Generic ecological impact assessments of alien species in Norway: a semi-quantitative set of criteria

Abstract

The ecological impact assessment scheme that has been developed to classify alien species in Norway is presented. The underlying set of criteria enables a generic and semi-quantitative impact assessment of alien species. The criteria produce a classification of alien species that is testable, transparent and easily adjustable to novel evidence or environmental change. This gives a high scientific and political legitimacy to the end product and enables an effective prioritization of management efforts, while at the same time paying attention to the precautionary principle. The criteria chosen are applicable to all species regardless of taxonomic position. This makes the assessment scheme comparable to the Red List criteria used to classify threatened species. The impact of alien species is expressed along two independent axes, one measuring invasion potential, the other ecological effects. Using this two-dimensional approach, the categorization captures the ecological impact of alien species, which is the product rather than the sum of spread and effect. Invasion potential is assessed using three criteria, including expected population lifetime and expansion rate. Ecological effects are evaluated using six criteria, including interactions with native species, changes in landscape types, and the potential to transmit genes or parasites. Effects on threatened species or landscape types receive greater weightings.

Appendix

Estimation of expected population lifetime and expansion rate

Following Leigh (1981; Lande et al. 2003:38–40), expected population lifetime can be estimated as:

$$ \bar{T}_{\text{ext}} = 2\int_{C}^{{N_{0} }} {s\left( x \right)} {\kern 1pt} \;\int_{x}^{\infty } {\frac{1}{s\left( N \right)V\left( N \right)}} \;dN\;dx , $$

(1)

where

$$ s\left( N \right) = e^{{ - 2\int_{C}^{N} {{{M\left( x \right)} \mathord{\left/ {\vphantom {{M\left( x \right)} {V\left( x \right)}}} \right. \kern-\nulldelimiterspace} {V\left( x \right)}}\;dx} }} , $$

(2)

$$ M\left( N \right) = \left( {\uplambda - 1} \right)N\left[ {1 - \left( {\tfrac{N}{K}} \right)^{\uptheta } } \right] , $$

(3)

$$ V\left( N \right) = \upsigma_{\text{d}}^{2} N + \upsigma_{\text{e}}^{2} N^{2} + 2\uprho \upsigma_{\text{e}} \upsigma_{\ln K} \tfrac{\uplambda - 1}{{\bar{K}}}N^{3} + \left( {e^{{\upsigma_{{_{\ln K} }}^{2} }} - 1} \right)\left( {\tfrac{\uplambda - 1}{{\bar{K}}}} \right)^{2} N^{4} , $$

(4)

with C, quasi-extinction threshold; K, carrying capacity; λ = e ^r, annual multiplicative population growth rate; N ₀, current population size; ρ, correlation between growth rate and environmental noise; $\upsigma_{d}^{2}$, demographic variance; $\upsigma_{e}^{2}$, environmental variance; $\upsigma_{\ln K}^{2}$, temporal variance of the carrying capacity; θ, form of the density dependence. An R-script (R development core team 2011) that carries out the estimation of expected population lifetime as described here, is available from the first author (http://www.evol.no/hanno/12/lifetime.htm).

In our analyses, we assumed the quasi-extinction threshold to be 10; carrying capacity to be 100 times current population size; temporal variance of the carrying capacity to be negligible (i.e., $\upsigma_{\ln K}^{2}$ = 0); and density regulation to be logistic (i.e., θ = 1). Simulations have shown that the estimation of expected population lifetime is quite insensitive to variation in these parameters and in demographic variance (Sandvik and Sæther unpublished data).

Population size was based on reported counts and observations available, accounting for the unreported or undetected fraction of the population (i.e., including levels of uncertainty by dividing known numbers by the estimated or suspected detection rate). Growth rates and variances were based on what is known about the life history of the taxa considered and, were available, on the actual trends of the populations in Norway. In order to obtain intervals, we used the best available estimate as well as a realistic upper limit for each parameter. Table 7 summarises the information used as input to Table 3 including the references consulted.

Table 7 Input values used for the estimation of expected population lifetime of the five example species

An alternative way to estimate expected population lifetime is by use of population viability analyses. Given estimates on extinction risk within a given timeframe, as used in criterion E of the international Red List criteria (IUCN 2001), expected population lifetime can be obtained as

$$ \bar{T}_{\text{ext}} = {{ - \Updelta t} \mathord{\left/ {\vphantom {{ - \Updelta t} {\ln \left( {1 - p} \right)}}} \right. \kern-\nulldelimiterspace} {\ln \left( {1 - p} \right)}} , $$

(5)

where p is the probability of extinction within time interval Δt.

Expansion rate was estimated as the speed $\bar{v}$ of an assumed invasion front starting from the position of first observation of the species. Corresponding to our definition of expansion rate, the assumed invasion front was, in turn, inferred using all individual observations of the species, irrespective of how the species might have ended up there (locomotion, dispersal, anthropogenic transport etc.). The speed of the invasion front was then obtained using linear regression under the assumption of sampling error and no process variance. An R-script (R development core team 2011) that carries out the estimation of expansion rate as described here, is available from the first author (http://www.evol.no/hanno/12/expans.htm).

The method described may produce unrealistic estimates in cases where an alien species has been introduced a few times at locations that are very far from each other. An alternative definition of expansion rate for such cases would be the sum of the rates estimated from each of the introductions.