Our current best conceptual understanding of the stressor ‘biota removal/disturbance’ is shown in Figure 1.
Figure 1. Potential causes of a change to biota removal/disturbance and the condition responses observed as a result of this change.
The removal, loss or disturbance of individual organisms of a specific species results in loss of biodiversity. Biodiversity is the variety of life forms: the different plants, animals and microorganisms, the genes they contain and the ecosystems they form. Maintaining biological diversity is about the sustainable use of biological resources and safeguarding the life-support systems on earth. The direct removal of biota through commercial and recreational fishing, bait collection and aquarium fish collection is a major cause of a change to this stressor. Fishing results in the removal of large numbers of individuals of the target species. The effects on these target species are undoubtedly significant and may be one of the more important stressors in an estuary. The improvement in target species stocks in areas that have been temporarily closed to fishing is evidence of this.
In addition, many species of endangered or threatened marine mammals, sharks and reptiles are particularly at risk from boat strike, shark nets or drum lines. Shark nets and drum lines offer some protection to swimmers by ‘fishing’ for potentially dangerous sharks and reducing their numbers around protected beaches. Unfortunately, they also indiscriminately capture other marine life, including harmless sharks, rays, dolphins, dugong, whales, turtles, etc., some of which are critically endangered. Boat strike is another human related cause of death for marine mammals and reptiles. Theses animals are air breathers and must surface regularly, therefore, putting them at risk of boat strike.
The inadvertent loss of even a small number of critically endangered animals a year due to humans will have a high impact on the chances of survival of the species. Boat strike, shark nets and drum lines thus pose a large threat to many marine mammal, shark and turtle species, particularly the threatened or endangered ones.
Habitat connectivity and presence of species refugia, as well as the size and population structure of affected species are natural mediating factors which influence the impact of biota removal/disturbance on the ecosystem.