Human Impacts on Marine Ecosystems I. Types of Impacts II. Overharvesting III. Invasions IV. V. Physical Alterations VI. What we see today is not the pristine state of nature How does this observation affect our ability to understand evolution and ecology? Is it meaningful or even possible to think about nature without humans? Log Abundance Living habitat structure Large mobile animals Input from land then now Time Indirect effects of overfishing Near elimination of megaherbivores and megapredators From Knowlton and Jackson 2001 Types of Human Impacts 1. Overharvesting 2. Invasions 3. 4. Physical Alterations 5. Diverse Human Impacts cod Acropora coral oyster Jackson et al. 2001 Jackson et al. 2001 Overharvesting Overharvesting is widespread and fast is relatively recent for most species, coinciding with advent of new technologies Then Now Myers & Worm 2003 1
1. Growth Overfishing Effects of Overfishing 1. Growth Overfishing 2. Recruitment Overfishing 3. Ecosystem Overfishing average size of individuals declines, by... selective removal of large fish or shortened lifespan associated changes younger average & maximum ages decreased reproductive output skewed sex ratios Growth overfishing of Nassau groupers in Jamaica (Thompson & Munro 1978) 2. Recruitment Overfishing adult biomass reduced to point where recruitment declines Lightly fished area Abalone in California Allee effect? Heavily fished area Recruits per collector 2.5 2 1 9000 Adults per ha 0 S. Cal N. Cal 0 S. Cal N. Cal Rogers-Bennett et al. 2004 3. Ecosystem Overfishing decline in fish population alters community & ecosystem structure removal of apex predators: fishing down food webs (Pauly) causes trophic cascades: - e.g., the sea otter & killer whale story Fishing has altered trophic structure of Hawaiian coral reefs Friedlander & DeMartini 2002 2
Invasive Species Types of invasion Natural (rare) - exceptional dispersal (e.g., island colonization) - new corridors Human Mediated ( Introductions ) vessels biological industry (aquaculture, bait, fisheries, aquarium) scientists canals other common: est.15,000 spp. per week (Carlton 1999) unpredictable Invasive Species Examples Human Mediated ( Introductions ) invasive Caulerpa in Agua Hedionda Lagoon - vessels: most known marine invasions (e.g., Chinese mitten crab, European green crab) - biological industry (e.g., sabellid polychaete from abalone farms; Caulerpa algae in S. Cal) - scientists (e.g., Fungia coral into Jamaica) - canals (e.g., Suez Canal: >250 spp. established in Mediterranean invaded from Red Sea) - other (e.g., introductions for fisheries: snappers & a grouper in Hawaii, striped bass in California) Invasive Species Consequences none for most introduced species (most don t survive) for some species, severe consequences: - modify habitat - competition - predation invasive Spartina in SF Bay Types Toxicants Nutrients Inputs Point-source Large scale Cuyahoga River burns in 1969 Types Toxicants pesticides (DDT) industrial chemicals (PCBs) metals (mercury, lead) - sources: discharges, coal burning, gasoline combustion Nutrients nitrogen (ammonia, nitrites, nitrates) carbon (e.g., sewage) - sources: agriculture, sewage, animal wastes, runoff, car exhaust Inputs Point-source (e.g., sewage) - effects relatively easy to measure Large scale (e.g., mercury input) - effects difficult to measure (baseline?) 3
Habitat Alteration Intentional land reclamation - estuarine habitats - mangroves - beach & sand dune Unintentional fishing - dredge & trawl damage - dynamite fishing boating - anchor damage - propeller scars - grounding Habitat Alteration Severe impacts on ecosystems (due to facilitation/ support by habitat producing organisms) Difficult to mediate (need to restore lost habitats) Background Causes Predicted Climate Change Consequences Background climate of earth continually changes temperatures now are cooler than they were during 95% of the last billion years Causes increase in greenhouse gases (CO 2 & CH 4 ) traps heat in the atmosphere scientific consensus: human activities are the main cause Predicted Climate Change warmer (but how much?) higher sea level (but how much? 0.5-1.5 m by 2100) increased intensity or frequency of storms changes to ocean circulation? 4
Consequences Measured + 0.6 C in last 100 years several cm increase in sea level poleward shifts by warm water species coral bleaching Possible altered primary production ecosystem-wide responses shoreline retreat Effects of global climate change on marine ecosystems is extremely difficult to predict because the magnitude of climate change is difficult to predict how living organisms will respond to it can be very complex 5