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Article for Biological Invasions:
Competitive exclusion after invasion? (title and running head)
Thomas Bøhn
1
, Per-Arne Amundsen
2
and Ashley Sparrow
3
1
Norwegian Institute of Gene Ecology, The Science Park, P.O. box 6418, 9294 Tromsø,
Norway. Email: thomas@genok.org
2
Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway.
Email: pera@nfh.uit.no
3
Dept. Natural Resources and Environmental Science, University of Nevada, Reno, Mail
Stop 186 / 1000 Valley Road, Reno, NV 89012-0013, USA. Email:
asparrow@cabnr.unr.edu
KEYWORDS
Community structure, competitive exclusion, introduced exotic species, long-term
empirical data, resource limitation
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ABSTRACT
The ‘Competitive Exclusion Principle’ is a foundation stone in the understanding of
interspecific competition and niche relationships between species. In spite of having the
status of a biological law, the principle has limited empirical support. In this study, we
document strong effects of competition from the invading fish species vendace Coregonus
albula over a 14-year period in the sub-arctic Pasvik watercourse. The native d.r. whitefish,
that shared food and habitat niche with the invader, was displaced from its original niche
and showed a more than 90% decline in population density over the study period. The study
thus provides a unique record of how an exotic fish species excludes a native species from
its original niche. Our data support the competitive exclusion principle, but also indicate
that the vulnerability of the inferior competitor depends on a lack of alternative resources
and on indirect ecological interactions.
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INTRODUCTION
The theoretical principle of competitive exclusion predicts the outcome of interspecific
competition as elimination or extinction of one of two species that occur together without
niche differentiation (Volterra 1931; Lotka 1932; Gause 1934; Hardin 1960; Pianka 2000;
Webb et al. 2002). The reason is simply that one species will be at least slightly more
efficient at capture of the shared resource or will be able to sustain a viable population at
lower minimum resource levels than can the second species (Tilman 1982). The principle is
described by differential equations and has obtained the status of a biological law
(Ekschmitt and Breckling 1994; Weber 1999). However, the empirical support is limited,
and mainly based on simple laboratory experiments (Gause 1934; Wang et al. 2002). The
lack of empirical support has raised the question about its domain of application (Keddy
1989; Peters 1991), but interspecific competition is difficult to study because its effects are
mostly hidden as the temporary endpoint of a complex community structure. However,
during periods of change, competition can be shown or indicated empirically by a number
of different measures (Ross 1991); directly on the competitors, or on any shared resource
pools or prey community affected (top-down effects). More specifically, competition is
supported if a study shows:
(i) interactive niche shifts (e.g. in habitat or diet) (Nilsson 1967, 1978)
(ii) density reduction or extinction (negative population growth) (Hardin 1960)
(iii) reduced individual growth (Diehl and Eklov 1995)
(iv) reduced food intake (Bøhn and Amundsen 2001; Wauters et al. 2002)
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(v) altered community composition of prey towards smaller species (Brooks and
Dodson 1965; Gliwicz and Pijanowska 1989)
(vi) altered size structure of prey populations towards smaller individuals (Lazzaro
1987)
Biological invasions provide ecological scenarios that may facilitate more empirical insight
to competitive interactions. Subsequent to an invasion, the processes of interspecific
competition, rather than its steady-state outcome, can be studied in the receiving ecosystem
(Simberloff 1981; Pimm 1989). Introduced exotic species thus provide large-scale ‘natural
experiments’ where ecological theory (e.g. the ‘Competitive Exclusion Principle’) may be
tested empirically. Elucidative in situ studies on ecological processes require comparisons
between systems or changes due to disturbance within systems. Biological invasions
provide both these possibilities and may produce unique long-term datasets. Suitable data
from invasion biology cannot be obtained easily from manipulated experimental systems,
for a number of reasons. Firstly, exotic invaders are largely unwanted and strong measures
are taken to avoid further spread of exotic species (Glowka et al. 1994; Sandlund et al.
1999). Secondly, only about 10% of the introduced exotic species establish, and only one
percent produce strong effects on the receiving community (Williamson 1996). Thirdly, the
effects of exotic species may come after long time delays, often too slowly for the short-
term-funded researcher to follow. Finally, for most studies of exotic species that have had
significant ecological impact, there are weak or no data on the pre-invasion native
community structure, simply because many exotic organisms are not discovered before late
in the process. Invasion biology has mainly been operating in retrospect on long time scales
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with relatively low precision data. Such data may however be successfully combined with
modelling of the process (Moyer et al. 2005). In the literature, we were able to find just a
few long-term studies that, in a large-scale ecosystem, followed the consequences arising
from an invasion, and that also provided evidence of competitive exclusion during the
process (Wilson et al. 2004; Geiger et al. 2005).
This study documents the successive stages of interspecific competition between a native
and an exotic fish species over a 14-year period (1991–2004) in the Pasvik watercourse in
the Norwegian Subarctic. The native whitefish Coregonus lavaretus in the watercourse
consists of two sympatric morphs, densely and sparsely rakered whitefish (hereafter
denoted d.r. whitefish and s.r. whitefish), with distinct ecological and morphological
differentiation (Amundsen et al. 1999, 2004). The two whitefish morphs belong to the
Siberian clade (Ostbye et al. 2005) and are most likely the result of sympatric ecological
speciation within the system, partly due to lack of trophic competitors (Ostbye et al. 2006).
The d.r. whitefish naturally occupies the same ecological niche (in the pelagic habitat) as
the closely related, introduced vendace Coregonus albula, whereas the sparsely rakered
whitefish prefers the littoral habitat (Amundsen et al. 2004). Vendace was introduced into
the Inari watercourse in Finland around 1960 (Mutenia and Salonen 1992). After a 25-year
delay in Lake Inari, this exotic species reached a high population density during the second
half of the 1980s, then subsequently migrated downstream into the Pasvik watercourse,
where it was recorded for the first time in 1989 (Amundsen et al. 1999). The gradual
expansion of vendace into the Pasvik watercourse provided the opportunity to study the
mechanisms and impact of competition throughout the period of change by comparing