Unveiling a mechanism for species decline in fragmented habitats: fragmentation induced reduction in encounter rates

Abstract

Several studies have reported that fragmentation (e.g. of anthropogenic origin) of habitats often leads to a decrease in the number of species in the region. An important mechanism causing this adverse ecological impact is the change in the encounter rates (i.e. the rates at which individuals meet other organisms of the same or different species). Yet, how fragmentation can change encounter rates is poorly understood. To gain insight into the problem, here we ask how landscape fragmentation affects encounter rates when all other relevant variables remain fixed. We present strong numerical evidence that fragmentation decreases search efficiencies thus encounter rates. What is surprising is that it falls even when the global average densities of interacting organisms are held constant. In other words, fragmentation per se can reduce encounter rates. As encounter rates are fundamental for biological interactions, it can explain part of the observed diminishing in animal biodiversity. Neglecting this effect may underestimate the negative outcomes of fragmentation. Partial deforestation and roads that cut through forests, for instance, might be responsible for far greater damage than thought. Preservation policies should take into account this previously overlooked scientific fact.

Robustness of optimal random searches in fragmented environments

“…Finally, we observe that our findings are relevant to a better understanding of the stochastic mechanisms [58] of efficient searches in multifragmented heterogeneous environments in several practical contexts. In particular, the searches performed by animals for food resources and even mates (animal foraging) is a highly significant problem with potentially drastic ecological implications [8,42].…”

And yet it optimizes

“…We start providing a very short historical perspective about LWs ideas applied to animal foraging, also addressing the initial motivations for the optimization perspective on the random search processes applied to biological encounters [2,5]. We then outline the "Lévy flight foraging hypothesis", highlighting its main assumptions and limits of validity.…”

Efficient search of multiple types of targets

“…In fact, the steps truncation upon finding targets [rule (B) above] imposes a dichotomy in-out. Note that inside (as expected from resource-rich regions forming the patches) the density of targets is high enough to induce a very large number of truncated paths (see, e.g., [34,37]). Thus the behavior within a fragment effectively is much less diffusive than out, and regardless of the strategy used (unless for μ → 3), the searcher displays different locomotion patterns inside and outside the patches.…”

“…For instance, coming back to the animal foraging context, in which the patches (representing, say, food source) often vary in (i) quantity and (ii) quality, individuals can get more benefits from visits to patches with the highest density of resources to explore [31][32][33]. Even then, if (i) is the case, at least for certain aspects [34] the combination of statistical physics ideas [35,36] with simple biologically based mechanisms [34,37]-as to assume the time spent inside a patch to be proportional to its size [34]-still can allow a simple metric like Eq. (1) to render a proper indicator of advantageous strategies.…”

“…So, the appreciation of which RS (if any) brings more assets to the forager cannot be possible analyzing just N/L. On the other hand, the drive to visit many patches may be restricted by survival conditions [37]. Indeed, long interpatch displacements in depleted areas may imply high exposition to predator, so that the animal must balance the risks and rewards of leaving a patch [34,[40][41][42].…”