Foraging decisions with conservation consequences: Interaction between beavers and invasive tree species

Abstract

Herbivore species can either hinder or accelerate the invasion of woody species through selective utilization. Therefore, an exploration of foraging decisions can contribute to the understanding and forecasting of woody plant invasions. Despite the large distribution range and rapidly growing abundance of beaver species across the Northern Hemisphere, only a few studies focus on the interaction between beavers and invasive woody plants.We collected data on the woody plant supply and utilization at 20 study sites in Hungary, at two fixed distances from the water. The following parameters were registered: taxon, trunk diameter, type of utilization, and carving depth. Altogether 5401 units (trunks and thick branches) were identified individually. We developed a statistical protocol that uses a dual approach, combining whole-database and transect-level analyses to examine foraging strategy.Taxon, diameter, and distance from water all had a significant effect on foraging decisions. The order of preference for the four most abundant taxa was Populus spp. (softwood), Salix spp. (softwood), Fraxinus pennsylvanica (invasive hardwood), and Acer negundo (invasive hardwood). The diameter influenced the type of utilization, as units with greater diameter were rather carved or debarked than felled. According to the central-place foraging strategy, the intensity of the foraging decreased with the distance from the water, while both the taxon and diameter selectivity increased. This suggests stronger modification of the woody vegetation directly along the waterbank, together with a weaker impact further from the water.In contrast to invasive trees, for which utilization occurred almost exclusively in the smallest diameter class, even the largest softwood trees were utilized by means of carving and debarking. This may lead to the gradual loss of softwoods or the transformation of them into shrubby forms. After the return of the beaver, mature stages of softwood stands and thus the structural heterogeneity of floodplain woody vegetation could be supported by the maintenance of sufficiently large active floodplains.The beaver accelerates the shift of the canopy layer's species composition toward invasive hardwood species, supporting the enemy release hypothesis. However, the long-term impact will also depend on how plants respond to different types of utilization and on their ability to regenerate, which are still unexplored issues in this environment. Our results should be integrated with knowledge about factors influencing the competitiveness of the studied native and invasive woody species to support floodplain conservation and reconstruction.

Keywords: Castor fiber; alluvial forest; central‐place foraging strategy; ecosystem engineer species; floodplain; invasion ecology; optimal foraging strategy.

FIGURE 1

Eurasian beaver (Castor fiber). Photo: Juhász, E

FIGURE 2

Map of the study sites in Hungary. Sites were located along six rivers: Danube (D1–D7), Mura (M1), Ipoly (I1), Tisza (T1–T5), Zagyva (Z1), and Körös rivers (K1–K5). Source of base maps: ArcGIS 10.1. (ESRI 2012). Main rivers: thin blue lines; national borders: thick brown lines; capital of Hungary: gray; main lakes: blue (Source: Natural Earth; http3: https://www.naturalearthdata.com/downloads/)

FIGURE 3

Types of utilization: (a) debarking – surface damage to a depth of 0.5–3 cm; (b) carving – damage with a depth over 3 cm; and (c) felling. Photos: Juhász, E

FIGURE 4

Percentage of each utilization type within the supply groups (combinations of taxon and diameter class). Whiskers display the standard deviation of transect‐level values. Key: F—Felled, C—Carved, D—Debarked, I—Intact, WBT—waterbank transect, OT—outer transect, n—average number of units of the given taxon in the diameter class

FIGURE 5

The mean diameter of felled, carved, debarked and intact units in the case of the most abundant four taxa. Whiskers display the standard deviation of transect‐level values. Significance groups (i.e., letters) were generated by means of Tukey according to the GLMM defined in Equation 2, where two groups sharing no common letter(s) are significantly different at level α = 0.05. Key: WBT—waterbank transect, OT—outer transect

FIGURE 6

Relation of the Jacobs selectivity index (D_i) and relative supply (p_i) calculated for the most abundant four taxa. (Relative supply means the proportion of a given taxon in the total supply.) Values were considered for transects where at least two units were utilized. Key: black circle—Salix spp., black square—Populus spp., blue circle—A. negundo, blue square—F. pennsylvanica, WBT—waterbank transect, OT—outer transect, filled marker—significant Jacobs index value, empty marker—nonsignificant Jacobs index value. Significance level: α = 0.05