Mixed-species groups –everything is about predation... or isn’t it?: a hypothesis and field evidence

In many species we can see individuals forming groups, and these individuals can be group members for life or just for specific periods of time. So, what are groups and why do they matter? In order to form a group it is necessary for at least two individuals to overlap spatially and temporally. However, the duration, frequency and structure of these associations can vary considerably. Groups matter in evolutionary ecology because they are major “game changers” of fitness.  In their book “Living in Groups”, Jens Krause and Graeme Ruxton review how forming groups affects fitness; for example, they provide protection from predation and may increase the acquisition of resources (i.e. food, mates and all those things that increase survival and reproductive output). But not everything is great about being part of a group, and costs also come in as part of the bargain. One of the most prominent costs is that there is a bigger chance of getting a parasite from other group mates. To make matters worse, larger groups may also increase the amount of competition among individuals and increased competition may lead to lower resource acquisition and therefore to a decreased ability to mount an immune response. Forming groups, as with so many things in evolution, is all about trade-offs. 

There is another kind of group that has elicited somewhat less attention: mixed-species groups (a.k.a. heterospecific groups). As with any other group we are talking about temporal and spatial overlapping of individuals, but in this specific case it requires that those individuals are at least of two different species. These groups might lead to asymmetric benefits for the participating species (only one of them benefits while the other doesn’t ) and requires the active behavioural choice of at least one of the species. It is in the arguments for why mixed-species groups are formed where we run into some complications. The most frequent explanation is that mixed-species groups provide anti-predator advantages, but the mechanisms through which they provide such advantages are the same as those for single-species groups, which should lead to conclude that there is nothing special about mixed-species groups and most likely that they just happen when there are not enough conspecifics to form a large group of the same species. Still, under some situations where conspecifics are abundant, we find mixed-species groups. Another possibility less explored but with some support, is that mixed-species groups reduce intra-specific competition or enhances the ability of one species to find resources. In this case, the heterospecific characteristic of the group is really what matters. But, why then do we still find mixed-species groups where resources and conspecifics are abundant?

A more interesting hypothesis (to me at least) as to why mixed-species groups are formed is that they provide protection against host-specific contagious parasites. As I mentioned earlier, increasing group size leads to more contact among group members and therefore to higher parasite transmission. So, when parasites do better on a specific  host-species (i.e. they are host-specific), an efficient way to reduce parasite transmission and their growth rate is to form mixed-species groups. The main idea here is that individuals in mixed-species groups, relative to a single-species one, should gain similar advantages in terms of protection from predators while reducing the costs of parasitism. So, can mixed-species groups reduce individual parasite load? Find out here. In a recently published study, we found that infection patterns of two Poeciliid species (Poecilia reticulata and Poecilia picta) infected by host specific Gyrodactylus spp ectoparasites support our hypothesis. Both P. reticulata and P. picta have lower Gyrodactylus prevalence and abundance in the field when they form mixed-species groups than when they are in single-species groups. As these groups are formed in sites where food and conspecific abundance is high, it is unlikely that either improved foraging efficiency or lack of conspecifics explain the formation of mixed-species groups. Independently of the mechanism by which the group is established, mixed-species groups provide an advantage to individuals by reducing parasite loads.