In our study, we investigated how water beetles responded to different successional stages of beaver ponds. Our results show that the beetles in new beaver ponds were significantly more abundant and species-rich than in other types of wetlands. Water beetle richness decreased when beaver ponds became mature, but they retained high diversity compared to non-beaver ponds. Our results provide support for the role of beavers as ecosystem engineers promoting biodiversity, as beaver wetlands, especially newly created ones, supported more diverse assemblages of water beetles than non-beaver wetlands. We will discuss how the beaver alters habitats and the succession of beaver-created wetlands in the following sections.
Beaver creates suitable habitats
Our results reveal that beaver ponds support water beetle assemblages of higher diversity than non-beaver ponds, suggesting that beaver dams create habitats suitable for water beetles. Habitat suitability of beaver ponds can be manifested both in the form of habitat structural amelioration and resource enhancement (Table
2; sensu Bruno et al.
2003; Nummi and Hahtola
2008). Intact beaver dams slow down the hydrologic fluxes (Schlosser and Kallemeyn
2000), resulting in habitats with lentic water favoured by most water beetle species (Holmen
1987; Nilsson and Holmen
1995). Beaver construction also creates an inundated area behind the dam. The flooded areas often characterize beneficial habitat features, such as shallow water zones with emergent vegetation (Nummi and Hahtola
2008; Johnston
2017) and an increase in plant and woody debris, which are favoured by water beetles (Nilsson et al.
1994; Willby et al.
2018; Liao et al.
2020). These physical pond characteristics are often associated with biotic changes in beaver ponds. Beaver ponds can also have high within-patch heterogeneity with the sub-habitats including the relatively deep water behind the dam and in the former creek channel, inundated shrubs-swamps, shallow marshes of emergent vegetation, and vernal wet meadows (Nummi
1989; Wissinger and Gallagher
1999; Bush and Wissinger
2016; Johnston
2017).
Table 2
Habitat features of new and old beaver ponds beneficial to water beetles
Habitat amelioration | Lentic water | + | + | Favoured by water beetles due to their swimming behaviour; change of physical, chemical, and biological conditions of habitats | Gioria ( 2014) and Bush and Wissinger ( 2016) |
Shallow shores | + | + | Shelter from predators | Nilsson et al. ( 1994) and Liao et al. ( 2020) |
Aquatic vegetation | + | + | Shelter from predators, breeding sites | |
Fish abundance | − | + | Low predation pressure in new beaver ponds | Schlosser and Kallemeyn ( 2000) and Bush and Wissinger ( 2016) |
Food resource enhancement | Dead terrestrial invertebrates | + | − | Food sources for water beetle adults | Lindberg ( 1944) and Eberle and Stanford ( 2010) |
Coarse dead organic matter | + | − | Food resource for water beetle prey: benthic shredders (e.g. Isopoda) and nektonic filter feeders (e.g. Cladocera) | Nummi ( 1989) and Juliano and Lawton ( 1990) |
Fine grained organic matter | − | + | Food resource for water beetle prey: benthic collectors, e.g. Chironomidae | McDowell and Naiman ( 1986) and Nilsson and Holmen ( 1995) |
Resource enhancement caused by pond physical characteristics occurs in beaver ponds in a way that can benefit water beetles (Table
2). In our traps,
Ilybius ater, a species preferring habitats with muddy bottoms rich in detritus harboring numerous prey (Verberk et al.
2001; Frelik and Pakulnicka
2015), was found in both new and old beaver ponds but not in non-beaver ponds. In previous research, Law et al. (
2019) found that
Ilybius ater was highly associated with their beaver ponds. In beaver ponds, floods usually lead to abundant organic matter accumulating in pond bottoms (Naiman et al.
1986; Johnston
2017; Nummi et al.
2018). This organic matter accumulation results in an increase in the abundance of Chironomidae larvae as well as other detritivorous invertebrates, such as the isopod
Asellus sp. (Hodkinson
1975; McDowell and Naiman
1986; Nummi
1989; Law et al.
2016; Bush and Wissinger
2016). Furthermore, beaver-created floods can also lead to increasing availability of terrestrial invertebrates (see Swanson et al.
1985; Eberle and Stanford
2010), which is an additional food resource for adult dytiscids, the most abundant water beetles in our traps (Nilsson and Holmen
1995). These invertebrates, both live and dead (Lindberg
1944; Juliano and Lawton
1990), are a food resource to water beetles which as early colonisers can benefit from resource enhancement at the early successional stage of beaver ponds.
Water beetles can take advantage of these beneficial changes of habitat features via various means of dispersal. In our study, good flier species, such as
Acilius canaliculatus and
Rhantus exsoletus, were early colonizers and frequently occurred in new beaver ponds (Fig.
2). In North America, Larson et al. (
2000) discovered that
Acilius, a genus of good fliers, was especially associated with beaver ponds. These early colonisers have the advantage to colonise newly available suitable habitats. In our study,
Graphoderus zonatus, a species known to be poor fliers (Eriksson
1972; Lundkvist et al.
2002), also had a very frequent occurrence in one of the new beaver ponds. We think this species could have dispersed from nearby wetlands through aquatic corridors created by beaver floods (Wissinger and Gallagher
1999; Hood and Larson
2015; Bush and Wissinger
2016), as shown for the area by Kivinen et al. (
2020). Availability of multiple dispersal pathways, therefore, results in the increase of water beetle species richness in beaver ponds compared to that in non-beaver ponds.
The succession of beaver-created wetlands affects water beetle assemblages
In our study, we found that new beaver ponds had higher numbers of water beetles and more diverse assemblages than other wetland types including old beaver ponds. Our result differs from the findings of Bush et al. (
2019, see also Bush and Wissinger,
2016) that showed taxa richness of invertebrates being the highest in abandoned (homologous to the category ‘former’ in our analysis) beaver ponds. Bush et al. (
2019) investigated a broad range of macroinvertebrate families that were identified to the family level, while our study focused specifically on water beetles identified to the species level. Therefore, the differing results are likely due to the macroinvertebrate taxonomical level of identification and the ecology of different taxa. Furthermore, it should be noted that the activity traps we were using are especially suitable for capturing mobile water beetles. Sedentary species may, thus, be underrepresented in our sampling. When generally studying the effect of pond age on invertebrates, Fairchild et al. (
2000) found predatory dytiscids to be especially dwellers of ponds of less than 10 years of age. It is to be noted, though, that according to the variance in the beta diversity, the old beaver ponds of our study were more dissimilar to each other than the new ones.
The first year(s) of beaver impoundments are characterized by abundance of inundated, dying plants providing suitable structure for many aquatic invertebrates (Table
2). In an experiment imitating beaver flooding, Nummi (
1989) showed that the group of detritivores especially increasing during the first flood summer are cladocerans. They are important prey for some dytiscids, such as
Hydroporus (Nilsson and Holmen
1995) found in some of our new beaver flowages. During the second year of flooding, benthic detritivores, especially the shredder
Asellus sp. as well as chironomids, increased; they are preyed upon e.g. by
Hyphydrus (Juliano and Lawton
1990). The role of shredders in the invertebrate community can remain high for some years if there are trees in the flowage (especially
Salix sp.) which continue to produce leaf litter for detritivores eating coarse matter (Nummi
1989).
From the perspective of invertebrates, an important characteristic of the early stage beaver ponds is the reduced presence of invertivorous fish (Bush and Wissinger
2016). In the short term after damming, the populations of invertivorous fishes are small and recolonization by fish is delayed by the barrier effect of beaver dams (Snodgrass and Meffe
1998). This has the effect of diluting the fish predation pressure on beetles as many water beetles are known to be sensitive to the presence of predatory fish (Nummi et al.
2012; Liao et al.
2020). In our results, the assemblage difference of water beetles found in the early and later successional stages of beaver ponds indicates that some water beetles species indeed could have benefitted from the possible time lag in fish colonization of beaver flowages, partly delayed by the barrier effect of beaver dams.
In later stages of succession, the water beetle species composition seems to partially become a subset of the species pool of the new beaver ponds (Fig.
1), which we think resulted from the higher predation pressure in old beaver ponds. According to North-American studies, large gaped predatory fish replace small fish as beaver ponds become mature, (Snodgrass and Meffe
1998; Bush and Wissinger
2016), and fish abundance reaches its peak in mature beaver ponds (Schlosser and Kallemeyn
2000). Still, these old beaver ponds provide habitats to species, such as the
Haliplus spp. (Fig.
1); they are not very active dispersers since they do not often fly, and they are not very vulnerable to fish because of their habit of crawling among vegetation (Yee and Kehl
2015). As noted above, the water beetles of the old beaver ponds occurred in a more uneven way than in the new ones. The presence of the species in the old beaver ponds, which were not found in the new ones (three in all), adds to the species turnover “β” in the landscape and reveals the value of also the old beaver ponds in biodiversity conservation at the landscape level.
The species composition change we found in water beetles is likely also partly due to changes in overall productivity of the beaver pond. At the very early stage, water beetles may utilize the abundance of dead terrestrial animal matter (Margolis et al.
2001). As succession progresses water beetles may switch to feeding primarily on detritivores which colonize the pond and benefit from the inundated plant matter (McDowell and Naiman
1986; Nummi
1989). As the beaver pond progresses to later successional stages, detritivore productivity decreases as the effect of the pulse of the organic matter washed from the flooded shores decreases. It has been found to decrease after some years of flooding (Vehkaoja et al.
2015; Nummi et al.
2018) along with the biomass of the former terrestrial vegetation (Nummi
1989).
Water beetles and other aquatic invertebrates that benefit from newly created beaver wetlands (Nummi
1989; McDowell and Naiman
1986; Bush and Wissinger
2016) appear to attract biodiversity at higher trophic levels. Waterbirds, such as common teal
Anas crecca and green sandpiper
Tringa ochropus (Nummi and Pöysä
1993; Nummi and Holopainen
2014), are known to benefit from the availability of invertebrates as food in new beaver wetlands. The available aquatic invertebrates in beaver wetlands also facilitate the abundance of mammals, such as bats, via the food web (Nummi et al.
2011). When beaver wetlands become mature, fishes colonize the flooded areas and strongly compete for food with the birds (Schlosser and Kallemayn
2000; Nummi et al.
2012).
Apart from beaver created floods, there are also other temporarily flooded areas in the same study area, boreal vernal pools. Vernal pools harbour even greater abundance and diversity of water beetles than beaver ponds do (Liao and Nummi, pers. obs.) but are even more variable habitats. Vernal pools are very susceptible to drying out early during summers with low amounts of precipitation. Synchronized fluctuations in environmental conditions can reduce the likelihood of insect metapopulations persisting, as could happen from the effect of an extensive drought as an example. The addition of more permanent habitats to a system, such as new beaver ponds, could sometimes enable metapopulation persistence even in cases in which the metapopulation would otherwise go extinct (Frouz and Kindlmann
2015). In our landscape setting, new beaver ponds can act as the “permanent” habitats of the source-sink model. Modifying from Wissinger and Gallagher (
1999), we could state that the loss of productive semi-permanent beaver ponds from the wetland complex of the landscape could reduce the pool of cyclic colonizers that seasonally invade temporary habitats. Thus also reducing diversity in those temporary vernal pools. Future studies are needed to investigate the function of and interaction of beaver ponds and vernal pools in water beetle metapopulations.