trophic levels and their interactions is crucial for predicting future global-change scenarios. Despite the conceptual advantage of a multitrophic approach, this has been rarely applied in the context of biodiversity and ecosystem functioning of ecosystems undergoing land-use change. In addition, while there has been considerable evidence established for the role of biodiversity in maintaining ecosystem functioning in local-scale experiments, there is still very limited knowledge of how this relationship scales up to landscapes in real-world ecosystems. In this thesis, I aimed to achieve a conceptual advance in biodiversity-ecosystem functioning (BEF) research within the context of global environmental change by investigating responses of complex multitrophic communities to land-use change and the resulting consequences for ecosystem functioning. 
\nFirstly, in Chapter 2, I combined data from a wide taxonomic range of trophic groups to test how communities of interacting species respond to tropical land-use intensification in Sumatra, Indonesia. I employed structural equation modelling to test if land-use intensification directly impacted all trophic groups or, alternatively, if it affected only lower trophic levels, resulting in bottom-up trophic cascades. Results from this model suggested that direct land-use impacts were generally much stronger than bottom-up trophic effects. Interestingly though, the number of direct effects from land-use intensification decreased considerably from plants to predators, whereas the number of bottom-up trophic effects increased dramatically with increasing trophic level. These findings suggest that the underlying mechanisms of land-use intensification that alter communities highly depend on the trophic level in question, indicating the need for trophic level-specific conservation management strategies. 
\nThe results from Chapter 2 provided strong evidence for the importance of species interactions in moderating community responses to land use, leading to the question of how ecological processes carried out by multitrophic communities are resultantly affected. One major challenge of BEF research has been to fully incorporate species interactions across multiple trophic levels to quantify a trophically broad measure of ecosystem functioning. In Chapter 3, I overcame this challenge by developing a measure of ecosystem functioning that integrates food web and metabolic theory to calculate community energy flux across multiple trophic levels. By calculating energy flux of multitrophic macroinvertebrate communities, I demonstrated that declining species diversity with increasing land-use intensity led to concomitantly strong declines in community energy flux. Furthermore, I found that the relationship between species richness and energy flux was steeper in the most intense land-use system, oil palm, but this result did not hold when trophic guilds were analysed independently. Thus, these findings suggest that if trophic groups are omitted, it is possible that BEF relationships could be misinterpreted in response to anthropogenic land use.
\nIn order to extend the previous chapter’s findings beyond the provisioning of ecosystem functioning of multitrophic communities, in Chapter 4, I investigated the functional stability and resilience of the macroinvertebrate communities to future perturbations. Using a trait-based approach, I determined how communities were assembled among different land-use types. I then calculated functional stability and community resilience by measuring the number of functionally redundant species within functional effect groups (based on traits that determine species’ influence on ecosystem processes) and the dispersion of traits within functional response groups (based on traits that determine species’ responses to disturbances). In doing so, I found that litter invertebrate communities in oil palm plantations were more randomly assembled, as well as having significantly fewer functionally redundant species. However, the jungle rubber agroforest system harboured communities with considerably higher functional redundancy than in oil palm. These results indicate that communities in high-intensity land-use systems are more susceptible to functional collapse given future perturbations, but low-intensity agroforests could help to maintain higher functional stability in anthropogenic landscapes.
\nFinally, in Chapter 5, I investigated how ecosystem functioning varies across spatial and environmental gradients and the mechanisms that give rise to spatial turnover in ecosystem functioning. To test this, I used data on litter macroinvertebrate communities from landscapes in Indonesia and Germany and applied the energy flux calculations developed in Chapter 3 as a measure of multitrophic ecosystem functioning. I then employed structural equation modelling based on distance matrices to establish how environmental and geographic distance drive turnover in species composition, species richness, functional trait dispersion and community biomass, and how these factors consequentially drive spatial turnover in community energy flux in a tropical and temperate region. Environmental distance appeared to be more important in the Indonesian compared with the German region for driving species turnover. However, the mechanisms that determined spatial turnover in ecosystem functioning were remarkably similar between the tropical and temperate regions, such that species richness and community biomass were the most important variables explaining spatial variability in energy flux. These results suggest that mechanisms such as species identity and niche complementarity may become redundant for predicting ecosystem functioning at the landscape scale. Instead, species richness and biomass should be sufficient for predicting multitrophic ecosystem functioning at large spatial scales.
\nOverall, in this thesis I demonstrate that species interactions are important for mediating responses of multitrophic communities to land-use intensification and that the loss of species across trophic levels has drastic consequences for the provisioning of multitrophic ecosystem functioning. Furthermore, this species loss reduces the stability of ecosystem functioning in intensified agricultural landscapes. Finally, I demonstrate that species richness and community biomass are the key components for developing a framework aimed at predicting likely scenarios of functional losses in intensified land-use systems at the landscape scale. Ultimately, by incorporating real-world complexity into studies that integrate across multiple ecological concepts, this thesis presents a significant advance toward understanding how ecosystems respond to anthropogenic land-use change, thus highlighting important areas for future exploration.

Food-chain length (FCL) is a fundamental ecosystem attribute, integrating information on both food web composition and ecosystem processes. It remains untested whether FCL also reflects the history of community assembly known to affect community composition and ecosystem functioning. Here, we performed microcosm experiments with a copepod (top predator), two ciliate species (intermediate consumers), and bacteria (producers), and modified the sequence of species introduction into the microcosm at four productivity levels to jointly test the effects of historical contingency and productivity on FCL. FCL increased when the top predator was introduced last; thus, the trophic position of the copepod reflected assembly history. A shorter FCL occurred at the highest productivity level, probably because the predator switched to feeding at the lower trophic levels because of the abundant basal resource. Thus, we present empirical evidence that FCL was determined by historical contingency, likely caused by priority effects, and by productivity.

Summary With ever‐increasing human pressure on ecosystems, it is critically important to predict how ecosystem functions will respond to such human‐induced perturbations. We define perturbations as either changes to abiotic environment (e.g. eutrophication, climate change) that indirectly affects biota, or direct changes to biota (e.g. species introductions). While two lines of research in ecology, biodiversity–ecosystem function (BDEF) and ecological resilience (ER) research, have addressed this issue, both fields of research have nontrivial shortcomings in their abilities to address a wide range of realistic scenarios. We outline how an integrated research framework may foster a deeper understanding of the functional consequences of perturbations via simultaneous application of (i) process‐based mechanistic predictions using trait‐based approaches and (ii) detection of empirical patterns of functional changes along real perturbation gradients. In this context, the complexities of ecological interactions and evolutionary perspectives should be integrated into future research. Synthesis and applications. Management of human‐impacted ecosystems can be guided most directly by understanding the response of ecosystem functions to controllable perturbations. In particular, we need to characterize the form of a wide range of perturbation–function relationships and to draw connections between those patterns and the underlying ecological processes. We anticipate that the integrated perspectives will also be helpful for managers to derive practical implications for management from academic literature.

AimClimate change affects forest functioning not only through direct physiological effects such as modifying photosynthesis and growing season lengths, but also through indirect effects on community composition related to species extinctions and colonizations. Such indirect effects remain poorly explored in comparison with the direct ones. Biodiversity–ecosystem functioning (BEF) studies commonly examine the effects of species loss by eliminating species randomly. However, species extinctions caused by climate change will depend on the species’ vulnerability to the new environmental conditions, thus occurring in a specific, non‐random order. Here, we evaluated whether successive tree species extinctions, according to their vulnerability to climate change, impact forest functions differently than random species losses.LocationEleven temperate forests across a gradient of climatic conditions in central Europe.MethodsWe simulated tree community dynamics with a forest succession model to study the impact of species loss on the communities’ aboveground biomass, productivity and temporal stability. Tree species were removed from the local pool (1) randomly, and according to (2) their inability to be recruited under a warmer climate or (3) their increased mortality under drier conditions.ResultsResults showed that non‐random species loss (i.e., based on their vulnerability to warmer or drier conditions) changed forest functioning at a different rate, and sometimes direction, than random species loss. Furthermore, directed extinctions, unlike random, triggered tipping points along the species loss process where forest functions were strongly impacted. These tipping points occurred after fewer extinctions in forests located in the coldest areas, where ecosystem functioning relies on fewer species.Main conclusionsWe showed that the extinction of species in a deterministic and mechanistically motivated order, in this case the species vulnerability to climate change, strengthens the selection effect of diversity on ecosystem functioning. BEF studies exploring the impact of species loss on ecosystem functioning using random extinctions thus possibly underestimate the potential effect of biodiversity loss when driven by a directional force, such as climate change.

The order and timing of species immigration during community assembly can affect species abundances at multiple spatial scales. Known as priority effects, these effects cause historical contingency in the structure and function of communities, resulting in alternative stable states, alternative transient states, or compositional cycles. The mechanisms of priority effects fall into two categories, niche preemption and niche modification, and the conditions for historical contingency by priority effects can be organized into two groups, those regarding regional species pool properties and those regarding local population dynamics. Specifically, two requirements must be satisfied for historical contingency to occur: The regional pool contains species that can together cause priority effects, and local dynamics are rapid enough for early-arriving species to preempt or modify niches before other species arrive. Organizing current knowledge this way reveals an outstanding key question: How are regional species pools that yield priority effects generated and maintained?

Towards a multidimensional view of biodiversity and ecosystem functioning in a changing world

Human activity has dramatically accelerated both species extinctions and introductions, and the balance of these two processes is generally expected to reduce biodiversity and increase taxonomic homogenization. However, few tests of this hypothesis have been made. We tested whether new macroinvertebrate invaders in North American freshwaters can replace the recent loss of biodiversity, particularly focusing on molluscs. We found that both crustaceans and molluscs are overrepresented among endangered and recently extinct species, as well as among invaders. For molluscs, the number of recently extinct species (79 species) was more than twice that for exotic species (38 species). In addition, molluscan invaders are from different taxonomic families than recently extinct or endangered species. While most extinct and endangered molluscs are from streams and rivers, invaders preferentially colonize lakes and reservoirs. The impact of humans on species introductions and extinctions increases with spatial scale (from local to continental scales), resulting in the increased phylogenetic dissimilarity between introduced species and native communities. Construction of dams and alteration of the flow regimes of lotic systems will continue to lead to the extinction of native species, and promote the spread of invaders, resulting in a loss of biodiversity and taxonomic homogenization.

Grazing effects on arid and semi‐arid grasslands can be constrained by aridity. Plant functional groups (PFGs) are the most basic component of community structure (CS) and biodiversity & ecosystem function (BEF). They have been suggested as identity‐dependent in quantifying the response to grazing intensity and drought severity. Here, we examine how the relationships among PFGs, CS, BEF, and grazing intensity are driven by climatic drought. We conducted a manipulative experiment with three grazing intensities in 2012 (nondrought year) and 2013 (drought year). We classified 62 herbaceous plants into four functional groups based on their life forms. We used the relative species abundance of PFGs to quantify the effects of grazing and drought, and to explore the mechanisms for the pathway correlations using structural equation models (SEM) among PFGs, CS, and BEF directly or indirectly. Grazers consistently favored the perennial forbs (e.g., palatable or nutritious plants), decreasing the plants’ relative abundance by 23%–38%. Drought decreased the relative abundance of ephemeral plants by 42 ± 13%; and increased perennial forbs by 20 ± 7% and graminoids by 80 ± 31%. SEM confirmed that annuals and biennials had negative correlations with the other three PFGs, with perennial bunchgrasses facilitated by perennial rhizome grass. Moreover, the contributions of grazing to community structure (i.e., canopy height) were 1.6–6.1 times those from drought, whereas drought effect on community species richness was 3.6 times of the grazing treatment. Lastly, the interactive effects of grazing and drought on BEF were greater than either alone; particularly, drought escalated grazing damage on primary production. Synthesis. The responses of PFGs, CS, and BEF to grazing and drought were identity‐dependent, suggesting that grazing and drought regulation of plant functional groups might be a way to shape ecosystem structure and function in grasslands.

Priority effects are an important ecological force shaping biotic communities and ecosystem processes, in which the establishment of early colonists alters the colonization success of later-arriving organisms via competitive exclusion and habitat modification. However, we do not understand which biotic and abiotic conditions lead to strong priority effects and lasting historical contingencies. Using saprotrophic fungi in a model leaf decomposition system, we investigated whether compositional and functional consequences of initial colonization were dependent on initial colonizer traits, resource availability or a combination thereof. To test these ideas, we factorially manipulated leaf litter biochemistry and initial fungal colonist identity, quantifying subsequent community composition, using neutral genetic markers, and community functional characteristics, including enzyme potential and leaf decay rates. During the first 3months, initial colonist respiration rate and physiological capacity to degrade plant detritus were significant determinants of fungal community composition and leaf decay, indicating that rapid growth and lignolytic potential of early colonists contributed to altered trajectories of community assembly. Further, initial colonization on oak leaves generated increasingly divergent trajectories of fungal community composition and enzyme potential, indicating stronger initial colonizer effects on energy-poor substrates. Together, these observations provide evidence that initial colonization effects, and subsequent consequences on litter decay, are dependent upon substrate biochemistry and physiological traits within a regional species pool. Because microbial decay of plant detritus is important to global C storage, our results demonstrate that understanding the mechanisms by which initial conditions alter priority effects during community assembly may be key to understanding the drivers of ecosystem-level processes.

Abstract: Due to habitat loss, disease, and introduction of nonnative species, many native species in Hawai‘i have gone extinct or are at risk of extinction. As a result of interspecific interactions such as pollination, the decline, loss, or introduction of species can have cascading effects on island ecosystems. We studied Hylaeus spp., Hawai‘i's yellow-faced bees, the only native bees in Hawai‘i. This group of potentially important pollinators has been largely overlooked until recently, and its conservation status and ecological role are virtually unknown. We investigated how native (Hylaeus spp.) and nonnative (Apis mellifera) bees interact with flowering plants in a large-scale pasture-to-forest restoration system. We used pan traps and nets to collect bees in mature forest, remnant corridors, planted Acacia koa tracts, and open pastureland. We removed pollen from each specimen and identified it using pollen samples collected on-site. We found that Hylaeus spp. were more likely to carry less pollen a...

[Extract] Governments, businesses, financial institutions and local communities are increasingly using biodiversity offsets, also known as compensatory mitigation, as a putative mechanism to achieve 'no net loss' (NNL) of biodiversity as a result of specific development projects (McKenney & Kiesecker, 2010; Quetier & Lavorel, 2011; Gardner et al., 2013). The Business and Biodiversity Offsets Programme (BBOP), an international collaboration for the development of offset methodologies, defines biodiversity offsets as 'the measurable conservation outcomes resulting from actions designed to compensate for significant residual adverse biodiversity impacts arising from project development after appropriate prevention and mitigation measures have been taken. The goal of biodiversity offsets is to achieve no net loss and preferably a net gain of biodiversity on the ground with respect to species composition, habitat structure, ecosystem function and people's use and cultural values associated with biodiversity' (BBOP, 2009). Proposals are already proceeding in the European Union (EU) for a NNL initiative as part of the 'EU Biodiversity Strategy to 2020' – with possible operational principles that include offsetting schemes (see http://ec.europa.eu/environment/nature/biodiversity/nnl/index_en.htm). Madsen et al. (2011) identified legislation mandating compensatory biodiversity conservation mechanisms (including offsets) in 45 countries, with a further 27 under development and this number is likely to have grown since.