On Critical Thresholds in Landscape Connectivity: A Management Perspective

Critical thresholds imply that the effects of fragmentation on connectivity and, ultimately, on population persistence are non-linear. The loss of species with the increasing degree of fragmentation and habitat loss are subtle in the beginning, but after some threshold in the proportion of original habitat the decline in diversity may be very rapid. Andren (1994) reviewed the existing literature on mammals and birds in habitat patches in landscapes with different proportions of suitable habitat. He concluded that for these taxa critical threshold levels are between 10 and 30% of suitable habitat. In the landscape below this level of habitat availability the loss of species or decline in population size will be greater than expected from habitat loss alone. This result should, however, not be interpreted as a guide line in forest management and we cannot conclude from the review that the negative effects of fragmentation on terrestrial bird and mammal assemblages will only appear after up to 70 or 90% of the original habitat is lost. In this article we, first, argue by reanalysing the data compiled by Andren (1994) that the empirical generalised threshold levels for birds and mammals are not well substantiated. Second, we discuss more generally the problems involved in identifying generalised critical thresholds in landscape connectivity. Connectivity is, by definition, a species-specific characteristic determined by the interaction between the movement potential of each species and landscape structure. Third, we argue that generalised critical thresholds should be used with caution in landscape management. If management aims at maintaining viable populations of all species in an area, one obviously should take into account also more sensitive species in the habitat fragmentation process rather than focusing on average species. Finally, we discuss the critical threshold concept in relation to different spatial scales.