Table 1

Table 1. Techniques used to assess the impact of predatory invasive alien species

Technique Description/methods/outputs Advantages Disadvantages

Predictive techniques* Use of information from species that have invaded different geographic areas or ecosystems¹ Quick Cannot inform about new situations, impacts or mechanisms of impact.

Use of invasible ecosystems^{2, 3} and invasive species characteristics^4–6 Inexpensive

Food web analysis⁷ Aid for prioritizing action

Correlations in abundance and range Correlation between the timing of arrival or increase in range or abundance of IAS with the extirpation or decrease in range or abundance of a native species Data can be relatively easy to collect or past records of abundance or range may be used. Cannot derive causal relationship.
Cannot inform on the mechanism of impact.

Where experimental manipulation is not considered feasible, it may be the only source of information. Problem in some studies with independence of multiple sites^9–10

Can provide persuasive circumstantial evidence provided relationship is found at multiple independent sites and confounding variables can be eliminated⁸ Potential confounding variables (e. g., habitat loss, overhunting, pollution) over same period as introduction¹¹

Dietary Analysis Analysis of IAS diet used to assess which species may be affected through herbivory, predation, or competition Qualitative and quantitative information on potential species affected by introduction and spread of an IAS Bias in dietary analysis can lead to under- or over-representation of particular species or groups

Methods: direct observation,¹² prey items returned to den or nest sites,¹³ stomach contents,¹⁴ fecal/scat analysis,¹⁵ marks left on carcass/eggs,¹⁶ stable isotope analysis of IAS tisssue samples¹⁷ In combination with mathematical modeling, using a prey population model can provide an assessment of likely impact^{19, 20} Number of individuals, or proportion of prey population taken, is not a measure of impact although this is sometimes implied^{11, 17, 21}

Potential outputs: number of prey items taken,¹⁸ proportion of prey population taken,¹⁰ energetic contribution to IAS diet¹⁶

Demographic/behavioral studies Investigation of demographic or behavioral traits that may be attributed to IAS under laboratory, semi-field, or field conditions May provide first indication of impact Potential confounding variables (e. g., habitat loss, overhunting, pollution) over same period as introduction

Potential outputs: differences in survival,²² sex ratios,²³ population age structure, ²⁴ behavioral traits²⁵ between areas/replicates, etc. May inform on potential future impact²⁵ Without experimental manipulation, a causal relationship cannot be established

May inform on mechanism of impact²⁴

Experimental Removal Removal,²⁶ reduction, ^{27, 28} or exclusion²⁹ of IAS from monitored areas May be used: to test impact of IAS, to assess a trial eradication, as part of an adaptive management strategy³⁷ Lack of response from native population may result from: inadequate design or sample size, inappropriate timescale, need for other restorative measures^{38, 39}

Methods: differences in, e. g., population size, survival, etc. can be monitored using before/after or removal/non-removal areas. A combination of both provides the most rigorous design³⁰ May derive causal relationship Possibility of mesopredator release,⁴⁰ and other secondary effects⁴¹

Potential outputs: changes to survival, breeding,^{26, 31, 32} or foraging success, population size,^{27, 33–36} or changes in demographic characteristics May reduce impact for the duration of the study

* Examples cited as “Predictive techniques” are not specifically related to predatory IAS, but invasive species in general). As some of the techniques described represent a wide range of assessment tools, methods and potential outputs are not included for all those listed.

¹Global Invasive Species Database (IUCN), ²Lodge 1993, ³Lonsdale 1999, ⁴Williamson 1996, ⁵Kolar & Lodge 2001, ⁶Kolar & Lodge 2002, ⁷Memmott 1999, ⁸Caughley & Gunn 1996, ⁹Strachan et al. 1998, ¹⁰Ferreras & MacDonald 1999, ¹¹Newman & McFadden 1990, ¹²Brown et al. 1993, ¹³Redpath & Thirgood 1999, ¹⁴Drever & Harestad 1998, ¹⁵Chanin 1980, ¹⁶Jackson & Green 2000, ¹⁷Hobson et al. 1999, ¹⁸Apps 1984, ¹⁹Cuthbert & Davis 2002, ²⁰Dumitru et al. 2001, ²¹Cole et al. 1995, ²²Crossland 2000, ²³Leslie & Spotila 2001, ²⁴Cree et al. 1995, ²⁵McDonald et al. 2001, ²⁶Craik 1998, ²⁷Harding et al. 2001, ²⁸Allen et al. 2001, ²⁹Jackson 2001, ³⁰Parrish & Ussher 2002, ³¹Imber et al. 2000, ³²Cruz & Cruz 1987, ³³Nordström et al. 2002, ³⁴Nordström et al. 2003, ³⁵Kinnear et al. 2002, ³⁶Killion et al. 1995, ³⁷Innes et al. 1999, ³⁸Rushton et al. 2000, ³⁹Foin et al. 1997, ⁴⁰Courchamp et al. 1999, ⁴¹Zavaleta et al. 2001.


Technique	Description/methods/outputs	Advantages	Disadvantages

Predictive techniques*	Use of information from species that have invaded different geographic areas or ecosystems¹	Quick	Cannot inform about new situations, impacts or mechanisms of impact.

	Use of invasible ecosystems^{2, 3} and invasive species characteristics^4–6	Inexpensive

	Food web analysis⁷	Aid for prioritizing action

Correlations in abundance and range	Correlation between the timing of arrival or increase in range or abundance of IAS with the extirpation or decrease in range or abundance of a native species	Data can be relatively easy to collect or past records of abundance or range may be used.	Cannot derive causal relationship. Cannot inform on the mechanism of impact.

		Where experimental manipulation is not considered feasible, it may be the only source of information.	Problem in some studies with independence of multiple sites^9–10

		Can provide persuasive circumstantial evidence provided relationship is found at multiple independent sites and confounding variables can be eliminated⁸	Potential confounding variables (e. g., habitat loss, overhunting, pollution) over same period as introduction¹¹

Dietary Analysis	Analysis of IAS diet used to assess which species may be affected through herbivory, predation, or competition	Qualitative and quantitative information on potential species affected by introduction and spread of an IAS	Bias in dietary analysis can lead to under- or over-representation of particular species or groups

	Methods: direct observation,¹² prey items returned to den or nest sites,¹³ stomach contents,¹⁴ fecal/scat analysis,¹⁵ marks left on carcass/eggs,¹⁶ stable isotope analysis of IAS tisssue samples¹⁷	In combination with mathematical modeling, using a prey population model can provide an assessment of likely impact^{19, 20}	Number of individuals, or proportion of prey population taken, is not a measure of impact although this is sometimes implied^{11, 17, 21}

	Potential outputs: number of prey items taken,¹⁸ proportion of prey population taken,¹⁰ energetic contribution to IAS diet¹⁶

Demographic/behavioral studies	Investigation of demographic or behavioral traits that may be attributed to IAS under laboratory, semi-field, or field conditions	May provide first indication of impact	Potential confounding variables (e. g., habitat loss, overhunting, pollution) over same period as introduction

	Potential outputs: differences in survival,²² sex ratios,²³ population age structure, ²⁴ behavioral traits²⁵ between areas/replicates, etc.	May inform on potential future impact²⁵	Without experimental manipulation, a causal relationship cannot be established

		May inform on mechanism of impact²⁴

Experimental Removal	Removal,²⁶ reduction, ^{27, 28} or exclusion²⁹ of IAS from monitored areas	May be used: to test impact of IAS, to assess a trial eradication, as part of an adaptive management strategy³⁷	Lack of response from native population may result from: inadequate design or sample size, inappropriate timescale, need for other restorative measures^{38, 39}

	Methods: differences in, e. g., population size, survival, etc. can be monitored using before/after or removal/non-removal areas. A combination of both provides the most rigorous design³⁰	May derive causal relationship	Possibility of mesopredator release,⁴⁰ and other secondary effects⁴¹

	Potential outputs: changes to survival, breeding,^{26, 31, 32} or foraging success, population size,^{27, 33–36} or changes in demographic characteristics	May reduce impact for the duration of the study