Porcupines as ecosystem engineers in the central Negev
Bertrand Boeken
Ben Gurion University of the Negev, Ecophsiology,
The J. Blaustein Institute for Desert Research, Sede Boqer Campus 84990, Israel.
bboeken@bgumail.bgu.ac.il
Porcupines (Hystrix indica) create pits (ca. 15 cm diameter and 15 cm depth) in the
soil by digging for below-ground parts of perennial herbaceous plants. Because
diggings remain different from the undisturbed soil for many years, their cumulative
density can be quite high, from <5 to > 50 per 100 m2 soil surface, depending on the
density of the food plants. Besides having a direct effect on the food plant
populations, porcupines change the structure of the landscape, and thus influence plant
communities. Because of their effects on other (non-food) organisms, porcupines are
'ecosystem engineers'.
Studies in the central Negev have shown that porcupine diggings have higher density,
biomass production and species richness than undisturbed soil, because they capture
resources (runoff water with nutrients, soil and organic matter) and seeds (via runoff
and wind). Detailed studies on populations of three local annual plant species on a
north-facing slope in the central Negev hills near Sede Boqer showed that whereas
whole-community density increases in diggings arranged according to increasing
water availability, some species decrease in density and others increase.
Since increased capture of resources and seeds in diggings implies that they are 'sinks'
for these items, and that these items are captured by intercepting flows in the
landscape, communities in diggings (and, in fact, also on undisturbed soil) should be
related to the presence of 'sources' in the landscape. This was demonstrated in another
study on the same
north-facing slope, where community density, biomass and species richness correlated
with the structure of the landscape ('patchiness') at a larger scale (several 100 m2).
Two kinds of landscape patchiness were measured. Physical patchiness was measured
as the ratio of bare rock surface to soil area, which is highest on the middle slope
(Shivta formation), a little lower on the upper part (Netser), and lowest on the lower
part of the slope (Drorim). Biological patchiness was defined as the proportional cover
of shrubs relative to soil and surface. This is also highest in the middle part of the
slope, but is lowest in the upper part (Netser). Physical patchiness sugnifies the source
of resource flow, as the amounts of water and other resources flowing across the
surface in the respective parts of the slope depend on the amount of bare rock surface,
which generates runoff (while soil surface partly absorbes it). Biological patchiness
represents seed sources for annual plants, since shrubs generally have a dense and
species-rich annual plant understorey producing dispersing seeds.
Plant density and biomass production of annual plant communities in diggings did not
vary with the rock-soil ratio, but they did on undisturbed soil, although they were
lower than in diggings. Apparently, diggings collect maximum amounts of runoff, as
long as there is some runoff flow. On the other hand, species richness correlated with
shrub cover in the vicinity, but only within diggings. On undisturbed soil species
richness was very low, limited by low plant density. This study clearly demonstrated
that not only resource avialability, but also seed availability (or, in general, arrival of
the organisms themselves) is a crucial determinant of biological communities. In
addition, community structure depends on flows of the resources and the organisms
within the landscape, which in turn depend on the structure of the landscape at the
larger scale (its patchiness). Therefore, porcupines are ecosystem engineers, since
their digging activity create 'patches' that interact with these flows and alter
the abundance of other organisms.