Using spatial pattern analysis to distinguish causes of mortality: an example from kelp in north-eastern New Zealand

1 Spatial analysis techniques were used to differentiate between climate-induced and pathogen-induced mass mortalities of the kelp Ecklonia radiata in north-eastern New Zealand. We predicted that climate-induced effects would generate broad-scale patterns, whereas pathogen-induced mortality would be traceable among neighbouring thalli. 2 Spatial autocorrelation analysis was performed on the proportion of E. radiata affected by dieback in quadrats during an initial mortality event in 1991. The absence of any consistent spatial scale of affected thalli between 10 and 100 m suggested that small-scale spread of an agent might be occurring. 3 Individual thalli were therefore mapped at two sites during a subsequent mortality event in 1992/93, and the degree of damage recorded. Spatial analyses found little evidence of aggregation of either intact or affected thalli at scales of 1-150 cm. 4 The relative spatial patterns of healthy and affected plants in mapped quadrats during the 1992/93 mortality provided little evidence of spatial association or repulsion between these broad damage categories. 5 The large-scale mortality of 1992/93 was consistent with a physiological response to broad-scale light deprivation, although other agents, perhaps both a virus and amphipod grazing, might also have been involved. Potentially complex interactions among the candidate agents render interpretation of the spatial patterns difficult.