Table 3.

Invasion ecology is confronted with three domains of difficulties. Domain C: Conceptual and methodological difficulties, and measures to meet them. C1–C3 relate to the conceptual basis and theory of invasion ecology, C4 and C5 to empirical research, and C6–C8 to the need of integration with other scientific disciplines and societal groups. Letters and numbers in parentheses refer to difficulties given in Tables 1 and 2

	Difficulty	Measures
C1	Terminology: unclear concepts and definitions	Explicit definitions (see checklist in Box 1)
C2	Insufficient synthesis; sub-division of invasion ecology (e.g., taxonomic groups)	Hierarchy of hypotheses (HoH) with precise, testable hypotheses at lowest level
C3	Imprecise hypotheses (a) different versions of hypotheses (b) lack of testability	HoH
C4	Lack of data to test hypotheses (a) lack of data on unsuccessful introductions (b) lack of large-scale experimental data (c) lack of long-term data	Funding of large-scale and long-term research ‘Indirect’ methods (e.g., retrospective analyses and model simulations instead of long-term experiments) Online databases Citizen science and monitoring programs by the general public
C5	Bias in data collection (a) invasion events (most research on successful species in areas with high density of researchers) (b) methods of data collection	Frequent reviews with connection to HoH; aim: identification of gaps and biases
C6	Necessity of communication of research results to concerned stakeholders (A4)	Focus on output valuable for applications Up-to-date networks and platforms Joint conferences and discussions
C7	Complexity (B1) creates the need to integrate other biological subdisciplines	Integration of HoH into other disciplines Joint conferences and discussions
C8	Influence of socio-economic and cultural processes on invasions (A1, A2, B3) creates the need for transdisciplinary research	Communication and collaboration with researchers of humanities and social sciences Joint conferences and discussions