Wild reindeer, wolf and moose population dynamics in Eastern Finland

Background. The alternative prey hypothesis describes the mechanism for
apparent competition whereby the mortality of the secondary prey species
increases (and population size decreases) by the increased predation by
the shared predator if the population size of the primary prey decreases.
Apparent competition is a process where the abundance of two co-existing
prey species are negatively associated because they share a mutual
predator, which negatively affects the abundance of both prey. Here, we
examined whether alternative prey and/or apparent competition hypothesis
can explain the population dynamics and reproductive output of the
secondary prey, wild forest reindeer (Rangifer tarandus fennicus) in
Finland, in a predator-prey community in which moose (Alces alces) is the
primary prey and the wolf (Canis lupus) is the generalist predator.
Methods. We examined a 22-year time series (1996 - 2017) to determine how
the population size and the calf/female ratio of wild forest reindeer in
Eastern Finland were related to the abundances of wolf and moose. Only
moose population size was regulated by hunting. Summer predation of wolves
on reindeer focuses on calves. We used least squares regression (GLS)
models (for handling autocorrelated error structures and resulting
pseudo-R2s) and generalized linear mixed (GLMs) models (for avoidance of
negative predictions) to determine the relationships between abundances.
We performed linear and general linear models for the calf/female ratio of
reindeer. Results and Synthesis. The trends in reindeer population size
and moose abundance were almost identical: an increase during the first
years and then a decrease until the last years of our study period. Wolf
population size in turn did not show long-term trends. Change
in reindeer population size between consecutive winters was
related positively to the calf/female ratio. The calf/female ratio was
negatively related to wolf population size, but the reindeer population
size was related to the wolf population only when moose abundance was
entered as another independent variable. The wolf population was not
related to moose abundance even though it is likely to consist the
majority of the prey biomass. Because reindeer and moose populations were
positively associated, our results seemed to support the alternative prey
hypothesis more than the apparent competition hypothesis. However, these
two hypotheses are not mutually exclusive and the primary mechanism is
difficult to distinguish as the system is heavily managed by moose
hunting. The recovery of wild forest reindeer in eastern Finland probably
requires ecosystem management involving both habitat restoration and
control of species abundances.