Habitat changes and population dynamics of fishes in a stream with forest floor degradation due to deer overconsumption in its catchment area

Abstract

Forest degradation caused by deer overabundance has become a worldwide problem in recent decades. Overgrazing by deer not only affects terrestrial ecosystems but also spreads to aquatic ecosystems. Mass consumption of forest floor vegetation by deer creates denuded slopes and increases sediment inputs into adjacent rivers. In addition, rivers have upstream-downstream continuum structures, whereby the effects of degradation events in forests at upstream sites may spread to larger ecosystems downstream. However, few studies have examined the indirect effects of deer overabundance on downstream ecosystems. I examined the relationships between population dynamics of 13 fish species and habitat characteristics at a downstream site over the course of 11 years after forest floor degradation by deer overconsumption in a 36.5-km 2 catchment area of the Yura River in the Ashiu Research Forest, Japan, which is well-protected from anthropogenic influences. During my 11 years of observation, characteristics of stream habitats changed from a predominantly coarse substrate to a fine substrate. I observed a remarkable decrease in one species (Tribolodon hakonensis) and increase in another species (Pseudogobio esocinus), and these changes were reasonably consistent with the increase or decrease in their preferred habitat types in the sampling site. This study showed long-term habitat changes in a stream after forest floor degradation due to deer overconsumption in its catchment area and demonstrated that fish populations reacted to these changes.This study suggests that catchment-level management, including forest ecosystem conservation, is necessary to solve fundamental problems in stream ecosystems.

Ecological Traits of a Common Japanese Pit Viper, the Mamushi (Gloydius blomhoffii), in Kyoto, with a Brief Geographic Comparison

“…During the last several decades the environments of the Ashiu forest have been drastically altered by the extensive increase in the population density of deer (Cervus nippon) (e.g., Kato and Okuyama, 2004;Fukushima and Tokuchi, 2008), as well as the loss of local paddy fields (Mori, personal observation) and possible climate change due to the global warming. These drastic environmental changes may have affected basic ecological traits, such as growth, reproduction, and population dynamics of wild animals living in this forest (e.g., Kato and Okuyama, 2004;Sakai et al, 2012;Nakagawa, 2019). Fortunately, I did not detect any obvious change in adult body size of G. blomhoffii during this 31-year-research period.…”

Comparison of macroinvertebrate assemblages in a stream before and after fine sedimentation by deer‐induced forest floor degradation

“…Pseudogobio esocinus is a vacuum feeding cyprinid which forages for animals, including the larva of crane fly, in sandy bottom habitats (Kawanabe & Mizuno, 2001;Nakamura, 1969;Nakagawa, unpublished). These species increased after fine sedimentation (Nakagawa, 2019a). The absence of top-down suppression of the crane fly population might be explained by the low abundance of P. esocinus at the sampling site (≤10 individuals per survey vs. typically >30 T. hakonensis individuals), even after fine sedimentation (Nakagawa, 2019a(Nakagawa, , 2019b.…”

“…The continuous structure of rivers allows for transport of waterborne materials from upstream forests to downstream ecosystems. Thus, changes to forest and headstream ecosystems may greatly impact larger downstream ecosystems (Nakagawa, 2019a;Vannote et al, 1980). However, data on the effects of deer overabundance on aquatic organisms are scarce compared to those on terrestrial organisms and ecosystems, and more case studies are needed before adequate countermeasures against deer-induced ecosystem degradation can be realized.…”

“…Sakai et al (2012Sakai et al ( , 2013 reported an increase in fine sediment input into first-order streams and associated changes in stream habitat and macroinvertebrate fauna 2 or 3 years after the forest degradation event in 2006. Nakagawa (2019a) reported that increased sediment yields upstream resulted in a higher proportion of fine sediments on the river bottom and associated changes in the density of several fish species in fourth-order reaches in the lowermost parts of the research forest in the 11 years after forest floor degradation. Stream fishes are effective predators in stream food webs (Inoue & Miyayoshi, 2006;Katano et al, 2013;Power, 1990;Power et al, 2008); therefore, the effects of fine sedimentation may extend beyond the fish population to other aquatic organisms, such as the macroinvertebrates that are the major prey of fish at the research site (Nakagawa & Takemon, 2018).…”

“…I predicted that increased sedimentation from upstream forests would (1) decrease the population density of macroinvertebrates that favor coarse substrate habitats and increase the number that prefer fine substrate habitats, and (2) reveal effects beyond microhabitat change that influence taxonomic composition. Previously, I showed that the composition of fish species changed after fine sedimentation in streambeds (Nakagawa, 2019a); therefore, the effect of the increase in sediment inputs may be detected as the difference in composition between sampling periods, even considering habitat changes and seasonality.…”