Abstract
The biodiversity of ecosystems worldwide is changing because of species loss due to human-caused extinctions and species gain through intentional and accidental introductions. Here we show that the combined effect of these two processes is altering the trophic structure of food webs in coastal marine systems. This is because most extinctions (∼70%) occur at high trophic levels (top predators and other carnivores), while most invasions are by species from lower trophic levels (70% macroplanktivores, deposit feeders, and detritivores). These opposing changes thus alter the shape of marine food webs from a trophic pyramid capped by a diverse array of predators and consumers to a shorter, squatter configuration dominated by filter feeders and scavengers. The consequences of the simultaneous loss of diversity at top trophic levels and gain at lower trophic levels is largely unknown. However, current research suggests that a better understanding of how such simultaneous changes in diversity can impact ecosystem function will be required to manage coastal ecosystems and forecast future changes.
Highlights
The biodiversity of ecosystems around the world is being altered by species loss due to extinction from human activities [1] and gain through intentional and accidental introductions [2]
A comparison of the trophic distribution of extinctions with each invasion data set shows that species loss is skewed toward species from higher trophic groups relative to species gain, which is skewed towards lower order consumers (San Francisco Bay x210 = 163.03 p,0.0001, Gulf of the Farallones National Marine Sanctuary x29 = 126.64 p,0.0001, Australia x210 = 90.02 p,0.0001)
While species richness has remained nearly the same, there are 14% fewer predator species and 8.6% more primary consumer species. These results suggest that marine ecosystems have already experienced a shift in food web architecture, with shrinking numbers of predatory species being replaced by an increasing diversity of suspension and deposit feeders
Summary
The biodiversity of ecosystems around the world is being altered by species loss due to extinction from human activities [1] and gain through intentional and accidental introductions [2]. Extinctions due to anthropogenic stressors such as overfishing and climate change are thought to impact higher trophic levels first [7,8] Such a re-organization of trophic structure (i.e., trophic skew sensu Duffy [9]) may result in major changes to ecosystem structure and function, even if the total number of species in a region remains constant or even increases. These changes can impact a wide variety of ecosystem level processes [10,11] such as the total biomass and production and their distribution among trophic levels
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