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Review

Bridging the Gaps: Exploring Aquatic–Terrestrial Connectivity through the Trait-Based Ecology of Riparian Predatory Arthropods

by
Cristian Andrei Murgu
1 and
Geta Rîșnoveanu
1,2,*
1
Doctoral School of Ecology, University of Bucharest, 050095 Bucharest, Romania
2
Department of Systems Ecology and Sustainability, Faculty of Biology, University of Bucharest, 050095 Bucharest, Romania
*
Author to whom correspondence should be addressed.
Water 2023, 15(22), 3983; https://doi.org/10.3390/w15223983
Submission received: 20 October 2023 / Revised: 10 November 2023 / Accepted: 13 November 2023 / Published: 16 November 2023
(This article belongs to the Special Issue Monitoring and Evaluation for Ecosystem Management in Rivers and Streams)
Review Reports Versions Notes

Abstract

:
Riparian predatory arthropods represent one of the main trophic links between lotic and terrestrial ecosystems along riverine landscapes. The use of the trait-based approach promises to enhance our understanding of how these predatory communities interact with their environment through their response to various drivers of change and through their trophic interactions. We reviewed the scientific literature focused on the interaction between drivers of community change (natural and anthropogenic) and the functional traits and functional diversity components that characterize riparian ground beetles and spiders and, ultimately, on their role as cross-ecosystem trophic links. We highlight land use changes and river regulations as the strongest drivers that change the communities we study, often through various interacting mechanisms that favor the replacement of riparian specialists with generalist species, thus altering aquatic–terrestrial connectivity and the resilience of riverine arthropod consumers. Tropical regions and traits related to community responses to extreme climatic events (e.g., submersion tolerance and desiccation resistance) are less studied, while inconsistent patterns are noticed for well-studied traits, especially for spiders (e.g., their feeding preference response to aquatic subsidy availability and their body size response to flooding and bank hydrological connectivity). Future research should focus on the aforementioned drivers and knowledge gaps, along with the functional diversity changes in predatory arthropod communities along environmental and anthropogenic impact gradients, in order to improve riparian conservation.

1. Introduction

Riparian zones represent interfaces between lotic and terrestrial ecosystems [1,2] especially known for supporting and regulating ecosystem services [3]. As ecotones, they provide habitats for many specialized biotic communities, distinct from the uplands, as well as increase regional biodiversity [4] and help regulate multiple ecosystem processes [5]. Regardless, rivers and riparian corridors are still subjected to anthropogenic pressures such as hydro-morphological alterations and land use changes [6], which predominantly impact specialized taxa and the benefits they provide [7,8].
Riparian predatory arthropods link lotic and terrestrial ecosystems by consuming emergent insects that are further transferred along terrestrial food webs [9]. Through evolution, these predators have adapted to the dynamic riparian conditions and developed specific features (i.e., traits) that improve their capacity to avoid natural flooding and to capitalize on aquatic prey [10]. This specialization and their sensitivity to change have certified the use of riparian predatory arthropods as bioindicators that are important in riverine landscape management and conservation [7].
The increased use of the trait-based approach should improve our knowledge of organism–environment interactions [11]. This method proposes traits, defined as the functional phenotypic features of an organism that determine its fitness [12], as the common currency used to characterize biotic communities. As such, more research has begun to implement the trait-based approach, along with taxonomic diversity, in order to analyze the response of riparian predatory arthropods to various drivers and its implications for the ecosystem functions they regulate. Recent reviews have addressed the use of traits [13] and of the trait-based approach for the research of arthropods in general [11,14]. Closer to our topic, Ramey and Richardson [5] reviewed the mechanisms that shape the diversity of riparian arthropod communities, while Batzer and Wu [15] detailed the functional characteristics that make these communities unique. To the best of our knowledge, this is the first review that investigates the trait-based ecology of riparian predatory arthropods to understand the drivers and community responses that shape the capacity of these predators to connect aquatic and terrestrial ecosystems through trophic interactions. We believe this to be a step forward in fostering the research and conservation of riparian biodiversity and of the ecosystem services it provides.
We aim to critically analyze the scientific literature focused on (1) the interaction between drivers of change (natural and anthropogenic) and the functional diversity (FD) of riparian ground beetles and spiders; and (2) on their role as cross-ecosystem trophic links. To achieve this goal, firstly, we identify the most relevant metrics used to assess the functional changes in riparian predatory arthropods and showcase patterns in their response to the key drivers and acting mechanisms. We then discuss how the resulting changes in these communities shape aquatic–terrestrial connectivity. Finally, we suggest how existing knowledge can be used for riparian conservation and propose future research directions.

2. Literature Screening and Curation Methodology

We focused solely on ground beetles (the family Carabidae) and spiders (the order Araneae), as they represent relevant bioindicators and dominant riparian arthropod taxa [5,15]. The responses of riparian predatory arthropod communities were evaluated based on traits, FD components, and trait-mediated behaviors (for details, see Table 1). The drivers and acting mechanisms are also detailed in Table 1, while our overall conceptual approach to the response (i.e., environmental filtering and disturbance influence) and role (i.e., biodiversity–ecosystem interactions) of FD was based on the research of Wong et al. [11].
We searched and selected available research from indexed peer-reviewed journals in English, published until September 2023. We aimed to be as inclusive as possible and, as such, used complementary literature search methods to assure good coverage, whilst using selection criteria to obtain only relevant results. We used a search string on the Web of Science (WoS) database (Table 2), then applied the snowball method on the result of our query and a bibliographic search of the first authors. From each search, we selected only articles that measured and analyzed traits, components of FD, and trait-mediated behaviors, as previously described. Based on the query, we identified 21 articles out of 102 and found another 94 after complementary searches. A total of 115 articles were included in our analysis (partly cited in tables and partly in text; see the paragraph below) and discussions (listed in the Supplementary Material). We acknowledge the possibility of having omitted a few sources; however, we believe that the total research gathered faithfully represents the current state of knowledge on the topic. Furthermore, we present and discuss the main tendencies and patterns of community-weighted mean (CWM) responses to drivers and acting mechanisms, as reported from the literature, mentioning and discussing existing deviations when relevant.
Our results include three sections. The first section begins with our main findings, then details the effect of the identified drivers and acting mechanisms on (1) the main functional metrics that characterize riparian predatory arthropods; and (2) the capacity of these communities to link lotic and terrestrial ecosystems by preying on aquatic insects. We provide a brief overview of each relevant functional metric and highlight existing trends in their response to drivers and acting mechanisms. Next, we discuss how resulting community changes in riparian predatory arthropods influence cross-ecosystem connectivity. For the most researched functional metrics (feeding preference, body size, and dispersal capacity), we used graphical representations of the existing trends in the response of the studied communities, referenced with numbered citations, as provided in the Supplementary Material. The second section details knowledge gaps and suggests research directions, while the final section summarizes our main findings and their importance for the research and conservation of riparian predatory arthropods and their habitats.

3. The Effects and Consequences of Drivers and Acting Mechanisms on the Functional Diversity of Riparian Predatory Arthropods and Aquatic–Terrestrial Connectivity

3.1. Main Findings

We highlight land use changes and hydro-morphological alterations as the most damaging drivers that shape the FD of riparian predatory arthropods. We consider the biophysical integrity of the riparian habitat (i.e., its native vegetation integrity and substrate quality) to supersede aquatic subsidy availability in determining the choice and capacity of riparian predatory arthropods to consume aquatic prey and, hence, regulate aquatic–terrestrial linkages. Riparian habitat alteration, in terms of spatial availability and biophysical diversity, changes predatory arthropod communities by trait filtering and trait-mediated behaviors. Native vegetation removal and river regulation generally reduce the abundance and diversity of sensitive specialists and favor the occurrence of generalists with wider ecological niches [19,20]. The deterioration of biophysical structures and its cascading effects on aquatic subsidies also produce dietary shifts within the functional group, as compared to unaltered riparian habitats [21,22,23]. Research on FD components has further emphasized the importance of trait composition, instead of trait diversity, and of traits associated with flood resistance and aquatic feeding for the resilience and integrity of riparian predatory arthropod communities [24,25]. Overall, the trait-based approach represents an important framework for understanding the community dynamics of riparian arthropods and their role in ecosystem functioning along river corridors. This method should be used along with other approaches (e.g., taxonomic, phylogenetic, and genetic) to obtain the best results. More research is needed to fully understand the relationship between riparian predatory arthropods and their habitat, especially in tropical climates, which have been virtually unstudied, and with regard to their trait-mediated behaviors and least-researched traits (e.g., submersion tolerance, desiccation resistance).

3.2. Feeding Preference

We found that feeding preference is the most studied and important trait of riparian predatory arthropods, due to its direct role in determining aquatic–terrestrial connectivity. The stable isotope analysis (SIA) was the method of choice used to analyze this trait, followed by the polyunsaturated fatty acid (PUFA) analysis, which is increasingly popular due to its better capacity to discern food origin and nutritional quality [22,23].
The aquatic subsidy represents a major acting mechanism linked to the CWM feeding preference of riparian predatory arthropods. The emergent insect abundance [26,27], emergence timing and duration [9], FD of the prey [28,29], and its nutritional value [23,30] all influence the food choice of riparian consumers and cross-ecosystem connectivity. In general, higher aquatic subsidy availability unequivocally increases its consumption by ground beetles, though species-specific feeding preferences have been reported [31,32], while spiders exhibit fluctuating responses (Table 3). This can be explained by the different ecological preferences and life strategies among groups and by the diversity, identity of species, and habitats sampled [33]. Numerous riparian specialized ground beetles are known to feed mainly on aquatic prey, while spiders, with certain exceptions, seem to exhibit more plasticity in their feeding preference based on the availability of different food sources [9,21,22].
The effect of aquatic subsidy timing and duration has only been studied for spiders. The diversity of aquatic insects and river microhabitats prolongs the emergence of the former and strengthens aquatic–terrestrial connectivity by allowing underdeveloped web hunters to capitalize on aquatic subsidies [65]. Emergence timing also influences the fitness and community structure of active hunting spiders which varies with their development stage and body size [66]. It is very likely that anthropogenic alterations of the emergence timing can influence the feeding preference and trait-mediated community structure of riparian predatory arthropods [30], with more research still needed to explore the response mechanism of recipient communities.
Generally, the aquatic feeding preference of riparian predatory arthropods drastically decreases a few meters from the stream [67]. However, we observed this might not be universally true, as results seem to dependent on the method, with PUFA research usually reporting higher levels of aquatic food consumption further inland than SIA research [23,40,68]. This can be argued by the fact that PUFAs can be traced further inland due to their long-term storage in the cellular membranes of consumers [69]. A comparative study by Kowarik et al. [68] found PUFA analyses indicate the extent of aquatic feeding preferences significantly further than SIAs, highlighting the importance of complementary methods.
The traits and nutritional content of emergent insects influence their consumption by predatory arthropods. Larger, highly mobile emergent insects are especially valuable for inland predators due to their high fatty acid content [39]. The lateral extent of aquatic food consumption is mainly addressed for spiders, known to generally consume live prey [70]. Consequently, the aquatic feeding preference of inland spiders should be more influenced by the dispersal capacity of their prey and by the riparian biophysical structure [53] than would be the case for ground beetles, which are also known to forage on dead prey [36]. The riparian biophysical structure and meteorological conditions may be more important for carabids in open habitats, where weak flying emergent insects can be transported further inland by the wind and land on the ground [71,72]. Though less studied, it was observed that the emergence strategy (from water or from land) of aquatic insects can also influence the feeding preference of riparian consumers and their nutritive uptake [23], as can the body size of emergent insects [55], thus reinforcing the need for more research on this topic.
In general, biophysical alterations of riparian and lotic habitats not only influenced the CWM feeding preference of riparian predatory arthropods (Table 3) by inducing structural changes in their communities [60,73] and those of their prey [53], but also by changing trait-mediated behaviors of the consumers and prey [22,23]. The impact of vegetation removal was higher for riparian web hunting spiders [61] as opposed to ground-dwelling predators, which were shown to be more dependent on the availability and quality of river sediments and soil characteristics [8,20] (Table 3). These findings show the necessity of maintaining pristine riparian habitats and naturally flowing rivers in order to maintain the functional roles of riparian predatory arthropods.

3.3. Body Size

Aquatic subsidy availability constitutes a major mechanism that shapes the body size and growth rate of riparian predatory arthropods. Its characteristics influence the CWM body size of riparian predators and aquatic–terrestrial connectivity to various degrees and through multiple pathways. Aquatic subsidy availability seems to be directly correlated with the body size of riparian spiders (Table 4). The absence of aquatic subsidies or reductions in availability unanimously stunt the growth and development of riparian spiders [49,60,73]. Conversely, increased subsidies, delivered continuously or as post-starvation pulses, increase the growth rate and overall fitness of riparian spiders (Table 4), given that their body size matched that of their prey [55]. Marczak and Richardson [66] reported that spiders subjected to food pulses during early development stages had an overall smaller body size and growth rate compared to those that experience pulses later in their development, probably due to reduced metabolic efficiency induced by the initial food abundance. We found no experimental research linking subsidy characteristics to the body size and growth of riparian ground beetles. We assume that the body size of riparian carabids is more likely to be influenced by habitat characteristics and acting disturbances rather than prey consumption.
Aside from its effect on subsidies, flooding represents a direct life threat for riparian arthropods and an acting mechanism that structures shoreline microhabitats. CWM body size response to flooding showed divergent trends among taxa, likely based on habitat specialization and post-disturbance temporal dynamics (Table 4). Riparian post-flooding abundances of larger cursorial spiders usually increase, mostly due to the arrival of larger, well-dispersed generalist species [10,82] (Table 4) in search for food. The CWM body size of web hunters also increases (Table 4), likely based on the same type of aggregative response [83,84]. The CWM body size of specialized ground beetles generally decreases with flooding disturbances [8,74,83]; however, high variation among taxa was observed when comparing different subfamilies [10].
It is generally accepted that an increase in an individual’s body size broadens its food resource spectrum [85] and that subsidy availability increases the growth rate and body size of riparian predatory arthropods (Table 4). The body size of spiders, mostly generalists, negatively correlates with the amount of aquatic prey consumed, as grown individuals start to prey on larger terrestrial taxa [54], thus influencing aquatic–terrestrial connectivity through trait-mediated behavior. Increased subsidy availability also produces aggregative responses, attracting larger generalist predators [56,82] with higher degree of feeding plasticity, which influences aquatic–terrestrial connectivity through structural changes in riparian communities. Though more research is needed [32], in the case of ground beetles, body size does not seem a strong determinant of their feeding preference, and probably influences aquatic–terrestrial connectivity more in structural than behavioral ways. The reduced body size of carabid specialists provides substantial advantages, enabling them to inhabit interstitial spaces formed through sediment deposition, and as such, facilitates the development and survival of these aquatic-feeding predators.
Land use changes have generally increased the CWM body size of riparian predatory arthropods by favoring larger generalist species (Table 4). River regulation, native vegetation removal, and sediment or soil deterioration are all associated with increased abundances of larger predatory arthropods (Table 4). Drought was the only anthropogenic driver that decreased the body size of riparian consumers, acting by reduced subsidy availability and riparian substrate changes (Table 4). Overall, the body size of riparian predatory arthropods can influence aquatic–terrestrial connectivity by determining the prey choice of certain generalist species [54] and by increasing overall predation by specialized taxa due to increased metabolic needs. Except for post-flooding aggregative responses, an increased CWM body size of riparian predatory arthropods is equivalent to less dynamic riparian conditions caused by anthropogenic impacts and can be used as an indicator of habitat changes.

3.4. Dispersal Capacity

Flooding is arguably the most important natural driver that shapes the dispersal capacity of riparian predatory arthropods. It does so directly, by environmentally filtering processes [76], or indirectly, by its influence on the characteristics of riparian habitats and control of aquatic subsidies (Table 5). Anthropogenic impacts in the riparian zone also appear to increase the dispersal capacity of carabids, a trend that was also observed in other anthropogenically disturbed habitats [86]. Altogether, these aforementioned responses underline the general effect of habitat disturbances in filtering carabid communities, based on their dispersal capacity [86].
Unmanaged and minimally impacted riparian zones with natural flooding and sediment dynamics support predatory arthropod communities with a higher dispersal capacity (Table 5). Increased hydrological connectivity between rivers and riparian zones facilitates the presence of specific microhabitats and plant communities [5,33] and increases the abundance and density of highly mobile habitat specialists (Table 5). Exposed river sediments (ERSs) constitute a preferred microhabitat of riparian specialized ground beetles and cursorial spiders [91], both known to be highly mobile, aquatic feeding taxa. Alone, riparian vegetation has been shown to have no direct effect on the dispersal capacity of riparian spiders in floodplains and hills (Table 5), but rather an indirect effect through the increased abundance of spiders with an increased niche width in open habitats associated with increased prey abundance [79]. Vegetation reductions were reported to directly increase the abundance of macropterous ground beetles [80] and the diversity and richness of coleopterans and web hunting and ambush hunting spiders [33] in unflooded riparian forest habitats.
Dispersal capacity influences how riparian predatory arthropods regulate aquatic–terrestrial connectivity by determining their capacity to survive flooding (i.e., it controls the community structure) and to capitalize on the presence of post-flooding insects washed ashore (i.e., it enables behavioral responses). Well-dispersed riparian specialists feed preponderantly on aquatic prey [92]; therefore, suitable habitat conditions should facilitate biomass transfer between lotic and terrestrial ecosystems. Though understudied at larger spatial scales, the dispersal capacity of riparian predatory arthropods can also influence how these predators colonize suitable downstream or upstream habitats, up to a reasonable distance, through aquatic and aerial dispersal in the first case, and through aerial and terrestrial dispersal for the latter. Though dispersal capacity plays an important role in the community structure of riparian predatory arthropods, it is important to note that specialist species have also been documented to have additional survival strategies and behaviors [93,94,95].

3.5. Least-Studied Traits and Behaviors: Desiccation Resistance, Submersion Tolerance, and Flood Avoidance Behavior

Desiccation resistance is addressed for both spiders and ground beetles to explain the community structure and trophic interactions of riparian predatory arthropods in arid climates [62,64] and those experimentally measured in a few desiccation stress experiments [96,97]. Due to there being scarce research, few conclusions can be drawn; however, the existing results seem to reliably predict the response of riparian predatory arthropods to desiccation stress. DeVito and Formanowicz [97] showed that juvenile spiders were more resistant to increased desiccation stress than adults. However, in the field, no direct correlation between individual body size and desiccation-induced mortality could be found [74]. This suggests that other individual response mechanisms shape the desiccation resistance of riparian arachnids. Increased predation is unanimously shown to increase the survival rate of the consumers, while death rates are observed to be nonlinear, suggesting a community resistance threshold [62,64,74,97]. It is very plausible that larger predators will survive desiccation by consuming smaller prey, even resorting to cannibalism when needed [63,74]. In contrast, the reduced body size of juveniles could also prove beneficial for their survival by enabling them to access the deeper, less spacious layers of river sediment which could still maintain moisture during long periods of drought [98].
The desiccation resistance of riparian ground beetles strongly correlates with the ecological preferences and life strategies of the studied taxa. Yet, we found no research that addressed the effect of desiccation stress on their feeding preference and trophic interactions. Riparian specialized carabids of the genus Bembidion showed lower desiccation resistance compared to that of generalist species [96]. The transpiration rate of carabids was observed to vary according to their developmental stage and habitat specialization and to be inversely proportional to the humidity conditions experienced. A CWM biomass reduction was also shown to follow a nonlinear tendency with increasing desiccation stress [96,99]. So far, desiccation resistance research has emphasized the importance of predation for individual survival and community response [100], whilst also underlining the importance of body size and other survival adaptations for community structure. Additionally, we believe the lower desiccation resistance of riparian specialists further justifies the importance of riparian zone conservation in order to maintain its increasingly valuable microhabitat conditions.
The submersion resistance and flood avoidance behavior of both arthropod groups are briefly addressed. Lambeets et al. [94,101] found significantly different orientation behaviors between riparian specialists and generalist sympatric species, with the former category exhibiting more between-individual variability. This indicates that when avoiding rising water, specialists are guided more by internal factors, related to an individual’s origin [101], but also by individual experience, which allows them to better escape floods. Spiders as well as carabids seem to orient themselves based on lights and shapes that are associated with the higher vegetation characteristics of safe upland habitats [94,101,102]. The movement activity of ground beetles along riverbanks was found to be higher in and towards riparian zones with native vegetation [103,104] and with increased ERS availability [91,105], thus reaffirming the importance of habitat availability and diversity.
We found little research concerning the submersion tolerance of riparian taxa, though we acknowledge more species-specific research could exist. Most riparian arthropods were observed to resist flooding by relying on their dispersal capacity to avoid it [82]; however, few species can withstand significant periods of submersion by employing different strategies such as air pockets [106], floating or sailing [95], tree climbing [107], and overwintering in the sediment [106,108]. All of the species which exhibited such flood survival strategies were riparian specialists. This aspect raises the importance of the conservation of riverbank habitats, as they host unique and sensitive species functionally distinct from generalists. Future research on these traits can benefit from addressing their effect on the longitudinal connectivity and recolonization potential of restored habitats and also from examining how these traits determine the trophic interaction of riparian specialists, as some species might use submergence to avoid predation [95].

3.6. Functional Diveristy Components

FD still represents a novelty in the research on riparian predatory arthropods. It has always been considered in conjunction with taxonomic diversity, almost exclusively for carabids and only along large rivers. The effects of both natural and anthropogenic drivers were evaluated, while the mechanism studied was related to hydro-morphological connectivity and native vegetation integrity. FD was usually assessed based on the following: the diversity of traits and trait composition within communities, the degree of functional similarity, functional redundancy, functional evenness, functional divergence, and functional dispersion.
Studies on the effect of habitat restoration on the trait diversity of riparian carabids indicate a slight increase in this variable, although it was not statistically significant [109]. However, trait composition was positively and significantly correlated with environmental variables associated with habitat restoration and hydrologic connectivity, signaling the presence of traits associated with riparian specialists. In open riparian zones, Gerisch et al. [17] found the degree of functional similarity among carabid communities to increase with flooding disturbances up to a threshold and then abruptly decrease at the highest level of disturbance, probably due to the post-flooding arrival of upland opportunistic species. This functional response indicates that intermediate disturbances drive the resilience of riparian ground beetle communities and supports the hypothesis that disturbances act as trait filters, sorting individuals that possess specific traits [110]. The functional evenness and functional divergence of riparian ground beetle communities have been studied in regard to their response to flooding [17] and to riparian habitat characteristics [25]. Both variables have been observed to decrease with flood intensity and have been “slightly negatively correlated with the rarefied species richness” [17], thus reinforcing flooding as a discriminant sorting mechanism.
Functional dispersion is the most used FD component for the research of riparian predatory arthropods. It has been studied in response to the following: flooding [17,24], riparian bank type (i.e., cut-off or slip-off banks) [25], riparian vegetation characteristics (i.e., natural or managed vegetation; % of vegetation cover) [25,111], and land use change (i.e., landscape vegetation productivity index) [111]. Carabid functional dispersion, used to evaluate functional redundancy, was lower in flooded habitats [17,24], demonstrating the increased flood resilience of local communities. This tendency was explained by the non-random way in which floods filter traits, favoring mobile species and hygrophilous species, and supports the resilience of riparian communities that is apparent during natural intermediate disturbances [17,24].
Riparian vegetation management has been found to reduce the functional dispersion of carabid communities [25,111], as has densely forested habitats [111]. Landscape land use changes and vegetation management influence the functional dispersion of riparian ground beetle communities. The highest value of this component was found in landscapes with medium vegetation productivity [111]. The only study of riparian spider FD found functional evenness to be the most sensitive metric to environmental changes, slightly increasing with urbanization. This indicates the dominance of a few generalist species and an overall biodiversity reduction [112]. Altogether, these FD changes underline the importance of microhabitat and habitat diversity and integrity to support riparian predatory arthropods with a higher functional diversity and more specialized trait composition. Research on ground beetle functional dispersion also highlights the importance of larger-scale conservation. Such endeavors should, ideally, also include the upland habitats which serve to increase the landscape level functional diversity of these communities by fostering diversified habitat conditions in addition to providing shelter [94], regulation [113], and a starting point for post-flooding riparian recolonization [10,17].

4. Future Research Directions and Priorities

Most research focused on just a few functional traits (commonly, feeding preference, body size, and dispersal capacity) and took place almost exclusively in temperate regions (refer to the Supplementary Material). We believe that our understanding and capacity to conserve riparian predatory arthropods will benefit from addressing their least-studied traits, such as their submersion tolerance and desiccation resistance, and from more research on the traits exhibiting fluctuating responses to drivers (e.g., spider feeding preference and body size), likely based on species composition and particular habitat features. The study of tropical and subtropical riparian zones is also extremely important in the context of worldwide increased anthropogenic pressure and given that much of their tropical biodiversity is still unknown [114]. Riparian management and conservation would also benefit from more research using the better-studied traits of riparian predatory arthropods. The exploration of the importance of aquatic subsidy quality (in terms of the nutritive value of the emergent insects) for riparian consumers is already an emerging direction. PUFAs and stoichiometric analyses promise to provide better insights into the way that riparian predatory arthropods function and should consequently receive increased attention. More research on the effects of poor nutrition on community structure and the behavioral responses of riparian predatory arthropods is also needed [115].
Having a better understanding of the connection between FD and genetic diversity and between FD and the stoichiometry of riparian epigean arthropods may prove useful for understanding intra- and inter-community trait variations and how they respond and shape ecosystem processes. Functional traits result from the interaction between individual genetic information and the environment. As shown earlier, malnutrition and overfeeding during the early development stages affect the overall fitness of an organism and the structure of biotic communities [66]. However, less research, or more niched research with less exposure, has been carried out on how riparian arthropods adapt to stressors by developing traits and strategies that are passed down through generations [94,101]. Behavioral traits and trait-mediated behaviors constitute the results of the interaction between fitness and accumulated experience that have been rarely studied, yet so far are promising, thus prompting us to believe that this area will be one of future interest.
It is crucial that future research directions should analyze the traits of riparian epigean arthropods in order to better understand how these communities interact with other consumers with which they compete and which prey upon them. Of all the studies we know of, only a small fraction addressed the interactions of riparian arthropods, specifically ground beetles and spiders, with their competitors and predators, such as fish [38], reptiles [116], birds [117], and bats [118], and seldom accounted for their traits. Future research on this topic could be further split in two directions, namely, how traits influence the efficiency of riparian arthropod predators at the level of the overall ecosystem process of predation and how traits make these arthropods more or less susceptible to be preyed upon.
The increased frequency of extreme climatic events, resulting in more intense and longer floods and droughts, highlights the necessity for future research to address the capacity of riparian predatory arthropods to withstand such conditions and their role as efficient trophic links along riverine landscapes. Research on this topic is scarce but showed that extreme disturbances act as a restructuring mechanism that can, at times, surpass the resilience of riparian specialized communities. This also hints at the importance of adjacent habitats as shelters for riparian species, sources of biodiversity spillover [10,76], and elements of ecosystem regulation [113].
Ending this section, we point out that the future conservation of riparian zones and predatory arthropod communities would benefit from research addressing the functional diversity of these communities along different natural gradients, such as elevation or distance from water, and anthropogenic impact gradients, such as urbanization. Additionally, researchers should try to carry out studies on a broader spatial scale that include adjacent habitats and analyze landscape-scale drivers and mechanism, as these seem to influence arthropod groups differently [119]. Last but not least, more experimental research is still needed in order to understand the effects that anthropogenic pressures, such as land use changes, pollution, and invasive species introductions, have on the functional diversity and trait-mediated behavior of riparian predatory arthropods.

5. Conclusions

Riparian predatory arthropods represent valuable links that connect aquatic and terrestrial food webs along a river’s continuum. Their aquatic feeding preferences and the capacity of these predators to consume emerging aquatic insects are determined by their traits. These filter communities based on their adaptability to the riverbank habitat and influence individual and collective behavioral responses in less understood ways. Biophysical alterations of lotic and riparian habitats are the main drivers that change the functional and structural diversity of the communities of riparian predatory arthropods and that reduce their capabilities to act as cross-ecosystem trophic links and provide ecosystem services.
More research is needed to understand the response of riparian arthropods to increased climatic instability, characterized by extreme floods and severe droughts, and also in response to the growing occurrence of invasive alien species. The effects of anthropogenic alterations are mostly complex and often result from multiple interacting mechanisms, generated in many instances by more than one driver of ecosystem changes. The research and conservation of riparian predatory arthropods would benefit from more interdisciplinary, larger-scale research that could better address such complex interactions.
The FD of riparian predatory arthropods has been shown to be more critical in terms of the composition rather than the diversity of traits. The FD within these communities is frequently associated with their specialization for flood resistance. This ensures functional redundancy and the capacity of the constituent individuals to capitalize on aquatic subsidies when available, which is determined by a few dominant traits. Taxonomic diversity indices appear to be more sensitive to environmental changes than FD components. These observations strengthen the importance of complementary approaches that combine measures of taxonomic and functional diversity in order to understand and assess the response of biotic communities to various drivers, as the former seems more sensitive. At the same time, the latter helps to underline a mechanistic understanding of community structure and its role in ecosystem functions.
In conclusion, the trait-based approach represents a good and complementary framework used to understand the interaction of riparian predatory arthropods with their environment and its repercussions for ecosystem functioning. More research and increasingly complex research is needed to better understand and conserve these communities, their habitats, and the ecosystem services they provide for the benefit of society.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15223983/s1, Table S1: List of articles found from literature screening.

Author Contributions

Conceptualization, C.A.M. and G.R.; methodology, C.A.M. and G.R.; software, C.A.M. and G.R.; formal analysis, C.A.M. and G.R.; investigation, C.A.M.; data curation, C.A.M.; writing—original draft preparation, C.A.M.; writing—review and editing, G.R.; visualization, G.R. and C.A.M.; supervision, G.R.; project administration, G.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available in the article and Supplementary Materials.

Acknowledgments

This paper was supported by the Doctoral School of Ecology, the University of Bucharest and by the Council for Doctoral Studies (CSUD), the University of Bucharest. The authors wish to thank the anonymous reviewers.

Conflicts of Interest

The authors declare no conflict of interest.

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Table 1. Description of the general terms used, with examples and references.
Table 1. Description of the general terms used, with examples and references.
TermDescriptionExamplesReferences
TraitsIndividually measured phenotypic features that affect an organism’s interaction with its environment, hence its fitness.Feeding preference; body size; dispersal capacity; desiccation resistance; submersion tolerance; reproductive strategy; activity period.[11,13]
Trait mediated behaviorBehavioral responses to changed environmental factors that are based on specific traits of individuals.Flood avoiding behavior; aggregative responses; stress-induced shifts in feeding preference or feeding rate.[11]
Functional diversity componentsMeasures of functional diversity used to describe how biotic communities interact with their environment; they are based on the intra community distribution and spread of selected traits.Functional richness; functional dispersion; functional evenness; functional divergence; degree of functional
similarity.		[16,17]
DriversNatural or anthropogenic-induced factors that lead to ecosystem changes.Flooding; fire; land use changes; pollution; invasive species; climate changes.[18]
Acting mechanismsDynamic and often interacting properties that determine the characteristic of the riparian habitat and that are the result of the aforementioned drivers.Flooding disturbance; aquatic subsidy availability; native vegetation integrity; substrate quality (related to sediment or soil); bank hydrological connectivity; desiccation stress.Adapted from [5]
Table 2. The search string used on Web of Science to select the relevant papers for our review.
Table 2. The search string used on Web of Science to select the relevant papers for our review.
(Title) riparian OR wetland$ OR ecoton* OR floodplain$ OR shore* OR “land-water interface” OR alluvi* OR gravel OR steam$ OR rive* OR ERS OR sediment*
AND
(Title) “predatory arthropod*” OR spider* OR beetle* OR arachnid* OR carabid*
AND
(Topic) unction* OR trait$ OR preference$ OR service$
Table 3. The effect of acting mechanisms on the community-weighted mean (CWM) feeding preference of riparian predatory arthropods: GB = ground beetles; AS = active hunting spiders; WS = web hunting spiders. Arrow pointing up = overall tendency of increase; Arrow pointing down = overall tendency of decrease; Bi-directional arrow = inconsistent reported response tendencies.
Table 3. The effect of acting mechanisms on the community-weighted mean (CWM) feeding preference of riparian predatory arthropods: GB = ground beetles; AS = active hunting spiders; WS = web hunting spiders. Arrow pointing up = overall tendency of increase; Arrow pointing down = overall tendency of decrease; Bi-directional arrow = inconsistent reported response tendencies.
Acting MechanismTendencyTaxaFeeding PreferenceTendencyReferences
Aquatic subsidy availabilityWater 15 03983 i001GBAquaticWater 15 03983 i002[9,34,35,36]
AS and WSWater 15 03983 i003[9,25,33,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53]
GB and ASTerrestrialWater 15 03983 i004[32,54,55,56,57]
WSWater 15 03983 i005
Native vegetation integrityWater 15 03983 i006GBAquaticWater 15 03983 i007[28]
AS and WSWater 15 03983 i008[21,22,28,58]
Aquatic subsidy availability and substrate quality/native vegetation integrityWater 15 03983 i009GB, AS and WSAquaticWater 15 03983 i010[59,60,61]
Desiccation stress/DroughtWater 15 03983 i011AS and WSTerrestrial and
Cannibalism		Water 15 03983 i012[62,63,64]
Table 4. The effect of acting mechanisms on the community-weighted mean (CWM) body size of riparian predatory arthropods: GB = ground beetles; AS = active hunting spiders; WS = web hunting spiders. Arrow pointing up = overall tendency of increase; Arrow pointing down = overall tendency of decrease; Bi-directional arrow = inconsistent reported response tendencies.
Table 4. The effect of acting mechanisms on the community-weighted mean (CWM) body size of riparian predatory arthropods: GB = ground beetles; AS = active hunting spiders; WS = web hunting spiders. Arrow pointing up = overall tendency of increase; Arrow pointing down = overall tendency of decrease; Bi-directional arrow = inconsistent reported response tendencies.
Acting MechanismTendencyTaxaBody Size TendencyReferences
Aquatic subsidy availabilityWater 15 03983 i013AS and WSWater 15 03983 i014[49,54,65,66,74,75]
Flooding disturbance, substrate quality, and bank hydrological connectivityWater 15 03983 i015GBWater 15 03983 i016[8,76,77,78]
ASWater 15 03983 i017[10,77]
WSWater 15 03983 i018
Native vegetation integrityWater 15 03983 i019GB, AS and WSWater 15 03983 i020[79,80,81]
Aquatic subsidy availability and substrate qualityASWater 15 03983 i021[81]
Table 5. The effect of acting mechanisms on the community-weighted mean (CWM) dispersal capacity of riparian predatory arthropods: GB = ground beetles; AS = active hunting spiders; WS = web hunting spiders. Arrow pointing up = overall tendency of increase; Arrow pointing down = overall tendency of decrease; Zero = no significant correlation.
Table 5. The effect of acting mechanisms on the community-weighted mean (CWM) dispersal capacity of riparian predatory arthropods: GB = ground beetles; AS = active hunting spiders; WS = web hunting spiders. Arrow pointing up = overall tendency of increase; Arrow pointing down = overall tendency of decrease; Zero = no significant correlation.
Acting Mechanism TendencyTaxaDispersal Capacity TendencyReferences
Flooding disturbanceWater 15 03983 i022GB and ASWater 15 03983 i023[82,87]
Flooding disturbance, substrate quality, and bank hydrological connectivityGB, AS and WS[8,10,77,88,89]
Flooding disturbance, substrate quality, and native vegetation integrity[90]
Substrate quality and native vegetation integrityWater 15 03983 i024GBWater 15 03983 i025[91]
Native vegetation integrity[81]
AS and WSWater 15 03983 i026[79]
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Murgu, C.A.; Rîșnoveanu, G. Bridging the Gaps: Exploring Aquatic–Terrestrial Connectivity through the Trait-Based Ecology of Riparian Predatory Arthropods. Water 2023, 15, 3983. https://doi.org/10.3390/w15223983

AMA Style

Murgu CA, Rîșnoveanu G. Bridging the Gaps: Exploring Aquatic–Terrestrial Connectivity through the Trait-Based Ecology of Riparian Predatory Arthropods. Water. 2023; 15(22):3983. https://doi.org/10.3390/w15223983

Chicago/Turabian Style

Murgu, Cristian Andrei, and Geta Rîșnoveanu. 2023. "Bridging the Gaps: Exploring Aquatic–Terrestrial Connectivity through the Trait-Based Ecology of Riparian Predatory Arthropods" Water 15, no. 22: 3983. https://doi.org/10.3390/w15223983

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