Indirect effects of excessive deer browsing through understory vegetation on stream insect assemblages

Abstract

Over the past decade, the abundance of sika deer has rapidly increased around Japan. Previous studies have showed overabundance of deer causes drastic reduction of forest understory vegetation, leading excessive soil erosion. However, no study has investigated the effects of excessive deer browsing on aquatic insect assemblages via sediment runoff. These effects are important to understand whether the terrestrial alteration by deer influences aquatic ecosystems. In a primary deciduous forest catchment in Ashiu, Kyoto, a deer exclusion fence has been in place since 2006. We compared forest floor cover, overland flow, stream environment, and aquatic insect assemblages in first-order streams and catchments inside and outside of the deer-exclosure from May-2008 to April-2009. The floor inside the deer-exclosure catchment was covered by lush understory vegetation, whereas outside was almost bare. The overland flow runoff rate at midslope and the dominancy of fine sediment deposition in the streambed were higher outside than inside. Among aquatic insects, burrowers, which are tolerant against fine sediment deposition, were significantly more abundant outside than inside, whereas clingers exhibited the opposite patterns. Collectorgatherers, which feed on fine detritus, were significantly more abundant outside than inside. Meanwhile, filterers were more abundant inside. The Simpson's diversity index of the aquatic insect assemblages was higher inside than outside. These results suggest that the demise of understory vegetation due to excessive deer browsing has indirectly caused changes in the aquatic insect assemblages of this catchment via increased sediment runoff and subsequent sandy sedimentation of the streambed.

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

“…This pattern contrasts with the rapid response to forest degradation seen in headwater streams in the Ashiu Research Forest. Sakai et al (2012Sakai et al ( , 2013 reported that an increase in fine sediments in a streambed was clearly observable in first-order streams 2 or 3 years after the forest degradation event in 2006. This difference may be due to the time lag of downstream sediment transportation, which depends on the distance between upstream and downstream sites and seasonal and annual variations in water discharge, including flood disturbance events (Church, 2002;Hamilton, 2012).…”

“…Deer overabundance not only affects terrestrial ecosystems but can also spread to aquatic ecosystems. These increased sediment inputs affect the community structure of macroinvertebrate assemblages via changes in the physical characteristics of stream habitats (Sakai et al, 2012. These increased sediment inputs affect the community structure of macroinvertebrate assemblages via changes in the physical characteristics of stream habitats (Sakai et al, 2012.…”

“…denuded slopes and increases sediment inputs to nearby rivers (Sakai et al, 2012Biligetu et al, 2013). These increased sediment inputs affect the community structure of macroinvertebrate assemblages via changes in the physical characteristics of stream habitats (Sakai et al, 2012. In addition, rivers are continuous structures that transport waterborne materials downstream (Vannote, Minshall, Cummins, Sedell, & Cushing, 1980).…”

Ecological functions of persistent Japanese cedar litter in structuring stream macroinvertebrate assemblages

“…Excessive deer browsing has caused drastic decreases in the abundance and diversity of understory plants since the late 1990s (Kato and Okuyama 2004;Tanaka et al 2008). Soil erosion via denudation by deer has increased sediment runoff and subsequent sandy sedimentation of streambeds in deciduous forests (Sakai et al 2012).…”

Peer Review #1 of "Deer herbivory reduces web-building spider abundance by simplifying forest vegetation structure (v0.1)"

“…Through selective feeding, deer directly affect forest communities by altering species composition and vegetation structure (Côté et al 2004;Takatsuki 2009;Habeck and Schultz 2016). These direct effects have the potential to indirectly alter the abundance of co-occurring animal species (Rooney 2001;Rooney and Waller 2003;Sakai et al 2012). For example, through resource competition, deer can negatively affect the abundance of small granivorous mammals.…”