How latitude affects biotic interactions.

A re biotic interactions, such as competition and predation, stronger in the tropics than at higher latitudes? Biologists have long speculated that more intense interactions tend to occur at lower rather than at higher latitudes as a result of year-round warm and wet conditions near the equator, particularly in lowland tropical rainforests (1). This idea is now well established in the ecological literature (1, 2), but recent debate has questioned whether such a latitudinal gradient in biotic interactions actually exists (3). On page 1389 of this issue, LaManna et al. provide evidence that the intensity of at least some types of biotic interactions are indeed stronger closer to the equator (4). In a recent study, Roslin et al. (5) experimentally assessed global patterns of predation rates on insects by using fake (plasticine) caterpillars placed at sites on six continents spanning a latitudinal gradient of >11,000 km. They found increasing rates of insect predation toward the equator. The higher predation risk at lower latitudes was driven by arthropod predators (such as ants) rather than birds or mammals. Use of standardized methods to assess predation across multiple sites allowed Roslin et al. to make robust comparisons among sites and validate the existence of a latitudinal gradient in predation. Similarly, LaManna et al. used data collected with standardized methods at multiple sites from around the world to test for a latitudinal gradient in the strength of biotic interactions. Rather than predation, they investigated interactions between neighboring trees, focusing on a phenomenon referred to as conspecific negative density dependence (CNDD), where an individual is negatively affected by the local density of other individuals of its own species (conspecifics). Biologists have long hypothesized that plants should have lower performance (e.g., ECOLOGY