We investigated how water depth affects emergent plant cover, and how the plant cover affects dytiscid diversity in ponds, both with fish and without. We found that emergent plant cover decreases with increasing water depth. The effects of plant cover on dytiscids are different at the pond level than the microhabitat level. At the pond level, increasing emergent plant cover has significantly positive effects on the presence of dytiscids in ponds both with and without fish. At the microhabitat level, the effects of plant cover on dytiscid diversity has noticeable differences between ponds with and without fish. In ponds with fish, both dytiscid species richness and abundance are positively correlated with increasing plant cover (Fig 3 a & c). In ponds without fish, plant cover has no effect on dytiscid species richness and abundance at the microhabitat level (Fig 3 b & d).
Emergent plant cover decreases with water depth
Our results indicate that when the water is deeper than 50 cm, there is a low probability of finding emergent plants. Yet, we found no association between plant species richness and water depth, which is contradictory with the findings of 55 urban wetlands in Japan that emergent plant species richness is negatively associated with increasing water depth [28]. Emergent plants are only capable of establishing in shallow but not deep waters, although they can expand into deeper water via rhizomal growth [27]. In urban ponds, water turbidity can be high, due to the effects of intensive land use [28], and the action of benthic fish [41], which limits light penetration [42] and consequently restricts emergent plant growth [27].
Emergent plants as prey refuges to promote species coexistence
The importance of emergent plant cover to dytiscids can be different at the pond level from the microhabitat level (trap level). At the pond level, the presence or absence of emergent plant cover determines the occurrence probability of dytiscids, because most dytiscid species prefer water with vegetation [30]. Larger areas of emergent plants at the pond margins are more important in ponds with fish than in ponds without fish, due to the heavy predation pressure from fish. This reflects the importance of emergent plants as prey refuges for dytiscids, in order for them to establish a stable population. Aquatic plant species richness has a positive correlation with aquatic invertebrate species richness [8] [29]. Our result of the positive effects of plant cover on dytiscids (in the presence of predators) emphasizes the importance of prey refuge quantity as a conservation tool to support aquatic biodiversity.
Dytiscid population dynamics at the microhabitat level is different in ponds with fish than in fishless ponds. These differences indicate that predation pressure is different in the two types of ponds. Fish are known to have strong negative effects on dytiscid species richness and abundance [24]. In fishless aquatic habitats, however, dytiscids are top predators in the food chain, although not completely free from other predators [43]. Emergent plants can serve as dytiscid refuges, providing sanctuary from predators [44] and reducing attacking rate and success [4][45].
Prey refuges can also facilitate utilization of alternative prey in both quality and quantity for generalist predators. Emergent plants are often associated with other aquatic invertebrates, such as Hemiptera [37] and Diptera [46]. These invertebrates are alternative prey for generalist predators, such as Crussian carps, which are found in some of our ponds [24], and which can reduce predation pressure on dytiscids. When generalist predators switch their diets, the switching behaviour can reduce the risk of local extinctions of scarce prey species as a result of competition with dominant prey species [5]. Prey refuges, thus, can stabilise the predator-prey dynamics of an ecosystem and promote the coexistence of prey and predator species, as well as the coexistence of different prey species.
Predation pressure and dytiscids’ use of prey refuges
In fishless ponds, dytiscids are not free from predators. Intraspecific and interspecific predation is common among dytiscids, especially among their larvae [43]. Other potential predators of dytiscids include aquatic insects [47], amphibians [48], waterbirds [49], and mammals [50], summaried in Table 1. However, these predators exert a lower level of predation pressure on dytiscids compared with predatory fish, because predatory insects have a lower gut capacity, and dytiscids comprise only a small proportion of the diet of the other predators mentioned.
The presence of refuges can increase habitat complexity and lead to shifts of hunting modes in predatory insects, including dytiscids and Odonata larvae. Some predatory insects shift from an active hunting mode in habitats with a simple structure to an ambush strategy in vegetated habitats [51][52]. With an ambush strategy, Dytiscus decreases its predation on Graphoderus spp. [45][52]. The presence of refuges can balance encounter rates between prey and predators and capture rates of predators [53].
The presence of emergent plants can also influence the foraging preferences of non-fish vertebrate predators (Table1). Many non-fish predators of dytiscids prefer to forage in vegetated microhabitats [54][55][56]. This foraging preference of predators indicates that, in fishless ponds, the predation pressure on dytiscids in vegetated microhabitats is not necessarily lower than in non-vegetated microhabitats Dytiscids need instantaneous responses and escape to non-vegetated microhabitats to avoid predation when these non-fish predators feed in vegetated microhabitats. In this case, non-vegetated microhabitats serve as refugia for dytiscids. Habitats with structural heterogeneity, thus, can facilitate co-existence of predator and prey species.
As non-fish predators exert low predation pressure on dytiscids, in fishless ponds, dytiscids can spend more time foraging in open water, which is favoured by other invertebrates, such as Trichoptera [37][58]. Refuges help prey species to avoid predators, but safe habitats do not necessarily provide them with good foraging [3]. Emergent plants also provide dytiscids’ prey with refugia as dytiscids’ attacking rate and success is often higher in habitats without vegetation than in vegetated habitats [45]. Dytiscids have more opportunities to forage in non-vegetated microhabitats in fishless ponds than in ponds with fish. We assume that predation pressure and predators’ foraging behaviours may have contributed to dytiscids’ lack of preference for vegetated or non-vegetated microhabitats in fishless ponds.
Implications for wetland management
On the basis of this study, we recommend managing ponds for habitat heterogeneity, with different levels of structural complexity. Although vegetation can increase habitat complexity, both vegetated and non-vegetated microhabitats are beneficial to support aquatic biodiversity. For instance, emergent plants provide valuable habitats and act as refugia in predator-prey dynamics for many dytiscid species and other aquatic invertebrates that often occur in vegetated habitats. However, not all aquatic invertebrates are associated with vegetation. For instance, other important taxa such as waterboatmen (Corixidae) and caddisfly (Trichoptera) larvae seem to prefer open water [37][58]. Therefore, variation in provision of plant cover and open water can benefit different taxa and support higher levels of biodiversity.
We recommend maintaining a diversity of microhabitats, characterized by different water depths. As mentioned above, the provision of both heterogeneous aquatic vegetation and open water can benefit different taxa. Also from a human perspective, diversity of aquatic vegetation and open water are two key factors determining aesthetic appreciation regarding blue infrastructure [60]. Dense emergent vegetation promotes a sense of naturalness, but urban residents seem tolerant of only a certain level of naturalness in urban wetlands [61]. Management of vegetation is only necessary when vegetation cover is excessive, particularly in shallow ponds where there would be potential for homogenizing vegetation spread.