Temporal Variation in Trophic Cascades

The trophic cascade has emerged as a key paradigm in ecology. Although ecologists have made progress in understanding spatial variation in the strength of trophic cascades, temporal variation remains relatively unexplored. Our review suggests that strong trophic cascades are often transient, appearing when ecological conditions support high consumer abundance and rapidly growing, highly edible prey. Persistent top-down control is expected to decay over time in the absence of external drivers, as strong top-down control favors the emergence of better-defended resources. Temporal shifts in cascade strength-including those driven by contemporary global change-can either stabilize or destabilize ecological communities. We suggest that a more temporally explicit approach can improve our ability to explain the drivers of trophic cascades and predict the impact of changing cascade strength on community dynamics

The effect of lizards on spiders and wasps: Variation with island size and marine subsidy

Introduced predators can have dramatic effects on island ecosystems, the magnitude of which are likely to vary with island characteristics. We investigated the influence of two important properties of islands-size and amount of resource subsidy-on the effects of an introduced predatory lizard (Anolis sagrei) on three groups of arthropod prey. Lizards were experimentally introduced to 16 islands that spanned gradients in vegetated area and seaweed deposition (a marine resource subsidy); 16 similar islands served as lizard-free controls. The abundance of web spiders, salticid spiders, and wasps was estimated prior to lizard introduction and again four months after lizard introduction. Lizard introduction reduced the average abundance of all three groups of arthropods. The effect of lizards on salticid spiders- which was very large (94% reduction in salticid abundance)-decreased with island size. In contrast, the effect of lizards on wasps-which was also very large (88% reduction in wasp abundance)-tended to increase with island size, but with only marginal significance. There was no evidence for variation in the effect of lizards on web spiders with island size. This variation between prey taxa may be related to the relative importance of environmental stress (such as wind and wave exposure, which tend to be more pronounced on smaller islands) in determining abundance. Salticids seem to tolerate the stressful environmental conditions that characterize smaller islands, allowing for larger lizard effects; wasps seem to be limited by these conditions (either directly or indirectly via reduced prey availability), minimizing lizard effects on smaller islands. There was a marginally significant tendency for the effect of lizards on salticid spiders to be weaker on islands with more seaweed deposition, suggesting that subsidies may play a role in reducing predator effects on islands. Our results highlight the importance of ecological context in determining the top-down effects of introduced predators and underscore the need to extend existing theories relating island area and community characteristics toward an explicit consideration of species interactions

Consumer responses to experimental pulsed subsidies in isolated versus connected habitats

Increases in consumer abundance following a resource pulse can be driven by diet shifts, aggregation, and reproductive responses, with combined responses expected to result in faster response times and larger numerical increases. Previous work in plots on large Bahamian islands has shown that lizards (Anolis sagrei) increased in abundance following pulses of seaweed deposition, which provide additional prey (i.e., seaweed detritivores). Numerical responses were associated with rapid diet shifts and aggregation, followed by increased reproduction. These dynamics are likely different on isolated small islands, where lizards cannot readily immigrate or emigrate. To test this, we manipulated the frequency and magnitude of seaweed resource pulses on whole small islands and in plots within large islands, and we monitored lizard diet and numerical responses over 4 years. We found that seaweed addition caused persistent increases in lizard abundance on small islands regardless of pulse frequency or magnitude. Increased abundance may have occurred because the initial pulse facilitated population establishment, possibly via enhanced overwinter survival. In contrast with a previous experiment, we did not detect numerical responses in plots on large islands, despite lizards consuming more marine resources in subsidized plots. This lack of a numerical response may be due to rapid aggregation followed by disaggregation or to stronger suppression of A. sagrei by their predators on the large islands in this study. Our results highlight the importance of habitat connectivity in governing ecological responses to resource pulses and suggest that disaggregation and changes in survivorship may be underappreciated drivers of pulse-associated dynamics

Temporal Variation in Trophic Cascades

The trophic cascade has emerged as a key paradigm in ecology. Although ecologists have made progress in understanding spatial variation in the strength of trophic cascades, temporal variation remains relatively unexplored. Our review suggests that strong trophic cascades are often transient, appearing when ecological conditions support high consumer abundance and rapidly growing, highly edible prey. Persistent top-down control is expected to decay over time in the absence of external drivers, as strong top-down control favors the emergence of better-defended resources. Temporal shifts in cascade strength-including those driven by contemporary global change-can either stabilize or destabilize ecological communities. We suggest that a more temporally explicit approach can improve our ability to explain the drivers of trophic cascades and predict the impact of changing cascade strength on community dynamics