IS IT ENEMY-FREE SPACE - THE EVIDENCE FOR TERRESTRIAL INSECTS AND FRESH-WATER ARTHROPODS

1. Enemy-free space (EFS) was defined by Jeffries & Lawton (1984) as ' ways of living that reduce or eliminate a species' vulnerability to on e or more species of natural enemies'. EFS has emerged in the literatu re as a significant niche-moulding factor. However, the lack of consis tency among the empirical studies as to how EFS should be defined, and what hypotheses should be tested in order to evaluate its relative im portance, prompted us to review the literature and to propose a workin g definition that results in a general set of testable hypotheses. 2. To test the relative importance of EFS in structuring the communities of organisms, we propose a set of three falsifiable null hypotheses th at must be tested sequentially and rejected. Ho1: The fitness of the o rganism in an original habit (e.g. on an original host plant) in the p resence of natural enemies is equal to the fitness of the organism in that habit in the absence of natural enemies. Acceptance of the altern ative hypothesis that the fitness of the organism in the presence of n atural enemies is less than in the absence of natural enemies is neces sary to demonstrate the importance of natural enemies. Ho2: The fitnes s of the organism in an alternative habit with natural enemies is equa l to the fitness of the organism in the original habit with natural en emies. Acceptance of the alternative hypothesis that the fitness of th e organism in the alternative habit with natural enemies is greater th an that in the original habit with natural enemies is necessary to dem onstrate that the alternative habit provides EFS. Ho3: The fitness of the organism in an alternative habit without natural enemies equals th e fitness of the organism in the original habit without natural enemie s. Acceptance of the alternative hypothesis that the fitness of the or ganism in an alternative habit without natural enemies is less than in the original habit without natural enemies is necessary to demonstrat e the relative importance of EFS compared with other co-occurring nich e-moulding factors such as competition or host nutritional quality. 3. We searched the literature and evaluated fifty-three references (nine teen references to seventeen different terrestrial systems and thirty- four references to twenty-four different freshwater systems) to test o ur hypotheses. 4. Of the forty-one systems examined, nineteen (46%) te sted only for differences in vulnerability of the prey or host species between EFS and non-EFS options (our Ho2); sixteen (39%) tested for t he importance of natural enemies and the effectiveness of the alternat ive habit in providing EFS (our Ho1 and Ho2); and only ten systems (24 %) tested for Ho1, Ho2 and the relative importance of EFS in the syste m as' measured by fitness (our Ho3). 5. Of the systems that tested for EFS, sixteen of nineteen (84%), thirteen of sixteen (81%) and seven o f ten (70%) showed evidence in support of the existence of EFS accordi ng to hypothesis Ho2 only, hypotheses Ho1 and Ho2, and our three worki ng hypotheses, respectively. 6. These results indicate that very few s tudies have actually tested for the existence of EFS. Nevertheless, re sults from this Limited number of natural systems suggest that EFS may be important in moulding the niches of arthropods. Because of the lar ge number of claims for EFS in systems where none of the basic hypothe ses were investigated, we suggest that authors test for EFS experiment ally, be judicious in selecting articles to cite in support of EFS, an d exert care in attributing it as a selective force in the evolution o f arthropods in specific systems.