Our latest paper, published this week in the journal Ecology and Evolution, describes how we detected the ‘signature’ of disturbance on tropical freshwater fish communities, using the streams of Trinidad’s Northern Range as a model system.
“Liming” is a popular pastime on the Caribbean islands of Trinidad & Tobago. It describes the act of relaxing with friends and family – usually accompanied by food, drink and loud music. The island’s many picturesque streams are favourite locations for liming.
Although it is a positive thing that people are enjoying and connecting with the natural environment, it also means that certain stretches of river are exposed to very high levels of recreational use. In this study we sampled 8 pairs of sites in the Northern Range of Trinidad, multiple times over a 2 year period; each matched pair of sites consisted of one well-known liming spot and a nearby, less-disturbed, site.
We also decided to put the spotlight on one species within the community – the Trinidadian guppy (Poecilia reticulata) – to see if this well-studied, fast-breeding, widespread species would reveal any patterns at the population level. We looked at the proportion of guppy biomass (in relation to total biomass) and the sex ratio of guppies at our sites.
Overall we found that the more disturbed sites were associated with significantly greater biomass and species richness than the less disturbed sites. This could be due to greater nutrient input at the disturbed sites, which may allow it to support more individuals and consequently more species (known as the “more individuals hypothesis”).
We also found that higher levels of disturbance tended to correspond with more female-biased guppy populations. This may be due to a greater sensitivity to physiological stress in male guppies, perhaps as a result of males increased investment in bright colouration.
Initially increased biomass and increased richness sound like positive effects. However, it is important to remember that the knock-on effects of human-induced changes are complex and still poorly understood, so it is not yet possible to say whether detecting increases in these parameters is reassuring, neutral or something to be concerned about.
What we do know, is that as human populations increase, recreational use of natural habitats is set to do the same – not least for tropical streams - making it increasingly urgent that we understand the effects we have on such ecosystems.
These results from Trinidad’s streams also add to our understanding of other tropical ecosystems globally, and will help us learn more about how to conserve and manage those that are most vulnerable. Importantly, the fact that some of the most commonly employed indices of diversity did not detect any effect of disturbance in this system, emphasises that we should not rely on these ‘classical’ measures alone, and that further research into the implications of changes at all levels of community properties is urgently needed.