Causes and consequences of consumer interactions across latitude in a marine ecosystem

Background/Question/Methods

One of the clearest patterns in ecology is the latitudinal diversity gradient that shows most species on Earth occur in the tropics, with far fewer species occurring in higher-latitude temperate environments.  However, we know surprisingly little about the ecological and evolutionary mechanisms that underlie both the origin and maintenance of this biodiversity gradient.  One long-standing hypothesis suggests that species interaction strengths are stronger in the tropics than in higher-latitude environments, leading to higher diversification rates and ultimately the rich tropical biodiversity observed today.  While multiple lines of evidence support this hypothesis, we know very little about how this change in interaction strengths with latitude shapes contemporary patterns of species diversity.  Our prior research using marine epifaunal communities in a simplified model habitat demonstrated that stronger consumer pressure in the tropics relative to the temperate zone can affect the maintenance of prey species diversity in ecological time.  Our goal was to translate these advances to a complex natural ecosystem of conservation importance, seagrasses.  Using standardized experiments in New Jersey and Panama, we tested the hypothesis that consumer pressure is stronger in the tropics than the temperate zone and will predictably shape patterns of biodiversity in seagrass ecosystems.

Results/Conclusions

Using standardized consumer exclusion experiments, we found that consumers had a stronger effect on sessile invertebrate richness on seagrasses in tropical Panama in comparison to temperate New Jersey over an 8-week time period.  Consumers also had a stronger effect on invertebrate recruitment in Panama than New Jersey after only two weeks.  Using consumer exposure experiments, we observed maximum consumer activity rates that were higher in Panama than New Jersey, suggesting a possible ecological driver of the strong aggregate effects of consumers in Panama.  Recruitment rates, however, were several orders of magnitude higher in temperate New Jersey.  Therefore, in New Jersey, high recruitment, likely due to both high propagule supply and low mortality at early life stages, may also compensate for any post-settlement losses by consumers and weaken observed consumer effects. This research provides insight into the causes and consequences of latitudinal variation in consumer interactions for patterns of biodiversity in a complex natural system, and also highlights the utility of standardized comparative studies to understanding macroecological patterns.